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Fixing gears onto Motor shaft

Which is the most used/desirable method?

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James Paton17/02/2011 17:28:24
3 forum posts
I am making a very simple, small robotic machine. Its function is simply to produce two separate outputs of linear motion.
 
I'm using a tiny little motor than runs off of a 9v battery. I purchased it as part of a gear ratio system, which it is still part of. The operational shaft that has been geared down is purely cylindrical and polished so I am a little puzzled about how to fix gears onto the shaft.
 
I guess I could glue them on, but before I proceed I thought I'd check with the professionals here!
 
What's the usual way of fixing gears to a small shaft? Also just for knowings sake, whats the best way?
 
Many thanks!
James 
 
 

Edited By James Paton on 17/02/2011 17:29:54

James Paton17/02/2011 17:35:58
3 forum posts
James fortin17/02/2011 18:40:45
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46 forum posts
12 photos
you could put a keyway onto the motor shaft and in the gear or you could maybe ream it to a push fit and lock-tight it on.
JasonB17/02/2011 18:53:27
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25215 forum posts
3105 photos
1 articles
You can fix a small pinion onto the shaft with a grub screw, this is how its usually done on model cars etc.
 
 

 
I doubt the shaft is big enough for a keyway, probably only a mm or two dia.
 
J

Edited By JasonB on 17/02/2011 18:54:53

Jeff Dayman17/02/2011 20:08:11
2356 forum posts
47 photos
It does depend on the shaft size and amount of power to be transmitted. For a very small DC motor as used in model cars etc with a 2 mm dia or smaller shaft, in my opinion a high precision heavy press fit is the best most concentric way to mount a gear or pulley IF the gear or pulley is made of durable material ie steel, hard brass, bronze etc.
 
If the gear or pulley is zinc or aluminum die casting alloy the best method is Loctite cylindrical fit locating compound. Press fit is not recommended for zinc or aluminum alloys because these materials will creep away from a high stress press fit and loosen over time.
 
Loctite cylindrical fit locating compound will also work for steel brass or bronze gears and pulleys but in my opinion press fit is better because it requires no part to shaft clearance so concentricity control is better.
 
For plastic gears a press fit onto a splined shaft is not bad if power is low, but slots in the plastic hub engaging with a pin in a cross drilled hole is far better if the shaft is large enough to drill. If designing such a joint from scratch, I have had good luck with hexagonal shaft press fitted into a hexagonal hub on the plastic gear or pulley, but care must be taken with corner radii, and to design a very strong hub. Loctite is NOT recommended for use with plastic gears or pulleys.
 
If you use key and keyway or grub screws on small shafts it is just about guaranteed your gear or pulley will not run concentric to the shaft, at least that has been my experience.
 
JD
Dinosaur Engineer17/02/2011 21:54:52
147 forum posts
4 photos
Loctite 603 is perfectly adequate for this type of application as used throughout industry.
You may damage the fragile motor if you attempt a heavy press fit . Alternatively an allen key type cup point set screw ( Grub screw) is more than capable of handling the drive from this type of motor.
Keith Long17/02/2011 23:04:20
883 forum posts
11 photos

Hi James

If you look at the "Hobbies" - or even the "Hobby's" catalogues I think you'll find that the plastic gears meant for those type of motors are actually moulded with a slightly under-size bore so that they are an interference fit on the shafts and don't need much to hold them securely for normal torques that the motors can generate. A spot of superglue wouldn't hurt as well.

For the more "engineered" motor / gear units around then they usually use a grub screw or Loctite, but concentricity may be an issue - it all depends on your application. Where concentricity is important then the gears can be obtained with a split hub that you then fit a clamp ove,r and the clamp is pulled up with a small nut and bolt compressing th gear hub onto the shaft. As usual the solutions that give the best results cost a fair bit more.

Keith

Ian S C18/02/2011 00:33:50
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7468 forum posts
230 photos
James, best way I find is a firm sliding fit on the shaft, and secure with a grub screw, if possible, file or grind a little flat on the shaftto locate the screw. On my hot air engines where I am changing pullies, gears often I use brass screws instead of steel so that the shaft remains undamaged, its also ok for holding flywheels of at least 6" diameter, and motors to at least 5 watts. Ian S C
James Paton20/02/2011 17:41:44
3 forum posts
Thanks all for your informative replies! Just for clarity, I should mention that I am using a 3mm shaft, and I have a set of plastic gears already so as I am not applying much load, I will use them. They have a cross cut into them and I have some crossed shafts on the way also, so maybe I can fix the crossed shaft to the cylinder shaft for a more secure fit, as it has a larger internal surface area.
 
If this fails I at least have some sound advice to fall back on!
 
Thanks again!
 
James

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