Rubber properties : Freezing

The effects of temperature so far described take place instantaneously with any change of temperature. There is , however at least with certain types of rubber including natural rubber, another effect which is produced by long exposure to moderately low temperature. This is a gradual hardening, often referred to as ‘freezing’ , due to the tendency of part of neighbouring molecules to line up in parallel ; this permits stronger intermolecular attractions which bind the parallel molecule segments into a more or less rigid ‘crytallite’ , to use a familiar, though not very accurate, name. This process is quite gradual and proceeds over periods of many days; it is most rapid at a particular temperature which in the case of natural rubber about –25 *C . Vulcanisation reduces, though it does not eliminate, this technically undesirable effect, which must be noted especially in connection with the storage of rubber articles. The change from rubber-like to glass-like properties occurs at a temperature called the glass transition temperarture. Certain other properties also change suddenly at this temperature, for instance, the coefficient of thermal expansion is less below the glass transition temperature, but rubber is never used in practice below this temperature. Another inportant consequence of the influence of temperature on deformation rate is that, broadly speaking, the behaviour of rubber is change in the same way either by increasing the rate of deformation or by lowering the temperature; conversely, reducing the deformation rate produces the same effect as raising the temperature. There is, in fact, a fundamental relationship between temperature and strain-rate effect.

Cellular rubber (Foams)

There are two processes for the manufacture of foams from natural and synthetic latices. The introduction of air or gas into the latex to create foam is common to both processes. This foam is then gelled, cast into moulds, vulcanised and dried. With latex beating the latex mix is suspended in soap or gelatine and beaten from a 7- to 14-fold vulcanisable volume. For this machine wire beater with a selected steplessly variable beater speed is used. Gelling with sodium silicate fluoride keeps the foam for an extended time in a castable condition. The foam is cast into a mould and vulcanised at approximately 373 K . The cured foam rubber is removed from the mould washed free of clinging chemicals and dried.

With blown latex, hydrogen peroxide is added together with blowing agents to a vulcanisable latex mix which is decomposed. The created oxygen foams up the volume of the latex from 8 to 14 fold, the foam consisting of small regularly spaced cells. It is frozen at 258 to 263 K and carbon dioxide is passed through the rigid foam. After thawing out, the foam becomes liquid. It is then vulcanised, washed and dried in the usual way.

The manufacture of plastic foams is basically different from natural and synthetic latex foams. Polystyrene foam contains a blowing agent which expands by heating the mass to over 350 K and produces a cell structure. With polyether or polyester respectively the foam structure is built by a chemical reaction which releases carbon dioxide; the mass rises and sets simultaneously.

Rubber properties : Endurance limit

The strength of rubber under a continuous oscillating stress is of particular significance because such an application occurs so frequently in practice. By endurance strength is meant that stress which a rubber can sustain indefinitely under an oscillating load without damage. It is experimentally ascertained with help of suitable fatigue-tensile machines through a stress-frequency curve. Modern rubber testing machines permit the rubber to be subjected to a loading which corresponds to the one occurring in practice. Thus a good insight is achieved into the elastic and thermal relationship, especially into the fatigue life.

Thermal properties of rubber compound : thermal diffusivity

Thermal diffusivity can be conveniently determined by observing temperature change as a function of time for sample geometrical shapes under heating condition. Alternatively of course if k r and Cp are known or separately determine. Thermal diffusivity can be calculated from its definition as thermal diffusivity = k/r*Cp. As specific heat is an additive property, it is generally convenient to calculate specific heats of rubber compound than to measure them. Specific heat of a rubber compound is given by

Cp = w1*C1 + w2*C2 + w3*C3 + …..

where w1, w2 and w3 are weight fractions of the ingredients and C1, C2 and C3 are their specific heat. In general for rubber compound above the grass transition thermal diffusivity tends to decrease slightly with increasing temperature. This is attributed to an increase of specific heat as temperature increase. However some author stated that there is no significant change in thermal diffusivity over the temperature range from room temperature up to 140 *C.

How to increased thermal conductivity of rubber compound.

Thermal conductivity is the basic parameter for defining heat flow in a material. Normally the thermal conductivity of rubber compound is inversely propertional to temperature but in the range of 20-90*C, the value of thermal conductivity of both gum and carbon black filled compound are slightly changed, therefore it can be assumed that the thermal conductivity of rubber is independent of temperature without any significant error. The inclusion of filler has a marked effect on thermal conductivity of the rubber compound which postulated that 10 phr of carbon black may be expected to increase the thermal conductivity by about 17% at room temperature. In the meantime we founded that thermal conductivity was an additive property depending on the volume fraction of the ingredient by an appropriate conductivity and adding these product to the thermal conductivity of gum vulcanisate. A large dependence of conductivity on loading of carbon black was reported that thermal conductivity of rubber compound increased almost linearly with black content in the range of 10-50 phr. An experimental result clearly showed that thermal conductivity increased linearly with carbon black loading and therefore the mathermatical relations between thermal conductivity and carbon black loading were introduce as

k(w) = k(0) + 0.32w

or

k(j) = k(0) + 0.4jf

where w is the weight fraction and j is the volume fraction of crabon black.

Apart from carbon black loading carbon black structure seems to have an influence on the thermal conductivity of rubber as well. Effect of carbon black structure on thermal conductivity of NR compound we found that higher carbon black structure gives higher value of thermal conductivity. However the degree of change in thermal conductivity as a function of carbon black structure is small and is not straight forward.

Thermal Properties : Basic principal of heat transfer

Holman had proposed a difinition of heat transfer as “science which seeks to predict the energy transfer which may take place between material bodies as a result of temperature difference”. The energy transfer is known as heat . There are three mode of heat transfer : conduction, convection, and radiation.

1. Conduction heat transfer : When a single body is subjected to a temperature gradient heat will be transferred from the hight temperature region to the low temperature region. This mode of heat transfer is call “conduction” and from Fourier’s law for steady state condition, the heat transfer rate per unit area is proportional to the normal temperature gradient, as shown in equation 1

when dQ/dt is the heat transfer rate and partial derivative of temperature respective to x is the temperature gradient in the direction of the heat flow. The constant k is called the thermal conductivity of the material which is define as the heat transport in a material per unit temperature gradient per unit area between two isothermal planes. In case of unsteady state (transient) conduction, where the temperature at any point in a body varies with both time and position, the basic Fourier equation becomes a patial differential :

where C and r are specific heat and density, respectively. The expression k/(rC) is called “thermal diffusivity” of the material and is represented by a .A hight value of a could either result from a high value of thermal conductivity (k), which would indicate a rapid energy transfer rate, or from a low value of the thermal heat capacity rC . The low value of heat capacity indicate that less of the energy moving through the material would be absorbed and used to raise the temperature of the material thus more energy would be available for further transfer. Therefore, the larger the value of a , the faster will heat diffuse through the material.

