Let us address your questions one by one.
1) Am I missing something ?...
I referred to the Simulink model attached by you in the comment section and found that the “torque sensor” was not placed appropriately.
The torque sensor must be placed in “series” with the driving shaft. In this case, it must be placed between “C” port of “Rotational Spring” and “Inertia” block as shown in the below figure.
2) Increasing the load torque on mechanical drive shaft.
Here, an additional “rotational damper” is placed in order to simulate increase/decrease of load torque.
In order to increase the load torque on the mechanical drive side, increase the value of friction coefficient “B” of the “rotational damper” block and in order to decrease the load torque, decrease the value of “B”.
The below given simulation results are corresponding to the value of B = 0.05.
Conclusions (at steady state):
- Pressure sensor reading: 1.458e+05
- Rotational speed at “O” port of “gear box”: 103.8
- Rotational speed at “S” port of “gear box”: 51.9
In order to simulate increased load torque, I made “B” = 0.1 and got the following results:
Conclusions (at steady state):
- Pressure sensor reading: 2.911e+05
- Rotational speed at “O” port of “gear box”: 51.89
- Rotational speed at “S” port of “gear box”: 103.8
Conclusions about the simulation:
As we increase the load torque on the mechanical drive side, we can see that the pressure of the “isothermal fluid” has now become equal to “2.911e+05” as compared to its previous value of “1.458e+05”. Also, the speed at “O” port “gear box” is non-zero
I am also attaching the reference Simulink model with this answer.
I hope this helps !