What is a pillow block bearing?

A pillow block bearing consists of a mounting bracket (pillow block) that houses a bearing and is used in low-torque, light load applications.

With this configuration, the pillow block is bolted to a foundation, securing it, while the shaft and the inner ring of the bearing are free to rotate. Usually made of gray cast iron, pillow blocks come in two types, split or unsplit. With split pillow blocks the housing element or cap can be separated from the base.  An unsplit pillow block is made from one solid piece.

Often, the terms pillow block bearing and plumber block bearing are used synonymously but the two are different devices.  Both are designed to be mounted to a surface through mounting holes on the base of the block. They differ in that plumber block bearings do contain an inner bearing and they are also designed for higher loads and more corrosive environments.

Permissible misalignment

Static misalignment	There is an initial alignment error between the two supports of a shaft.
	Shaft deflection creates misalignment between bearing inner and outer rings that is constant in magnitude and direction.
Dynamic misalignment	Varying shaft deflection creates misalignment between bearing inner and outer rings that is continuously changing in magnitude or direction.

Static misalignment

Ball bearing units can accommodate static initial misalignment by tilting in the housing. The permissible values are:

• where relubrication is not required: 5°
• where relubrication is required: 2°
• for units with a back seal (designation suffix DFH): 1°

When using pressed steel housings, misalignment cannot be accommodated once the attachment bolts have been fully tightened, unless a rubber seating ring is used. 

Dynamic misalignment

Ball bearing units cannot accommodate dynamic misalignment.

Locating/non-locating support

Locating support

Most ball bearing units are designed to serve as locating supports. 

Non-locating support

Most ball bearing units are not intended to accommodate axial displacement of the shaft relative to the housing. The internal clearance of the bearing units is sufficient to compensate for minimal displacements. The distance between bearing positions should be kept short to avoid inducing excessive axial loads as a result of thermal expansion of the shaft.

Design for small axial displacement

To accommodate small axial displacements, the bearing units should be supported by resilient surrounding structures.

Design for larger axial displacement

In applications where there are low speeds and light loads, bearing units with set screw locking can be used to accommodate axial displacement. The shaft at the non-locating bearing position should be provided with one or two grooves, 120° or 62° apart, to engage a modified set screw.