I understand that double-helical gears are used over regular helical gears to balance axial forces in a shaft but, depending on the direction, it seems that these balanced forces can be either compressive or tensile.

A quick Google search shows that many suppliers recommend the compressive configuration over the tensile one when installing these gears, because it is more "stable." However, most of the websites I have looked at have similar wording when saying this, and I have not yet found any source that is credible (academic textbooks, well-know suppliers, etc.). This makes me suspect that this is either not a very important consideration, or it is not true. From what I understand, if the forces are compressive, they will correct any axial misalignment, but I would think that tensile configuration would also correct the misalignment. In other words, I can't really picture the "unstable" scenario.

Moreover, I do not understand how both the pinion and the gear could be both in a "stable" configuration, since any compressive forces in the gear would have to cause a tensile force in the pinion. If one direction is truly "stable," which of the two (pinion or gear) should be stabilized?

If anybody has any credible online sources or intuitive explanations on this stability problem, that would be a great help

  • $\begingroup$ what gears and gearboxes have you looked at ? Google is good but real machines may give you a better insight.... Go to a scrap yard and get a couple of old transmissions : take them apart and work from there... $\endgroup$
    – Solar Mike
    Nov 24, 2017 at 21:56
  • $\begingroup$ I'm actually trying to analyze the transmission system of a mill. There's a way to reverse the double-helical gears such that they work in either the "stable" or "unstable" configuration. I'm not sure that we installed the gears correctly. 1 out of our 4 mills have a transmission in the "stable" configuration, and the other 3 are in the "unstable" configuration. I'm not sure if that actually is a problem, but I can't take our setup as a reference because it's what I'm trying to analyze. $\endgroup$ Nov 24, 2017 at 22:10

1 Answer 1


I imagine this may have something to do with compressive forces being favorable to tension forces in respect to fatigue failure. Pulling apart bonds vs smashing them together.

  • $\begingroup$ That makes sense, but most of these websites speak about problems with misalignment. The conclusion they make is that, if the forces are tensile, then a little axial misalignment will cause a force imbalance that will exacerbate the axial misalignment. However, if the forces are compressive, then a little axial misalignment will cause a force imbalance that is restorative, so that the axial misalignment is corrected. I do not know if this is true, and I can't picture how it would work anyway. $\endgroup$ Nov 27, 2017 at 20:53
  • $\begingroup$ This might depend on the orientation of the piece in question. It sounds like tensile forces will widen the gap and compressive forces close the gap as a result of the geometry. If a free body diagram of the piece in question can be provided I might be able to help. $\endgroup$
    – Necrodox
    Dec 13, 2017 at 0:11

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