How to find the moment of an arm

[RogerR]

Originally Posted by Matthew_H

Our team is working on a 5 foot arm made out of PVC pipe in a scissor lift configuration. The motor needs to be mounted at the beginning of the 5 feet. We are trying to find out how much weight we need to place behind the arm to counter balance the 5 feet of PVC. The back end needs to be as short as possible. How do we find how much weight we need to mount to the back of the arm?

assuming uniform weight distribution (of the PVC arm), the moment will be: (weight of arm)*1/2(length of arm)

once you have that, divide it by the distance from the fulcrum to (roughly) the center of gravity of the counter weight. that will give you the amount of weight you need. this doesn't factor in the material supporting the counterweight. if you're concerned about it being slightly off balance because this, you can use the above equation to find the moment of the material supporting the counter balance, subtract it from the moment of the 5 ft PVC arm, and then divide by the distance from the fulcrum to the center of gravity of the counter weight.

final equation:

1/2(distance from fulcrum to end of arm)*(weight of arm)=1/2(distance from fulcrum to end of arm supporting counterwieght)*(weight of arm supporting counterweight)+(distance from fulcrum to CG of counterweight)*(weight of counterweight)

welcome to the wonderful world of statics.