So you are saying the springs have no part in the resistance of the hammers to movement? Somehow I doubt that.
I don't think Ed said that precisely. He said "inertial resistance of the hammers" which I interpret as "moment of inertia
" which doesn't have anything to do with the springs.
Anthony-Yes, your terminology is correct.
The hammer return springs matter most for faster return of the hammers from the strings, since the hammers are almost vertical when they are touching the strings. This is also the point in the cycle when the springs are most compressed and are exerting the most force.
I think you'll find that when the springs are loosened from their loops (or the loops are broken), there is not much perceptible difference at the beginning of the keystroke, but there may be a slower repetition, especially in the low tenor.
If, on the other hand, you randomly add weight to some of the hammers, you will probably feel a difference and find it difficult to control dynamics.
Weighing off the hammers and evening the strike weight line is very fast and easy, well worth trying when replacing hammers on a good vertical like the U-3.
And if the spring cords are breaking, they need to be replaced.