Rolling-Element Bearings: Selection, Fits and Installation: Shaft Raceway Tolerance Limits

Shaft Raceway Tolerance Limits — Drawn Cup Needle Bearings

Per ANSI/ABMA 18.2-1982 (R1993)

Machined Ring Needle Bearings (Type NIA with Inner Rings Type NIR)

For machined ring types, the housing bore shape requirements are equally stringent:

• Mean bore diameter variation across planes: Must not exceed 0.0005 inch (0.013 mm) or one-half the bore tolerance limit
• Radial deviation from circular form: Must not exceed 0.00025 inch (0.006 mm)
• Housing bore surface finish: Must not exceed 125 micro-inches (3.2 micrometers) arithmetical average

Machined Ring Shaft Diameter Tolerances

Key Insight: Notice the fit direction reverses depending on which ring rotates relative to the load. The rotating ring always gets the interference fit. This is the single most important principle in all of bearing mounting.

Needle Roller and Cage Assemblies (Types NIM and NM)

For boundary dimensions, tolerance limits, and fitting/mounting practice for these types, reference should be made to ANSI/ABMA 18.1-1982 (R1994) and ANSI/ABMA 18.2-1982 (R1993).

Bearing Mounting Practice

Rolling contact bearings are made of fully hardened steel. They do not "wear in" like journal bearings. They do not forgive misalignment. They do not absorb deflection gracefully.

This is the fundamental reality that separates successful bearing installations from catastrophic failures.

The DN Value Rule

The baseline for acceptable mounting practice begins with the DN value:

$$DN = D \times N$$

Where:

• D = bearing bore diameter (mm)
• N = bearing speed (revolutions per minute)

For medium-speed or slower applications (DN values of 400,000 or less) and medium to light loads (C/P values of 7 or greater, where C is the bearing specific dynamic capacity and P is the average bearing load), misalignments equivalent to those acceptable for high-capacity precision journal bearings can be tolerated.

Maximum Allowable Shaft Deflections

The "Aha" Moment: the practitioner realized that a deflection of 0.0002 inch per inch means that on a 4-inch-diameter bearing seat, the total allowable deflection across the face is only 0.0008 inches — less than the thickness of a human hair. At that scale, a fingerprint of contamination on the mounting surface can create enough local deformation to exceed the tolerance.

Assembly Methods — The Three Non-Negotiable Requirements

Every bearing assembly must ensure:

1. The faces are square before the cavity is closed
2. The cover face is square to the shoulder and pulled in evenly
3. The cover will be located by a face parallel to it when finally seated against the housing

These are not guidelines. These are the three conditions that must be verified with a dial indicator on every installation that matters.

Alignment Tolerances

Commercial Application Alignment Specifications

For commercial applications with moderate life and reliability requirements:

For preloaded and precision applications, cut these tolerances in half.

The Rolling-Contact Bearing Reality Check

Rolling-contact bearings do not wear in like journal bearings when carefully applied. At C/P values of 6 or less, the rolling element-race deformation is generally not over 0.0002 inch. This means the bearing itself provides almost zero accommodation for mounting errors.

Consequence: Proper mounting and control of shaft deflections are imperative. Aside from inadequate lubrication, mounting-related alignment problems are the most frequent cause of premature bearing failures.

Squareness and Alignment

End Faces and Shoulders

• End face and shoulder squareness: 0.0001 inch full indicator reading per inch of diameter
• Fillet eccentricities: Must fall within specified limits for radii tolerances to prevent interference and cocking of the race
• Shoulder height: Must be sufficient to properly support the races — hardened steel races are less capable of absorbing shock loads

Shaft Design Guidelines

What works:

• Bearing seats and locating shoulders turned and ground with the shaft held on centers
• Sufficient contact area between shoulder and bearing face for positive, accurate location
• Undercuts ending in fillets rather than sharp corners (sharp corners initiate fatigue cracks)
• Clamping nut threads cut true and square for even pressure distribution
• Shaft diameter between bearings great enough to resist bending

What fails:

• Threads cut so far into the bearing seat that part of the inner ring sits on threads instead of solid shaft
• More than two bearings on a single shaft (alignment becomes nearly impossible)
• Thin-walled housing sections that deflect during finish machining
• Housing designs where radial load is not transmitted directly to supporting walls or ribs
• Diaphragm walls connecting offset housings (they deflect unless thick and well-braced)
• Opposed bearings in separate housings without fins or webs to prevent axial-load deflection

