Rolling-Element Bearings: Selection, Fits and Installation: Soft Metal and Resilient Housings (Aluminum,...

Soft Metal and Resilient Housings (Aluminum, Magnesium, Sheet Metal)

These materials present a special challenge:

• Outer races can loosen and turn as the soft housing wears, deforms under load, or changes dimension with temperature
• Rotating unbalance forces (which exist even after balancing) initiate precession that aggravates loosening through wear, pounding, and abrasion
• No foolproof "fix" exists for securing outer races in housings that deform significantly
• The only reliable solution: Press the race into a housing of sufficient stiffness with the heaviest fit consistent with installed and operating clearances
• Where soft housings are unavoidable: Use cast iron or steel inserts/liners to maintain the desired fit and extend bearing and housing life

Clamping and Retaining Methods

Shaft Clamping

The most common method uses a locknut screwed onto the shaft end and secured with a tongued lockwasher. Critical requirements:

• Shaft threads must be cut in accurate relation to bearing seats and shoulders to avoid inducing bearing stresses
• Threads are American National Form, Class 3 — special diameters exist per AFBMA standards
• Where closer accuracy is required, locknut faces and washers can be obtained ground for tighter alignment with threads
• For highest accuracy, shaft threads are ground and more precise clamping methods are employed

Shoulder Requirements

• Sufficient shoulder height is required to properly support the races — hardened steel races are less capable of absorbing shock loads
• Low shoulders (where the difference between bore and max shaft diameter is small) require a shoulder ring that extends above the shoulder into the shaft corner
• Snap rings can prevent endwise movement away from locating shoulders, but should not be used where a shaft groove might lead to fatigue failure

General Bearing Handling Precautions: The 23-Point Checklist

This is the checklist that separates professional bearing work from amateur assembly. Every point here addresses a documented cause of premature bearing failure:

Storage and Preparation

1. Use the best bearing available for the application — the cost of a premium bearing is small compared to replacement costs of rotating components destroyed by bearing failure
2. Consult the bearing manufacturer's representative when questions arise in application design
3. Keep bearings in sealed, original containers until ready to use
4. Follow manufacturer's instructions for handling and assembly
5. Work with clean tools, clean dry hands, and clean surroundings
6. Do not wash or wipe bearings prior to installation unless specifically required
7. Place unwrapped bearings on clean paper and keep covered if original container is unavailable

Assembly

8. Never use wooden mallets, brittle or chipped tools, or dirty fixtures
9. Never spin uncleaned bearings, and never spin any bearing with an air blast
10. Do not scratch or nick bearings — even minor surface damage initiates spalling
11. Do not strike or press on race flanges
12. Use adapters for mounting that provide uniform steady pressure — never hammer on a drift or sleeve
13. Start races evenly onto shafts and into housings to prevent cocking
14. Inspect shafts and housings before mounting to confirm proper fits will be maintained

Disassembly and Reuse

15. Clean housings, covers, and shafts before exposing bearings during removal — all dirt is an abrasive dangerous to reuse
16. Treat used bearings (that may be reused) as carefully as new ones
17. Protect dismantled bearings from dirt and moisture
18. Use clean, lint-free rags if bearings must be wiped
19. Wrap bearings in clean, oil-proof paper when not in use
20. Use clean, filtered, water-free Stoddard's solvent or flushing oil to clean bearings

Critical Assembly Warnings

21. When heating bearings for shaft mounting, follow manufacturer's instructions exactly — never exceed 250°F
22. When assembling onto shafts, never strike the outer race or press on it to force the inner race — apply pressure on the inner race only. The same applies during dismantling.
23. Never press, strike, or force the seal or shield on factory-sealed bearings

Bearing Failures: Recognizing the Symptoms Before Catastrophe

the practitioner's failure taught him to listen for the warning signs. Here are the general classifications of failures and deficiencies that require bearing removal, along with their most common causes:

Overheating

Vibration

Noisy Bearing

Shaft Binding

Shaft Displacement

Engineering takeaway

the practitioner went from a competent mechanic to a bearing-installation expert by learning one fundamental truth:

The bearing is only as good as the geometry that supports it.

