The mating hardware can make or break the life and effectiveness of a polymer bushing.
The key characteristics of the mating hardware that affect bushing life are the hardness and the surface finish. In this article we are going to discuss the general guidelines for surface roughness, finish, and hardness when using polymer bushings.
Surface Roughness and Surface Finish
Surface finish is important because it plays a major part in the friction and the wear rate of the bushing. Of course, a shaft that is too rough can result in abrasion and premature wear, but did you know there can be even worse issues if the shaft is too smooth?
A shaft that is too rough can result in abrasion and premature wear, but did you know there can be even worse issues if the shaft is too smooth?
When a shaft is too smooth, the bushing is likely to adhere to it. For example, squeaking coming from a bushing is usually caused by stick-slip which occurs when the shaft is too smooth. This happens because an extremely smooth shaft results in a significant difference between the coefficients of static and dynamic friction. In addition, if the surface of the shaft is too smooth, it lacks the necessary roughness to enable self-lubricating bushings to release the lubricant.
The general rule of thumb for the surface finish is 8 32 Ra for the dynamic surface. However, if your goal is to maximize the life of the bushing you want to aim for 20 Rms as your average surface roughness. If, on the other hand, your goal is to minimize friction, some designers suggest you aim for a surface roughness of 32 to 64 Rms.
In general, a ground surface is to be preferrd over a turned surface finish and polishing is also recommended. In addition, the finishing operation really should be performed in the same direction as the bearing motion relative to the shaft.
Surface Hardness of the Mating Shaft
Keep in mind that fillers in the bushing material are sometimes abrasive (e.g., glass fiber) even if the main bushing material is not. For that reason, the recommended surface hardness is Rockwell 40C. Thats why softer materials like aluminium are not typically recommended for the shaft material unless the have been hardened or anodized.
When shafts are not hard, they will be smoothed during the break-in phase and, as a result, lose some of the necessary surface roughness they need. This leads to the problems discussed in connection with shafts that are too smooth. Another problem that can arise is damage to the surface of a softer shaft if particles get caught between the bearing and the shaft. This can be avoided, though, by use of self-lubricating bearings because they do not pull debris and contaminants into the mating interface like bushings lubricated by grease will do.
If the bearing is for a light load, experts indicate that you can use a shaft material with a hardness of Rockwell 120B. In general, however, harder mating surfaces result in improved wear.
In general, harder mating surfaces result in improved wear.
Two of the key factors that determine bushing life and effectiveness involve the shaft surface finish and hardness. The general rule of thumb is ground and polished 8 32 Ra surface roughness and a hardness of about Rockwell 40C.
Rockwell Hardness conversion: http://www.engineersedge.com/hardness_conversion.htm
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