Short Answer:

Mechanical efficiency is the ratio of the useful output work (or power) to the input work (or power) of a machine. It indicates how effectively a machine converts the input energy into useful output without losses.

In simple words, mechanical efficiency tells us how much of the input energy supplied to a machine is actually used to do useful work. Since some energy is always lost due to friction and other resistances, mechanical efficiency is always less than 100%. It is usually expressed as a percentage.

Detailed Explanation:

Mechanical Efficiency

Every machine is designed to convert energy from one form to another to perform useful work. However, in all real systems, part of the input energy is lost due to friction, heat, vibration, and other resistances. Therefore, not all of the supplied energy is converted into useful output work.

The mechanical efficiency of a machine measures how effectively the input energy or work is converted into useful output work. It gives an idea of the machine’s performance and helps engineers evaluate how much energy is wasted and how much is used productively.

Hence, a highly efficient machine performs more useful work for the same amount of input energy.

Definition

Mechanical efficiency can be defined as:

“The ratio of useful output work or power to the total input work or power of a machine.”

Mathematically,

or,

Since not all input energy is converted into useful work,

To express it as a percentage,

Units of Mechanical Efficiency

Mechanical efficiency is a dimensionless quantity, as it is a ratio of two similar quantities (work/work or power/power).

• SI Unit: None (it has no unit)
• Expressed as: Percentage (%)

Example:
If a motor gives 900 W output from 1000 W input,

This means 90% of the input energy is converted into useful work, and 10% is lost.

Explanation with Work and Power

Let:

•  = Work done on the machine (input work)
•  = Useful work delivered by the machine (output work)
•  = Input power
•  = Output power

Then,

If a machine is ideal (no losses), the output equals input, and the efficiency becomes 100%. However, in practical machines, there are always energy losses, so the efficiency is less than 100%.

Energy Losses Affecting Mechanical Efficiency

The main reason why mechanical efficiency is less than 100% is because of energy losses. These losses occur due to:

1. Friction:
   Friction between moving parts converts some mechanical energy into heat energy, which is wasted.
2. Air Resistance:
   Moving parts such as flywheels and rotors face air drag, reducing the useful output.
3. Vibration and Noise:
   Vibrations in machines waste energy as sound and unwanted motion.
4. Deformation:
   Slight deformation of parts under stress absorbs energy, reducing useful output.
5. Transmission Losses:
   In systems like gearboxes and belt drives, some energy is lost in transmitting motion from one part to another.

Due to these losses, only a portion of the input energy is available as useful output energy.

Expression for Efficiency in Terms of Frictional Losses

Let:

•  = Total input power
•  = Useful output power
•  = Power lost due to friction and other causes

Then,

So,

 

This expression shows that the smaller the losses, the higher the mechanical efficiency.

Examples of Mechanical Efficiency

1. Electric Motor:
   Converts electrical energy into mechanical energy. Efficiency depends on losses due to friction, resistance, and magnetic effects.
   Typical efficiency: 85% – 95%.
2. Internal Combustion Engine:
   Converts chemical energy of fuel into mechanical work. Efficiency depends on fuel combustion, friction, and heat losses.
   Typical efficiency: 25% – 35%.
3. Pulley System:
   Converts input mechanical effort into useful load lifting. Efficiency is reduced by friction between rope and pulley.
4. Gearbox or Transmission System:
   Efficiency is affected by tooth friction, lubrication, and load on gears.
5. Pump or Compressor:
   Efficiency is reduced due to friction, leakage, and fluid resistance.

Measurement of Mechanical Efficiency

In experiments and practical systems, efficiency is measured using the ratio of power:

where,

• Brake Power (BP): Useful power output measured at the shaft.
• Indicated Power (IP): Total power developed inside the engine.

The difference between IP and BP represents the friction power (FP) lost due to mechanical resistance:

Hence,

This concept is especially used in internal combustion engines and turbines to find mechanical efficiency.

Improvement of Mechanical Efficiency

Mechanical efficiency can be improved by:

1. Reducing Friction: Using proper lubrication between moving parts.
2. Using Light and Strong Materials: Reduces inertia and energy losses.
3. Proper Maintenance: Ensures smooth operation and minimizes wear.
4. Balancing and Alignment: Prevents vibration and uneven loading.
5. Streamlining: Reduces air drag and resistance in moving parts.

By minimizing losses, machines can operate closer to their maximum possible efficiency.

Importance of Mechanical Efficiency

1. Performance Evaluation:
   Efficiency helps in comparing the performance of different machines doing similar work.
2. Energy Saving:
   Higher efficiency means less energy wastage, resulting in cost savings and better energy management.
3. Design Optimization:
   Helps engineers design machines that produce more output with less input.
4. Economic Operation:
   Reduces running costs by minimizing energy losses.
5. Environmental Benefits:
   High-efficiency machines consume less energy and produce fewer emissions.

Conclusion

In conclusion, mechanical efficiency is a measure of how effectively a machine converts input energy into useful output work. It is defined as the ratio of output work to input work and is always less than 100% because of energy losses like friction, heat, and vibration. Improving efficiency is essential for saving energy, enhancing performance, and achieving economic operation. Hence, mechanical efficiency is one of the most important parameters for evaluating and designing mechanical systems.