Oops. I can see that how I explained the question I gave you the wrong impression. All failures we experienced involved the original window balances. And all failure modes were the same: the balance separates at the point where the metal shaft connects to the pivot/brake at the base of the sash. The balance then fully contracts and hangs at the screw plug connect point. In about half the cases this happened when not operating the window.
The pictures I sent are of one of two failures where the channel first separated from the pivot/brake and then contracted with such force that it then also separated at the upper attach point. Those two balances we found laying on the floor after hearing a "bang".
So considering the large number of original equipment failures we experienced, I was wondering if the replacements you provide are constructed using the same materials as the original.
Ah, I see. I'm not sure what materials your old balances were made from, exactly, but I imagine that they're pretty similar. This kind of hardware isn't really made with different levels of durability in mind; it's all about spring strength. How old were the balances when they failed? If they were very old, I can see how this might happen in a few cases.
Can you also confirm that they had the correct spring strengths? This is determined by the stamp. Two balances with a 36C stamp can carry a window sash weight of 22 through 27 pounds, while two balances with a 30B stamp can carry a window sash weight of 13 through 19 pounds. How heavy were your two sashes? Do they correspond with these stamps?
We are the original homeowners. The first known 3-4 failures occurred after 5 1/2 years. The remaining failures occurred in the following 6 months.
I say known failures because I have since learned that there have been failures and the channel is being held under tension only by the tilt release latch. So when someone opens the window, the channel pops that latch off and fully retracts. Scared the mess out my wife one day. Me too!
I weighed 2 downstairs sashes and they are ~24 lbs each. So the 36C seems to be properly sized.
I will be placing an order for all 8 of our downstairs windows - 16 36C balances. Will also order some screw plugs (only two popped out) and some tilt latches because some of them were cracked when popped off by the channel.
You have been extremely helpful. Appreciate the follow-up and advice.
Sure, it's my pleasure. To be honest, a balance with a lifespan of 6 years isn't totally out of the ordinary. You can expect somewhere around 5 to 10 years, depending on how often the windows are opened. It sounds like your old balances just reached their limit.