# Math Help - surface rpm is 20, motor rpm is 30...what is bit rpm?

1. ## surface rpm is 20, motor rpm is 30...what is bit rpm?

have a simple math question.
If you have looking at a drilling rig. and you are trying to calculate downhole but rpm (revolutions per minute). and you have a motor right above the bit.
So your surface rpm say is 20 and your motor rpm say is 30....what would your bit rpm be?
is it just a straight addition? 20+30 = 50

for some reason I am thinking that if you have a surface rmp of 2 and a motor rmp of 30 you are spinning the motor twice...so when its still it rotats at 30 rpm but if you are spinning it twice wouldnt that be 62?

2. Hello luckyc1423
Originally Posted by luckyc1423
have a simple math question.
If you have looking at a drilling rig. and you are trying to calculate downhole but rpm (revolutions per minute). and you have a motor right above the bit.
So your surface rpm say is 20 and your motor rpm say is 30....what would your bit rpm be?
is it just a straight addition? 20+30 = 50

for some reason I am thinking that if you have a surface rmp of 2 and a motor rmp of 30 you are spinning the motor twice...so when its still it rotats at 30 rpm but if you are spinning it twice wouldnt that be 62?
You will add the two rpm's, not multiply.

It's easiest to see if you imagine the two motions to happen separately. Suppose that in one minute the bit makes $r$ revolutions, relative to the motor, and the motor makes $s$ revolutions relative to the earth. Then if we let the bit rotate first, while holding the motor still, the bit will make $r$ revolutions per minute relative to the earth.

Now hold the bit still relative to the motor, and allow the motor to rotate. The bit will rotate through a further $s$ revolutions in one minute relative to the earth.

So it will make $(r+s)$ revolutions relative to the earth altogether in one minute.