Basic Definitions- Kinematics, Mechanisms

Kinematic Mechanism Definition

What is Mechanism

It can be defined as when the assembly of different bodies is done in such a way that the motion of one causes constrained or predictable motion to other members.

In other words, when one of the links of a kinematic chain, a combination of kinematic pairs (a combination of two links), is made fix the chain can be called as a mechanism.

Read Also: Theory of Machine Objective Questions

  • A mechanism may be used for transmitting the motion or for modifying the motion.
  • A machine itself is a mechanism or a combination of mechanisms and a machine apart from imparting motion also transmits and modifies the available mechanical energy to some other desired work.
  • A mechanism can be a Simple Mechanism or a Compound Mechanism.

  • When the number of links forming a mechanism is four, it is a case of Simple Mechanism whereas a mechanism consisting of more than four links is regarded as a Compound mechanism.
  • Four bar mechanism (4 links & 4 turning pairs), a slider-crank mechanism (4 links, 3 turning pairs and 1 sliding pair) and double slider crank mechanism (4 links, 2 turning & 2 sliding pairs) are the three types of simple mechanisms.

Kinematic Pair

  • Two links or elements when in contact have constrained motion (successfully or completely).

Types of Kinematic Pair

Based on the Nature of Contact

  • Lower Pair- having surface or area contact. e.g., shaft motion in bearings
  • Higher Pair- point or line contact types. e.g., cam and follower pair

Based on Nature of Relative Motion

  • Sliding Pair- relative motion is sliding.
  • Turning Pair- having turning or revolving motion.
  • Rolling Pair- having rolling motion relative to each other.
  • Spherical Pair- spherical shaped link turning inside a fixed link.
  • Screw (Helical) – having turning as well as sliding motion. e.g., lead screw

Based on the type of closure or nature of Mechanical Constrain

  • Closed/Self closed- elements held mechanically. e.g., all lower pairs are closed pair.
  • Open/forced closed- links are made in contact by other means or some external forces like gravity, spring action. e.g., cam & follower.

Read Also: Classification of Gears

What is Kinematic Links

A part of a machine or a member of a mechanism, connecting other parts or members and having a relative motion with one another.

  • It can be a binary (2 joints), ternary (3 joints) or Quaternary (4 joints)

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