A small Hardened drive shaft

**simmo** · 27th July 2015, 08:45 PM

Hi guys i'm working on a small home project and need a small hollow drive shaft , say 10 mm OD and about 40 or 50 mm long, with a bore of 5 or 6 mm.

My initial idea was to use SS or carbon steel heavy wall tube or even hydraulic/ instrument tube, it's available in those sizes, so dimensionally and strength wise its fine.

Now comes the tricky bit, it needs to be Rockwell C 58-64 hardness for about 15 mm on one end, and very smooth.

I'm investigating hard chroming say 4 thou or 0.1 mm, or case hardening.
Or uses 8 mm heavy wall tube, (4 mm ID), and press on a hardened sleeve

Is there a metallurgist or someone who knows a bit about the subject out there ?
cheers simmo

**POD** · 27th July 2015, 09:13 PM

Is there any reason why the whole shaft can't be hardened after being machined to size? Wouldn't cost much for something in that size, Hills Heat Treating in Melbourne is the only mob I know of but there might be someone closer to you (easy enough to post it anyway) if the shaft is made in a suitable, known material they will be able to harden it to your specs. How precise do the dimensions need to be? Particularly the bore?

**p38arover** · 27th July 2015, 09:19 PM

Being such a short length, it would be relatively easy to drill (in a lathe) through a length of 10mm silver steel rod. Silver steel is very easy to harden at home. http://www.homews.co.uk/page337.html

See https://en.m.wikipedia.org/wiki/Silver_steel

I used to use it to make lathe tool bits. It's precision ground for diameter and is easy to machine.

I also used to use it for propellor shafts in my model racing boats.

**Blknight.aus** · 27th July 2015, 10:20 PM

turn down a chisel with a good set of bits in a lathe?

Ive got to admit I'm almost curious enough to do it at work just to find out what its for.

the silver steel would be the best answer but then you'd have to have a lathe, if you had a lathe......

the real question is how are you going to test the hardness on the end if you do it as a DIY? Id reccomend something along the lines of making up the part ad then sending it to a small hardening business as mentioed previously and if you cant make it then just have the whole thing made from a bit of bar of the appropriate hardness.

Just a side thought.....

a lightened hollow or sodium filled valve from a performance engine might just be what you need.. hold the collet end in a drill spin it up and cut it with a 1mm wheel on grinder.

**Slunnie** · 27th July 2015, 10:33 PM

I would also be thinking along the lines of what Ron is saying.

SS is a royal pain to machine and mild carbon steel wont give you that hardness unless you go to a higher carbon content like Silver Steel etc. dia12mm Silver steel is about $50 per meter length. With the silver steel you can heat treat it though the full thickness, though I'm not sure if you will reach that level of hardness required. Likewise, with the case hardening, I don't think you'll get the level of hardness required as tested during Rockwell testing due to the supporting material although you may (doubtfully) get that hardness on the surface.

**p38arover** · 28th July 2015, 07:50 AM

Originally Posted by Slunnie

SS is a royal pain to machine

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

**cuppabillytea** · 28th July 2015, 08:08 AM

How much torque is going to be applied to this shaft?

**p38arover** · 28th July 2015, 08:17 AM

Good question, cbt. Also, why the need for a specific hardness>

**simmo** · 28th July 2015, 09:33 AM

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

**simmo** · 28th July 2015, 09:52 AM

The small shaft has to run as the inner surface for a roller bearing, NSK advised the hardness range, and the surface finish.

The shaft has to be hollow to allow some oil to pass through it, min ID say 3 mm.

Torque is not expected to be high, but the speed is up to 5000 rpm, also I want the shaft to have a controlled breaking section.

I'm designing a proto type for testing, and have started to gather the bits and make the drawings. Silver steel would be fine for a prototype but later if the design is functional, a more economical engineering material would be needed.

I think a common engineering grade steel that can be hardened to the required level I believe even 304 and 316 SS can be nitrided to achieve the hardness required. cheers simmo.

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