Traditional round shaft and profiled rail linear guides can be joined in sections to provide virtually unlimited travel. But some applications require a linear guide that can extend into and retract out of the work area, and with profiled rail and round shaft guides, there’s not a simple, easy way to do this. For these applications, telescoping linear bearings fit the bill, providing strokes that can be significantly longer than the length of the guide rail, with high load capacity and good rigidity. If you have an application that needs extending and retracting motion, here are some key things to know about telescoping linear guides.
Telescoping slides are made up of an outer rail and one or more inner rails. Designs with longer strokes (equal to the rail length or greater) typically incorporate an intermediate element, which improves stiffness and reduces deflection.
Balls or cam rollers guide the movement and support the load. Cam roller versions can be supplied with wipers or seals that protect against contamination and provide lubrication to the slide. Steel balls provide higher load capacity and can be preloaded, but they’re more sensitive to contamination.
Unlike profiled rail and round shaft linear recirculating linear guides, telescoping slides are offered in a variety of materials—from aluminum or stainless steel to hardened cold rolled steel. Each material offers different advantages in cost, weight, inertia, load capacity, and stiffness. Manufacturers also offer various surface treatments, such as hard chrome or electrolytic zinc plating, to withstand harsh environments.
Telescoping linear guides are grouped into three general categories, depending on their stroke length: partial stroke, full stroke, and extended stroke.
Partial stroke: The extension length is typically 50 percent of the rail length.
Full stroke: The extension length is equal to the rail length. Full stroke versions may include an intermediate element.
Extended stroke: The extension length is greater than the rail length. (Also referred to as “over-extension.”) For most extended stroke slides, the extension length can be up to 150 percent of the rail length. These designs often require an intermediate element.
Within these three categories of stroke type, many telescoping designs allow stroke in both directions, referred to as “double stroke” or “dual stroke.” A double-stroke slide provides twice the stroke for the same base length and be a compact and cost-effective solution for some applications.
The load capacity of a telescoping slide depends on the rolling elements and the rigidity of the intermediate element. In many telescoping guide applications, static load capacity is more important than dynamic load capacity, since the purpose of the guide is to move a load to a position and hold it there, resulting in intermittent, rather than continuous, operation.
However, for those applications with dynamic (continuous) operation, the life equation for telescoping slides is similar to that for ball or roller recirculating bearings, but it includes correction factors that are specific to the operation of a telescoping design, such as the frequency of direction change. Virtually all manufacturers specify static load capacities, but not all specifications include dynamic load capacities. Be sure, when evaluating the load or calculating life, that you’re using the correct load capacity (static or dynamic) for the calculation.
It’s important to note that in most telescoping slide product lines, only partial stroke versions can withstand moment loads.
Due to their design, telescoping guides have higher radial load capacity than axial load capacity. Therefore, manufacturers often recommend that these guides be mounted on their sides, so that the most significant loading is in the radial direction. The typical design for telescoping linear guides is to use two guides in parallel, with the load centered between the rails.
The mounting surface requirements for telescoping slides are less stringent than those of profiled rail guides. The most important considerations are the flatness of the mounting surface, and when two guides are used in conjunction, the parallelism of the guides.
Because telescoping rails are not fully supported, deflection is a particularly important consideration. When the rail experiences a vertical load, the expected deflection should be calculated and checked to ensure it falls within acceptable limits for the application.
Feature image credit: Rollon Corporation