Every tolerance in this guide exists because someone, somewhere, learned the hard way what happens when it's violated. Shaft shoulder squareness. Housing bore roundness. Race fit interference. Mounted clearance. Assembly cleanliness. Each one is a link in a chain, and the chain breaks at its weakest point.

The Universal Principles

These principles apply regardless of bearing type, application, or era:

1. Squareness of shoulders and faces is of primary importance to bearing life
2. The rotating ring gets an interference fit; the stationary ring gets a clearance fit
3. 80% of the interference fit shows up as lost internal clearance — account for it
4. Roller bearings cannot tolerate preload the way ball bearings can — verify clearance post-assembly
5. Three-point measurement catches what two-point measurement misses
6. Shaft deflection under load is an alignment problem, not a bearing problem
7. More bearings are killed during installation than die during service — use proper tools, clean conditions, and uniform force application
8. Check running clearance after assembly, every time, on every critical application

Your Next Step

Pick one machine in your facility — the one that eats bearings, the one that's always warm, the one that hums a little louder than it should.

Pull the engineering drawings. Check the shaft shoulder squareness with a dial indicator. Measure the housing bore with a three-point method. Compare the interference fit to the tables in this guide. Check the mounted clearance.

You may discover that the "bad bearing" problem you've been living with is actually a geometry problem that's been hiding in plain sight for years.

The bearing isn't broken. The installation is. Fix the installation, and the bearing will take care of itself.

Have you ever traced a chronic bearing failure back to an installation issue? What did you find? Share your experience — your lesson might save someone else's production line.

Overview

• Topic: Rolling element bearing selection, life calculation, and design methodology
• Scope: Covers bearing types, load rating systems, life prediction methods (basic, adjusted, and advanced), bearing selection procedures for multiple bearing categories, and supporting reference data
• Core Principle: Bearing life is a statistical estimate influenced by load, lubrication, cleanliness, and material — the more factors accounted for, the more accurate the prediction
• Key Takeaway: Three progressively refined methods exist for calculating bearing life, each incorporating additional real-world factors beyond basic load capacity

Dynamic Load Rating and Bearing Life

• Basic Dynamic Load Rating (C): The constant radial load under which a group of identical bearings will achieve a basic rating life of 1 million revolutions
• L₁₀ Life: The life that 90% of a sufficiently large group of identical bearings can be expected to attain or exceed under given operating loads
• The average life of a bearing is approximately 5 times the L₁₀ life
• L₁₀ life is expressed in millions of revolutions

Three Methods for Determining Bearing Life

• Method 1 — Basic L₁₀ Life: Accounts only for the loads on the bearing; simplest and most conservative
• Method 2 — Adjusted Rating Life (Lna): Extends Method 1 by incorporating reliability, material type, and lubricant viscosity
• Method 3 — New Life Theory (Lnaa): Further extends Method 2 by adding the concept of a fatigue load limit and contamination factor, enabling infinite life prediction under ideal conditions

Factors Affecting Bearing Life

• Steel type: Standard bearing steels meet international specifications; premium steels can exceed standard life properties
• Lubrication: Encompasses lubricant type (oil/grease), additives, viscosity, temperature, circulation method, and change intervals
• Cleanliness: Encompasses environmental contaminants, particulate ingress, water contamination, lubricant filtration, and sealing method

Bearing Types Overview

• 37 common types of rolling element bearings exist, categorised into radial and thrust families
• Selection depends on factors such as: load direction, shaft speed, accuracy, noise, friction, self-alignment capability

Radial Bearings

• Deep groove ball bearings — Single row, double row, with shields or seals, with snap ring groove in outer ring
• Self-aligning ball bearings — Cylindrical or tapered bore, with seals, with extended inner ring
• Angular contact ball bearings — Single row, paired mounting, precision, double row, four-point contact
• Cylindrical roller bearings — Multiple sub-types (NJ, NJP, NNU, NN), single/double/four row, full complement
• Needle roller bearings — Drawn cup, open/closed ends, with flanges, with/without inner ring, with seals
• Spherical roller bearings — Cylindrical or tapered bore
• Taper roller bearings — Single row, four row, crossed

Thrust Bearings

• Thrust ball bearings — Single/double direction, with flat/spherical housing washers, with sealing rings
• Angular contact thrust ball bearings — Single/double direction
• Cylindrical roller thrust bearings
• Needle roller thrust bearings
• Spherical roller thrust bearings
• Taper roller thrust bearings — Single/double direction

