Coil balance

This is kind of a weird setup. I wonder if the window company that made your windows were changing their balance systems. You have a pivot shoe like our 15-046, which usually goes goes with spiral balances. However, you very clearly have coil springs attached to it. Definitely a weird setup, but if it works, it works.

I would recommend looking at our 15-143 bushing, 15-144 bushing cap, 15-145 support barrel, and our Series 150 coil spring. You will need the number off the coil spring near the end that is hook to the shoe to determine the proper Series 150 option.

Yes, 640 would represent 6 pounds of support per spring. So if you use one coil on each side, the sash would likely weigh approximately 12 pounds. If you use two springs on each side, your sash weight would be 24 pounds, instead, and so on.