Define a link mechanism.

Short Answer:

A link mechanism is a combination of two or more rigid bodies, called links, that are connected by joints to form a system capable of transmitting motion and force. Each link in the mechanism moves relative to the others to produce a desired type of motion, such as rotation or translation.

In simple words, a link mechanism converts one type of motion into another or transmits power from one part of a machine to another. Examples include a four-bar chain, crank and slider mechanism, and steering mechanism used in automobiles and machinery.

Detailed Explanation :

Link Mechanism

A link mechanism is a fundamental concept in the study of kinematics of machines. It is formed when two or more links are connected in such a way that they can move relative to one another. These connected links transmit motion and force from one part of a machine to another in a controlled and predictable manner.

Every mechanical device, from simple hand tools to complex machines, is made up of one or more link mechanisms. For example, the piston-crank system in an engine converts the reciprocating motion of the piston into rotary motion of the crankshaft using a combination of connected links.

A link mechanism can perform various tasks such as lifting, pushing, pulling, rotating, or oscillating depending on its design and the type of motion required.

Definition

A link mechanism can be defined as:

“A combination of two or more rigid bodies (called links) connected by kinematic pairs or joints to form a closed or open chain capable of transmitting motion and force is known as a link mechanism.”

In short, it is a system of interconnected links that can convert one form of motion into another, such as rotary to linear motion or linear to oscillatory motion.

Components of a Link Mechanism

A typical link mechanism consists of the following components:

  1. Links:
    A link is a rigid body that forms part of the mechanism and transmits motion or force. Each link has at least two points for making connections with other links.

    • Example: In a crank and slider mechanism, the crank, connecting rod, and piston are links.
  2. Kinematic Pair (Joint):
    When two links are joined together in such a way that they can move relative to each other, the connection is called a kinematic pair or joint.
    Common types of kinematic pairs include:

    • Revolute (turning) pair – rotation about a fixed axis (e.g., crank and frame).
    • Sliding pair – linear motion between two links (e.g., piston and cylinder).
    • Screw pair – rotational and translational motion combined (e.g., lead screw).
    • Cylindrical pair – both rotational and translational freedom.
  3. Frame or Fixed Link:
    It is the stationary link of the mechanism to which other links are connected. It provides the reference for motion of other links.
    Example: The engine block in a crank and slider mechanism acts as the frame.
  4. Input Link (Driver):
    It is the link to which motion or force is initially applied.
  5. Output Link (Follower):
    It is the link that delivers the required output motion or force as a result of the input.

Types of Link Mechanisms

Based on the way links are connected and their motion, mechanisms can be classified into the following types:

  1. Four-Bar Chain Mechanism:
    • Consists of four rigid links connected by four turning pairs.
    • One link is fixed while the remaining three links move relative to it.
    • Example: Coupler mechanism and lever mechanism.
    • Used in linkages, engine valves, and folding arms.
  2. Slider-Crank Mechanism:
    • One of the most common mechanisms used in engines.
    • Consists of a crank, connecting rod, and slider.
    • Converts rotary motion into reciprocating motion or vice versa.
    • Example: Internal combustion engines, reciprocating pumps, and compressors.
  3. Double Slider-Crank Mechanism:
    • Contains two sliding pairs and two turning pairs.
    • Example: Elliptical trammel and Oldham’s coupling.
  4. Quick Return Mechanism:
    • Used in shaping and slotting machines.
    • Converts rotary motion into reciprocating motion with different speeds in forward and return strokes.
  5. Steering Mechanism:
    • Used in automobiles for changing direction of the front wheels using interconnected links.

Each type of mechanism serves a specific purpose based on the required type of motion and power transmission.

Classification of Links in a Mechanism

Links in a mechanism are classified based on the number of joints they form with other links:

  1. Binary Link: Connected to two other links.
    Example: Connecting rod in a slider-crank mechanism.
  2. Ternary Link: Connected to three other links.
    Example: Crank lever connecting three different links.
  3. Quaternary Link: Connected to four other links.
    Example: A link joining four points of contact in a mechanism.

Applications of Link Mechanisms

  1. Automobiles:
    • Used in steering systems, suspension linkages, and engine valve mechanisms.
  2. Machines and Tools:
    • Found in shaping machines, slotting machines, and presses.
  3. Robotics:
    • Used in robot arms for transmitting controlled motion.
  4. Aerospace:
    • Employed in landing gear mechanisms and control linkages.
  5. Everyday Devices:
    • Found in door handles, bicycle pedals, and scissors.

Advantages of Link Mechanisms

  1. Motion Conversion:
    Easily converts one type of motion into another (e.g., rotary to linear).
  2. Mechanical Advantage:
    Amplifies force or displacement according to the need.
  3. Compact Design:
    Provides efficient motion in a smaller space compared to belt or gear drives.
  4. Flexibility:
    Can be used in various machines for different motion requirements.
  5. Smooth Operation:
    Ensures continuous and predictable motion without jerks.

Importance in Mechanical Engineering

  • Link mechanisms form the foundation of kinematics and dynamics in mechanical design.
  • They are essential for understanding how machines transmit and transform motion.
  • Proper design ensures efficiency, balance, and smooth power transmission.
  • They are the building blocks for creating more complex systems such as robots, engines, and automated machinery.
Conclusion

A link mechanism is a combination of interconnected rigid links joined by kinematic pairs to produce a desired motion or force transmission. It forms the backbone of all machines, from simple tools to complex mechanisms. By understanding the working and types of link mechanisms, engineers can design systems that efficiently convert and control motion. Mechanisms like the slider-crank, four-bar chain, and quick return are widely used in modern engineering for performing various mechanical tasks with accuracy and reliability.