Rubber wedge mounting

When flat rubber sandwich springs are placed symmetrically to form a wedge, compressive and shear strains will be imposed in the rubber simultaneously under action of the load. This combined compression and shear stress in the rubber is specially advantageous to its strength. Additional bending stresses which are unavoidable in simple shear loading are reduced by the superimposition of the compressive stresses. Bending stresses can even be completely eliminated if the rubber bodies take the form of a parallelogram whose centerline follows the direction of the compression-shear resultant. The same compressive and shear stress will then arise at every point.

Wedge rubber spring are particularly interesting for the designer as they defect in three directions and at the same time possess very different stiffness in each of the directions of their three main axes. It is a wedge rubber spring designed with special care. The rubber is protected against mechanical damage and attack from oil. The rubber spring can easily be attached both to the machine and to the floor.

Rubber mould

Rubber product can be produced reliably and economiclly only if moulds are designed to suit the rubber moulding process. The three major moulding methods used in rubber product manufacture are compression rubber moulding, transfer rubber moulding and injection rubber moulding. rubber mould in general are made of metal such as steel, cast iron, aluminium and aluminium alloy. The moulds must be so designed that they can withstand large forces without deformation. Further there must be ready access for the introduction of blanks and the easy removal of finished rubber product. Apart from two-part rubber moulds, multi-plate rubber moulds with loose cores and split inserts, are also in use. Vertical face are given a small taper. Large undercuts are to be avoided. Ribs which protrude into the cavity should not be too thin as they may bend or be damaged during compression. Rubber mould must be provided with appropriate vents to avoid faults in rubber product because of incomplete filling of the cavity or voids in the rubber moulding caused by the entrapped air.

Rubber product are mostly produced by compression moulding. The numbers of cavities frequently amount to 1 to 12 or more. With transfer moulding up to 90 cavities may be served. This is possible because transfer moulding does not call for the insertion of prepared uncured blanks into cavities. Instead, the mixed blank is placed into transfer pot from where the mix flows through sprue and runner into single closed cavities under pressure of the transfer plunger. Rubber injection moulding of rubber product is as yet a comparatively new process. It draw on the processing of plastics and allow extensive automation with moulding times which are at the most up to a tenth of the time needed for compression or transfer moulding.

Rubber product : rubber fender

Quay structures are today successfully protected from impact by ships with the help of rubber fender. This involves large systems of rubber fender to make the fender elastic. Different designs are used such as bonded rubber ring or loose hollow or cylindrical rubber fender. In every case extraordinarily large overall measurements are encountered in individual spring. On the new quay in Puttgarden on the Fehmarnbelt the special form of a loose hollow rubber fender has been chosen. The rubber fender has an outer diameter of 402 mm, a height of 500 mm , a hardness of 70 +/- 5 IRHD and a progressively rising rubber fender characteristic. It can be compressed to maximum of abount 340 mm with an expended force of 700 kN and an expended energy of about 55 kJ.

Rubber properties : Ageing

Rubber ageing is that process by which natural rubber grades are exposed over a period of time to the attack of oxigen, ozone and ultraviolet light. Under these conditions cracks develop on the exposed surface. In bonded rubber products the rubber remains largely protected by the vulcanised-on metal part and antioxidants. Temperatures over 350 K (abount 80*C) accelerate ageing. To retard ageing, antiozonants are added to the rubber mixing process. The application of lacquer to exposed rubber surface is not of great significance. Spring of natural rubber should be compounded with waxes and thus always protected from the action of oil or petrol because these cause swelling, loosening of structure and reduced strength. Rubber product in systhetic rubber show greater resistance to the factors mentioned than those made from natural rubber.

Rubber pad

The circular rubber pad.

The circular rubber pad is the oldest form of bonded rubber spring. In design it is characterised by the diameter and the height. In practice it is given the name of “metalatic” pad mounting. Its range of application is very large. There are manufacturer who make circular rubber pad in over 100 different size and each in five different rubber grade for loading ranges up to 100 kN per rubber spring. In the moulding process the rubber pad are bonded to adjoining metal insert etc. Some design the rubber pad have metal plates bonded on both ends or have plates only at one end.

The hollow rubber pad.

The ejector spring which are required in cutting and formimg press tools for sheet metal work are frequently designed with hollow rubber pad instead of metal spring. They are cheap and reliable in operation and make simple assemblies. The German AWF Committee of the press manufacturers has published a set of guidelines for the choice, calculation and arrangement of hollow rubber pad. Polychoroprene, nitrile and natural rubber have become the eatablished material. Nitrile rubber is stable under the influence of oil, polychoroprene are reasonably stable, and natural rubber is unstable but natural rubber does not tend to creep as much as nitrile rubber. Creep is understood to be the relaxation of the spring force under continuous stress.

The rubber ring spring

The rubber ring spring, known as a low-frequency stud mounting, can be, as shown in Figure 1 (a), stress by three types of loading A, B, and C. The elasticity of rubber ring has different values in the separate directions. It is largest in direction B and smallest in direction A, as shown in the characteristic lines in Figure 1(b). In compressive loading (direction A), the compression may be so large that the hole is completely closed. The description ‘low-frequency mounting’, mean that this rubber ring is suitable for such cases where a low natural frequency is required. It can be loaded up to 100 N per mounting. Thus rubber ring uses lies in the field of precision instrument applications.

Figure 1 Rubber ring spring

Rubber product : Rubber supported rails

In general, rails used to be supported by wooden sleepers. Wood was preferred because of its elasticity and its damping action. With modern permanent way, however, preference is given to reinforced concrete sleepers which facilitate the use of continuous welded rails. Their use eliminates rail joints and reduces the maintenance expenses due to joint ends, fishplates and securing bolts. However, the concrete sleepers are rather inelastic and because of this use is made of rubber sheet rail mats of special design and characteristic. These are mats with trapeze shaped grooves provided at the top and bottom faces to ensure the required compressive and shear elasticity. Because of the wide temperature variation, which on European railways can rang from 243 to 333 K (-30 to 60 *C), use has to be made of synthetic rubber. The mats are made in two size, 166x123x4.5 mm and 141x125x4.5 mm. The end are flanged to prevent displacement. The Geman Federal Railways(DB) have successfully applied these pad and also at high speeds. Similar pads are also used experimentally by the French State Railways and the Stockholm Underground.

Rubber mounts : Relay boxes

Example of rubber mounts that presents in this article is relay boxes. The relay box of the numerically controlled machine tool in Figure 1 is fixed to the machine itself. To protect the sensitive relays from impact and vibrations of the machine, the box is insulated with the help of rubber product. With regard to load application, two different types of elements are used. A moment is produced by the weight of the box, which applies tension to the upper rubber mounting point. But tensile applications are unfavourable to rubber. Therefore a bell shape, such as in Figure 1, is chosen for the upper rubber mounting point, in which the rubber is then stressed in compression. The lower ‘W’ shape is likewise stressed in compression by the moment. The shear load is taken up in almost equal parts by both elements.

