Technical summary of linear-motion guides, rails, and systems

Linear-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.

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.

Heavy-duty slides from HepcoMotion work for longlength applications such as pick-and-place and robottranslation stages. V-slide rails are of bearing-grade steel in sections to four meters long. The slides typically bolt to aluminum extrusions or supporting back plates. A guide wheel bearing with matching V geometry rides the raceway. Image courtesy Bishop-Wisecarver

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.

This linear plain bearing is PBC Linear Uni-Guide with a Frelon self-lubricating liner. The liner serves to lower the coefficient of friction, reduce wear, and boost load capacity.

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.

O-shaped arrangements can withstand higher torque than X arrangements. In general, the number of load-bearing rolling-element rows influences the load capacity … so more rail rows means more load capacity and rigidity. However, more rows make systems more complex and costly. Here are more details on these rolling-contact options:

Rolling elements are either linear rollers or balls. Because the rolling elements recirculate in recirculating rollingelement guides, they have a nearly infinite stroke length. They are available on flat guide ways and guide way rails. Flat guide ways are available in single or double row rolling elements. Guide way rails are often square rails. Non-recirculating roller type units have limited stroke length. Flat guide ways are dominant here and have either a grooved race compatible with crossed rollers, or non-grooved race, which uses cage and roller-type rolling elements.

The low friction also lets these systems move at high speeds without generating too much heat. That minimizes wear to help machinery maintain a level accuracy for much of the linear system’s operating life.

Recirculating elements (ball or roller bearings) between the rail and the bearing block enable precise linear motion. The coefficient of friction with roller-elementbased systems is much less than with slide based linear motion guides … about 1/50th that of non-recirculating systems.

Ball-type rolling element units are also subdivided into recirculating and nonrecirculating types. The flat guide ways here typically use double row recirculating rolling elements. The guide way rail can be either round or square. If the raceway is not grooved, the rolling element is typically a linear ball bushing. If the raceway is grooved, the unit usually uses a ball spline. For square rails, the raceway is usually grooved. For ball-type rolling element units that are non-recirculating, the flat guide ways are grooved and use linear ball guides. The guide ways are round rail, without a grooved raceway, and use stroke bearings.

Shown here are ball bushing bearings, 60 Case shafting, square and round rail, and other linear-motion components from Thomson Industries Inc.