POWER TRANSMISSION REFERENCE GUIDE
Technical summary of
linear-motion guides, rails, and systems
L
inear-motion systems are essential in all sorts of applications, including everything from manually operated industrial drawers to advanced Cartesian robots. Mechanisms that include the former operate without power, using inertia or manual power to move loads. Components to complete the latter include ready-to-install drive and guidance designs … in the form of self-contained actuators or linear-motion machinery subsections. Some designs simply rely on the rotary-to-linear mechanism or actuator structure for total load support. However, most industrial linear designs have pneumatics, linear motors or motor-driven, rotary-to-linear mechanisms to advance attached loads, as well as rails that guide and support the loads. Here, linear rails, rotary rails, guide rails, linear slides and linear ways are just a few options to facilitate single-axis motion. Their main function is to support and guide load with minimal friction along the way. Typical linear-motion arrangements consist of rails or shafts, carriages and runner blocks, and some type of moving element. Engineers differentiate these systems by the type of surface interaction (sliding or rolling), the type of contact points, and (if applicable) how the design’s rolling-element recirculation works. In fact, slides and rails are more advanced than ever, with advances in materials and lubrication setups (to help designs last longer in harsh applications), innovative rail geometries (to help designs withstand more misalignment and load than ever), and modular guide mounts (to boost load capacity and minimize deflection). No matter the ultimate installation, linear-motion rails, guides, and ways enable motion along an axis or rail either through sliding or rolling contact. Myriad moving elements can produce either sliding or rolling support: ball bearings, cam roller sliders, dovetail bearings, linear roller bearings, magnetic bearings, fluid bearings, X-Y tables, linear stages and machine slides. One classic rail with sliding contact is a dovetail slide, and one classic rail with rolling contact is a ball rail with a recirculating system. Sliding-contact bearings are the more straightforward type of linear-motion component. These consist of a carriage or slide that rides over a surface known as a rail, way or guide. Sliding contact occurs when the moving part directly contacts the rail section. Newer versions have self-lubricating sleeves and other features to boost positioning accuracy and repeatability.
38
DESIGN WORLD — MOTION
Linear Guide Rails Slides & Ways — Power Transmission HB 05.19 V3.indd 38
5 • 2019
In contrast, rolling-element linear-motion systems are either recirculating or non-recirculating. Non-recirculating types use rolling elements such as bearing balls, rollers and cam followers for movement. Recirculating types use some type of moving platform that houses a bearing block. This bearing block contains raceways with rolling elements that let the platform move along the rail with little friction. Recirculating types include linear guides and ballbushing bearings. More specifically, rolling-element linear guides come in two basic versions — those with circular arc grooves and those with Gothic arc grooves. These groove choices are a result of industry evolution that’s enabled new geometries for better load handling. Circular arc grooves contact bearing balls at two points. The Gothic arch contacts the balls at four points for bidirectional load capacity. Another option for rolling-element linear motion is ball bushings that have a bushing nut lined with recirculating bearing balls. This nut rides along a round shaft to allow axial movement.
Sliding-contact rail geometries A distinguishing feature of sliding carriage-and-rail setups is that manufacturers typically incorporate a ground groove in a rectangular track’s geometry (to serve as a working surface). Manufacturers typically build these rails in one of three shapes: Rails with a boxway shape or square shape are simplest. Square rails excel at carrying large loads without a lot of deflection. Manufacturers often preload square rails, and most linear systems based on square rails do not self-align. Square rails often have a smaller envelope size; the boxway rails handle the highest loads in all directions. Round rails deflect less under load. In addition, systems based on round rails are inherently self-aligning, so are easier to install than the other options.
Rolling-contact functions and options Rolling-element linear systems need little force to initiate motion. In addition, friction-force variations due to speed are minimal, so these systems can position loads with small and precise steps. The low friction also lets these systems move at high speeds without generating too much heat. That minimizes wear to help
motioncontroltips.com | designworldonline.com
5/14/19 1:39 PM
