Limits and fits

To ensure that an assembly will function correctly, its component parts must fit together in a predictable manner. Now, in practice, no component can be manufactured to an exact size, and one of the problems facing the designer is to decide the upper and lower limits of size which are acceptable for each of the dimensions used to define shape and form and which will ensure satisfactory operation in service. For example, a dimension of 10 ± 0.02 means that a part will be acceptable if manufactured anywhere between the limits of size of 9.98 mm and 10.02 mm. The present system of manufacture of interchangeable parts was brought about by the advent of and the needs of mass production, and has the following advantages.

1 Instead of 'fitting' components together, which requires some adjustment of size and a high degree of skill, they can be 'assembled'.

2 An assembly can be serviced by replacing defective parts with components manufactured to within the same range of dimensions.

3 Parts can be produced in large quantities, in some cases with less demand on the skill of the operator. Invariably this requires the use of special-purpose machines, tools, jigs, fixtures, and gauges: but the final cost of each component will be far less than if made separately by a skilled craftsman.

It should be noted, however, that full interchangeability is not always necessary in practice; neither is it always feasible, especially when the dimensions are required to be controlled very closely in size. Many units used in the construction of motor vehicles are assembled after an elaborate inspection process has sorted the components into different groups according to size. Suppose, for example, that it was required to maintain the clearance between a piston and a cylinder to within 0.012 mm. To maintain full interchangeability would require both the piston and the cylinder bores to be finished to a tolerance of 0.006 mm, which would be difficult to maintain and also uneconomic to produce. In practice it is possible to manufacture both bores and pistons to within a tolerance of 0.06 mm and then divide them into groups for assembly; this involves the gauging of each component.

A designer should ensure that the drawing conveys clear instructions regarding the upper and lower limits of size for each dimension, and Figs 19.1 to 19.4 show typical methods in common use.

The method shown in Fig. 19.1 is perhaps the clearest

60.05 60.00

way of expressing limits of size on a drawing, since the upper and lower limits are quoted, and the machine operator is not involved in mental arithmetic. The dimensions are quoted in logical form, with the upper limit above the lower limit and both to the same number of decimal places.

As an alternative to the method above, the basic size may be quoted and the tolerance limits added as in Fig. 19.2. It is not necessary to express the nominal dimension to the same number of decimal places as the limits.

Fits can be taken directly from those tabulated in BS 4500, 'ISO limits and fits', and, in order to indicate the grade of fit, the following alternative methods of dimensioning a hole may be used:

90 H7

90.035 90.000

(first choice)

90 H7 or 90 H7

Similarly, a shaft may be dimensioned as follows:

90 g6

89.966

(first choice)

or 90 g6 or 90g6

In cases where a large amount of repetition is involved, information can be given in tabulated form, and a typical component drawing is shown in Fig. 19.3.

Dia.

Size

A

40,05 40,00

B

50,02 49,97

C

45,03 44,98

D

In many cases, tolerances need be only of a general nature, and cover a wide range of dimensions. A box with a standard note is added to the drawing, and the typical examples in Fig. 19.4 are self explanatory.

General casting tolerance ± 0.3

General casting tolerance ± 0.3

Engineering fits between two mating parts can be divided into three types:

1 a clearance fit (Fig. 19.5), in which the shaft is always smaller than the hole into which it fits;

2 an interference fit (Fig. 19.6), in which the shaft is always bigger than the hole into which it fits;

3 a transition fit (Fig. 19.7), in which the shaft may be either bigger or smaller than the hole into which it fits—it will therefore be possible to get interference or clearance fits in one group of assemblies.

It will be appreciated that, as the degree of accuracy required for each dimension increases, the cost of production to maintain this accuracy increases at a sharper rate.

Figure 19.8 shows the approximate relationship between cost and tolerance. For all applications, the manufacturing tolerance should be the largest possible which permits satisfactory operation.

Minimum clearance

Minimum clearance

Maximum clearance

Fig. 19.5 Clearance fits—allowance always positive

Maximum clearance

Fig. 19.5 Clearance fits—allowance always positive

Minimum interference

Fig. 19.6 Interference fits—allowance always negative

Maximum interference

Minimum interference

Fig. 19.6 Interference fits—allowance always negative

Maximum clearance

Maximum clearance

Interference Fit

Maximum interference

Fig. 19.7 Transition fit—allowance may be positive or negative

Maximum interference

Fig. 19.7 Transition fit—allowance may be positive or negative

Tolerance (mm)

Fig. 19.8 Approximate relationship between production cost and manufacturing tolerance

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Responses

  • EEMIL
    What is limits in engineering?
    3 years ago
  • azzurra
    How to calculate limits in engineering drawing?
    3 years ago
  • gabriel
    What is limit in engineering drawing?
    3 years ago
  • Orazio
    What is limit and fits?
    2 years ago
  • gloriana
    What is the upper limit and the lower limits of the. drawing?
    2 years ago
  • aatifa
    What is limit of size in assembly drawing?
    2 years ago
  • MELBA
    How are tolerances on engineering drawing determined?
    1 year ago
  • Asmait Kiros
    How are tolerances shown on an engineering drawing?
    1 year ago
  • demsas
    How many types of limits in engg drawing?
    10 months ago
  • Saimi
    How to mention interference fit in an assembly drawing?
    10 months ago
  • MARLENE
    How limits and fits are applied?
    6 months ago
  • felipe
    Why are limits and fits generally used in engineering drawings?
    6 months ago
  • tauno
    How should a limit or fit be indictaed on an engineering drawing?
    5 months ago
  • Katharina
    How would a limit and fit be shown on an engineering drawing?
    4 months ago
  • Sandra
    How a limit or fit should be indicated on engineering drawings?
    4 months ago

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