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.

Energy tires : How to improve tire design for rolling resistance

Energy tires has become very important becuase with the goverment requirements for better fuel consumtion. The rolling resistance is performance charactoristic and energy tires have be low rolling resistance. The general ideas change to improve tires to energy tires but will certainly affect other properties as well , for better or for worse.

Effect of Rubber Material

ranked various rubber material regarding their effect on energy tire (low rolling resistance) . The rolling resistance index was as the table

Rubber material	rolling resistance index
Natural Rubber	100
Hight cis-BR	94
Solution SBR	93
Emulsion SBR	90
IIR	73

* high rolling resistance index indicated that low rolling resistance coefficient.

Energy Tires Design

Energy tires can be radial construction that it have better rolling resistance than bias construction becouase the radial carcass is more flexible. Energy tires can reduces the gauge thickness of tread to reduce energy dissipation to lead to poorer rolling resistance.

Loading

Energy tires rolling resistance improves when tire’s load is decreased

Inflation Pressure high or low?

To design energy tires to operate with a higher air pressure.

Vihicle Speed.

Energy tires rolling resistance improves when vehicle speed is decreased.

Running Temperature.

Tire’s running temperature has effect on impart rolling resistance. Lower running

temperature may impart more rolling resistance.

Rubber Seals : Stress Relaxation Model

Rubber material is classified as viscoelastic material that mechanical behavior depends on the time and temperature. The decreasing of stress in the rubber material while under the constant deformation, is one of the most important characteristics. Because rubber material is used as a rubber seals to prevent leakage of fluids such as rubber gaskets O-Ring, etc. Stress decreasing through the critical value during operation resulting in the leakage of fluid. The decreasing of stress in rubber material, while under the constant deformation is called stress relaxation of the rubber material.

Materials model

Linear viscoelastic model was developed to describe the behavior of rubber material such as Maxwell model , Kelvin-Voigt model as Figure 1.

Fig.1 Linear viscoelastic model

Equations of motion of Kelvin-Voigt model and Maxwell model is shown in equation (1) and (2), respectively.

Solution of equation (1) and (2) can be expressed as Figure 2, which showed that Maxwell's model can simulate the behavior of stress relaxation, while the Kelvin-Voigt model for the simulation is not consistent with the behavior of stress relaxation. Thus, in the modeling of stress relaxation behavior of rubber material is used Maxwell's model .

Fig.2 Response of Maxwell’s Model and Kelvin-Voigt Model

Generally Maxwell's model can not easily predict the behavior of stress relaxation of rubber material which is very accurate. It has developed the Generalized Maxwell’s model, which consists of a maxwell’s model to a simple parallel, as shown in Figure 3.

Fig.3 Generalized Maxwell’s model

Solution of the Generalized Maxwell’s model is described in a pronie’s series is shown in Equation 3.

We can describe the solution of the Generalized Maxwell’s model in the form of a relaxation of shear modulus in the rubber material, as shown in equation (4).

When

G0 is the shear modulus at time t = 0.

gi is a coefficient of pronie’ series order i

ti is the relaxation time order i.

To develop a rubber compound to analyze the behavior of rubber material or rubber products. Stress relaxation testing of rubber material in order to determine coefficient of Pronie series, this is what the design engineer will be doing before design rubber seals etc.

Relate topic

Rubber seals : Seal Friction

Stress relaxation

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

How to improve rubber process : Reducing Backrinding in compression moulding

In Compression moulding process , when the expansion of the cure rubber is concentrated in a limited area and is greater than the ultimate cure elongation of the compound the mould is opened after cure , the phenomenon is called Backrinding The resulting in a rupture at the part line (o-ring ,rubber seals, solid tire,rubber mounts ,rubber gaskets etc.) . There are some ideas which may help prevent or eliminate backrinding but these general ideas may not work in all specific situation.

1. Mould Temperature : Curing a compound at a lower temperature for a longer period of time will allow more rubber to exit the mould from thermal expansion before scorch and cure are reached. The rubber compound will most likely have better tear resistance at lower temperature.
2. Mould Design : Backrinding can be severe in the moulding of lage, bulky articles with a lage amount of mass .In design the rubber mould, if the ration of surface area to volume can be increased then backrinding may be less of a problem. In design the mould place the part line in a noncritical area the does not affect product performance .
3. Cooling the mould to lower the pressure that will be released on demoulding is another option to consider to reduce or eliminate backrinding.
4. Preheat the preform before placing it in the mould
5. Extending scorch time through compound modifications should favor reduced backrinding becouse it provides more time for rubber to exit mould cavity before curing occur.
6. Improving tear resistance of rubber compound (hot tear resistance)

Warning : Change to reduce backrinding will certainly effect other properties.

Recycled rubber storage

Recycled rubber from whole tires into chips and crumbs can generate heat up to 115 *C in the ambient grinding process. This can on storage without sufficient cooling lead to spontaneous combustion. The presence of iron in the natural rubber compounds can catalyze the oxidation process, causing rubber degradation. This degradation is accelerated with heat. During grinding processing generation of crumbs and storage, volatiles and hot fumes may be generated.

Thus the recycled rubber must be sufficiently cooled, either by water or air to avoid spontaneous combustion. Rubber crumbs should be stored at ambient temperature in dry areas but not in tin sheds or tin roof warehouses.

Relate Topic

Evaluation of recycled rubber in compounds.

