Mechanical devices · GCSE Design and Technology

Types of movement

Linear: movement in a straight line, such as a paper trimmer.
Rotary: movement in a circle, such as a wheel.
Reciprocating: backwards and forwards in a straight line, such as a sewing machine needle.
Oscillating: swinging backwards and forwards in an arc, such as a pendulum.

First order levers have the fulcrum between the effort and load (a see-saw); second order have the load in the middle (a wheelbarrow); third order have the effort in the middle (tweezers).
Mechanical advantage = load ÷ effort. A mechanical advantage above 1 means less effort is needed.
Linkages transfer force and change direction: a reverse motion linkage makes the output move the opposite way to the input; a bell crank changes the direction of motion through 90 degrees.

Cams and followers change rotary motion into reciprocating motion. The shape of the cam (such as circular/eccentric, pear or snail) decides how the follower moves.
Gear trains: when a small driver gear turns a large driven gear, the output is slower but more powerful. Gear ratio = teeth on driven gear ÷ teeth on driver gear.
Rack and pinion turns rotary motion into linear motion, such as in a car's steering.
Pulleys and belts transfer rotary motion between shafts; a belt can slip if overloaded, which protects the machine.

A driver gear with 10 teeth turns a driven gear with 40 teeth. The gear ratio is 40 ÷ 10 = 4 (4:1). If the driver turns at 200 rpm, the output speed is 200 ÷ 4 = 50 rpm.

Linear motion: Movement in a straight line.
Rotary motion: Movement in a circle.
Reciprocating motion: Backwards and forwards movement in a straight line.
Oscillating motion: Swinging backwards and forwards in an arc.
Cam: A shaped part that turns rotary motion into reciprocating motion.
Gear ratio: Teeth on the driven gear divided by teeth on the driver gear.
Rack and pinion: A gear system that turns rotary motion into linear motion.
Bell crank: A linkage that changes the direction of motion through 90 degrees.