The balance or equilibrium of a crane can be described as: F1 x d1 = F2 x d2
The load applied at the end of the stick
The distance from the load to the pivot point
The load of the counterweight
The distance from the counterweight to the pivot point
To be balanced at any given position, the counterweight has to continuously compensate for the load. As the load increases, the counterweight must also increase in weight. Alternatively, instead of increasing the counterweight, it could be moved further rearward from the center of rotation to compensate for the increase in the load.
Since adding counterweight is often neither practical nor, in some cases, recommended, true balance of a fixed counterweight crane can only be achieved at one specific radius. Any variation caused by changing the radius at which the load is lifted, destroys the balance.