Epicyclic gears and gear trains

Epicyclics made up of spur gears 

 A simple epicyclic train

 A is the annular wheel which has internal teeth P is a plant wheel L is an arm, star or spider which carries pins on which the planet wheels can rotate freely. S, the sun wheel, rotates about the same axis as A The input and output shafts are then connected to any two of the sun, arm or annulus.

Epicyclics made up of bevel gears

 Epicyclic bevel gear train

In bevel gearing, the axes of the shafts intersect. Q represents the fixed wheel which meshes with R, the pinion. T is the wheel found on the driven shaft and meshes with the pinion shown at S. R and S pinions are keyed to a shaft revolving in bearings on the arm P. P, the arm, is then keyed to the driving shaft.

 Application of epicyclic gears

 Epicyclic gear trains are used when there is the need for large reductions in speed and limited space availability. Some examples of where these can be found are:

 • Gearboxes

 • Steam driven generators or gas turbines

 • Compressor drives
 • Mine hoists

 • Water turbine drives and marine drives

Principle of a typical epicyclic system

Epicyclic gears, also known as planetary gears, utilize gears which rotate about their own axis and at the same time rotate bodily around a main axis. The planetary gears are mounted on a carrier and mesh with a gear in the centre, called the sun gear, as well as with an internally toothed large ring gear. The planetary gears therefore rotate around the sun gear. If the sun gear were to be held stationary, and the ring gear rotated, the planetary gears move around the sun gear and therefore take the carrier with them. The carrier rotates in the same direction as the ring gear, however, at a reduced speed, which depends on the gear ratio. On the other hand, if the carrier is held stationary, the plant gears will drive the sun gear in the reverse direction to the ring gear and results in having a normal gear train.

 Simple two-speed epicyclic gearbox

The sun gear is driven by an engine and the ring and two planet gears or pinions are mounted on a carrier. The carrier is connected to a shaft which drives the road wheels. The planet gears mesh with the sun gear and the ring gear. When required, a brake band enables the ring gear to be locked stationary. Assume the ring gear is free to revolve. When the sun gear is driven, the planet gears will freely spin and therefore slowly rotate the ring gear. This means the carrier will not move and therefore the gearbox is in neutral. If the brake band locks the ring gear, this results in the planet gears being forced to rotate inside it. The planet carrier is carried around in the same direction as the sun gear at a reduced speed which is dependent on the ratio of the number of teeth in the sun and ring gears. If the ring and sun gears are locked with use of a clutch system and the brake band is released, the planet gears cannot spin and the carrier therefore rotates at the same speed as the sun and ring gears. This results in a direct drive from the engine to the road wheels. In the following epicyclic gear train, the final drive is connected to the ring gear and the carrier is locked to provide drive through the planet gears. Although a different method is used to rotate the planet gears inside the ring gear, the principles involved are the same. Two sun gears of different diameters are used; one is the forward sun gear and the second the reverse sun gear. Mounted to the single carrier are two sets of planet gears; one set is known as the planet pinions short which meshes with both the sun gear and the other set known as the planet pinions long.

The cyclometer mechanism

 This mechanism is used to measure the distance travelled by a bicycle. C and D are the internal wheels and are coaxial. C is fixed. A-B is the compound wheel and is free to revolve on a pin at E. The wheel A meshes with C whereas wheel B meshes with D. The star wheel, S, is operated by a striker fixed to the bicycle wheel and is carried on the driving shaft. For each revolution of the bicycle wheel, the star S makes one fifth of a revolution. The wheel D completes one revolution for every 0, 6 km travelled by the bicycle wheel.

Humpage’s gear

Bevel wheel epicyclic The driving and driven shafts A and B are coaxial, each carrying a bevel wheel C and D. The wheel C meshes with E and F with the fixed wheel G. Wheel D meshes with which is compound with E. The compound wheel E-F freely revolves on the arm attached and revolves about the same axis as the shafts A and B. In the actual gear there are either 2 or 3 arms on H each carrying a compound wheel identical with E and F. This gear is used sometimes on lathes. The mechanism is compact and can be accommodated inside the cone pulley.