What is Degree of Freedom? - Definition, Formula, Examples

What is the Degree of Freedom?

The set of independent displacements and/or rotations that specify completely the displaced or deformed position and orientation of the body or system are known as degrees of freedom (DOF) in mechanics.

Degree of Freedom Definition

“A system's degree of freedom can be conceived of as the minimum number of coordinates needed to specify a configuration.”

Using this definition, we get the following:

• An unconstrained object in space has six degrees of freedom: three translational and three rotational.
• An unconstrained object in a plane has three degrees of freedom: two translational and one rotational.

Degree of Freedom of a Mechanism

“It is the minimum number of independent inputs required to get a constrained output from a mechanism.”

If any mechanism has two degrees of freedom, then two independent inputs are required to get a constrained output. Example: Epicyclic gear train.

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Degree of Freedom Formula

Kutzback was a scientist who gave the relation to determine the degree of freedom of a 2-D planar mechanism. The degree of freedom formula is:

DOF = 3(L-1) – 2j – h

where,

• L = number of links
• j = number of joints
• h = number of higher pairs

When one or more links of a mechanism may be moved without causing the rest of the mechanism's links to move, this is referred to as a redundant degree of freedom. The planar mechanism's degree of freedom formula has been updated as a result of this.

DOF = 3(L-1) – 2j – h – Fr

where, Fr = redundant motion

Now, Grubler was another scientist who used the Kutzback equation and came up with his own formulation, in which he employed the equation and set the degree of freedom to 1 and the higher pair to zero for a kinematic chain. The Grubler criterion was an extension of the Kutzback equation, and it was written as

DOF = 3(L-1) – 2j – h

1 = 3(L-1) – 2j – 0

3L – 2j – 4 = 0 (derived equation of Grubler’s criterion.)

According to Grubler's criterion, the number of links required to hold the equation should be even, i.e. the minimal number of links required is four.

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Degrees of Freedom Example

Question 1: What are the degrees of freedom for a rigid body?

Solution: A rigid body's position and orientation in space are determined by three components of translation and three components of rotation, giving it six degrees of freedom.

Question 2: What is the number of degrees of freedom in a four-bar linkage?

Solution: Grubler's criterion determines the degree of freedom (F) for a simple mechanism:

F = 3 (n - 1) - 2j - h

Here, j = number of revolute joints, n = number of links, h = number of higher pairs

Given: n = 4, j = 4, h = 0

F = 3 (4 - 1) - 2 × 4 - 0

F = 9 - 8 = 1