Final Drive Centre

Planetary

Planetary / Epicyclic.

These gearboxes are by far the most common type used on modern excavators.

The units work on a very simple principal.

	A high speed and (relatively) low-torque input shaft drives a centre gear, this is often called a sun gear or input gear.
	The sun gear drives a set of gears that are mounted on a plate that that is fixed to the static part of the gearbox. These intermediate gears are generally known as planetary gears. There are commonly 3 or 4 planetary gears although both 2 and 5 gear configurations are not unknown.
	The planetary gears mesh with gear teeth on the inside surface of the gearbox casing or hub. This is sometimes called an annulus gear,
	If the sun gear is removed the gearbox hub is free to rotate. With the sun gear inserted the sun gear and hub gear are linked to each other through the intermediate, planetary gears. The centre sun gear then indirectly drives the hub.
	Gear stages can be 'ganged' together to achieve even lower gear ratios. If the centre plate of the first reduction is left free to rotate it can dive the sun gear of a second set of planet gears.
	This configuration offers a wide range of gear reduction ratios; 25:1 - 100:1 are common. If 2 or 3 stages of reduction are used then gear reduction ratios of well over 100:1 are readily achievable.

These types of gearboxes are fairly straightforward to work on. Obviously configurations vary but watch out for:

	There cannot be any movement in any bearings.
	Chipped teeth may look OK but remember that a chipped planet gear tooth meshes with both the replaceable sun gear and the often irreplaceable hub or annulus gear. Save a little here and you could spend a lot in a few days time!
	Depending on the particular arrangement the sun / planet / hub gears may need timing! There is no universal guide but we would be pleased to undertake the work for you.

	One of the principle advantages of this type of gearbox is its compact size and high reductions ratios.
	Many modern planetary gearboxes have an integrate hydraulic motor or the ability to semi-integrate a standard hydraulic motor.
	The main gearbox bearings usually take a significant part of the weight of the machine and are prone to impact damage. ·
	The metal to metal case seals are subject to wear. Oil loss often goes unnoticed and leads to rapid failure of the gearbox.

This type of gearbox can also be found in truck hub-reduction units, high power drilling machines and even Sturmey-Archer bicycle 3 speed units.

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