Hey Arthur,
the conditions under which stepper motors work reliably, both open- and closed loop, is depending on the mechanical load. If the motors are sized to fit the expected load, you will not loose steps. On the other hand, if you overload them, the closed-loop stepper will not always be a real advantage. A closed loop stepper is a stepper with a glass encoder disc that feeds a closed-loop controller. If the steps sent to the motor deviates more than a certain amount, this closed-loop controller signals an error to the CNC controller wich can then stop the g-code program. But the amount of deviation between steps sent and sensed is relatively high before the error is triggered. The closed-loop controller gets a chance to try to keep up. Usually it takes time until the controller uses the incoming motor error to stop the entire machine. So if you encounter, let’s say, a very hard knothole that overloads the stepper so much that it looses its position, usually you can’t avoid the bit breakage anyway. The difference is if the deviation between steps received by the motor and sensed by its encoder is too big, an error is triggered, while with an open-loop stepper it is not, so the machine continues with a wrong position and may ruin more of your blank. But this is just an example. Usually knotholes rather make bits break than overloading the stepper, and a bit break is nothing a closed-loop stepper can sense.
So if you size your steppers strong enough for the expected application, usually you don’t loose steps, but closed-loop steppers are steppers too in the end, so they have the same speed and torque limitation than open-loop steppers. But the stronger PRO steppers reduce the risk to mechanically overload the steppers in any case, compared to the weaker X-50 steppers.
Usually you can try to mechanically overload a machine’s steppers if you select a much too high feedrate, depth of cut, stepover and ramp down with too big bit diameter and very hard workpiece. But usually the first thing that is to expect is that the bit breaks then.
The other thing that influence the mechanicial load limits is the ball screw pitch. For the older Z-16 assembly with its 1004 ball screw a small motor was enough to move a heavy spindle while the steeper pitch of the Z-20 assembly requires a stronger motor with more torque.
I think in this forum if steps were lost the cause was usually not mechanically overload but EMI due to non-shielded cables, unsuited cables like the “curly Z” cable or missing connector’s strain relief on old X-50 and Original Series, or the unsuited tin-plated Molex-Amphenol connectors that are not made for external connections (instead of using industrial circular gold-plated connectors) (more info here. Note that the PRO Series now have the 30 mm drag chains from the Elite, while all machines still have non-shielded cables with Molex/Amphenol connectors with tin-plated contacts)