Mechanisms and control systems
Mechanism questions test whether you can trace motion and torque through a system and calculate ratios; control questions test whether you can describe a system as input–process–output and say whether feedback exists.
Motion and mechanisms
Four motion types: linear, rotary, reciprocating, oscillating. Conversions: crank and slider (rotary ↔ reciprocating), cam and follower (rotary → oscillating or linear, the profile setting the motion law), rack and pinion (rotary ↔ linear), screw thread (rotary → linear), bell crank (changes direction). Levers: class 1 fulcrum between load and effort, class 2 load in the middle, class 3 effort in the middle. Mechanical advantage = load ÷ effort; velocity ratio = distance moved by effort ÷ distance moved by load; efficiency = MA/VR × 100%.
Gears and drives
For a meshing pair, velocity ratio = teeth on driven ÷ teeth on driver, and output speed = input × driver teeth ÷ driven teeth. An idler changes rotation direction only and is excluded from the ratio; a compound train multiplies the ratios of each pair. Worm drives give large reduction and are self-locking; belt drives slip, so real output speed falls short of the calculated value.
Control
Open loop: no sensor, output unchecked (a timed garden sprinkler). Closed loop: sensor → comparator → controller → actuator, with feedback (a thermostat). Draw systems as input → process → output with a feedback arrow. Sensors: switch, LDR, thermistor, ultrasonic, strain gauge. Actuators: DC motor, servo, stepper, solenoid, pneumatic cylinder. Logic gates: AND (both inputs 1), OR (either), NOT (invert), NAND; always supply a truth table. Pneumatics: a 5/2 valve has five ports and two positions.
Worked example
A motor runs at 1,400 rpm. A 20-tooth gear drives a 60-tooth gear; on the same shaft a 15-tooth gear drives a 45-tooth gear. Overall velocity ratio = (60/20) × (45/15) = 3 × 3 = 9, so output speed = 1,400 ÷ 9 ≈ 156 rpm, and torque is multiplied by about 9 minus losses. If the train is 90% efficient, output power = 0.9 × input power, so the usable torque gain is about 8.1, not 9.
Traps
- Inverting the gear ratio by dividing driver by driven.
- Counting idler gears in the ratio.
- Mixing MA and VR, or ignoring efficiency when power is transferred.
- Labelling a system closed loop when nothing measures the output.
- Confusing a 3/2 valve (three ports, two positions) with a 5/2 valve.