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Does anyone know of a good (low profile) way to attach the threaded (chamfered) end of a M5 screw shaft to a 30MM (30T 1M) plastic cog wheel? Would it be a good idea to widen the shaft hole and just let the threads grip the hole?

EDIT The torque will be around 6kg-cm and the plastic I'm looking at is POM.

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A full answer depends on what plastic and what the load will be. Having a depth of 30 mm does give you a lot of options.

You can drill and tap ABS just fine, and I've had success drilling and tapping acrylic as long as I kept the tool slow so as not to overheat and warp. We have also used self-tapping screws for Delrin and UHMW with good results (and some Loctite plastic bonder). For plastics which are too brittle or too soft for direct threading, a very standard method is to place a threaded insert into a through-hole in the plastic. Helicoil, Dodge, Acme, and others make these inserts. The method of adhering the insert to the plastic depends on the process. Look at the expansion inserts on pages 24 & 25 of this Dodge catalog: http://www.afi.cc/contentonly.aspx?file=images/vendors/DodgeInserts.pdf Those seem like they would work well for you, as would the self-threading ones on page 28.

EDIT (load identified): The 6 kg cm value is approximately 0.59 N m. PEM's data for their M5 press-in threaded insert (pages 15 and 18 of http://www.pemnet.com/fastening_products/pdf/sidata.pdf) lists a torque limit of 4.02 N m for insertion into a polycarbonate plastic. That is plenty of margin. Given the large margin you could likely eliminate the Loctite step altogether.

EDIT 2 (plastic identified): With POM you can use brass threaded inserts. It is also a good candidate for direct drilling and tapping since it has low creep. If you have cyclic loading I would go with the threaded brass inserts to ensure the threads remain consistent over the device's life. If you are simply fastening items, and just need them to hold for your 6 kg cm torque load without other significant stresses, you should be fine with using thread-forming screws or the drill and tap method.

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  • $\begingroup$ The torque for my application is around 6kg-cm. If I tap a hole in the plastic gear and turn the shaft in, will the Loctite hold? It'll just be Loctite that will be holding the shaft in place. $\endgroup$ – WKleinberg Dec 30 '15 at 15:37
  • $\begingroup$ Could you also edit your question to show what plastic material you are using? $\endgroup$ – SteveO Dec 30 '15 at 15:38
  • $\begingroup$ I've updated the OP now. $\endgroup$ – WKleinberg Dec 30 '15 at 17:09

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