What are flywheels used for?

Short Answer:

Flywheels are mechanical devices used to store rotational energy and release it whenever needed to maintain a uniform speed in machines. They help to reduce speed fluctuations caused by variations in torque during operation.

In simple words, a flywheel is a heavy rotating wheel attached to a shaft that stores kinetic energy when the energy supply is more than required and releases it when the supply is less. Flywheels are widely used in engines, machines, and vehicles to ensure smooth and continuous motion by balancing energy input and output.

Detailed Explanation :

Flywheels Used For

A flywheel is an important mechanical component designed to regulate the rotational speed of a machine. It stores excess energy when the torque input is higher than the resistance torque and releases it when the torque input falls below the resistance. This process ensures smooth operation of the machine and reduces the effects of torque fluctuations.

Flywheels are generally heavy disks or rims made of steel, cast iron, or composite materials. They are mounted on the crankshaft or rotating shafts of engines and mechanical systems. The energy stored in a flywheel is in the form of rotational kinetic energy and depends on its moment of inertia and angular velocity.

Definition

A flywheel can be defined as:

“A heavy rotating wheel used to store rotational energy and to regulate the speed of an engine or machine by balancing fluctuations in energy supply and demand.”

It acts as an energy reservoir, absorbing energy when the power supply exceeds the requirement and delivering it when there is a shortage.

Energy Stored in a Flywheel

The energy stored in a rotating flywheel is given by the formula:

Where,

  •  = Energy stored (Joules)
  •  = Moment of inertia of the flywheel (kg·m²)
  •  = Angular speed of rotation (rad/s)

Since the moment of inertia depends on the mass and shape of the flywheel, a heavier or larger flywheel can store more energy.

Working Principle

The working of a flywheel is based on the law of conservation of energy.

When the input torque to the machine increases, the flywheel accelerates and stores the excess energy in the form of kinetic energy. When the input torque decreases or the load increases, the flywheel decelerates and releases the stored energy to the system.

This process helps in maintaining a uniform angular velocity of the shaft, minimizing fluctuations during each cycle of operation.

Example:
In an internal combustion engine, during the power stroke, the flywheel stores excess energy, and during the compression or exhaust strokes, it supplies energy to keep the crankshaft rotating uniformly.

Functions of a Flywheel

Flywheels serve several important functions in mechanical systems:

  1. Regulation of Speed:
    Flywheels reduce speed fluctuations in engines and machines caused by variable torque or power input.
  2. Energy Storage:
    They store energy during periods of light load and release it during high load conditions.
  3. Smooth Operation:
    They help to maintain a continuous and steady motion by balancing energy variations.
  4. Reduction of Vibrations:
    Flywheels minimize mechanical vibrations and shocks in machines, ensuring smoother performance.
  5. Control of Acceleration:
    By resisting sudden speed changes, flywheels prevent abrupt acceleration or deceleration of rotating parts.
  6. Energy Transfer:
    Flywheels transfer stored rotational energy to other mechanical systems when required.

Applications of Flywheels

Flywheels are used in various fields of engineering and mechanical systems for different purposes. Some of the most common applications are:

  1. Internal Combustion Engines:
    Flywheels are mounted on the crankshaft to ensure uniform rotation by storing energy during power strokes and supplying it during other strokes.
  2. Press Machines:
    In mechanical presses, flywheels store energy from the motor and release it rapidly to perform the pressing or punching operation.
  3. Energy Recovery Systems:
    Flywheels are used in hybrid and electric vehicles to store braking energy (regenerative braking) and reuse it during acceleration.
  4. Textile and Metalworking Machines:
    They maintain steady motion in looms, rolling mills, and lathes by balancing torque variations.
  5. Electric Power Plants:
    Flywheels maintain the uniform speed of turbines and generators, ensuring stable power generation.
  6. Uninterruptible Power Supplies (UPS):
    In some UPS systems, flywheels act as short-term energy storage devices to provide backup power during supply interruptions.
  7. Spacecraft and Satellites:
    Flywheels, called reaction wheels, are used to control the orientation and stability of spacecraft without using fuel.

Design Considerations

The design of a flywheel depends on several factors:

  1. Mass and Shape:
    Heavier flywheels store more energy but require more power to accelerate. The shape (rim-type or solid disk) affects the moment of inertia.
  2. Material:
    Materials with high strength-to-weight ratios, such as steel or composites, are used for better performance.
  3. Speed of Rotation:
    Higher angular velocity increases stored energy but also introduces stresses due to centrifugal force.
  4. Moment of Inertia (I):
    The energy storage capacity directly depends on the moment of inertia of the flywheel.
  5. Safety:
    Proper balancing and containment are essential to prevent accidents due to high-speed failures.

Advantages of Using Flywheels

  1. Improves uniformity of machine speed.
  2. Provides smooth and continuous energy output.
  3. Reduces the power fluctuations in engines and generators.
  4. Can temporarily replace power supply during short interruptions.
  5. Helps improve machine life by minimizing shocks and stresses.

Limitations of Flywheels

  1. Large size and heavy weight increase system inertia.
  2. High rotational speeds can cause material stress and vibration.
  3. Not suitable for continuous energy storage over long durations.
  4. Difficult to balance perfectly at very high speeds.
  5. Occupies significant space in compact machines.

Example Calculation

If a flywheel with a moment of inertia  rotates at 300 rpm,

Then energy stored,

Hence, the flywheel stores approximately 4.9 kJ of energy at that speed.

Conclusion

Flywheels are essential mechanical devices used for energy storage, speed regulation, and power stabilization in rotating machines. By storing excess energy and releasing it when required, flywheels help maintain uniform motion and efficiency in systems like engines, presses, and power generators. They reduce mechanical vibrations, improve performance, and extend the life of machinery. With advancements in materials and design, modern flywheels are becoming more compact, efficient, and widely used in various mechanical and energy applications.