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.