# Question about press-fitting a gear to a shaft...



## mjolner (Feb 9, 2009)

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

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

I have a steel gear, 0.173" i.d., and am trying to fix it on a nominal 3/16" brass rod, that has an actual diameter of 0.188". 
This has to be a firm attachment, as this gear will drive the rod, rotating another fixed gear.

That's the other problem, the second gear is plastic, about 0.125" thick, with a bore of 0.125". 
I could either ream that out to match the brass rod and use some sort of epoxy, but that seems messy and unlikely to hold up to the torque. 
Another option would be to cut a slot in a section of the brass rod, and another slot in the bore of the plastic gear and insert a square key. 
That seems like the best option, the challenge there is the tiny dimensions of such a key and slots. 

Back to the first problem - the steel gear with the 0.173" bore. 
I'm thinking I can turn the 0.188" brass rod down to a diameter that would allow a press fitting of the gear.
But to what diameter? 
And could/should I use heat to expand the gear and then slip it onto the cold shaft, making a much tighter fit? 

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


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## joeby (Feb 9, 2009)

You'll get about a hundred different answers on how to figure your press fit; but as a rule of thumb, I always used .001 per inch of diameter. Metal to metal (shaft and bore the same size) to .0002 interference should work without too much worry on getting it together.

 As for the plastic gear, how about cross drilling the shaft to take a pin, and slotting the gear bore to slide over the pin? Another method would be to assemble and drill at the intersection of the bore and shaft and inserting a pin (half the pin dia. in the shaft and half in the gear). A key and keyseat in a 1/8" or 3/16" shaft would not be the easier approach.

Kevin


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## mjolner (Feb 10, 2009)

Thanks for the reply!
I like your idea of cross-drilling the shaft and slotting the bore of the plastic gear - seems much better than fab'ing a microscopic key. 

What about heat for the metal gear? One consideration is that with a bore that small, the amount of expansion gained by red-heating may be minimal. 

So maybe if I turn the brass rod to 0.174-5" and *whack* the 0.173" bore gear onto it....
This isn't going to be an extremely high torque application, but there is some.
The metal gear is 16 teeth 7/16" o.d. being driven off a 110VAC 5W 6rpm brass motor with a 10 tooth 1/4" drive gear.
The plastic gear is 40 teeth, roughly an inch and a mumble across, which then drives a 27 tooth 1 inch gear that then is turning a crank at 5.55 rpm. 

I'm nearly finished and I'm really excited, just a few more fussy little details!
Thanks!
-Kris


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## ksouers (Feb 10, 2009)

Kris,
Since it's low torque could you make it a press fit and Lock-tite it? Otherwise Kevin's suggestion of drilling a small hole at the shaft/gear interface and using a piece of piano wire for a key is probably your best bet.


The other Kevin


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## Stan (Feb 10, 2009)

The thickness of the steel gear has a bearing on the method of attachment. If it is not a very thin gear I would turn the shaft to the the same size as the gear id and then put a light straight knurl on the shaft. Make the turned down part long enough to put a small taper for a lead in and then press together in an arbor press or vise.

Pressed together parts are obviously permanent so Loctite is added security.

I hate working with plastic gears!


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## Marinesteam (Feb 10, 2009)

Here is something else to chew on...

Press fits in plastic are generally a bad idea. Plastic (painting with a broad brush) will creep over time if a force is being applied to it (even gravity in some cases). So what is now a press fit may not always be a press fit.

Good luck

Ken


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## mjolner (Feb 10, 2009)

Well I got the metal gear onto the shaft following some of the tips here, and I was able to pin the plastic gear to the shaft....
Then, when everything was assembled it became evident that my chosen motor didn't have enough torque for the load. 
The 'transmission' I built worked fine, but when I attached the load to the output shaft it came to a standstill.
Arggh! There goes about 20 hours of work.  

So "Plan B", I found an old motor from a Littermaid that has torque-galore and came up with a way to retrofit that to my design. 

It works! Quite noisy, but it works!

When I finish the last 6 things, I will post up some pictures. 
(This project is my own scaled down version of a huge kinetic sculpture I saw on a TV show.)

Thanks again for the help! -Kris


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