My third set of damper motors showed broken teeth on the brass gear wheel, once again. The motor is not burned out, it still turns under 24v, but with two or three teeth missing, it doesn't turn the damper blades. Any idea what could cause this?

I have a two zone system mounted on the main trunkline with Enerstat opposing blade dampers 24x10", spring open/power closed. The zone control board is Enerstat System 2, which was state of art in 1994 when the system was installed in my newly built home. Now Enerstat is extinct and no parts available. The contractor tested the motors and saw the broken gears, switched out with two new Honeywell motors and threw away the metal housing that came in the package. He explained that motors are all the same and fit into the exact spot. I did not have a broken spring and my main gear was metal not cheap plastic like the ones that are made now. He slid out the damper, fiddled around with it and it seems to be functioning like its supposed to- nothing obstructing, springs open smoothly. It's the teeth on the small brass gear that is located on the motor that is broken. As if it were forced. And on both motors. I've done this twice in 2 years. Six broken motors in all. And no warranty. Obviously, there is something else that is wrong. I think the problem must be in the zone control, or the damper system should be replaced or both.

Any alternative diagnosis before I write another check?