IGCSE Design and Technology - Systems and Control: Mechanical Systems

This document provides detailed notes on mechanical systems, covering levers, linkages, gears, pulleys, cams, and cranks, as part of the IGCSE Design and Technology 0445 syllabus.

1. Levers

1.1 What is a Lever?

A lever is a simple machine that uses a rigid bar (the lever arm) pivoting about a fixed point (the fulcrum) to multiply force. It allows a small force to exert a larger force.

1.2 Parts of a Lever

• Fulcrum: The fixed point about which the lever pivots.
• Effort: The force applied to the lever.
• Load: The resistance that the lever overcomes.

1.3 Classes of Levers

Class	Fulcrum Position	Effort Position	Load Position	Examples
Class 1	Fulcrum	Load	Effort	Seesaw, Crowbar, Scissors
Class 2	Load	Fulcrum	Effort	Wheelbarrow, Nutcracker, Bottle Opener
Class 3	Effort	Fulcrum	Load	Tweezers, Fishing Rod, Human Arm

1.4 Mechanical Advantage (MA)

Mechanical advantage is the ratio of the output force (load) to the input force (effort).

$$MA = \frac{Load}{Effort}$$

• MA > 1: Effort is multiplied.
• MA < 1: Effort is increased, but load is reduced.
• MA = 1: No mechanical advantage.

2. Linkages

2.1 What are Linkages?

A linkage is a system of rigid bars connected by joints (pins) that transmit motion. Linkages are used to convert rotary motion into linear motion or vice versa.

2.2 Types of Linkages

• Four-bar linkage: A common linkage consisting of four rigid bars connected by four joints. Used in car suspensions and bicycle pedals.
• Crank-and-slider linkage: Converts rotary motion of a crank into linear motion of a slider. Used in engines.
• Reciprocating linkage: Converts rotary motion into back-and-forth linear motion.

2.3 Applications of Linkages

Linkages are used in various applications, including:

• Bicycle pedals
• Car suspensions
• Door opening mechanisms
• Printing presses

3. Gears

3.1 What are Gears?

Gears are toothed wheels that mesh together to transmit rotary motion and torque. They are used to change the speed and torque of a rotating shaft.

3.2 Types of Gears

• Spur gears: Simple gears with straight teeth. Efficient for transmitting power over short distances.
• Bevel gears: Used to transmit motion between shafts that are at an angle to each other.
• Worm gears: Used to transmit motion between shafts that are at a right angle.

3.3 Gear Ratio

The gear ratio is the ratio of the number of teeth on the driving gear to the number of teeth on the driven gear.

$$Gear Ratio = \frac{Number\ of\ Teeth\ on\ Driving\ Gear}{Number\ of\ Teeth\ on\ Driven\ Gear}$$

• Gear ratio > 1: Speed is reduced, torque is increased.
• Gear ratio < 1: Speed is increased, torque is reduced.

4. Pulleys

4.1 What are Pulleys?

A pulley is a wheel with a grooved rim around which a rope, cable, or belt passes. Pulleys are used to change the direction of a force and can provide a mechanical advantage.

4.2 Types of Pulleys

• Fixed pulley: Attached to a fixed support. Changes the direction of the force but does not provide a mechanical advantage.
• Movable pulley: Attached to the load. Provides a mechanical advantage of approximately 2.
• Compound pulley: A combination of fixed and movable pulleys. Provides a greater mechanical advantage.

4.3 Mechanical Advantage of Pulleys

The mechanical advantage of a pulley system is determined by the number of rope sections supporting the load (for movable pulleys).

$$MA = \frac{Number\ of\ Rope\ Sections\ Supporting\ Load}{Effort\ Distance}$$

5. Cams

5.1 What is a Cam?

A cam is a rotating disk with an irregular profile. When the cam rotates, its profile forces a follower to move linearly. Cams are used to convert rotary motion into linear motion.

5.2 Follower

A follower is a part that moves linearly as a result of the cam's rotation. It is connected to the mechanism that needs to be moved.

5.3 Applications of Cams

Cams are used in:

• Internal combustion engines (to control the opening and closing of valves)
• Printing presses
• Automated machinery

6. Cranks

6.1 What is a Crank?

A crank is a lever that converts rotary motion into reciprocating (back-and-forth) linear motion. It is typically used in conjunction with a connecting rod.

6.2 Crank-and-Slider Linkage

A crank-and-slider linkage consists of a crank (a rotating arm) and a slider (a moving part connected to the crank by a connecting rod).

6.3 Applications of Cranks

Cranks are used in:

• Internal combustion engines (to convert the rotary motion of the crankshaft into the linear motion of the pistons)
• Pumps
• Bicycles (pedals are connected to a crank)