Limiting PV, Wear Factor and Coefficient of Friction

Limiting PV
Limiting PV is a description of the capability of a bearing material to resist the pressures and velocities of use. It is defined as the product of the unit load P (psi) based on the projected bearing surface area and the velocity of the shaft expressed in feet per minute.

PV is measured by short duration tests in which the velocity is held constant while the load is increased in fixed increments. Transversely, load can be held constant while velocity is stepped up. Properties such as friction, torque, temperature and wear are measured. When these fail to stabilize at equilibrium, it is an indication that the bearing will fail. The PV at the highest load where equilibrium occurred is recorded as the limiting PV for the material.

Materials must be compared under the same test conditions to be fair and accurate, so test conditions should be specified when reporting PV. Interpreting PV data as design data can be misleading if factors such as shaft speed, shaft finish, temperature, etc., are not the same as test conditions.

For example: Initial measurements of a cotton phenolic composite were made on a steel mill bearing tester. It consisted of a 1 ¼” dia. Shaft, 12-22 Rockwell C (hardness) and a 12-16 microinch finish rotating against the test specimen. The specimen was a 1” x ½” x ½” piece with a ⅝” radius cut into it and fixed to an arm that distributes the load against the shaft. Initial tests were run at 22.5 ft/minute with the load on the arm varied every 24 hours. The test specimen thickness was checked periodically with a micrometer and when wear increased excessively, the next lower load of 22,000 psi was recorded as the limiting PV. The average test result of the same cotton phenolic composite tested with a 1141 carbon steel shaft 1 ¼” dia., 12-22 Rockwell C and a 12-16 microinch (μin) finish was 35,000 psi.

Wear
A bearing material will wear in time even if it is used at pressures and velocities under its Limiting PV. To predict this, wear formulas and test methods have been developed for comparing plastics materials.

A wear factor (K) can be determined by analytically measuring weight loss and change in thickness. The wear factor (K) is stated in terms of in3 minute/ft.lb.hr. It means that a material with a wear factor (K) of one (1) will wear one cubic inch in one hour supporting a one pound load at a velocity of one foot per minute.

As an example of determining the wear factor (K) on a trust washer application; A thrust washer, molded or machined from the test material is mounted in an anti-friction bearing equipped with a torque arm. The thrust washer runs against a carbon steel wear ring which has a 12-16 microinch (μin) finish and 18-22 Rockwell C (hardness). A new wear ring is used in each evaluation because the wear resisting additives in the composite rub off on the steel and could affect subsequent runs. Tests are run below the limiting PV of the test specimen. The test specimen usually goes through a period of 12 to 24 hours until equilibrium temperature is attained and full contact is realized between the steel wear ring and the composite test specimen surface. The test is run until three reproducible values are aobtained for the wear factor (K). Test results (in3 minute/ft.lb.hr) for various cotton phenolic bearing grades range from 300 x 10-10 to 2000 x10-10.

Materials with a lower wear factor (K) will have the longest life in use compared to other similar composites.

Coefficient of Friction
The coefficient of dynamic friction between two surfaces is the ratio of the force necessary to move one surface over the other with uniform velocity normal to the force holding the surfaces together. Since the coefficient of friction is a ratio of two forces, it is a pure numbers and have no unit of measure. Both dynamic and static coefficient of friction are measured on the thrust washer apparatus from the example given at 8500 PV and 300 ft./minute. Different speeds and loads will cause changes in frictional values. These conditions are used because they best represent values attained on other methods of measuring coefficient of friction.

Typical values for various cotton phenolic bearing grades range from .09 to .20 dynamic and .12 to .21 static coefficient of friction.

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