coil balance

Hey Zhilin. I believe this is a 15-087 Pivot Shoe, which uses Series 180 Coil Balances. Let me know if you have any questions about any of this hardware.

There should be a stamp printed on the original spring that you can use as a reference to determine the proper Series 180 option. 

If not, then the only other method is to weigh the sash on a scale and divide that by how many springs support it. In your case, that'd be 2, so you would have to divide the weight in half and pick accordingly.