Hello,

What is the best way of preventing small plastic compound gears from moving up/down their metal axle?

(attached is image)

Details =

Compound gear 24mm dia.
Gear bore hole = 2.1mm dia.
Axle = 2mm dia.

Current solution =

Metal axles are encased in 2 halves of plastic housing (encapsulating hole gear assembly). Axles slot into plastic protrusion (1.5mm thickness) on each side, leaving just enough space for the gear in the middle. The plastic protrusions stop the gear moving up/down the shaft.

Will this be ok, or will friction be too high?
If the protrusion is nylon, will it be better?
Is a metal washer needed between the gear and the protruded surface?
Is a custom axle needed with "insets" for mounting circlips?

I'd like the simplest - cheapest solution

many thanks

Attached Images

gear_setup.jpg (212.2 KB, 29 views)

 

Your method will work if these gears are not spinning too fast. It really helps that the gears are nylon and if the spacers are nylon too, the friction will be much lower and wear less than if the spacers were metal since nylon is "self-lubricating" and is slippery against itself. A nylon washer between the spacer and gear might help, but it has to be nylon also- not metal.

If they are spinning fast, use two shaft collars on either side of the gear to keep it from sliding and grip on them to make enough force to make the gear spin with the shaft (you don't need too much grip as long as it is the shaft isn't transferring torque). Then mount the shaft on some ball bearings on both sides. THen the gear spins the shaft with it and the shaft spins on ball bearings. Of course, with this method only one gear can be used per axle since the grip might not be strong enough to keep the gear from slipping and all gears sharing an axle must spin at exactly the same speed.

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appreciate your reply.

the motor's rpm is around 8000.... do you think a nylon gear setup with nylon spacers will be alright?

i wonder how remote controlled cars set up their gear spacing?

thanks

 

I recall from my son's RC cars that E clips were used if friction wasn't sufficient. That requires a groove in the shaft. You might consider sandwiching the assembly between two plates rather than having the shafts cantilevered. This will help maintain shaft alignment, gear spacing and allow spacers on the other side.

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stevez

 

Hard to say. Instinct says thats far too fast and lubrication would definately be required.

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Where might i find images which show RC car gear construction as I think this suits the application. Unfortunatley I'm not somewhere with easy access to RC cars.

Would the lubrication be a one time application do you think, or need consistent re-applications?

sorry for so many questions - bit stumped how this minor detail is halting the whole thing.

 

I have posted another picture here, simpler, better describing my question.

Attached Images

gear_setup_2.jpg (144.3 KB, 7 views)

 

I'd lubricate it periodically. But at 8000 RPM the friction might be enough to melt the nylon.

http://www.tracgear.com/product/rc4w...on/R2/g2/1.jpg
http://tracgear.com/product/rc4wd/cu...2/g1/v2-05.jpg

And both ends of the shaft sit on ball bearings mounted on slots in the sides of the case.

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I think a close look at RC cars might help to answer the question. As already mentioned, I'd think at 8,000 rpm that the nylon would soften if not melt completely. Often something like this has a metal shaft with a gear pressed on - the metal shaft rotates in a metal bushing at the very least - if not a proper bearing.

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stevez

 

thank you for your answers, I'm a bit confused as I've seen other plastic gear applications at similar RPM's, please see here ;

http://www.robotshop.ca/home/supplie...r-gearbox.html

The motor is running at around 9000 - 12000 rpm, only heavily geared down with plastic gears.

How is this possible? Is it becuase POM is used for the gear material as opposed to Nylon?

thanks

 

You can look at some RC manuals here at this website.
http://www.tamiya.com/english/rc/manuals.htm

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The gears aren't rubbing against a nylon spacer continuously. THe only thing rubbing are the gear teeth as they come into contact and out of contact.

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hi dknguyen

if you see here, some gears are assembled directly onto shafts, just with extra grease. This is how my gears would be mounted, directly onto a metal shaft, pushed up against the plastic casing (similar to the below link)

http://www.elex.camosun.bc.ca/roboti...oxAssembly.pdf

I'm really confused now.