Design of Bearings free study notes for Diploma / BTech.

1. What is Bearing ?

A bearing is a mechanical component designed to reduce friction between moving parts and support loads. Its primary job is to allow for smooth rotation or linear movement while ensuring that parts stay in their correct alignment.

Without bearings, the heat and wear generated by metal rubbing against metal would cause machinery to fail almost instantly.

2. Types of Bearings

(a) Sliding Contact Bearings (Plain Bearings)

• Surfaces slide over each other
• Examples: Journal bearing, Footstep bearing
• Used in heavy load & low-speed applications

(b) Rolling Contact Bearings (Anti-friction Bearings)

• Rolling elements (balls/rollers) reduce friction
• Types:
  • Ball bearings
  • Roller bearings (cylindrical, tapered, needle)
• Used in high-speed applications

3. Functions of Bearings

• Support shaft and guide motion
• Reduce friction and wear
• Carry radial and/or axial loads
• Maintain shaft alignment

4. Design Considerations

Key factors in bearing design:

(1) Load Carrying Capacity

• Radial load (Fr)
• Axial (thrust) load (Fa)
• Combined loading

(2) Speed of Operation

• High speed → rolling bearings preferred
• Low speed → sliding bearings

(3) Lubrication

• Essential for reducing friction and wear
• Types:
  • Hydrodynamic lubrication
  • Boundary lubrication
  • Hydrostatic lubrication

(4) Material Selection

• Good fatigue strength
• Low friction coefficient
• Good thermal conductivity
• Common materials:
  • Babbitt metal
  • Bronze
  • Steel

(5) Clearance and Fit

• Proper clearance avoids overheating
• Too tight → friction ↑
• Too loose → vibration ↑

(6) Operating Temperature

• Affects viscosity of lubricant
• Thermal expansion must be considered

5. Design of Sliding Contact Bearings

(a) Bearing Pressure

$$P = \frac{W}{L \cdot d}$$

Where:

• $W$W = Load
• $L$L = Length of bearing
• $d$d = Diameter of shaft

(b) PV Factor

$$PV = P \times V$$

• Indicates heat generation
• Must be within allowable limits

(c) Sommerfeld Number

• Used in hydrodynamic lubrication design
• Helps determine oil film thickness

(d) Heat Dissipation

• Heat generated = Heat dissipated
• Ensures safe operating temperature

6. Design of Rolling Contact Bearings

(a) Static Load Capacity

• Maximum load without permanent deformation

(b) Dynamic Load Capacity

• Load that bearing can carry for a specified life

(c) Bearing Life (L10 Life)

$L_{10} = \left( \frac{C}{P} \right)^p$

Where:

• $C$C = Dynamic load capacity
• $P$P = Equivalent load
• $p = 3$p=3 for ball bearings
• $p = \frac{10}{3}$p=310​ for roller bearings

(d) Equivalent Dynamic Load

$$P = X F_r + Y F_a$$

• $X, Y$ depend on bearing type

(e) Reliability Factor

• Life is adjusted based on required reliability

7. Lubrication of Bearings

• Reduces friction and wear
• Removes heat
• Prevents corrosion

Types:

• Oil lubrication
• Grease lubrication
• Solid lubrication

8. Failure of Bearings

Common causes:

• Fatigue failure
• Wear and scoring
• Overheating
• Corrosion
• Misalignment

9. Advantages and Disadvantages

Sliding Bearings

Advantages:

• High load capacity
• Quiet operation

Disadvantages:

• High starting friction
• Requires continuous lubrication

Rolling Bearings

Advantages:

• Low friction
• Easy replacement

Disadvantages:

• Lower load capacity
• Sensitive to shock

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