Figure 1 Example of rubber mounts : relay boxes

Rubber Properties : Damping Properties

Damping is the difference between deforming work and elastic recovery. The energy loss is caused by friction and is determined by the loading and unloading phase of a process. Damping due to surface friction between the rubber product and a mating surface may be present. But there is also internal friction within the elastic body itself, call material damping. With internal friction, unlike surface friction, no appreciable variations occur in the fraction forces. In both cases the work of damping is changed into heat, which thus account for the energy lost in vibration. The case of oscillating loading is presented by Figure 1. The area of the loop U1 – U2 is a measure of damping. It is equal to the energy loss per oscillation and is called absolute damping in joules but published values so far are given mainly in kgf cm . The ratio of the area of the strip U1-U2 to the area under the upward curve U1 is the percentage damping. The area U1 is the total strain energy, also called absorbed energy. The Roelig percentage damping is thus given by

damping[%] = 100x(U1-U2)/U1

Figure 1 Damping Properties

A percentage damping of, for example, damping = 30% mean that 30% of the total energy imposed on the rubber material is absorbed by the rubber material, i.e. damped. The percentage damping of natural rubber grows from 6 to 30% with increase rubber hardness. With synthetic rubber, e.g. butadiene rubber, the value are higher for soft grades but are almost identical for harder grades. The damping capacity of rubber is considerably greater than that of steel. The percentage damping is not greatly used in the calculation of dynamic problems. The damping has no constant value. It depends on the grade of the rubber, temperature, strain rate(velocity) and acceleration, shape, and type of stress (compression, tensile, or shear). Generally valid damping values which would be approximately dependent on IRHD can't be stated. The magnitude of damping has to be obtained in given cases by enquiry from the rubber product manufacturer or ascertained dynamically with the help of a suitable apparatus. The determination of dampingand dynamic modulus is governed by ASTM D945-55. A preloaded probe is stressed dynamically. The damping and the dynamic modulus are determined from the area of the hysteresis loop and its position. The Yerzley oscillograph is particularly well suited for determination of damping under impact load when the oscillation curves are plotted. The behaviour of rubber product which are subject to forces caused by oscillating masses is tested on the Barry impact machines. If energy tires is designed , its damping properties is lower than general tires becuase energy tires must have lower energy lost.

Why use rubber material in vibration dampers ?

Vibration dampers is mechanical part that absorb energy from working condition. The ability to absorb energy is dependent on the damping of material. Rubber material have high damping when compared with other material thus it has been used as material in vibration dampers . Its properties that indicate high damping is energy absorption. Which must be considered in the design of vibration dampers. Energy absorption must define with specific energy , some time referred to inadvisably as energy density, is the capacity to store elastically the spring energy U which is absorbed by 1 kg of spring mass. It is calculated from

Usp = U/m (J/kg)

where m is the mass of the spring material. Table 1 shows that the specific energy storage of a rubber spring is greater than that of steel. This mean that in practice the use of rubber material represents an appreciable saving in weight. Specific energy storage is greater in simple shear than in compression or tension. The best performance is shown by rubber material which are subjected simultaneously to shear and compression, yielding both the best exploitation of material and highest endurance strength.

Table 1 Specific energy Usp of rubber material in comparison with steel

Table 1 shown that vibration dampers that absorb energy depends on material and type of loading. In design vibration dampers must be consider type of loading which affect in performance of vibration dampers.

The conical rubber ring vibration mounts

A form of the conical rubber vibration mounts design is given again in Figure 1. It is described as a machine suspension and is used to isolate vibrations of heavy machine tools, e.g. eccentric presses becuase the specific energy storage of rubber vibration mounts is greater than that of steel ring . The machine support absorbs vertical force very softly but at the same time is sufficiently stiff to cope with horizontal forces. It thus meet the requirements it will prevent any undersirable floating of the suspended machine that would arise with compression loaded elastic rubber vibration mounts. Furthermore the low overall height and full protection of the rubber from oil leaks are worth emphasising as well as the complete freedom from maintenance and the great durability, which is where a machine support derives most benefit from the favourable combination of shear and compressive stresses in the rubber vibration mounts.

Figure 1 conical rubber ring vibration mount

Sound isolated piping

The example of sound attenuation from water pipes and the consequent reduction in sound reflection off the walls shows the extent to which rubber can reduce the volume of sound. The situation is reproduced schematically in Figure 1 . An open tap causes a loud noise. The sound of the following water is conducted by the piping and then by wall brackets to the wall, from where it emanates into a neighbouring room. If the pipe is isolated from the brackets by rubber bands, the sound volume is considerably diminished, as Table 1 shows. This depends on the force exerted in the tightening of the rubber strip. The measurements show that rubber is more effective than cork. Light to average tightening of the bracket clip screws give an optimum noise reduction.

Figure 1 Sound-deadened water pipes

No.	Sound proofing material	Tightening of isolating material	Sound volume (phon)	Improvement with insulating material (phon)
1	without	-	55.5	0
2	rubber	light	39	16.5
3	rubber	average	39	16.5
4	rubber	strong	42	13.5
5	cork	light	42.5	13
6	cork	average	46	9.5
7	cork	strong	48.5	7

Table 1. Reduction of sound volume with rubber and cork in water pipe

Sound damping

Sound describes those mechanical oscillations in acoustics which lie within the audible range of man’s ear. It is the range between abount 10 Hz and 16 kHz. These oscillation spread in solid liquid and gaseous bodies. Noise according to BS661 is sound undesired by recipient and DIN 1320 defines it as disturbing sound.

In modern sound technology a sound baffle is understood as something that ia an obstacle to the spreading out of sound by a reflecting structure. Sound damping material is sound absorption, i.e. its convert into heat. The first case deals with damming, the second with attenuation.

Rubber materials act as effective sound barriers. The aim of sound damping material is to prevent the sound produced by a machine from spreading further, i.e. sound isolation. Also, transmitted reflected sound originating from orther bodies in thus obstructed. The grade of sound damping material is mostly judged with the help of sound meters. Thus the noise strength is measured before and after isolation. The sound strength is plotted for the individual frequencies of which the noise consists.

The processes involed in the damming of the sound which is being propagated through a body are truly complicated. To this day there are no extensive general references which would enable the designer to calculate the effectiveness of a given rubber material barrier. The problems occurring in this field up to the present have had to be handle purely empirically. It has been shown that each dynamically solved rubber damping also carries with it appreciable sound damming. A survey of the incidence of loudness is given in table 1 . For the inportance of sound damming in the framework of sound damping material. The ability of rubber to dam sound is proved by the slowness with which the sound is propagated. (Sound propagation in rubber is approximately 1/70 that in steel.)

