Power transmission — gears, bearings, and shafts

This block of the Machine Design & Materials exam is about getting torque and motion from one place to another. The relationships are clean once you have them, which makes these fast points.

Gears and gear trains

• Speed and torque trade off with the gear ratio: gear up for torque, gear down for speed, power (ideally) conserved.
• Gear trains multiply ratios stage by stage; track direction and which gears are idlers.
• Know the basic geometry terms (module/diametral pitch, pitch diameter) well enough to connect tooth counts to sizes and speeds.

Bearings

• Rolling-element bearing life follows a load-life relationship: life scales with the (capacity ÷ load) ratio raised to a power — 3 for ball bearings, 10/3 for roller bearings. That exponent is the classic trap.
• L10 life is the rating at which 90% survive; equivalent load combines radial and thrust components.

Shafts and keys

• Power, torque, and speed are linked: power equals torque times angular velocity — mind the units (and the SI/USCS switch this exam can throw at you).
• Shaft sizing usually means a combined bending-and-torsion stress state (back to the stress-analysis and failure-theory deep-dives), often with a fatigue check.
• Keys transmit torque through shear and bearing on the keyway — a standard sizing question.

Where people lose points

• Bearing-life exponent — using 3 for a roller bearing or vice versa.
• Gear-train ratio direction or miscounting stages/idlers.
• Power = torque × speed unit errors, especially converting rpm to rad/s or across unit systems.
• Treating shaft sizing as pure torsion when bending is also present.

How to study it

Lock in the bearing-life exponents and the power-torque-speed relationship cold, then practice shaft problems that combine bending and torsion so the link to stress analysis is automatic. These are bounded, repeatable question types — ideal for timed reps.

Part of the Machine Design & Materials exam. → Start practicing free — the whole practice bank, free with an account.