There are various types of bearing fits used in mechanical assemblies, each serving specific purposes based on the requirements of the application. Two common types of fits are:
In a sliding fit, there is intentional clearance or allowance between the mating parts. This allows for relative movement between the parts, often providing ease of assembly and disassembly. Sliding fits are used when a certain amount of play or movement between components is acceptable or necessary. They reduce the risk of binding or jamming while enabling lubrication between the parts. Examples include the fit between a shaft and a bearing.
A press fit involves the interference between the mating parts, where one component is slightly larger than the other, requiring force to assemble them. The interference creates a tight joint without the need for additional fasteners. Press fits are chosen for applications where a secure, rigid connection is necessary to prevent relative movement between parts. This fit ensures better alignment, improved load distribution, and higher torque transmission. For instance, gears on a shaft often employ a press fit to ensure a secure connection.
The reasons for using these fits depend on the specific requirements of the application:
With a clearance fit, the shaft is always smaller than the hole. This enables easy assembly and leaves room for sliding and rotational movement.
Max clearance – left; min clearance – right
When the shaft diameter is at its minimum and the hole diameter at its maximum, we have a situation of maximum clearance. When the shaft diameter is at its max and the hole diameter at its minimum, we have a situation of minimum clearance.
Clearance fits come in 6 sub-categories. Starting from the loosest:
Fit with the largest clearance. It is suitable for applications where accuracy is not of the utmost importance and contamination may be a problem.
Example uses in engineering: Fits exposed to dust contamination, corrosion, thermal and mechanical deformations. Pivots, latches, etc.
Example fits: H11/c11, H11/a11, H11/d11 (all hole-basis), C11/h11, A11/h11, D11/h11 (all shaft-basis). Using a 25 mm diameter, an H11/c11 fit gives a minimum clearance of 0.11 mm and a maximum clearance of 0.37 mm. In this case, the shaft diameter can fall between 24.76 and 24.89 mm, while the minimum hole size is 25 mm and the max is 25.13 mm.
Suitable where no special requirements apply to the accuracy of matching parts. It leaves room for movement in environments with heavy temperature fluctuations, high running speeds, and heavy plain bearing pressures.
Example uses in engineering: Applications where maintaining a film of oil lubrication is essential. For example, the shaft and plain bearing fit with little rotational movement.
Example fits: H9/d9, H9/c9, H9/d10 (all hole-basis), D9/h9, D9/h8, D10/h9 (all shaft-basis)
Using a 25 mm diameter, a H9/d9 fit gives a minimum clearance of 0.065 mm and a max clearance of 0.169 mm.
Close-running fits are a good choice for applications that require smaller clearances and moderate accuracy. They are also suitable for withstanding medium speeds and pressures.
Examples used in engineering: Machine tools, sliding rods, machine tool spindles, etc.
Example fits:H8/f8, H9/f8, H7/f7 (all hole-basis), F8/h6, F8/h7 (all shaft-basis)
Using a 25 mm diameter, a H8/f7 fit gives a minimum clearance of 0.020 mm and a max clearance of 0.074 mm.
It leaves a small clearance for high accuracy while maintaining ease of assembly. Parts will turn and slide quite freely.
Examples used in engineering: Guiding shafts, sliding gears, slide valves, automobile assemblies, clutch discs, parts of machine tools, etc.
Example fits:H7/g6, H8/g7 (all hole-basis), G7/h6 (shaft-basis)
Using a 25 mm diameter, a H7/g6 fit gives a minimum clearance of 0.007 mm and a max clearance of 0.041 mm.
Location clearance provides minimal clearance for high accuracy requirements. The assembly does not need any force, and the mating parts can turn and slide freely with lubrication, helping with assembly by hand. The fit is snug for stationary parts.
Examples used in engineering are roller guides, shaft guidance, etc.
Example fits: H7/h6, H8/h7, H8/h9, H8/h8 (all hole-basis)
Using a 25 mm diameter, a H7/h6 fit gives a minimum clearance of 0.000 mm and a max clearance of 0.034 mm.
A transition fit encompasses two possibilities. The shaft may be a little bigger than the hole, requiring some force to create the fit. At the other end of the spectrum is a clearance fit with a little bit of room for movement.
Clearance – left; interference – right
Specifying a transition fit means that both outcomes are possible even inside a single batch.
Transition fits come in 2 forms – similar fit and fixed fit.
It leaves a small clearance or creates a slight interference. Assembly is possible using a rubber mallet.
Examples include Hubs, gears, pulleys, bearings, etc.
Example fits:H7/k6 for hole-basis and K7/h6 for shaft-basis
Using a 25 mm diameter, a H7/k6 fit gives a max clearance of 0.019 mm and a max interference of 0.015 mm.
It leaves a small clearance or creates a slight interference. Assembly is possible using light force.
Examples used in engineering: Driven bushes, armatures on shafts, etc.
Example fits: H7/n6 for hole-basis and N7/h6 for shaft-basis
Using a 25 mm diameter, a H7/n6 fit gives a max clearance of 0.006 mm and a max interference of 0.028 mm.
Interference fits, also known as press fits or friction fits, always have the same principle: a larger shaft compared to the hole size.
Max interference – left; min interference – right
The assembly stage requires force, sometimes lubrication, heating of the hole, and freezing of the shaft. These help increase/decrease the hole and shaft sizes, respectively, to make for a more straightforward process. The interference helps secure the relative positioning of the shaft and hub even during rotation, making this type of fit good for transmitting rotational speed and power.
Minimal interference. Assembly can be performed with cold pressing.
Examples used in engineering: Hubs, bushings, bearings, etc.
Example fits:H7/p6 for hole-basis, P7/h6 for shaft-basis.
Using a 25 mm diameter, an H7/p6 fit gives a minimum interference of 0.001 mm and a maximum interference of 0.035 mm.
Needs higher assembly forces for cold pressing. Another way is by using hot pressing. This interference fit is more prominent than with a press fit.
Example uses in engineering: Permanent mounting of gears, shafts, bushes, etc.
Example fits:H7/s6 for hole-basis, S7/h6 for shaft-basis
Using a 25 mm diameter, an H7/s6 fit gives a minimum interference of 0.014 mm and a maximum interference of 0.048 mm.
High interference fit. Assembly requires heating the part with a hole and freezing the shaft to force the mating parts together. Disassembly can result in broken parts.
Examples used in engineering: Shafts, gears, etc.
Example fits:H7/u6 for hole-basis, U7/h6 for shaft-basis
Using a 25 mm diameter, an H7/u6 fit gives a minimum interference of 0.027 mm and a maximum interference of 0.061 mm.