Evaluation of recycled rubber in compounds

Recycled rubber is blended with virgin rubber compound in two ways

1. The total elastomer content from the virgin compound plus recycled rubber is kept at 100 phr (part per hundred rubber)
2. The recycled rubber ia added on top of the virgin compound , where the total elastomer content becomes more than 100 phr.

Researcher found that at more than 20 phr natural rubber reclaim, tensile strength are higher when reclaim rubber is added on the top of 100 phr than when elastomer content of the blend is constant at 100 phr.

The amount of recycled rubber used in a rubber compound must be optimized. Experiments can be used to determine the effect of varying amount of recycled rubber on desired properties.

The natural rubber compound containing 30 phr recycled rubber had shorter scorch time , higher cure rate, and lower maximum torque than the control compound with no recycled rubber.

In the case of virgin rubber compound was mixed , cure and cryogenically ground. The ground recycled rubber was added to the virgin rubber compound at different levels. It appears that 5 phr ground butyl rubber content is the optimum level. At 10 and 15 phr tensile properties and air permeability are reduced significantly.

Recycled Rubber Techniques

Recycled rubber is defined as recyclable , vulcanized rubber that has been processed to give particulates or orther forms of different shapes size and size distributions.

Techniques are used to generate recycld rubber

Reclaiming

This is done by a chemical digestion process. Aryl sulfides and other chemicals are added to rubber , and then the mixture is chemically digested. Over the years, there have been significantly decreases in use of reclaim rubber.

Ground Rubber

Vulcanized scrap rubber is first reduced to approximately 2.5 cm by 2.5 cm to 5 cm by 5 cm chips by shredding. This material is further reduced in size by use of ambient grinding mills. The finer particle size needed, the longer the rubber must be milled. Multiple grinds reduce the particle size. Any fiber is removed by air and metal is removed by magnetic seperation. The heat generated in this process can degrade rubber if the product is not cooled properly before storage or shipping.

Cryogenic Ground Rubber

This process , small rubber chips or crumb rubber , 10 to 20 mesh are cooled by chiller using liquid nitrogen and then put through a grinding mill. For finer particle, multiple cooling and grinding are usually needed. Little heat is generated in the process, thereby decreasing degradation of the rubber. All fiber and steel are separated by freeze-grinding to give a higher yield of usable product.

Wet Ground Rubber

Ground rubber is produced by passing a water suspension of rubber particle through a flour-grinding type mill.

Devulcanization Process

In sulfur-accelelator vulcanization of diene rubbers,sulfur-sulfur bonds are formed. Devulcanization is the reverse process in which either via external energy or by addition of chemicals some of the sulfur-sulfur bonds are broken. There may also be cleavage of some C-S and C-C bond in these process.The resulting devulcanized rubber can be compound as is , or mixed with virgin rubber and recured. Chemical devulcanization involves addition of certain accelerators and sulfur to the crumb rubber on the mill or in Banbury.

Relate Topic

Evaluation of recycled rubber in compounds.

Rubber seals : Seal friction

The coefficient of friction of rubber material ranges from 0.001 to 10 which depending on the interface conditions.

In case of shaft seals or O-ring

Hydrodynamics film thickness is about 150 mm. Shearing of this film is the prime cause of dynamic or running friction, so that the dynamic coefficient of friction is a function of lubricant viscosity and sliding velocity. Running friction can be mimimized by optimizing vicosity and velocity effects.

Causing

Seal friction create a heat buildup , often causing premature failure due to excessive heat aging of the rubber material.

Rubber Formula :

nitrile seal formulations often include a hight percentage of graphite as a filler. becouse it increased the thermal conductivity , thus carrying heat away from the rubbing surface.

Warning :

Graphite is not used to reduce friction and it is no more than a bulk filler that hardness is increased only.

Trend

Concurent engine oil temperature have increased with smaller engines operating at higher frequency . This combination of more serve operating conditions and changing seals requirements at oil operationg temperatures. But the additional thermal load created by seal frictions can raise this value to 175 to 200 *C at the seal contact .The temperature in excess of the capabilities of nitrile rubber material. Finite element analysis of heat transfer in rubber products is increasingly needed in the design stage, using a specific heat value of the rubber material to maximize service life.

Rubber : Thermal properties

Most important of these is conductivity. The reason is that the conductivity of rubber material is very small, so that when heat is generated in rubber material as a result of deformations, as explained in vicoelastic properties this is not readily dissipated. Some increase in the conductivity is possible by incorporation of suitable filling materials having inherently high conductivity, the possibilities of this procedure is limited and it must be accepted that any technically useful rubber material will have low heat conductivity.

The temperature developed in the rubber material depends also on its specific heat. This is an essentially additive property which can be calculate from the proportions of rubber material and other constituents. ; thus for a compounded rubber comprising 60 part(by weight) of china clay per 100 parts rubber material the specific heat would be {(60 x 0.21) + (100 x 0.50)} / (60+100) = 0.39 cal/g approximately. A similar procedure can be applied to thermal conductivities .

Another thermal property of practical importance is the coefficient of expansion ; this is much greater than for metal. An incidental consequence of the big difference in expansion between steel and rubber material is that the differential expansion must be taken into account in designing moulds for precision part.

All comercially available rubber material are organic meterials which become unserviceable at temperature much below those at which metal will operate. Nevertheless the range of operating temperatures is being rapidly extended upwards by the development of special polymers, especially the silicone rubbers and those containing fluorine.