Mechanical Devices Exam Practice

This lesson brings together everything you have learned about mechanical devices and movement for AQA GCSE Design and Technology (8552), Section 3.1.5. It provides exam-style questions, model answers and revision strategies.

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Key Topics to Revise

Topic	What You Must Know
Types of motion	Linear, rotary, reciprocating, oscillating — definitions and examples
Input → Process → Output	Identify each stage in a mechanical system
Levers	Three classes; fulcrum, effort, load positions; MA calculation
Linkages	Reverse motion, parallel motion, bell crank, crank and slider
Gears	Simple, compound, worm, rack and pinion, bevel — gear ratio calculations
Cams	Pear, eccentric, snail, heart — profiles and displacement diagrams
Pulleys	Single fixed, single movable, block and tackle — MA calculations
Belt drives	Speed ratios, flat/V/toothed/crossed belts
Calculations	MA, VR, efficiency, gear ratios, output speed, torque

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Practice Question 1: Identify the Type of Motion (2 marks)

Q: Name the type of motion exhibited by each of the following: (a) A car piston moving up and down in a cylinder. (b) A playground swing.

Model Answer: (a) Reciprocating motion — the piston moves back and forth along a straight line. (1 mark) (b) Oscillating motion — the swing moves back and forth along a curved arc around the pivot point. (1 mark)

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Practice Question 2: Lever Classification (3 marks)

Q: A pair of tweezers is used to pick up a small component. (a) Identify the class of lever used in tweezers. (1 mark) (b) Identify the position of the fulcrum, effort and load. (1 mark) (c) Explain why tweezers have a mechanical advantage of less than 1. (1 mark)

Model Answer: (a) Third class lever. (1) (b) The fulcrum is at the joined end, the effort is applied by the fingers in the middle, and the load is at the tips. (1) (c) Because the effort is applied between the fulcrum and the load, the effort arm is shorter than the load arm. This means the mechanism multiplies distance and precision rather than force, giving an MA less than 1. (1)

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Practice Question 3: Gear Ratio Calculation (4 marks)

Q: A simple gear train consists of a driver gear with 12 teeth meshing with a driven gear with 48 teeth. The driver gear is connected to a motor running at 2400 RPM.

(a) Calculate the gear ratio. (1 mark) (b) Calculate the output speed. (1 mark) (c) State the effect on the output torque compared to the input. (1 mark) (d) State the direction of the output gear relative to the input gear. (1 mark)

Model Answer:

(a) $GR = \frac{48}{12} = 4$GR=1248​=4 (or 4:1) (1)

(b) $\text{Output speed} = \frac{2400}{4} = 600 \text{ RPM}$Output speed=42400​=600 RPM (1)

(c) The output torque is 4 times greater than the input torque (because the gear ratio is 4:1 and speed has decreased). (1)

(d) The output gear rotates in the opposite direction to the input gear (meshing spur gears rotate in opposite directions). (1)

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Practice Question 4: Cam Identification (3 marks)

Q: A displacement diagram shows a follower that rises gradually at a constant rate over most of the cam rotation, then drops suddenly back to its starting position.

(a) Name the type of cam that would produce this motion. (1 mark) (b) Give one real-world application for this type of cam. (1 mark) (c) Explain why a return spring is needed in most cam mechanisms. (1 mark)

Model Answer: (a) Snail cam (spiral cam). (1) (b) A music box — the snail cam gradually lifts a hammer, which then drops suddenly to strike a tuned metal tooth, producing a musical note. (1) (c) The spring keeps the follower in contact with the cam surface at all times, especially during the sudden drop, preventing the follower from bouncing off the cam. (1)

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Practice Question 5: Pulley Calculation (4 marks)

Q: A block and tackle pulley system is used to lift a crate weighing 800 N. The system has 4 rope sections supporting the load.

(a) Calculate the effort required to lift the crate (ignore friction). (1 mark) (b) State the velocity ratio of the system. (1 mark) (c) If the actual effort required is 250 N (due to friction), calculate the efficiency of the system. (2 marks)

Model Answer:

(a) $\text{Effort} = \frac{800}{4} = 200 \text{ N}$Effort=4800​=200 N (1)

(b) VR = 4 (equals the number of rope sections supporting the load). (1)

(c) $MA = \frac{800}{250} = 3.2$MA=250800​=3.2 (1)

$\text{Efficiency} = \frac{3.2}{4} \times 100\% = 80\%$Efficiency=43.2​×100%=80% (1)

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Practice Question 6: Linkage Selection (4 marks)

Q: A designer is creating a mechanism for an automatic door closer. The door handle is pressed downwards, and this must cause a horizontal bolt to slide sideways to release the door.

(a) Name the type of linkage that could convert the downward handle movement into horizontal bolt movement. (1 mark) (b) Explain how this linkage achieves the direction change. (2 marks) (c) State one other product that uses this type of linkage. (1 mark)

Model Answer: (a) Bell crank linkage. (1) (b) The bell crank is an L-shaped lever with a fixed pivot at the corner. When the downward input force is applied to the vertical arm, the L-shape rotates about the fixed pivot, causing the horizontal arm to move sideways. This converts the vertical (downward) motion into horizontal (sideways) motion — a change of direction of 90°. (2) (c) A bicycle brake lever — squeezing the horizontal lever applies a vertical force to pull the brake cable. (1)

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Practice Question 7: Compound Gear Train (4 marks)

Q: A compound gear train has the following configuration:

• Gear A (driver): 15 teeth, input speed 1800 RPM
• Gear B: 45 teeth, meshes with A
• Gear C: 10 teeth, on the same shaft as B
• Gear D (output): 40 teeth, meshes with C

Calculate the output speed.

Model Answer:

Step 1: $GR_1 = \frac{45}{15} = 3$GR1​=1545​=3 (1)

Step 2: $GR_2 = \frac{40}{10} = 4$GR2​=1040​=4 (1)

Step 3: $\text{Overall GR} = 3 \times 4 = 12$Overall GR=3×4=12 (1)

Step 4: $\text{Output speed} = \frac{1800}{12} = 150 \text{ RPM}$Output speed=121800​=150 RPM (1)

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Exam Strategy Reminders

For Identification Questions (1–2 marks)

• Give a precise name (e.g. "snail cam" not just "cam").
• Refer to the key distinguishing feature (e.g. "the follower rises gradually and drops suddenly").

For Calculation Questions (2–4 marks)

• Write the formula before substituting values.
• Show all working — even simple arithmetic.
• Include units in every answer (N, m, RPM, %, Nm).
• Check your answer — does it make sense? (e.g. a gear ratio of 0.001 is probably wrong).

For Explanation Questions (3–4 marks)

• Use connective phrases: "because", "this means that", "as a result".
• Give specific examples or product references.
• Link your answer to the mechanical principles (force multiplication, direction change, motion conversion).

For Evaluation Questions (6 marks)

• Give at least 3 advantages and 3 disadvantages.
• Use specific technical detail — not vague generalisations.
• Include a conclusion that weighs up the evidence.