HP and Torque

People use the words hp and torque quite a bit, however, they don’t always understand the relationship between the two units.  I’m going to attempt to clear that up a little bit, and maybe make designing a little easier in the process.

Torque (Q) is basically a force at a distance.  If you think of a ratchet that is one foot long, and you apply 2lbs of force at the end of the ratchet, you are applying 2ft*lbs of torque (1ft x 2lbs).

Horsepower (hp) is a unit of measure of the rate at which work is done.  Let’s take the example above.  We are going to assume that applying the 2lbs of force will allow the ratchet to turn the bolt.  We count every time we make a complete circle (revolution) over a specific amount of time.  If we use minutes, we have RPM (revolutions per minute).  To calculate hp from torque and rpm you can use the following equations:

hp = (Q * RPM)/5252      This equation is when Q is given as ft*lbs

hp = (Q * RPM)/63025   This equation is when Q is given as in*lbs

You can see that hp is always a function of torque and rpm.  In the case above, if we can turn the ratchet 300 time per minute, with 2lbs of force, we are creating a huge 0.114hp.  These equations really start to become useful when you need to calculate the amount of torque that can be produced at a driven sprocket after a gearbox.  This will allow you to determine if your motor is big enough, and your reduction ratios are correct.