How Do You Make Something Spin Freely? Replace Sliding With Rolling

A shaft turning inside a plain hole rubs against it, and that rubbing dissipates energy as heat, wears both surfaces and requires continuous lubrication to be tolerable at all. Interposing hard spheres changes the contact from sliding to rolling, and rolling resistance is far lower, typically by a large factor, because the surfaces at the point of contact are momentarily stationary relative to each other rather than sliding past. Energy is still lost, to deformation of the materials at the contact point, to the cage holding the spheres and to the lubricant, and the total is small enough that a well made assembly spins freely for a long time from a flick of the hand.

The assembly is simple and every component matters:

• •An inner ring, fitted to the shaft and turning with it
• •An outer ring, fitted into the housing and stationary
• •Hardened spheres running in grooves machined into both rings
• •A cage keeping the spheres evenly spaced so they do not touch each other
• •A seal or shield keeping contamination out and lubricant in
• •Grease or oil, which separates the surfaces with a microscopically thin film

Manufacturing tolerances in this field are far tighter than in most engineering, for reasons that follow from how the load is carried. The load passes through whichever spheres happen to be in the loaded zone, over contact areas smaller than a pinhead, so the pressure at those points is enormous and any sphere slightly larger than the others carries a disproportionate share and fails early. Spheres are therefore graded to tolerances measured in fractions of a micrometre and sorted so that a single assembly uses matched ones. The grooves must be equally accurate and must have the correct profile, since the contact shape determines how the pressure spreads. Surface finish matters because the lubricant film separating the parts is thinner than the roughness of an ordinary machined surface.

Spheres are one option among several and each suits a different loading. Cylindrical rollers contact along a line rather than at a point, which spreads the load over a much larger area and carries far more weight radially, at the cost of tolerating almost no thrust along the shaft. Tapered rollers are cones arranged so that they carry load both radially and along the shaft at once, which is why they appear in wheel hubs where both occur. Needle rollers are long thin cylinders used where radial space is tight. Spherical rollers are barrel-shaped and tolerate a shaft that is not quite aligned, which matters in large machinery where perfect alignment is impractical. Plain bearings, with no rolling element at all, remain the choice for very high loads at low speed.

The characteristic failure mode is fatigue rather than wear, which is why these components have a rated life expressed statistically rather than a wear-out point. Repeated loading of the same spot on a ring eventually produces cracks below the surface that grow and break out as flakes, after which the assembly becomes noisy and rapidly destroys itself. Ratings therefore state the load at which a stated proportion of a population survives a stated number of revolutions, which is a probability rather than a guarantee. Contamination is the commonest cause of early failure, since a hard particle rolled into the contact dents the surface and starts the process. Poor lubrication, misalignment, corrosion and electrical current passing through the assembly all cause characteristic and recognisable damage patterns.

Hard spheres between two rings convert sliding contact into rolling contact, which cuts friction by a large factor because the surfaces at the contact point are momentarily stationary relative to each other. Load passes through tiny contact areas at enormous pressure, so spheres are graded to fractions of a micrometre and matched within an assembly. Failure comes from fatigue rather than wear, and contamination is the usual cause of an early one.