Thursday, April 28, 2011

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.

Monday, April 25, 2011

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.

Thursday, April 7, 2011

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.

Wednesday, April 6, 2011

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.

Tuesday, April 5, 2011

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.

polyurethaner lining pulley

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

Monday, April 4, 2011

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.