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I see that the housings of most planetary gearboxes consist of 3 parts: a front cap, a ring gear and a rear cap. Here's a common design:

enter image description here enter image description here

In designs similar to this, the rear cap and the ring gear are held together by several (torx) screws. But how about the front cap and the ring gear? There's no sign of any screw or fastener. There is a circlip on the shaft, but I believe that's for holding the bearings in and not for holding the front cap. Does anyone know how the front cap is held fixed? Here's a video: https://www.youtube.com/watch?v=cMr9ciatm2s

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  • $\begingroup$ There are two holes shown where screws go through into the casing, seen clearly left side on the outer ring of the cap. $\endgroup$
    – Solar Mike
    Jun 23, 2020 at 19:30
  • $\begingroup$ @SolarMike I believe those are tapped holes for fastening fixating the gearbox to something like a bracket. Also, the ring gear won't have enough material for tapped holes in that direction. $\endgroup$
    – John M.
    Jun 23, 2020 at 19:32

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There appear to be 4 thru holes on the front cap (1 is missing due to the section view). There are probably tapped holes in the ring gear, screws keep the cap in place.

This could also sit in a housing with stops next to the front and bottom housings.

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  • $\begingroup$ Those 4 holes are tapped for mounting the gearbox to something like a bracket. $\endgroup$
    – John M.
    Jun 23, 2020 at 19:34
  • $\begingroup$ Then there is probably a similar fixture point for the rear cap, with that and the front cap bracket fixed to a chassis. $\endgroup$
    – jko
    Jun 23, 2020 at 19:36
  • $\begingroup$ That's what I thought, but there doesn't seem to be any screw near the front cap: img.directindustry.com/images_di/photo-mg/7056-13952899.jpg $\endgroup$
    – John M.
    Jun 23, 2020 at 19:38
  • $\begingroup$ Could just be press fit then. All spur gear train, doesn't generate any axial force. $\endgroup$
    – jko
    Jun 23, 2020 at 19:42
  • $\begingroup$ You mean the front cap is press fit to the ring gear? I wonder why they all follow this design rather than use screws like they do to the back cap. $\endgroup$
    – John M.
    Jun 23, 2020 at 19:44
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A possible method is that the circlips hold the front cap to the assembly which is slid into the housing and the rear cap holds it togther with the 4 cap screws, given there is little axial force generated by the gearing system.

The frint cap can also be a press fit into the casing especially as the section does not show any o-rings...

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  • $\begingroup$ But it's often the case that the gearbox's front cap bracket holds the motor's weight as well, which can easily be a few kg. That'd be a lot of weight for a circlip, wouldn't it? $\endgroup$
    – John M.
    Jun 23, 2020 at 19:42
  • $\begingroup$ You'd be surprised the load a retaining ring can handle $\endgroup$
    – jko
    Jun 23, 2020 at 19:46
  • $\begingroup$ But the circlip wouldn't stop the the shaft along with the front cap from sliding out, right? $\endgroup$
    – John M.
    Jun 23, 2020 at 19:49
  • $\begingroup$ @JohnM. Have you dismantled any of these? Or are you just working from pictures? $\endgroup$
    – Solar Mike
    Jun 23, 2020 at 19:49
  • $\begingroup$ @SolarMike I've been doing that, but the front cap and ring gear are together. I didn't want to cause too much damage. $\endgroup$
    – John M.
    Jun 23, 2020 at 19:52

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