Bearing Clearance Information

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Bearing clearance is the play that exists between the balls and the inner and outer races of the bearing. The bearing clearance of a ball bearing is a measure of the geometrical space between these three parts: outer ring, ball, and inner ring. Clearance is important for a bearing as it can have an effect on bearing life and other factors related to the cohesiveness of the bearing. Therefore, before selecting your bearings, it’s important to have an understanding of what kind of clearances you are working with. What follows is some basic information about internal clearances and related issues, as well as some general clearance measurement information for standard NMB ball bearings.

Clearance Fits

The shaft and housing you choose will determine the amount of clearance you experience with your ball bearing. This is known as the “fit”. An interference fit is one in which there is virtually no play between the parts. In a loose clearance fit, there is a great deal of play within the parts.

Radial and Axial Clearance

As with many aspects of ball bearings, it’s important to differentiate between radial and axial. Radial clearance refers to the give between ball and races perpendicular to the bearing axis, while axial clearance refers to the play between ball and races parallel to the bearing axis. Clearance issues become particularly important in applications that are subjected to temperature extremes, as extremely hot or cold weather may expand or contract shafts and housings.

Thermal Expansion and Cooling

Thermal expansion is the basic response of matter to expand in volume as a result of higher temperatures. Solid matter exists in a dense state where molecules are fairly close together. Increasing temperature excites these molecules and causes them to move faster. They bounce against each other and have wider ranges of motion, with the end result being to expand the matter and increase the volume size of the object. Different types of matter will expand at different rates due to their structural composition, which is why thermal expansion may create a tighter fit within a ball bearing. Fortunately, the rate of expansion among similar types of matter is relatively consistent, so it is possible to calculate the rate and result of thermal expansion at various temperatures and pressures. With these calculations, it’s possible to predict the level of fit that thermal expansion will produce for your ball bearing within a reasonable measurement in most cases.

Cooling follows a similar concept. Decreasing temperature causes the molecules to move more slowly, binding closer together and becoming denser, with the result that certain parts will contract, creating more clearance. This is also possible to calculate, because although cooling metal is often preferable, since it is less likely to alter the other basic properties of the substance, this form of alteration is less common because most applications that use ball bearings produce heat.

Press Fit

Another way in which an interference fit can be achieved that does not involve temperature adjustment is through a very large amount of pressure (often with hydraulic presses) that pushes the parts together. This type of interference fit is called a press fit. In a press fit situation, the edges of the shafts and the holes may be chamfered to smooth out the pressing operation and make it easier to control.

As should now be apparent, internal clearance of a ball bearing affects life, noise, vibration and temperature generation, so it is extremely important to select the optimum radial clearance for each application.

Internal Clearance

The three types of internal clearance of a ball bearing are radial clearance, axial clearance, and moment clearance.

Radial Clearance (Gr)

The displacement generated by moving the outer ring in the radial direction while securing the inner ring
(Figure 1)

Axial Clearance (Gt)

The displacement generated by moving the inner ring in the axial direction while securing the outer ring
(Figure 2)

Moment Clearance (θ)

The angular displacement generated by tilting the inner ring in the axial direction while securing the outer ring
(Figure 3)

Generally, radial clearance is considered as a specified value, as shown in the table below by NMB part number and JIS part number. When a ball bearing is fitted to either a shaft or housing with interference, internal clearance decreases.
The fits and temperature affect the internal clearance. Therefore, actual application conditions need to be reviewed when internal clearance is selected.

Radial Clearance Radial clearance (Gr)
Figure 1

Axial Clearance Axial clearance (Gt)
Figure 2

Moment Clearance
Moment clearance (θ)
Figure 3

 

Radial Clearance Standard for NMB Part Numbers

Radial Clearance Code P13 P24 (Standard)
P25
P58
Dimensions in µm 2.5-7.5 5-10 5-12.5 12.5-20
Dimensions in inches 0.0001-0.0003 0.0002-0.0004 0.0002-0.0005 0.0005-0.0008

Raidal Clearance Standard for NMB Part Numbers (JIS Series Bearings only)

Radial Clearance Code M2 (Standard)
M3
M4 M5
Dimensions in µm 3-8 5-10 8-13 13-20
Dimensions in inches 0.0001-0.0003 0.0002-0.0004 0.0003-0.0005 0.0005-0.0008




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