How a Linear Bushing Works

A linear bushing or a ball bushing is a linear bearing that rides on a round hardened shaft that uses low friction to provide linear guidance. 

A Quick History of Linear Bushings 

The first designs for linear bushings received a patent in 1945 and it was during the 1950s when these became commercially available. Until profiled rail linear guides were introduced in the 1970s, the linear bushings were used as the main linear motion form for applications that didn’t call for extreme accuracy and load capacity of machined ways.

Linear bushing KH
Linear bushing KH has most compact design

With the evolution of the designs of linear bushings, features like self-alignment capabilities and improved load capacities have helped retain their place as among the primary options for linear guides in the motion applications. 

How Linear Bushings Work 

Linear bushings are made up of load-bearing plates, steel balls, a ball retainer, seals, and end rings or caps. External loads get transferred from load plates that are analogous to a radial bearing’s outer race through steel balls then to the shaft that is analogous to a radial bearing’s inner race. 

Linear guides, ball screws, bearings & more

Traditional designs of a linear bushing produced the point contact between the load plates and the balls. The point contact offered very low friction and limited the bushing’s load capacity as well. Even though linear bushings remain available with this particular design, many of today’s designs feature a groove in load plate to offer improved conformity with load-carrying balls. Change from the point contact to the conformity boosts the load capacity by up to three times that of the conventional design, resulting in 27 times the life of dynamic bearing. 

Benefits of a Linear Bushing

In spite of the stiffness of the profiled rail guides and the increased load capacities, among the primary advantages that linear bushings offer is the fact that they are suitable for applications in which high-precision machining of the mounting surfaces and the alignment of components is not possible.

To support the moment loads, for instance, linear guides that can either be profiled rails or shafts, are used in pairs most of the time. When the guides are not properly aligned with regard to their height, the profiled rail versions are going to bind, suffer from high friction, and have significantly reduced life. 

On the other hand, linear bushings can roll or rotate around the shaft that compensate for the misalignment of the shaft. The general rule of thumb is that a height difference of 1 to 2mm can get compensated once the shafts get mounted apart at 300mm or more. 

Aside from their natural roll capability, the linear bushings could be self-aligning as well, which means that these can rotate in yaw or pitch directions, letting them compensation for alignment or flatness errors or the shaft’s deflection. The capability to self-align is made possible thanks to the design of load bearing plates that can be gradually crowned, radiused, or sharply crowned. 

Aside from making up for any inaccuracy in the mounting surfaces, shaft deflection, or alignment, a self-aligning linear bushing also ensures even load distribution over the bushing’s full length. It prevents the edge pressure between the shaft and end of the bushing for smooth running and low friction characteristics. 

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