Engineering Fits

I have been working on some optomechanical parts that require a hole-and-shaft style mating. During their design, I realized I really didn't have any theoretical background on how big the holes and shafts should be so that they fit together. This lead me to do some basic research into engineering fits.

Engineering Fits

According to Building Scientific Apparatus, 4th ed.1, fit should be specified when the absolute size of two mating parts is not important, but the clearance between them is critical.

To understand fits, it helps first to think in terms of active surfaces and tolerances.

An active surface is a region where two surfaces touch and either move against each other or have a static fit 2. (Interestingly, an active surface is really two physical surfaces by this definition.) The tolerances on the size of two mating parts determines the type of fit. An example of the tolerances on a hole-and-shaft assembly is shown below.

Fit definitions

In this context, we can define three types of fits:

1. Clearance fits : Tolerance zones do not overlap
2. Transition fits : Tolerance zones partially overlap
3. Interference fits : Tolerance zones fully overlap

These fits exist on a continuum and are not neatly distinguished in practice. The continuum can be seen by plotting the force required for mating vs. the allowance. The allowance in this context can be defined as follows3:

\[ \text{allowance} = \text{smallest hole} - \text{largest shaft} \]

Clearance fits

1. Sliding fit : Some lateral play
2. Running fit : More fricition, but more accurate motion

Transition fits

1. Keyring fit : Slight force required for mating and easy to remove
2. Push fit : More force required; possible to remove by hand

Interference fits

1. Force fit : Hand tools likely required for mating
2. Press fit : Requires more force, likely using a press