Page 234 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

Hole basis system: In a standard system of limits and fits, where the size of the hole is kept constant and the
size of the shaft is varied to get the different classes of fits, it is known as the hole basis system.
The fundamental deviation symbol `H' is chosen for the holes, when the hole basis system is followed. This is
because the lower deviation of the `H' hole is zero. It is known as the `basic hole'. (Fig 10)
Shaft basis system: In a standard system of limits and fits, where the size of the shaft is kept constant and the
variations are given to the hole for obtaining different classes of fits, then it is known as shaft basis system. The
fundamental deviation symbol `h' is chosen for the shaft when the shaft basis is followed. This is because the
upper deviation of the `h' shaft is zero. It is known as the `basic shaft'. (Fig 11)

Fig 10 Fig 11

The hole basis system is followed mostly. This is because, depending upon the class of fit, it will be always easier
to alter the size of the shaft as it is external, but it is difficult to do minor alternations to a hole. Moreover the hole
can be produced by using standard toolings.

The three classes of fits, both under the hole basis and the shaft basis are illustrated in figure 12.

Fig 12

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CITS :Engineering Drawing (Mechanical) - Exercise 9 CITS :Engineering Drawing (Mechanical) - Exercise 9