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darryl
10-28-2012, 08:46 PM
I've become interested of late in the operation of gears, specifically the way the surfaces work against each other to transmit torque. At the moment I'm considering the way worm gears work, then working backwards towards plain spur gears. A worm gear has the surfaces sliding along each other to about the greatest degree, while spur gears are about the opposite. If you consider a ring gear with a slightly smaller pinion working inside of it, you get about the least 'wiping' action there could be. Between those extremes there's a varying contact area, speed of wiping action, and requirement for lube, which also depends on the gear material or combination of materials.

Now that we're onto materials, the main question that has popped up, and the reason for this thread, is regarding the use of acetal for something like hypoid gears. A couple of application ideas have come up where I would need a right angle drive with an offset and some speed reduction- the exact situation that exists in a drive axle. Power levels would be around 100-300 watts, in a hand-held size package. The options would be- metal pinion, acetal ring gear, metal-metal, or acetal-acetal, which is the one I'd like to use. I see a lot of examples of plastic-plastic, with some sort of non-odorous grease, and pretty much every configuration (not hypoid though) and that seems to work well.

I'm just wondering what situations you would try to avoid- spur gears have a small area, line contact which is fairly slow in wiping speed. Typical bevel gears would be about the same, while worm gears would have both a larger contact area and a faster wiping speed. I'm thinking that if both gears are plastic (acetal) then something in the middle might be optimal- medium wiping speed with a decent area of contact. Offhand, it seems that a hypoid gear set would provide this, although more difficult to produce. Anyone have any thoughts on this?

Black_Moons
10-28-2012, 09:41 PM
Spur gears should have next to no rubbing when correctly spaced and designed.
The teeth 'roll' on eachother, not wipe. Hence why you need a diffrent tooth profile depending on the tooth count of your gear.

Its also why you can't use laping compound to improve the meshing, You only wear from where the tooth is incorrect and make it more incorrect, where it was correct it won't wear.

strokersix
10-28-2012, 09:56 PM
Acetal/acetal is a bad combination. High friction and general bad behavior as a sliding couple. Nylon/acetal is much better. Nylon/nylon well greased works good too.

darryl
10-28-2012, 11:34 PM
So acetal on acetal is not good- good to know. I could do one in nylon, or in metal. Probably the small gear if metal. I wonder which would wear less of the two plastics- that would be for the small gear then.

I'm aware that spur gears ideally aren't supposed to rub, but nothing is perfect, especially if these would be made in the home shop. I'm thinking that as soon as you offset the shafts, which is going to add something helical to the tooth profiles, there is going to be some rub.

Seems I should make the ring gear from acetal and choose a compatible material for the pinion. Could be an interesting machining job, without cnc or driven rotary tables, etc.

oldtiffie
10-29-2012, 12:17 AM
I've become interested of late in the operation of gears, specifically the way the surfaces work against each other to transmit torque. At the moment I'm considering the way worm gears work, then working backwards towards plain spur gears. A worm gear has the surfaces sliding along each other to about the greatest degree, while spur gears are about the opposite. If you consider a ring gear with a slightly smaller pinion working inside of it, you get about the least 'wiping' action there could be. Between those extremes there's a varying contact area, speed of wiping action, and requirement for lube, which also depends on the gear material or combination of materials.



Helical, spiral and hypoid gears are the most efficient load (read: "torque") transmitters and they all use "wipe" - same for worms and wheels. But all have side/end thrust which spur gears don't. All are multi-start helixes.

Jpfalt
10-29-2012, 12:50 AM
So acetal on acetal is not good- good to know. I could do one in nylon, or in metal. Probably the small gear if metal. I wonder which would wear less of the two plastics- that would be for the small gear then.

I'm aware that spur gears ideally aren't supposed to rub, but nothing is perfect, especially if these would be made in the home shop. I'm thinking that as soon as you offset the shafts, which is going to add something helical to the tooth profiles, there is going to be some rub.

Seems I should make the ring gear from acetal and choose a compatible material for the pinion. Could be an interesting machining job, without cnc or driven rotary tables, etc.

You will find that the metal gear will wear faster than the plastic. This is because bits of metal and any dust will embed in the plastic and will lap away the metal.

Worm gears are usually run in an extremely thick extreme pressure oil or a grease for slower speed operation. With worm gears, heat is the biggest problem. An easy system for low loads is to use an involute spur gear in plastic or Zamax or bronze and make a hardened worm that looks like a piece of acme screw thread, but with dimensions that match the gear tooth profile of the spur gear. The axis of the worm is not at 90 degrees to the axis of the spur gear, but cocked at an angle equal to the helix angle on the worm at the pitch diameter. This system will start out with not much more than point contacts, but will wear in to line contacts as it runs.

willmac
10-29-2012, 06:50 AM
Spur gears essentially roll without rubbing, but only if the material they are made from is inelastic and only if the tooth profiles are perfect. In reality all materials and especially all materials that that you can make gears from are elastic to differing degrees and this introduces a significant complicating factor. The deflection of the tooth faces caused by the torque transmitted by the gears makes their relative motion much more complex. This is quite significant for acetal gears.

Paul Alciatore
10-29-2012, 12:32 PM
I also once thought that spur gears did not rub but more reading has lead me to doubt that. And I am also not sure what differences the profile of the gear faces makes (involute, cycloid, ?). In any case, the rubbing in a spur gear is certainly very small as compared to a worm gear.

If anybody has any definitive references on rubbing in spur gears I would very much like to see it.

strokersix
10-29-2012, 01:15 PM
Spur gear teeth slide on entry and exit. Sliding velocity is zero when contact point is on a line between gear axes. For proof look at used gears and you will see witness marks that support my statement.