How do mechanisms change the type, direction and magnitude of movement and force?
Types of motion (linear, rotary, reciprocating, oscillating) and mechanical devices - levers, linkages, gears, pulleys, cams and followers, cranks - that change motion and provide mechanical advantage.
A focused answer to WJEC A-Level Design and Technology Unit 3 mechanical devices and movement, covering the four types of motion (linear, rotary, reciprocating, oscillating) and mechanisms such as levers, linkages, gears, pulleys, cams and cranks that change the type, direction and magnitude of motion and force, including mechanical advantage and velocity ratio.
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What this dot point is asking
WJEC wants you to name the four types of motion and explain how mechanisms change the type, direction and magnitude of motion and force, including simple calculations of mechanical advantage and gear ratio. The exam asks you to identify motions, describe a named mechanism, and calculate a ratio with its effect on speed and force. You need the mechanisms, what each does to the motion, and the speed-force trade-off.
The answer
The four types of motion
Levers and linkages
A lever is a rigid bar pivoting about a fulcrum, giving mechanical advantage: a small effort over a large distance moves a large load over a small distance. WJEC covers the three classes of lever (by the order of effort, load and fulcrum). Linkages connect levers to change the direction of motion (reverse motion linkage), make parts move together (parallel linkage) or change the path of movement.
Gears
Gears are toothed wheels that mesh to transmit rotary motion. They change speed and torque and reverse direction between meshing gears. The gear ratio is the teeth on the driven gear divided by the teeth on the driver gear. A large driven gear turns slower but with more torque (a reduction); a small driven gear turns faster with less torque. Idler gears change direction without changing the ratio; bevel gears turn drive through 90 degrees; a worm and wheel gives a large reduction.
Pulleys and belts
Pulleys linked by a belt transmit rotary motion between shafts, can change speed (by differing pulley diameters) and allow some slip. A block and tackle of pulleys gives mechanical advantage for lifting.
Cams and cranks
A cam and follower turns rotary motion into reciprocating (or oscillating) motion; the cam's profile sets the pattern of rise and fall (engine valves, automata). A crank and slider converts rotary motion into reciprocating linear motion or the reverse (engine piston and crankshaft, sewing machine needle).
Examples in context
Example 1. A bicycle. The pedals and chainset drive the rear wheel through gears: a small rear sprocket gives a high gear (fast but harder to turn) for flat roads, and a large rear sprocket gives a low gear (slow but easier) for climbing, a direct use of the gear-ratio trade-off between speed and force.
Example 2. A car engine. The crank and slider converts the reciprocating motion of the pistons into the rotary motion of the crankshaft, while cams on a shaft open and close the valves in time, combining two mechanisms to turn combustion into useful rotary drive.
Try this
Q1. Name the type of motion produced when a cam and follower is driven by a rotating shaft. [1 mark]
- Cue. Reciprocating (or oscillating) motion.
Q2. A driver gear of 30 teeth meshes with a driven gear of 90 teeth. Calculate the gear ratio and state whether the output is faster or slower than the input. [2 marks]
- Cue. Gear ratio = 90 / 30 = 3 to 1; the output is slower than the input (and has greater torque).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20185 marksA gear train has a driver gear of 20 teeth meshing with a driven gear of 60 teeth. Calculate the gear ratio and state the effect on speed and turning force (torque).Show worked answer →
The gear ratio compares the teeth on the driven gear to the teeth on the driver gear.
Gear ratio = teeth on driven / teeth on driver = 60 / 20 = 3, written 3 to 1. This means the driver turns three times for every one turn of the driven gear.
Effect: because the driven gear is larger, it turns more slowly than the driver (the output speed is one third of the input speed). Since power is conserved, the loss of speed is matched by a gain in turning force, so the output torque is about three times the input torque. This is a reduction gear, used where more force at lower speed is wanted.
Markers reward the correct ratio (3 to 1), the statement that the output is slower, and the trade-off that lower speed gives proportionally greater torque.
WJEC 20204 marksExplain the difference between a cam and follower mechanism and a crank and slider mechanism, giving one product that uses each.Show worked answer →
A cam and follower converts rotary motion into reciprocating (or sometimes oscillating) motion. As the shaped cam rotates, the follower resting on its profile rises and falls, and the cam profile controls the pattern of movement. It is used in engine valves and in automata to make parts rise and fall.
A crank and slider converts rotary motion into reciprocating linear motion (or the reverse). A rotating crank is linked by a connecting rod to a slider that moves back and forth in a straight line. It is used in a car engine (piston to crankshaft) and in a sewing machine needle drive.
Markers reward the input and output motion of each mechanism (rotary to reciprocating via a profile for the cam; rotary to reciprocating linear via a crank and rod for the slider) and a correct product example for each.
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Sources & how we know this
- WJEC AS/A Level Design and Technology specification — WJEC (2017)