Calculating Spring Force for a Multi Plate Clutch

[HNCMATHS]

A multi plate clutch has 4 frictional surfaces, imternal diameter 120mm and external diameter 200mm. The coefficient of friction is 0.3

What spring force will be required to transmit 20KW of power at 1440 RPM if the clutch is under uniform pressure conditions?

 

[HNCMATHS]

Using constant wear therory R1 + R2 / 2

I have managed to reach an answer of 1381.597N or 1.381KN (Can anyone confirm please?)

Thanks,

 

[mender]

After converting the 20 KW at 1440 rpm to 132.6 Nm of torque and using that with this:
http://www.engineering.com/calculators/torque_transmitted.htm
I get 0.0916N/mm² or 1841N of force needed.