Types Of Fits In Engineering
When designing an engineering product, most parts are assembled together at a final stage. This requires careful dimensions of two mating parts which allows them to join without any obstructions. Engineering fits are designed specifically for this purpose and different types of fits define the clearance between two mating parts or the dimensioning of the pair of parts for a loose or tight fit.
In this article, I will explore the different types of fits, hole and shaft system and the criteria for choosing a fit for your project.
What Is An Engineering Fit?
In engineering ‘fit’ is a term used whenever there’s an assembly of two or more components. A standalone part does not have a ‘fit’. Engineering fit refers to the amount of clearance between two components that are designed to come together and mate. The dimensions of the mating parts determine the fit.
For example, a shaft and bearing have an interference fit. The dimensions and fit are so tight, that the shaft rotates the bearing. Using a clearance fit between a shaft and a bearing would invalidate the design because the bearing would simply slide off the shaft.
There are many types of fit and most assemblies have predetermined engineering fits for the best design and to avoid clearance issues with other multiple-part assemblies.
Importance of Tolerances in Fits
Before I explain all the engineering fits, it is better to understand tolerance and tolerance ranges. Tolerance is the maximum or minimum allowable variation from a reference measurement. Dimensional tolerance exists due to machining capabilities as no machine can cut a piece to the exact dimension shown on the technical drawing.
Nominal Dimension
Nominal dimension is the basic dimension you expect your part to have. The nominal dimension is not accurate and manufacturers will try to stay close to this specified dimension. Nominal dimension has no tolerance variation mentioned.
Tolerance
Adding deviations next to the basic dimension is a way of giving tolerance. This bilateral deviation tells the manufacturer that the allowable variation from the nominal dimension is only 0.05 units.
So the part can have a maximum width of 25.05 units and a minimum width of 24.95 units.
3 Main of Types of Fits
In mechanical assemblies, there are three types of fit.
Clearance Fit
Interference Fit
Transition Fit
Clearance fit
In a clearance fit, the attaching components are always loose. There is a tiny gap between the two mating parts and clearance fit allows the parts to move in and out, and freely rotate freely.
In a hole and shaft basis system, this is defined as the maximum clearance or minimum clearance. Maximum clearance has the smallest shaft diameter while the hole has its largest diameter. In minimum clearance, the shaft has the largest diameter and the hole has the smallest diameter.
Types of Clearance Fits
Slide fit:
Sliding fit is a type of clearance fit that allows two parts to slide on each other while having minimal clearance. This means a sliding fit is just loose enough for components to slide freely making it ideal for close alignment and accuracy.
Application: Sliding doors, sliding gears, shafts and bushings, sliding rods
Example: H7/g6
Loose running fit:
Loose running fit has the largest clearance and it is used where the accuracy of mating parts is not important. This is usually done for applications where unwanted debris can get inside the fitting.
Application: construction equipment and dust prone environments.
Example: H11/e11
Free running fit:
Free running fit is also known as easy slide fit is a type of clearance fit that ensures free movement of parts and has relatively large room for aligning parts. Free running fits can accommodate mating parts where a misalignment is expected.
Application: For easy disassembly, parts in environments having large temperature variations
Example: H8/f7
Close running fit:
Close running fits are used in applications requiring small clearance and good accuracy. They are mainly for controlled movement without slippage.
Applications: Tools, drills, CNC machine tool spindles at high running speeds
Example: H8/g7
Interference fit
Interference fits are types of fit where one component has a larger nominal dimension than the other. In most cases, the shaft is always larger than the hole. The force holding together the mating components is the friction between the faces.
Interference fit requires some kind of external process to assemble the parts. For example, a larger diameter shaft can slide into a smaller hole using a hydraulic hammer. The shaft is pressed into the hole and that’s why another name for interference fit is a press fit.
Many types of processes can lead to an interference fit and each categorizes to a different interference fit.
Types of Interference Fit
Force fit:
Force fit requires the mating parts to have interfering dimensions. The parts are joined using mechanical force by either a press or hammer and are intended for mostly permanent assembly. Force fit is different from shrink fit because it may or may not require heating or cold shrinking.
Applications: Gear wheels and machinery
Examples: H7/s6
Shrink fit:
Shrink fit is a type of interference fit where the larger diameter shaft is shrunk using freezing temperatures and if required the hole thermally expands to facilitate the shaft.
