MECHANISM

1. What is a Mechanism ?

In the subject Theory of Machines (TOM), a mechanism is a fundamental concept used to understand how motion is transmitted and transformed in machines.
A mechanism is a combination of rigid bodies (links) connected by joints in such a way that they have definite relative motion.

In simple words:
A mechanism is a kinematic chain with one link fixed to produce useful motion.

2. Machine vs Mechanism

Aspect	Mechanism	Machine
Function	Transmits motion	Transmits motion + power
Forces	Not considered	Considered
Example	Four-bar linkage	Engine

Every machine is a mechanism, but not every mechanism is a machine.

3. Basic Elements of Mechanism

(A) Link (Kinematic Link)

A link is a resistant body that transmits motion.

Types of Links:

1. Rigid Link – No deformation (e.g., crank)
2. Flexible Link – Slight deformation (e.g., belt)
3. Fluid Link – Motion through fluid (e.g., hydraulic system)

(B) Kinematic Pair

When two links are connected for relative motion → Kinematic Pair

Types:

1. Based on Contact:

• Lower Pair → Surface contact (e.g., piston-cylinder)
• Higher Pair → Line/point contact (e.g., cam-follower)

2. Based on Motion:

• Sliding pair
• Turning pair
• Rolling pair
• Screw pair
• Spherical pair

4. Kinematic Chain

A kinematic chain is a combination of links connected in such a way that relative motion is possible.

Example:

• Four-bar chain
• Slider-crank chain

When one link is fixed → it becomes a mechanism

5. Types of Mechanisms

(A) Four-Bar Mechanism

• Consists of 4 links and 4 joints
• Types:
  • Crank-rocker
  • Double crank
  • Double rocker

Applications:

• Pump mechanisms
• Engine linkages

(B) Slider-Crank Mechanism

• Converts rotary motion ↔ reciprocating motion

Applications:

• IC Engines
• Compressors

(C) Cam and Follower Mechanism

• Converts rotary motion into specific follower motion

Applications:

• Valve timing in engines

(D) Gear Mechanism

• Transmits motion between shafts using gears

Applications:

• Automobiles
• Machinery

6. Degrees of Freedom (Mobility of Mechanism)

It defines number of independent motions.

Kutzbach Criterion:

$$F = 3(n – 1) – 2j – h$$

Where:

• $n$n = number of links
• $j$j = number of lower pairs
• $h$h = number of higher pairs

7. Inversion of Mechanism

Fixing different links of a kinematic chain produces different mechanisms.

Example:

• Slider-crank inversion:
  • Reciprocating engine
  • Oscillating cylinder engine

8. Constrained Motion

Types:

1. Completely constrained motion
2. Incompletely constrained motion
3. Successfully constrained motion

9. Mechanical Advantage (MA)

$$MA = \frac{Output\ Force}{Input\ Force}$$

• Important in mechanism performance

10. Applications of Mechanisms

• Automobiles
• Robotics
• Engines
• Manufacturing machines
• Aerospace systems

---