3.3 Load Carrying Ability of Plastic Bearings

In Section 2.2 of sintered metal bearings, the meaning and formulas for calculation of PV factor was dealt with.

For different plastic materials, the following values of PV and load capacities apply:

A PV limit of 15000 ordinarily can be used for dry operation of carbon bearings. This should be reduced for continuous running with a steady load over a long period of time to avoid excessive wear. When operating with liquids which permit the development of a supporting fluid film, much higher PV values can be used.

A hard, rust-resistant shaft with at least a 10 min. finish should be used. Hardened tool steel or chrome plate is recommended for heavy loads and high-speed applications. Steel having a hardness over Rockwell C50, bronzes, 18-8 stainless steels, and various carbides and ceramics also can be used.

Certain precautions should be observed in applying carbon-graphite. Since this material is brittle, it is chipped or cracked easily if struck on an edge or a corner, or if subjected to high thermal, tensile, or bending stresses. Edges should be relieved with a chamfer. Sharp corners, thin sections, keyways and blind holes should be avoided wherever possible. Because of brittleness and low coefficient of expansion (about 1/4 that of steel), carbon-graphite bearings are often shrunk into a steel sleeve. This minimizes changes in shaft clearance with temperature variations and provides mechanical support for the carbon-graphite elements.

The PV factor, used as a load-speed limit also provides a basis for estimating relative wear rates. The total volume of material worn away is approximately proportional to the total normal load multiplied by the distance traveled in a length of time.

Thus,

R = K(PV) T

where:

• R = radial wear in a sleeve bearing (in)

• K = wear factor (in3•min/ft•lb•hr)

• P = load (psi)

• V = surface velocity (fpm)

• T = time (hrs)

This equation does not always provide accurate absolute values for wear rate, but it is useful for estimating relative wear rates for alternative materials. In general, K wear values with fillers are lower than unfilled materials. If wear values are important for specific components, life tests should be made. These might employ moderately accelerated load and speed conditions to obtain a K value representative of the plastic, the shaft and its finish, and the application conditions.

K values should be increased by 50% for cast iron and bronze shafts, and more than 5 times with soft stainless steel or aluminum alloys. Increased surface hardness can markedly reduce wear, while surface roughness of the shaft often has an optimum value in the 4 to 14 min. rms range. Lubrication also has a pronounced influence on wear. With oil impregnation, wear rates commonly drop to negligible values with plastics, wood, and porous metals.

The wear factor K values are shown as follows:

Comparative values for plastics often used as bearing materials are given in the following table:

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