<limit velocity="6.5" effort="1000" lower="0" upper="0" />

The problem is, now the simulation does not run at all, with the sdf file missing all of the joints and links after the joint I converted from fixed to revolute (I checked with rosrun xacro xacro my_urdf.urdf.xacro > test.urdf followed by gz sdf -p test.urdf > test.sdf). Very frustrated. Any help would be greatly appreciated!