Metals for bearings

Aluminium in mild steel is about the worst combination I could envisage, especially with the possibility of dirt ingress. Poor friction coefficients, rapid wear and galvanic corrosion are just some of the issues. Hardened steel in aluminium bearings is ok if the system is enclosed and well lubricated such as inside an engine, camshafts and piston gudgeon pins being examples. King pins are usually case hardened steel pins in bronze bushings or roller bearings with dirt seals at each end and provision for lubrication via grease or oil nipples.

You'd never choose aluminium over steel for a load bearing axle or pin anyway. Why go for a soft weak material over a hard strong one?

-- Dave Baker

by "bearings" the OP has apparently selected the sub-set of bearings that all involve one plain metal or alloy running inside another plain metal or alloy, basic principle of this is to maintain a fairly regular difference in hardness between the two materials, eg if one is super hard the other must be hard, if one is hard the other can be soft, but the main point is off the top of my head I cannot think of a single plain bearing application that does not have some sort of lubrication, even if it is only a pickup ring in an oil bath and grooving...

for an adult trike why reinvent the wheel? why not simply specify off the shelf standard ball / roller / taper bearings, esp sealed ones.....

Buy a bicycle for $20 from a garage sale (perhaps carboot sale, given your spelling of "aluminium"), cut it up and use the headset and ball bearings it contains. It will be a lot easier to steer if it has good geometry and good bearings.

Or use steel rod in aluminum bearings, or bronze bearings, or nylon, or HDPE, or herdwood boiled in oil, whatever. Do NOT use aluminum shaft in steel bearing (bushing)

I use headsets..

Thanks for that. Can you explain why not aluminium shaft in steel bushing? Is there some rule of thumb that I'm not aware of? Or does something nasty happen that I've not thought of? I have rethought and will probably use aluminum in aluminum, anyway. The shaft will be someting like 1" in diameter. The bearing surfaces will be large and well lubricated with heavy grease with a cover to prevent ingress of dirt as much as possible. I need a lot of meat to screw the M12 HT bolt stub axles into it, and also insert the control arms. I've seen headsets used for the two front steering bearings. It just seems a rather complex way of doing what I want. I want to avoid welding if I can, and I want it to be light and cheap. These bearings don't need to be very free, in fact a bit of friction in them will be advantageous. Save me installing a steering damper.