What shaft material works?
The shaft has a big effect on plain bearing life, because it is the other half of the sliding pair. Cold-rolled carbon steel is a suitable mating surface for polymer bearings. Ceramic shaft surfaces induce more wear, though they are sometimes chosen for their ability to stand up to harsh environments. Aluminium shafts are light and easy to machine, but they wear the bearing faster, and anodising helps only slightly. Wear rates from different shaft and bearing pairings can differ by as much as a hundredfold (Design World), so the pairing is never a detail.
How hard should the shaft surface be?
Hard enough that the bushing cannot wear it. Core hardness and surface hardness both matter, but the surface matters most, because that is the layer in contact. If the surface is softer than about 30 HRC, a metal bearing or even a plain bushing with a high Shore D rating can damage the pin and make the bearing fail early.
A field case shows what that looks like. A tillage equipment maker tested a field cultivator pivot at 5,000 psi (about 34.5 MPa) per bushing, on a cold-drawn 1045 steel shaft of about 20 HRC. The bushings failed, and on inspection they had worn into the shaft. Hardening the shaft surface gave much better results.
For outdoor places such as agriculture and construction, where abrasive dirt is always present, a surface hardness of at least 50 HRC is a good target, because hard grit then has a harder time scoring the shaft. Three ways to get it:
| Method | Note |
|---|---|
| Carburizing (case hardening) | No corrosion protection |
| Hard chrome plating | Good surface hardening, with good corrosion resistance. An extra step adds more |
| Nitrocarburizing | Hardness and corrosion protection in the same process |
How smooth should the shaft be?
Not too smooth and not too rough. When a self-lubricating bushing first runs, material from the bushing and its solid lubricant transfer into the valleys of the shaft surface, and after the break-in this gives a stable, low-friction running surface. If the shaft is too smooth, the transfer cannot take place and friction rises through adhesion, and a mirror finish also invites stick-slip (K02-Q1-13). If it is too rough, the surface acts like a file and removes too much material.
The numbers differ from maker to maker. Some recommend Ra 0.2 to 0.4 µm and warn that a very low roughness can cause stick-slip and very fine nicks in the plastic. Others suggest Ra 0.2 to 0.8 µm, and some give 16 to 64 µin RMS for oscillating or pivoting use and 8 to 16 µin RMS for linear use. Taken together the range is of the order of Ra 0.2 to 1.6 µm, which is why the datasheet for your grade decides. Mind the units: 16 to 64 µin is about 0.4 to 1.6 µm, and 8 to 16 µin is about 0.2 to 0.4 µm. RMS and Ra are also different scales, with RMS reading about 10% higher than Ra for the same surface, so do not swap one for the other.
What about stainless steel and corrosion?
Corrosion resistance matters alongside hardness and roughness. Hard chrome and nitrocarburizing both give some protection, and carburizing gives none, so a carburized shaft in a wet place needs another protection.
Stainless steel is the obvious answer in a wet place, but it brings a hardness trap. The common austenitic grades, such as 304 and 316, cannot be hardened by heat treatment and are normally much softer than a hardened steel shaft, so they can fall below the 30 HRC line above. Where wear matters, stainless shafts are specified in a hardenable type or with a surface treatment, and the hardness is read from the shaft supplier's certificate, not assumed from the grade name. See K01-Q1-07 for corrosion in a wet place.