March 2011
Setting any object in motion makes it necessary to transfer power and overcome friction. For millennia mankind has used rollers and slides for this purpose. But how do modern linear guides actually work?
Linear technology is the interface between stationary and moving parts. The function is to transmit power, to guide machine components and to position them exactly. Linear guides can essentially be classified according to the nature of the movement and the contact area. Thus movement can be along an axis or rotate around an axis – referred to as linear and rotary guides. 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. The latter is, however, a special case and hardly ever used in practice.
Rolling guides
Rolling guides without (fig. A) and with (fig. B) rolling element recirculation. (1 Carrier block 2 Rolling elements 3 Guide rail)
In everyday life rolling-element guides using profiled rails are the standard choice for linear motion and will be found in the majority of all applications. The rolling elements are balls or rollers and transmit the forces from the carrier block to the guide track. The 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, thus increasing the contact area and enhancing load distribution.
(Fig. C) Stress distribution for osculating contact areas. (Fig. D) Contact areas for balls and rollers at increasing loads.
The contact surface for rollers is far larger than that for balls. Consequently rollers can transmit far higher loads. In turn, they are able to achieve far more compact designs with the same load levels. The rolling elements move on a lubricating film in order 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
Structure of a ball guide as an example of the newest generation of profiled rail guides.
In addition to reduced rolling friction, profiled rail guides are also distinguished by superb precision. Their high load-carrying capacities and great stiffness qualify them for almost all tasks associated with precise linear motions. They comprise a profiled guide rail and a carrier block made up of the 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 either 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. Specialties in the Rexroth product line are the ball and roller guide rails with an integrated, direct, inductive longitudinal measurement system. This concept joins the “guidance” and “measurement” functions in a single unit and gives the mechanical engineer new options in machinery design.
Selecting the appropriate linear guide
Selecting the ideal profiled rail guide in accordance with the use properties.
Selecting the ideal guide will be influenced by many parameters. Among them are the environment where they are to be used and the task to be fulfilled. Essential criteria for the users are precision, loading capacity and running speed. The classification of carrier blocks and rails according to the precision level is especially important. Tolerances between the rail and the block will typically be in a range of from ± 5 to ± 120 micrometers. Quick assistance in selecting the correct guide is available from Rexroth online. A simple form asks questions about the properties needed – such as travel speed, acceleration, temperatures, loading and dirt and corrosion. The visitor will obtain a listing of suitable products with just a few mouse clicks.
www.boschrexroth.com/brl-selectionguide
Additional information on the fundamentals will be found in the “Linear Technology Handbook”, available at no cost in both German or English at the following Web address: www.boschrexroth.com/handbook_LT
More information:
www.boschrexroth.com/brl-selectionguide
www.boschrexroth.com/handbook_LT
Contact:
drive-control@boschrexroth.de