Cool spring

Your shoe looks like a match for our 15-177, which is itself compatible with our Series 152 spring. Look these parts over and let me know if you agree.

I'm sorry for missing that detail of your post. The S152-9 will likely not be strong enough; assuming you have one spring on each side of the sash marked with 10, then your sash is presumably 20 pounds (10 pounds + 10 pounds). 18 pounds would fall a little short of that.

Instead, you can use two 5 pound springs on each side of the sash. That would add up to 20 pounds without issue. The catch is that you'd need an extra retainer for these springs; I recommend the 15-143 bushing, 15-144 cap, and 15-145 barrel. These kind of sandwich over the spring and keep it in place.