Relate topic

Sound isolated piping

Table 1 Order of magnitude of sound volumes

Phon	Volume of sound
10	very soft whisper
20	slight rustle of leaves
30	ticking of a clock
50	normal conversation
60	vacuum cleaner noise
70	telephone ring, desk apparatus at 1 m distance
80	very busy road
90	machine shop with lathe and automatics
100	noise in a cotton and silk weaving shop
110	pneumatic riveters in a boiler shop
120	peening of welds with pneumatic hammers at 2 m distance
130	pain threshold, sound effect at the operator’s head during peening with pneumatic hammers

The flat rubber mounts

The flat rubber sandwich mounts both the loaded face and the rubber cross section are rectangular. As with circular sandwich rubber pads it is possible to have a pure compressive stress or a pure shear stress or a combination of both. Two flat rubber mounts shear springs would be used which are preloaded in compression to avoid possible tensile strain. The rubber layer is intentionally narrowed by 5 to 10% using compression. In this way it is possible to choose a shear loading three times higher than that for a flat rubber mounts without compression.

Flat rubber mounts of greater length are called rubber strips or mats. They are avilable in lengths up to 2,000 mm and are specially suitable for isolating the vibration of heavy machines.

Rubber strips are delivered in stock lengths, but can also be cut to any required length. The metal parts are sufficiently thick for holes to be drilled and tapped later for mounting purposes.

Installation example of flat rubber mounts are shown in Figure 1.

Figure 1 Example of flat rubber mounts sub-assemblies

Rubber properties : Creep and Set

The deformation of rubber is influenced by the length of time under stress. If the rubber is statically loaded to a give amount, as occurs for example by the support of a machine, then, in accordance with Figure 1, an elastic deformation takes place superseded after a longer period of time by creep. Creep behavior follows an exponential law and is conclude some time afterwards. If the spring is released then its shape return elastically and by creep to a new position at witch recovery is arrested. This represent an inelastic after-effect for a given rubber mix. High elastic grades show a small residual strain and minimal creep. With good elastic qualities, the residual strain (Permanent Set) after prolonged loading and unloading lies between 2 and 5 %, and creep lies between 5 and 10 % of the total elastic strain.

Figure 1. Creep Diagram for soft rubber

Creep and residual strain are proportional to the total elastic strain under a static load. Because of this they are expressed as a percentage of the elastic strain. Thus both effects are independent of the magnitude of load, but they are temperature dependent as show in Figure 2 . The diagram represents a butadiene mix; natural rubber mixes behave more favourably.

Figuer 2 . Temperature dependent of creep , permanent set and modulus for rubber cylinder 20 mm in diameter x 20 mm under stress of 1 MPa

When an oscillating (dynamic) load is superimposed on a static load a further undesirable deformation results from the oscillating load. This is described as Dynamic Set. The dynamic set is dependent on the number of stress reversals and the magnitude of the oscillating load. It also reaches and end value quickly. For vehicle springs the set may take up the same magnitude as creep at full static loading of the vehicle. The spring constants are not influenced by creep and set. The sole effect is a parallel displacement of the spring curve by an amount of the set. In rubber spring design it is useful to allow from 8 to 10 % of the elastic deflection of the spring for permanent deformation through creep and set.

Relate Content

1. What is the standard test for oil seal.

Tire design : Tire stiffness

The vertical tire force (Fz) can be calculated as a linear function of the normal tire deflection (z) measured at the tire center.

Fz = kz*z

the coefficient kz is call tire stiffness in the z direction. Similarly, the reaction of a tire to a lateral and longitudinal force can be approximated by

Fx = kx*x

Fy = ky*y

When kx , ky are called tire stiffness in the x and y directions.

The stiffness curve can be influenced by many parameter. The most effective one is the tire inflation pressure. Lateral and longitudinal force/deflection behavior is also determined experimentally by applying a force in the apporiate direction. The lateral and longitudinal force are limited by the sliding force when the tire is vertically loaded. The coefficient kx and ky are called tire stiffness . They are measured by the slope of the experimental stiffness curve. When the longitudinal and lateral force increases, part of the tire footprint creep and slide on the ground until the whole tire footprint starts sliding. At this point. the applied force saturates and reaches its maximum supportable value. Generally, a tire is most stiff in the longitudinal direction and least stiff in the lateral direction.

kx > kz > ky

Tire with a larger of plies have higher damping, because the plies’ internal friction generate the damping. Tire damping decreases by increasing speeds.

Polyurethane applied to Flexible Gaiters

Polyurethane is applied to automotive industry . The ball joints for steering linkages and front suspension mechanisms of cars have to be protected from wear otherwise play rapidly occurs, which is reflected back to the steering wheel with a resultant loss in steering control. The conventional method of protection is by packing the joint with grease and containing the grease with a flexible gaiter. These gaiters are subjected to the effect of the grease inside and the effect of moisture on the outside and have to operate over serveral years without deterioration. To operate satisfactority they also have to resist the damage caused by small stones thrown up by the tires. The high tear strength of polyurethane is an obvious advantage here, but another equally important advantage is that in the environment of grease, water, ozone and general exposure to weathering the polyurethane elastomers or nitriles. Initially these gaiters were made from millable types of polyurethanes and were two to three times the cost of similar gaiters made in neoprene. Even so the practical advantage gained dictated fairly widespread use, although it is considered that further material improvements are still required and injection-moulded gaiters are already replacing some millable types.

Tires : Effect of inflation pressure.

High inflation pressure increases stiffness, which reduces ride comfort and generates vibration. Tire footprint and traction are reduce when tires are over inflated. Over inflation causes the tires to transmit shock loads to the suspension, and reduces the tire ‘ s ability to support the required load for cornerability, breaking and acceleration. The rolling resistance friction coeficient decreases by increasing the inflation pressure. The effect of increasing pressure is equivalent to decreasing normal load.

Under inflation results in cracking and tire component separation. It also increases sidewall flexing and rolling resistance that causes heat and mechanical failure. A tire’s load capacity is largely determined by its inflation pressure. Therefore, under inflation results in an overloaded tire that operates at high deflection with a low fuel economy, and low handling. Proper inflation pressure is necessary for optimum tire performance, safety, and fuel economy (Energy Tires). Correct inflation is especially significant to the endurance and performance of radial tires because it may not be possible to find a 5 psi under inflation in a radial tire just by looking. However, under inflation of 5 psi can reduce up to 25% of the tire performance and life.

Polyurethane in Textile Machinery

The most importhant application to date in the textile field has been the use of polyurethane elastomers for loom pickers. Here the application is vary arduous, the shuttle weighing about 1 lb traversing a 4 ft loom approximately 150 to 200 times a minute. Previously the materials chiefly used were either raw hide or rubber-fabric composition. On the average polyurethane pickers outlast other types of pickers but they do exhibit a number of unexplained early faileres. The life of the picker is very dependent on the correct setting of the picker in relation to the shuttle, and if badly aligned, the picker will fail rapidly. The search for even better pickers is still proceeding, and the most recent design incorporate a wearing block of high density polyethylene inside a resilient body of a hard thermoplastic polyurethane.

