Levers, Linkages, Cams and Followers

Mechanisms do the maths

A mechanism changes movement: its size, its direction, or its type (turning a motor's spin into a straight push, say). Levers, linkages, cams, pulleys and gears are the building blocks. This is also the topic that carries most of the exam's calculation marks, so alongside naming parts you will practise the three formulae that come up again and again: mechanical advantage, velocity ratio and gear ratio.

Four types of motion

Every mechanism turns one type of motion into another. Learn the four exact names.

Match each motion to an example

Class 1 levers

In a class 1 lever, such as a seesaw, what sits between the effort and the load? (The fulcrum is the pivot point.)

• The fulcrum
• The load
• The effort
• Nothing - all three are at the same point

The three formulae

Learn these exactly - most of the calculation marks depend on them: - Mechanical advantage (MA) = load / effort (a ratio of forces, no units).\n- Velocity ratio (VR) = distance moved by effort / distance moved by load (a ratio of distances).\n- Efficiency = (MA / VR) x 100%.\n- For gears and pulleys, ratio = driven / driver (teeth or diameter). Always show your working: write the formula, substitute the numbers, then give the answer.

Calculate mechanical advantage

A lever is used to lift a load of 200 N using an effort of 50 N. Calculate the mechanical advantage (MA = load / effort). Give your answer as a number.

Mechanical advantage vs velocity ratio

This is the confusion examiners see most. MA and VR are both ratios, but of completely different things.

Force or distance?

Which quantity is a ratio of distances or gear teeth, rather than a ratio of forces?

• Velocity ratio
• Mechanical advantage
• The load
• The effort

Match each component to its job

Calculate the gear ratio

A gear train has a driver gear with 20 teeth turning a driven gear with 60 teeth. Calculate the gear ratio (ratio = driven teeth / driver teeth). Give your answer as a number (for a ratio of that number to 1).

Calculate the output speed

The same 20-tooth driver turns at 300 rpm and drives the 60-tooth driven gear. Calculate the output speed of the driven gear, in rpm. (Output speed = input speed x driver teeth / driven teeth.)

Design the drive

Three mechanism decisions. Pick the correct engineering choice each time.

• You want the output shaft to turn SLOWER than the input but with MORE turning force (torque). What do you do?
• You place an idler gear between the driver and the driven gear. What does it do?
• A motor spins, but you need a straight-line (linear) output, like a sliding gate. Which mechanism fits?

Show your working

A gear train has a driver gear of 20 teeth turning at 300 rpm, connected to a driven gear of 40 teeth. Work out the output speed of the driven gear, showing each step, and explain what happens to the turning force (torque).

• Write the formula: output speed = input speed x driver teeth / driven teeth
• Substitute the numbers: 300 x 20 / 40
• Give the answer with its unit (150 rpm)
• Explain the trade-off: a slower output shaft gains more torque

Keep MA and VR apart

Two habits win the calculation marks here. First, keep MA and VR separate in your head: MA is load / effort (forces), VR is a ratio of distances or teeth. Muddling them is the single most common error. Second, for gears always reason from driven over driver: more teeth on the driven gear means a slower output shaft but more torque, and an idler gear only flips the direction, never the ratio. Write the formula, substitute, then answer with the right unit - method marks are there even if the final number slips.