Friction of Die-Thane polymers against non-lubricated surfaces decreases with increasing hardness as shown in Figure 1.

FIGURE 1 COEFFICENT OF FRICTION AT VARIOUS HARDNESS

Since harder polymers have the lowest coefficient of friction, these materials have been used where sliding resistance is important.

Of all Die-Thane Urethane rubbers, P-675 has the lowest, unlubricated coefficient of friction. This characteristic, coupled with its superior abrasion resistance and load-carrying ability, is an important reason why 75 is used for bearings and bushings. Since the hardness of compounds of P-675 approaches some plastics, comparison of P-675 with various plastics is shown in Table I.

EFFECT OF ADDITIVES ON FRICTIONAL PROPERTIES

Additives may be used to alter the frictional properties of Die-Thane polymers. With P-675 urethane rubber, powdered TEFLON® TFE fluorocarbon resin and TEFLON® TFE fluorocarbon fiber flock significantly reduce the coefficient of friction of this elastomer. The frictional due to these additives is shown in Figure 2. P-675 without additives is used as a control.

FIGURE 2 DIE-THANE P-675 EFFECT OF ADDITIVES ON COEFFICIENT OF FRICTION

The additives mentioned above will affect other physical properties of Die-Thane P-675. property changes are presented on Table II.

Materials Used:

1. TEFLON® Powder – “ Rilube #63, Modern Industrial Plastics, Dayton, Ohio
3. TEFLON® Fiber – TFE TEFLON® fiber, flock, 1/64”, Du Pont, Textile Fibers Department.

All of the additives at a 10 part level (7.4% by weight of total compound) will reduce modulus, tensile and elongation. Additives which reduce the friction coefficient also improve abrasion resistance. The improvement in abrasion obtained with TEFLON® fluorocarbon fiber addition is significant and was also observed during long-term friction tests.

The 10 parts of additive is not necessarily the optimum. However, 10 parts offer significant frictional improvement over 5 parts and not significantly inferior to 15 parts. The optimum level of additive, considering a balance of physical and frictional properties, probably fails between 5 and 10 parts.

EFFECT OF ADDITIVES ON BEARING PERFORMANCE

TEFLON® improves the performance of Die-Thane P-675 in bearing applications. Pressure-Velocity (PV) limit data for bearings based on Die-Thane P-675 urethane rubber, Nylon 66 and DELRIN® acetal resins are shown in Figure 3.

FIGURE 3 PV LIMITS FOR DIE-THANE P-675

These curves represent the performance limits of the bearings. Bearings can operate at any combination of pressure and velocity below the respective curves without catastrophic failure. The PV limits predict nothing about the length of service however. Although Die-Thane P-675 may be expected to outwear most thermoplastics, its performance will be influenced strongly by temperature and other environmental conditions. The best determination of bearing durability is a service test.

Friction data listed in this chapter were obtained with an apparatus which utilizes a thrust washer principle and the apparatus is shown schematically in Figure 4. It consists of a table mounted drill press, variable speed drive and simple machined parts to accommodate test samples. Friction force and wear can be measured with this device. The use of standard components and small, easily fabricated test samples make this an inexpensive testing apparatus. The sample is a disc with a diameter of 1-1/3”, on one side is a rim of 1/16” width; this rim constitutes the area of contact.

FIGURE 4 THRUST WASHER TESTER