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Bushings and Plain Bearings - Clearances

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Bushing Clearance

Typically bearing internal clearance is defined as the total distance through which one ring can be moved radially (radial internal clearance) or axially (axial internal clearance) in relation to the other ring under a defined measuring load. In the case of bushings, this is not so. Since bushings are of single piece construction, any clearance found would be from the radial movement of the shaft that rides within the bushing.

It is necessary to distinguish between the internal clearance of a bushing before it is mounted and the internal clearance of a mounted bushing when in operation (operational clearance).The initial clearance will always be greater than the operational clearance because the bushings are expanded or compressed by interferences fits and as a result of the differences in thermal expansion of the bearing rings and mating components. In the event that a wrapped bushing is utilized, clearance before mounting is not measurable due to the construction of the bushing.

The bushing internal clearance shall be selected so that when bushings are mounted with recommended fits, and operate under normal conditions a suitable operational clearance will be obtained. For other conditions, e.g. where both rings are mounted with an interference fit or where unusual temperatures prevail, bearings with greater or smaller internal clearance than normal may be required.

Please see the associated Bushing Tolerances and Bushing Fitting and Mounting, Technical Information Sheets for further information on bushing clearance.

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Bushing Tolerances

Wrapped Bushings

 

It is not possible to accurately measure the external and internal diameters of a wrapped bushing in its free condition. In the free state, a wrapped bushing will not be perfectly cylindrical. The bushing will conform to the housing when the split is tightly closed. For this reason the OD & ID of a wrapped bushing can only be checked with special gauges and test equipment.

 

Below is an example on how to determine the bushing’s ID & OD tolerances with respect to the mating shaft and housing tolerances after installation:

 

Units: mm

 

The chart and diagram shows that after installation this bushing will have a total OD tolerance of 0.030 mm. Also, the ID tolerance is 0.065 mm after installation.

 

Standard Bushings (non-wrapped)

 

Standard bushing tolerances are specified on their respective dimension tables. For example:

 

 

Units: mm

 

If a bushing with an 8 mm ID (d) and a 12 mm OD (D) is used, the ID (d) dimension specifies an F7 tolerance which corresponds to 13 µm / 28 µm. The OD (D) dimension specifies an m6 tolerance which corresponds to 7 µm / 18 µm.

 

Note:  Uppercase tolerances refer to different values from lowercase tolerances.

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