Short notes on Fits
The fir between two mating parts is the relationship between them with respect to the amount of play or interference which is present when they are assembled together. Accordingly, the fit may result either in movable joint or a permanent joint. For example, a shaft running in a bush can move in relation to it and so forms a moving joint, whereas a pulley mounted on the shaft forms a fixed joint. The nature of joint or "fit" is characterized by the presence and size of 'clearance' (for movable joints) or 'interference' (for fixed joints).
There are three basic type of fits:
- Clearance fit
- Interference, Press or Force fit
- Transition or Sliding fit
(1) Clearance fit:
In clearance or running fit, the shaft is always smaller than the hole. A positive allowance exists between the largest possible shaft and the smallest possible hole, i.e. when the shaft and hole are at their maximum metal conditions. The tolerance zone of the hole is entirely above that of the shaft.
- Minimum clearance: It is the difference between the maximum size of the shaft and minimum size of the hole.
- Maximum clearance: It is the difference between the minimum size of shaft and maximum size of the hole.
(2) Interference, Press of Force fit:
In this type of fit, the shaft is always larger than the hole. The tolerance zone of the shaft is entirely above that of the hole.
- Minimum tolerance: It is the difference between the maximum size of the hole and the minimum size of shaft prior to assembly.
- Maximum tolerance: It is the difference between the minimum size of hole and the maximum size of shaft prior to assembly.
(3) Transition or Sliding fit:
It occurs when the resulting fit due to the variations in size of male and female components due to their tolerance, varies between clearance and interference fits. The tolerance zones of shaft and hole overlap.
Hole basis system: In this system, the hole is kept constant and the shaft diameter is varied to give the types of fits. The basic size of the hole is taken as the low limit of size of the hole, i.e. the maximum metal condition (MMC) of the hole. The high limit size of the hole and the two limits of size for the shaft are then selected to give the desired fit. It is clear, that in this system the actual size of a hole that is within the tolerance limits is always more than the basic size; it can be equal to the basic size as a particular case but can never be less. In the 'Basic Hole System', the holes get the letter 'H' and shafts get different letters to decide the position of tolerance zone to obtain a desired fit.
Shaft basis system: Here, the shaft is kept constant and the size of the hole is varied to give the various fits. The basic size of the shaft is taken as one of the limits of size (maximum limits) for the shaft, i.e. its maximum metal condition (MMC). The other shaft limit of size for the hole are then selected to give the desired fit. It is clear that in this system, the actual size of a shaft that is within the tolerance limits is always less than the basic size. As a particular case, it can be equal to the basic size but can never be larger. In the 'Basic Shaft System', the shaft gets the letter 'h' and holes get different letters to decide the position of tolerance zone to obtain a desired fit.
Hole: The upper limit refers to the greatest diameter at any point in hole, the lower limit refers to the diameter of the inscribed circle or cylinder which will just pass through the hole.
Shaft: The upper limit refers to the escribed ring which will just pass over the shaft, the lower limit refers to the minimum diameter at any point on the shaft.
Normally limits refer to the extreme dimensions which may occur on a component, a more recent outlook on the matter is that the limits refer to the dimensions inside which all but a small percentage of parts must lie.