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Four Bar Chain Linkage Mechanism

Four Bar Chain Linkage Mechanism

A Four Bar Chain Linkage Mechanism is simply known as four – bar. It is the simplest closed moving mechanism connected by bars. These are connected closely in the form of a loop with four joints. The assembly of joints in such a way that links move in parallel planes is known to be planar four-bar linkage.

This four – bar chain linkage mechanism has four degrees of freedom. When all the linkages are locked and when given freedom to one joint, it moves in concentric spheres. This mechanism is known as Spherical Four – Bar Linkage Mechanism.

Four Bar Chain Linkage Mechanism

There are three types of Four Bar Linkage Mechanisms. They are as follows –

  • Four Bar Chain Linkage Mechanism with four revolute joints.
  • Four Bar Chain Linkage Mechanism with three revolute joints and a prismatic joint.
  • Four Bar Chain Linkage Mechanism with two revolute joints and two prismatic joints.

These are very important mechanisms found in heavy machinery involving both kinematics and dynamics.

Four Bar Chain Linkage Mechanism with four revolute joints –

The revolute joints were represented by the capital letter ‘R’ and the prismatic joints were represented by capital letter ‘P’. This mechanism has four revolute joints and it is represented by RRRR and consists of two cranks and a coupler.

 

Four Bar Chain Linkage Mechanism with three revolute joints and a prismatic joint –

This mechanism consists of three revolute joints & a prismatic joint and it is represented by RRRP. It consists of two cranks and a slider. This is also known as Inverted Slider Crank.

 

Four Bar Chain Linkage Mechanism with two revolute joints and two prismatic joints –

This mechanism consists of two revolute joints & two prismatic joints and it is represented by RRPP. It consists of two sliders and a couple. This is also known as Trammel of Archimedes or Elliptical Trammel.

There are many examples of Four Bar Chain Linkage Mechanism in our daily life. The design of Four Bar Chain Linkage Mechanism includes time ratio and timing charts. A suspension in a bicycle is also an example for Four Bar Chain Linkage Mechanism.

There are four linkages in suspension of a bicycle. They are as follows –

  • Chain Stay
  • Seat Stay
  • Rocker
  • Seat Tube

Many theories like Bona fide single pivot, Split pivot are used for this suspension. When the link is behind the axle, it is known as Bona fide single pivot. When the link combines the seat, it is known as Split Pivot. Two triangles will be created using the four links and one link is directly the suspension. At the second & third joint, two triangles will be created and at the first & fourth joint, a suspension is placed. The movement at one joint creates a movement in one triangle, which further creates a movement in another triangle and also the another joint. The joints come back to their position once it is complete.

Advantages of Four Bar Chain Linkage Mechanism –

  • This mechanism helps to create the lightweight design.
  • This mechanism is simply versatile
  • It is simple in design and easy to understand.
  • Last but not the least, it is economical but also has high standards.
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