Because it lets the worm run at a lower ratio, where a worm is much more efficient.

A worm's efficiency depends strongly on its lead angle, and the lead angle falls as the ratio rises. A single-stage worm producing a very high ratio has a shallow lead angle, enormous sliding per unit of output, and correspondingly poor efficiency.

Splitting the job changes that. If a helical stage provides part of the reduction, the worm only has to provide the remainder, so it operates at a lower ratio with a steeper lead angle - where its efficiency is considerably better. The helical stage itself loses very little.

The net effect is a unit with the same overall ratio and right-angle output as a plain worm, but wasting significantly less power and running cooler. The trade is a second gear stage to buy and a slightly longer unit.

It sits between them on efficiency, cost and running temperature, which is exactly why it exists.

Against a plain worm: better efficiency, cooler running, higher continuous rating for the same size, and a higher price. Where a worm unit is being oversized for thermal reasons, a helical-worm is often the cheaper answer overall.

Against a helical-bevel: lower efficiency and more heat, but a lower purchase price and generally higher available ratios in a compact package. Helical-bevel remains the choice for genuine continuous heavy duty where running cost dominates.

The practical decision rule: light intermittent duty takes a plain worm; heavy continuous duty takes helical-bevel; the substantial middle ground - moderate hours, high ratio, right-angle output, cost-sensitive - is where helical-worm is the right answer.

Often, because the worm stage is still a worm - but the same warning applies with even more force.

Self-locking comes from the worm stage's friction, and that stage is still present. However, because the worm in a helical-worm unit typically runs at a lower ratio than it would in a single-stage design, its lead angle is steeper - and a steeper lead angle is less likely to self-lock.

So a helical-worm unit may or may not hold, and whether it does depends on the specific ratio split, which is a design detail rather than something visible from outside.

If holding matters, ask the manufacturer for a written answer for the specific unit and ratio, and even then do not rely on it for safety. As with any worm drive, friction-based holding degrades with heat, wear and vibration. Where a load must be held - particularly a suspended one - fit a mechanical brake.

Better than a single-stage worm of the same overall ratio, and below a comparable helical-bevel - with the exact figure depending on how the ratio is split.

The helical stage contributes very little loss. Almost all of the loss is in the worm stage, and that depends on the worm's own ratio: the more of the total reduction the helical stage takes, the more efficient the unit overall.

Manufacturers publish efficiency for each specific ratio rather than a single figure for the range, and the variation across the range is large enough to matter. It is worth comparing at the actual ratio you need rather than accepting a headline number.

Also ask for the figure at the intended load. Gearbox efficiency falls at part load, and a unit running at a fraction of its rated torque - which many oversized installations do - performs worse than the catalogue figure suggests.

Broadly yes - the worm stage dictates the oil, and it is the demanding element.

Worm gearing needs a lubricant that performs under sliding contact at high pressure, and synthetic polyglycol oils are widely specified because they reduce friction measurably, which lowers temperature and raises efficiency. Since the helical-worm unit shares one oil bath between both stages, the worm's requirement governs.

Polyglycols are not miscible with mineral oils and are not compatible with every seal or paint material, so changing oil type is not a simple drain and refill - the system must be flushed and compatibility confirmed.

Follow the manufacturer's specification rather than substituting a general industrial gear oil. The efficiency gain that justifies buying a helical-worm over a plain worm can be partly given back by using the wrong lubricant, which is a frustrating way to lose it.

When it is running too hot, when it is being oversized for thermal reasons, or when the duty cycle has increased beyond what it was selected for.

The clearest signal is temperature. A worm gearbox running near or above its oil temperature limit is degrading its lubricant and its bronze wheel continuously, and the usual response - fitting a larger worm unit - buys thermal headroom at the cost of size, weight and price.

A helical-worm of the same ratio wastes less power in the first place, so it addresses the cause rather than the symptom, and is frequently smaller than the oversized worm it replaces.

The other trigger is a change in use: a drive originally specified for occasional operation that has become a continuous one as the process changed. Recalculate the thermal rating for the actual duty rather than the original one - it is a common and expensive oversight.

The usual range - foot, flange and face mounting, with solid or hollow output shafts - and the mounting position must be specified because it sets the lubrication arrangement.

Hollow output shafts with keyway, taper bush or shrink disc are widely used, letting the unit mount directly on the driven shaft with a torque arm rather than a base and coupling. That suits the conveyor and agitator drives these units commonly serve.

On the input side, units are supplied with a solid input shaft for a coupled motor or with an IEC or NEMA motor flange so the motor bolts directly on.

As with any gearbox, the mounting position determines the oil level, filling point and breather location, and a unit installed in a position other than the one it was supplied for may not lubricate correctly. Confirm the orientation at the order stage, and tell the supplier if the machine layout changes afterwards.