What to consider when specifying a linear slide

Here are some considerations when specifying a linear slide for a new or existing application.

When a design engineer has fully defined a linear-motion application’s requirements regarding travel length, speed, force, and accuracy as well as repeatability, the next question to answer is this:

Does an off-the-shelf linear slide (or a stock linear slide from a catalog) fully satisfy the application?

Off-the-shelf linear slides are advantageous for their quick delivery and lower cost than that of custom components. No wonder most linear-motion designs do in fact employ off-the-shelf linear slides from various component and system suppliers.

But what about those designs for which there are no suitable off-the-shelf linear slides? Such designs may have parameters that might necessitate unacceptable compromises if reliant on stock product … these parameters can include nontraditional drive methods, unusually long lengths of travel, exposure to overturning loads and torques, and high force, load, and speed requirements. Elsewhere, the retrofitting of existing machine designs may significantly complicate the integration of the linear slide. Any such parameters that necessitate modifications to off-the-shelf linear slides will cause actual cost to escalate — sometimes to an unacceptable degree.

Custom linear slides can serve as a more practical solution in these situations, as well as designs requiring linear slides that are:

Compliant with FDA/USDA/3A-Dairy requirements
Subjected to caustic chemicals or an otherwise corrosive environment
Exposed to a radioactive environment
Required to operate in below-freezing or high temperatures
Destined to be submerged in caustic or corrosive liquids
Regulated by industry guidelines that preclude the use of certain materials or plating

Where no stock catalog linear slides satisfy all of a machine design’s requirements, a custom-engineered linear slide may be the most cost-effective solution. Such linear slides are often machined and assembled by the manufacturer to include the base material, rail and bearing system, stage or table, and drive system.

Mounting-base material: The base material may be made of steel, stainless steel, aluminum, anodized aluminum, aluminum tooling plate, or granite. As this is the foundation it has to be rigid to support the load — and securely anchor the linear components.

Drive system: The drive for the type of linear axis we discuss here may be driven by a servo motor or stepper motor coupled to a lead screw, ball screw, or belt. The linear system can also be powered by a hydraulic or pneumatic cylinder. The selection of the drive system is dictated by accuracy, repeatability, acceleration, speed, load, and environmental requirements.

Unacceptable compromises on travel, speed, force, and accuracy can result in degradation of overall machine throughput as well as shorter maintenance intervals and premature failure — not to mention complicated modifications to off-the-shelf linear slides. In contrast, custom solutions can help linear axes deliver on all counts — even on exceptionally long axes such as the one here. Image courtesy LM76

Shafting (round rails) and linear bearings: Shafting and linear bearings offer the greatest flexibility in design for linear slides for high speeds and light to medium loads. Shafting or round-rail iterations include ceramic-coated aluminum shafting (having a Rockwell hardness of 60 Rc) and 440C, 303, or 316 stainless-steel shafting. Shafting is available without support and with support for heavier loads.

Here’s a closeup of round-rail linear slides on a ballscrew-driven linear stage.

Linear bearings for use with round rails include stainless steel variations and PTFE-composite-lined linear bearings — as well as ceramic-coated and solid-ceramic linear bearings. Ceramic-coated PTFE-composite lined linear bearings feature a low coefficient of friction (0.09 to 0.12) and are available in closed or open configurations. They can deliver speeds to 400 ft/min. — plus can handle 10 times the load of linear ball bearings. They’re also smooth and quiet and suitable for applications requiring washdown or submersion … and can operate in contaminated environments.

In contrast, ceramic-coated linear bearings have coefficients of friction of 0.04 to 0.08 for smooth and efficient travel at a lower cost than bearings — with essentially unlimited travel speeds and acceleration. That makes them particularly suitable for long strokes and oscillating applications. Other benefits include their self-lubricating and inert nature — even when exposed to caustic chemicals.

Linear-slide carriages: These can be based on plain bearings (including round bushings) with PTFE, filled PTFE, ceramic, and metal filled elements. Case in point: Certain carriages for mating with round rail are actually solid blocks of self-lubricating material — Ertalyte PET-P, a semi-crystalline thermoplastic polyester based on polyethylene terephthalate — for ruggedness and simplicity. Other designs with less demanding environmental requirements may make use of pillow blocks and flange blocks having plain aluminum, electroless nickel, ceramic coated, and stainless-steel sections.

Of course, in other designs the carriage can mount on profiled rail and take the form of a linear guide with ceramic or steel ball-bearing elements. The latter include a wide array of materials and configurations (beyond the scope of this article) each having strengths and weakness.

Profile rails: The best choice for heavy loads, profile rails are sold by various component suppliers. All profiled rails have attributes that should be evaluated based on the application.

Radial support bearings: These (like linear bearings) can integrate steel or corrosion-free ceramic balls. In fact, ceramic ball-bearing elements are stronger than steel balls and exhibit less rolling resistance — plus impart longer design life. They’re also not subject to spalling and or caustic chemicals — so often impart longer design life than steel balls.

Stage or table: Made from a variety of materials (including aluminum and stainless steel) the carriage in a linear stage often attaches to a round-rail system via pillow blocks … or (in a linear stage with profiled rail) blocks with profiled elements. In some cases, closed and open round rail linear bearings can insert directly into a solid stage metal stage. Typically the stage has a pattern of threaded holes for mounting of tooling — or (for Cartesian arrangements) perhaps mounting of Y, Z, or theta motion axes.

In summary, design engineers should look for linear-slide suppliers that evaluate every request for a linear-motion solution according to specific requirements — and recommend various linear components as deemed most appropriate. After all, every power-transmission and motion component has its benefits and drawbacks. Fully understanding the application, its working environment, and the demands on the linear slide (and its components) and carefully selecting the best components ensures the design fully leverages all its component advantages.

Shown here is a machine integrating profiled rail linear guides.

Shown here is a machine integrating round-rail linear slides.

Custom-engineered linear-slide specification begins with analysis load and speed, anticipated stresses on the slide, and the application environment. LM76 engineers draw on the experience gained from designing and building thousands of linear slides … but treat each linear-slide as a fresh concept and not a redesign.