Fitting rods/shafts in plastic gears

[John MunroeParticipant @johnmunroe13164]

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!

[John MunroeParticipant @johnmunroe13164]

[Dennis DParticipant @dennisd]

Mark out on the shaft where the gears will be and file a small flat

[Paul LousickParticipant @paullousick59116]

Machine (or file) a small flat on the shaft for the set screw to clamp against.

Another option is to drill a hole thru the gear and shaft and fit a roll pin (like spring dowel pin)

Paul.

Edited By Paul Lousick on 31/01/2016 07:32:13

[capnahabParticipant @capnahab]

if the rod is not hardened you could drill a small hole for a dog point grubscrew ? (no affiliation).

[John MunroeParticipant @johnmunroe13164]

Thank you for all the input.

I have a related question: the shaft of my motor is actually just 3MM in diameter. Given that the gear bores are 6MM in diameter, what's a good way to fit the 3MM shaft in a 6MM bore? I'm hoping to avoid using a shaft coupler. Could I add a metal ring of some sort inside the bore such that the resulting bore diameter will be reduced to 3MM?

[Ian S CParticipant @iansc]

John, how about a 6 mm dia bush, with a 3 mm hole. Don't know if Loctite works on plastic, but the outside of the bush could be lightly knurled with a fine straight knurl, a hole drilled through and tapped a suitable size. That's how this 3 mm shaft is fitted in a 5 mm bore bearing on my smallest hot air engine. Ian S C

[John MunroeParticipant @johnmunroe13164]

Posted by Ian S C on 31/01/2016 10:44:06:

John, how about a 6 mm dia bush, with a 3 mm hole. Don't know if Loctite works on plastic, but the outside of the bush could be lightly knurled with a fine straight knurl, a hole drilled through and tapped a suitable size. That's how this 3 mm shaft is fitted in a 5 mm bore bearing on my smallest hot air engine. Ian S C

That's interesting! Have you got pictures of the knurled bushings used? Did you make them or buy them off the shelf? I guess you used Loctite on yours – right?

Thanks

[Ian S CParticipant @iansc]

The bushings are aluminium, and just lightly pushed into the bearings, and a sliding fit on the shaft. With these wee bearings you have to be careful not to either push them into too smaller hole, or make them too tight on the shaft, it tightens up the clearance in the bearing. I was looking at some of my collection of small electric motors, and the plastic pulleys and gears seem to be just pressed on the shaft, as are some brass gears.

Ian S C

[Clive HartlandParticipant @clivehartland94829]

One aspect of plastic gears is that the hub of the gear often splits, you should use a boss which fits the gears hole and sliding fit on the 3 mm shaft. Then, secure the boss to the plastic gear in such a way as to ensure no force is on the gear hub. Perhaps a flange on the boss and then pin that to the gear. Make sure there is no movement of the gear on the boss and that you have full width gear meshing as the plastic gear will wear.

Clive

[John McNamaraParticipant @johnmcnamara74883]

Hello John Munroe

One possible solution is to straight Knurl the shaft, However for a 6mm shaft maybe that will be difficult as the shaft may bend if a single knurl is used. the clamp type knurling tool with two knurls one pressing form each side may work reducing the side load.

**LINK**

There are many different sizes of knurling wheel. available individually fairly cheaply you can make your own holder simply if you do not have one..

**LINK**

An alternative if you have an indexing head would be to cut the teeth into the shaft, however there will be no increase in shaft diameter you will need to allow and turn down the rest of the shaft. or make a smaller bore in your gear.

Using a knurling tool the teeth are formed no metal is removed this will increase the diameter of the shaft. enabling a press fit. Many of the plastic gears and other bits in laser printers are attached with this method.

The many teeth hold very well giving a very strong grip.
The method also works very well with Aluminium or brass gears.

Regards
John

Edited By John McNamara on 01/02/2016 12:23:13

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