Bearing Selection

1) Introduction

This transcript introduces bearing selection concepts and the aims of the session:
- Identify the different parts of a single-row radial ball bearing
- Identify the different types of rolling element bearings
- Know the application of different types of bearings
- Explain life, rating life, basic load rating, and equivalent load
- Select bearings based on load, life, and speed
Topics listed in the agenda include: Bearing Numbering System, Types of Rolling Element Bearings, The Life Equation, Bearing Mounting, and Worked Examples on Bearing Selection (two sets)
Function of bearings (Page 4):
- (a) To reduce friction between two members
- (b) To help carry radial and axial loads

2) Types of Bearings

2.1 Plain Bearings
- Metallic porous impregnated bearings (sintered metals such as iron and bronze) impregnated with oil or polymer lubricants
- Non-metallic plain-dry bearings based on nylon, PTFE, carbon
- Hydrodynamic Fluid Film Bearings: surfaces kept apart by lubricant pressure generated hydrodynamically in the fluid film by rotation; lubricant may be fed under pressure
- Hydrostatic Fluid Film Bearings: surfaces kept apart by externally generated pressure in the lubricant; can support non-rotating shafts
2.2 Rolling Element Bearings (REB)
- Ball and Roller Bearings: bearings with rolling elements (ball, cylindrical, or spherical rollers)
Advantages of REB (Page 6):
- Higher speed capability
- Do not require constant lubrication
- Low wear rate
Disadvantages of REB (Page 6):
- Higher initial cost
- More radial space required
- Not ideal for overload/shock loading
- Noisier after wear; more susceptible to foreign matter

3) Bearing Nomenclature and Basic Geometry

Page 7 lists common nomenclature for bearings:
- Width, Ball Shoulders, Bore, Corner, Inner race, Ball race, Separator/Cage, Outer race, Face, Inner race, Outer diameter
Page 8 describes rolling loads as:
- (a) Radial load
- (b) Axial load
- (c) Radial and Axial load
Page 9 provides the key concept:
- Rolling bearings are classified as ball bearings or roller bearings based on rolling element
- Ball bearings use point contact with raceways (small contact area) limiting heavy loading; rollers provide line contact allowing higher load
- Ball bearings generally have lower friction than roller bearings

4) Common Bearing Types (selected highlights)

Deep Groove Ball Bearings (single row):
- Bearing No: 6xxx, 16xxx
- Capable of carrying radial and axial loads; high speeds; minimal lubrication required
- Example description (Page 10): large balls in raceways; high-speed capability
Angular Contact Ball Bearings (single row and duplex):
- Bearing No: 7XXX, 7XXX(B)DB, etc. with various contact angles
- Raceways offset to allow higher axial load capacity in one direction; duplex arrangements can be back-to-back, face-to-face, or in other configurations
Self-Aligning Ball Bearings (double row):
- Bearing No: 1XXX; two rows of balls with a common sphered outer raceway; permits angular misalignment (1.5° to 3° depending on size/series)
- Suitable for light axial and radial loads
Thrust Bearings (Groove Raceway Type):
- Thrust ball bearings (Flat Raceway Type) with high axial load capacity and low torque
Cylindrical Roller Bearings:
- Greater radial load capacity than radial ball bearings of the same size; NU/N series limitations on axial load
Double Row Spherical Roller Bearings:
- Spherical raceways allow misalignment; can accommodate both radial and axial loads

5) Bearing Designations and Coding (Overview)

Page 16 shows that bearing designations are built from a combination of type, series, and bore codes; example: 6310 → TYPE CODE / SERIES CODE / BORE CODE
Series Code (Page 17):
- Ball bearings and roller bearings have series codes indicating load capacity (e.g., Extremely Light, Extra Light, Light, Medium, Heavy)
Bore Code (Page 18-19):
- Two-digit bore code defines bore diameter in mm
- For bore diameters below 20 mm, special bore codes exist (e.g., 00 → 10 mm, 01 → 12 mm, 02 → 15 mm, 03 → 17 mm)
- For bore codes 04 and above, bore diameter = 5 × code number (e.g., 04 → 20 mm, 06 → 30 mm, 09 → 45 mm, 20 → 100 mm, 67 → 335 mm)
Example: Designation 6310 indicates a Deep Groove Ball Bearing, Medium load series, bore 50 mm (Page 19 example)

6) The Life Equation (Overview)

