Apptech wrote:

> Useful rule of thumb.

For some values of "useful" :)

>     Torque x RPM = power
> 
>             Torque in kg.m
>             Power in Watts.

FWIW, I don't know any measurement system in current use where torque is
measured in kg m. There has been one in use where torque was measured in kp
m (which would have a numeric value close to what kg m could mean if it
were torque :), but I'm not sure there are many on this list who have heard
of it. In any case, there's probably no place where you find torque
specified or graphed in kg m.

Currently, at least in the realm where one talks about kg and m, torque is
measured in N m. And in that system, the rule is not really useful.  

> Based on
> 
>     Power (Watt)  = 2 x Pi x Torque (kg x g x m) x RPM/60 x
> 
> The 2Pi.g and the 60 ALMOST cancel.

While this is correct, the thing is that after you cancel out the g (which
is part of the torque), the "torque" that remains is no torque anymore. Not
in units, not in numbers, not in meaning.

So given that one has to do some scaling anyway, it's possibly more useful
to give the (approximate) scaling factor:

  P [W] = speed [rpm] * torque [Nm] / 10

or

  P [hp] = speed [rpm] * torque [Nm] / 7000
  P [hp] = speed [rpm] * torque [lbf ft] / 5200

Gerhard

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