Bearing Type Selection Characteristics

Legend: ++ = excellent, + = good, — = poor/unsuitable, ½ = limited in one direction

Bearing Selection Procedures

Selection of Deep Groove Ball Bearings (Method 1 — Basic L₁₀ Life)

1. Determine required life in operating hours
   • Consider continuous vs intermittent operation and expected equipment life
   • Common default for mechanical design: 10 years
2. Convert design life from hours to millions of revolutions:
   • L = (60 × N × h) / 10⁶
   • Where: L = design life (millions of revolutions), N = average speed (rev/min), h = design life (hours)
3. Determine average radial load (Fr) and axial load (Fa)
   • Loads are typically shaft reaction forces at the bearing
   • Apply shock factors to steady loads where dynamic loads occur
4. Calculate the ratio Fa/Fr
5. Select a bearing from data tables for the known shaft size
   • In the absence of other knowledge, select a bearing around the mid-range of those available
   • Record the static load rating Co and dynamic load rating C
6. Calculate the ratio Fa/Co and read off the value of e from the calculation factors graph
   • For normal clearance bearings, project from the Fa/Co value to the Y line and down to the e scale
7. Calculate the equivalent dynamic bearing load P:
   • If Fa/Fr ≤ e → then P = Fr
   • If Fa/Fr > e → then P = X·Fr + Y·Fa where X = 0.56
   • Values of Y can be read from the graph by projecting across from the Fa/Co value to the Y line
   • Note: This graph is drawn for normal clearance bearings
8. Calculate the bearing L₁₀ life:
   • L₁₀ = (C / P)³
9. Compare L₁₀ to L:
   • If L₁₀ < L → bearing is too small; repeat steps 5–8 with a higher load capacity bearing
   • If L₁₀ ≈ L → bearing is acceptable
   • If L₁₀ > L → bearing may be oversized; repeat with a lower load capacity bearing
10. Calculate equivalent static bearing load Po:
    • Po = 0.6·Fr + 0.5·Fa
    • Check that Po < Co; if not, select another bearing
    • Note: If Po < Fr then set Po = Fr
11. Check minimum radial load:
    • For satisfactory operation: Fr > 0.01·C
    • More accurate methods based on lubricant viscosity and operating speed exist in manufacturer catalogues
12. Check maximum shaft speed does not exceed the speed rating of the bearing

Selection of Self-Aligning Ball Bearings (Method 1 — Basic L₁₀ Life)

• Steps 1–4 are identical to the deep groove ball procedure
• Step 5: Use bearing data tables specific to self-aligning ball bearings (with or without adaptor sleeves)
  • Record C, Co, and calculation factors: e, Y₁, Y₂, Y₀
• Step 6: Calculate equivalent dynamic bearing load P:
  • If Fa/Fr ≤ e → then P = Fr + Y₁·Fa
  • If Fa/Fr > e → then P = 0.65·Fr + Y₂·Fa
• Steps 7–8: Calculate L₁₀ life using L₁₀ = (C / P)³ and compare to required life
• Step 9: Calculate equivalent static bearing load:
  • Po = Fr + Y₀·Fa
  • Check that Po < Co
  • Note: If Po < Fr then set Po = Fr
• Step 10: Minimum radial load check: Fr > 0.01·C
• Step 11: Check maximum shaft speed
• Step 12: If tapered bore with adaptor sleeve, check axial load limit:
  • Fa < 3·B·d
  • Where: Fa = axial load (N), B = bearing width (mm), d = internal diameter (mm)

Selection of Cylindrical Roller Bearings (Method 1 — Basic L₁₀ Life)

• Steps 1–4 follow the same pattern
  • Note: Fa/Fr ratio should not exceed 0.5
• Step 5: Use cylindrical roller bearing data tables; record Co and C
• Step 6: Calculate equivalent dynamic bearing load P:
  • For bearings without flanges (type NU): P = Fr
  • For bearings with flanges:
    • If Fa/Fr ≤ e → then P = Fr
    • If Fa/Fr > e → then P = 0.92·Fr + Y·Fa
    • Where: e = 0.2, Y = 0.6 for series 10, 2, 3, and 4; e = 0.3, Y = 0.4 for series 22 and 23
• Step 7: Calculate L₁₀ using: L₁₀ = (C / P)^(10/3)
  • Note: Roller bearings use the exponent 10/3 rather than 3
• Steps 8–9: Compare life, check static load:
  • Po < Co/1.5 (for cylindrical roller bearings)
  • Note: Po = Fr for cylindrical roller bearings
• Step 10: Minimum radial load: Fr > 0.02·C
• Step 11: Check maximum shaft speed