The Life Equation (L10h) relates basic dynamic load rating C, dynamic bearing load P_r, and speed n (rpm):
- The standard relationship for ball/roller bearings:
 $L{10h} = \bigg(\frac{C}{Pr}\bigg)^3 imes 10^6 \bigg/ n$
Where:
- $P_r$ = dynamic equivalent bearing load (N)
- $C$ = basic dynamic load rating (N)
- $n$ = speed in rpm
The dynamic equivalent load is computed from radial and axial loads using X and Y factors depending on bearing type and load ratio:
$Pr = X Fr + Y F_a$
Factors X, Y depend on bearing type, contact angle, and Fa/Fr ratio; they are obtained from manufacturer data or nomography. A common rule of thumb:
- If Fa/Fr ≤ e, then X = 1, Y = 0
- If Fa/Fr > e, then X and Y take values from charts/tables (examples shown in the transcript include X = 0.56, Y = 1.47 or X = 1, Y = 0.55 depending on arrangement)
The transcript shows practical use with nomographs and example values for fn (speed factor) and f (life factor) extracted from charts (nomography) to adjust results for given life and speed targets

7) Bearing Mounting (Axial Securing)

Inner ring axial securing (examples):
- (a) Locknut and lockwasher: locknut threads engage the shaft, with a tab in the lockwasher bent into a slot to prevent rotation; components include locknut, lockwasher, and an adaptor sleeve sometimes
- (b) Retaining ring: for small bearings, a retaining ring in a shaft groove can axially locate the inner ring
Outer ring axial securing:
- End covers are commonly used to clamp the outer ring axially; features include a housing, end cover, lock screws, and gaskets/seals to ensure clamping without leakage

8) Worked Examples on Bearing Selection (Summary of Approach and Results)

Worked Example 1 (Q1): “A deep groove ball bearing is required to carry a radial load of 2000 N with shaft speed 4000 rpm for a life of 10,000 hours. Determine minimum size and bearing number for bore sizes:
- (a) 40 mm
- (b) 50 mm
- (c) 60 mm”
- The session provides bearing numbers by bore size (from the table):
- (a) 40 mm bore → bearing number 6208 (Cr ≈ 29.1 kN)
- (b) 50 mm bore → bearing number 6210 (Cr ≈ 35 kN)
- (c) 60 mm bore → bearing number 6012 (Cr ≈ 29.5 kN)
- These choices reflect the minimum dynamic load ratings corresponding to the specified bore sizes to meet life requirement under the given loads and speed
Worked Example 2 (Q2): “Life of deep groove bearing no. 6208 under a radial load of 3200 N and axial load of 1650 N at 650 rpm, for life L10h = ?”
- Step 1: Compute dynamic equivalent load with Fa/Fr:
- Fa = 1650 N, Fr = 3200 N
- Fa/Fr = 1650/3200 ≈ 0.52
- From the table, since Fa/Fr > e, use X and Y appropriate for this arrangement (example used: X = 0.56, Y = 1.47)
- Step 2: Compute Pr: $Pr = X Fr + Y Fa = (0.56)(3200) + (1.47)(1650) = 4217.5 ext{ N}$
- Step 3: Use life equation with C (Cr) and P_r, along with speed factor fn and life factor f from nomographs (example provided):
- For the given data, the calculation yielded L10h ≈ 8100 hours for bearing 6208 under the specified loads and speed
Worked Example 3 (Q3): “A pair of deep groove bearings is required for a 40 mm diameter shaft which runs at 750 rpm. The shaft carries a stationary radial load of 1000 N at its central span and an axial load of 1500 N acts on it. The rating life required is 6000 hours. The loads are shared equally by the two bearings. Determine the bearing number.”
- Load sharing: per bearing, Fr = 500 N, Fa = 750 N (since the total is split between two bearings)
- Trial-and-error approach used starting with smallest bearing size to find suitable X, Y values and P_r, then compare required life using the life equation
- The session shows the process using sizes around 40 mm bore with bearing choices such as 6808 and 6908 to meet life and load requirements, concluding on the selection process rather than a single fixed final number in the transcript
Worked Example 4 (Q4): “Determine the life of a duplex 60 mm bore, 73 series angular contact ball bearings (40° contact angle and face-to-face arrangement) under a stationary radial load of 16,000 N and an axial load of 18,000 N. Shaft speed 1000 rpm.”
- Configuration: duplex angular contact, 60 mm bore, 73 series, 40° contact angle, face-to-face
- Load components: Fr = 16,000 N, Fa = 18,000 N
- For a duplex arrangement, Pr is calculated using dynamic load factors X and Y for the arrangement (single/dual contact configuration; single DT vs DB/DF variants). The example uses: Pr = X Fr + Y Fa with X ≈ 1 and Y ≈ 0.55 (from the nomograph data in the transcript)
- Calculation: P_r = (1)(16000) + (0.55)(18000) = 25900 N
- Choose the appropriate bearing (7312 BDF in the example) and compute life factor using the nomograph:
 $fh = fn imes \frac{Cr}{Pr}$
- With the given data, fn ≈ 0.32 and Cr ≈ 146 ext{ kN} (for 7312 BDF in the table), P_r = 25900 N
- f_h = 0.32 × 146000 / 25900 ≈ 1.80
- Life: L10h ≈ 2900 hours (for the duplex angular contact arrangement with the given loads and speed)

