Clearance versus preload
For most applications, bearings operate with some residual clearance. Normally, a positive operating clearance near zero is the optimum (diagram 1).
A somewhat greater clearance may be more suitable for:
- high-speed applications to reduce frictional heat
- form errors on the shaft or housing seat such as ovality
The initial internal clearance prior to mounting and permissible reduction after mounting, depend on the type and size of the bearing. The reduction in clearance due to an interference fit may require greater initial internal clearance than Normal to avoid preloading the bearing (fig. 1).
Preload (negative operating clearance) has advantages, but can also be risky. If a high degree of stiffness is required, light preload can be suitable (→ Bearing preload).
A light preload may also be required when there is a very light or no external load on the bearing in operation.
However, there is a risk that too much preload causes the bearing to overheat, which further increases preload, friction and heat. This cycle can continue until the bearing seizes.
It could be argued that preload is acceptable, provided the bearing operates in a zone that does not exceed light preload (diagram 1, zone between 0 and -1). In this case, however, there is an increase in friction and frictional heat.
Although all bearing types can run with some preload, SKF recommends a positive operating clearance. This is particularly important for roller bearings such as cylindrical roller, needle roller, spherical roller and CARB toroidal roller bearings.