1. Introduction to power transmission elements

Mechanical power transmission elements are the components of the process of transferring energy from a power source, such as a motor or engine, to a point where it can perform useful work through physical machine elements. Within a machine, these systems move power as either rotational motion to drive shafts and pulleys or linear motion to move items in a straight line. The primary elements used in mechanical power transmission include:

1. Gear Drives

Gears use interlocking toothed wheels to transmit power between rotating shafts and are the standard for applications requiring precise speed and torque control. Gear trains define the velocity ratio by dividing the number of teeth on the driven gear by those on the driving gear. Common configurations include:

2. Belt Drives

These transmit power through belts wrapped around pulleys (sheaves) and are valued for quiet operation, shock damping, and minimal maintenance.

3. Chain Drives

Consisting of linked segments meshing with sprockets, chain drives handle heavy loads and high torque with positive (non-slip) engagement. They are highly efficient (98–99% when lubricated) and excel in harsh, dirty, or high-temperature environments where belts might fail.

4. Couplings

Couplings join two shafts together for torque transmission while managing misalignment and protecting internal components.

5. Power Screws

Also known as translation screws, these convert rotary motion into linear motion. They provide high mechanical advantage, allowing small input forces to raise heavy loads or provide precise linear positioning in devices like screw-jacks and lathe lead-screws. Common thread types include square, trapezoidal (Acme), and buttress threads.

2. Types of Power Transmission Elements

1. Light drives: These are used to transmit small powers at belt speeds upto about 10 m/s as in agricultural machines and small machine tools.
2. Medium drives. These are used to transmit medium powers at belt speeds over 10 m/s but up to 22 m/s, as in machine tools.
3. Heavy drives. These are used to transmit large powers at belt speeds above 22 m/s as in compressors and generators.

(e) Couplings

• Connect two shafts

3. General Design Considerations

• Power to be transmitted
• Speed of operation
• Nature of load (steady, shock, fluctuating)
• Efficiency
• Space and cost constraints
• Maintenance requirements

4. Design of Belt Drives

Velocity Ratio

$$\frac{N_1}{N_2} = \frac{D_2}{D_1}$$

Belt Tension Relation

$$\frac{T_1}{T_2} = e^{\mu \theta}$$

Where:

• $T_1, T_2$​ = tight and slack side tensions
• $\mu$μ = coefficient of friction
• $\theta$θ = angle of contact

Power Transmitted

$$P = (T_1 – T_2) \cdot v$$

5. Design of Chain Drives

• Based on:
  • Chain pitch
  • Number of teeth on sprocket
  • Speed ratio

Velocity Ratio

$$\frac{N_1}{N_2} = \frac{T_2}{T_1}$$

6. Design of Gear Drives

Gear Ratio

$$\text{Gear Ratio} = \frac{T_2}{T_1}$$

Lewis Equation (Beam Strength)

$F_t = \sigma \, b \, y \, \pi m$

Where:

• $F_t$Ft​ = tangential force
• $\sigma$σ = allowable stress
• $b$b = face width
• $m$m = module
• $y$y = Lewis form factor

Wear Strength

• Based on surface durability
• Depends on material hardness

7. Design of Shafts

Torsion Equation

$\frac{T}{J} = \frac{\tau}{R} = \frac{G \theta}{L}$

Where:

• $T$T = torque
• $J$J = polar moment of inertia
• $\tau$τ = shear stress

Combined Loading

• Shaft subjected to:
  • Bending moment
  • Torsion

Equivalent torque method used for design

8. Design of Couplings

• Must transmit torque safely
• Types:
  • Rigid (flange, sleeve)
  • Flexible (bush pin, Oldham)

Torque Transmitted

$$T = \frac{P \cdot 60}{2\pi N}$$

9. Material Selection for Power Transmission Elements

• High strength and toughness
• Wear resistance
• Common materials:
  • Steel
  • Cast iron
  • Alloy steel

10. Failure of Power Transmission Elements

• Fatigue failure
• Wear
• Overloading
• Misalignment
• Lubrication failure

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External References

NPTEL course on design of power transmission elements.