Counterbalance weights are used to equal out -- or counteract -- the amount of force pulling down on one side of a lever in order to keep the weight from applying too much torque, the type of force that acts on rotating objects. Perhaps the most visible application of this principle is in modern construction cranes, in which the fulcrum lies at the point where the horizontal and vertical sections of the crane meet, the long arm holds the weight and the short arm bears a counterweight. This counterweight must be enough to balance the weight or the crane can become unstable.
Determine the amount of weight being applied to one end of the lever (or, simply put, the weight that needs to be balanced).
Example: a construction crane lifting a 2-ton steel girder. The weight is 4,000 pounds.
Measure the distance between the weight and the fulcrum, or pivot point, of the lever. This is how far the weight is from the balance point.
Example: the steel girder is being held 50 feet from the crane's support structure. The distance is 50 feet.
Multiply the amount of weight by the distance.
Example: 4,000-pound girder at 50 feet equals 200,000 pounds of applied torque. This is also the amount of torque that must be applied to the opposite side of the crane.
Measure the distance of the opposite side of the level. This is the space available to hold the counterbalance.
Example: the crane has a maximum distance of 20 feet with which to support a counterbalance weight. The distance of the counterweight is 20 feet.
Divide the total applied torque by the distance of the counterbalance weight.
Example: 200,000 pounds of torque divided by 20 feet (200,000 / 20) = 10,000 pounds of counterbalance weight.
Therefore in order to balance the weight of the 2-ton steel girder 50 feet from the crane's fulcrum the opposite side of the crane must have a 10,000 pound, or 5 ton, weight placed 20 feet from the fulcrum.