Selection of Spherical (Self-Aligning) Roller Bearings (Method 1 — Basic L₁₀ Life)

• Steps 1–4 follow the same pattern
• Step 5: Use spherical roller bearing data tables; record Co, C, and calculation factors e, Y₁, Y₂, Y₀
• Step 6: Calculate equivalent dynamic bearing load P:
  • If Fa/Fr ≤ e → then P = Fr + Y₁·Fa
  • If Fa/Fr > e → then P = 0.67·Fr + Y₂·Fa
• Step 7: Calculate L₁₀ using: L₁₀ = (C / P)^(10/3)
  • Roller bearing exponent 10/3 applies
• Steps 8–9: Compare life, check static load:
  • Po = Fr + Y₀·Fa
  • Check that Po < Co/1.5
• Step 10: Minimum radial load: Fr > 0.02·C
• Step 11: Check maximum shaft speed

Method 2 — Adjusted Rating Life

• Uses the formula: Lna = a₁ × a₂ × a₃ × L₁₀
• Where:
  • a₁ = life adjustment factor for reliability
  • a₂ = life adjustment factor for material
  • a₃ = life adjustment factor for operating conditions

Reliability Factor (a₁)

• For the standard 90% reliability: a₁ = 1
• For higher reliability levels, manufacturer handbooks provide corresponding a₁ values (always < 1)

Material Factor (a₂)

• For standard bearing steels (as defined by international standards): a₂ = 1
• Premium manufacturer steels may have higher life properties

Operating Conditions Factor (a₃)

• Determined primarily by bearing lubrication (assuming normal operating temperatures and cleanliness)
• Some manufacturers combine a₂ and a₃ into a single factor a₂₃

Determining a₂₃

1. Calculate the mean diameter of the bearing: dₘ = 0.5 × (d + D)
   • Where d = bore diameter, D = outer diameter
2. From Diagram 1, read the required kinematic viscosity (ν₁) for adequate lubrication at the given rotational speed and mean diameter
3. Calculate the viscosity ratio: κ = ν / ν₁
   • Where ν = actual kinematic viscosity of the lubricant at operating temperature
4. From Diagram 3, read the value of a₂₃ based on κ
5. Apply: Lna = a₁ × a₂₃ × L₁₀

Notes on Viscosity and Temperature

• Viscosity and temperature of lubricants are interrelated; for liquid lubricants, viscosity decreases with temperature
• Typical pre-lubricated deep groove ball bearing grease has a viscosity of 100 mm²/s at 40°C
• Operating temperature depends on ambient temperature, bearing load, shaft speed, housing design, and similar factors
• Measurement of similar bearings under operating conditions is a good method for estimating operating temperature
• Diagram notes: shaded area on Diagram 3 is for lubricants with additives; diagrams are valid for mineral oils and greases under normal cleanliness conditions

Method 3 — New Life Theory

• Introduces the concept of a fatigue load limit (Pu) — the load below which fatigue will not occur (given adequate lubrication and cleanliness)
• For loads below Pu, the bearing theoretically lasts indefinitely
• The formula: Lnaa = a₁ × a_the bearing supplier × L₁₀
• Where:
  • a₁ = reliability factor (same as Method 2)
  • a_the bearing supplier = life adjustment factor based on new life theory

Determining a_the bearing supplier

1. Calculate the viscosity ratio κ (same as Method 2)
2. Determine the contamination factor ηc from the contamination table
3. Calculate: ηc × Pu / P
4. From Diagram 4 (ball bearings) or Diagram 5 (roller bearings), read a_the bearing supplier based on κ and ηc·Pu/P
5. Apply: Lnaa = a₁ × a_the bearing supplier × L₁₀ (assuming a₁ = 1 for 90% reliability: L_the bearing supplier = a_the bearing supplier × L₁₀)

