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I have some issues regarding a lead screw.

It's used as the main driving system for a scissor lift which requires about 2Nm torque to operate.

This is the exact part if it matters:

https://www.amazon.co.uk/TOOGOO-Optical-Housings-Aluminum-Coupling/dp/B07PFQKQV6/ref=sr_1_2?keywords=200mm+opticalis&qid=1583887143&sr=8-2-spell

due to space constraints in the design I have to use a universal joint and place the motor at a very slight angle to clear a few things:

https://www.amazon.co.uk/sourcing-map-Rotatable-Universal-Steering/dp/B07H9ZKBSB/ref=sr_1_4?keywords=6mm%2Bto%2B8mm%2Buniversal&qid=1583887292&sr=8-4&th=1

Regardless of how much I tighten the grub screws, they will eventually come loose (only on the lead screw end, not the motor end) and slide over the the lead screw. I've tightened them so much that even one of them became stripped and I had to replace it.

I have a bottle of medium strength loctite, does using that for the grub screws solve the issue or are there better ways of doing it?

Another idea was using a file to make a flat spot on the lead screw but I'm not sure how well that'd work or if it makes things worse. Since I only have 1 lead screw, I thought it'd be best to ask before destroying it.

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  • $\begingroup$ Is there space to add a jubilee clip to act as a retainer for the grub screws? $\endgroup$ – Gonzonator Mar 11 at 14:47
  • $\begingroup$ Have you tried a spline end connector , keyway or a pin. The reason to making flats is not really to hold better but to get the connection off later if the holes are tightly matched. $\endgroup$ – joojaa Mar 11 at 20:18
  • $\begingroup$ unfortunately not, the grub screw is way too small for that. $\endgroup$ – OM222O Mar 12 at 4:22
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Loctite works well and this application, while not the 'normal' application, I would expect to give good results. If it's necessary to be sure that your solution works over time, I would go with a physical connection. The trouble with the flat spot idea is to locate it properly and not mess up the treads such that the parts can not be screwed together. I would try assembling and then using a marker through the set hole mark the spot so that you can see it after dissembling. The threads can be repaired by careful filing between them with a small triangular shaped file.

Or, perhaps put the Loctite into the bottom of the set screw and the set the screw. This would have the same affect as you proposed but instead of removing material to get the set screw to grab, you're adding material, so to speak

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    $\begingroup$ how about drilling a small hole so the grub screw goes into it and acts as a sheer pin? As long as the hole is reasonably small and perpendicular I think it should be fine? $\endgroup$ – OM222O Mar 11 at 1:20
  • $\begingroup$ That would be the best solution. But drilling the hole will be difficult unless you have a vise and a drill press. You can start the hole with a punch to create a divot that will allow the drill bit to stay centered as you start the hole $\endgroup$ – Ack Mar 11 at 2:38
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There are a range of options for such a connection. I would put loctite for a plain grub screw at the bottom of the list. Depending on the safety aspects of the joint breaking loose, it might be very important for it not to slip. Your other options (in rough order of reliability):

  • Grub screw to a flat-spotted shaft
  • Grub screw into a drilled and tapped hole
  • Through-hole with a tapered pin
  • keyway and key
  • Spline fit or other shape-matched fit
  • welded-on connectors (like an automobile)
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  • $\begingroup$ I’ve been told years ago by a professor that a grub screw is only used long enough to allow a hole to be drilled and a pin inserted. The the grub screw is removed. $\endgroup$ – Eric S Mar 11 at 14:15
  • $\begingroup$ @Eric Shain, I think the main idea of a grub screw is that it's out of the way on the shaft. How it connects to the shaft is full of options. $\endgroup$ – Tiger Guy Mar 11 at 15:01
  • $\begingroup$ I think my prof was suggesting that you shouldn’t trust a grub screw to reliably work when there are better options. $\endgroup$ – Eric S Mar 11 at 17:32
  • $\begingroup$ Both keyways and pinning seem quite easy in this case. Although you could be more clear which way your liat ia more reliable. $\endgroup$ – joojaa Mar 11 at 20:23

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