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.

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.

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.

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

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.

Supporting detail: Addendum 1 — Manufacturer reading for bearing selection.

Practice identifying the load

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

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.

Read the basic designation before the extra letters

A bearing number is a compact description, not a universal sizing formula. Start with the maker and the bearing family. A basic designation identifies a series and bore; prefixes and suffixes add details. The same letters can mean different things in different families or manufacturers' catalogs.

For the ordinary metric deep-groove example 6205, read 6 | (0)2 | 05: type 6 is deep-groove ball; 02 is the dimension series, with width-series 0 omitted in the written number; bore code 05 means 25 mm. The diameter-series digit 2 is not an outside diameter in millimetres.

Width-series digits can be omitted differently: NSK lists 6005 as 6 | (1)0 | 05, and 6305 as 6 | (0)3 | 05. Parentheses here explain the omitted digit; they are not characters to add to an order. See NSK's bearing-series table.

Recognize the geometry first in Bearing Types and Loads. A matching bore alone does not make two bearings interchangeable.

00, 01, 02 and 03 are the bore-code exceptions

Thus 6202 has a 15 mm bore, not 10 mm; 6205 has a 25 mm bore. This rule supplies only the bore. It does not calculate D or B.

Do not apply it blindly to miniature, inch, letter-coded or special bearings. Slash forms can state the bore directly: NSK gives 63/22 as 22 mm and 230/560 as 560 mm. Its miniature 688 example has an 8 mm bore. NSK excludes double-direction thrust ball bearings from its general 04–96 rule.

Supporting detail: Addendum 2 — Primary references for the bore-code scope.

Series identify a family; the catalog supplies the envelope

A dimension series selects a standardized size family; it is a table key, not a digit-to-millimetres formula. Series examples follow NSK's designation guide. Different types also use different dimension letters. A tapered bearing distinguishes inner-ring width B, outer-ring width C and assembled width T. Thrust catalogs may label axial height T or H: use that manufacturer's diagram rather than substituting B automatically.

The 15 mm inner-ring width of HR30205J is not its 16.25 mm assembled width. These examples are not added to the three-row deep-groove viewer above. NSK's boundary-dimension diagrams also distinguish radial width from thrust height.

C3 means internal clearance, not accuracy or a loose shaft fit

Internal clearance is movement between a bearing's rings under the specified measuring conditions. A fit is the relationship between a ring and its shaft or housing seat. A precision class specifies dimensional and running accuracy. These are different quantities; see NSK's explanation of fits and clearance.

Prefixes matter too: NSK uses HR for its high-capacity tapered roller series. Read the complete maker-specific designation, including cage and lubrication details where required. Seals and shields affect exclusion of contaminants, friction and speed limits; they do not replace the catalog's load ratings. Sources: NSK TI/E/0111, auxiliary-symbol tables; NSK Bearing Designation Systems, sections 2 and 3.

For load direction and the existing basic rating-life calculation, return to Bearing Types and Loads. For how the bearing is located on a machine, see Shaft Support and Axial Adjustment.

Locate the shaft at one end and allow growth at the other

In a locating/non-locating arrangement, also called fixed/floating here, both supports carry radial load. The locating support also carries the applied axial load and establishes the shaft's axial position. The non-locating support allows axial movement as the shaft expands relative to the housing.

Location needs a load path from shaft to bearing to housing: shoulders, suitable spacers, nuts, retaining rings or covers can retain the rings. Which end is fixed follows the required position of the driven parts; it is not automatically the left end or the motor end.

The floating end can accommodate movement inside a suitable bearing, such as an NU cylindrical roller bearing, or by controlled sliding of a ring in its seat. Do not loosen an arbitrary fit: see Tolerances and Fits and the bearing maker's mounting guidance.

Supporting detail: Addendum 3 — Why two locating supports can create unwanted load.

Keep shaft support separate from torque transmission. Bearings support the shaft relative to the housing; a keyed shaft-hub connection transfers torque to or from a pulley, sprocket or gear. Securing a hub against sliding along the shaft is a separate assembly requirement.

End play is free movement; preload is an initial internal load

End play is the axial free movement available before the bearing arrangement takes up its clearance. Preload is a deliberately applied internal load before the external working load arrives. Reducing end play to a specified value and applying preload are different adjustments.

Do not tighten a bearing until it merely “feels solid.” Excess preload increases friction and temperature. Some arrangements require positive operating clearance; others use specified preload for stiffness or running accuracy. Temperature and fits change the operating condition.

A shim is a thin, accurately sized spacer. Changing its thickness sets a position in a rigid stack. Whether adding a shim increases or decreases end play depends on where the shim sits and which faces it separates.

Avoid burrs and rubbing contact that could release debris. Use the material and placement specified for the assembly.

A linear guide leaves travel along one axis

A linear bearing guides sliding motion just as a rotary bearing guides turning. Let x point along the travel. The guide supports transverse loads and, where its geometry allows, resists moments that would tilt or twist the carriage. A screw, belt or other actuator supplies travel force; the guide does not drive itself.

In the ideal model, translation along x is free. The desired constraints depend on whether rotation about x is acceptable. Real guide selection also checks load, moment capacity, stiffness, clearance, lubrication and alignment.

A round shaft does not provide an anti-rotation key

A plain round sleeve or linear ball bushing guides motion along a cylindrical shaft. In the ideal geometry it leaves two freedoms: T_x and R_x. The four other motions are restrained by the support's length or spaced bearings. A second parallel guide can stop the carriage rotating.

Geometric freedom is not a rotary service rating. A recirculating ball bushing may resist or wear under rotation even though its round shaft provides no positive anti-rotation feature. Use a product explicitly rated for combined linear and rotary motion when both are required.

LMU is a common catalog family label associated with round-shaft linear ball bushings. Treat it as a catalog name, not a universal standard or proof of rotary capability. Check the supplier's construction, dimensions and limits.

A profile rail constrains twist as well as sideways motion

A shaped rail and matching carriage have separated contact paths that resist twisting and tipping. Within their moment ratings, they leave one degree of freedom: T_x. Mounting surfaces must keep the rail aligned; adding constraints can also make an assembly sensitive to misalignment.

MGN commonly identifies a miniature, narrow-block profile-guide family. For example, HIWIN uses MGN within its miniature MG series, with recirculating balls. The label does not make every supplier's product interchangeable, and does not describe every plain or roller guide.

Opposed V-wheels guide a carriage; one wheel does not

A V-groove wheel runs on a matching edge or rail. A stable carriage commonly uses three or more wheels with contacts opposed across the rail and spaced along it. A triangular three-wheel arrangement can use an adjustable wheel to take up clearance.

The assembled carriage should leave only travel along the rail. Three wheels placed arbitrarily do not guarantee that result: orientation, spacing, contact retention and adjustment determine which loads and moments it can resist. A carriage that can lift away from its track has not constrained that motion.

Supporting detail: Addendum 6 — Manufacturer reading and the rotary-motion qualification.

Compare rotary support on Bearing Types and Loads. Practice counting freedoms