I believe that Alex Dahm’s article The Right Gear (May 2002, p40) misrepresents several key benefits of planetary gears. I worked for Brevini for 15 years, founded Brevini UK in 1986, and subsequently acted as consultant to SOM and Reggiana.

The multi-contact arrangement provides perfectly balanced tooth loads within the gearbox, transmitting no stresses to the input and output bearings, so that they have only external loading to contend with. Hence they are far less susceptible to shock loads.

To obtain a right-angle drive, it is necessary only to mount a bevel gear sub-assembly on to the planetary gearbox, usually on the input side. The resultant drive unit will probably be no longer than any other type of gearbox, should be at least as efficient and will certainly be considerably lighter! Most worm gears have to be selected on their thermal rating (the amount of heat that can be dissipated by the gear casing) thus the unit may be several sizes larger than would be necessary were torque the selection criterion. Efficiency losses convert directly into unwanted heat and wasted energy.

David Makin says that planetaries are less easy to service in the field. A correctly selected planetary unit needs no spares, nor are any recommended. A supplier of ‘conventional’ gearboxes will almost certainly propose a set of spare gears and bearings. In the event of a catastrophic failure, a smaller, lighter unit will be much easier to replace.

Maybe when the real truth gets out engineers will be less suspicious of the planetary solution and this simple and efficient gear system will take its rightful place alongside ‘conventional’ gearboxes.