Replacement coil balance and pivot shoe

Thanks for posting. This looks like our 15-111 Pivot Shoe. If the coil springs are 1/2" (or 9/16" as you report) wide, then the Series 160 should be what you need. Note however that the Series 160 Coil Balances have weights in X-1/2" increments. So the S160-5 supports 5.5 pounds, whereas the S160-6 6 supports 6.5 pounds. To be safe, I'd like to know the exact weight of your window sash that these coil springs support, just to ensure these spring weights are correct for you. 

I'm happy to help and thanks for confirming. Given that your sash weighs 18.5lbs, I would suggest using the S160-4. These coils individually can support a sash weight of 4.5lbs, so four of them (2) on each side of the window, would work fine to support your 18.5lb sash.