Linear ball bearings and round shafts are often used in applications that call for ease of use and simple mounting requirements. For example, where profiled rail guides require full support along their length, linear ball bearings, or bushings, can be used in many applications with shafts that are supported only at their ends. However, unlike profiled rail guides or bearings that ride on spline shafts, linear ball bearings have no built-in mechanism that prevents them from rotating around the shaft, so they’re typically mounted in a bearing housing. This housing, or pillow block, also facilitates mounting the external load to the bearing.
Image credit: Bosch Rexroth
Linear ball bearing pillow block housings
The inner diameter of the pillow block affects the clearance between the bearing and shaft. Too much interference can reduce bearing life, while too much clearance can reduce load capacity and rigidity.
Image credit: Lee Linear
Like linear ball bearings, pillow blocks are available in both closed and open versions, with open versions being suitable for use on shafts with support rails. Various materials are also available, including aluminum, cast iron, and stainless steel, depending on the application’s requirements for rigidity and use in harsh environments. And to facilitate easier mounting and alignment of multiple bearings, some aluminum pillow blocks come in twin, or tandem, versions, allowing two bearings to be mounted in one housing.
The amount of clearance between the bearing and shaft is an important factor in bearing performance and is determined by a combination of factors — the radial clearance of the bearing, the tolerance of the pillow block bore, and the tolerance of the shaft diameter.
Too much interference between the bearing and shaft can cause excessive friction and lead to premature wear on the shaft and the bearing load plates. Alternatively, too much clearance between bearing and shaft can result in fewer rolling balls being in contact with the raceway, leading to reduced load capacity and lower stiffness.
Manufacturers provide guidance for the expected clearance between bearing and shaft when standard pillow blocks and shafts are used. However, if pillow blocks are manufactured in-house by an OEM or end user, it’s important to follow the manufacturer’s recommendation for the bore tolerance. Not only does this affect clearance between the bearing and the shaft, but too much interference between the inner bore of the pillow block and the outer bore of the bearing can also reduce bearing life.
Adjustable pillow blocks allow the clearance between the bearing and the shaft to be reduced or eliminated.
Image credit: Bosch Rexroth
Although too much interference can cause rough operation and reduced life, most linear ball bearings are able to operate with zero clearance or light preload between the bearing and the shaft. To achieve this, some linear bearings — and most pillow block designs — are available in adjustable versions, with a slot in the outer housing of the bearing or in the pillow block.
Adjustment of this slot — whether directly on the bearing or in the pillow block — slightly reduces the diameter of the bearing and removes clearance between the bearing inner diameter and the shaft outer diameter.
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