Linear Guides Provide Smooth, Precise Positioning

Engineers have a bewildering array of choices for linear guides. Does the application require round rails or profile rails? Should the sliders ride on plain bearings or ball bearings?

There are many variables to consider when specifying linear guides. These variables include load, accuracy, rigidity, smoothness, speed, acceleration, deceleration, travel length, envelope, environment, duty cycle, life expectancy, installation and maintenance. Even the product to be moved is important. For example, a plain bearing made from a self-lubricating polymer is good for food-handling applications, because it withstands cleaning and won't contaminate the food.

Of these variables, the load is the most important. When specifying a guide, engineers will want to state the actual load, not the load plus a safety factor. They’ll also need to share the dimensions of the load, which will determine how many rails and sliders will be needed to support it.

Knowing the load helps to select the bearings. Plain bearings require more force to move than ball bearings, so they’re limited to supporting small loads. They’re good for short-stroke, high-cycle applications. Ball bearings are good for long-stroke, high-speed applications. Rollers spread the load over a larger contact area than ball bearings, so they can carry heavier loads.

The load is important when choosing between round rails or profiles, but it's not the only factor. Profile rails are flat on the top and bottom. The sides are concave with ball-conforming races where the bearings roll. This stiffens the rail and enables the guide to handle heavy loads. Round rails can be mounted with end supports, continuous support or no support. They are smoother and more forgiving than profile rails, but not as accurate. Profile rails are more rigid and accurate than round rails, and they’re more resistant to roll moments. However, profiles must be mounted to a highly flat surface with straight reference edges, so there are no induced loads on the carriages.

Travel length is another consideration. Is a standard length available to meet your need? If not, some rails accommodate butt joints better than others. Similarly, the envelope can govern which guide to use. For example, if you have 2 feet of mounting space and 4 feet to move, you need a telescopic rail.

Another factor to consider is the environment. Will the guides be exposed to debris or contaminants? Will they need to be washed or disinfected? For example, polymer bearings have limited ability to withstand temperatures above 300 F, while steel bearings can be exposed to 500 F. If the guides will be used in a vacuum, outgassing is a concern. For clean rooms, rails can be plated with Armaloy, black oxide or electroless nickel.

Precision is one variable that engineers often overspecify. Linear guides for opening and closing doors on a machine tool do not have to be as stiff or precise as guides for positioning sensors in a medical imaging application.

Bosch Rexroth's high-precision linear guide ball rail system, or BSHP, was designed for high-performance applications. Originally designed for high-performance machine tool applications, BSHP ball rails are available with many options, such as running blocks with high-speed ceramic balls, and all stainless-steel options with FDA-certified plastics.

The company's basic ball rail system, BSCL, fills the role for applications requiring linear bearings in the midperformance range. BSCL ball rails are available in six sizes, six runner block types, three preload classes and three accuracy classes.

Runner blocks and rails are freely combinable within the BSHP or BSCL families, which simplifies design, inventory and support considerations. Early in the design phase, designers can plan to use either mid- or high-performance ball rail systems from Bosch, since their overall dimensions are the same.

Bosch's miniature ball rail system was developed specifically for precision applications—such as assembly of optical, electronic or medical devices—that require extremely compact linear motion guideways with high load capacities. The guideways have equal load capacities in all four directions of load application.

Bosch's roller rail systems, or RSHP, provide smooth transport of extremely heavy loads with optimal precision and rigidity, which is essential for demanding machine tool and industrial automation environments. These robust rails offer various options for sealing and lubrication delivery to maximize uptime.

Bosch's Tychoway linear roller bearings provide extremely low friction and high load capacity. They ensure accurate positioning in machine slides by virtually eliminating the stick-slip phenomenon.

Bosch Rexroth's integrated measuring system (IMS) is a linear encoder that can be seamlessly integrated into its profiled rail systems (ball or roller), providing exact linear positioning feedback.

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