Friction in the gripper not working, objects slipping

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A couple suggestions:

1. This problem may be caused by the PositionJointInterface (See this Gazebo Answer).
   What happens if you use an EffortJointInterface for the grippers?
2. Try giving your gripper contact links a small <minDepth> value of say 0.002-0.003 and a <maxVel> value of 0. This may provide more consistent contact between the gripper links and the object being grasped.

<gazebo reference="GRIPPER_LINK_NAME">  
   <minDepth>0.003</minDepth>  
   <maxVel>0</maxVel>  
 </gazebo>

I was experiencing the same issue and after trying many things (changing joint controller types, increasing mu values, decreasing slip, decreasing minDepth, modifying the physics properties of the block that my robot was grasping etc.), What ended up solving this problem for me was to specify the kp and kd values for the gripper fingers and to increase the kd value until the block stopped slipping out of the robot's grasp. Below is the example section from my robot's urdf file (using the LoCoBot Gazebo package):

<gazebo reference="finger_r">
    <mu1>1000</mu1>
    <mu2>1000</mu2>
    <kp>1000000.0</kp>
    <kd>100000.0</kd>
    <slip1>0</slip1>
    <slip2>0</slip2>
    <minDepth>0.001</minDepth>
    <maxVel>0</maxVel>
    <torsional>
        <ode>
            <coefficient>1000</coefficient>
            <slip>0</slip>
        </ode>
    </torsional>

</gazebo>

<gazebo reference="finger_l">
    <mu1>1000</mu1>
    <mu2>1000</mu2>
    <kp>1000000.0</kp>
    <kd>100000.0</kd>
    <slip1>0</slip1>
    <slip2>0</slip2>
    <minDepth>0.001</minDepth>
    <maxVel>0</maxVel>
    <torsional>
        <ode>
            <coefficient>1000</coefficient>
            <slip>0</slip>
        </ode>
    </torsional>
</gazebo>

You might have to experiment with these values a bit, but I hope this is helpful to anyone who is having this same issue!

I had the same problem and tried different physics engine parameters and none of the approaches worked properly for me. I have changed the interia, PID gains, frictions, maxVel, minDepth and none of them worked properly for me. I also didn't want to use EffortJointInterface. Therefore, I decided to not waste more time on having a realistic grasp in gazebo simulator (of course you can have a realistic grasp in a real robot using this approach). Instead, I have used the following approach.

• When the gripper fingers are closed enough to hold the object, you can attach the object's link, and the finger tips link with a fixed joint dynamically. As an example, you can use this package for your purpose. This way there is no need for changing the JointInterface or controller type. In order to have a more realistic grasp, you can also use the contact_points published by the gazebo.