Bearing Types and Loads

A bearing allows useful motion and restrains unwanted motion

A loose rigid part can move in six independent ways: translate along three axes and rotate about those three axes. These are its degrees of freedom. A bearing transfers load to a support while allowing the motion the machine needs, with low friction.

For a rotating shaft, the goal is usually one remaining freedom: rotation about its own axis. Two spaced radial supports resist sideways movement and tilting; axial location stops the shaft walking lengthwise. A single short bearing does not automatically provide that whole constraint system.

Six free-body motions become one intended shaft rotation when spaced radial supports and axial location are provided. — Let x run along the shaft. Translation T means movement along an axis; rotation R means turning about it. This is an ideal rigid-body model; real supports deflect and have clearance.

Bearings guide motion; motors and drives supply it. Belt, chain and gear drives load their supporting shafts. See Torque for turning effort and Shaft Support and Axial Adjustment for the complete support arrangement.

Radial load crosses the shaft; thrust acts along it

Radial load acts perpendicular to the shaft axis. Axial load, or thrust, acts parallel to it. Combined load has both components. A bearing's orientation and internal geometry determine which it can carry.

Three horizontal shafts show perpendicular radial force, parallel axial force, and both force components together. — Load direction is relative to the shaft, not to the room: a vertical shaft can have a horizontal radial load.

First selection filter, not a load rating. Check the exact catalog design.
Type	Radial load	Axial load	Combined load
Plain journal, without thrust faces	Yes	No intended capacity	Needs a thrust bearing
Plain footstep or thrust collar face	Needs a journal guide	Yes, against the loaded face	With a radial guide
Deep-groove ball	Yes	Both directions, within rating	Yes, within rating
Angular-contact ball or tapered roller, single row	Yes	Primarily one direction	Yes; opposing bearing needed for reverse thrust
Cylindrical roller, NU or N design	Yes	No intended locating duty	Use another bearing for thrust
Pure thrust ball or cylindrical thrust roller	No	Yes; direction depends on arrangement	No

Do not turn “roller” into “radial only,” or “thrust” into a rule for every product bearing that name. Tapered rollers carry combined loads; some spherical roller thrust designs also accept radial load under specified conditions.

Plain and rolling bearings trade friction, upkeep and cost

A plain bearing supports sliding surfaces, often with a replaceable sleeve called a bushing. A rolling-element bearing places balls or rollers between rings. “Anti-friction” is a traditional name, not a promise of zero friction.

Compare the operating conditions as well as the purchase price.
Question	Plain bearing	Rolling-element bearing
Starting versus running resistance?	Breakaway friction can exceed running friction. A hydrodynamic film is not fully established at rest.	Often low starting resistance and a smaller start/run difference; seals, grease and preload still matter.
Lubrication and maintenance?	Dry grades may need no added lubricant; oil-fed designs require the specified supply and cleanliness.	Usually grease or oil lubricated. A sealed, prelubricated bearing still has a finite service life.
Speed?	Dry bushings have material-specific speed and pressure limits. Fluid-film bearings can serve high-speed machinery.	Many suit moderate to high speeds; type, size, lubrication and heat limit speed.
Load and cost?	Simple bushings often cost little initially. Capacity depends on size, material, speed and heat removal.	Precision rings and elements add cost. Roller designs often carry more load than ball designs of similar overall size.

Rolling contact generally reduces resistance compared with sliding contact. It does not guarantee higher efficiency than every fluid-film bearing, nor does a cheap bushing guarantee lower lifetime cost. Include lubrication equipment, replacement, downtime and power loss. Bearing losses contribute to Power and Efficiency and the stage efficiencies discussed in Gear Ratios.

Plain bearings differ in both lubrication and load direction

Dry or self-lubricating: A suitable polymer, composite or lubricant-containing material supplies a low-friction sliding surface. Follow that material's instructions; adding grease is not automatically an improvement.
Hydrodynamic: Relative motion draws lubricant into a converging gap, forming a pressure-supported film. Starts and stops require attention because the running film depends on motion.
Hydrostatic: An external pressure supply supports the load on a fluid film, including at rest while the supply operates. The pump, flow control and loss-of-pressure behavior are part of the system.

Those are lubrication descriptions. Shape answers a different question: a journal surrounds a shaft to carry radial load; a footstep supports axial load at a shaft end; a thrust collar presents an annular face to carry axial load. An assembly can combine a journal and a collar to handle both components.

Rings carry the load; the cage spaces the rolling elements; Practice

Rings carry the load; the cage spaces the rolling elements

Front view of a ball bearing identifying the inner ring, outer ring, balls, cage and ring raceway surfaces. — Schematic with seals omitted. A raceway is the track on a ring that the rolling elements contact; ring and raceway are not quite the same thing.

