Module 6.9-6.10 Transmissions and control cables

Knowledge levels

Sub-module	A	B1 / B3	B2 / B2L
6.9 Transmissions: gear types, ratios, idlers, mesh patterns, belts, pulleys, chains, sprockets	1	2	2
6.10 Control cables: types, end fittings, turnbuckles, tension regulators, pulleys, Bowden and flexible controls	1	2	1

6.9 Gear types

Gear	Shafts	Features / uses
Spur	parallel	straight teeth; simple, noisy at high speed; accessory gearboxes
Helical	parallel (or crossed)	angled teeth; smoother and quieter; produces axial thrust
Double helical (herringbone)	parallel	cancels axial thrust
Bevel	intersecting (usually 90°)	change direction of drive; spiral bevel for smoothness (engine radial drive shafts)
Worm and worm wheel	non-intersecting at 90°	very high reduction in one stage; often self-locking; trim actuators
Rack and pinion	rotary ↔ linear	steering, some actuators
Planetary (epicyclic)	coaxial	sun gear, planet gears on a carrier, ring (annulus) gear; compact high reduction; propeller reduction gearboxes, helicopter main gearboxes
Internal gear	parallel	meshes inside a ring; same direction of rotation

Gear ratios

• Gear ratio = number of teeth on the driven gear ÷ number of teeth on the driver = driver speed ÷ driven speed.
• Small driver → large driven = reduction (speed down, torque up). Large driver → small driven = multiplication (speed up, torque down).
• Two meshing external gears turn in opposite directions.
• An idler gear between driver and driven changes direction but not the ratio.
• Compound gear trains: overall ratio = product of the individual ratios.

Worked example - compound train

Driver A (15 teeth) drives B (45 teeth). Gear C (20 teeth) is on the same shaft as B and drives D (60 teeth). A turns at 3 000 rpm.

• Ratio A-B = 45/15 = 3; ratio C-D = 60/20 = 3; overall = 9 : 1
• Speed of D = 3 000 / 9 = 333 rpm
• Direction: A → B reverses, C (same as B) → D reverses again, so D turns the same way as A.

Mesh patterns and backlash

• Backlash: small clearance between meshing teeth to allow lubrication and thermal expansion. Too little → binding, heat; too much → noise, shock loads.
• Tooth contact pattern is checked with marking compound (engineer's blue): the correct pattern is centred on the tooth flank; patterns at the toe, heel, top or root indicate misalignment or wrong shimming (especially bevel gears).

Belts, pulleys, chains and sprockets

• Belts (flat, V-belts, toothed/timing belts) transmit power between pulleys; speed ratio = driven pulley diameter ÷ driver pulley diameter (inverse of speeds). Slip is possible except with toothed belts.
• Chains and sprockets give positive drive without slip: used in some control systems (around sprockets at control columns, trim systems). Ratio = driven teeth ÷ driver teeth.
• Check belt and chain tension and wear (Module 7.12).

Worked example - pulleys

A 100 mm pulley at 1 500 rpm drives a 250 mm pulley.

• Driven speed = 1 500 × 100 / 250 = 600 rpm