Fits:- It is defined as the degree of tightness or looseness between two mating parts. The fits are further divided into three types depending upon the degree of tightness or looseness

**1. Clearance fit

1. Transition fit

2. Interference fit**

I. *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 :

a. 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 machines etc.

b. 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 boring may be used.

c. 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

II. 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.

The interference fit can be sub-classified as follows

a. Shrink Fit or Heavy Force Fit:- 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.

b. 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.

c. 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

III. 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 :

a. 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.

b. 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.

c. 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.

TAYLOR’S PRINCIPLE A GO Gauge will check all the dimensions of the work piece in what is called the maximum metal condition (indicating the presence of the greatest amount of material permitted at a prescribed surface)

That NOT GO Gauges shall check only one dimension of the work piece at a time, for the minimum metal conditions (indicating the presence of the least amount of material permitted at a prescribed surface) size. In case of hole, the maximum metal condition obtains when the hole is machined to the low limit of size, & minimum metal condition results when the hole is made to the high limit of size. In case of shaft the limits taken would be inverse of hole