Good fit with the transition fit
The transition fit lies between the interference fit or interference fit and the clearance fit. Assembly is possible with this type of fit, as is a certain degree of rotatability. To calculate the transition fit, both components must be considered in detail.
The transition fit between clearance fit and press fit
A fit is generally the relationship between two components with tolerances. It determines the force and friction that two adjacent components exert on each other. The fits are mainly used in the definition of shafts and bores. However, they can also be used for grooves and sliding blocks or springs. A distinction is made between three types of fit:
- Oversize fit/press fit
- Transition fit
- Game fit
The oversize fit or interference fit is characterized by the fact that the components do not slide into each other straight away. Tricks or force are required to join them. One trick, for example, is the use of heat or cold. A component can be stretched or shrunk until it can be assembled with the other component. One example of this is the wings of Tornado fighter bombers: the bolt is cooled with liquid nitrogen to assemble the pivoting wings. Shrunk in this way, it slides cleanly into the hole provided. After warming up to normal ambient temperature, the piston was again oversized and ensured a particularly tight fit.
One disadvantage of the interference fit is that it greatly hinders the rotation of the inserted shaft. If the shaft is turned with force, there is a risk of seizure damage. However, if the shaft is too loose in its bore, the components can come loose from each other. It therefore always depends on the application in which you have to choose between the individual types of fit.
Calculation of the fit types
A fit basically means that the components do exactly that – they “fit” together. Even if some force is required for assembly, it may still be an acceptable fit. As described, a tight connection is often desired after assembly. This includes, for example, gear wheels mounted on shafts. These are not only held in position by the internal spring. The fit also ensures a firm connection and thus ensures the functioning of the component.
Fits are always characterized by the specification of a tolerance range. This is specified in the familiar, standardized notation. The tolerances of fits are specified in EN 20286. They always refer to the diameter of the shaft or bore.
Shaft tolerances are defined with a lower case letter, bore tolerances have capital letters. The upper and lower tolerance of a component (regardless of whether it is a bore or shaft) result in the “tolerance zone”. The tolerance zones of bores and shafts normally overlap slightly. This prevents excessive play between the components.
The first value required is the nominal dimension. This is always identical for the shaft and bore. The associated tolerance defines the permissible actual dimensions of the components. The tolerance is derived from the drawing and refers to the standard value.
Example:
A shaft has a nominal diameter of 10 mm and is to be manufactured with a tolerance of h6. The tolerance h6 means a dimension of 9 nm in relation to the nominal diameter. This means that the shaft must not exceed the nominal dimension of 10 mm and must be at least 9.991 mm wide.
The corresponding bore is manufactured to tolerance H9. This type stands for a minimum interference of 15 nm and a maximum interference of 24 nm. The bore therefore has a diameter between 10.015 nm and 10.024 nm. The two tolerance points are referred to as the “maximum dimension” and the “minimum dimension”.
If these values are known, they are offset against each other.
- Maximum dimension of the bore – maximum dimension of the shaft = 10.024 mm – 10.00 mm = 0.024 mm
- Minimum dimension of the bore – maximum dimension of the shaft = 10.015 mm – 10.00 mm = 0.015 mm.
- Minimum dimension of the bore – minimum dimension of the shaft = 10.015 mm – 9.991 mm = 0.024 mm.
- Maximum dimension of the bore – minimum dimension of the shaft = 10.024 mm – 9.991 mm = 0.033 mm.
When is there which fit?
- If the result of all four calculations is positive, there is always a clearance fit if the tolerance is maintained.
- If positive and negative results alternate, the transition fit is obtained.
- However, if there are only negative results, you have an interference fit.
Why calculate in advance?
It is very important to calculate the fit types before production. This is the only way the worker can prevent a design error from being incorporated into the component. With the small dimensions, it can always happen that the engineer or technician slips by one cell in the fit table. As you can see, even the smallest differences are enough to produce a completely wrong result.
If the mechanic is familiar with the installation situation, he will be able to detect any anomalies when analyzing the fits. If, for example, a gear-shaft connection only allows clearance fits, you should ask. After all, press fits are preferred in these assembly situations.
Properties of the transition fit
With a transition fit, the components can be fitted together under slight pressure. If necessary, a wooden mallet can be used to help drive the shaft into the hole. However, the use of high pressure, heat or cold is never necessary for a transition fit.
A shaft-bore connection manufactured with a transition fit can be twisted against each other under force. If this is desired, some measures should be taken to prevent galling.
Supporting the transition fit
A connection between a shaft and a bore that is as firm as possible but still flexible can be supported by the following measures:
- Use of different materials
- Bore or shaft made of materials with self-lubricating properties
- Facilitate lubrication
Material uniformity in moving components always leads to seizure. This connection will self-destruct in a very short time. A steel bolt is therefore ideally combined with a bronze bore. Bronze bushes can also be designed to be self-lubricating. External lubrication, for example by means of a grease nipple attached to the side, reliably prevents galling or internal abrasion.
