mujoco

<mujoco model="Connect and Weld defined with sites">
  <option viscosity="1">
    <flag contact="disable"/>
  </option>

  <visual>
    <scale framewidth="0.05" framelength="0.4"/>
  </visual>

  <default>
    <default class="free">
      <geom type="box" size=".2" fromto=".2 .2 0 .2 .2 -2" rgba=".4 .7 .6 1"/>
    </default>
    <site rgba=".0 .7 0 1"/>
  </default>

  <worldbody>
    <geom pos="0 0 -2" type="plane" size="10 10 .01"/>
    <light pos="0 0 20"/>

    <site name="a1" pos="-2 0 0" euler="-90 0 0"/>
    <body pos="-2 1 0">
      <freejoint/>
      <geom class="free"/>
      <site name="a2"/>
    </body>

    <site name="b1" euler="-90 0 0"/>
    <body pos="0 1 0">
      <freejoint/>
      <geom class="free"/>
      <site name="b2"/>
    </body>

    <site name="c1" pos="2 0 0" euler="-90 0 0"/>
    <body pos="2 1 0">
      <freejoint/>
      <geom class="free"/>
      <site name="c2"/>
    </body>
  </worldbody>

  <equality>
    <connect name="connect" site1="a1" site2="a2"/>
    <weld    name="weak weld" site1="b1" site2="b2" torquescale="0.2"/>
    <weld    name="weld" site1="c1" site2="c2"/>
  </equality>
</mujoco>