Travel length and limits

In a Studroller creep preventer, studs on a roller engage with raceway depressions. The design costs almost the same as a standard slideway —nearly half the cost of gear-based or exterior anti-creep devices — as there are no redesign costs for customizing the slideway.

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

Here, metal cages hold the rollers via notches on top and bottom. Metal cages are useful in high-temperature, washdown, and vacuum applications where resin cages deteriorate or outgas.

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.