Topic 4 of 4 · Form 5 · Lower 6
Structures and mechanisms
Frame and shell structures; forces: tension, compression, shear, bending, torsion. Triangulation; reactions at supports; simple beam calculations using moments; factor of safety = ultimate stress ÷ working stress.
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Structures
Frame and shell structures; forces: tension, compression, shear, bending, torsion. Triangulation; reactions at supports; simple beam calculations using moments; factor of safety = ultimate stress ÷ working stress.
Worked example: A 4 m beam is simply supported at its ends and carries a 6 kN load 1 m from the left support. Find the reactions.
Moments about left: R_R × 4 = 6 × 1 → R_R = 1.5 kN; R_L = 6 − 1.5 = 4.5 kN
Mechanisms
- Levers (three classes): mechanical advantage MA = load ÷ effort; velocity ratio VR = distance moved by effort ÷ distance moved by load; efficiency = MA ÷ VR × 100%.
- Gears: gear ratio = driven teeth ÷ driver teeth; compound gear trains multiply ratios; worm gears give large reductions.
- Pulleys and belts, chains and sprockets, cams and followers, cranks and sliders, screw threads, ratchets.
Worked example: A compound train: 20T drives 60T, and on the same shaft a 15T drives 45T. Find the overall ratio and output speed for 900 rpm input.
Ratio = (60/20) × (45/15) = 3 × 3 = 9 : 1 → output 900 ÷ 9 = 100 rpm
Pneumatics and hydraulics
Cylinders, valves and circuits; Pascal's principle; force = pressure × area.
Energy and power transmission
Work, power, torque (T = F × r), and mechanical efficiency.
Check yourself
- A machine has MA 4 and VR 5. Find its efficiency.
- A cylinder has a piston area of 0.002 m² and a pressure of 500 kPa. Find the force.
- What does a factor of safety of 3 mean?
Show answers
- 80%
- 1 000 N
- The structure can withstand three times the working load (or stress) before failure.
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