How to model a non-backdrivable joint

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I have an arm that is effort driven and each joint is not backdrivable. I'm having a hard time figuring out the right way to model this in the URDF. Right now I just have a SimpleTransmission with an EffortJointInterface hardware interface and am using JointEffortControllers. The problem is that when no effort is being applied, the arm flops to the floor in gazebo since the simulation doesn't know the joints don't work like that.

I've read a few places to just jack up the damping and friction in the joint specification. I've tried that. As I raise and raise it, there is a fine line between where the arms still just drop to the floor, or they spin off into oblivion. Additionally I don't think this is the right way because my simulated arm would require much more effort to move it that it would in reality.

Any suggestions? I would have thought there would be something in the transmission tag to support this but I haven't seen anything.

Thanks,