Mechanical Motion and Force Conversion Systems

Mechanisms: Transmitting Force and Motion

Mechanisms are devices used to transmit and convert forces and motion from a driving force or input to an output.

a) Levers

A lever is a rigid bar that is supported by a fulcrum. When a lever is balanced, it complies with the Law of the Lever:

F × d = R × r

Force (F) and Resistance (R) can be measured in Newton [N] or kilograms [Kg], but both must always be in the same unit.

There are 3 kinds of levers according to the position of the Force (F), Resistance (R or Load), and Fulcrum (P or pivot):

• Class 1: Force – Fulcrum – Resistance
• Class 2: Force – Resistance – Fulcrum
• Class 3: Fulcrum – Force - Resistance

b) Pulleys

We will study three kinds of pulleys:

Fixed Pulley

A fixed pulley is a wheel that has a groove around it into which a rope, chain, or belt fits. It rotates around an axle that is fixed to an immobile surface. A fixed pulley is balanced when the effort (F) is equal to the resistance of the load (R): F = R.

Movable Pulley

A movable pulley is a set of two pulleys—one is fixed while the other can move in a linear direction. The effort required to move a load with a movable pulley is half the effort needed to move the same load with a fixed pulley: F = R/2.

Compound Pulley (Block and Tackle)

This is a system of fixed and movable pulleys used in lifts, goods lifts, and cranes. If we use more pulleys, the system becomes more complex, but less effort is needed to move the load. The relationship is: F = R/2 × n or F = R/2ⁿ (where n is the number of movable pulleys).

Rotary Motion Mechanisms

Friction Drives

Friction drives are made up of two or more wheels that are in contact. The first wheel is called the primary drive wheel. When it moves, it turns or drives the second or output wheel, causing it to move as well.

Pulleys with Belt

These consist of two pulleys or wheels that are a certain distance apart. Their axles are parallel, and they rotate simultaneously due to the effect of the belt.

Gear Mechanisms and Cogwheels

Cogwheels are sets of wheels that have teeth called cogs. The cogs fit into the spaces between the cogs of another wheel, so that one wheel moves the other.

Gear Mechanisms with Chain

These involve two cogwheels with parallel axles that are a certain distance apart; they rotate simultaneously by means of a metal chain or a toothed belt stretched over both wheels.

Gear Train

This is a system of more than two gears connected together. The rotary motion of the first wheel (1) drives the second wheel (2), and so on. In this system, the rotary movement of the first axle is transmitted to the second by means of wheels 1 and 2. Wheel 3 rotates with the same velocity as wheel 2 and drives wheel 4, to which it is connected.

The gear ratio between the drive wheel (1) and the driven wheel (4) depends on the number of geared teeth in the system:

N4/N1 = (Z1 × Z3) / (Z2 × Z4)