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.