Bearings enable components to move together smoothly while offering both support and alignment. Linear bearings, in particular, support the load of a carriage while providing a low friction surface for guide rails to slide against. Guide rails are fitted and inserted into the linear bearing while the carriage travels back and forth along its length. Linear bearings feature either a rolling element or fluid device to reduce friction, and they ensure superior mounting, additional precision, and smoother motion. Because of their positive capabilities, they are often found in 3D printers, sliding doors, and many automated applications in which guiding rail motion is necessary.
Part of a linear guide assembly, linear bearings can be found on cutting machinery, XY positioning tables, machine slides, instrumentation systems, and more. In order to drive the motion, either a motor-driven ball screw, lead screw, pneumatic cylinder, hydraulic cylinder, or manual force must be used. Linear bearings tend to be divided into two classifications, rolling bearings and plain bearings, both of which will be distinguished below.
Rolling linear bearings are used most often, as they provide the least frictional surface for linear motion. They use balls or rollers as the rolling elements that are positioned in between the mating grooves of the bearing and guide rails. The diameter of these balls or rollers is proportional to the linear speed of the linear guide, meaning that as the ball diameter increases, so does the linear speed of the guide. The contact angle is measured horizontally and it impacts the loading capacity of the linear bearing in a specific direction. It is important to note that the contact angle is directly proportional to the radial loading capacity and inversely proportional to the lateral loading capacity.
On the other hand, plain linear bearings require a sliding contact between two surfaces, but they do not employ rolling elements. These are much cheaper and simpler to operate than their counterparts due to the fact that a linear bearing’s contact area is larger, causing the surface pressure to drop. Moreover, plain bearings are able to absorb shocks and vibrations more effectively while also possessing a higher load capacity and lighter weight. Despite these benefits, plain linear bearings do have higher friction, that of which limits the speed of the linear guide while increasing its amount of wear. For this reason, lubrication must be consistently maintained, whether that is through the use of different sliding materials or a self-lubricating coating in order to reduce the coefficient of friction. They tend to be unfit for high precision systems because of their lower travel accuracy.
Linear bearings can be constructed of steel, aluminum, plastic, bronze, ceramic, or composite materials. To start, steel is most often used as the material for linear bearings because of their excellent mechanical properties which are able to support heavy loads and produce smooth and precise movement. Aluminum is a strong, lightweight material that resists corrosion and chemical damage while costing less than steel. Despite these benefits, aluminum has a lower load capacity than steel. On the other hand, plastic bearings feature a lower coefficient of friction than metallic bearings. Materials commonly used include nylon, polyethylene, and PVDF, all of which tend to be lined with a self-lubricating coating. Bronze bearings generate more friction than plastic bearings, and this makes lubrication maintenance necessary. Finally, ceramics are quite rigid, allowing for travel accuracy and precision at high speeds.
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