Transmissions, Module 7, Certify66

Notes

Main ideas

Transmission systems transfer drive and change speed, torque or direction using gears, belts and pulleys, chains and sprockets, and screw-jack or push-pull rod mechanisms.
BACKLASH is the free play between meshing gear teeth. Too little causes binding and overheating; too much indicates wear and causes lost motion and shock loading. It is measured with a dial gauge against a locked mating gear.
Gear wear and damage are inspected for: pitting and spalling of the tooth flanks (fatigue), scoring or scuffing (lubrication breakdown), and chipped or cracked teeth.
Belts are checked for correct tension, fraying, glazing and cracking; over-tension overloads the bearings, under-tension lets the belt slip and glaze. Pulleys are checked for wear and alignment.
Chains and sprockets are inspected for stretch (a chain 'grows' as the pins and rollers wear), tight or stiff links, and hooked or worn sprocket teeth.
Screw-jacks (e.g. a stabiliser trim actuator) are checked for end-play and for the condition of the ballscrew or acme thread; a worn primary thread relies on the secondary/no-back nut, so any measurable free play is significant.
Correct lubrication is fundamental, the specified grade, at the specified points and intervals.
⚠ Exam trap: backlash that is too SMALL causes binding and heat; the wear fault is too MUCH backlash.

Key formulas

Solved examples

A gear train is noisy and running hot with almost no measurable backlash. What is wrong?
The backlash is too small, the teeth are binding, which generates heat and accelerates wear. The mesh or centre distance must be corrected to restore the specified clearance.
How is a roller chain checked for wear, and what does the wear represent?
By measuring its length over a number of links against the limit, a worn chain elongates as the pins and roller bushes wear. Excess elongation makes the chain ride up the sprocket teeth and must be replaced, usually with the sprockets.

Transmissions