Help to understand the code for gazebo plugin "gazebo_ros_vacuum_gripper"?

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The source code for the Gazebo_ros_vacuum_gripper is available in the following link

http://docs.ros.org/jade/api/gazebo_p...

I understand that the gripper is supposed to be activated by calling the ros service "on".

I am doing exactly the same as I explained here

http://answers.ros.org/question/24308...

The problem is that upon contact and after calling the ros service "on" , I cannot see any applicatio of force on the object link. I feel that I am missing something here, please take a look at the code, and let me know what are the conditions for the application of force.

I am unable to grasp the numbers there as well, in the force application section , such as line 00207 says normal force >20 so what is 20 here. similarly on lines 00195 and 00201.

I would really appreciate any help, I have been working to make this plugin work for weeks now. Please help me.