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If I understand correctly, there are two separate forces acting on the gearbox: One of them (windage) is loosely proportional to the speed of the motor, caused by churning of the lubricant and friction we can't get rid of. The other is loosely proportional to the forces on the gear teeth, and thus to the torque on the gearbox.
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They are all simplified models of very complex interaction forces... There are many many more than two forces/torques at work. Every model has it's boundaries.
This one (load dependent plus windage) actually has some undesirable behavior about zero speed. You would expect the gearbox to stay stationary until the torque exceeds some threshhold torque boundary. This won't happen here and the slightest torque still transmits 90% and will result in motion. This aberant behavior may or may not be important to your application. A "constant parasitic torque" model you've described does prety well in this case and can even approximate some non-back-drivable gearbox behaviors.
The "constant parasitic torque" is often the dominant force in tight fitting pin joint type rotational systems. Here the coulomb friction is a function of the "clamp load" in the joint and approximated as constant. Unless you have an air bearing, this behavior is always present to some extent. A "next best" model could be to apply all three. As you combine effects you need more measurements/observations to determine the values of the parameters.
The bottom line is that we can say "bad things" about any model. A good model minimizes complexity of understanding, implementation, and/or measurements while giving a desirable approximation. A bad model is one which gives inferior results to models of similar or lesser complexity. I haven't seen any bad models in our discussions, and we certainly don't want to steer anybody away from what has been proven to work!
Hopefully that about does it for the "simple" models discussion.