Despite the cold pressing they need to be carefully dimensioned with tolerance to avoid the parts breaking or deforming more than needed. Usually, the variation is fractions of a millimeter.
Applications: Gears, blades, pulley and shaft assemblies and permanent mounting
Examples: H7/p6
Press Fit:
Press fit is based on pressing the two mating parts together that have minimal interference.
Applications: Camshafts, bearings
Example: H7/m6
Driving Fit:
Driving fit is similar to other interference fits but here the force required is moderate. Driving fits are often done using a cold press. Driving fit is good for disassembly as the interference is not prominent.
Applications: Brake discs and gear assemblies
Example: H7/n6
Transition fit
Transition fit is something between interference fits and clearance fits. Transition fit can have interference or clearance. In both cases, the variation or tolerance range is quite small and it is used in applications requiring accuracy and tiny play.
When there are parts with transition fits, some parts may interfere while others have a clearance.
Types of Transition Fits
Push fit:
Transition fits are also sometimes called push fits. Push fits allow the assembly of parts with mild force. Push fits can have a small interference or a small positive clearance.
Applications: piston pins, valve guides and switchgears, shaft key interference
Example: H7/j6
Wringing fit:
Wringing fit is a less popular name of a type of fit that requires force to join two components. Wringing fit requires more force than press fits and the parts have a tight interference fit.
Applications: Gear fittings, rudders
Example: H7/u6
Similar Fit:
Similar fit gets its name from both parts having similar dimensions and requires just a light force for the mating parts to join. Similar fits can have either a small clearance or an interference.
Applications: furniture assembly, bearing, bushes and sleeves
Examples: H7/k6
Fixed Fit:
Fixed fit is a transition fit with tight interference and require more force than similar fits. Similar fits can be assembled using a rubber mallet but fixed fits require a press.
Applications: clutch disks, brake disks, motor shafts
Examples: H7/s6
Basis of Fits: Hole and Shaft System
Engineering fits are always based on either a hole basis system or a shaft basis system. These systems tell us
Tolerance range/dimensional variation
Which part has the fixed dimension
Which part has allowable variation/tolerance range
Standardizes engineering drawings
Hole-basis system
In a hole basis system, the hole’s size is constant and the shaft has allowable dimensional tolerances.
In a hole basis system, the shaft can have an upper or lower deviation but the hole cannot have a lower deviation from the nominal dimension.
Hole Basis System Example
For a hole basis system clearance fits can be defined using the ISO tolerance table. The hole diameter is fixed to for example 40.00 mm and the shaft is given a tolerance range.
Here the tolerance is defined H9/e9 and corresponds to a clearance fit.
The capital letter refers to the hole and the lowercase letter is for the shaft.
H9 gives a lower limit of 0.00 and an upper limit of 0.062 mm
e9 gives a lower limit of -0.112 and an upper limit of 0.050 mm.
So the hole can have a maximum dimension of 40.062 mm and the shaft can have maximum dimension of 39.950 and a minimum dimension of 39.888 mm.
Such a tolerance range gives clearance fit.
Shaft-basis system
Hole and shaft basis systems are similar. However, in a shaft basis system, the shaft diameter is kept constant and the tolerance of the hole determines the fit.
In this example, the shaft size is fixed and the hole size is allowed to have a bilateral tolerance of +0.012 mm and -0.012 mm.
How To Decide Between Clearance, Interference, and Transition fit?
Many factors can help you decide what engineering fits are suitable for your project. The type of engineering fits depends on the final application and precision of the assembly.
Precision
You can achieve a very close and tight interference fit which perfectly joins the two parts but this will result in hot pressing or shrink fitting which have additional costs. Having a very precise tolerance and using interference fit and transition fits is useful for applications needing high accuracy.
Application
Different types of fits can be used depending on the application of your project. For instance, if your project involves free rotation and play between the mating faces, a clearance fit is ideal but if you’re designing for permanent mounting an interference fit is a better choice.
Conclusion
Engineering fits are extremely important for shop floors, designers, and most importantly for mechanical applications. There are three types of fits and each is used for a different type of application. As you go for closer and tighter fits with higher precision, the cost of production increases, and the freedom of movement between the parts decreases.
Shaft and hole basis system are used as an international reference for tolerance and define the range of tolerance for different fits.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.