Frictional Characteristics

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

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

This characteristic, coupled with its superior abrasion resistance and load-carrying ability, is an important reason why urethane is used for bearings and bushings. Since the hardness of compounds of urethane approaches some plastics, comparison of urethane with various plastics is shown in Table I.

TABLE 1
  Urethane Nylon 66 2.5% water Acetal Cast Phenolic Unfilled
Specific Gravity 1.20 1.14 1.42 1.30
Hardness, Rockwell R90 R108 R120 M110
Tensile Strength, psi 10,000 11,200 10,000 7,000
Elongation at Break, % 270 200 15 --
Modulus of Elasticity in Tension, 10^3 psi 52 260 410 700
Flexural Modulus of Elasticity (Instron), 10^3 psi 81 175 410 --
Compressive Modulus, 10^3 psi 68.75 -- -- --
Impact Resistance, ft lb/in Notched Izod, 75F 15 2.0 1.4 0.3
Head Deflection Temp, % at 66psi 365 300 338 260
At 264 psi 135 150 255 --
Taber Abrasion, CS-17, 1000g, mg loss/1000 rev 5 7 20 --
Water Absorption, 24 hrs at 75F, % 1.2 0.4 0.25 0.4


FIG 2

Effect of Additives on Friction Properties

Additives may be used to alter the frictional proportion of urethane polymers. With urethane, powdered Teflon® TFE fluorocarbon resin and Teflon® TFE fluorocarbon fiber flock significantly reduce the coefficient of friction of this elastomer. Incorporation of graphite increases the coefficient of friction. The frictional change due to these additives is shown in Figure 2. Urethane without additives is used as a control.

The additives mentioned above will affect other physical properties of urethane. Property changes are presented in Table II.

TABLE II
Effect of Lubricant Additives on Physical Properties
Plain Urethane + Graphite 10 parts + Teflon® Powder 10 parts + Teflon® Fiber Flock 10 parts
100% Modulus, psi 4300 4200 4100 4100
Tensile Strength, psi 9400 6500 5000 5200
Elongation @ Break, % 265 180 115 125
Durometer D Hardness 72 72 72 72
ASTM D470 Tear, lbs/in 115 120 105 95
National Bureau of Stds Abrasion Index, % 425 365 500 890
Compression Set, Method A 1350 psi, 22 hrs @ 158F, % 6 9 8 4

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

Effect of Additives on Bearing Performance

Both graphite and Teflon® improve the performance of urethane in bearing applications. Even graphite, which results in an increased friction coefficient, will provide better bearing service. This unexpected improvement is probably due to the eventual formation of a graphite film at the load interface, and to the better thermal conductivity of the graphite-filled compound. Pressure-Velocity (PV) limit data for bearings based on urethane, Nylon 66 and Delrin acetal resins are shown in Figure 3.

FIG 3

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 noting about the length of service however. Although urethane 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 appraratus which utilizes a thrust washer principle and the apparatus is shown in figure 4. It consists of a table drill press, variable speed drive and 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 inexpensive to run tests. 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.