This is a model of an ideal stiff coupling: One device connected
to the coupling is turning clockwise (looking at the shaft end),
the other device connected to the coupling is turning
counter-clockwise (looking at the shaft end). The torque at
flange_b has the same magnitude as the torque at
flange_a but opposite sign. This is achieved by using
an ideal
gear with ratio = -1.
Variable w represents the angular velocity of
flange_a and tau represents the torque
transferred from flange_a to
flange_b.
This behaviour is essential when coupling electric machines. The usage is demonstrated in the example Modelica.Electrical.Machines.Examples.DCMachines.DCPM_Drive.