Clearance, preloading, and fit are common mechanical engineering concepts that also apply to the rolling bearing, for which they have a crucial importance. At Emerson Bearing we want to make sure that your bearing selection is perfect for your design. In this post, we will discuss clearance fits and preloads.
First, it is important to delineate between initial and operating clearance. The former refers to the bearing before it’s mounted and the latter when the bearing is mounted and has reached a steady temperature. As a general rule, a ball bearing should have an almost-zero operational clearance, while needle, spherical, cylindrical and CARB toroidal bearings should always have some residual radial operational clearance.
Radial clearance is the play between the ball and raceway perpendicular to the bearing axis. Axial clearance is the play parallel to the bearing axis and is typically at least 10 times greater than the radial clearance. Too little or too much internal clearance will significantly influence factors such as heat, vibration, noise, and fatigue life. Radial bearing clearance is broken down into several groups: C1, C2, CN, C3, C4, and C5. CN is the “normal clearance” and is defined by DIN 620 and ISO 5753 and is used for bearings in normal operating conditions; C1 and C2 class bearing have radial clearances smaller than CN.
It is important to note that, in applications that have to withstand particularly high or low temperatures, the clearance has to be taken into account early in the design process, in order to make sure that the clearance is compensated for, due to the thermal expansion and contraction of the housing and shaft.
How a bearing fits into your assembly is also a major factor in its performance. In general, the bearing’s inner ring should have a tight fit on the shaft and the outer ring should have a tight fit in the housing. For more details about mechanical fits, you can refer to mechanical tolerance standard charts, or contact us at Emerson Bearing. We would be delighted to guide you through this process and make sure that your bearing and shaft have the appropriate size for your design.
As a general rule, once the bearing is fit into place there should still be some residual clearance inside the bearing. However, in some cases that clearance may not be ideal and the bearing should be preloaded – a load should be applied to the bearing, pushing the ring and effectively removing any clearance.
Preloading is a good idea for bearings under a very light load at high speeds, for preventing bearing slip or creep in machine tools, or for pinion bearings in automotive axle drives where some extra rigidity in the bearing is needed.
There are two main methods for preloading: position preload and constant pressure preload. The type of preload that’s best for you depends on your application. Generally speaking, position preloading increases the rigidity of the bearing and constant pressure preloading reduces axial vibrations in high speed applications.