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I'm self-learning a bit about preferred fits and tolerances and was wondering why transition fits were even a thing.

Clearance fits I understand, applications where you don't want parts to touch and "clear" each other.

Interference fits are for snug, more rigid connections which require pressing.

However, why would you want a transition fit which sometimes is clearance, and sometimes is interference? Doesn't that make your end product variable and not consistent?

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A transition fit actually makes the end product more consistent than the clearance fit. In transition fit clearance (if occurring) is less compared to the clearance fit. Also, if interference occurs, we don't need to used press fit or temperature based shrink fit. We can simply use mallet for transition fit. This makes transition fit a preferable choice in case of the higher quality grade equipment such as Gears (cycloidal), Hubs, driven bushes where more accuracy is needed.

Transition fit is mostly used for accurate location, a compromise between clearance and interference.

For example consider two gears with cycloidal profile where you need the center distance between the shafts to be exact or with the maximum accuracy. Here, a clearance fit my cause less accuracy resulting in more vibrations in cycloidal gears. While the interference fit my cause the replacement of the gear difficult as shaft can be damaged while replacing the gear. Here a compromise is made between clearance fit and interference fit.

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  • $\begingroup$ "In transition fit clearance (if occurring) is less compared to the clearance fit." this would be solely based on the spec. We could specify a clearance fit with less clearance than allowed by a transition fit. $\endgroup$
    – Tiger Guy
    Aug 27 at 15:07
  • $\begingroup$ @TigerGuy The point is that a transition fit always specifies a possible interference, but a clearance fit never does. Of course you can define words to mean whatever you like, but the purpose of standard terminology is communication - i.e. everyone thinks the same words (or the same symbols on a diagram) mean the same thing. $\endgroup$
    – alephzero
    Aug 27 at 15:26
  • $\begingroup$ @alephzero, my point is the clearance/interference spec of a transition fit is not necessarily smaller than that of an clearance/interference fit. $\endgroup$
    – Tiger Guy
    Aug 27 at 15:33
  • $\begingroup$ I think I kind of understand better. So, essentially in a transition fit the "clearance" option is more acceptable then the largest clearance case of a regular clearance fit. Meanwhile, the "interference" option of the transition fit is acceptable than a normal interference fit since you can just manually press things in. $\endgroup$ Aug 27 at 16:58
  • $\begingroup$ @TwoWaySpeedOfLight that pretty much sums it up. $\endgroup$
    – Ashmark
    Aug 28 at 3:37

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