The inner ring fits around the shaft; the outer ring fits in the housing. The rolling elements transfer load between their raceways. The cage maintains spacing and guides the elements.

In a common arrangement the inner ring turns with the shaft while the outer ring stays still. The balls spin about their own axes and orbit around the shaft. The rings do not have to rotate in opposite directions.

Balls have idealized point contact; rollers have idealized line contact. Under load these become finite contact patches. The longer roller contact generally supports more load in a similar overall size. Contact angle, not just element shape, determines thrust capability. Thrust ball and roller bearings place their contact geometry to support axial force.

Manufacturer reading for bearing selection

Use the exact product's current catalog to check its load, speed and lubrication limits. These references introduce the design choices.

NSK: Introduction to Bearings identifies rings, elements and cage.
NSK: Roller Bearings compares line and point contact.
Kingsbury: Hydrodynamic Bearings describes oil-film support and high-speed applications.
igus: self-lubricating plain bearings describes embedded solid lubricants.
SKF: Rolling Bearings, bearing type and arrangement, for product-specific selection.

Practice identifying the load

Recognize the bearing family by its contact geometry; Interactive

Recognize the bearing family by its contact geometry

Choose a family and compare its section with its end view. A radial bearing surrounds the shaft; a thrust bearing places its load-carrying faces across the shaft axis. Ball, straight roller, tapered roller and barrel roller shapes lead to different contact paths.

Fᵣ is radial load; Fₐ is axial load. Arrows show directions, not ratings. Sections omit the cage; end views expose the elements.

Plain journal / bushing

Plain thrust face

Deep-groove ball

Angular-contact ball

Self-aligning ball

Thrust ball

Cylindrical roller · NU

Needle roller · radial

Tapered roller

Spherical roller

Cylindrical roller thrust

Spherical roller thrust

These are original schematic sections, not interchangeable catalog parts. Cage details and seals are omitted from the sections to expose the contact geometry. A self-aligning raceway allows a limited tilt; it does not excuse arbitrary misalignment or eliminate the need for a correctly designed support arrangement.

Read the type, dimensions and suffixes in Bearing Designations and Sizes. The type tells you which catalog to consult; the complete designation and catalog row tell you what fits.

A pillow block is a housing arrangement, not another rolling-element shape

End views of a pillow or plummer block bolted beneath a shaft and a four-bolt flange housing attached to a facing support; both contain a separate bearing. — The outside casting provides mounting points. Identify the bearing or insert inside it separately.

A pillow or plummer block supports a shaft from a base. A flange-mounted housing attaches around the shaft to a facing support. Different housings can contain different bearing families. For example, NSK Self-Lube units combine a sealed ball-bearing insert with a pillow-block or flanged housing.

These housings support rotation. For bearings that guide a carriage along a straight path, see Linear Bearing Guides, and see Shaft Support and Axial Adjustment for shaft location and support arrangements.

Double a ball bearing's load and its basic rating life falls to one eighth; Interactive; Practice

Double a ball bearing's load and its basic rating life falls to one eighth

L10 is the life that 90% of a large group of identical bearings are expected to reach or exceed. It is neither a guaranteed life nor an average. Change one input below and watch the life curve.

Example bearing: C = 14.0 kN, given for this exercise. A real bearing's C comes from its manufacturer's catalog; this example does not represent a catalog part.

Basic rating life: L10 = (C/P)^p in millions of revolutions, with p = 3 for balls and p = 10/3 for rollers. C is the basic dynamic load rating; P is the equivalent dynamic load, taken as given here (for pure radial loading in this exercise, P = F_r). Use the same force units for C and P.

At constant speed n in r/min, L10h = L10 × 10⁶/(60 × n) hours. The years conversion assumes a duty schedule you can change; it does not predict calendar aging.

Static worked example (also used for printing).

Example ball bearing: C = 14 kN (given), P = 2 kN, n = 1500 r/min
C/P	7
L10 (million revolutions)	343
L10h (hours)	3811
Years at assumed duty	1.906

Duty assumption: 8 h/day × 250 days/year. Years = hours ÷ annual operating hours.

L10h versus P: logarithmic vertical scale. Equal vertical steps represent equal life ratios. C and speed stay fixed.

Marked points: current 2 kN → 3811 h; double load 4 kN → 476.4 h. Double the load → 1/8 the life.

C/P = 14/2 = 7; L10 = 7³ = 343 million revolutions. L10h = 343 × 10⁶/(60 × 1500) = 3811 h, or 1.906 years at 2000 h/year. At P = 4 kN, L10 = 42.875 million revolutions and L10h = 476.4 h.

Adjusted life factors (a1, aISO), contamination, lubrication and combined-load X/Y factors are beyond this page.

Practice: Ball rating life · Life in hours · Double the load · Roller rating life · Required dynamic rating ·