Mechanical Systems
This section covers Mechanical Systems: simple machines, levers, pulleys, gears, belts, chains, bearings, couplings, cams and linkages. The exam tests velocity ratio, mechanical advantage, efficiency and basic fault recognition.
Core rules
- Mechanical advantage MA = load/effort. Velocity ratio VR = distance moved by effort/distance moved by load. Efficiency = MA/VR × 100%.
- For gears, speed ratio = driven teeth/driver teeth; a 20-tooth driver at 600 rpm driving a 60-tooth gear gives 200 rpm. Torque increases in the same ratio, less losses.
- Belt drives are smooth and absorb shock but can slip; chain drives are positive but need lubrication; gear drives are positive and compact.
- Bearings support rotating shafts. Deep-groove ball bearings take radial and some axial load; taper-roller bearings take combined heavy loads.
- A cam converts rotation into reciprocating motion; a follower’s displacement depends on the cam profile.
Worked example
A 20-tooth driver turns at 600 rpm and drives a 60-tooth gear. Speed ratio = 60/20 = 3, so output speed = 600/3 = 200 rpm. If input torque is 10 N·m and efficiency is 90%, output torque = 10 × 3 × 0.90 = 27 N·m. The gear train does not create energy; it trades speed for torque.
Traps
Do not multiply speed by the gear ratio when the ratio is defined as driven/driver. Do not ignore efficiency. A loose belt may cause speed loss, not a gear-ratio change.