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