Other application in the textile and allied field include falling bars for selector needles on the knitting machines, cot roller covering, and drive couplings for bobbins.

Polyurethane in Paper-making and Printing

Solid polyurethane are being used successfullyfor the top of vacuum boxed on paper-marking machinery. A gauze holding the paper fibers travels across the vacuum box and excess water is removed by vacuum. The polyurethane is less harsh to the guaze than other materials and is itself resistant to wear. This is an example where polyurethanes are used sucessfully in water at a temperature around 50 *C .

Special grades of very soft polyurethane elastomers have been used for many year for printing rollers. The hardness of these materials vary from around 10 to 30 Shore A and the physical properties are poor compared with what is normally expected from polyurethane elastomers. However, solid polyurethane around a hardness of 60 to 70 Shore A are being used for scraper blades on silk-screen printing due to their solvent resistance and low ink pick up.

Polyurethane properties : Coefficient of friction

The frictional properties of polyurethane elastomers are generally consistent with the established friction properties of rubber and vary from a coefficient of friction approximately 0.2 for the harder to around 2 to 3 for softest grades. The soft grades exhibit high values for coefficient of friction owing to the large true area of contact developed and the friction coefficients drop as the hardness increases. As with abrasion tests, however, values of friction obtained in the laboratory can be misleading and should only be taken as a rough guide as to actual service behavior. Such factors as surface cleanliness, lubrication by airborne dust or abrasion debris, and small traces of fluids can greatly influence the frictional charecteristics in practical applications.

The slip velocity has a small effect on the coefficient of friction, higher slip velocities giving slightly higher coefficient of friction. The coefficient of friction is higher on smooth moulded plastic surfaces than on the relatively rough surface of rough ground steel. The application of higher load increases the frictional force although there is a tendency for the coefficient of friction to fall with time. This effect is most probably due to lubrication of the interface by abrasion debris.

The coefficient of friction can be considerably reduced by lubrication, and when using polyurethanes for bearing applications it is generally beneficial to use oil or grease for this purpose. Where the provision of external lubrication is undesirable it has been claimed that additive can be incorporated into the polyurethane which makes them essentially self-lubricating. The additives generally employed are molybdenum disulphide, graphite and silicone oils. Caution must be observed when making use of these additives since in certain case it is known that their presence can have an adverse effect upon the ageing charecteristics of the polyurethane.

Polyurethane Properties : Effect of low temperature

Low temperature environments affect the properties of polyurethane elastomers, but no degradation occurs and effect is completely revesible. The major effect is an increase in Young’ s modulus below 0 *C with a corresponding increase in hardness, tensile strength, tear strength and torsional stiffness and a decrease in resilience.

The two properties that can have an important bearing on the use of polyurethane at low temperature are the decrease in resilience which has already been mentioned and the increase in stiffness. In the use of polyesterurethane at low temperatures another phenomenon can sometimes be observed. This is that of crystallization of the polyurethane when held at moderately low temperature foe some period of time. When crystallized the polymers are considerably harder and their stiffness charecteristics are similar to those normally observed at much lower temperatures. The effect is reversible and can be removed by the application of heat or by the generation of internal heat by flexing. The use of mixed polyesters reduces this tendency to crystallization.

Polyurethane in Shoes and Shoe Machinery

The use of solid polyurethane rubber for ladies’s shoe heel top pieces has been known for several year. Cast grades of medium to high hardness have generally been employed for this purpose, but more recently the thermoplastic polyurethane have been graining ground. The material cost of polyurethane are high compared with more conventional heeling materials and to be competitive the processing cost must be as low as possible. The high cost has so far prevented the use of solid polyurethanes for the soles of shoes but the development of the cellular varieties of the solid materials has reawakened interest in this outlet. By using cellular grades with a specific gravity of 0.4, material cost are reduce to a third of those of the solid. Provided the process can be refined sufficiently to keep the costs low, soles for hard-wearing applications are a distinct possibility. In this case closed cell matreial is necessary to prevent excessive uptake of water.

The process for shoe manufacture is becoming highly automated. The high hardness, toughness and flexibility of the solid polyurethanes make them excellent materials for many items in this application. Replacement of a natural rubber diaphragm by polyurethane increased the diaphragm life from two months to a yeas.

Tire footprint

The contact area between a tire and the road is called the tire footprint and is shown by Ap. At any point of a tire footprint , the normal and friction forces are transmitted between the road and tire. The effect of the contact force can be described by a resulting force system including force and torque vectors applied at the center of the tire footprint. The area of the tire footprint is inversely proportional to the tire pressure. Lowering the tire pressure is a technique used for off-road vehicle in sandy, muddy. or snowy areas, and for drag racing. Decreasing the tire pressure causes the tire to slump so more of the tire is in contact with the surface, giving better traction in low friction conditions. It also helps the tire grip small obstacles as the tire conform more to the shape of the obstacle, and makes contact with the object in more places. Low tire pressure increase fuel consumption, tire wear, and tire temperature. In most vehicles, the front and rear tires will wear at different rates. So, it is advised to swap the front and rear tires as they wear down to even out the wear patterns. This is called rotating the tires. Front tire, especially on front-wheel drive vehicles, wear out more quickly than rear tires.

Figure1 Tire footprint**

** Figure Ref. http://www.aircti.com

Tires behavior : Hydroplaning

Hydroplaning is sliding of a tire on a film of water . Hydroplaning can occur when a car drives through standing water and the water cannot totally escape out from under the tire. This causes the tire to lift off the ground and slide on the water. The hydroplaning tire will have little traction and there for, the vehicle will not obey the driver’s command.

Deep grooves running from the center front edge of the footprint to the corners of the back edges, along with a wide central channel help water to escape from under the tire.

There are three type of hydroplaning; dynamic, viscous, and rubber hydroplaning. Dynamic hydroplaning occurs when standing water on a wet road is not displace from under the tire fast enough to allow the tire to make pavement contact over the total footprint. The tire ride on a wedge of water and loses its contact with the road. The speed at which hydroplaning happens is called “Hydroplaning speed (VH) ” that is estimated in knots by equation 1

VH = 9*P^0.5 -------(1)

Where P is tire inflation pressure in psi

For a matric system would be

VH = 5.5753 x 10^-2*P^0.5

Where P is tire inflation pressure in Pa

VH is in m/s

Rubber hydroplaning is generated by superheated steam at high pressure in the tire footprint, which is causes by the friction generated heat in a hard braking.

Viscous hydroplaning occurs when the wet road is covered with a layer of oil, grease, or dust. Viscous hydroplaning happens with less water depth and at a lower speed than dynamic hydroplaning.

Ref. Vehicle Dynamics: Theory and Applications , Reza N. Jazar

Using polyurethane in mining and quarrying industries.