Key insight: If κ > 4, use the κ = 4 curve. As ηc·Pu/P tends to zero, a_the bearing supplier tends to 0.1 for all values of κ

Contamination Factor (ηc) Reference Table

Worked Example — Comparing All Three Methods

Given:

• Shaft diameter: 45 mm
• Speed: 5000 rev/min
• Bearing designation: 6309 (deep groove ball)
• Radial load: 8 kN, no axial load
• Lubrication: oil with viscosity 20 mm²/s at operating temperature
• Reliability: 90% (normal)

Method 1 — Basic L₁₀ Life:

• From bearing tables for 6309 (45 mm shaft): C = 52.7 kN
• Since no axial load: P = Fr = 8 kN
• L₁₀ = (52.7 / 8)³ = 286 million revolutions

Method 2 — Adjusted Life:

• a₁ = 1 (90% reliability)
• From tables: D = 100 mm → dₘ = 0.5 × (45 + 100) = 72.5 mm
• From Diagram 1 at 5000 rpm: required viscosity ν₁ = 7 mm²/s
• Actual viscosity ν = 20 mm²/s → κ = 20/7 = 2.9
• From Diagram 3 with κ = 2.9: a₂₃ = 2
• Lna = 1 × 2 × 286 = 572 million revolutions
• The longer life is due to lubricating oil viscosity being ~3× the minimum required

Method 3 — New Life Theory:

(a) Clean conditions:

• From tables: Pu = 1.34 kN, ηc = 0.8 (normal cleanliness)
• ηc × Pu/P = 0.8 × 1.34/8 = 0.134
• From Diagram 4 with κ = 2.9: a_the bearing supplier ≈ 8
• Lnaa = 8 × 286 = 2288 million revolutions
• This is 4× higher than the adjusted life method prediction

(b) Contaminated conditions (ηc = 0.2):

• ηc × Pu/P = 0.2 × 1.34/8 = 0.0335
• From Diagram 4 with κ = 2.9: a_the bearing supplier ≈ 1.2
• Lnaa = 1.2 × 286 = 343 million revolutions
• Contamination causes a considerable reduction in predicted life

Comparison Tables

Life Calculation Methods Comparison

Bearing Exponents by Type

Equivalent Dynamic Load Formulas in the supplied reference

Equivalent Static Load Formulas in the supplied reference

Note: For all types, if Po < Fr, then set Po = Fr

Minimum Radial Load Requirements

Self-Aligning Ball Bearing Adaptor Sleeve Axial Load Limit

Mermaid Diagrams

Bearing Selection Decision Process

flowchart TD
    A[Start: Define Operating Requirements] --> B[Determine required life in hours]
    B --> C[Convert hours to millions of revolutions<br/>L = 60·N·h / 10⁶]
    C --> D[Determine radial load Fr and axial load Fa]
    D --> E[Calculate Fa/Fr ratio]
    E --> F[Select bearing type based on<br/>load direction, speed, alignment needs]
    F --> G[Select specific bearing from data tables<br/>Record C, Co, and calculation factors]
    G --> H[Calculate equivalent dynamic load P]
    H --> I[Calculate L₁₀ = C/P raised to p<br/>p=3 ball, p=10/3 roller]
    I --> J{L₁₀ vs L?}
    J -->|L₁₀ < L| K[Bearing too small<br/>Select higher capacity]
    K --> G
    J -->|L₁₀ ≈ L| L[Bearing OK]
    J -->|L₁₀ >> L| M[Bearing oversized<br/>Select lower capacity]
    M --> G
    L --> N[Check static load Po < Co or Co/1.5]
    N --> O[Check minimum radial load]
    O --> P[Check maximum shaft speed]
    P --> Q[Bearing Selection Complete]

Three Life Calculation Methods

flowchart LR
    subgraph Method1["Method 1: Basic L₁₀"]
        M1A[Load data only] --> M1B["L₁₀ = (C/P)^p"]
    end

    subgraph Method2["Method 2: Adjusted Life Lna"]
        M2A[Load data] --> M2D
        M2B[Reliability a₁] --> M2D
        M2C[Material + Lubrication a₂₃] --> M2D
        M2D["Lna = a₁ · a₂₃ · L₁₀"]
    end

    subgraph Method3["Method 3: New Life Lnaa"]
        M3A[Load data] --> M3E
        M3B[Reliability a₁] --> M3E
        M3C[Viscosity ratio κ] --> M3E
        M3D[Contamination ηc + Fatigue limit Pu] --> M3E
        M3E["Lnaa = a₁ · a_the bearing supplier · L₁₀"]
    end

