LIMIT, TOLERANCE AND ALLOWANCE

 

LIMIT, TOLERANCE AND

ALLOWANCE

Limit and tolerance permit

interchangeability of parts. These give

the maximum and minimum sizes

acceptable for a certain nominal size.

Allowance gives an extra dimension.

Nominal size

It is the normal size of a part or a component say 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. 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 either positive or negative limit exists
2. Bilateral deviation or Bilateral limit
When both positive and negative limits exist

Tolerance

It is the difference between the maximum and minimum actual sizes. This is the 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 series of processes to convert raw components into finished products. It uses National or International standards.

INTERCHANGEABILITY IN MANUFACTURE

 Use standard specifications for the parts manufacture.  Use these for each and every component of a particular size. This makes repair, replacement convenient and cheap. Limits, tolerances and allowances help to achieve standard specifications. Interchangeable parts are 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

 

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