Linear Bearing Pillow Block Selection Criteria

Pillow blocks, as with flange mounts, are a type of mounted bearing that contains a set of mounting holes that line up perpendicular to the mounting shaft. Through these mounting holes, pillow blocks can be secured to a foundation to facilitate a rotating shaft, or part of a bearing assembly that moves along a linear shaft. LEE Linear offers three basic types of pillow blocks that utilize either sliding plain bearings, recirculating balls, or rollers where the bearing meets shaft. Which one you decide on will depend on your specific needs that are defined by careful assessment of the application and the resulting set of criteria.

We often encourage customers to apply the P.O.S.T.L.U.D.E.S acronym, which is a convenient guide to asking the right questions for determining the best product for the job. The acronym stands for Precision, Orientation, Speed, Travel, Load, Unknowns, Duty Cycle, Environment, and Safety. The sections below offer some of the P.O.S.T.L.U.D.E.S. criteria for each of the three types of pillow blocks. Additionally, this article touches on the more complicated load and orientation dynamics. Should the reader feel inclined to further investigate these helpful criteria, they may find more satisfactory bearing selection outcomes.

Pillow block construction and applications

Pillow blocks from LEE Linear are constructed with an aluminum alloy housing and are offered in many sizes as well as metric or imperial configurations. Certain combinations of housing type, bearing type, and shafting will provide the best pillow block bearing for a given application or condition. For example, all three types can be purchased in the open format, which accommodates support rails for shafts requiring longer or more rigid travel. Plain and recirculating ball bearings also have closed designs for use in end-supported applications.

Standard plain and ball type pillow blocks from LEE Linear combine a self-aligning housing with either a Simplicity® plain bearing or recirculating ball bearing. This self-aligning crown allows up to 1/2 degree of mis-alignment in the application, making it helpful in avoiding binding caused by misalignment. For single shaft applications demanding more rigidity, both plain and ball type bearings can be purchased with a straight bore ID in the housing. The basic rundown and P.O.S.T.L.U.D.E.S. criteria on our three types of pillow blocks are as follows:

Establish your load needs

Load refers to the forces acting upon a system and includes either static, dynamic, or impact components. A static load condition occurs when the system is fully loaded and at rest, and is the easiest condition to define. Dynamic loads, which look to quantify forces applied during motion, better reflect the actual loads experienced by the bearing during machine operation. This is important because when bearing systems are in motion, they need to contend with a larger set of variables that can affect overall performance. This set of variables makes calculating dynamic loads trickier, but not impossible. For example, plain bearings can offer static loads up to 20-times higher than ball bearings. However, in dynamic situations, the larger surface area of plain bearings is more sensitive to variations, and subsequently require additional calculations such as PV value: where P= Pressure (applied load divided by reflected surface area), and V = Surface Speed. Plain bearings are rated by their limiting PV, which is a combination of load over a given surface area and the velocity (PV = P x V).

P = Maximum pressure or load in pounds per square inch (psi) or kilograms per square centimeter (kg/cm²)

V = Maximum velocity or surface speed in square feet per minute (sfm) or meters per minute (m/min)

This dynamic load chart offers an example of the different dynamic load ratings using the example of a 1-inch shaft diameter. As is illustrated, the double and twin pillow blocks initially offer the most load capacity for pillow block configurations, while plain bearings are dependent on the PV value derived from the specific application.

Depending on the type of application, moment loads should be factored into the decision on bearing selection. Moment loads apply a force that causes a bearing to rotate about a specific point or axis, and will increase relative to the distance of the load from the bearing’s center line. Moment load forces are defined as pitch, yaw, or rotational movement. Ball bearings offer modest resistance to moment loads, but can sustain increased wear if loads are too high. Plain bearings are limited to the 2:1 ratio and often related to cantilevered loads. The 2:1 ratio or “Binding Ratio” is officially defined as the maximum ratio of moment arm distance to bearing length which will not bind (prevent motion). The Binding Ratio is often displayed numerically as “X:Y,” where “X” is the moment arm distance and “Y” is bearing length. With roller pillow blocks, they are less resilient to moment loads because the cantilever or moments will load the side rollers and cause premature failure.

Orientation can affect load capacity and performance

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