Degrees of freedom is the one of the most important concept in mechanics. This concept is widely used in robotics and kinematics. D.O.F means how many variables are required to determine position of a mechanism in space. In this video lecture we will understand how to predict degrees of freedom of a mechanism.Detailed description of the video lecture is given below.
Degrees of Freedom – Examples
Consider the mechanism shown in first figure of Fig.4. Position of this 4 bar mechanism can be completely determined just by knowing angle or position of any one of the member. So degree of freedom is one. Similarly degree of freedom of the cam and follower mechanism is also one. But to determine position of the slider crank mechanism shown, we should know angle or displacement of at least 2 members. So here degrees of freedom is 2.
|Fig.1 Examples of degrees of freedom of different mechanisms|
Degrees of Freedom of a Rigid Body
Consider the rigid body shown below, which is situated in space. It could have 3 translatory motions as shown. Also it could have 3 rotary motions as shown.In total we need 6 inputs to determine its position. So degree of freedom of rigid body in space is 6.
|Fig.2 A rigid body in space can have total 6 degrees of freedom|
|Fig.3 A rigid body on plane can have total 3 degrees of freedom|
Degrees of Freedom of a Mechanism
A mechanism is a collection of rigid bodies or links, connected through pairs, provided one link is grounded. Consider the mechanism shown below.
|Fig.4 An example of mechanism, it is necessary that one link should be groudnded|
|Fig.5 If links were not connected each link would have 3 D.O.F, except the ground|
If joint between 2 links is having surface contact as shown below, then both the links will have same translatory motion, in X and Y directions.So for each such pairs, there will be a deduction of 2 mobility from total mobility. Where LP represents number of pairs with surface contacts. Such pairs are called lower pairs. In this case we have 2 lower pairs.
Now consider the joint which is having a line contact. If joint between 2 links is having line or point contact, both the link should have same translational motion along the common normal. However it could have different motion, in tangential direction. So for each such pairs, there will be deduction of 1 mobility from total mobility. This kind of pair is called higher pair. Here we have got 1 higher pair.
|Fig.6 Lower pairs and higher pairs in a mechanism, a lower pair arrests 2 D.O.F, while a higher pair arrests one D.O.F|
The general equation to find out degrees of freedom of a planar mechanism is given below. This equation is also known as Kuthbach equation.
4 Bar Linkage
Back to same old planar mechanisms.This mechanism is having 4 links, and 4 lower pairs. So you can predict from Kuthbach equation that mobility of the mechanism is 1.
|Fig.7 4 bar linkage, it is having 4 links and 4 lower pairs|
Cam and Follower
Cam and follower is having 3 links, 2 lower pairs, and one higher pair.So mobility is again one.
|Fig.8 Cam and follower, 3 links, 2 lower pairs, 1 higher pair|
5 bar linkage
This mechanism is having 5 links and 5 lower pairs. So mobility is 2.
|Fig.9 5 bar linkage, 5 links, 5 lower pairs|
A 3 Dimensional Mechanism
If the mechanism is 3 dimensional in nature, you could easily derive an equation for mobility using the same concept. So equation for degree of freedom would be as follows
|Fig.10 A 3 dimensional mechansim|