9) Key Formulas and Relationships (Summary)

Dynamic load for bearing: $Pr = X Fr + Y F_a$
- X, Y depend on bearing style and load ratio Fa/Fr; see nomograms/tables for specific values
Life equation (basic dynamic load rating): $L{10h} = \bigg(\frac{C}{Pr}\bigg)^3 imes 10^6 \bigg/ n$
- C = basic dynamic load rating (N)
- P_r = dynamic equivalent load (N)
- n = shaft speed in rpm
Conversion/nomogram adjustments: life factor f and speed factor f_n are used to adjust life calculations for different life targets and speeds (from the nomograph). Example values used in the transcript include:
- At 4000 rpm, f_n ≈ 0.203; Life factor for 10,000 hours ≈ 2.7
- For other speeds, f_n and f values vary; consult the nomograph for exact values at the given life and speed
Load sharing in multi-bearings systems: when multiple bearings support a load, distribute Fr and Fa appropriately and compute P_r for each bearing. For example, with two bearings sharing radial and axial loads evenly, each bearing sees half of the total radial load and axial load

10) Quick Reference: Representative Bearing Numbers and Cr (From Transcript Tables)

For bore sizes used in the examples:
- 40 mm bore: bearing number 6208 (Cr ≈ 29.1 kN)
- 50 mm bore: bearing number 6210 (Cr ≈ 35 kN)
- 60 mm bore: bearing number 6012 (Cr ≈ 29.5 kN)
For angular contact duplex arrangements and 60 mm bore examples: complex designations such as 7312 BDF are used (with Cr ≈ 146 kN in the example table) and are selected based on the required Pr and life

11) Practical Takeaways

Life and speed requirements drive the selection of bearing size and type; larger bore sizes typically offer higher dynamic load ratings but add weight and cost
The dynamic load rating C must be sufficient to carry the dynamic equivalent load P_r at the operating speed to achieve the required life
For mixed radial/axial loading, calculating P_r accurately requires the appropriate X and Y factors; incorrect factors lead to erroneous life predictions
Two bearings sharing loads requires careful distribution of Fr and Fa to avoid overloading a single bearing
Bearing mounting details (locknuts, lockwashers, end covers, retaining rings) are critical to proper axial positioning and preventing unwanted movement during operation
The nomograph-based approach (for fn and f) is essential to translate life and speed targets into usable P_r and C requirements; when a nomograph is not available, use manufacturer data tables or software to obtain X, Y, fn, and f values

12) Notes on Exam-Style Practice

Be able to compute P_r given Fr and Fa for common bearing types (deep groove vs angular contact) and common configurations (single, duplex)
Be able to determine if a given bearing size (by bore) meets life requirements using the formula for L10h and the known rpm
Be able to interpret a bearing designation (6310, 6208, 7312 BDF, etc.) and identify basic type, approximate bore, and general load capacity
Be prepared to apply axial securing concepts to select suitable mounting methods for inner and outer rings in a given assembly

13) Quick Glossary

Dynamic load rating (C): the maximum load a bearing can endure for a specified life under standard conditions
Dynamic equivalent load (P_r): the radial/axial load equivalent that accounts for direction and combination of loads
Life (L10h): the expected bearing life in hours at a given speed, based on the life equation
Fa: axial load
Fr: radial load
X, Y: coefficients used to compute P_r depending on bearing type and load ratio
e: a parameter in X/Y selection derived from bearing type and geometry
Nomograph: a chart used to determine factors like f_n and f given operating life and speed
Duplex arrangement: two bearings mounted in a configuration (back-to-back DB, face-to-face DF) to handle combined loads with proper misalignment tolerance