By Naoki Yamaguchi, Assistant Technical Manager, NB Corporation of America
As newer motion equipment demands more in terms of precision, the crossed roller bearing has gained prominence for use in friction reducing devices. Here’s a closer look at its features and suitability for your motion system design.
Rollers in a resin cage can be closer together. Cage creep can be prevented by studs that roll into the raceways depressions.
Linear bearings have evolved rapidly to carry heavier loads, take up less space, and deliver precise positioning reliably. In cancer research, for example, to find one cancer cell on a slide holding a billion cells, the microscope depends on sample positioning, bringing the images into focus and moving the proper filters into place to produce an image in three colors. The automated movements within the instrument work in concert with one another to take multicolored images of the cellular sample that is in the chamber. The linear bearings have to meet specific requirements to achieve perfect positioning. These are determined by the amount of space within the product envelope, distance to be traveled, load to be carried, and most critical of all, the degree of accuracy needed.
When referring to linear bearings, the reference usually is to devices that incorporate ball bearings – meaning friction is reduced between moving parts by recirculating (usually metal) balls. Bushings, ball splines, linear guides and slides typically incorporate recirculating ball bearings.
Resin retainer fits around the roller so the whole shape can be in contact with the load
As newer motion equipment demands more in terms of precision, another type of bearing has gained prominence in friction reducing devices – the crossed roller bearing. High-tech microscopes, wafer manufacturing equipment, robots and vision inspection systems increasingly rely on crossed roller bearings.
Crossed-roller slideways or bearings are the most accurate form of mechanical linear motion component. Also known as crossed roller slides, these bearings work similarly to ball bearing slides, except that those housed within the carriage are cylinder-shaped instead of ball shaped. The rollers crisscross each other at a 90° angle and move between the two parallel guides (alternately termed table and bed) that surround the rollers. The rollers are between “V” grooved bearing ways or raceways ground out of the guides. The travel of the moving guide/table, ends when it meets an end cap, a limiting component.
Crossed roller slides offer a line of contact, versus a ball bearing’s point contact. The line of contact creates a broader contact surface that can carry a heavier load. Such a feature offers more rigidity, less deformation, and thus more accuracy compared to the point contact of balls. Plus, due to the crossed rollers’ consistency of contact between the carriage and the base, erosion is slower. The recirculation of ball bearings creates vibration when the balls leave the load-carrying path to recirculate. Because most crossed roller bearings do not have recirculating components, this source of vibration is eliminated.
Load capacity: resin versus metal cages
Roller-to-rail contact determines load capacity. Of course, rollers provide a large contact area, since they usually do not recirculate they are all carrying the load, which produces greater rigidity as well as high load capacity. Balls typically contact at a single point, though a raceway’s surfaces can be designed as curves to increase the points of contact.
There is a direct correlation between the contact area of the crossed rollers and load capacity. Load capacity can be increased as much as 250% by designs with greater roller-to-rail contact. The closer together the rollers are, the more rollers can be fitted into the same space and the more weight per inch the linear device can carry.
How the cage holds the rollers also governs the amount of roller contact area per inch. Since the amount of space between the rollers is a factor in calculating the roller surface per inch available to carry a load, the design of the cage surrounding the rollers is important.
Traditional metal cages limit, some-what, the rollers’ contact area because of the way the cage holds in the rollers. However, the more recent development of resin retainers offers more design options. Resin retainers/cages can be shaped to allow a larger contact area with less space between rollers. They can also be thinner in critical parts.
Metal and resin cages hold the rollers in different ways. Metal cages hold the rollers through a notch on the top and on the bottom of the rollers. However, the resin retainer fits around the roller. The resin retainer is able to harvest the whole shape to be held. The whole shape can be in contact with the load because the resin retainer exposes the necessary contact areas. It doesn’t compromise the contact area when holding the rollers.
Rollers in a resin cage can be closer together. This increases the number of allowable rollers within the cage length. So either the cage can be shorter while maintaining the same load capacity, or while maintaining the same cage length the load capacity will increase with the increase in the number of rollers per cage. A resin cage can afford at least a 30 to 58% increase of the contact area as compared to a metal cage.
Metal cages are less expensive and can be all stainless or steel. Therefore they can be used in high temperature or medical applications involving water. Metal is also better suited in a vacuum since resin can have out-gas emissions.
Deeper V-grooves allow greater roller contact.
The longer the travel the longer the rail has to be. However, as opposed to recirculating-ball bushings, which require a shaft only as long as the travel distance, with crossed roller bearings, the whole rail assembly must be twice as long as the stroke. That’s because both rails containing crossed roller bearings move in opposite directions. So the whole assembly has to move within a space that is twice as long as the travel length. (The exceptions are in the minority of cases in which crossed roller linear guide products have recirculating crossed rollers or roller guide products that are not crisscrossed – having four circulations with opposite roller orientations. These products don’t need rails moving in opposite directions.)
