Linear bearings - from Pyramids to Levitation

A survey of linear bearings "From the Pyramids to Magnetic Levitation" by H G Stevens Chairman, Unimatic Engineers Ltd.

We will probably never know when exactly one of our forebears dropped the carcass of a deer, or babysaurus, on the broken-off branch of a tree for the very first time, but that was almost certainly the first linear bearing! The coefficient of friction was high, except when the sub-surface was snow and ice.

By the time the Egyptians built the pyramids, linear roller bearings had been invented.

Rollers were put under the pre-cut stone blocks, but loads could only be moved slowly because the rollers move at half the speed and through half the distance (you can check this out by rolling a pen or pencil between the two palms of your hands).

So, to move the blocks through any distance, one-by-one, the rollers popping out at the trailing edge of the load had to be manhandled to the front to be re-inserted.

In effect, the Recirculating Roller Bearing had been invented! It would, however, be a few more thousand years before this idea re-appeared as a practical engineering device in the form of the "roller shoe" for reciprocating linear mechanisms.

Two main problems had to be overcome: the provision of hardened and ground steel ways, and a mechanism which ensured that the load bearing rollers remained parallel to one another to inhibit crabbing and lock-up.

Incidentally, the Egyptian "recirculating-log" roller bearing anticipated not only the modern rotating ball bearing, but it may have even preceded the invention of the wheel.

The wheel of course had a central bearing and the next step forward was lubrication in the form of animal axle grease.

But the concept of precision, the next important feature, had to await the making of clocks, navigational instruments and, finally, the discovery of steam power.

Here the need arose to translate the linear motion of a steam cylinder into rotary motion, and linear bearings in the modern sense were born! Cast iron slideways and lots of lubrication did the trick.

For ship propulsion these soon became quite large, accurately machined, and brass or bronze were added to the engineer's armoury (some of this knowledge was derived from gun-making and I suppose a cannon ball shot out of a barrel is also a linear motion device!).

But the railway-age, and the industrial revolution generally, spurred human inventiveness to create ball bearings, and, for heavy loads, rotary roller bearings.

However, many decades had to go by before the same technology was applied to linear motion, and only when the "limited stroke" Ball Sleeve began to replace sliding bronze bushings was the way open to the Recirculating Ball type Linear Bush, capable of moving at high speeds along hardened and ground steel shafts.

Although there are some special linear and rotary balls, the standard type is not free to rotate.

Accordingly, a complete ball-bush type linear bearing usually involves two accurately aligned parallel shafts on which two or more Ball Bushes will move in unison.

A typical linear bearing carriage will incorporate four bushings, two to each shaft.

Shafts do, of course, bend, flex and vibrate and so, for greater rigidity, they are often continuously supported.

Open type ball-bushes must then be used.

These are usually supplied in adjustable mounts, or housings, either singly or in tandem, in twin units.

For high precision applications and for higher loads, the cross-roller type linear bearings were introduced in the 1950's.

Here cylindrical bearing rollers, whose axial length is fractionally less than their diameter, move between hardened and precision ground V-track rails.

These had the advantage that they could be pre-loaded (usually by lateral pressure-screws), but they only worked well when mounted on rigid accurately machined beds or assembled into "zero-friction" sliding tables.

For higher loads needle-rollers held in nylon or brass cages which could be joined together are also available.

Here, the hardened and ground track rails have a "male" and "female" V-configuration.

Engineers also often designed rectangular-section linear tracks with cam-followers moving along two or more accurately machined track-rail faces.

The recirculating roller "shoes" mentioned earlier were used in a similar manner where loads were too great for cam-followers.

But crabbing and skidding of rollers can occur, especially at higher speeds (when rollers skid the units not only need replacing in the field, but they may also have damaged the precision track on which they run).

Absence of even minute vibrations is essential for linear grinding machines.

This led to the development of hydro-static slides, where oil is pumped at high pressure into the gap between mating elements of accurately machined slides which are now also available commercially in modular units.

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