Linear bearings are rolling-element and fluid-film devices that reduce friction in motion systems where the motion acts along a straight — or sometimes curved — pathway. They are distinguished from radial bearings in which motion is rotary. Linear bearings are used in machine tool applications such as sliding doors, 3D printers, and automation settings where reducing friction and guiding linear motion is needed. They can be loosely grouped as ball and roller types that use the rolling motion of rolling elements and sliding types that rely on lubricant and low-friction surfaces. This article will discuss the distinct forms that linear bearings take and highlight some general applications.
Sometimes called linear bushings, linear bearings are manufactured in sintered aluminum bronze, metal/polymer composites, carbon insert styles, polymer-lined sleeves, etc. and rely on a lubricating film to form between the bearing and the shaft while the two are in relative motion. Powdered-metal bronze bushings have been used for many years as die-post bushings. Their porous constructions, often with carbon plug inserts, are vacuum impregnated with oil that flows to and from the bearing under the frictional heat developed as the shaft moves and stops, providing a film of oil between the bearing and the shaft.
Polymer, self-lubricating linear bushings have captured some of the market that had been dominated by sintered metal bushings. The polymer itself — typically PTFE — provides slipperiness for the running shaft. These are popular in food packaging for their ability to run without lubricants and their ability to withstand washdown chemicals. They can run on unhardened shafts having high corrosion resistance. Because polymers are subject to cold flow at high loads and do not dissipate heat well, polymer is often bonded in thin sections to metal backers in the construction of these bearings.
Linear bushings can be crowned slightly to permit some angular adjustment of the bushing relative to the housing to adapt to shaft deflection. In more extreme cases, the bushing is supported in the housing by O-rings. Linear bushings are available as naked units or housed in pillow blocks and flanged units.
Bushings can be arranged as tandem installations in common housings to provide higher axial spread for the bearing surfaces. They are available as open and closed varieties, with the selection depending on how the shaft is supported. Shafts supported continuously over their lengths require open bushings that can clear the rod rails. Shafts supported on their ends can use closed bushings. Linear bushings are tolerant of dirt as a result of having no moving elements.
Open ball cages — sometimes called guidepost or leader-pins — are used with die posts for guided, precise motion of the upper and lower dies as they close. These bushings are close-toleranced to provide accurate location for mating dies.
For linear motion control, both recirculating and non-recirculating ball and roller bearings are available. They both provide low starting friction as compared with slide-type bushings. Non-recirculating types sandwich the balls or rollers along the complete lengths of paired rails or ways. The rails move relative to each other as the rolling elements rotate between them. These designs can provide very accurate linear motion and some designs use crossed rollers for even higher accuracies. The stroke length is fixed by the number of elements, however, making long strokes somewhat impractical.
Recirculating ball and roller bearings have no theoretical limits on travel distance because the rolling elements travel the length of the bearing as it moves along the rod but then double back through the carrier, creating an endless loop of load-bearing elements. These designs operate on round rails or profiled tracks, depending on the design of the bearing.
Recirculating ball bushings are perhaps the most familiar as they are direct descendants of sliding bushings. As with the sliding varieties, recirculating ball bushings can be open or closed based on the support of the linear rails; they can be mounted in pillow blocks, flanged units, etc. and they are often sized to directly correspond to the dimensions of sliding bearings.
Care must be taken in dirty environments to protect the many moving balls within the bearing. In some instances, the entire rod is covered. Manufacturers will often supply seals, wipers, etc. as standard or optional with these bearings.
Ball bushings are stiffer than linear bushings and consequently less compliant with regards to shaft misalignment. Ball bushings can run on hardened round rails. Recirculating roller bushings usually run along profiled tracks that have flat ways. Ball bushings are sometimes available with different levels of precision with the looser versions able to compensate for misalignment of the shaft. Self-aligning ball bushings also are available. Ball bushings have some ability to rotate on the shafts, so if used with parallel shafts, one shaft can sit lower than the other without noticeably affecting operation.
Ball bushing rails can be end-mounted successfully and can span some distance between supports. Mounting of profile rails usually requires support over the full length of the rail, demanding careful preparation of the mounting surface.
Other roller-type forms include V-groove rollers and rails capable of negotiating curved tracks, recirculating roller-chain bearings used for heavy loads on round rails, and bearings that employ ceramic elements as a means of reducing system weight. There also exist linear bearings that use air as the lubricating medium.
Most linear bearing suppliers offer selection guides that step the user through the design problem. Considerations include the precision of the application, from lowbrow operations such as drawer slides to semi-precision operations such as saw guides, to exacting applications such as machine tools. Bronze bushings might be appropriate where loads are high and smooth motion is required but neither speed nor accuracy is important. On the other hand, where loads are high and accuracy is necessary, a recirculating roller bearing and profile rail might be the preferred choice.
Ball bushings are easily contaminated by particulates — the sealing problem being somewhat more difficult than that of radial bearings because of the long lengths of exposed shafting involved. As was suggested, seals, wipers, and way covers all offer means of minimizing contamination but in very dirty environments, sometimes a sliding bushing makes a better fit. The friction of these can be quite high when compared with rolling-element linear bearings.
In addition to offering the components to piece together linear-bearing systems, many manufacturers sell assembled systems under various names such as linear slides, linear tables, etc. Many manufacturers can supply systems with a variety of actuating options.
In sizing a system, engineers will need to have some information at hand including the maximum expected speed and acceleration, the maximum force the system will see, and the length of stroke. Repeatability is a concern. Moment loads acting on the system along the x, y, and z axes also must be considered. Maintenance plays a part in system selection, as do vibration and noise constraints.
This article was contributed by Thomasnet, New York, NY. For more information, visit here .