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36. ASSEMBLY

•the mating of parts to give a combined operation.

•In previous centuries, and before, fit in assemblies was often not considered, or when it did matter, each piece was custom fitted.

•Modern methods of mass production means that some fundamental methods of fitting are necessary.

•The three basic methods of fitting are,

-Fitting - One part is made to size, and then a second part is made to fit it.

-Selective Assembly - parts are made to loose general tolerances, and then the results are sorted into tolerance ranges. (e.g. bearings, solar cells, etc)

-Interchangeable Assembly - Parts are made to tighter tolerances, and as a result assembly of randomly selected parts will yield a good fit. This is essential for modern assembly lines.

•The basic categories of fits are,

-Clearance - a gap is always present between parts

-Transitional - the parts will have a gap sometimes, other times the parts will touch

-Interference - both parts will always be in full contact

36.1 THE BASICS OF FITS

• A set of reasonable fits is suggested below,

shafts rotating under 600 rpm with ordinary loads; >RC5 shafts rotating over 600 rpm with heavy loads; < RC5 shafts sliding freely; approx. LC

push fits with keyed shafts and clamp, no fitting; LT parts assmemble with some basic fitting; LN Permanent assembly with no freely moving parts; FN1 permanent assembly with severe loading effects; FN3

permanent assembly with press needed for assembly; FN5

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36.1.1 Clearance Fits

•A clearance fit always has a gap between the two mating parts.

•The diagram below shows a clearance fit between a shaft and a hole

Hole

Tolerance

Minimum clearance

shaft tolerance

Minimum Maximum

metal metal Hole

Shaft

36.1.2 Transitional Fits

•This type of fit may result in interference, or clearance

•This type of fit can be used for items such as snap fits

•The figure below illustrates this condition for a hole shaft pair