Mechanism and Machines: Lecture Notes- 02

1. Mechanism

If the number of bodies assembled in such a way that the motion of one body causes the motion of other constrained or predictable. This is called Mechanism.

For More Details About Mechanism and Kinematic Pair, Click Here

2. Machine

A machine is basically a mechanism or combination of mechanisms, which apart from imparting motion to the part, also transmits and modifies the available mechanical energy into some kind of the desired work.

(It may also be called as a combination of rigid or resistant bodies).

3. Constrained Motion types

  • Completely Constrained Motion
  • Incompletely Constrained Motion
  • Successfully Constrained Motion

4. Rigid Body/Resistant Body

  • The rigid body doesn’t suffer any type of distortion under the action of a force.
  • In real life no body is a rigid body, so for engineering application concept of the resistant body comes to play.
  • A resistant body doesn’t deform for the purpose it is used. It means if a body is designed to withstand a force of 50 N, and under this amount of force if the body doesn’t deform then it is called a resistant body; however, it may deform if a force of 51 N or more is applied.

5. Link

A member of a mechanism that connects other members and having relative motion.

6. Degrees of Freedom

Degrees of Freedom = 6 – no. of restraints

Remember no. of restraints can never be zero (means joint disconnected), and can never be 6 (means joint becomes rigid).

Formula to Find D.O.F of Space Mechanism (3-D)

F = 6(L – 1) – 5d1 – 4d2 – 3d3 – 2d4 – d5

Where L = total number of links in a Mechanism

F = Degrees of Freedom

d1 = Pairs having 1 D.O.F

d2 = Pairs having 2 D.O.F

d3 = Pairs having 3 D.O.F

d4 = Pairs having 4 D.O.F

d5 = Pairs having 5 D.O.F

Formula to find D.O.F of Plane Mechanism (2-D)

F = 3(L – 1) – 2d1 – d2

Kutzback’s Criterion

F = 3(L – 1) – 2j – h

j = binary joints (lower pair)

h = no. of higher joints (higher pairs)

Gruebler’s Criterion

If h = 0, then formula reduced to

F = 3(L – 1) – 2j

Note –

  • If F = 0 (Frame)
  • F < 0 (Redundant frame)
  • F > 0 (Constrained/Unconstrained frame)

7. Simple Mechanism/Compound Mechanism

In simple words, mechanisms having four links are called simple mechanism.

i. Four bar mechanism

ii. Slider crank mechanism

iii. Double slider crank mechanism

Mechanisms having more than four links are called Compound Mechanisms.

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