Apr 052013

In kinematic pairs, constrained motion (or relative motion) can be broadly classified into three types. They are

  1. Completely constrained motion
  2. Incompletely constrained motion
  3. Partially (or successfully) constrained motion

1. Completely constrained motion:

Completely constrained motion is a type of constrained motion in which relative motion between the links of a kinematic pair occurs in a definite direction by itself, irrespective of the external forces applied.

Square bar in a square hole undergoes completely constrained motion

Square bar in a square hole

Even when any external force is applied, a square bar always slides inside a square hole. It does not turn.

2. Incompletely constrained motion:

In incompletely constrained motion, the relative motion between the links depend on the direction of external forces acting on them. A good example of incompletely constrained motion is the motion of a shaft inside a circular hole. Depending on the direction of external forces applied, the shaft may slide or turn (or do both) inside the circular hole.

Incompletely constrained motion is undesirable in any mechanical system. It leads to improper mechanical outputs.

Shaft in a circular hole

Shaft in a Circular Hole

3. Partially (or successfully) constrained motion:

A kinematic pair is said to be partially or successfully constrained if the relative motion between its links occurs in a definite direction, not by itself, but by some other means. A good example of successfully constrained motion is piston reciprocating inside a cylinder in an internal combustion engine.

Normally, when a piston is placed in a cylinder, it may undergo reciprocating motion (upward and downward motion) and turning motion, depending on the external forces applied. It is incompletely constrained.

However, if the piston is connected to a connecting rod, its motion is successfully constrained i.e., it can only undergo only reciprocating motion inside the cylinder. Here, some other means (i.e., connecting rod) is used for successfully constraining the motion of the piston.

A Piston


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I am a mechanical engineer with a passion for technical stuff. I am the founder and former editor-in-chief of Mechteacher.com.

  • MechEngineer

    Hi. i’am a T.E. Mech student… I’ve been assigned to a make a project on all the 3 types of constrained motion… so should i just make these examples of the square block with square bar, a square block with shaft & a square block with a key way, for my project ? can u give me some more examples in which i could apply the three motions ? & also can u tell me, with which material should i make my project? with wood, mild steel, cast iron, plz ?….

    • http://mechteacher.com/ Surjeet Sankararaj

      For your project, you can make use of the examples discussed in this article.

      It seems like you wish to demonstrate the 3 types of constrained motion with your project. As long as this is your project’s objective, material doesn’t matter. You can use any rigid material.

      To demonstrate successfully constrained motion, you can also use a foot step bearing (with load on its top).

      • Sumit Rana

        Its too gud

  • Shreenandan Sundara

    Description on the contrained motion is very helpful. Thanls alot

  • Kavitha

    Thank u so..much

  • ps kaladhar

    Respected friend,
    your ideas of teaching is greately gratified by all. I am thankful to u and ur team, for the contributions u have put forward.
    my certain request to u is that ,it would be more widely accepted if u put ur work forward
    for designing a ” working model” for the figures shown above ,with explaination of their charecterstics in a well defined manner . So that it would be more precisely understandable for the the viewers in one gasp.
    For the present work ,we are thankful to u.