Fitting a rods in plastic gears

[jokmu]

Hello everyone, what a wonderful forum this is! (Long time lurker...)

I am working on a project and am down to the last few operations; and one of them is stumping me.

I have a plastic (POM) 15T bevel gear, 16MM in diameter and 15MM thick, and am trying to fix it on a steel rod (shaft) with a diameter of 6MM. The bore on the gear is also 6MM.

This has to be a firm attachment, as the rod/shaft will drive the gear, rotating another bevel gear (45T).

That's the other problem, the second gear is also plastic, about 14MM thick, with a bore of 6MM as well. The output torque on that rod will be around 0.2N-m, maximum torque around 0.4N-m.

Both gears have M4 set screw holes.

What would be a good way to attach the rods to the gears? I've heard that press fitting metal into plastic may not be a good idea. The 6MM rods I'm using are round, so there's no flat surface for the set screw to hold though. Would it be a good idea to still use set screws even if they'll only be gripping on a round surface?

I've never fit a gear before, so all opinions are helpful. Thanks!

 

[Swifty]

Once the position of the gears are established, you could use the set screw to mark the shaft lightly, then move the gear and drill a small indent for the set screw to locate in.

Paul.

 

[Parksy]

If you have a mill, you can use an end mill to create a flat surface. Failing that, some careful filing will also create a flat surface to stop the gears spinning.

 

[Mad_Mike]

There are several ways to do this and since you are not transmitting much torque, where I come from your torgue figures would be 1.776 - 3.54 in/lbs. In that case I would just pin it and use the set screw. Drill a small hole, about .5 - .75 MM in diameter 90 degree's to the set screw and use a roll pin. You might know it as a split pin or spring pin. To even further the stability of the connection you might considered sleeving the bore of the gears. On the pin make sure to have the length equal the diameter of the gear hub, thus the whole hub takes the force and flatten a small patch of the shaft for the screw you can use a bench grinder, all you need is a small, flat surface for the screw to contact. You did not mention the rpm these gears will be traveling at.


Mad Mike
I like anything with enough power to do something stupid with!

 

[Dalee]

Hi,

I like the small flat for a setscrew. Should hold well enough. Super glue will also work, though it's more of a hassle to disassemble.

Dalee

 

[Bastelmike]

Hi jokmu,

I would file a flat area on the rod for the setscrew.

Additionally I would add some Loctite to the Joint. But check before if the type of Loctite is suitable to Your POM.

Both methods combined should be sufficient for Your low torque application.

Mike