It may be defined as the allowable variation of a dimension or other measured or controlled criterion from the specified value.
Tolerances must be allowed because of our fallibility and that of our machines which preclude ideal attainment in manufacture.
In order to effect economic production and proper assembly of components, a restricted variation from the designed size must be permitted.
Because of its inevitability, tolerance contains an engineering legality of deviation from the ideal value, and like any other legal matter, formulation of tolerance.
Design of tolerances requires careful study and planning, since the designer wants them to serve his purpose in the best and fairest possible manner.
To illustrate this, let us say it is our intention to cut off a small shaft to a diameter of 50 mm. (basic or nominal diameter).
Be that as it may, the owner of the shaft will feel satisfied if the diameter of the finished shaft lies anywhere between to 50 + 0.06 = 50.06 mm (the upper limit) and 50 − 0.06 = 49.94 mm (the lower limit).
The difference between the upper and lower limit is called tolerance.
Thus: Tolerance = 50.06 – 49.94 = 0.12mm, which, as a matter of fact, is always a positive quantitative number.
Types of Tolerances
Following are the three types of tolerances used in measurements:
- Unilateral tolerances
- Bilateral tolerances
- Compound tolerances.
Unilateral Tolerances
If the two limit dimensions are only above the nominal size as in the figure or only below the nominal size, the tolerance is said to be unilateral.
Bilateral Tolerances
If the two-limit dimension is above and below the nominal size, the tolerances are said to be bilateral.
Compound Tolerances
Tolerance which is determined by the established tolerances, i.e., the union of more than one kind of tolerance is called compound tolerance. The different types of tolerance may be angular, lateral, etc.
For example: In the figure, the tolerances on length/are dependent upon those of L, h and θ. This compound tolerance on ‘l’ is the resulting effect of these three tolerances. Minimum tolerance on ‘l’ will be corresponding to L-b, θ+∝ and h+c.
Schematic Representation of Tolerances
Different Between Unilateral and Bilateral Tolerance
Unilateral Tolerance
- Variation is made in only one direction from the nominal or basic dimension.
- The tolerances for the same allowances or type of fit can be changed without altering the basic or nominal size of the shaft or hole.
- These are suitable for any type of production.
- GO gauges for holes and NOGO gauges for shafts can be standardized.
- Ex: Ø50
Bilateral Tolerance
- Variation is made in both directions from the nominal or basic dimension.
- The tolerances for the same allowance or type of fit are changed by altering the nominal size of the shaft or hole.
- These are suitable for mass production.
- It is impossible to provide GO-NOGO gauges that can be standardized.
- Ex: Ø50