In the mining and quarrying industries, however, polyurethane is being used for many other applications with advantage and replacing rubber or even hard steel. In general these applications are connected with movement of very abrasive slurries of relatively small particle size such as in the ball and rod mills, flotation plant, hydrocyclones and piping.

Ball or rod mills have generally been lined with manganese steel replaceable linings, but in recent years the use of rubber replaceable linings has been developed. Dependent on the condition existing in a given mill these rubber linings can last several times longer than the manganese steel, and polyurethane linings can last considerably longer than the rubber linings. Further advantages are that the tooling cost with polyurethane are less and the fact that, due to the availability of the harder grades, the polyurethane can be used as a structural material and does not require reinforcement by metals.

Similarly in flotation plant the impellers, rotors, stator and scrapers are made of steel centers often covered in conventional elastomers. Polyurethane again can be used in certain cases and will generally outlive natural rubber. The main precaution to be observed are limitation in temperature and also extremes in acidity or alkalinity of the slurries.

Hydrocyclones used in the separation of ores and cray are also subjected to harsh abrasive condition, and polyurethane have been successfully used in these conditions.

In mines there is often a considerable fire risk, and comprehensive precaution are taken to avoid spark which can start a fire or explosion. For this reason the use of track laying vehicle with metal tracks is prohibited. Covering of the tracks with natural rubber goes only part of the way to solving the problem due to highly abrasive conditions causing relatively rapid breakdown. The polyurethane track pad do not wear so rapidly and are being used successfully. A further advantage of polyurethane as opposed to metal is the increase in traction obtained which enable the vehicle to climb steeper slopes.

Polyurethane in Automotive Industry : Gear change levels

Polyurethane has replaced both rubber and metal in several components used in the modern car. The properties attractive to the design engineer have again been the general wear and tear characteristics coupled with oil and fuel resistance. The remote control gear change mechanism on many car are subject to considerable vibration and can be the source of irritating noise. Wear on the ball joint can also lead to a less positive action. Both these effects can be minimized by the covering of the balls with polyurethane or by the use of polyurethane housings. A further advantage in the case of the ball joint is that no accurate machining of special mating surfaces is required. If the socket and ball are both metal they require accurate machining. The polyurethane housing eliminates one surface completely and the elastic nature of the material overcomes the necessity of the machining the ball itself to highly accurate dimensions. The gear change mechanism shown is for floor-mounted levers. Gear change mounted on the steering column generally involve a highly articulated machanism, and the use of polyurethane housings in all the ball joint can reduce metallic clatter to a minimum and provide a cushioning effect on the system.

Polyurethane tires are used on rotary screen plant

Polyurethane tires are used on rotary screen plant. Rotary screen plant is used extensively in the washing and grading of gravel and sand and also for heat drying of road stones. Other user include the preparation of natural and artificial fertilizers. Where the drum loading and temperature are such that the natural rubber can be used the substitution of polyurethane gives the expected advantages of higher load capacity and longer life. In many case however steel rollers have been in the past used in place of natural rubber due to the high loading required. In these case because of the highly abrasive environment, severe wear can take place both on the steel roller and the steel track on the drum itself. This wear leads to increased vibration in the plant which in turn can result in fractures of welds and rivets. To avoid serious damage the drum tracks have to be remachined at intervals during which the plant is inoperable. More recently these steel rollers have been successfully replaced by polyurethane tires in a number of cases. Taking a typical plant, for example, having an output of 50 to 60 tons per hour of washed gravel the weekly output is equivalent in value to ten times the cost of a set of polyurethane tires to replace the steel tires. The use of polyurethane tires reduces the vibration considerably and substantially reduces the maintenance time required. It also considerably reduces the noise level. In certain case the rotation of the drum can be carried out using polyurethane tires, with a further reduction in vibration.

Polyurethane tires : Overhead cranes

Polyurethane tires are being successfully used on overhead cranes. In these instances the conventional flanged wheel and rail can be replaced by a polyurethane tires which runs directly on the top girder or structure to which the rail would previously be fixed. A polyurethane side rollers is used at the same time to prevent undue sideways movement. The crane is driven by power transmitted througth the tires and the overall result is a noticeable reduction in noise and vibration. Noise elimination is an important factor in modern workshop, and reduction of vibration result in saving on maintenance cost.

A similar use for polyurethane tires is on sewang plant settling tanks. The conventional method is the use of steel wheels running on the circular track, both wheels being driven by a fairly complicate drive mechanism. The substitution of polyurethane tires for the steel tires gives greatly increased traction and allows the replacement of the normal drive mechanism by a single motor on one wheel. This still gives better traction than by driving the two steel wheels separately, especially when the plant is operating in wet condition.

Pneumatic and Hydraulic Polyurethane Seals

One Example of the successful use of polyurethane for sealing is an Oleo-Pneumatic buffer for use on railway rolling stock. The pressures encountered in this application are not particularly high but an efficient seals is of prime importance. In this buffer compressed air or nitrogen is utilized as the spring medium for providing static resistance and recoil, the gas being sealed into the buffer behide the oil by a floating piston. It is therefore very important for the seal carried on the floating piston to be highly efficient throughout the working lift of the piston. Operational experience showed that the original seal in nitrile rubber had inadequate wear resistance and this is one of the main factor in buffer service life. After three years operational experience with polyurethane seals the outstanding properties of polyurethane were quite apparent. Seals examined from these buffers, having operated under the most arduous conditions which would have completely destroyed the nitrile seal, showed virtually no wear.

Polyurethane seals can also be used to advantage in hydraulic pit-props for roof supports and other applications. These props operate at relatively high pressure, but the seal must also prevent leakages at low pressures when the prop is out of operation.

Another use for polyurethane seals in the mining industry is found in the control valve unit of chocks. The seal in this case also acts as a seal for high pressure bleed valve. There is a high velocity flow of hydraulic fluid across the sealing face which rapidly scours nitrile rubber, whereas the polyurethane, being more wear resistant, operates satisfactorily.

Other uses for polyurethane hydraulic seals are found where the equipment is used in dirty condition, such as in the case of earthmoving equipment where the efficient operation of seals is of importance in view of hire charges and cost of down-time arising from failures.

Polyurethanes : Block and Sheet shape

Cast polyurethan elastomers, is the ease with which blocks of different sizes can be manufactured. The tooling costs are very low and due to the simplicity of shape they can be quickly made. Coupled with this is the suitability of most types of polyurethane, in particular the harder grade, for many machining operations, as already discussed in the next article. This enable in certain cases different designs to be turned out quickly fo evaluation and choice of optimum design.

Apart from blocks there is a considerable call for sheeting in various thicknesses for a wide variety of applications. Sheets are usually manufactured in a cast grade in a centrifuge. This method limits the length and width obtainable, and the largest sheets available aer of the order of 15 x 3 ft with thicknesses from 0.030 to 0.50 inches. Longer sheets can noe be made by peeling from a cylindrical block, although the thickness is limited in such cases to around 1/8 inch.

