Whenever the drive runs continuously or is heavily loaded. A bevel stage transmits torque largely by rolling contact and stays highly efficient, while a worm stage works by sliding and converts a substantial part of the input into heat, increasingly so at higher ratios. Over long running hours that difference dominates the cost of ownership and can also limit output thermally.

Helical-bevel units commonly operate in the mid nineties per cent overall, while a worm unit can fall well below that, with the gap widening as ratio increases. On a small intermittent drive that is an academic difference. On a drive of any size running most of the day, it appears directly in the electricity bill and in the amount of heat the gearcase has to shed.

Because lost efficiency becomes heat inside the gearcase, and that heat must be dissipated through the housing. In a warm ambient, an enclosed guard or a dusty environment where the casing cannot shed heat, the thermal rating rather than the mechanical rating becomes the limit. A more efficient unit produces less heat for the same output and is therefore less likely to be thermally constrained.

Foot, flange and face mountings, with solid output shafts for couplings, sprockets and pulleys, and hollow output shafts with keys or shrink discs for mounting directly onto the driven shaft with a torque arm. The right-angle geometry makes them particularly useful where a drive must be fitted beneath or beside equipment with limited clearance in one direction.

Conveyors where the drive must come in from the side, cranes and hoists, agitators and mixers on process tanks, aerators, cooling tower fans, bucket elevators and bulk handling plant. The pattern is a layout that requires a corner combined with a duty long enough or heavy enough that the losses of a worm unit would matter.

Yes, usually noticeably so, because bevel gears are more complex to manufacture and require accurate setting of the gear mesh. That first cost is what keeps helical-worm units in the market. The judgement is between capital cost and running cost, and it turns almost entirely on running hours: the more the drive runs, the more clearly the bevel unit wins.

Generally yes, because their high efficiency works in both directions, so a load can drive the output shaft and turn the motor. That is normally desirable, but it means these units provide no inherent braking. Where a load must be held, such as on a hoist or an inclined conveyor, a motor brake or a dedicated holding device is required and should not be assumed from the gearing.