Mechanical Power Transmission, Alignment, and Couplings
Drives
Speed ratio equals the driven size divided by the driver size. Gears: spur (parallel shafts), helical (smoother, thrust load), bevel (right angle), worm (high ratio, self-locking, low efficiency). Belt drives need matched belt lengths on multi-belt sheaves, correct tension, and aligned sheaves - over-tensioning destroys bearings as surely as under-tensioning glazes belts. Chain drives need correct sag (roughly 1–2% of span) and lubrication.
Power relationships: torque and horsepower convert by HP = (torque in lb-ft × rpm) ÷ 5252.
Alignment
Check for soft foot first - a foot that rocks will bend the frame and ruin any alignment you set. Distinguish parallel (offset) misalignment from angular (gap) misalignment. Methods run from straightedge and feeler gauge, to rim-and-face and reverse-dial indicators, to laser systems. Account for thermal growth: a machine aligned cold may be misaligned at operating temperature.
Couplings
Rigid couplings demand near-perfect alignment. Flexible types - elastomeric, jaw, grid, gear, disc - absorb small misalignment, shock, and vibration but are not a fix for gross offset. Lubricated gear couplings need the correct grease; elastomeric inserts wear and should be replaced as sets.
Traps
Inverting the ratio, forgetting that compound reducers multiply ratios, using a flexible coupling to cover bad alignment, skipping the soft-foot check.
Worked example
A 1,750 rpm motor with a 3 in sheave drives a 9 in sheave. Driven speed = 1,750 × (3 ÷ 9) = 583 rpm. The driven shaft turns slower and carries about three times the torque.