- Practice · 8 marks
Basic terms of limits, fits and tolerances
Short answer
Basic size is the nominal dimension. Limits are max/min permissible sizes; tolerance is their difference. Upper/lower deviation is the algebraic difference from basic size. Clearance: hole larger than shaft; interference: shaft larger than hole. Allowance is the minimum clearance or maximum interference. Fundamental deviation (letter) positions the tolerance zone; IT grade gives its magnitude.
Define the following terms with a neat sketch: basic size, actual size, limits of size, deviation (upper and lower), fundamental deviation, tolerance, allowance, hole and shaft, zero line, and clearance and interference.
+ ------------------ upper limit
| tolerance zone | (upper deviation)
------------------ lower limit
0 ============================ zero line
(basic size) (lower deviation
- if below the line)
Deviations are measured from the zero line: positive above, negative below.
| Term | Meaning |
|---|---|
| Basic (nominal) size | The size from which limits are derived; the theoretical size on the drawing, e.g. 40 mm |
| Actual size | The size found by measurement of the finished part |
| Limits of size | The two extreme permissible sizes: maximum limit and minimum limit |
| Deviation | Algebraic difference between a size and the basic size: upper deviation = max. limit - basic size (ES for hole, es for shaft), lower deviation = min. limit - basic size (EI, ei) |
| Fundamental deviation | The deviation (upper or lower) nearest to the zero line; it fixes the position of the tolerance zone, given by a letter (A to ZC for holes, a to zc for shafts) |
| Tolerance | Difference between the maximum and minimum limits = upper deviation - lower deviation; the permissible variation in size. Size of zone is given by IT grade (IT01 to IT16) |
| Allowance | The intentional difference between the dimensions of the mating parts; the minimum clearance (positive) or the maximum interference (negative). It is the difference between the maximum material limits of hole and shaft |
| Hole / shaft | The internal and external features of a part; a hole is not necessarily round (e.g. a keyway width), a shaft includes any external feature |
| Zero (reference) line | The line representing the basic size; deviations are positive above and negative below |
| Clearance | The positive difference between the hole size and the shaft size (hole larger than shaft) |
| Interference | The negative difference (shaft larger than hole before assembly) |
Tolerance zone: the zone between the upper and lower limits, defined by the grade (size) and the fundamental deviation (position), as in .
✓ Answer checked by Claude Opus 5.5 (high effort)
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