Mechanical systems · GCSE Engineering

Motion and levers

Four types of motion: rotary (turning in a circle), linear (in a straight line), reciprocating (backwards and forwards in a line) and oscillating (swinging back and forth in an arc).
A lever turns about a pivot (fulcrum). First class: the fulcrum is between the effort and the load (a crowbar). Second class: the load is between (a wheelbarrow). Third class: the effort is between (tweezers).
Moment = force × perpendicular distance from the pivot. In balance, clockwise moments = anticlockwise moments. An effort of 50 N at 2 m balances a load of 200 N at 0.5 m.
Mechanical advantage = load ÷ effort. Lifting 200 N with 50 N gives a mechanical advantage of 4.

Gear ratio = number of teeth on the driven gear ÷ number of teeth on the driver gear. A 20-tooth driver turning a 60-tooth driven gear gives a ratio of 3.
Output speed = input speed ÷ gear ratio. With a ratio of 3, 300 rpm in gives 100 rpm out, but with three times the turning force (torque).
Meshing gears turn in opposite directions; an idler gear between them makes the output turn the same way as the input.
Compound gear trains multiply ratios. A worm and wheel gives a large reduction: a single-start worm with a 40-tooth wheel gives a ratio of 40. A rack and pinion changes rotary motion to linear motion.

Pulleys and belts transfer rotary motion between shafts; the ratio uses pulley diameters. Chains and sprockets don't slip.
Cams and followers change rotary motion into reciprocating motion. Cranks and sliders convert between rotary and reciprocating, as in an engine.
Bearings reduce friction on rotating shafts, and lubrication reduces wear.