Motor Bearing Clearance: What is it and How to Choose?

Generally, we consider the components of a bearing to include: the inner ring, outer ring, rolling element, cage, seals, and lubrication. Indeed, during past lectures to motor engineers, the clearance of a motor bearing has been listed as one of the components of a bearing.

Of course, clearance is an air gap, not a physical component, but the attention engineers pay to clearance in design, installation, use, and maintenance is no less than any physical part of the bearing.

What is the clearance of a motor bearing?

Motor Bearing Clearance: What is it and How to Choose?

Bearing clearance typically refers to the movement of one ring relative to a fixed ring on the bearing. If this movement is axial, it is called axial clearance; if it is radial, it is called radial clearance.

The following figure shows the radial and axial clearance of a deep groove ball bearing:

radial and axial clearance of a deep groove ball bearing

In the figure:

  • Radial clearance of the bearing = δ
  • Axial clearance of the bearing = δ1 + δ2

Clearances for bearings are governed by corresponding national and international standards. For example, the commonly used radial clearance for rolling bearings, GB/T4604-93, categorizes clearances as follows:

  • Group 2, a clearance group smaller than Group 0
  • Group 0, the standard clearance group
  • Group 3, a clearance group larger than Group 0

The standard also specifies other clearance groups. In practical applications, the clearances most often used for industrial motor bearings are from the standard clearance group (Group 0) and Group C3.

Table 1: Radial Clearance of Deep Groove Ball Bearings (Cylindrical Bore) – Unit: µm

Nominal Inner Diameter of the BearingClearance
d mmC2StandardC3C4C5
ExceedingToMinMaxMinMaxMinMaxMinMaxMinMax
25607213823
6100721382314292037
101809318112518332545
1824010520132820362848
2430111520132823413053
3040111620153328464064
405011623183630514573
5065115828234338615590
658011510302551467165105
8010011812363058538475120
10012022015413666619790140
1201402231848418171114105160
14016022318534691811130120180
16018022520615310291147135200
10200230257163117107163150230
200225235258575140125195175265
225250240309585160145225205300
2502802453510590170155245225340
28031525540115100190175270245370
31535536045125110210195300275410
35540037055145130240225340315460

Radial clearance of deep groove ball bearings

(1) Round Housing Bore Tolerance – Unit in Micrometers (μm)
Nominal Inner Diameter of the BearingClearance
d mmC2StandardC3C4C5
ExceedingToMinMaxMinMaxMinMaxMinMaxMinMax
10025204535605075
10240252045356050756590
24300252045356050757095
304053025504570608580105
4050535306050807010095125
506510404070609080110440140
6580104540756510090125130165
801001550508575110105140155190
1001201555509085125125165180220
120140156060105100145145190.20245
14016020707012011516516521.5225275
160180257575125120170170220250300
180200359090145140195195250275330
20022545105105165160220220280305365
22525045110110175170235235300300395
25028055125125195190260260330375440
28031555130130205200275275350410485
31535565145145225225305305385455535
355400100190190180280370370460510600
400450110210210310310410410510565665
450500110220220330330440440550625735

For motor engineers, the most commonly used bearing types are deep groove ball bearings and cylindrical roller bearings. If clearances for other types of bearings are required, they can be obtained from the relevant national standards or manufacturers.

How should the clearance of a motor bearing be selected?

Firstly, the bearing clearance values mentioned in the above standards are initial values, that is, the clearance values when the bearing has not been installed or used. When the bearing is installed, put into operation, and in a working state, the clearance value is a critical factor in bearing operation.

Generally, the fit between the rotor of the internal-rotation motor and the bearing is relatively tight, while the fit between the bearing housing and the outer ring of the bearing is relatively loose. In this way, the inner ring of the bearing will expand outward, which will reduce the clearance. We call this the reduction of installation clearance.

When the bearing is in the working state, there is usually a temperature difference between the inner and outer rings of the bearing, which leads to different degrees of thermal expansion of the inner and outer rings of the bearing, thus changing the clearance. We call this the reduction of clearance caused by temperature.

Therefore, the actual working clearance of the bearing is the initial clearance minus both the reduction of installation clearance and the reduction of clearance caused by temperature.

δwork = δinitial – δinstall – δtemperature

The lifespan of a typical motor bearing is related to its operational clearance as follows:

The relationship between lifespan and clearance.
  • Vertical Axis: The ratio of the actual lifespan to the expected lifespan of a bearing under various effective clearances (operational clearances).
  • Horizontal Axis: The effective clearance (operational clearance) of the bearing.

It’s clear to see that the highest lifespan of the bearing occurs when its clearance is a value less than zero. This implies that theoretically, when the operational clearance of the bearing is negative, the bearing’s life is at its longest.

However, operating at this clearance value carries a risk. By examining the slope of the curve on both sides, it becomes apparent that when the actual operational clearance is less than the clearance at maximum lifespan, the slope of the curve is steep, and with further decrease in clearance, the lifespan drops dramatically.

In contrast, an increase in clearance results in a much slower decrease in bearing life.

Therefore, if the bearing’s operational clearance is at the optimal value for maximum lifespan, any further reduction due to changes in operating conditions will rapidly decrease the bearing lifespan. This is what we commonly refer to as ‘bearing seizure’.

Considering all the above, in actual operating conditions, we often choose an operational clearance slightly greater than zero for the bearing. This is a safe value, ensuring the impact on bearing life, regardless of changes in operational clearance, remains within a controllable range.

Typically, motor engineers opt for clearances from the Normal or C3 group when selecting bearing clearances. (This is associated with the tolerance fit we commonly use and the internal temperature distribution in the motor.)

Naturally, if operating conditions are unique, the bearing clearance value would need to be verified through calculations to ensure the bearing operates under a safe clearance condition.

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Shane
Author

Shane

Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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