Broken coil spring on vinyl tilt-in window sash. Unknown manufacturer - generic.

Hey Sam. This looks like a 15-180 Pivot Shoe, which uses Series 190 Coil Balances. These parts are modular, meaning you only need to replace what's damaged. For each coil spring you replace, you have to choose a spring weight, which indicates the strength of the spring. Look for a stamp on the spring, near the tongue, which indicates this. If there is none or you are unsure, give me the exact weight of the window sash and I can work it out for you. For details on how to replace this type of coil balance system, see the video below. 

Sure thing! If your old spring doesn't have a stamp then it probably wore away from time and friction. It happens, but it does mean you have a little more homework to do before you can place an order. 

What you want to do next is weigh your sash on a scale and then divide that number by how many individual springs support it; in your case, 4. So, for example, if you have a 20 pound sash then you would want to get four 5 pound springs, two for each side.

Looking at the stamp options for the Series 190, you'll note that they all have three digits. You can ignore the last two digits (which are all "40") and just focus on the first. That's the weight. In the above example, the 540 stamp represents 5 pounds of support. 640 represents 6 pounds of support, 740 represents 7 pounds, and so on.

Hopefully that makes sense. I'm happy to answer any follow up questions you may have.