Most linear bearings incorporate ball bearings (usually of metal) to reduce friction between moving parts. Of these designs, most bushings, ball splines, linear guides and slides recirculate balls.
As evidenced by their ubiquitous use, recirculating ball bearings have many advantages, including unlimited travel and low cost. However, they are limited in life and load capacity: Though their raceway surfaces are sometimes curved to increase contact, most ball bearings have only point contact with races, so can only carry limited weight. Oscillations and noise are other issues — as friction fluctuates and the balls excite resonant frequencies when they leave the load-carrying path to recirculate.
In contrast, crossed-roller bearings that use cylinder-shaped rollers are more accurate and rigid mechanical linear components. Also known as crossed-roller slides, in these mechanisms, cylindrical rollers are sandwiched between two parallel guides called the table and bed. Here, the rollers nest in V-grooved raceways ground out of the guides, and crisscross at 90° angles.
The rollers make lines of contact that are broader than a ball bearing's point of contact. In addition, most rollers do not recirculate, so their motion is smoother than that of designs with recirculating balls. Deflection under load is reduced and rigidity increased; consistent (slip-free) contact between carriage and base also reduces wear.
Cages and load capacity
Crossed-roller linear bearings do require cages to keep their rollers evenly spaced and prevent jamming. In fact, the cages in any bearing prevent ball-to-ball or roller-to-roller contact, which can increase friction and wear. Various designs exist: Most are either metal or plastic.
In bearings with metal cages, the cages have tabs that pin the rollers at each end, to secure them. What's the drawback? The rollers must be spaced more widely than in other designs, and that reduces load-carrying capacity.
Even so, metal cages are less expensive and can be plain steel or stainless — useful in high-temperature, washdown, and medical applications where rust is unacceptable. Metal is also more suitable for vacuums, as resin can outgas here.
Resin cages envelop the rollers in shaped pockets while leaving working areas open to allow the rollers to contact the bearing table and bed. This cage design allows rollers to be spaced more tightly, for at least 30 to 58% increase in contact area as compared to that associated with metal cages — allowing for more rollers per inch — and an increase in load capacity of up to 250%.
No matter what type is used, all cages effectively float between the bearing's rails, and so most need a mechanism to prevent cage creep: This is the tendency of the roller cage to drift from the bearing's longitudinal center.
In short, a cage creeps over time if the linear bearing only makes partial strokes, especially when mounted vertically. Then the cage can restrict slide travel — because once the bearing makes its next full stroke, an off-center cage will hit a rail endstop and is forced to skid, to center itself again.
The forces associated with this impact require a strong motor, and can damage the retainer, rollers, and slideway. (With higher preload, it is even more difficult and damaging to skid the cage back in place.) What is more, every time a cage creeps, rolling elements are not rolling, but slipping, and causing metal-to-metal rubbing, which causes wear.
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 often occurs as a result of high acceleration and uneven preloading or load distribution, or inclined or vertical orientation.
Anti-creep mechanisms eliminate any slippage of the retainer by holding the crossed rollers between the two V-grooved slideway rails … allowing slideways to be used in any mounting direction, and with lower-momentum motors such as linear motors.
One rack-and-pinion mechanism uses external plastic gears and another metal gear inside the rail. Though effective, these designs obviate interchangeability and are costly.
Another mechanism uses a roller with round balls studded around its surface: Depressions in the raceway track the nodules to prevent slippage in any position. This arrangement has smoother tracking motion than gear-based designs, so is quieter and more accurate. The studs are on a center roller and ride along the rail's middle, so this retainer also never slips.
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Travel length and limits
One distinct advantage of recirculating ball bushings is that the shaft must only be as long as the required travel, because only the bushing moves.
In contrast, with a crossed-roller bearing, the rail assembly (and workspace) must be twice as long as the application's travel length — because the two rails that contain its rollers move in opposite directions. (There are rare exceptions: A few crossed-roller linear guides recirculate crossed rollers. In other products, the rollers aren't crisscrossed, so have four circulations with opposite roller orientations.)
The length of travel is, first of all, limited by the space available for the rails within an application. Because 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. They can only extend (in opposite directions) until they hit the endstop. This makes crossed-roller bearings unsuitable for applications that require long strokes. However, 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.
Wear, smoothness, accuracy
For motion control applications with extremely fast acceleration and deceleration crossed-roller bearings (at dimensions ranging from 30 to 600 mm long in 2 to 12-mm rollers) can last 150 million cycles. Crossed-roller bearings are almost as quiet as cage-limited non-recirculating linear bearings using balls.
Crossed-roller bearings are inherently accurate but their rigidity makes them less forgiving of mounting-surface inaccuracies. Recirculating ball bearings accomodate 5 to 10 µm of deflection. In contrast, crossed-roller bearings are specified with mounting tables hone to 2 µm (or better) of mounting surface deflection.
Consult with manufacturers about stroke length before ordering. Stoppers, cage design, and other factors affect requirements. For more information, call (800) 521-2045 or visit www.nbcorporation.com.
Application: Cancer research
In the medical field, automated microscopes are used to examine collected samples of cells for signs of cancer. One slide may hold a billion cells, so samples must be precisely positioned and brought into the microscope's focus.
Coordinated microscope movements work in concert to take images of the cellular sample in the chamber, using filters to produce cell images in three colors.
Linear bearings are used here, because they are compact, travel set distances, carry the loads involved, and most critical of all, deliver extraordinary accuracy.