    Method1 -.->|Adds reliability,<br/>material, lubrication| Method2
    Method2 -.->|Adds fatigue limit<br/>and contamination| Method3

Factors Affecting Bearing Life

flowchart TD
    A[Bearing Life] --> B[Steel Type]
    A --> C[Lubrication]
    A --> D[Cleanliness]

    B --> B1[Standard ISO steel: a₂ = 1]
    B --> B2[Premium steels: a₂ > 1]

    C --> C1[Lubricant type: oil / grease]
    C --> C2[Additives present?]
    C --> C3[Viscosity at operating temp]
    C --> C4[Circulation and filtration method]
    C --> C5[Change interval]

    D --> D1[Environmental contaminants]
    D --> D2[Metallic particles / dirt / dust]
    D --> D3[Water contamination]
    D --> D4[Lubricant filtration quality]
    D --> D5[Sealing method effectiveness]

Viscosity Ratio (κ) Determination Process

flowchart TD
    A[Calculate mean diameter<br/>dₘ = 0.5 × d + D] --> B[Use Diagram 1:<br/>Find required viscosity ν₁<br/>from speed and dₘ]
    B --> C[Determine actual lubricant<br/>viscosity ν at operating temp]
    C --> D[Calculate κ = ν / ν₁]
    D --> E{κ value?}
    E -->|κ < 1| F[Inadequate lubrication<br/>Reduced life]
    E -->|κ ≈ 1| G[Marginal lubrication<br/>Standard life]
    E -->|κ > 1| H[Good lubrication<br/>Extended life]
    E -->|κ > 4| I[Use κ = 4 curve<br/>Maximum benefit reached]

Imperial-Metric Equivalents Reference

Key Terms Glossary

• L₁₀ Life: The rated life at which 90% of a group of identical bearings will survive under a specified load; expressed in millions of revolutions
• Basic Dynamic Load Rating (C): The constant radial load that produces a basic rating life of 1 million revolutions for a bearing
• Basic Static Load Rating (Co): The static load that produces a specified permanent deformation at the most heavily stressed rolling element/raceway contact
• Fatigue Load Limit (Pu): The load below which metal fatigue will not occur in a bearing with adequate lubrication and cleanliness (used in Method 3)
• Equivalent Dynamic Bearing Load (P): A calculated constant radial load that would produce the same life as the actual combined radial and axial loads
• Equivalent Static Bearing Load (Po): A calculated static radial load that would cause the same total permanent deformation as the actual combined loads
• Viscosity Ratio (κ): The ratio of actual lubricant kinematic viscosity to the required minimum kinematic viscosity at operating temperature (κ = ν/ν₁)
• Contamination Factor (ηc): A factor (0 to 1) representing the level of particulate contamination in the bearing operating environment
• Adjusted Rating Life (Lna): Bearing life calculated using Method 2, incorporating reliability, material, and lubrication adjustment factors
• a₁: Life adjustment factor for reliability (= 1 for 90% reliability)
• a₂: Life adjustment factor for material (= 1 for standard bearing steels)
• a₃: Life adjustment factor for operating conditions (primarily lubrication)
• a₂₃: Combined material and operating conditions factor
• a_the bearing supplier: Life adjustment factor used in the new life theory (Method 3)
• Deep Groove Ball Bearing: Most common bearing type; handles radial and moderate axial loads; low friction; high speed capability
• Self-Aligning Ball Bearing: Accommodates shaft misalignment and deflection; uses a spherical outer ring raceway
• Cylindrical Roller Bearing: Handles heavy radial loads; higher capacity than ball bearings of equivalent size; line contact between rollers and raceways
• Spherical Roller Bearing: Handles heavy radial and moderate axial loads while accommodating misalignment; uses barrel-shaped rollers
• Adaptor Sleeve: A tapered sleeve used to mount bearings with tapered bores onto cylindrical shafts
• Shock Factor: A multiplier applied to steady loads to account for dynamic/impact loading conditions