For rolling element bearings, the L10 life calculation is a simple way to determine the expected bearing stroke life. But for self-lubricating bearings, determining life expectancy is not so simple.
Plain bearing life is based on the amount of wear the bearing experiences, which depends on factors unique to each application, such as bearing material and operating conditions. Furthermore, the amount of wear that indicates failure (or failure to perform as required) in one application may be perfectly acceptable in a different application. This variability makes it important to understand what factors cause self-lubricating bearings to wear out and therefore shorten their service life.
The most significant factor in plain bearing wear is the material. Plain bearings are made of softer materials than the shafts or rails they sit on and are “sacrificial,” meaning the bearing will wear out more (or faster) than the shaft. Common bearing materials include plastic, composites and bronze.
Which materials resist wear best?
Plastic bearings have good wear resistance because they are usually soft enough that particles are trapped in the bearing rather than trapped between the bearing and the shaft. They are also self-lubricating and can maintain a very low coefficient of friction between the bearing and shaft during starting and continuous operation. Both attributes contribute to good wear resistance. However, as heat is generated in the bearing, the bearing can expand, reducing the clearance between it and the shaft, causing more friction and wear. Another vulnerability of plastic bearing materials is moisture, which can also cause plastic bearings to expand. And exposure to chemicals can change the properties of the bearing material, typically decreasing the material's hardness, resulting in increased wear.
Composite bearings are often constructed with an inner coating that contributes to a low coefficient of friction and provides self-lubrication. Just like plastic bearings, composite designs are susceptible to decreased hardness when exposed to certain chemicals. But they can dissipate heat better than plastic bearings and have better dimensional integrity in high-temperature applications, which helps prevent excessive wear due to reduced clearances.
Bronze bearings can be self-lubricating under some conditions, but generally require external lubrication. If sufficient lubrication is not maintained, metal-to-metal contact will occur, resulting in high friction, high heat generation, and excessive wear. And this external lubrication can attract and retain particles, causing greater wear between the bearing and the shaft.
What is the role of shafts in the wear of plain bearings?
Shafts are usually made of aluminum, steel or ceramic. Ground and polished shafts have fewer surface imperfections that cause friction and wear on the bearings. But if the surface is too smooth, lubrication (provided externally or by the self-lubricating properties of the bearing) will not adhere to the shaft and wear will occur more quickly.
Causes of plain bearing wear include insufficient lubrication, excessive heat, and overloading.
