Life Prediction of O-Rings

Life prediction of O-Ring can use compression stress relaxation test because most polymeric part are intended to be in service for several year. O-Ring experience changes in properties under heat , high temperature, mechanical stress and fluids. Service life prediction of  an O-Ring is of practical and engineering interest to predict the behaviour of the component over the service life cycle. Main factor for O-Ring degradation can shown in picture 1

The two important properties of O-Ring are compression stress relaxation and resilience. Resilience is a measure of the ability of the seal to retract to its original shape after a constraint has been removed. The force decay of elastomer component under constant compressive strain is known as compression stress relaxation. The test measures the sealing force exerted by seal or O-Ring under compression between two plates.

Test condition for compression stress relaxation. 

• Accelerated aging : Accelerated Oven Aging at 100 , 125 and 150 degree celsius. 
• Sample were aged inside jigs with 25% compression.
• Failure Criteria : Compression stress relaxation 40%

Compression stress relaxation test data is used to Service Life prediction by Arrhenius equation and WLF equation. 

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

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.

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.

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.

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.

Stress Relaxation

Stress relaxation is the more relevant property for sealing products such as O-ring , rubber seals and rubber gaskets. When rubber is held at constant deformation, there is a decrease in stress as a function of time.This phenomenon can be of great impoertance in sealing application,where the material of the seal is required to maintain a specific level of sealing force to prevent leakage. Stress relaxation can be the dominant factor that limits the effect life of rubber seals and rubber gaskets.

Stress relaxation is usually defined as the loss in stress expressed as a percentage of the initial stress. Thus

stree relaxation = 100*(So – St)/So

So is initial stress

St is stress at time t

Stress relaxation and creep rate are related to one another if the shape of the force-deflection curve is known. According to this the relationship between the two parameter is determined by the incremental stiffness at the point on the force-deflection curve relevant to the stress relaxation or creep measurement .Thus

where C is the creep rate, S is the stress relaxation rate .Since it has been established that creep and stress relaxation can be related in this way,in the next topic i will be following discussion both are referred to as relaxation processes.

Pic. Stress relaxation phenomenon

**Ref. http://www.engin.umich.edu/class/bme332/ch10ligten/bme332ligamenttendon.htm