Housing Design Rules

• Plenty of metal in wall sections — large, thin areas deflect during boring
• Direct load paths — radial loads should transmit as directly as possible to supporting walls or ribs
• Rigid boring setups — deep housings with long tool overhang produce out-of-roundness and taper unless very rigid tools with light finishing cuts are used
• Watch for peening — in a too-roughly-bored housing, metal ridges can peen down under load, eventually loosening the outer ring fit

Soft Metal and Resilient Housings

This is where most "experienced" engineers get caught. Aluminum, magnesium, light sheet metal, and thermally-expanding housings present a unique and dangerous mounting challenge.

The Failure Cascade

Here is the sequence of events that destroys bearings in soft housings:

1. Race loosening begins — whether from thermal expansion, material creep, or wear
2. Residual unbalance creates a rotating force — balancing processes ensure "acceptable maximum" unbalance, not zero
3. The rotating force initiates precession — the outer race begins to move within the housing
4. Precession causes further wear — pounding, abrasion, and attrition accelerate the loosening
5. The loosening accelerates exponentially — the rotating force exceeds friction forces between outer race, housing, and closures

The Only Reliable Solution

No foolproof method can be recommended for securing outer races in housings that deform significantly under load or after appreciable service wear.

The only sure solution is to press the race into a housing of sufficient stiffness with the heaviest fit consistent with installed and operating clearances. In many cases, inserts or liners of cast iron or steel are provided to:

• Maintain the desired interference fit
• Increase useful life of both bearing and housing
• Provide a dimensionally stable mounting surface

Quiet or Vibration-Free Mountings

In applications where vibration or smoothness is critical — instruments, medical equipment, precision spindles — the mounting approach appears to contradict everything stated above.

The Isolation Paradox

Here, bearing outer races are often supported on elastomeric or metallic springs to isolate shaft excursions from the frame, housing, or supporting structure.

But this creates a fundamental conflict:

• The primary bearing objective is location and restraint of the rotating body
• The vibration objective is isolation and decoupling from the structure
• Loose-mounted races risk rolling element skidding — where elements slide instead of roll, generating heat and surface damage

This is a specialist problem. It generally requires special or non-catalog bearings. Consult bearing manufacturer engineers directly — this is not a design-by-table exercise.

Precision Bearing Mounting Requirements

For ultra-precise or quiet applications:

• Shaft and housing roundness: Grinding chatter, lobular out-of-roundness, waviness, and flats of less than 0.0005 inch deviation from mean diameter can cause significant roughness
• Inspection method: Three-point electronic indicator inspection is mandatory; for critical applications, use a "Talyrond" or similar continuous recording instrument measuring to millionths of an inch
• Rolling element control: Precision bearings use rolling elements controlled to less than 0.000005 inch (5 millionths) deviation from roundness
• Shaft deformity transfer: Shaft deformities are reflected through inner races shrunk onto them; tight-fit outer races pick up housing deviations

For missile guidance and high-end instrument applications, deviations may need to be limited to less than 0.00002 inch.

Housing Response to Bearing Wobble

A critical discovery in vibration analysis: Housing response to axial excursions forced by bearing wobble (itself a result of out-of-square mounting) is a major source of noise and howl in small electric and rotating equipment.

The fix: Stiffer, more massive housings combined with careful alignment of bearing races.

General Mounting Precautions

More bearings are abused or "killed" during mounting than wear out under the conditions for which they were designed.

This is not a cautionary anecdote. This is the statistical reality of bearing failure analysis. The mounting and closing operations are the last steps in the bearing's journey from factory to operation — and they are where the majority of damage occurs.

The Damage Checklist — What Must Be Avoided

• Nicks — from careless handling or improper tools
• Dents — from hammer blows transmitted through the wrong race
• Scores — from sliding a bearing across a contaminated surface
• Scratches — from using non-precision tools or abrasive work surfaces
• Corrosion staining — from fingerprints, humidity exposure, or contact with dissimilar metals
• Dirt and contamination — from inadequate workspace cleanliness

The Design Engineer's Responsibility

It is the design engineer's responsibility to provide for proper mounting in:

• The design itself (access for tools, clearance for fixtures)
• Advisory notes on drawings
• Mounting instructions in assembly documentation
• Service manuals for field maintenance

If your design requires a hammer to install a bearing, you have designed the failure mode into the machine.

