Simulating linked joints in Gazebo [closed]

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I am trying to simulate some mechanically linked joints in gazebo. On the real robot the joints are connected with a chain so their positions and velocities are tied. Usually some of the connected joints are unloaded while others are loaded. Should I use a custom transmission here? If so how would I link the joints in the transmission? Is there a place I can find documentation regarding how the transmission is processed?

Thanks Brian