WOW, a circle saw...you must have kerf to spare.

Although I have never felt a differential after running, I would say yours might be too hot.

I can't imagine that it was engineered to rely on air passing over for cooling.

If your mill is being driven by one axle, what have you done to the other axle? Restricting its rotation might create extra heat in the spider gears.

The normal scenario would be for both axles to turn about the same speed, but utilizing it as you are, I don't know how that would be possible.

What are you cutting, and what diameter is the saw? Your critical speed will be FPM on the rim of the saw, not RPM. Good luck.

Stuart