Where are the screw joints in Indigo?

edit

<robot name="pr2"> <link name="base_link"> <inertial> <mass value="116.0"/> <origin xyz="-0.061 0.0 0.1465"/> <inertia ixx="5.652232699207" ixy="-0.009719934438" ixz="1.293988226423" iyy="5.669473158652" iyz="-0.007379583694" izz="3.683196351726"/> </inertial> <visual> <origin rpy="0 0 0" xyz="0 0 0"/> <geometry> <mesh filename="package://pr2_description/meshes/base_v0/base.dae"/> </geometry> </visual> <collision> <origin rpy="0 0 0" xyz="0 0 0"/> <geometry> <mesh filename="package://pr2_description/meshes/base_v0/base_L.stl"/> </geometry> </collision> </link> <joint name="torso_lift_joint" type="prismatic"> <axis xyz="0 0 1"/> <limit effort="10000" lower="0.0" upper="0.33" velocity="0.013"/> <safety_controller k_position="100" k_velocity="2000000" soft_lower_limit="0.0115" soft_upper_limit="0.325"/> <calibration falling="0.00475"/> <dynamics damping="20000.0"/> <origin rpy="0 0 0" xyz="-0.05 0 0.739675"/> <parent link="base_link"/> <child link="torso_lift_link"/> </joint>

 <link name="torso_lift_link">
<inertial>
  <mass value="36.248046"/>
  <origin xyz="-0.1 0 -0.0885"/>
  <inertia ixx="2.771653750257" ixy="0.004284522609" ixz="-0.160418504506" iyy="2.510019507959" iyz="0.029664468704" izz="0.526432355569"/>
</inertial>
<visual>
  <origin rpy="0 0 0" xyz="0 0 0"/>
  <geometry>
    <mesh filename="package://pr2_description/meshes/torso_v0/torso_lift.dae"/>
  </geometry>
 </visual>
<collision name="torso_lift_collision">
  <origin rpy="0 0 0" xyz="0 0 0"/>
  <geometry>
    <mesh filename="package://pr2_description/meshes/torso_v0/torso_lift_L.stl"/>
  </geometry>
</collision>
</link>

  <!-- actuated motor screw joint -->
<link name="torso_lift_motor_screw_link">
  <inertial>
    <mass value="1.0"/>
    <origin rpy="0 0 0" xyz="0 0 0"/>
    <inertia ixx="0.001" ixy="0.0" ixz="0.0" iyy="0.001" iyz="0.0" izz="0.001"/>
  </inertial>
 <visual>
    <origin xyz="0 0 0" rpy="0 0 0" />
    <geometry>
      <box size="0.5 0.7 0.01" />
    </geometry>
  </visual>
  <collision>
    <origin xyz="0 0 0" rpy="0 0 0" />
    <geometry>
      <box size="0.5 0.7 0.01" />
    </geometry>
   </collision>
   </link>


      <joint name="torso_lift_motor_screw_joint" type="continuous">
      <origin xyz="-0.15 0.0 0.7"/>
      <axis xyz="0 0 1"/>
      <parent link="base_link"/>
      <child link="torso_lift_motor_screw_link"/>
      <dynamics damping="0.0001"/>
    </joint>


       <gazebo>
     <joint name="torso_lift_screw_torso_lift_joint" type="screw">
        <parent>torso_lift_link</parent>
       <child>torso_lift_motor_screw_link</child>
       <axis>
       <xyz>0 0 1</xyz>
       </axis>
         <thread_pitch>3141.6</thread_pitch>
        </joint>
      </gazebo>

       <transmission name="torso_lift_trans" type="pr2_mechanism_model/SimpleTransmission">
         <actuator name="torso_lift_motor"/>
          <joint name="torso_lift_joint"/>
          <mechanicalReduction>-47641.53</mechanicalReduction>
            <simulated_actuated_joint name="torso_lift_motor_screw_joint" simulated_reduction="3141.6"/>
        </transmission>
      </robot>

This is the filtered out code from pr2.urdf for realising the usage of screw joint in torso. This screw joint lifts the torso from the base_link , visual for the screw is a cuboid.

Hope this helps in understanding the implementation of screw joint.