Once the required demand torque in a particular application has been determined, the tendency is to determine motor power using the standard equation: kW = Nm x r/min / 9550. However, gearbox efficiency must also be considered in calculations, and is often overlooked.
Gearbox Efficiency
Gear type, bearing type, seal type and lubrication all contribute to the inefficiency of gearboxes. As gearboxes lose efficiency, so the input power required also increases to compensate, to deliver the required torque at the output.
Gear Type
The type of gear primarily determines how efficient a particular gearbox is going to be. Inefficiency per stage can amount to:
Spur gears – 0.5% to 3%
Helical gears – 0.5% to 3%
Bevel gears – 0.5% to 3%
Hypoid gears – 2% to 10%
Planet gears – 2% to 10%
Crossed helical gears – 5% to 50%
Cylindrical worm gears – 10% to 50%
Spiroid gears – 3% to 50%
Worm gearboxes are renowned for having the possibility of a high ratio in a small form factor, but also having a relatively low efficiency. As a result, consideration must always be given to application demand torque.
With multiple stage units, inefficiency per stage is compounded.
Bearings and Seals
With gearbox type being the key determining factor in finding efficiency, bearings and seals have a slightly smaller contribution and usually are not able to be changed for each gearbox type.
In bigger design applications, losses for bearings and seals are obviously less of a concern when compared with smaller applications.
Lubrication
Lubrication type (grease or oil), viscosity, and amount also contributes to gearbox inefficiency, although, like bearings and seals, are only a secondary consideration to gearbox type.
Design Considerations
The main thing to think about when looking at gearbox application design is the overall power loss from design inefficiency, and to compensate accordingly. Planetary, inline helical and bevel helical gearboxes are the most efficient, whereas worm gearboxes are usually the least efficient.
