Pillow Block Selection: Loads and Sealing

Pillow block units - cast housings with an inserted ball bearing, locked to a plain shaft - are the workhorses of conveyor, fan, agricultural and packaging machinery. They look simple, and most of them are, but a surprising share of field failures comes from selecting a unit by shaft size alone and never checking the loads it will actually see. A pillow block that survives ten years on one conveyor can fail in eight months on the next line that looks identical, because the belt tension, the pulley weight or the ambient dust were quietly different. Selection, done properly, is a short but ordered walk through load, speed, environment and mounting.

This guide works that walk in order: the load case first, then the insert rating and the housing limit, then speed and grease life, then the sealing and material choice that the environment dictates, and finally the mounting and lubrication details that decide whether any of it lasts. The general family of housed units is described in mounted units; here I'll add the selection arithmetic.

Why shaft size is the last input, not the first

The shaft diameter is usually given to you by the driven element, so it feels like the starting point. It is not. The shaft only sets the bore; it says nothing about how hard the unit will work. What the unit actually sees is a load vector whose direction, magnitude and rotation relative to the housing decide the fatigue life, and a set of environmental conditions that decide how long the grease and the seal survive. Treat the shaft as a constraint on the answer, not as the question.

What the shaft does and does not fix

The shaft dimension pins the bore and therefore the insert series, but it leaves the housing style, the insert rating and the locking method open. A 30 mm shaft can be served by a two-bolt or four-bolt base, by a light or heavy insert, and by set screws or a collar. All three choices change the load the unit can survive, so treating the bore as the whole decision discards most of the available engineering.

Writing the load case for a conveyor shaft

The load is the number you must establish, and it is worth writing it down explicitly. For each position, estimate the radial load from the driven element - belt pull, chain tension, gear forces, the weight of the rotating assembly - and add any axial component from helical gearing, inclined conveyors or thermal growth of the shaft. Then record the peak: the load at start-up, or when a belt jams.

Rotating load versus stationary load

One subtlety changes life more than most people expect. A load that rotates with the shaft cycles every point of the inner-ring raceway once per revolution; the same magnitude standing still in the housing cycles the inner ring only where the load zone sits. Rotating-in-shaft loads therefore shorten ball bearing life faster. Two conveyors that carry the same tonnage can impose quite different lives on identically sized units because of this alone.

Insert bearings, housing limits and the spherical seat

Pillow block ratings are quoted as dynamic and static load capacities of the inserted bearing, but the housing itself can become the limit at high load, because the insert sits in a spherical seat that is only as strong as the cast iron around it. Manufacturers list maximum permitted radial loads per housing size, and a bearing that calculates fine can still overload a small housing by pressing against the seat lips. Check both the insert rating and the housing limit before you sign off on a size.

Matching speed and grease life to the duty

Insert bearings are deep groove ball bearings with a sphered outside surface and usually an extended inner ring with two set screws or a locking collar. Their speed capability is high enough for almost all conveyor and fan duty, but the practical limit is often grease life rather than the bearing itself. Above roughly half the catalogued limiting speed, or in hot environments near ovens and dryers, relubrication intervals shorten dramatically, and the housing must be specified with a grease nipple and a way for old grease to escape. Choosing the next larger housing does not buy speed; choosing the right seal and lubrication plan does.

Relubrication provision

A relubrication interval is only useful if the housing lets you carry it out. Specify a grease nipple and a relief path for the old grease, and confirm that a grease gun can reach the fitting without dismantling the line. The most common reason a scheduled relubrication is skipped is simply that the point is hard to reach.

Temperature and clearance

Temperature deserves the same attention. Cast pillow blocks tolerate the temperatures their grease does; above roughly 100 °C at the bearing, standard seals harden and standard greases thin out, and a high-temperature grease, a larger internal clearance such as C3, or a graphite-lubricated insert becomes necessary. A dryer section that cooks one unit a year is almost never a bearing-quality problem - it is a specification problem that was visible on the drawing before the first unit was installed.

Dust, washdown and shock each change the spec

The environment decides the sealing and the housing material more often than the load does. Three families cover most cases.

• Dusty plants. Quarries, cement and grain need contact seals renewed at relubrication, plus a routine that actually purges the seal cavity.
• Washdown. Food, beverage and pharma call for stainless inserts, thermoplastic or stainless housings, and lubricants rated for incidental food contact.
• Shock and vibration. Crushers, shakers and screens need heavier housings, an insert with wider internal clearance, and set screws torqued and re-checked, because loose set screws are the most common root cause of spun inner rings.

Where shock loads are severe, some users specify eccentric locking collars instead of set screws; the collar grips progressively and resists loosening. Each locking style has trade-offs in mounting speed and grip, and mixing styles on one shaft is acceptable only when the fit conditions are understood. For washdown lines the sealing and material choices follow the practice set out for food and beverage bearings.

Sizing against C, C0 and housing ratings

Once load and speed are known, sizing is a straight calculation: required basic rating life from the dynamic load rating, checked against the static rating for the peak condition. Most conveyor practice selects for a life of 20,000 to 30,000 hours; fans and long-life installations go higher. Do not forget the housing limit on top of the bearing arithmetic.

A worked line makes the arithmetic concrete. A conveyor pulley passing 12 kN radial at 300 rpm, with a 1.1 kN peak during a belt jam, selects an insert whose basic dynamic rating keeps the ISO 281 life above 20,000 hours at the steady load, while the static rating covers the 1.1 kN peak with margin. If the calculated life is fine but the housing maximum radial load is exceeded, the next housing size is the answer, not a heavier insert.

The load case sets the size; the environment sets the seal, material and clearance.

Matching the locking style to the duty prevents the most common field complaint, the spun inner ring.

Bores and outside diameters follow ISO 15; the housing height sets the support structure and the bolt pattern that carries the load into the frame.

Mounting, locking and fixed-versus-expansion layout

A correctly selected unit still fails if it is mounted carelessly. Set screws must meet clean, burr-free flat spots on the shaft and be torqued to specification; a second torque pass after a few hours of running is cheap insurance because the shaft and collar bed in. The shaft itself must be straight, sized to tolerance and free of paint or rust under the collar. The base must be flat: shimming a rocking base before final tightening prevents the housing distortion that preloads the insert and runs it hot. And on long shafts, one unit must be the fixed (locating) end and the others expansion units, so thermal growth has somewhere to go instead of loading a ball bearing axially until it fails.

Relubrication completes the picture. A grease gun applied on a fixed schedule, with the old grease purging visibly from the relief, keeps the seal cavity clean and the raceways coated. The interval comes from speed, temperature and grease type - not from the calendar - and a plant that documents intervals per machine consistently beats one that greases everything on the same monthly round. The arithmetic behind those intervals is set out in grease re-lubrication intervals.

A field-ready pillow block checklist

• Loads. Radial and axial, steady and peak, and whether the load rotates with the shaft.
• Insert and housing. Both the bearing rating and the housing maximum radial load are satisfied.
• Shaft. Size, tolerance and condition suit the chosen locking style.
• Speed and grease. The duty sits inside the grease life, with relubrication provision confirmed.
• Environment. Temperature, contamination and washdown requirements are matched by seal and material.
• Layout. Fixed versus expansion positions are assigned along the shaft.

Dimensions for the common insert and housing series can be compared in our technical library, and individual units filtered in the bearing selector. Where the environment is aggressive, the material and sealing options overlap with deep groove and stainless practice, and the housing choices mirror what is offered under the FYH and comparable mounted-unit ranges, with the wider family browsable under ball bearing units. Ten minutes on this checklist converts pillow block selection from guesswork into a repeatable procedure.