A small Hardened drive shaft

**p38arover** · 28th July 2015, 10:05 AM

Do you need a roller bearing for the prototype? My racing model boats used 5mm silver steel prop shafts about 200mm long running at up to 21,000 rpm from 1 to 1.5BHP. The bearings were either Delrin or phosphor bronze, one at each end of the prop tube. Lubrication was unburnt castor oil being taken from a tap on the exhaust.

**simmo** · 28th July 2015, 11:05 AM

Hi Ron, yes it definitely needs to be a specific roller bearing, and I'm aiming for a service life of 1000 hours @3000 rpm.

cheers simmo

**cuppabillytea** · 28th July 2015, 04:38 PM

Curiouser and curiouser

Could the hardened part be shrunk fit to the shaft?

**Blknight.aus** · 28th July 2015, 04:55 PM

Originally Posted by simmo

Hi Ron, yes it definitely needs to be a specific roller bearing, and I'm aiming for a service life of 1000 hours @3000 rpm.

cheers simmo

I''m not sure if it will help but you might be able to make your life easier if you go ad speak to the bearing place agai and discuss the options if you do an interference fit to the shaft and discuss various loctiting options as well.

bearings that sit in place like wheel bearings require a much stronger surface to sit on the ones that are press in fits like the layshaft bearings in gearboxes (which is in an ally housig) this might alleviate the need to harden up the shaft so much and give you more options.

Im really really curious because short of some printing and pressing gear I cant think of anything thats oil fed that size and runs that speed and has 1000 hours as the expected service life....

or are you looking at having the OD of the shaft be the inner race for the bearing? in which case

a, Brave man, kudos
and
b, this brings a whole new level of curious.

**simmo** · 28th July 2015, 09:04 PM

I think I'll end up going either trying to make a hardened sleeve and press it onto the shaft made of more common material. or go back to the bearing catalog and see if I can use bearing and press it onto the shaft.
I think i had best go to a heat treatment specialist in Brisbane and ask their advice, they probably have solved the problem before. one of the issues faced is getting people to sell you small quantities of things for prototyping. If you want a hundred or thousand of them it's Ok. Its a long way to go yet, build the prototype 1 st, then test for 1000 hrs on a test rig. thanks for your ideas . When i get it running on the test rig will be the most interesting to see if it can last the distance, I'll update. cheers simmo

**Slunnie** · 28th July 2015, 09:54 PM

Originally Posted by p38arover

Simon, I assume you mean stainless steel not silver steel in this context?

Yes, thats correct. Silver steel is fine as long as the tools are sharp and the material doesn't get hot.

**Slunnie** · 28th July 2015, 10:02 PM

Originally Posted by simmo

Thanks guys, your point about machining a small shaft on the lathe and material are well received. I did some research and there is heat treatment facility in Brisbane as well. The reason I chose tube is to minimize machining time.

Hardening the whole shaft might make it too rigid and lead to it cracking, and there is the problem that if you harden the surface and the substrate is too soft the hardened layer will de-laminate later, ( unless you make the hardening fairly thick) It looks like you need careful selection of the steel to be hardened, Silver steel is a good example, and would be a good choice.
Thanks for your advice. cheers simmo

If you're using Silver Steel, then you can harden it yourself with a gas flame.

Do the machining, heat it over a flame until it is cherry red for a while (to heat through the material) and then quench it by swirl it around in water. Thats all the hardening process is for that material. To reduce the brittleness and to try to prevent cracking you can reheat treat it by polishing it up and reheating it over the flame until the material heats to about 215 degrees which is when the metal turns to a a straw colour - don't go hotter! Then requench it immediately in water. This reduces some of the stress from the hardening process and takes out the brittleness.

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