Table of Contents
In this article we would study limits, fits and tolerances regarding machine parts (to be assembled).
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Limit
When we have to assemble two machine parts (for example nut and bolt) then accuracy in the dimensions of each part is necessary. Otherwise it would be very difficult (sometimes impossible) to assemble them.
The exact (or actual) dimension is known as basic or normal size.
Unfortunately, due to inaccuracy of manufacturing methods it is almost impossible to make a part with exact dimensions. Hence it is necessary for us to allow some deviation in the dimensions of machine components.
Types of deviations (or limits)
- Maximum (or upper) deviation
- Minimum (or Lower) deviation
The maximum allowed size is known as upper limit and the minimum allowed size is known as lower limit.
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Tolerance
Tolerance is the difference between maximum and minimum allowable size of the manufactured part.
Types of tolerances
- Unilateral tolerance
- Bilateral tolerance
Unilateral tolerance
Unilateral tolerance is a type of tolerance in which either only lower or only upper deviation from the normal size is allowed.
Examples of unilateral tolerance are 200(+0.05/+0.00) and 300(+0.00/+0.02)
Bilateral tolerance
Bilateral tolerance is a type of tolerance in which both upper and lower deviation from the basic size is allowed.
Examples of bilateral tolerance are 200(+0.05/-0.03) and 300(+0.08/-0.02)
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Fit
The relationship between the assembled parts due to the difference in their sizes before assembly, is known as a fit.
Types of fits
- Clearance fit
- Interference fit
- Transition fit
Here we will take example of shaft and hole to understand the types of fits.
Above diagram shows types of fits. The shaded portions show the tolerances of either shaft or hole.
Clearance fit
In clearance fit, upper limit of the shaft dimension is always less than the lower limit of the hole dimension. It always provides a positive clearance between the hole and the shaft, over the entire range of tolerance.
Interference fit
In interference fit, lower limit of the shaft dimension is always more than the upper limit of the hole dimension. It always provides interference over the entire range of tolerance.
Transition fit
Transition fit can either be a clearance fit or interference fit. It depends on the actual values of the dimensions.
There are two systems for giving tolerances to the shaft and the hole.
- Shaft basis system
- Hole basis system