Larger sheets can also be calendered using either the millable or thermoplastic grades and the blown film extrusion of the latter materials gives very thin sheeting in long lengths. Polyurethane sheets are also available with a pressure sensitive adhesive and a non-stick removable backing paper.

Sprayable Polyurethanes

The sprayable types of polyurethane are being used where large or awkwardly shaped surfaces require covering. Before the advent of these materials castable polyurethane were used, but either the moulds required were complicated and therefore expensive. Many of the applications therefore are not entirely new but the use of the sprayable materials has resulted in lower costs and more widespread use.

One example of successful use of sprayable polyurethane is the lining of vibrator finishing barrels. The barrel are filled with an abrasive substance and untrimmed forgings and casting with the object of deburring and polishing the metal parts. The whole action is very abrasive but the sprayed coating is giving a long useful life. A slide valve on a sand hopper is another proven application, as also is the lining of a cement mixing barrel. Other uses includes the covering of wear plates in shot-blast equipment, linings of earth dumper trucks, fertilizer hoppers and even ships’s propellers.

The process is also suitable for the covering of large rollers where the tooling cost for cast polyurethanes would be prohibitive. When spraying, the rollers only need supporting between centres and rotating slowly. Similarly, the sprayable materials offer a quick and economic method for the coating of long lengths of fabric.

Polyurethane elastomer in Aircraft Industry

One problem specific to the aircraft industry is the protection of leading surfaces against rain, dust and stone damage. The main surfaces in question are propellers and helicopter bladed, although leading edges of the mainplane and even the taiplane and fin are now being protected. On propellers de-icing mats are used to prevent icing up, and these are generally covered with a thin sheet of polyurethane. This protects the outer layer of nitrile rubber on the mat from stone and dust damage when landing or taking off and also against rain erosion during flight.

Helicopter blades can be made from either a special alumenium alloy or stainless steel, both of which at high speeds aer susceptible to damage, espectively in desert condition. One report indicates the the lift of the rotor blade increased from 40 to 1,000 hours after installation of protective polyurethane mats. This result in a very considerable saving in cost for replacement and also increases the operational life between changes.

Polyurethane lining : Pulleys

In both coal-mining and quarrying, belt conveyors are used to convey the coal or rock from one location to another. One type of conveyor used is a continuous belt to which a steel rope is attached at either side. This rope runs on pulleys and provided the motive power for belt itself. When using metal pulleys two problem arise, wear on the pulley and wear on the steel rope. The latter problem may prove to be more important, since repair or replacement involves considerable maintenance time and the rope itself is very expensive. Lining the faces of the pulley with polyurethane has a twofold advancetage. Wear on the rope is significantly reduced with a resultant saving on the rope cost. Breakdown time is also less, and since output rates of up to 1,000 tph. Another factor is that the pulley life is increased and there is a very marked drop in noise level, which is very real advantage in coal mines.

Another application for polyurethane lining is on the pulleys employed in the installation of overhead electrical transmission lines. The aluminium sheated transmission line are atthached to a relatively small diameter steel rope during the installation and the pulley has to be capable of taking the high unit load from the steel rope and yet be soft enough not to damage the alumenium sheathing.

Figure 1 Pulley (Polyurethane lining) http://www.indiamart.com

Why use polyurethane tires(PU). (2)

To date most polyurethane tires are used for load-carrying wheels where their advantages are immediately obvious. A more recent development is use of softer polyurethane for cushion tires on indrustial trucks. Polyurethane of approximately 75 to 80 shore A hardness are used for this application and give a degree of cushioning together with increased traction.

In all types of solid tires the constant deflection of the tread cause heat build-up in the mass of rubber materials. The higher the speed of the truck the higher is the frequency of deflection and the higher is the consequent heat build-up in the tires.Tires are normally rated at 10 mph and for higher speeds the loading rate has to be reduced, although by careful consideration of wheel size, tread thickness and tread profile higher speeds can be obtain and polyurethane solid tires are operating in certain application up to 30 mph.

In this instance rubber solid tires would not have been capable of carrying the load and still provide the required manoeuvrability. Steel or hard plastic wheels on the other hand would not have had the required traction and would have damage the pile-cap surface. This is possibly an extream example but it does illustrate how polyurethane can permit completely new design approaches.

Rubber seals : Effect of friction and Hardness

From previous article we analyzed the performance of the rubber seals by finite element method. We founded performance of rubber seals depending on the viscoelastic properties of rubber materials . In this article we study the effect of rubber friction and hardness to the performance of rubber seals by finite element method because the assembly load is dependent upon the friction coefficient and hardness. The result are shown in table 1 was based upon a dry assembly condition. If the coefficient of friction is alter the assembly load varies , see table 2

Table 1 Sealing stress at room temperature of rubber seals

	50 IRHC	60IRHC	70IRHC	80IRHC	90IRHC
maximum contact pressure	-1.18	-1.63	-4.24	-4.90	-6.60

The possible change in friction coefficient , i.e. from 0.10 to 0.15 , produce an increase in the assembly load grater than that produce by a 10 degree increase in the hardness of the rubber materials.

Table 2 Assembly load (N) of rubber seals.

Friction Coefficient	50 IRHC	60IRHC	70IRHC	80IRHC	90IRHC
0.010	5.46	6.45
0.050	5.84	6.81
0.100	6.30	8.57	20.26	30.07	45.87
0.125	8.08	9.59
0.150	9.16	10.96

Why use polyurethane tires(PU). (1)

One of the most successful applications for polyurethane elastomer is that of tires for industrial truck. A list of the important advantages that the use of polyurethane tires can offer over rubber tires or steel tires is given below.

1. Polyurethane tires have higher load bearing capacities than similar size rubber tires. In all these case quoted from a 6 inch to a 22 inch diameter tires, the polyurethanes are rate approximately 600% higher than the corresponding natural rubber. When truck are designed specifically for polyurethane tires it is more customary to reduce both the width of the tread and also the overall diameter. Example it will be seen that a 10 x 3 x 6 1/4 (10 inch OD , 3 inch width , 6 1/4 inch metal OD) polyurethane tires has a higher load-bearing capacity than a 22 x 8 x 16 natural rubber tyre. The reduce in overall diameter of the tire can prove very advantageous in warehouse and factories where the floor space is at a premium . In general where the full advantage can be taken of the higher load-bearing capacity of polyurethane, its use can result in considerable cost savings.
2. The general life of a polyurethane truck tires even at higher load is noticeably longer than that of a natural rubber tires due to the substantial improvement in resistance to cutting and abrasion.
3. Industial truck tires have to operate over widely different terrain including rough factory yard , smooth warehouse pass-ages and oil-soaked machine shop. The good oil resistance of polyurethane can be a contributory factor in giving longer life when compare with rubber tires. Compare with steel tires the use of polyurethane tires can give improved traction and does much to eliminate damage to concrete and vinyl floor. Although rubber tires cannot break up the floor surface as can steel, the carbon black filler present can mark the floor surface.
4. Polyurethane tires have considerably lower rolling resistance than rubber tires, mainly as a consequence of the different hardness of two materials employed. In electrically propelled truck this result in a reduction in frequency of battery recharging with a resultant improvment in truck utilization.
5. Many industial trucks are used at sub-zero temperatures in cold storage rooms. The hard polyurethane tires are capable of cutting through the ice formed at these temperature and thus improving the traction. Furthermore the resilience-temperature characteristic are such that polyurethane tires have lower power consumption than rubber tires at these temperature, an important consideration in cold storage rooms.

