In this article, we are going to discuss different types of fits in engineering. After reading this article, you will know, what is fit, the fit types used in mechanical engineering, the clearance fit examples, transition fit examples, interference fit examples, the basic system of fit, and what is allowance.
Table of Contents
What is Fit?
When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit.
In engineering terms, the fit is referred to as the tightness or looseness between the mating parts of an assembly. Generally, the mating parts are shafts and holes.
An engineer must have knowledge about the fits. It is very important to know because in real-life applications fits play an important role during their motion. The fit between two mating components decides the tightness or looseness in the assembly.
Examples of such fits can be given as, piston and cylinder, assembly of shaft and bearing, nut and bolts, splines shaft and spline disc, etc.
Types of Fits
Depending upon the limits of the shaft and the hole, fits are broadly classified into three groups. The types of fits are
- Clearance fit
- Interference fit
- Transition fit
In this article, we have discussed various types of fits in engineering with reference to the hole basis system. The reason for using a hole basis system is discussed further in this article.
Clearance Fit
A clearance fit is a fit, which always provides a positive clearance between the hole and the shaft over the entire range of tolerances. In this case, the tolerance zone of the hole is entirely above that of the shaft.
In other words, clearance fit occurs when two mating parts always leave a clearance when assembled. There is always a gap or clearance between the mating component.
The mating components can have sliding movement, rolling movement, or rotating movement as per their application.
The difference between the maximum size of the hole and the minimum size of the shaft is known as maximum clearance.
Maximum clearance = HLH – LLS
The difference between the minimum size of the hole and the maximum size of the shaft is known as minimum clearance.
Minimum clearance = LLH – HLS
HLH = Higher limit of hole
LLS = Lower limit of shaft
LLH = Lower limit of hole
HLS = Higher limit of shaft
Types of Clearance Fit
Clearance fit is further divided into 5 types
- Slide fit
- Easy slide fit
- Loose running fit
- Free running fit
- Close running fit
Slide fit is used where high accuracy and small clearance is needed. Hence, in a sliding fit, the mating parts slide freely. The tailstock of a lathe machine is an example of a slide fit.
A loose running fit is used where accuracy is not important. The loose running fit has the highest clearance among all types of clearance fit.
Free running fit is used in applications where accuracy is having less importance, but the movement of the parts is required with little consideration.
Close running fit is used where small clearance with some accuracy is required.
Example of Clearance Fit
The common examples of clearance fit are
- Piston and cylinder
- Valves
- Nut and bolts
- Door hinge assembly
- Tool slide
- Tailstock slide of the lathe
Interference Fit
An interference fit is a fit, which always provides a positive interference over the whole range of tolerances. In this case, the tolerance zone of the hole is completely below that of the shaft.
In an interference fit, there is no gap between the mating parts. There is an intersection of materials of the mating parts and interference is of the tight fit. Hence tools are required to precisely fit them into an assembly.
When mating parts are assembled using an interference fit, the assembly will be almost permanent. If one wants to remove these assembled parts, cutting those parts is the only option. Hence, it cannot be used where continuous removal of parts is required.
The difference between the maximum size of the shaft and the minimum size of the hole is known as maximum interference.
Maximum interference = HLS – LLH
The difference between the minimum size of the shaft and the maximum size of the hole is known as minimum interference.
Minimum interference = LLS – HLH
Types of Interference Fit
Types of interference fit are
- Force fit
- Tight fit
- Shrink fit
In force fit, one part is forcefully pressed into another part to form a single unit. A considerable amount of force is required to fit the parts.
The tight fit provides less interference than that of force fit. A considerable amount of force but less than force-fit is needed for the assembly of parts. The fitting of gear on a shaft is a well-known example of a tight fit.
In shrink fit, one of the mating parts is heated and then assembled in another part to form a unit. Permanent mounting of gears on a shaft, mounting of train wheels on a shaft are some examples of a shrink fit.
Examples of Interference Fit
The common examples of interference fit are
- Bearing Bush assembly
- Dowel pin in casting assembly
- Permanent mounting of the wheel on a shaft (Railway wheel).
- Permanent mounting of gear on a shaft.
- Pulley mounting on a shaft
Transition Fit
A transition fit is a fit, which may provide either a clearance or interference, depending upon the actual values of the individual tolerances of the mating components. In this case, the tolerances zones of the hole and the shaft overlap.
Transition fit lies in the region between clearance fit and interference fit and is neither tight nor loose like those two fits. The tolerance zones of the shaft and the hole will be overlapped between clearance and interference fit.
This type of fit is used in applications where a higher accuracy between mating parts is required. It is also known by another name, slip fit or push-fit. It has high clearance compared to interference type fit but it does not provide safety of movement of the joint.
Types of Transition Fit
Types of transition fit are
- Push fit
- Wringing fit
As the name implies, push fit is used where a negligible amount of force is required to fit the mating parts.
Wringing fit is used where the parts need to be replaced without any difficulty. A common example of wringing fit is an assembly of pulleys on a drive shaft and a driven shaft.
Examples of Transition Fit
The common examples of transition fit are
- Shaft key
- Coupling ring and recess
- Bearing assembly
- Gear – shaft assembly
- Pulley assembly
- Shaft – armature assembly
- Plain or taper dowel in a machine assembly
- Gear slip bushing
Basic Systems of Fit
There are two basic systems for giving tolerances to the shaft and the hole namely, the hole-basis system and the shaft-basis system.
Hole basis system
In the hole basis system, the different clearances and interferences are obtained by associating various shafts with a single hole, whose lower deviation is zero. The system is shown in the figure below.
In this case, the size of the hole is basic size, and the clearance or interference is applied to the shaft dimension. The system is denoted by the symbol ‘H‘.
This system has an advantage over the shaft basis system because holes are drilled by standard drills or reamers having fixed dimensions, while the shafts can be turned or ground to any given dimension. Due to this reason, the hole-basis system is widely used.
Shaft basis system
In the shaft basis system, the different clearances or interferences are obtained by associating various holes with a single shaft, whose upper deviation is zero. The system is shown in the figure below.
In this system, the size of the shaft is the basic size, while the clearance or interference is applied to the dimension of the hole. The system is denoted by ‘h‘.
The shaft basis system is popular in the industries using semi-finished or finished shafting, such as bright bars as raw material.
What is Allowance?
Very often the term ‘allowance’ is used along with fits and tolerances. An allowance is an intentional difference between the maximum material limits of mating parts.
Allowance = LLH – HLS
where, LLH stands for the lower limit of the hole, and HLS stands for the higher limit of the shaft.
It is minimum clearance (positive allowance) in case of clearance fits and maximum interference (negative allowance) in case of interference fit between mating parts.
FAQ
Q. Why hole basis system is widely used?
Answer- In the hole basis system, the size of the hole is basic size, and the clearance or interference is applied to the shaft dimension. The holes are drilled by standard drills or reamers having fixed dimensions, while the shafts can be turned or ground to any given dimension. Machining of holes along with drilling or reaming operations increases the cost of machining.
Due to this reason, the hole-basis system is widely used.
Q. What is fit?
Answer- When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit.
Q. Give classification of fits.
Anwer- Fits are classified into three groups clearance fit, transition fit, and interference fit.
Clearance fits are further classified into Slide fit, easy slide fit, loose running fit, close running fit, and free running fit.
Transition fits are classified into push-fit and wringing fit.
Interference fits are classified into force fit, tight fit, and shrink fit.
Reference:
Design of Machine Elements – V.B. Bhandari
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