Channel Balances breaking under tension

A quick learner from Santa Clarita says:
I ordered and tried to install 2 S385-27 2650 channel balances.

Once the channel balances were installed, I tested the window a few times. After about the third time one of the channel balances broke. The pin that holds the spring into the assembly sheared off. I then installed the old channel balance in its place and after a few tests the other one broke the same way.

It appears that the pin and the rope cannot handle the required spring tension as ordered for my window.

My original balances are marked 26 5. And the window weight is approximately 35 ish pounds. That does not explain why the pin would shear and the rope would break. The channel balance should be able to Handel the ordered spring rate.
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Paul from SWISCO responded:
Thank you for posting on our discussion board. It sounds like your stamp option is not correct. Are you sure your original balance said 26 5? An S385-27 channel balance with a 2650 stamp option has a sash weight capacity of 26 through 30 pounds. If your sash is 35 pounds, it would explain why your balance broke.

Please confirm your original balance's stamp. Then, please give us the exact weight of your sash. An approximate weight may be inaccurate; please get as exact a weight as you can.

Keep in mind that the weight range capacity information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate
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