Kinematic Pair, Classification of Kinematic Pair

The basic understanding of kinematic pair and its types is very important for mechanical engineering students. this is just the starting of theory of machines so you should know about its definition and its classification. kinematic pairs are classified based on some categories. You need to be aware all of them without any confusion.
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What is Kinematic pair

In a mechanism or a machine, when two links or elements are joined with some type of contact, the combination of these two elements is called kinematics pair. The motion between the elements of pair should be completely or successfully constrained. The theory of kinematics pair is very important in the field of machine building. In the kinematics pair, the relative motion between the element has the key role for selection of an appropriate type of pair for a specific machine.

In the theory of machine to understand that type of kinematics pair is very important.There is a Classification of kinematic pairs based on different categories which are described below.

Type of Kinematic pairs

The type of Pears is defined as following considerations

1. Based on Nature of contact

Lower pair

When two links are joined with an area or surface contact between them, then the pair of links is called lower pair. In the relative motion of these two elements of the pair, elements slides on one other surface.

Sliding pair in crank slider mechanism, screw joint, shaft in a hole, pin joints are some of the example of lower pair.

Higher pair

If there is a point contact between the joints of two elements of pair, the kinematics pair is called higher pair. In the higher pair both turning and sliding motion are present partially

 For example, if two spheres are connected then they will have point Contact. The ball bearing are usually connected with point Contact. In the toothed Gears meshing with each other, that type of contact is point contact.

2.Based on nature of mechanical constraint

Closed pair or Self Closed pair

In the closed pair the elements are held together mechanically. The relative Motion is possible between the elements in the closed pair. All The lower pairs are closed pairs and some of the higher pairs comes in the category of closed pair. In the two elements of closed pair one is necessary to be a hollow part in which other element will be inserted. This is also called self closed pair. The example of closed pair is a Prism sliding on a hollow part.

Unclosed pair or Force closed pair

In the unclosed pair, the external force is required to keep the element of a pair connected to each other. Due to this unclosed pair is called force closed pair. The pair is not held mechanically in the unclosed pair. The example of enclosed there is cam and follower mechanism.

3. Based on the type of relative motion between the links

Prismatic Pair or Sliding pair 

When the elements of a joint constitute in such a way that one element can slide in the other element relatively. This kinematic pair is called sliding pair. Since in a sliding pair one type of relative motion so it is completely constrained motion between the links.

Sliding pair is generally a lower pair. Crank slider or Piston cylinder mechanism, prism sliding on a hollow part are the examples of sliding pair.

Revolute pair or Turning pair 

If the element only have capability to revolve inside the other elements in a fix access then the kinematic pair is called turning pair. In this pair also, the completely constrained motion will be there.

Lathe spindle revolves in the headstock, the bicycle wheel rotates with respect to a fixed axis, a shaft in a bearing. These are some of the examples of turning pair.

Rolling pair

When one element has rolling motion by keeping other element fix then the pair is called rolling pair.

Examples: a wheel rolling on a flat surface, roller bearing, ball bearing.

Screw pair

When screw Threads are provided in an element and one element can move along these threads, then the kinematic pair is called screw pair.

For example nut and bolt, lead screw in the lathe.

Spherical pair

In the spherical pair one link is fixed and other will be in the shape of sphere. The spherical element will turn on the fixed link.

The example of spherical pair is ball and socket joint.

Cylindrical pair

If the elements has both linear and turning motion then the pair can be said as cylindrical pair.

So that is all about the kinematic pairs and its classification. This topic is very important in theory of Machines. The basic mechanisms start from here. You Should be well aware of kinematic pairs to understand the basic mechanism like four bar mechanism, slider crank mechanism, double slider mechanism etc. Because as I mentioned earlier the mechanisms and the machines are built with the combination of links and the links are joined through pairs. So the kinematic pair is basics during the analysis of machines. 


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