Metal combinations for plain bearings

A while (2-3 years?) back I made some things called 'rollets' for a ceramics teacher at a local school. They are like single wheel knurling tools - but in this case used to emboss a pattern in leather-hard clay. They guy asked me to make them in brass because steel would rust, so that's what I did. 25mm dia brass disks, about 5mm thick bearing on a 5mm brass axle. They're beginning to fail because of bearing wear.

I'm proposing to bore and bush the wheels with leady bronze and use stainless steel (304) axles as an improvement. Questions are (a) would that be longer lived and (b) can anyone suggest a better solution?

Doesn't matter too much I suppose, the originals stood up to 2+ years of classroom use, but I want to learn more about this metalmangling malarkey which is why I take jobs like this on.

Rob.

I would make the rollets hard wearing and the axle a soft material rather than the other way around. I imagine the axle would be easier to replace?

Robin - rather than using bronze for the bushes I think I'd go for one of the plastics, ptfe, oilon, acetal, delrin, etc. so they can run un-lubricated. The operating environment for them is likely to be very abrasive with clay dust. The dust will tend to embed into the softer material and wear the harder one away so yes make the axles from stainless. 304 will be OK if you can get the diameter that you want, if you envisage machining the axles at all 303 is much easier to work with. The original combination of brass on brass isn't recommended, most bearing systems work much better with different metals for the axle and bushing. I think you did well to get 2 years especially if they were getting used much by youngsters.

There are lots of pairs of materials that work well together as a bearing, phosphor bronze and hardened steel work together and may suit you job. Phosphor bronze would have the corrosion resistance required but a hardened steel pin would be vulnerable to corrosion. Maybe a hardenable stainless steel would be suitable? Two corrosion resistant materials may be hard to find without using exotic materials like titanium. Would fitting an Oilite bush be a possibility?

Mike

I don't know if I'm right, but I feel like saying, polished stainless pin, with a PTFE bushing in the brass wheel.

Another option would be to turn a bore in the 25mm diam wheel to suit the OD of a fully sealed ballrace 12x5x5, the ID of the race could be 5mm to suit the existing axle, fit race to axle with Loctite or similar, you could also use the same fixing method on the OD of the bearing.

Emgee

Some years ago I had to design a pressure roller to finish form a radius on a cast iron part. I can't remember the the details but it used a hydraulic cylinder to push the roller into the part with about 15,000 lbf. The roller itself was about 6mm wide and 20mm diameter running on a fixed 6mm shaft. Both parts were made from dissimilar tool steels from Carrs in Sheffield. These parts produced 1000's of parts befor being replaced. Whilst this is rather more extreme than your use it shows that correct material combination may be better than trying to include a soft bearing material.

Maybe go back to a good old tight grained hardwood that has served craftsmen for years on their "rouletting" tools

Is the clay getting into the bearing and acting as a fine abrasive?
Putty would wear out my glazing tools in no time.

Leaded bronze sleeve in the wheel running on a high-tensile steel bolt for an axle would be an easy fix.

Stainless is not a great bearing material. The surface tends to gall and carry on sometimes.

I'd follow Hopper,s suggestion. A steel wheel should be ok, it will only rust if it's not used. Any grit that enters the bearing area may embed it's self in the softer of the two metals, and grind the hard part quite quickly.

Ian S C

I'd push the boat out and make a phosphor bronze wheel and stainless spindle.

Neil

I misquoted the suggested bearing size, I see 5x16x5 double sealed are £4.11 inc post for 10 on ebay so not much outlay for a long term repair.

Emgee

The problem with a staionary axle/rotating bearing set up is that instead of the axle bedding into the bearing and making a good bearing surface the bearing just gets bigger and bigger. That's why old fashioned wheelbarrow bearings lasted a long time, new ones don't, but they are so cheap no-one cares. I don't know what your set up is, but if you can have a fork arrangemnent with non rotating bearings and an axle rotating with the wheel it might be a lot better. Having said that almost anything will be better than brass on brass

Check out Igus plastic bearings. The tool is going to get wet so use a stainless steel shaft.

As said it's easy to replace the pin so if you make any more maybe use stainless steel for the wheel and brass for the pin? Just a thought.

As others have mentioned above, I would be concerned about the abrasive effect of the clay particles being embedded in a soft material and abrading the harder material in the bearing.

On a house move, we once acquired a pond including a pump to run a fountain. This ran extremely roughly - on investigation, it used an impeller using a stainless steel shaft located in cylindrical recesses in the thermoplastic enclosure. The plastic recesses were not significantly worn, but the stainless steel was worn down to near half of its original 1/4" diameter!

So don't expect SS to wear well if the application does allow clay slurry into a softer bearing!

Many years later, I still have 3 off 24V 50W pond transformers attached to 50W halogen machine lights (bought cheaply at the Midlands model engineering exhibition a few years ago) to light my machines. The pond pumps are history.....

Martin

Thanks for the suggestions, all of which are much appreciated.I hadn't thought of using a using a polymer bush, but as I have some PTFE I've used that on a brass axle. Easy enough to press it out and go down one of the other routes suggested if it doesn't work out.

I drilled/reamed the bore to 10mm in the roulette (thanks for the spelling correction Jason - seems that's standard now, tho I'm sure they were rollets in the early C18 source material the copied to me) and pressed the PTFE bush in. I'm sort of OK with metal to metal fits, but this was new to me so I just turned the PTFE to a tad above 10mm and pressed it in, then drilled and reamed 5mm for the shaft. Of course the bore was undersized - I suppose the PTFE relaxed. More to learn!

Emgee - thanks for taking the time to research ball bearings. I shall keep the idea in mind.

Rob.