🛠️Mechanical Engineering Design Unit 9 – Power Transmission: Shafts & Bearings

Key Concepts and Terminology

• Shafts transmit power and torque between machine elements such as gears, pulleys, and bearings
• Bearings support and guide rotating shafts while reducing friction and wear
• Torque ($T$) is the twisting force applied to a shaft, measured in Newton-meters (N·m) or pound-force feet (lbf·ft)
• Bending moment ($M$) is the reaction of a shaft to external loads, causing the shaft to bend or deflect
• Torsional stress ($\tau$) is the shear stress resulting from torque acting on a shaft's cross-section
  • Calculated using the equation: $\tau = \frac{Tr}{J}$, where $r$ is the distance from the neutral axis and $J$ is the polar moment of inertia
• Fatigue is the weakening of a material caused by repeated cyclic loading, potentially leading to failure
• Shaft critical speed is the rotational speed at which a shaft becomes dynamically unstable and vibrates excessively
• Bearing life is the number of revolutions or hours a bearing can operate before failure or reaching a specific wear limit

Types of Shafts and Their Applications

• Transmission shafts transmit power between a source (motor) and a driven machine (pump or generator)
• Countershafts are intermediate shafts used to change the direction or speed of rotation in a system
• Line shafts are long shafts that transmit power to multiple machines or workstations along their length
• Flexible shafts consist of a flexible core (wire or polymer) capable of transmitting torque while bent or curved
  • Used in applications requiring variable alignment or vibration damping (automotive speedometers, power tools)
• Hollow shafts have a bore through the center, reducing weight and allowing for the passage of fluids or cables
• Splined shafts feature longitudinal grooves (splines) that mate with internal teeth in a hub, allowing torque transmission and axial movement
• Stepped shafts have varying diameters along their length to accommodate different machine elements (bearings, gears)
• Stub shafts are short, compact shafts used in space-limited applications or for connecting adjacent machine components

Shaft Design Considerations

• Material selection based on strength, durability, and cost (common materials include steel, stainless steel, and aluminum alloys)
• Shaft diameter is determined by considering the maximum allowable torsional and bending stresses
  • Larger diameters provide greater resistance to stresses but increase weight and cost
• Keyways and splines are used to secure machine elements (gears, pulleys) to the shaft and transmit torque
• Shoulder locations and fillet radii are designed to minimize stress concentrations and improve fatigue life
• Surface treatments (heat treatment, plating, or coating) enhance wear resistance, corrosion resistance, or appearance
• Balancing is critical for high-speed shafts to minimize vibration and extend bearing life
  • Methods include static balancing (single-plane) and dynamic balancing (two-plane)
• Alignment is essential for efficient power transmission and reduced wear, achieved through precise machining and assembly
• Shaft connections (couplings, universal joints) allow for misalignment, thermal expansion, or vibration isolation between connected shafts

Bearing Fundamentals

• Bearings support rotating shafts and reduce friction by providing a rolling or sliding interface between moving parts
• Radial bearings support loads perpendicular to the shaft axis, while thrust bearings support axial loads
• Bearing friction is influenced by factors such as load, speed, lubrication, and bearing type
  • Friction generates heat, which can lead to bearing damage or failure if not properly managed
• Bearing clearance is the gap between the bearing and shaft or housing, allowing for thermal expansion and lubricant flow
  • Insufficient clearance can cause binding or overheating, while excessive clearance may result in vibration or reduced load capacity
• Bearing lubrication is essential for reducing friction, dissipating heat, and preventing wear
  • Lubrication methods include grease, oil bath, oil mist, and forced oil circulation
• Bearing seals prevent contamination and retain lubricant, extending bearing life
  • Seal types include contact seals (lip seals) and non-contact seals (labyrinth seals)
• Bearing mounting and fit (interference or clearance) affect bearing performance and life
  • Proper fit ensures secure attachment to the shaft and housing, while allowing for thermal expansion

