Originally Posted by
Mondeo Man
I followed your generalised link and it pointed to the page below
Read the first line
http://en.wikipedia.org/wiki/Driveshaft 
Hoisted by your own petard
I'd love to know how knowing something is rated to 100 horsepower is useful.......i could apply 100 horsepower to something by hitting it with my knob repeatedly for ten years. Or in 4 seconds by mashing it with a huge press press.
An engineer knows a rotating shaft transmits torque. A gearbox converts torque. The force applied to the shaft is lbft/Nm
That is all.
if you mashed it with a huge press you would not be transmitting either power or torque through it, you would be applying compressive forces to the item, completely different!
a gearbox and diff is indeed a torque covertor but a shaft TRANSMITTS POWER ACROSS IT by experianceing a TORQUE APPLIED TO IT!