Selection Criteria

Information Courtesy: Mr. G. Downing - SAIMH

Selection Criteria
Before any gearbox selection can be made, there are certain specific pieces of information that need to be known.

1. Prime Mover (e.g. Electric Motor) power and speed.
2. Speed of driven equipment (e.g. conveyor head shaft)
3. Absorbed power of machine either in kW or Nm torque.
4. Ambient Temperature.
5. Operating Conditions (e.g. dusty, wet etc.)
6. Operating hours per day or required gearbox life (hrs.)
7. Driven machine characteristics (e.g. steady, medium or heavy shock loads)
8. Number of stop-starts per day.
9. Overhung or Radial loads (if any)
10. Any space restrictions.

General :

Armed with the above information, certain design factors ( which vary from manufacturer to manufacturer)
can then be applied, and a suitable selection can be made.

We have mentioned power in kW or torque in Nm
What are the connections ?

TORQUE = (9550 x kW) / n

where n= speed in r.p.m.

So the torque at the shaft of a 150kW electric motor running at 1440 r.p.m. is equal to

TORQUE = (9550 x 150) / 1440 = 994.8 Nm

We have also mentioned gearbox efficiencies and for normal Industrial gears the accepted efficiency is 98 % per set of gears.
So the overall efficiency of a triple reduction gearbox is 0.98 x 0.98 x 0.98 or 94%

Thus, with 150 kW input into a gearbox, the output power would be 150 X 0.94% or 141 kW.

However, with torque, the input torque is increased at the output shaft by the ratio and efficiency factors
i.e. a 150kW motor has a torque of 994.8 nM

With a 125:1 ratio and a triple reduction gearbox, the output torque , is calculated as

994.8 X 125 X 0.94 or 116889 Nm