Because the load is divided between several gear meshes working in parallel instead of one working alone.

In a conventional parallel-shaft reducer, all the torque passes through a single pair of meshing teeth at any instant. The size of the gearbox is set by what those teeth can carry.

In a planetary, the sun gear meshes with three or more planets simultaneously, and each planet meshes with the ring gear. The torque is split between those paths, so each tooth carries only a fraction of the total, and the gears can be much smaller for the same output torque.

The geometry also helps. The planets are arranged symmetrically around the sun, so the radial forces they exert largely cancel rather than loading a bearing in one direction. That allows smaller bearings and a lighter housing.

The practical result is a reducer that is typically far smaller and lighter than an equivalent parallel-shaft unit - the reason it dominates weight- and space-constrained applications.

Backlash is the small rotational free play between input and output - the amount the output can move before the input starts to drive it. It matters wherever position, reversal or control stability is involved.

In a conveyor drive that always turns one way, backlash is irrelevant. In a machine tool axis, a robot joint or a servo positioning system it is critical: every reversal loses motion equal to the backlash, so positioning accuracy suffers directly, and the free play can cause instability in a closed-loop control system.

Planetary units can be built to very low backlash because the load-sharing geometry allows tight tolerances without the tooth loading that would otherwise cause. Low-backlash and zero-backlash planetary reducers are made specifically for servo duty.

Backlash is specified in arc minutes, and lower costs more. Specify what the application genuinely needs rather than the lowest available - precision is expensive and unnecessary precision is wasted.

By stacking planetary stages, with each stage adding its reduction to the previous one.

A single planetary stage covers a limited ratio range, set by the geometry of how small the sun can be relative to the ring. Beyond that, a second stage is added - the carrier of the first stage drives the sun of the second - and a third for higher ratios still.

Because the arrangement is coaxial, stacking stages adds length but no width, so a three-stage planetary remains a compact cylinder. That is a real advantage over stacking parallel-shaft stages.

Each stage adds a little loss, but planetary efficiency per stage is high - comparable with helical gearing - so even multi-stage units remain efficient. That distinguishes them sharply from worm drives, where high ratio and poor efficiency go together.

More stages also mean more backlash, since each stage contributes its own, which is why very high ratio combined with very low backlash is the expensive corner of the range.

It depends heavily on the output bearing arrangement, and it is a specification point that is easy to overlook.

A planetary reducer's output shaft is supported by bearings in the housing, and their size and arrangement determine the radial and axial load the shaft can take externally - from a pulley, sprocket, pinion or the weight of a driven component.

Standard units are designed primarily to transmit torque, with modest external load capacity. Where the output drives through a pinion or a chain, the radial load can be substantial and continuous, and a standard unit may be adequate on torque while being overloaded at the bearing.

Manufacturers publish permissible radial and axial loads, usually at a defined distance from the output face - and the permissible load falls as that distance grows.

State the external loads when specifying. Where they are high, reinforced-bearing or output-flange versions are available, and they are much cheaper than a failed bearing in service.

Wherever high torque must come out of a small, light, coaxial package - or wherever positioning accuracy matters.

The space and weight cases are mobile and vehicle drives, wheel and track drives, aerospace and defence actuation, wind turbine pitch and yaw systems, and winches and hoists where the reducer must fit inside a drum.

The precision cases are servo-driven machine axes, robot joints, indexing tables and any closed-loop positioning system, where low backlash and high torsional stiffness matter as much as ratio.

They are less obviously the answer for ordinary industrial fixed-plant duty - a conveyor or a pump in a plant room with space around it - where a helical or parallel-shaft unit does the same job for less money. Planetary earns its price where the constraint is genuinely space, weight or accuracy.

High per stage, comparable with good helical gearing, because the teeth roll rather than slide.

Each mesh in a planetary train is a conventional involute gear contact with mostly rolling motion, so losses are small. A single stage typically operates in the high nineties per cent, and multi-stage units remain efficient because each stage adds only a small loss.

That is a significant advantage over the other route to high ratio in a compact package, which is a worm drive: worms achieve their ratio by sliding and lose a large fraction of the input as heat, whereas a planetary achieves it with rolling contact and stays cool.

The practical implication is that a planetary can be used for continuous high-power duty in a small package without a thermal problem, where a worm of the same ratio would need to be oversized for heat dissipation alone.

As with any gearbox, efficiency falls at part load, so check the figure at the actual operating torque.

Less than most, because many are sealed for life - but the ones that are not need the correct oil and level like any other gearbox.

Smaller planetary units, particularly servo reducers, are commonly grease-filled and sealed at the factory with no service requirement other than monitoring for leakage and noise. That suits their applications, where the unit may be buried inside a machine.

Larger industrial and mobile planetary units are oil-filled and need scheduled changes, with the level dependent on mounting orientation - a unit mounted vertically requires a different fill from the same unit mounted horizontally.

Watch for backlash increasing over time on precision units. A measurable rise in backlash indicates wear in the gear teeth or the carrier bearings, and on a positioning system it shows up as deteriorating accuracy before it shows up as noise.

Unusual noise from a planetary often means one planet is carrying more than its share, which is worth investigating promptly.