Back to Basics: Motion, guidance, positioning

Setting any object in motion makes it necessary to transfer power and overcome friction. For millennia humankind has used rollers and slides; modern technologies are much better. Here’s a quick guide to types of linear guides.

09/16/2011


Structure of a ball guide illustrates the newest generation of profiled rail guides. Courtesy: Bosch Rexroth AGLinear technology, the interface between stationary and moving parts, transmits power, guides machine components, and positions them exactly. Linear guides can be classified according to the nature of the movement and the contact area. Movement can be along an axis (linear guide) or circle an axis (rotary guide). The nature of the contact point defines the subdivision into rolling, sliding, and magnetic guides. Rolling element guides are particularly fast and will tolerate high loads, while sliding guides have excellent damping properties, and magnetic guides are distinguished by long service lives. Magnet guides are special cases and rarely used in practice.

Rolling guides

In everyday life rolling-element guides using profiled rails are the standard choice for linear motion and will be found in the majority of applications. Rolling elements are balls or rollers and transmit the forces from the carrier block to the guide track.

Rolling elements recirculate inside the block, in the direction of its travel. The guide grooves exhibit an arc-like profile. The balls “hug” the edges, increasing the contact area and enhancing load distribution. Because the contact surface for rollers is far larger than that for balls, rollers can transmit far higher loads. In turn, they can achieve far more compact designs with the same load levels. Rolling elements move on a lubricating film to reduce friction. Beyond that, the lubricant protects the metallic components against corrosion and extends the service life of the linear guide.

Profiled rail guides

Rolling guides without (Figure A) and with (Figure B) rolling element recirculation. 1) Carrier block, 2) Rolling elements, 3) Guide rail. Courtesy of Bosch Rexroth and design hoch drei GmbH & Co. KGIn addition to reduced rolling friction, profiled rail guides are also distinguished by superb precision. High load-carrying capacities and great stiffness qualify them for most tasks associated with precise linear motions. They comprise a profiled guide rail and a carrier block of individual components. One major component is the carrier block body with its hardened raceways, since the rolling elements circulating inside the block transfer the motion-related loads from the block to the rail. There is a wide variety of profiled rail guides. Depending on the rolling elements used, they may be ball rail systems, roller rail systems, or cam roller guides. They may exhibit any of many combinations of narrow, long, or tall carrier blocks; they may have two, four, or six rows of rolling elements, and may be in an X or O configuration. Ball and roller guide rails that integrate a direct, inductive longitudinal measurement system join the “guidance” and “measurement” functions into one and give the mechanical engineer new options in machinery design.

Linear guide selection

Figure C: Stress distribution for osculating contact areas. Figure D: Contact areas for balls and rollers at increasing loads. Courtesy of Bosch Rexroth and design hoch drei GmbH & Co. KGMany parameters influence selecting the ideal guide, such as application environment and task to be fulfilled. Essential criteria for the users are precision, loading capacity, and running speed. Classification of carrier blocks and rails according to the precision level is especially important. Tolerances between the rail and the block will typically range from ± 5 to ± 120 micrometers. Online guide selection assistance is available. A form asks questions about properties needed: travel speed, acceleration, temperatures, loading, and dirt and corrosion, leading to a list of suitable products.

- Kevin Gingerich is manager, communications and eBusiness, Bosch Rexroth, www.boschrexroth-us.com. Edited by Mark T. Hoske, CFE Media, Control Engineering, www.controleng.com

Machine Control Channel 

Bosch Rexroth Linear Technology Handbook