Detailed Classification Of Fits Used In Engineering Metrology

INTRODUCTION

In the early days, majority of the components were actually matted together, their dimensions being adjusted until the required type of fit was obtained. But with the passage of time, engineers and workers realized that the variations in the sizes of the parts had always present and that such variations could be restricted but not avoided. It has also been realized that exact size components are difficult to produce. Any attempt towards very closed dimensions of a product will increase cost of the production. The functional aspects of the component may be achieved even without going for its exact dimensions using limits, fit and tolerances. This reduces the unit cost of production and increases the rate of production.

For example, a shaft of exact 10.00 mm diameter is difficult to produce by machining process. But if you provide tolerence, i.e. the amount of variation permitted in the size, then such parts can be easily produced. A dimension 10 plus/minus  0.05 means a shaft may be produced between 10.05 and 9.95. These two figures represent limit and the difference, (10.05 – 9.95) = 0.10 is called tolerance.

FITS AND THEIR CLASSIFICATIONS

When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit. A fit may be defined as the degree of tightness and looseness between two mating parts.
Fits depend upon the actual limits of the hole and or shaft and can be divided into three general classes :
(i) Clearance Fit.
(ii) Interference Fit.
(iii) Transition Fit.

Clearance Fit

In clearance fit, an air space or clearance exists between the shaft and hole as shown in Figure .Such fits give loose joint. A clearance fit has positive allowance, i.e. there is minimum positive clearance between high limit of the shaft and low limit of the hole.

Clearance fit can be sub-classified as follows :

Loose Fit
It is used between those mating parts where no precision is required. It provides minimum allowance and is used on loose pulleys, agricultural machinery etc.
Running Fit
For a running fit, the dimension of shaft should be smaller enough to maintain a film of oil for lubrication. It is used in bearing pair etc. An allowance 0.025 mm per 25 mm of diameter of boaring may be used.
Slide Fit or Medium Fit
It is used on those mating parts where great precision is required. It provides medium allowance and is used in tool slides, slide valve, automobile parts, etc.

Interference Fit

A negative difference between diameter of the hole and the shaft is called interference. In such cases, the diameter of the shaft is always larger than the hole diameter. In Figure Interference fit has a negative allowance, i.e. interference exists between the high limit of hole and low limit of the shaft.
In such a fit, the tolerance zone of the hole is always below that of the shaft. The shaft is assembled by pressure or heat expansion.

Shrink Fit or Heavy Force Fit

The interference fit can be sub-classified as follows :

It refers to maximum negative allowance. In assembly of the hole and the shaft, the hole is expanded by heating and then rapidly cooled in its position. It is used in fitting of rims etc.
Medium Force Fit
These fits have medium negative allowance. Considerable pressure is required to assemble the hole and the shaft. It is used in car wheels, armature of dynamos etc.
Tight Fit or Press Fit
One part can be assembled into the other with a hand hammer or by light pressure. A slight negative allowance exists between two mating parts (more than wringing fit). It gives a semi-permanent fit and is used on a keyed pulley and shaft, rocker arm, etc.
Transition Fit
It may result in either clearance fit or interference fit depending on the actual value of the individual tolerances of the mating components. Transition fits are a compromise between clearance and interference fits. They are used for applications where accurate location is important but either a small amount of clearance or interference is permissible. As shown in Figure , there is overlapping of tolerance zones of the hole and shaft.
Transition fit can be sub-classified as follows :
Push Fit
It refers to zero allowance and a light pressure (10 cating dowels, pins, etc.) is required in assembling the hole and the shaft. The moving parts show least vibration with this type of fit. It is also known as snug fit.
Force Fit or Shrink Fit
A force fit is used when the two mating parts are to be rigidly fixed so that one cannot move without the other. It either requires high pressure to force the shaft into the hole or the hole to be expanded by heating. It is used in railway wheels, etc.
Wringing Fit
A slight negative allowance exists between two mating parts in wringing fit. It requires pressure to force the shaft into the hole and gives a light assembly. It is used in fixing keys, pins, etc.