Mechanisms, structures and control
What the domain tests
Mechanisms, structures and control tests how motion and force are transmitted and how structures stay stable. Expect levers, linkages, gears, pulleys, belts, chains, cams, types of motion, force diagrams, triangulation, and simple open- and closed-loop control.
Core rules and concepts
- A lever has effort, load and fulcrum; moments are force × perpendicular distance. Increase effort distance or reduce load distance to gain mechanical advantage.
- For two meshing gears, ratio = driven teeth ÷ driver teeth. Output speed is input speed ÷ ratio; torque rises roughly by the ratio, less friction. External gears reverse direction.
- Pulleys with a belt transmit motion quietly; slip limits torque. Chain drives do not slip but need lubrication. A cam converts rotation to reciprocating or oscillating motion.
- Structures need a load path. Triangulation stops a frame racking; ties are in tension, struts in compression. A wide base and low centre of gravity improve stability.
- Open-loop control does not check output; closed-loop uses feedback from a sensor to reduce error.
Worked example
A motor turns at 600 rev/min and drives a 20-tooth gear meshing with a 60-tooth gear. Ratio = 60 ÷ 20 = 3:1. Output speed = 600 ÷ 3 = 200 rev/min. The output torque is about three times the input torque, ignoring losses, and the output rotates in the opposite direction. If the mechanism must lift a heavy load slowly, this is useful; if high speed is needed, the gear order should be reversed.
Traps that cost marks
Putting the gear ratio upside down, forgetting opposite rotation, ignoring friction, or confusing a tie with a strut loses marks. A structure is not stable merely because it is strong; check load paths and triangulation. Do not call a feedback system open-loop if a sensor adjusts the output.