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SHAFT TOLERANCES & FITS | MECH MINUTES | MISUMI USA

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Previously on MechMinutes: Shafting series, we covered primary considerations when selecting material and surface treatments. All past videos are posted for your viewing pleasure to get caught up to this week’s episode of Tolerances & Fits.

A ‘tolerance’ is the allowable amount of variation from a specified dimension. When tolerances are combined between a shaft and bore, they can be classified into three categories known as ‘Fits’. The Clearance [Fit], Interference [Fit], and Transition [Fit] can either assist in assembly, or prevent them from being disassembled easily.

A Clearance fit ensures a shaft can be freely inserted into the intended bore. This generally means the maximum limit of the shaft is less than the minimum limit of the bore diameter.

An Interference fit guarantees the shaft and bore will interfere at every point within their tolerance zone. This is commonly referred to as a ‘Press Fit’. Often the two components must be joined by excessive force, or by altering the atmospheric temperature to slightly expand or shrink the diameter for assembly.

The Transition fit is a combination between the Clearance and Interference Fit. Depending on the actual dimension of the shaft and bore, it is possible that either could occur. This fit is best used when a shaft must be held in a precise location. If interference should occur, it will not be to the extent where disassembly is overly difficult.

Misumi shafts and bores follow the JIS shaft fit tolerances. Selecting the proper tolerance is critical to achieve the desired fit between two mating components.

This tolerance table can be found in the ‘Technical Reference’ section in each catalog. The columns represent the tolerance grade; while the rows are organized by dimeter ranges. Finding where the two intersect will indicate the corresponding tolerance range. If we look at a 10mm shaft with an h6 tolerance (10h6), we can see the expected tolerance is no larger than the basic size, but is capable of being 11microns under 10mm. This brings the expected minimum to 9.989, and the maximum to 10.000mm.

Misumi offers a useful table [Fig. 2.2] to help you choose the appropriate tolerance combination is your application. If we would like this 10h6 shaft to exhibit a clearance fit, we can see each tolerance class that would satisfy this condition.

Now that we’re familiar with shaft selection, join us next episode where we will take a look at linear shaft applications and related alterations!

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SHAFT TOLERANCES & FITS | MECH MINUTES | MISUMI USA

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