Page 228 - Mechanical Engineer's Data Handbook

216 MECHANICAL ENGINEER’S DATA HANDBOOK
Allowable stress for welded structural steels \

Grade 43 50 55

Stress (N mm-’) 115 160 195

5.13 Limits and fits

It is impossible to make components the exact size and makes interchangeability possible.
an allowance or ‘tolerance’ must be made which British Standard BS 4500: Part 1 : 1969, ‘IS0 Limits
depends on the process and the application. The and Fits’, gives a comprehensive system relating to
tolerance results in two extremes of size which must be holes and shafts; it can, however, be used for other
maintained. The tolerances of two fitting parts, e.g. a components, e.g. a key in a keyway.
shaft in a bearing, determines the type of ‘fit’ and

5.13. I Terminology Upper deviation
Lower deviation
Minimum limit Basic size size E-+ Tolerance
of
Taking the example of holes and shafts, there is a ‘basic Minimum limit of size
size’ and then maximum and minimum sizes for each,
their differences being the tolerances. Their differences
from the basic size are called the ‘maximum and
minimum deviations’. Maximum limit 01 size nca
r deviation
Maximum limit of size r deviation
Basic size

Maximum Maximum
Minimum
mum
rance

Clearance fit Transition lit Interference fit