Double fit
Double fit is a term for a certain mistake in the design of a technical product ( construction error ). As a rule, it is a matter of two components touching one another at two ( "double .." ) or several mutually parallel pairs of contact surfaces. In practice this is not possible. The contact actually takes place only at one of these contact points, whereby it is not possible to predict where the contact will occur.
The picture below shows an example of this error: The outer ring of a rolling bearing should be clamped axially between the bearing housing and the cover (the outer ring should not be allowed to rotate). However, the construction drawing shows that the cover should rest on both the rolling bearing and the housing. The production can be such that there remains a play between the cover and the ring and the retention of the bearing ring is not achieved. In the design drawing above, there is sufficient clearance between the cover and the housing: The bearing ring is clamped tight.
Double fits are not always random mistakes. Often two purposes should be fulfilled. In the example (above) - as is often the case - it could be desired that the external system should contribute to sealing the housing. But this only works with an inserted sealing ring made of flexible material. The tolerances must be chosen so that this ring is at least slightly compressed, but not so strong that it can no longer yield and contact of the cover with the bearing is prevented.
Remarks
- ↑ In general, the term static overdetermination describes this situation. In the present case, this is limited to one-dimensionality: only preventing a transverse movement.
- ↑ Even with the narrowest tolerance fields for the dimensions, the result is always such that either one or the other pair of surfaces is in contact, never both at the same time. Tolerated dimensions are mostly used for fits . The word part ... fit in double fit is misleading, because a double fit has nothing to do with a fit.