Adding revolute joint makes the wheels disappear

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Hello,

I am trying to create a 2 wheel mobile robot using xacro file.. I followed the tutorials when using SDF and I convert it to xacro and I have two links for the wheels and two joints of type revolute. when I add the revolute joints and I run the launch file I cant see the wheels on gazebo .. I have the following tags for the joints ?? is there any thing wrong ?? also, is it the right way to have a revolute joints of 2 wheels robot ??

<joint type="revolute" name="left_wheel_hinge">
 <origin xyz="0 -0.18 -0.03" rpy="0 1.5707 1.5707"/>
 <child link="left_wheel"></child>
  <parent link="base_link"></parent>
  <axis xyz="0 1 0"/>
  <limit effort="0.0" lower="0.0" upper="0.0" velocity="0.0"/>
   </joint>


<joint type="revolute" name="right_wheel_hinge">
 <origin xyz="0 0.18 -0.03" rpy="0 1.5707 1.5707"/>
 <parent link="base_link"></parent>
 <child link="right_wheel"></child>
  <axis xyz="0 1 0"/>
  <limit effort="0.0" lower="0.0" upper="0.0" velocity="0.0"/>
  </joint>