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Low Cost Automation Tutorial

#019 Selection of Bearing Fits

Category : Design hints
November13, 2009

Rotary bearings are machine elements that support rotating shafts accurately while providing low friction condition. In order to obtain stable bearing performances over prolonged duration, appropriate bearing fit design considerations are required to avoid any negative effects ([Chart 1]) that may arise from bearings' rotary operation.

[Chart 1] Designs for bearing fit and issues to be solved
Bearing mechanism issuesResultant behaviorsCountermeasure example
Temperature rise due to high speed rotation.Shaft diameter expansion causing moment loads, resulting in bearing seizure, uneven wear, and damages.Choose fitting dimensions with shaft expansion anticipated.
Thrust and radial direction clearance variations occurring from applied loads.Degradation of rotational accuracy. Abnormal noises.Choose fitting dimensions with clearance increase anticipated under load.

Design examples of rotary bearing application.

An example used here is based on "Inner race rotation : Outer race retained" arrangement shown in [Fig.1].

[Fig.1] Example - Inner race rotation : Outer race retained (loading in one direction)

Selection of bearing fits based on shaft diameter

Transition fit or interference fit is selected to ensure the shaft and the inner ring will always rotate together.

[Chart 2] Selection example of fits (case for radial bearings)
Shaft/bearing fitsApplicable for:Fitting standards
Transition fitFor light loads.
For varying loads.
h5, js6, k6, etc.
Interference fitFor general applicationsjs5, k5, m5, m6, etc.

Selection of fits for bearing holder bores

Clearance fit should be chosen for ease of assembly while preventing any deformation of bearings' outer/inner races. In general, [Clearance fit] (H7) is used. In case of light loading applications (H8) can be used, or (G7) when large temperature variations are to be considered.

[Chart 3] Fit selection examples for bores (case for radial bearings)
Bore fit selectionApplicable for:Fitting standards
Clearance fit.General rotation applications.H7
Same as above.Light loads.H8 (wider tolerance than H7)
Same as above.Large operational temp. rise.G7 (large clearance)

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