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 product 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 temperatures
- Destined to be submerged in various 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 suitable. Such linear slides are often machined and assembled by the manufacturer to include the platform, profiled rails, bearings and carriages, and drive system.
Linear-slide base: The linear-axis platform may be made of steel, stainless steel, or aluminum (such as tooling plate or 6061 alloy) with anodized or ceramic-coated elements.
Drive system: The drive for the type of linear axis we discuss here is usually a motor-driven arrangement that powers either a rack-and-pinion assembly or a screw technology — whether ballscrew, leadscrew, or roller screw. In fact, the drive system is the first element of any linear axis to be established … as it can affect the parameters and choices related to subsequent linear-slide selections.
Linear-slide shafting: Shafting in round-rail linear-slide iterations include ceramic-coated aluminum shafting having a Rockwell hardness of 60 Rc and 440C, 303, or 316 stainless-steel shafting.
Linear-slide tracks: The linear slides can have stainless-steel PTFE lined or ceramic-coated working surfaces. PTFE composite-lined linear slides have working surfaces with a low coefficient of friction — typically 0.09 to 0.12. They’re offered in closed or open configurations and can deliver speeds to 400 ft per 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.
In contrast, ceramic-coated linear slides 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 … making them particularly suitable for even 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.
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.
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 linear components offered by other manufacturers if 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.
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.