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The rollers crisscross each other at a 90° angle and move between the two parallel guides. The rollers are between "V" grooved bearing ways or raceways ground out of the guides.

Roller-to-rail contact is key to determining load capacity. Of course, rollers provide a larger contact area than ball bearings and, since the rollers usually do not recirculate they are all carrying the load, which produces greater rigidity as well as higher load capacity than ball bearings.

Load capacity correlates with contact area. So, the amount of space between the rollers is a major factor, making as much as a 250% difference.

Metal and resin cages hold the rollers in completely different ways. Metal cages hold the rollers via a notch on the top and on the bottom of the rollers. However, the resin retainer fits around the roller.

Rollers in a resin cage can be closer together. A resin cage can afford at least a 30 to 58% increase of the contact area as compared to a metal cage.

Rollers in a resin cage can be closer together.

Metal cages are less expensive and can be all stainless or steel. Therefore they can be used in high temperature or medical applications where there is a lot of water and rust potential. Also, resin can have out-gas - causing problems in high vacuum environments.

With crossed roller bearings, the whole rail assembly has to be twice as long as the stroke. That’s because the rails move in opposite directions. (However, a few crossed roller linear guide products have recirculating crossed rollers that are not criss-crossed - having four circulations with opposite roller orientations.)

With a resin cage, stroke length on a given length rail can be longer because the cage can be shorter for a given load. Because there is little or no difference between static and dynamic frictional resistances - perfect for minute motion - even under low-load conditions.

Resin retainer fits around the roller so the whole shape can be in contact with the load.

For motion control applications with extremely fast acceleration and deceleration (at dimensions ranging from 30 to 600 mm lengths, 2 to 12 mm rollers) endurance can be 150 million cycles.

In crossed roller bearings without anti-cage creep mechanisms, cage creep may necessitate the replacement of guides and readjustments. This affect often occurs as a result of high acceleration and uneven preloading, as well as orientation.

Crossed rollers larger contact area provides consistently precise movement and non-recirculation has less frictional resistance fluctuation making them extremely quiet and smooth.

The crossed roller is less forgiving of mounting surface inaccuracies because of the linear bearing’s rigidity and the way that they are designed. Often the bearings can be specified with mounting tables hone to exacting standards such as ultra precision, where 2 microns is the maximum allowable deflection.

A cage prevents the wear of ball to ball or roller to roller contact. Whether metal, resin or some other material, the cage alters, somewhat, a crossed roller bearing’s dimensions. Also affecting interchangeability is the design of stoppers as well as the anti-creep mechanism. External anti-creep mechanisms often obviate interchangeability. Internal mechanisms are much more accommodating.

The term “anti-creep” is used to describe the method of eliminating any slippage of the retainer holding the crossed rollers between the two V-grooved rails of the slideway.

There is one anti-cage creep mechanism, the Studroller (patent pending) that uses a roller with round balls studded around its surface. By placing studs in the centre roller and machining a path along the rail, this retainer will never slip. It has the smoothest tracking motion and therefore is quieter. It is suitable for high acceleration, vertical mounting and uneven load distribution.

Since the Studroller uses a ball bearing, as opposed to a gear or exterior control, its cost is almost half the cost of other anti-creep devices plus, there are no redesign costs to replace a standard slideway.

This article was contributed by NB Corporation.

www.nbcorporation.com

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