Because helical teeth mostly roll against each other, while a worm mostly slides.

In a helical mesh, contact starts at one end of a tooth and progresses along it as the gears turn. The relative motion at the contact point is largely rolling, and rolling friction is low. Losses per stage are small, so a single helical stage typically operates in the high nineties per cent.

A worm drive works by a screw thread sliding across a wheel tooth. That sliding is inherent to how it produces its ratio, and sliding friction is far higher than rolling friction. The energy lost becomes heat in the gearbox.

The practical consequence is running cost. On continuous duty the difference in electricity consumed over a few years can exceed the purchase price of the reducer, which is why helical units dominate process plant, conveyors and anything running long hours. On short-duty or intermittent applications the efficiency matters far less and other properties decide.

It means the output shaft is coaxial with the input - the drive goes straight through rather than being offset or turned.

That suits any layout where the motor can sit directly behind the driven shaft on the same centreline: pump drives, extruders, mixers and agitators, and many conveyor head shafts. The assembly is simple, there is no offset to design around, and the reducer is symmetrical about its axis so mounting is straightforward.

The cost is length. Stacking stages coaxially makes the unit long, and the whole drive train - motor, coupling, reducer, driven machine - occupies one straight run. In a machine where space is measured along that axis, that is exactly the wrong shape.

Where the driven shaft is offset from where the motor can go, a parallel-shaft unit is the direct answer. Where the drive has to turn a corner, a helical-bevel unit does it without the efficiency penalty of a worm.

From the required output speed and torque, then corrected by a service factor for the duty - and the service factor is the part most often skipped.

Start with the driven machine: the speed it must run at and the torque it demands. The ratio follows from the motor speed divided by the required output speed. The reducer's nominal torque rating must then exceed the required torque multiplied by a service factor.

That factor accounts for how the load actually behaves - hours run per day, the number of starts, whether the load is smooth or shock-loaded, and whether it reverses. A reducer sized on nominal torque alone will be adequate for a smooth eight-hour duty and will fail early on a shock-loaded, frequently-started, twenty-four-hour one.

Check the thermal rating separately. On continuous high-power duty a reducer can be within its mechanical torque rating and still exceed the power it can dissipate as heat, which needs a larger unit or forced cooling.

As many as the ratio requires, with each stage limited in how much reduction it can sensibly provide.

A single helical stage covers a modest ratio range - beyond that the gear diameters become impractical. Two stages cover the bulk of ordinary industrial requirements, and three stages reach the high ratios.

Each added stage costs a little efficiency, adds length and adds cost, so the lowest number of stages that achieves the ratio is generally preferred.

Where the ratio required is very high, a helical unit becomes a long three- or four-stage assembly, and other geometries become more attractive: a worm achieves a high ratio in one stage, and a planetary achieves it in a compact coaxial package. Very high ratios are also where a helical-worm combination earns its place, using a helical stage to do part of the work efficiently before the worm stage finishes it.

Foot-mounted, flange-mounted, and face-mounted, chosen by how the reducer attaches to the machine and what carries the reaction.

Foot mounting bolts the reducer to a base or bedplate through feet cast into the housing. It is the traditional arrangement, needs a rigid flat mounting surface, and requires alignment between the reducer output and the driven shaft through a coupling.

Flange mounting bolts the reducer directly to the driven machine through a flange on its output end, which locates the output shaft concentrically with the machine and removes the alignment task altogether. It suits pumps, mixers and any machine designed to accept a gearbox flange.

The input end has its own arrangement: a solid input shaft for a coupled motor, or a motor flange so the motor bolts straight on.

Specify the mounting position too - the oil level and breather position depend on which way up the unit runs, and a reducer supplied for horizontal mounting will not lubricate correctly if installed vertically.

The correct oil grade for the load and temperature, at the right level for the mounting position, changed on schedule - and a breather that works.

Most industrial reducers are splash lubricated: the gears dip into an oil bath and throw oil around the housing. That makes oil level critical. Too little and the upper bearings are starved; too much and the unit churns oil, which raises temperature and wastes power. The correct level depends on the mounting position, so a unit reconfigured from horizontal to vertical needs its level reconsidered.

Synthetic gear oils are widely used and extend change intervals considerably over mineral oils, particularly where the unit runs hot.

Check the breather. A blocked breather pressurises the housing as it warms and pushes oil out past the shaft seals, which is then diagnosed as a failed seal when the seal was doing its job.

Monitor running temperature and note any change; a reducer that suddenly runs hotter is reporting something.

A speed reducer is a gearbox on its own; a gearmotor is a motor and gearbox built as one unit. It is a real distinction and it determines which category you should be shopping in.

A reducer has an input shaft or a motor flange and is selected independently of the motor. You choose the gearbox for the ratio and torque, choose the motor for the power and supply, and connect them - which gives complete freedom to mix and match, to change the motor without changing the gearbox, and to source each from a preferred supplier.

A gearmotor arrives as a single assembly with the motor's rotor shaft feeding the gear train directly. It is more compact, has no coupling to align, and is usually cheaper as a package - but the two are matched by the manufacturer and are not separable.

On this marketplace, speed reducers sit here under Power Transmission and are cut by gear train type. Integrated gearmotors sit under Motors and are cut the same way.