Mechanisms of Motion Transmission

Mechanisms

Mechanisms are devices that transmit motion to perform a job. They are used to transmit motion in two primary ways:

Linear Motion Transmission

1. Fixed Pulleys

   A fixed pulley is a wheel with a grooved edge that rotates around a central axis. A rope or belt passes through the groove. The force applied at one end of the rope equals the resistance at the other (F = R). The advantage of a fixed pulley is the change in the direction of motion.

2. Movable Pulleys

   In a movable pulley, one end of the rope is fixed, and the resistance hangs from the pulley's axle. The force required is half the resistance (F = R/2).

3. Pulley Systems (Hoists)

   A hoist combines fixed and movable pulleys. The force required is calculated as F = R/(2n), where 'n' is the number of movable pulleys.

4. Levers

   A lever is a rigid bar that pivots on a fulcrum. There are three classes of levers:

   • 1st Class: The fulcrum is between the force and resistance (e.g., scissors).
   • 2nd Class: The resistance is between the fulcrum and force (e.g., wheelbarrow).
   • 3rd Class: The force is between the fulcrum and resistance (e.g., tweezers).

   The principle of levers states that the force multiplied by its distance from the fulcrum equals the resistance multiplied by its distance from the fulcrum (F x Bf = R x Br), where Bf and Br are the distances from the fulcrum to the force and resistance, respectively.

Circular Motion Transmission

1. Gears

   Gears are toothed wheels. When two gears mesh, the larger one is called the gear wheel, and the smaller one is called the pinion. Gears change the direction of rotation.

2. Chain Drives

   Chain drives use a chain to connect two or more gears. They offer the same gear ratio as directly connected gears but prevent slippage. The two gears rotate in the same direction.

3. Worm Gears

   A worm gear consists of a screw (worm) and a gear wheel. The screw's axis is perpendicular to the gear's plane. The worm effectively has a single tooth. This mechanism is often used for high gear ratios, typically to tighten wires or ropes.

4. Belt Drives

   Belt drives use a belt to connect two pulleys. The rotation of one pulley transfers motion to the other. The relationship between pulley diameters (D1, D2) and rotational speeds (N1, N2) is: D1 x N1 = D2 x N2. If the pulleys don't cross, they rotate in the same direction; if they cross, they rotate in opposite directions.

5. Friction Wheels

   Friction wheels are two wheels in direct contact, often with one made of rubber for better grip. They rotate in opposite directions and follow the same equation as belt drives: D1 x N1 = D2 x N2. They are used when there's a risk of breakage with other methods.

Gear Ratio

The gear ratio (i) is the ratio of the number of turns of the driving gear (nm) to the number of turns of the driven gear (nc): i = nm / nc.