Figure 1 Polyurethane Tires

Polyurethane rollers

These rollers are used to deliver case of beer to an automatic palletizing machine and are located at the end of a belt conveyor and at right angles to the conveyor. Polyurethane is also used extensively where rollers are handling sharp-edged or abrasive materials. Examples in this case are rollers for feeding thermoplastic strips to granulators, mills for dehusking rice, and rollers in a glass fibre chopping plant where the cutting of the glass fibre takes place on the rollers itself. Second examples include feed rollers in a plant for the sanding of plywood pannels, computer programme card feed rollers, and guide rollers in a plant for the manufacture of plate glass. The cement conveyor was originally fitted with cast iron rollers witch required replacement in less than 12 week. Substitution of polyurethane for the cast iron showed an immediate improvement and the useful life of the rollers was increased to more than 12 months.

These few example give an indication of the wide use of polyurethane rollers and the very definite advantages to be gained from their use.

Rubber Seals : Use of Polyurethane elastomers

Another large field of application for polyurethane is that of rubber seals .The rubber seals can be static , reciprocating or rotary and can be used in pneumatic or hydraulic system. Since the rubber seals can be in the form of O-ring ,lip seals or simple square section seals.

The economic advantages of polyurethane as a rubber seals are associated with its improved wear resistance and lower friction.Up to pressures of about 1500 psi nitrile rubber sealing are generally quite satisfactory but above this pressure the use of plastic or fabric reinforcement is required to avoid extrusion of the seal. The hard grades of polyurethane are able to seal quite successfully without reinforcement up to pressure of aproximately 6000 psi.

Since the seals are generally required to have the highest possible physical properties the cast polyurethane have been widely used in the past.

When hydraulic fluids are present the main limitation on the use of polyurethane is that of restricted temperature range. Under dry condition , whether pneumatic or hydraulic , an upper temperature limit of around 70 *C for continuous operation should not be exceeded. When the oil is contaminated with water or a water-base hydraulic fluid is used the upper temperature limit is reduced to 40 *C. Certain synthetic oils also attack polyurethane.

Although the abrasion resistance of polyurethane is exceptional at low abrasive speeds, high speeds can cause premature breakdown. For this reason the use of polyurethanes for rotary seals is generally restricted to maximum surface speeds 1 – 2 ft/sec.

Tire Tread Pattern Characteristics

The tread pattern is made up of tread lugs and tread voids. The lugs are the sections of rubber material that make contact with the road and void are the space that are located between the lugs. voids are also called grooves and lugs are also called slots or blocks.The tires tread pattern configurations affect the tire’s traction and noise level.

Width and straight grooves running circumferentially have a lower noise level and high lateral friction. More lateral grooves running from side to side increase traction and noise level.

Figure 1 Tire Tread Pattern

On dry road tire tread reduce grip because they reduce the contact area . This is the reason for using treadless or slick tires at smooth and dry race tracks.

Tire need both circumferential and lateral grooves because the water on the road is compressed into grooves by the vehicle’s weight, providing better traction at the contact area. The tire that without grooves the water would not be able to escape out to the side of the tire , This would cause a thin layer of water to remain between the tire and the road , which cause a loss of friction.

The design of tread pattern may be asymmetric and change form one side to other. Asymmetric patterns are designed to have two or more different fuctions and provide a better overall performance.

The direction tire is designed to rotate in only one direction for maximum performance. Direction tread pattern is designed foe driving on snowy muddy or wet surface. A non – directional tread pattern is designed to rotate in either direction without sacrificing in performance.

Rubber seals : Analyze the effectiveness of the rubber seals using Finite element method.

Rubber seals are designed for the prevention of leakage of fluid. The stress relaxation properties of rubber seals is a factor indicating the performance of the rubber seals. This article provides an example of analyzing the performance of rubber seals and comparison between the two compound by using the Finite element method. Shape of the rubber seals that are used in this article has a shape that axisymetric. as shown in Figure 1.

Figure 1. Finite element model of Rubber seals

Rubber materials model
Choose a neo-Hookean is strain energy function of hyperelastic model , while pronie’ series n = 1 is used as a model viscoelastic properties . The properties of rubber materials which both display Table 1.

Table 1 . Rubber material properties

Material name	Hyperelastic Model	Pronie’	series
	Neo Hookean (C10)instantaneous (MPa)	gi	ti (sec)
Rubber1	2	0.3	1
Rubber2	2	0.4	1

Stress relaxation behavior of rubber material in Table 1 are shown in Figure 2. It was found that the shear modulus of the Rubber1 is higher than Rubber2 (stress reduced to less than Rubber2).

Figure 2. Shear Modulus (G(t)) of rubber compound

Defined boundary conditions.
To simulate events of rubber seals (as O-Ring) will begin from the engagement Housing and Groove, which will simulate the speed of compression of Housing and Groove at 22.5 mm/s .The next step is to compare the pressure of the rubber seals (as O-Ring) compression with the Housing and Groove at the time of 10 seconds.

Simulation results.

Distribution of stress components in the radial direction.

Figure 3. Stress component in the radial direction of the back rubber seals (as O-ring).

Figure 4. Stress component in the radial direction of the rubber 1 material

Figure 5. Stress component in the radial direction of the rubber 2 material

From Figure 3 it was found that the stress in the radial direction is negative, represents the pressure rubber seals (as O-Ring) with Groove and Housing act to prevent leakage of fluid. It was found that the pressure rubber seal (as O-Ring) are similar, both rubber. (arising from the G (0) of these two materials are equal) over time. Pressure in these areas has decreased, as shown in Figure 4 and 5.

Considering the reactions of the rubber seals (as O-Ring), which acts on Groove and Housing will be found that compound Rubber 1 the reactions above, as shown in Figure 6. which is shown to be effective in preventing leaks better.

Figure 6 Reaction force from the rubber seals (as O-Ring) in the radial direction.

Simulation results show that viscoelastic properties affect the performance of the rubber seals to prevent leakage significantly. Although Hyperelastic properties of rubber compound is like a Viscoelastic properties that must be considered is the relaxation of the shear modulus (G (t)), as shown in Figure 2. At the same time improving efficiency of rubber seals also depend on other factors such as instantaneous modulus, operating temperature and seal friction.