Metal combinations for a plain bearing

A while ago I made an embossing tool for a friend who is a ceramicist. This is one of my efforts:

The metal bits in the MKI were all brass (CZ121) and when it came back with too much wear to be useable I just made a new brass pin and bushed the wheel with PTFE (not quite sure what informed that choice!)

It's back again now.

Can anyone suggest a more durable combination of materials? It has to be stainless or non-ferrous as it works in a wet environment, and of course clay slip is going to find its way in.

Apparently some historical examples of these things used a 'cone and socket' type of bearing so wear could be accommodated by pinching the yoke of the holder in. Might that be a better approach? Not sure if I could do it though....

Robin

The real issue is the clay slurry getting into the bearing. Sealing it with some silicone tubing on either side might be the answer. Clay particles are very hard. So no bearing material will cope with that sort of abrasion.

A stainless steel pin with bronze bushing in the wheel would be more durable than brass on brass. Brass on brass is never a great bearing combination.

As said, the clay slurry is the killer.

Is it possible to use the plain shank of a carbide drill as the shaft ?

Then 'throwaway' bushes made of virtually anything else.

Years ago I had to design a tool for pressure forming a radius on a cast iron part, the form wheel was about 20mm diameter and had to roll with around 2500psi applied pressure. Both the wheel and pin were made with heat treated steels from Carrs Steel. I don't know wether they still exist, I suspect they do. They supply special steels to the tooling industry, mounlds etc. Give them a call, I found they were happy to give you the best material suggestion and are happy to supply small pieces. My first material selection, using tool steels of may selection lasted for a couple of dozen parts, their selection lasted thousands with not determined wear.

Do miniature sealed ball bearings go down to a small enough size?

Brian

I agree with Hopper - its how I would do it.

Rik

I thought that was worth looking up...available smaller than i thought in stainless (0.6 x 2.5 x 1 mm) but get a little bigger before fitted with shields or seals. **LINK**

You can buy pretty stiff stainless bone pins down to these sizes as shaft material so it could be entirely practical to have easy replacement shafts and bearings. Hopper's approach is nevertheless simplest.

pgk

As has been said, the real problem is the clay slurry getting into the bearing. The particles imbed themselves into the softer material and then act as a lap on the harder part. Can your design be made to incorporate a V-Ring seal each side, this may give you a fighting chance for a longer life in such an aggressive situation. Simply Bearings sell these in quite small sizes. Might be worth considering.

Neil

I don't think any seal will last long in this environment. You need two metals which will effectively act as stone crushers on the small particles which make up the clay. So hardness depth would be critical, probably a through hardened steel for the roller and a slightly softer through hardened pin, the pin being sacrificial to some degree.

Wooden bearings?

Martin.

Thanks to all for replies. I sort of suspected that given the hostile environment I had an ongoing job here - I think I might just have to design the tool so that the pin/bush are expendable and as easily replaceable as possible. So next version will be stainless for the pin, phossy bronze for the bush in the hope that will last longer.

I'd though about miniature sealed bearings, but my gut feeling is that the clay will find its way in, and it'll just be more hassle and expense to replace.

Martin - thanks for the wooden bearings link! That might just work... back in the day (C18) the wheels were often made of wood I think - I'll follow that up. Maybe there was good engineering reason!

Robin

Maybe you can modify the design so your friend can replace the bushings and pin themself ? That way you could make extra parts while you are set up for it .

Wooden bearings in a hostile water based environment was my thought

lignum vitae being numero uno

You can find it on a well known auction site, old bowing balls are made of it for example

Water is one thing - it lubricates lignum vitae bearings - but grinding paste is another, so I would not consider it.

A single sealed bearing in the wheel is the obvious choice. No need for pin renewal if the shaft is a press fit (or at least non-moving) in the housings, so just another relatively cheap bearing change when the seals finally expire. A suitable grade of loctite could secure the bearing to pin and bearing in the wheel housing, so it is reasonably easily extracted for a bearing change.

Press out pin, replace bearing and pin. Job done. Cost will likely be far less than continued downtime for repair and a spare bearing can be held on stock. I would suggest a screwed pin with a close fit at the head end would retain the pin sufficiently rigid to avoid any abrasive wear to it

Have the bearings on either side of the "yoke" so they are further away from the slurry, use small rubber sealed ball bearings?

I seem to remember some model railway wagons used 'needle-point' bearings, steel axles with pointed ends running in brass cups. If suitably designed with a wide angle cup and narrow angle spindle it should be easy to wash out the clay slurry under running water before it does any harm; I assume there would be a tap in a ceramics workshop. Obviously the spindle would be fixed in the wheel.

It should be possible to make the bearings adjustable with a simple screw arrangement.

Mike.

I think Farmboy has the right idea, this cone type bearing arrangement is used a lot on toy gyroscopes. The spindle is fixed to the wheel and the outer bearings are adjustable screws, easy to take apart. I would not use a centre drill for the cone as it will give you a hole where the slurry will collect. You could also harden the parts as HOWARDT recommended. I also think the suggestion of washing it after use will help extend its life. It's going to have a hard life what ever you choose to do. Let us know how you get on.

Neil

I knew I'd seen them on something else, but couldn't think where . . .

Thanks for your further ideas all. I'm now thinking that having the shaft fixed to the wheel and running in cone bearings in the 'yoke' is a more sensible design - yes, like those toy gyroscopes. There are some constraints because the wheel has to be applied towards the base of the ware, which is still mounted on the potter's wheel, but that's not insuperable, just needs a rethink of the holder design. The current arrangement is 'historical' in that my friend first came to me with a request to make just the wheel (he'd had one 3D printed, but it didn't work) to fit a holder he'd made himself, so I just made him the wheel and a nicer holder to the same general pattern.

Maybe it would be possible to use hardened steel components. I think my friend has just assumed that that steel is a no-no because of the water - but maybe a swill with water then a spray with WD40 (there's got to be some use for the stuff!) would be OK.

Robin