In this article, you’ll learn what is Fit? And Types of Fits, System, & Reasons for using fits in metrology. And also download the PDF file of this article at the end.
What is Fit?
You know, every machine has a number of parts. And these parts are manufactured separately and then assembled together. There are some parts which move by sliding into other parts and there are some parts which fit tightly into other part and move them.
For the smooth functioning of these parts, it is essential that there should be adequate coordination between these parts.
Then this mutual connection between two joining parts is called fit. Based on the actual limits of the hole or shaft, the fit may be classified into the following types.
Types of Fits
Types of fits are:
- Clearance fit
- Transition fit
- Interference fit
1. Clearance Fit
In this fit, the size of the shaft or male part is kept comparatively slightly less than the size of the hole or female part so that the male part can move in the female part without any obstruction. Running and sliding fits come under the clearance fit.
1.1 Sliding Fit
Sliding means “to slip”. In this fit, the male part moves in the female part in a sliding manner. Very little allowance is kept for this fit. More variation is given to it as compared to Running fit.
1.2 Running Fit
In these types of fits, there is sufficient allowance between two parts such as shaft and hole that male part can revolve in the female part with ease. In this fit, the thin layer of lubrication given between the two parts is also taken into consideration.
This fit is used in the shaft, bush, or bearing. The allowance in this fit depends on the part’s diameter, hole, length, and surface. This fit is of three types,
- Slack running fit
- Normal running fit
- Close running fit
In the above fits slack running fit and the normal running fit are used for ordinary jobs. A close running fit is used for every accurate job.
2. Transition Fit
In this type of fit, the size of the male part or shaft is kept comparatively a bit thicker than a female part of the hole. But the parts of this type of fit can be fitted with some pressure. Push fits and ringing fits come under these types.
2.1 Push Fit
In this fit, allowance is kept less than that of running fit. One part is kept stationary while the other part is pushed with a hand or by using a wooden piece and striking it slightly to fit it such as a dowel pin, ring gauge, etc.
2.2 Ringing Fit
Ringing means to have friction. In this type of fit, female and male parts are found stuck together. If these parts are to be separated, it can be done only by using pressure. This fit is stationary to some extent.
3. Interference Fit
This fit is quite reverse to the clearance fit. In this type of fit, the size of the male part or shaft is bigger than the size of the hole or the female part. In order to join two parts or to fit them together a great amount of force is used.
For this purpose press machines or hydraulic machines are also used. Force fit and driving fit are included under this fit.
3.1 Force Fit
Normally this type of fit is used for round jobs. For example, to fit a shaft in a hole permanently or temporarily. In this fit male part is made slightly bigger or equal in diameter.
It is fitted by putting heavy pressure. Machine press or hydraulic press is used for this purpose such as cycle-hub or pulley of a motor.
3.2 Shrinkage Fit
It is a type of fit, the female part is kept slightly smaller than the male part. In other words, there is a negative allowance in this fit. In order to fit the male part into the female part, the male part is heated in order to fit it in the female part, by which its volume increases.
The male part is fitted when the female part is in hot condition. In this way when the female part becomes cool and returns to its normal temperature, it grips the male part permanently.
This type of fitting is done to mount an iron ring on the wheels with wooden spokes. Besides this, the male part can also be fitted by making the male part chilled cold. For example, cylinder liners of motor engines. This fit is called expansion fit.
3.3 Driving Fit
In these types of fits, a slight difference is kept between male and female parts. there are fitted by striking them with a hammer etc. It is used in dynamo pulley etc. We can divide this fit into three parts, Light driving fit, Medium driving fit, and heavy driving fit.
System of Fits
For obtaining various types of fits, the amount of maximum and minimum clearance either positive or negative must exist between the mating parts.
In practice, while providing the tolerance between two mating parts; from the production and economic point of view, one of the mating parts limit dimensions is fixed and while that of another limit dimension is varied, during so various types of fits are obtained based on this system of fits, are classified broadly into,
- Hole basis system
- Shaft basis system
1. Hole Basis System
In a hole base system, the hole limit dimensions are assumed to be constant and a variety of fits are obtained by changing the limit dimensions of the shaft. The following figure shows the clearance and interference fits in the hole basis system.
2. Shaft Basis System
In the shaft base system, the shaft limit dimensions are held constant and different types of fits are achieved by changing the limit dimensions of the holes. The figure below shows the clearance and interference fits in the shaft basis system.
Reasons For Adopting Hole Basis System
Suppose a shaft basis system is used to specify the unit dimensions, to obtain various types of fits, a number of holes of different sizes are required, which in turn requires tools of different types of sizes.
If the hole basis system is applied, there will be a decrease in the production costs as only one tool is needed to design the hole, and also the shaft can also be easily machined to any desired size. Hence, the whole basis system is preferred when compared to the shaft basis system.
By using the fits in metrology you can specify the size range of any parts. So now, we hope that we have clear all your doubts about Fits. If you have still any doubts about the “Types of Fits” you can contact us or ask in the comments.
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