GCSE Design & Technology: Mechanisms and Structures Revision Guide

Levers and linkages

A lever is a rigid bar that pivots around a fixed point called the fulcrum, used to change the size or direction of a force. In a first-class lever, the fulcrum sits between the effort and the load, as in a see-saw or a pair of scissors. In a second-class lever, the load sits between the fulcrum and the effort, as in a wheelbarrow, giving a mechanical advantage. In a third-class lever, the effort sits between the fulcrum and the load, as in a pair of tweezers, which trades force for a larger range of movement. Linkages connect several levers together so that a single input movement can produce a more complex output movement, such as the folding action of an umbrella.

Cams, gears, pulleys and belts

A cam mechanism converts rotary motion into reciprocating (up-and-down) motion, as a rotating, irregularly shaped cam pushes a follower up and down against it. Gears are toothed wheels that mesh together to transmit rotary motion between shafts, and by using gears of different sizes, a gear train can increase or decrease speed and torque, or change the direction of rotation. Pulleys connected by a belt transmit rotary motion between two shafts that are some distance apart; using pulleys of different diameters can also change the speed of rotation, in the same way that a gear train does.

Types of forces

Structures and components experience several types of force. Compression squeezes or pushes a material together, tension stretches or pulls a material apart, and shear occurs when two forces act in opposite directions, causing layers of a material to slide past each other. Bending combines compression on one side of a material with tension on the other, and torsion is a twisting force applied along the length of a material. Understanding which forces a component will experience helps a designer choose an appropriate material, shape and cross-section.

Structures

Structures can be classified as frame, shell or solid. A frame structure is built from thin structural members, such as struts (in compression) and ties (in tension), joined together, as in a bicycle frame or a pylon; frame structures are lightweight but rely on good joints. A shell structure gets most of its strength from its shape, such as a curved or folded surface, rather than from its thickness, as seen in an egg shell, a car body panel or a boat hull. A solid structure is filled completely with material, such as a brick, and can be very strong but heavy and wasteful of material for its size.

Reinforcement

Structures can be reinforced to improve their strength and stability. Triangulation, adding diagonal struts to form triangles within a frame structure, resists deformation because a triangle is a rigid shape that cannot change form without changing the length of its sides, unlike a square or rectangle. Gusset plates strengthen joints between members by spreading the load over a larger area. Corrugating (folding into ridges) or laminating (bonding layers together) can also reinforce a shell structure without significantly increasing its weight.

Common mistakes

• Muddling up compression and tension — compression pushes/squeezes a material, while tension pulls/stretches it apart.
• Assuming a frame structure is always weaker than a solid structure, when a well-triangulated frame can be both strong and far lighter than an equivalent solid structure.
• Forgetting that a cam converts rotary motion into reciprocating motion, not into purely linear or oscillating motion, which are different types of movement.