With a resin cage, stroke length on a given length rail can be longer because the cage can be shorter for a given load.
The length of travel is limited by the space available for the rails within an application. Since the rails move in opposition to each other, the space required is twice the distance that the load will be carried. An endstop limits travel length. Because there is little or no difference between static and dynamic frictional resistances – even under low-load conditions – crossed roller bearings are well suited for minute motion.
For motion control applications with extremely fast acceleration and deceleration (at dimensions ranging from 30-600 mm lengths, 2-12 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 readjustment of the machine or installation. This affect often occurs as a result of high acceleration and uneven preloading or load distribution, as well as orientation (inclined or vertical) can easily cause creep.
A benefit of not recirculating is less frictional resistance fluctuation making them extre-mely quiet and smooth. Crossed roller bearings are almost as quiet as cage-limited non-recirculating linear bearings using balls.
Accuracy and deflection
Crossed rollers provide a large contact area, which reduces elastic deformation. Because of this greater stiffness, crossed rollers provide consistently precise movement.
The crossed roller is less forgiving of mounting surface inaccuracies because of the linear bearing’s rigidity and the way they are designed. They’re much less forgiving of an inaccurate mounting surface than recirculating ball bearings. For recirculating ball bearings precision, 5 – 10 microns of deflection is accommodated.
Because crossed roller bearings are so precise, they must have a very accurate mounting surface to fully ensure their acc-uracy. Often the bearings can be specified with mounting tables hone to exacting standards. This makes sense since for ultra precision, 2 microns is the maximum allowable deflection from the mounting surface.
Fit and interchangeability
The cage in any bearing prevents ball-to-ball or roller-to-roller contact, which can cause more friction and wear. Whether the cage holding crossed roller bearings are metal, resin or other material, it alters, somewhat, a crossed roller bearing’s dimensions, which affects how they fit into a design. Stoppers can also affect their size and therefore their fit.
Resin, as opposed to metal cages, allow the crossed rollers to be much closer together allowing the total number of rollers to be greater in the same space. Also, because of the additional contact area allowed by a resin cage, load capacity will be high, as well. In any case, it is prudent to contact the manufacturer and provide them with the required stroke length to accurately determine rail and cage lengths before ordering.
In addition to stoppers and the cage material and design, another factor that affects interchangeability is the design of the anti-creep mechanism. External anti-creep mechanisms often obviate interchangeability. Internal mechanisms are much more accommodating.
In non-recirculating linear components, the retainer floats between the rails and can drift (or creep) from center position. As the roller cage creeps away from center, it will begin to restrict the slide’s travel. It will creep over time if a full stroke is not used, especially when mounted vertically. If, after the retainer drifts, the full stroke is once again used, the retainer will hit the endstop of the rail and be forced to skid in order to center itself again. This action requires a strong motor and can damage the retainer, the rollers and the slideway. With higher preload, it is even more difficult and damaging to skid the cage back in place. Cage creep is damaging because every time it creeps, rolling ele-ments are not rolling, instead they are slipping and causing metal to metal rubbing, which will wear out elements very quickly.
Metal cages hold the rollers through a notch on the top and on the bottom of the rollers.
Anti-Cage creep mechanisms
The term “anti-creep” describes the method used to eliminate any slippage of the retainer holding the crossed rollers between the two V-grooved rails of the slideway.
By eliminating ‘creeping’ of the retainer, the slideway can be used in any mounting direction and with lower momentum motors, such as linear motors. Several complex anti-creep devices have been developed and manufacturers have used a few different approaches, such as a rack and pinion mechanism; an external attachment made of plastic gears outside of the rail; and a metal gear inside of the rail. Some of these devices are quite expensive.
There is one mechanism that does not use a gear. It is an anti-cage creep mechanism that uses a roller with round balls studded around its surface. It has the smoothest tracking motion and therefore is quieter than an externally attached toothed-gear type anti-creep device. This Studroller™ mechanism, prevents creepage because the raceway has depressions that track the studs or nodules, preventing slippage – in any position.
By placing studs in the center roller and machining a path along the rail, this retainer will never slip. It is suitable for high acceleration, vertical mounting and uneven load distribution. The Studroller™, is a simple non-slip design and uses a ball bearing as opposed to a gear or exterior control, so its cost is almost the same as a standard slideway; plus, there are no re-design costs to replace a standard slideway.
NB Corporation of America
Filed Under: Factory automation, Bearings, Mechanical, Motion control • motor controls