Types of Bearings and Selection Criteria

• Rolling element bearings use rolling elements (balls or rollers) between races to reduce friction
  • Types include ball bearings (radial and angular contact), roller bearings (cylindrical, spherical, and tapered), and needle bearings
• Plain bearings (bushings) use sliding surfaces to support loads, typically made of bronze, babbitt, or polymer materials
  • Advantages include lower cost, compact design, and shock load tolerance
• Hydrostatic bearings use pressurized fluid (oil or water) to create a load-carrying film, providing low friction and high load capacity
  • Used in high-precision applications (machine tool spindles) or heavy-duty equipment (turbines, large pumps)
• Magnetic bearings use electromagnetic forces to levitate the shaft, eliminating mechanical contact and friction
  • Suitable for high-speed, clean room, or vacuum environments (turbomolecular pumps, flywheels)
• Bearing selection criteria include load (magnitude and direction), speed, operating temperature, environment (moisture, dust), and cost
• Bearing life and reliability requirements influence the choice of bearing type and size
  • Factors affecting bearing life include load, speed, lubrication, and contamination
• Noise and vibration constraints may dictate the use of precision bearings or specialized materials (ceramic, plastic)

Power Transmission Principles

• Power ($P$) is the rate of doing work, measured in watts (W) or horsepower (hp)
  • Calculated using the equation: $P = T \omega$, where $T$ is torque and $\omega$ is angular velocity
• Gear ratios determine the speed and torque relationship between the input and output shafts
  • Gear ratio ($i$) is the ratio of the output shaft speed ($n_2$) to the input shaft speed ($n_1$): $i = \frac{n_2}{n_1}$
• Belt drives use flexible belts (flat, V, or timing) to transmit power between pulleys
  • Advantages include low cost, quiet operation, and ability to absorb shock loads
• Chain drives use interlocking chains and sprockets to transmit power, suitable for high-torque applications
  • Advantages include high efficiency, positive engagement, and ability to operate in harsh environments
• Coupling types (rigid, flexible, or universal) connect shafts and accommodate misalignment, vibration, or thermal expansion
  • Rigid couplings (flanged, sleeve) provide a fixed connection, while flexible couplings (elastomeric, grid) allow for some movement

Stress Analysis in Shafts and Bearings

• Shear stress ($\tau$) in shafts is caused by torsional loading, calculated using the equation: $\tau = \frac{Tr}{J}$
  • Maximum shear stress occurs at the shaft surface and is used to determine the required shaft diameter
• Bending stress ($\sigma$) in shafts results from transverse loads and bending moments, calculated using the flexure formula: $\sigma = \frac{My}{I}$
  • Maximum bending stress occurs at the shaft surface and is superimposed with torsional stress for combined loading analysis
• Bearing contact stress is the compressive stress between rolling elements and raceways, influenced by load, geometry, and material properties
  • Hertzian contact theory is used to calculate contact stresses and predict bearing fatigue life
• Finite element analysis (FEA) is a numerical method for analyzing complex shaft and bearing geometries under various loading conditions
  • FEA helps optimize designs by identifying stress concentrations and predicting fatigue life
• Shaft critical speed calculations determine the rotational speeds at which a shaft becomes dynamically unstable due to resonance
  • Critical speeds are influenced by shaft geometry, support stiffness, and rotor mass distribution
• Bearing load capacity is the maximum load a bearing can support without exceeding the allowable contact stress or fatigue life
  • Static and dynamic load ratings are used to select bearings based on the application requirements

Maintenance and Troubleshooting

• Regular inspection of shafts and bearings helps identify wear, damage, or misalignment before failure occurs
  • Visual inspection, vibration analysis, and thermography are common monitoring techniques
• Lubrication maintenance ensures adequate lubricant quantity and quality to prevent wear and overheating
  • Lubricant selection based on bearing type, speed, temperature, and environment
  • Lubricant replacement intervals depend on operating conditions and manufacturer recommendations
• Bearing installation and removal procedures vary by bearing type and mounting method (interference fit, clearance fit)
  • Proper tools and techniques (bearing pullers, induction heaters) prevent damage to bearings and shafts during installation or removal
• Shaft alignment checks and corrections are essential for preventing premature bearing failure and ensuring efficient power transmission
  • Misalignment types include angular (shaft axes at an angle) and parallel (offset shaft axes)
  • Alignment methods include dial indicator, laser, or optical alignment systems
• Troubleshooting common issues such as vibration, overheating, or excessive noise requires a systematic approach
  • Possible causes include imbalance, misalignment, lubrication problems, or bearing damage
  • Root cause analysis helps identify and address the underlying issues to prevent recurrence
• Bearing failure analysis examines failed bearings to determine the cause and prevent future failures
  • Common failure modes include fatigue, wear, corrosion, and lubrication-related failures
  • Failure analysis techniques include visual inspection, microscopy, and metallurgical analysis