Bearings

In general, a bearing application involves the sliding action of two materials against each other under a given load, speed, and environmental conditions.  Engineering thermoplastics have been used as sliding materials for a long time, particularly in precision engineering applications, thrust bearings, sleeve bearings, and bushings.  Although plastic bearings do not have the high load-bearing capacity of metal bearings, they offer other advantages such as: the ability to run ‘dry’, inherent low friction and noise, maintenance free operation, chemical resistance, and broad design flexibility.

Materials
In general, semi-crystalline plastics are considerably more wear-resistant than amorphous materials, and therefore typically are used for plain bearings.  [In abrasion tests against steel, amorphous resins such as ABS and PC had abrasion factors of 3,500 and 2,500 respectively; while semi-crystalline resins such as POM and PBT had abrasion factors of 100-200].  More specifically, Ticona’s resins offer many possibilities for meeting the requirements of various bearing applications.

General Guidelines
The bearing behavior of a particular material combination is determined by the system (chosen materials, surface roughness, lubricant, history of handling, environmental exposure) and the stress factors (type of motion, sliding speed, movement pattern, surface pressure, temperature, loading time).  All systems are different: a material combination that is very successful in one system may not respond well in another system.  Many applications require that there be no external lubricant, i.e. the system is run ‘dry’.  The primary issue with dry running bearings is potential softening or melting due to the poor heat dissipation of plastics (although overheating can occur in lubricated bearings as well).  In these cases, the combination of bearing materials is very important.

Two material combinations are possible:

1. Plastic Against Plastic
Although many factors affect the bearing performance, some good plastic on plastic combinations are:

• POM against POM
• POM against PBT or PET
• POM against UHMWPE
• POM against PA
• PBT or PET against PA
• PBT or PET against UHMWPE

In many cases, materials/grades with different friction and surface hardness properties make better pairs (less sticking and noise) than do two surfaces of the same material.  Also it is highly recommended to test [trial is not a verb] the combination of plastic bearing materials.  [More information on material pairs is available in Chapter 3.2 of the "Plain Bearings Made From Engineering Plastics"guide.

2. Plastic Against Metal
In a bearing situation where one material will be plastic and the other steel (or another metal) the main issue is not heat build up.  Adhesion forces and bonding are more significant.  The coefficient of friction is dependent on the surface roughness of the metal, and there is usually an optimum surface roughness (see page 7, Figure 2.7 in “Plain Bearings Made From Engineering Plastics").  Typically, the frictional wear of plastic rises with increasing roughness of the metal counter surface.  Therefore, a balance must be met between the wear performance and the sliding friction for a particular application.

Lubricated Systems
Even though plastics such as acetal copolymer are naturally lubricious, there are times when still better performance is required and the use of lubricants can sometimes be very helpful.  Traditional lubrication can often be used since many engineering plastics are chemically resistant to lubricants.  However, with plastic bearings one also has the option of using an “internal” lubricant (already inside the plastic), such as PTFE, silicone, wax, or graphite.  This type of lubricant is especially useful for applications in which grease or oil is undesirable (such as children’s toys).  It can be challenging to figure out which type of traditional lubrication method to use or which grade of internally lubricated plastic to use (see the Celcon®  POM wear guide).  Some external lubricant suppliers also supply lubrication data.