Kinematic Pairs in Mechanism design

Summary of important concept of Kinematic pairs in design of mechanisms for definite motion

Mechanisms work on the principle of constrained degrees of freedom to obtain definite and controllable motion based on an input. This is possible through kinematic pairs.

The two links or parts of a machine when in contact with each other and having relative motion between them are said to form a pair

A kinematic pair is a connection between two adjacent links that permits a definite relative motion between them.

Classification of Kinematic pairs based on nature of contact:

  • Lower pairs – Surface to surface contact between two members.
    • Hinge or Pivot or revolute joints
    • Bearings
    • Linear guide rails
    • Ball joints
  • Higher pairs – Line to line or Line to surface contact between two members
    • Meshing gears
    • CAM and Followers
    • Wheel rollers

  • Wrapping pair – Contact is wrapping surface.
    • Belt systems

A Break down of various lower pairs based on degrees of freedom constrained.

Prismatic pair allows only one degree of freedom which is translation. Example is a piston moving inside a cylinder.

Planar pair allows 3 degrees of freedom , 2 translations in x and y direction and rotation in xy plane . Example: A block on a flat plate

Revolute pair which is one the most used, has one degree of freedom which is rotation. Bearings, pivots, clevis pins all are revolute pairs.

Screw pair is used to transform rotation to translation or vice versa. Two degrees of freedom are allowed

Cylindrical pair allows translation and rotation. Two concentric cylinders assembled can rotate relatively and also translate (move) laterally.

Spherical pair allows 3 degrees of freedom all in rotation about x ,y and z axes. Ball joints used in cars use this kinematic pair arrangement.

Higher pairs are characterized when there is a line to line contact ideally or theoretically as in the case of meshing gears where the contact is at the pressure point or pressure line. Wheels rolling on a flat surface also ideally have a line of contact so do CAMs with roller followers.

Wheel rolling on a flat surface

Categories: : Mechanism and Machine design