Seating Fits for Bearings

The AFBMA (Anti-Friction Bearing Manufacturers Association) standard shaft and housing bearing seat tolerances are the foundation of every bearing installation. Getting these right is non-negotiable.

Understanding Load Direction and Fit Selection

The required shaft and housing fits depend on three factors:

1. Type and extent of the load (light, normal, or heavy)
2. Bearing type (ball, cylindrical roller, or spherical roller)
3. Design and performance requirements

The Fundamental Fit Rule

For most normal applications where the shaft rotates and the radial load direction is constant, an interference fit on the inner ring is mandatory. The heavier the load, the greater the required interference.

For stationary shaft conditions with constant radial load direction, the inner ring may be moderately loose on the shaft.

For pure thrust (axial) loading, heavy interference fits are not necessary — only a moderately loose to tight fit is needed.

Shaft Tolerance Selection — Metric Radial Bearings (ABEC-1 / RBEC-1)

Per ANSI/ABMA 7-1995. For solid steel shafts. Hollow or nonferrous shafts may need tighter fits.

When greater accuracy is required, use j5, k5, and m5 instead of j6, k6, and m6 respectively.

Housing Tolerance Selection — Metric Radial Bearings (ABEC-1 / RBEC-1)

For cast iron or steel housings. Nonferrous alloy housings may need tighter fits.

Clamping and Retaining Methods

Once the bearing is seated, it must be positively retained against axial movement. The method you choose can enhance or destroy the precision of your installation.

Locknut and Lock Washer Method

The most common clamping method uses a nut screwed on the shaft end, held in place by a tongued lock washer.

Critical Requirements:

• Thread accuracy: The shaft thread must be cut in accurate relation to bearing seats and shoulders — inaccurate threads create bearing stresses
• Thread form: American National Form, Class 3
• Ground faces (when needed): For closer than average accuracy, lockwasher and locknut faces can be ground for better alignment
• Ground threads (for high precision): In precision applications, shaft threads are ground and more precise clamping methods are employed

AFBMA Standard Locknut Dimensions (Selected Sizes)

Per ANSI/AFBMA Standard 8.2-1991. Threads are American National Form, Class 3.

Shoulder Requirements

• Sufficient shoulder height is critical for positive and accurate bearing location
• If the difference between bearing bore and maximum shaft diameter gives a low shoulder that would enter the corner radius of the bearing, use a shoulder ring that extends above the shoulder and into the shaft corner
• A shoulder ring with snap wire fitting into a shaft groove can be used where no locating shoulder exists

Snap Ring Retention

• Snap rings in shaft grooves prevent endwise bearing movement away from the locating shoulder where tight clamping is not required
• Do not use snap rings where a groove slot in the shaft surface might lead to fatigue failure
• Snap rings are also used to locate outer bearing rings in the housing — dimensions per AFBMA and ANSI standards

Thread Design Warning

Never cut threads so far into the bearing seat that part of the inner ring is left unsupported or carried on threads. An unsupported inner ring will deflect under load, creating localized stress concentrations and premature spalling.

Bearing Closures

Closures are the last line of defense between your bearing's precision surfaces and the hostile environment outside. Shields, seals, labyrinths, and slingers each serve different roles depending on the application.

Shields vs. Seals — The Critical Difference

Seal Material Options

• Leather — Wide speed range; cup inward for best lubricant retention; cup outward at high speeds with abrasive dust to lead lubricant into contact area; only light pressure against shaft
• Rubber — Good general-purpose sealing; temperature limited
• Cork — Low-speed applications; limited sealing effectiveness
• Felt — Common in industrial equipment; moderate effectiveness
• Plastic composition — Custom-designed for specific applications

Seal Cartridges

Some seals are made as self-contained cartridge units that press into the bearing housing end. These simplify installation and replacement while providing consistent sealing performance.

Critical Seal Precaution

Excessive seal pressure must be avoided. The seal bears against the rotating member, and excessive contact pressure will:

1. Seize and burn the seal material
2. Score the rotating member surface
3. Generate heat that degrades the lubricant
4. Increase power consumption unnecessarily

Some lubricant must always flow into the seal contact area to prevent dry running.

Leather Seal Speed Considerations

• Low to medium speeds: Cup inward toward bearing for best retention
• High speeds: Cup inward arrangement risks burning the leather
• High speeds with abrasive dust: Cup outward to lead lubricant into contact area and flush contaminants

Method of Bearing Designation

The AFBMA identification code provides a universal, manufacturer-independent designation for every ball, roller, and needle bearing. Understanding this code eliminates confusion between company-specific part numbers.

