Coil Springs

Thanks for writing in, and thanks for providing so much detail up front! Your pivot shoe looks like an older version of our 15-180. This pivot shoe is compatible with our Series 190 Coil Balances. For a window sash that weighs 18.9 lbs, you'd be fine going with a S190-440 4 lb Coil Spring and S190-540 5 lb Coil Spring on each side of the sash. This would equal 9 lbs of support on each side of the sash, and 18 lbs of total support. This is close enough to 18.9 lbs that it should work just fine. 

The reason why some of your other windows only had one coil spring on each side of the sash is probably because those springs were calibrated to support a higher amount of weight--maybe 9 lbs apiece--and so only one per side was necessary. We don't carry 9 lb springs for the Series 190; but the S190-440 and S190-540 tandem should get the job done all the same. Let us know if you have any other questions.

Per each side of the sash, you'd install (1) 15-180 Pivot Shoe, (1) S190-540 and (1) S190-440 Coil Balances, and (1) 15-181 Double Support Detent Clip to support the Coil Balances. Essentially you'd have to ease the Pivot Shoe and Coil Balances into the cutout of the jamb track.

Thanks! Always happy to help, and many thanks for your business; we appreciate it. If you have any other questions, do not hesitate to ask.