Because helical teeth roll against each other with only limited sliding, whereas a worm gear works almost entirely by sliding contact between the worm and the wheel. Sliding generates friction, which is lost as heat, and the loss grows as the ratio increases. A helical stage typically runs in the mid to high nineties per cent; a worm stage can fall a long way below that at high ratios.

The output shaft is on the same centreline as the motor shaft, so the unit is a straight assembly with the gearbox extending beyond the motor. It gives the most compact cross-section and a simple load path. The alternative is a parallel-shaft unit, where the output is offset to one side, which shortens the assembly and lets it sit alongside the driven equipment.

As many as the ratio requires, and manufacturers combine two, three or more helical stages to reach high reductions. Each stage adds a small efficiency loss and some length, so for a given ratio fewer stages is better where the option exists. Very high ratios sometimes make a helical-worm or planetary unit more compact, which is worth comparing rather than simply adding stages.

Foot mounting onto a base, flange mounting to a machine face, and face mounting for closer coupling, with solid output shafts for couplings and sprockets or hollow output shafts that slide onto the driven shaft. Hollow shaft units with a torque arm avoid a bedplate and coupling alignment entirely, and are the usual arrangement on conveyors and similar drives.

Conveyors, mixers and agitators, pumps, extruders, packaging and processing machinery, and general industrial drives. The common factor is a continuous or repetitive duty at moderate torque where the output can be arranged in line with the motor. Where continuous running is involved, the efficiency of helical gearing is a direct and recurring saving against a worm alternative.

Quieter than equivalent spur gearing, because the gradual engagement of helical teeth avoids the abrupt tooth contact that makes spur gears whine. Noise rises with speed and load and with wear, so a change in noise from an installed unit is a useful maintenance indicator. Where noise is critical, check the manufacturer's stated sound pressure figures at the actual operating point rather than at nominal.

Yes. The angle of the teeth that gives helical gearing its smoothness also produces a force along the shaft, which the bearings must carry, and it is one reason gearmotor bearings are specified as they are. It is handled internally by the design and is not something the user needs to accommodate, but it does mean that substituting gearing types inside a housing is not a simple exchange.