Basic Number Structure

Every bearing has a basic number consisting of three elements:

1. Bore size — A one- to four-digit number indicating bore diameter in millimeters (metric series)
2. Type symbol — A two- or three-letter code indicating bearing type
3. Dimension series — A two-digit number identifying the series

Example: 50BC02 — bore is 50 mm, type BC (non-filling slot ball bearing), dimension series 02.

Supplementary Number

Additional characters are appended to designate specific features:

For Radial Bearings:

• Up to four letters for design modifications
• One or two digits for internal fit and tolerances
• One letter for lubricants and preservatives
• Up to three digits for special requirements

For Thrust Bearings:

• Two letters for design modifications
• One digit for tolerances
• One letter for lubricants and preservatives
• Up to three digits for special requirements

For Needle Bearings:

• Up to three letters for cage material, integral seal information, or crowned outside surface
• One letter for lubricants and preservatives

Complete Example: 50BC02JPXE0A10

• 50BC02 = basic number
• JPXE0A10 = supplementary number (design modifications, tolerance, lubrication, special requirements)

Dimension Series Explained

Annular ball, cylindrical roller, and self-aligning roller bearings are made in a series of:

• Different outside diameters for every given bore diameter
• Different widths for every given outside diameter

The two-digit dimension series number encodes both:

• First digit (8, 0, 1, 2, 3, 4, 5, 6, 9) = width series
• Second digit (7, 8, 9, 0, 1, 2, 3, 4) = diameter series

Bearing Tolerances — The Precision Hierarchy

Five classes of tolerances have been established to provide standards of precision for every application, from farm equipment to missile guidance systems.

Tolerance Classes at a Glance

What Each Class Controls

ABEC-1 / RBEC-1 through RBEC-5 control:

• Bore diameter and variation
• Outside diameter and variation
• Ring width
• Radial runout of inner and outer rings

ABEC-5 and higher additionally control:

• Parallelism of sides
• Side runout
• Groove parallelism with sides

Why You Need Precision Classes

Bearings closer than ABEC-1 / RBEC-1 are typically required for one or more of these reasons:

1. Very precise fits on shaft or housing — where dimensional accuracy of the mounting determines total system accuracy
2. Reduced eccentricity or runout — where the supported part must track true
3. Very high-speed operation — where even micro-inches of irregularity create unacceptable vibration and heat

ABEC-1 / RBEC-1 Tolerance Limits (Selected Values)

Inner Ring:

Outer Ring:

Per ANSI/ABMA 20-1987. All values in micrometers.

ABEC-3 / RBEC-3 — Tighter Control

ABEC-5 / RBEC-5 — Precision Grade

Note: ABEC-5 and higher add parallelism, side runout, and groove parallelism tolerances.

ABEC-7 — High Precision Grade

ABEC-9 — Ultra Precision Grade

Note: At ABEC-9, every dimensional parameter is controlled to single-digit micrometers.

Needle Roller Bearing Tolerances

Needle roller bearings have their own tolerance standards:

Drawn Cup Types (NIB, NIBM, NIY, NIYM, NIH, NIHM) — Inch:

Per ANSI/ABMA 18.2-1982 (R1993)

Bearing Fits — Preventing Slip and Creep

The Creep Problem

Bearing ring creep — the slow rotational slipping of a ring on its seat — occurs when the fit is too loose. Under dry, heavily loaded conditions, creep causes rapid wear of both shaft and bearing ring.

The Fit Principle

• Rotating ring → Press fit (interference)
• Stationary ring → Push fit (slight clearance or light transition)

For shock or vibratory loads, fits should be tighter than for ordinary service.

The stationary ring, if correctly fitted, is allowed to creep very slowly so that prolonged stressing of one part of the raceway is avoided. This controlled micro-creep actually extends bearing life by distributing load across the entire raceway circumference.

Thermal Mounting of Inner Rings

When a press fit is too tight for cold assembly, the inner ring may be heated to expand it:

• Method: Heat in clean oil or temperature-controlled furnace
• Temperature range: 200°F to 250°F (93°C to 121°C)
• Absolute maximum: Never exceed 250°F — overheating reduces ring hardness
• Prohibition: Prelubricated bearings must never be mounted by heating