The number of independent motions possessed by a component is called its degree of freedom. Obviously, a free component that performs planar motion has three degrees of freedom.

A mechanism is composed of many components connected in a certain way, which should ensure that there is a certain relative motion between the components. This type of connection that allows two components to come into direct contact and generate a certain relative motion is called a motion pair.

Low pair

The motion pair composed of two components through surface contact is called a low pair. It includes two types: rotating pair and moving pair.

The rotating pair can only rotate relatively in one plane, while the moving pair can only move along a certain axis direction. Therefore, a low pair introduces two constraints, namely reducing two degrees of freedom.

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The motion pair composed of two components in contact with each other through points or lines is called a high pair.

1. Fixed parts (rack)

The components used by humans to support moving components are called fixed components (racks). The cylinder block in an internal combustion engine is a fixed component used to support pistons, crankshafts, etc.

2. Prime mover

The active components with known motion patterns are called active components. The piston in an internal combustion engine is the driving component, and its motion is input from the outside world.

3. Follower

The other moving components that move with the motion of the original component are the driven components. Connecting rods, crankshafts, and other components in internal combustion engines are all driven components.

This type of multiple rotating pairs composed of three or more components with overlapping axes at one point is called a composite hinge.

The local independent motion that does not affect the motion of the entire mechanism is called the local degree of freedom.

In practical mechanisms, constraints that repeat other constraints without limiting motion are called virtual constraints. Virtual constraints should be disregarded when calculating the degree of freedom of the mechanism.

If one is a crank and the other is a rocker, then this mechanism is called a crank rocker mechanism.

In a hinged four bar mechanism, if both connecting rods are cranks, then this mechanism is called a double crank mechanism.

In a hinged four bar mechanism, the two connecting rods are both rocker rods, which is called a double rocker mechanism machine.

1. There must be one pole between the connecting pole and the rack as the shortest pole.

2. The sum of the lengths of the shortest and longest strokes is equal to the sum of the lengths of the other two strokes.

As can be seen from the above, when determining the form of a chain four bar mechanism, it is necessary to first determine whether the mechanism meets the rod length condition. If so, the form of the mechanism can be determined using the following method:

① If the short rod can be used as a connecting rod, a crank rocker mechanism can be obtained;

② If the shortest rod is a frame, a double crank mechanism is obtained

3. If the shortest rod is a connecting rod, a double rocker mechanism is obtained. A four bar linkage that satisfies the condition of being in a long chain, regardless of which member is used as the frame, does not have any curvature, so the mechanism can only be a double rich linkage.

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