Transmissions
Familiarisation with gear, belt and chain transmissions and the meaning of backlash and its wear limits.
Transmissions — summary notes
Free- 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.
- Backlash
- free play between meshing teeth — measured with a dial gauge
- Belt tension
- too tight overloads bearings · too slack slips and glazes
- Chain wear
- stretch (elongation) from pin and roller wear
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.
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