What is a Fit?
Before discussing about the types of fits let us first understand what is a fit. Fits refer to the assembly conditions between a Hole and Shaft, determining the level of tightness or looseness in their connection. It represents the mating relationship between the components. Engineering fits are geometric dimensions used when designing parts and other components. They define the clearance between mating parts based on size requirements.
In engineering, fits are described as the “shaft and hollow” relationship, indicating the level of tightness between mating parts. During machine assembly, proper fitting between two parts is necessary for successful assembly. Running fit allows the shaft to rotate easily within the hole, while sliding fit enables the shaft to slide within the hole. The design is determined accordingly.
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Minimum and Maximum Limit of Hole and Shaft
- Holes and shafts have minimum and maximum limits.
- The minimum limit refers to the smallest diameter of the shaft. The maximum limit represents the largest permitted diameter or size of the shaft.
- The dimensions of the hole and shaft can vary within the minimum and maximum limits.
- Removing material from a hole increases its size and corresponds to the maximum limit, indicating maximum material removal.
- The minimum limit for a hole indicates the maximum amount of material present.
- Holes and shafts have opposite relationships between material presence and size.
- In the case of a shaft, the size is minimum when the material is minimum, while in a hole, the minimum limit corresponds to maximum material presence. Removing material from the hole reaches the maximum limit.
Types of Fit
There are three types of fit which are as follows:
- Clearance Fit
- Interference Fit
- Transition Fit
Clearance Fit
Clearance fit involves a hole and a shaft with upper and lower limits, both falling within the tolerance zone. There is a big gap between the tolerance zone of the hole and shaft. When the hole is larger than the shaft, allowing rotation or sliding of the mating parts, it is known as a clearance fit.
In a clearance fit, the minimum hole size is always greater than the maximum shaft size. When assembling the shaft and hole, clearance is achieved, enabling easy sliding and rotation of the shaft inside the hole. Clearance fits allow for running fits and sliding fits, such as in the case of a piston and valve.
What is the Tolerance zone?
To ensure proper fit and interchangeability between parts, tolerance zones are employed. Let’s consider the example of a 10 mm nut and a 10 mm bolt. However, due to human and machining error, the nut’s inner diameter measures 9.98 mm. This deviation prevents the nut from fitting onto the bolt, potentially compromising the joint.
Tolerance zones establish a range where slight variations in the tolerances of nuts and bolts are allowed. This range enables achieving a fit between the parts and maintains interchangeability.
Types of Clearance Fit
There are five types of clearance fit which are as follows:
- Slide fit
- Running Fit
- Loose Running Fits
- Easy Slide and
- Location clearance fit
Slide Fit
In applications where very minimal clearance is required, precision and accuracy are paramount for the smooth movement of parts.
Examples of such cases include:
- Sliding gears
- Automobile assemblies
- Slide valves
- Clutch discs
- Parts of machine tools
- Tailstock spindle of a lathe machine
- Guiding of shafts
In these scenarios, the almost zero clearance ensures precise and accurate functioning of the components involved.
Running Fit
In situations where moderate-speed rotation of components is required and high precision is not necessary, a running fit is employed.
Examples of components that commonly use running fits include
- Gears
- Couplings
- Other Similar mechanisms
A running fit is characterized by a relatively large clearance between mating parts. This type of fit is suitable for applications involving large temperature variations, high running speeds, and heavy journal pressures.
Loose running Fits
Loose running fits are used when high-speed rotations of parts are involved, prioritizing speed over accuracy.
These running fits feature larger clearances, accommodating factors like heat, corrosion, contamination, and other environmental considerations.
Examples of components that typically employ loose running fits include
- Latches
- Pivots
- Parts that are susceptible to heat, corrosion, and contamination.
Easy slide
Easy slide fits are utilized when there is a small clearance between the hole and shaft, allowing for smooth and effortless sliding motion. Easy slide fits are suitable for applications involving non-regular motions or slow regular motions, where precision and tight tolerances are not critical.
An example of a component that commonly utilizes an easy slide fit is a piston, where a smooth and controlled sliding motion is required.
Location clearance fit
Close clearance fits are employed when accurate and precise requirements are necessary, providing a minimal clearance between mating parts. These fits ensure precise alignment and optimal functionality.
With the aid of lubrication, parts can be assembled smoothly without requiring excessive force. This allows for easy turning and sliding of the components, promoting proper functioning.
Examples of applications that commonly use close clearance fits include
- Guiding of shafts
- Roller guides
- Other systems where precise alignment and smooth movement are crucial.
Read More: Chain Drives
Interference Fit
In an interference fit, the tolerance zone of the shaft exceeds the tolerance zone of the hole. This means the shaft is larger in size while the hole is smaller. Achieving a fit between these two requires applying high force during assembly and disassembly, often necessitating the use of a hammer. A hydraulic press can be used to press the shaft into the hole.
Another method involves heating the hole, inserting the shaft, and allowing them to cool. This causes the hole to shrink, creating a tight joint between the components. Interference fits are tighter than clearance fits and are also known as friction fits or press fits because external force is required to join the components together.
Types of Interference Fit
There are three types of Interference fit:
- Force Fit
- Tight Fit
- Shrink Fit
Force fit
In order to achieve a high interference fit, a significant increase in temperature is necessary to heat the components and facilitate the assembly of the shaft into the hole. This process relies on external force to mate the parts together.
Examples of components where high interference fits are commonly used include
- Gears
- Shafts
- Other similar applications where a secure and rigid connection is essential.
Tight Fit
In force fits, the level of interference is higher compared to other types of fits. However, in minimal interference fits, the level of interference is reduced.
Examples of components that often utilize minimal interference fits include
- The stepped pulley of a conveyor system and the cylindrical grinding mechanism of a machine.
- These applications require a precise and secure fit while minimizing the amount of interference between the mating parts.
Driving fits
Medium interference fits require a moderate level of interference between mating parts, which can be achieved through the application of higher forces during assembly. Cold forging or hot forging techniques are commonly used for this purpose. Driving fits, which fall under the category of medium interference fits, are known for their reliability and secure connections. They provide a balance between tight fits and looser fits.
Examples of components that often utilize driving fits include
- Shafts
- Gears
- Bushes
- Other similar applications where a reliable and strong connection is required.
Transition Fit
In a transition fit, the tolerance zone of the shaft falls between the lower and middle limits of the tolerance zone of the hole, indicating that the shaft is larger than the hole. To achieve this fit, slight pressure is applied to insert the shaft into the hole. Transition fits are also referred to as push fits.
Transition fits offer excellent precision and accurate alignment between the mating parts. An example of a component that commonly utilizes a transition fit is a shaft key, where precise positioning and secure connection are crucial.
Read More: Kinematic Link
Types of transition fits
There are three types of transition fit:
- Similar fit
- Fixed fit
Similar Fit
It offers very little clearance or minimal interference, and assembly can be achieved using a rubber mallet. Examples of components that often utilize similar fits include
- Gears
- Hubs
- Pulleys
- Bearings
Fixed Fit
It offers a small clearance or minimal interference fit that can be assembled or disassembled with light pressing force. Examples include
- Driven bushes
- Armatures on shafts
- Plugs
- Similar components
Video on Types of Fit
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