LIMIT, TOLERANCE AND ALLOWANCE

LIMIT, TOLERANCE AND ALLOWANCE

Nominal size
It is the normal size of a part or a component. For example 10 mm diameter shaft
Deviation (limit)
The difference between actual size and the nominal size.
Positive deviation (upper limit)
It is the difference between the maximum permissible actual size and the nominal size. For example the size is 30.0+0.20 and 30.0–0.10. Positive deviation (upper limit) is 0.20.
Negative deviation (lower limit)
It is the difference between the minimum permissible actual size and the nominal size. The negative deviation (lower limit) is 0.10.
Types of deviations (limits)
1. Unilateral deviation or Unilateral limit
When only positive or negative deviation is allowed on the nominal size
2. Bilateral deviation or Bilateral limit
When both positive and negative deviations are allowed on the nominal size
Tolerance
It is the difference between the maximum and minimum actual sizes (Difference between upper and lower limits) of a SINGLE mating part. It is either for a shaft or for a hole. Tolerance will be 0.20 – (-0.10) = 0.30
Allowance
It is difference between the minimum hole size and maximum shaft size within the given deviations. It is for the TWO MATING PARTS taken together. Suppose shaft sizes are 30.0 + 0.20 and 30.0 –0.10 and hole sizes are 31.0 +0.10 and 31.0 –0.30. The allowance will be 30.70 –30.20 = 0.50
Now depending on the limits or deviations, there will be different types of fits (fittings) between the mating parts.

MANUFACTURING

It is a process (or series of processes) to convert raw materials or raw components into finished products as per National or International standards.
INTERCHANGEABILITY IN MANUFACTURE
Parts are made of standard specifications such that these can be used in each and every application of a particular size for making repair and replacement convenient and cheap. This is achieved by use of limits, tolerances and allowances. Interchangeable parts can be shafts, rims, tires, bottle caps, bolts and nuts etc..
Advantages of Interchangeability of Parts
(i) Makes repair easy.
(ii) Makes replacement easy.
(iii) Reduces the time of repair.
(iv) Reduces the cost of repair or replacement.
(v) Reduces the cost of manufacture by having mass production