The Difference Between Anti-friction Bearings and Hydrodynamic Bearings

August 6, 2019

Classed as rolling contact devices, anti-friction bearings operate on a well-recognized principle. Their rolling elements are in contact with one another. Set in motion, the lubricated rollers power along, and between, two rings while locked inside a caging mechanism. For hydrodynamic bearings, a whole other process is taking place. Instead of a group of solid rolling balls, there’s a stiff and unyielding pressurized fluid forming between this bearing type and its underlying sliding surface.

The Hydrodynamic Fluid Difference

That’s really the main difference, that anti-static devices use solid components. From the outer race all the way down to a spinning shaft, there are circular races and solid rolling elements in contact with one another. Fluid-based bearings replace hardened rolling components with a pressurized film. Given the need for a strong fluid force, which inserts itself between the outer ring and the inner shaft surfaces, some kind of a pump is needed to deliver hydraulic pressure to this rolling fluid film. Out with the breakable rollers, the stiff film supports heavy loads.

A Reordering Of Material Properties

In anti-friction bearings, it’s the material build that comes first, then the lubricant is added to the mechanism as a friction-inhibiting additive. Hydrostatic bearings turn the conventional order of things around. A powerful friction cancelling effect is still taking place in the device, but it’s the fluid characteristics that are responsible for the contact-less spinning action. Therefore, rather than look at the properties of an alloy-reinforced rolling element, designers are expected to assess the hydrodynamic qualities of different oils. The viscosity of the lubricating oil becomes more relevant, as does its filtration mechanism. Remember, the oil is flowing because it’s being driven by a pump.

Splitting the Behavioural Differences Apart

Friction develops between the point contact zones in anti-friction devices. This effect can become so strong that micro-welds develop between the balls. Worse still, overloaded rollers can cause physical damage. Race spalling is an example of this undesirable effect. Then there’s the fact that the rollers can deform when overloaded. Meanwhile, fluid systems also have weaknesses. Placed under an excessive load, hydraulic hammering and fluid cavitation repercussions attenuate hydrodynamic performance. There’s also the fact that fluid bearings can’t function without an ancillary pumping mechanism.

Conventional rolling elements and anti-friction bearings support moderately heavy loads and move at high velocities. There is, however, an at-rest resistance to overcome when employing them. That’s not an issue when using a similarly specced hydrodynamic bearing, for there are no weighty balls or rollers to give an initial push to, not when the interposing friction mitigator is fluid-based. Working on this principle, hydrostatic devices are used on slow-moving shafts, which provide inertia-less motion, unparalleled reliability, plus a fantastic amount of precision-based rotational control.

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