I Have a pinion that is made from the equivalent of 4340 steel (BS EN24). This will be hardened to about 50-52HRc to reduce wear from the gear that it runs with. The pinion runs on a shaft made from drill rod/Silver steel in a splash lubricated gearbox. Should I harden the shaft or leave it soft?
For context, it's one of the pinions/shafts in the apron gearbox of the Hardinge HLV that I'm rebuilding. The original gears were pretty well buggered.
Neither material fits the description of embedable or conformable that is preferred for a bearing metal. On the other hand I hope that I can keep the oil fairly clean (unlike the previous owners, who used it as a polishing lathe)
Harden it. The best and longest-wearing bushing is hardened steel on hardened steel. It also requires the best finish and the best fit, because it won't conform to anything.
Some of the worst are soft steel on soft steel. Hardened steel on soft steel is only slightly better.
Before ball bearings were common, 10,000 rpm internal grinders used hardened-steel on hardened-steel bushings. They also used no oil, but that's another story.
Ok, It'll get hardened and tempered to about 64HRc and then polished then. I wasn't sure, because I was under the impression that one should run soft against hard, but that didn't seem to make sense with steel on steel.
The "soft against hard" rule of thumb is based on the idea that you want the bushing to wear in preference to the journal. The reason soft steel makes a poor bearing is twofold: its static coefficient of friction against steel is very high; and it tends to gall. When you have high static (startup) friction in a gall-prone material, you typically have short bearing life.
The reasons hard steel makes long-lasting bushings is, again, twofold: it resists galling, and it resists contact wear. If steel contacts steel, you'll still have a high coefficient of friction from any contact. The hardened steel just resists galling better. The hard and polished surfaces you can obtain with steel resist contact wear, but it still has high friction if there's any contact. Lubrication is very important with any steel-on-steel bearing for these reasons. The advantages of a hard steel bearing occur mostly at high relative speeds.
The very high-rpm "no-oil" bearings I was talking about actually were aerodynamic bearings. They operated with a film of air as lubricant, which the journal maintained by dragging air around with it as it spun (thus, the "dynamic" part. Aero*static* bearings maintain the bearing with high-pressure air supplied by a pump). They were very tricky to make in such a way that they'd maintain the air bearing but there was little choice at the time. They did start up with contact and that required very hard and well-finished steel to resist galling upon startup.
Machinery's Handbook contains excellent info on plain bearings but it doesn't cover steel-on-steel, except with one general note about the high coefficient of friction. At least, that's true of the 26th Edition. Too bad, because it's an important issue.
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