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Definition of Torque vs Power

Definition of Torque vs Power

Definition of Torque vs Power

I am struggling with the meaning of Torque.  I know the equation of HP=Torque*rpm/constant.  It is when I apply that to a gear box or a motor, I get confused.
If a gear box is rated for a certain torque, what happens when I speed up the output shaft by changing the gear ratio.  Is the torque rating of the gear box changed?  Or is there just a new calculation using the above equation and since I have sped the box up, I can now put a larger motor on the box.
For my particular case, I have a adjustable torque limiting coupling between the motor and the gear box.  I have sped up the output shaft from 300 to 450 rpm and I am changing the motor from 150 HP to 250 HP.  Do I need to increase the torque setting on the coupling?  By what percent?

Bob Sander

RE: Definition of Torque vs Power

Hi Bob,

The equation you need is P=Tw where:

           P = Power (kW)
           T = Torque (Nm)
           w = Speed (rad/sec)

You know the output speed and the power generated, and therefore you can find the torque required.

I hope this is what you are looking for?



RE: Definition of Torque vs Power

Power is the rate of doing work, ie. the speed the motor can turn or move the load.  Torque is the is the turning moment that causes the motor to accelerate or overcome an opposing force.  You can increase the power of a motor, ie the speed it can turn under load, with out increasing the torque.  If you accelerate the motor at the same rate the torque coupling need not be adjusted.  If you want the accelerate the motor faster, then you will need a higher torque and thus need to adjust the coupling to transfere that torgue.  This a theory with out formula, if you need equations let me know.

Nigel Waterhouse

A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

RE: Definition of Torque vs Power

As Nigel stated, Power is the rate of doing work, and work is Force X Distance (in-lbf).  All motors or engines have a Torque vs. Speed Curve.  This tells you what happens to the torque when you vary the speed or vice versa.  In a gearbox, you have different gears or "Gear Ratios".  depending on the arrangment, it could increase the speed or increase the torque.  The input power HP into the "GearBox" is constant so if the conservation of energy principle is to be beleived, then the speed must decrease in order for the torque to increase, because the Input Power is constant.
The torque rating on the gear box is to avoid breaking the gear box.  This has some safety factor in it but if you are increasing the speed and the Power, I would think twice.

Edson Campos

RE: Definition of Torque vs Power

Hi Bob,

The Gear box is always rated for constant torque. When you speed up the output shaft with the same motor then you are decreasing the torque so in order to speed up and obtain the same torque you need to use a higher hp rated motor. This is what you propose doing when you say you are speeding up the output shaft from 300 to 450 rpm and changing the motor from 150hp to 250hp. However, check your calculations using the same equation mentioned by you and you will find that to maintain the same torque at 450 rpm you need an input of 225hp and not 250hp. As far as the torque setting on the coupling is concerned, you need not reset the coupling since the coupling is toque rated and will disconnect whenever the torque increases.
I will be pleased to get a feed back from you.

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