Metal plain bearings have been the industry standard for many years, but the use of plastic bearings is steadily increasing, especially in contaminated environments. Developed over the last 25 years, these self-lubricating plastic bearings offer performance advantages over metal, namely improved lubricity, elasticity, and corrosion resistance. As a result, more engineers turn to self-lubricating plastic bearings.

Maintaining lubricity

A proliferation of self-lubricated plastic bearings for contaminated environments.

All bearings must be lubricated, not only to reduce friction and wear, but, in the case of plain bearings, to prevent them from seizing the shaft which they support. Metal bearings rely mainly on oil and grease lubricants. But in dirty environments, these wet lubricants trap solid contaminants, causing a loss of lubricity or slipperiness. This increases friction and accelerates wear between shaft and bearing, a condition that is especially critical in low-load, low-speed, stop-and-start, and linear applications. As the lubricants become loaded with dust or dirt, they also lose their ability to dissipate heat, and friction rises significantly. Inevitably, the moving parts lock together or seize, causing failure and downtime.

By contrast, self-lubricating plastic bearings contain a mix of dry lubricants, which are not easily fouled by solid contaminants. In operation, movement between shaft and bearing causes microscopic abrasion of the dry lubricant, filling and smoothing the shaft surface to reduce friction.

If the shaft surface is too smooth, it inhibits the abrasion of the dry lubricant. As a result, the lubricant is ineffective and stick-slip can occur. For this reason, don’t use plastic bearings with chrome-covered shafts. If stick-slip occurs, roughening the shaft with fine sandpaper usually produces the necessary abrasion of dry lubricant to restore free movement.

Most plastic bearing materials expand when exposed to heat and moisture. But this factor is only significant when the running clearance between the bearing and shaft is less than 0.001 in. Plastic bearing manufacturers recommend larger clearances to eliminate potential problems, such as excessive wear or seizing of the shaft, from expansion of the bearing material.

Elasticity

Plastic bearings have less strength and load capacity than metal types. But their resilience enables them to absorb vibration and moderate shock loads that would cause metal bearings to squeak. In addition, plastic bearings can be more forgiving of misalignment. They bend and return to their original shape when the misalignment force is released.

With plastic bearings, especially those that contain no reinforcing fibers, the material can deform under edge loading without affecting bearing performance. The lubrication capabilities of the bearing material are effective even during misalignment. Plastic materials that contain reinforcing fibers possess more load capacity but are less able to deform and return to their original shape.

Combating corrosion

Plastic bearings do not corrode as a result of moisture. Some of the plastic materials commonly used in these bearings, such as PEEK, Torlon, and PTFE (Teflon), offer resistance to many chemicals, even etching acids, sulfuric acid, and chlorine gas in concentrations up to 65%.

Other bearing materials, such as nylon and Delrin, contain a blend of thermoplastic alloys combined with reinforcing composite fibers to provide high strength for applications involving extreme pressure and heat. The lubricants imbedded in the plastic material can’t be washed away or eaten away by chemicals, as can occur with wet lubricants.

Continue on page 2