Efficiency, and therefore running cost and heat - both types turn the drive through ninety degrees, but they pay very differently for it.

A spiral bevel gear set achieves the turn with rolling contact, so a helical-bevel unit typically operates in the nineties per cent. A worm produces its ratio by sliding, and a high-ratio worm can lose a substantial fraction of the input power as heat.

On continuous duty that difference is money. The electricity a worm wastes over a few years of running can exceed the extra purchase price of the bevel unit several times over. The wasted energy also has to be got rid of, so worm units frequently have to be oversized for thermal reasons rather than torque reasons.

The worm keeps two advantages: it is cheaper to buy, and it can be made self-locking. Where the duty is intermittent, the budget is tight or the self-locking characteristic is wanted, a worm remains a reasonable choice.

A bevel gear with curved teeth set at a spiral angle, which engage gradually instead of all at once.

A straight bevel gear has teeth cut radially, and each tooth pair comes into contact along its whole length simultaneously. That produces an impact at every tooth engagement - noisy, and limiting on both speed and load.

Spiral bevel teeth are curved and oblique, so contact starts at one end and sweeps along the tooth. More than one tooth pair is in mesh at any moment, load is shared, and engagement is smooth. The result runs far quieter, carries more torque for the same size, and tolerates much higher speeds.

The cost is manufacturing precision. Spiral bevel sets must be cut and lapped as matched pairs and set up with correct tooth contact, which is why they are more expensive and why the bevel stage is usually kept small with helical stages providing most of the ratio.

The bevel stage usually makes the turn and contributes a modest part of the ratio, with helical stages providing the rest.

The reason is cost. A spiral bevel set is the expensive component, requiring matched cutting, lapping and careful setting. Making it large enough to carry the full reduction would be costly and would also make the whole unit larger, because bevel gearing is less compact than helical for a given torque.

So a typical unit uses one bevel stage plus one or two helical stages. Low ratios may be achieved with the bevel stage alone; high ratios stack helical stages behind or ahead of it.

For the buyer this is mostly invisible - you specify the overall ratio and the manufacturer arranges the stages. It matters when comparing units, because two reducers with the same overall ratio may have quite different efficiencies and lengths depending on how the stages are divided.

Wherever the driven shaft runs perpendicular to the space available for the motor - which is extremely common in conveying and process plant.

On a belt conveyor the head drum shaft runs across the belt, while the space for a motor is alongside the conveyor frame. A right-angle unit puts the motor parallel to the belt and turns the drive into the drum.

The same applies to mixers and agitators driven from the side, to screw conveyors, to travelling and slewing drives on cranes and machinery, and to any machine where the driven axis is fixed by the process and the motor position is fixed by access.

Right-angle units are also used to bring a drive down or up - a vertical output from a horizontal motor - where headroom or floor space dictates.

Where the driven shaft is parallel to the available motor space but offset, a parallel-shaft unit solves the same problem in a different plane and is usually cheaper.

Yes - right-angle units are very commonly supplied with hollow outputs, because the applications they suit often involve mounting directly on a driven shaft.

A hollow output shaft slides onto the driven machine's shaft, held by a keyway, a taper bush or a shrink disc, and the reducer is prevented from rotating by a torque arm. That removes the coupling, the alignment task and the separate base.

Shrink disc connections are worth specifying where torque is high or where the connection must be free of backlash and fretting. A keyed connection can work and fret under reversing or shock loads; a shrink disc grips by friction around the whole circumference and does not.

Remember the torque arm. It must be anchored to something rigid, with a resilient bush to absorb shock, and positioned so it works in tension. A poorly arranged torque arm produces vibration and cracked brackets regardless of how good the gearbox is.

Yes, more than most, because a right-angle unit has two shaft axes and the oil has to reach the bearings on both.

Splash lubrication depends on gears dipping into an oil bath and throwing oil where it is needed. A right-angle housing has bearings at several levels and in two planes, and which of them sits above the oil line depends entirely on how the unit is mounted.

Manufacturers therefore supply these units for a specified mounting position, with the oil level, filling point and breather located to suit. Installing a unit in a position other than the one it was supplied for can starve a bearing while the sight glass still shows oil.

If a machine layout changes after ordering, tell the supplier rather than simply rotating the gearbox. In many cases the same unit can be reconfigured with a different oil quantity, plug arrangement and breather position - but it has to be done deliberately.

Low for the type - spiral bevel and helical gearing together are among the quieter mechanical drives, and considerably quieter than straight bevel or spur gearing.

The reason is the same progressive tooth engagement that gives the efficiency: contact sweeps along each tooth instead of striking it, and several teeth share load at any moment, so the excitation that produces gear noise is much reduced.

In practice the reducer is often not the loudest part of the drive. Motor cooling fans, the driven machine and resonance in the mounting structure frequently dominate, and a gearbox bolted to a thin sheet-metal frame will radiate noise the frame amplifies.

Where noise is a genuine specification, ask for measured sound power data at the intended speed and load rather than a general figure, and pay as much attention to the mounting structure as to the gearbox. Isolating a rigid drive from a resonant frame often achieves more than specifying a quieter unit.