I am applying a torque to an axle like this:

apply_effort = rospy.ServiceProxy("gazebo/apply_joint_effort", 
ApplyJointEffort)
apply_effort("axle", 0.01, rospy.Time(), rospy.Duration(-1))
rospy.sleep(0.05)
apply_effort("axle", 0, rospy.Time(), rospy.Duration(-1))

I expected the axle's angular velocity to accelerate for 0.05 seconds, 
then remain constant. In reality, the axle continues to accelerate. It 
appears that apply_joint_effort is cumulative, and my code actually 
needs to be like this:

apply_effort("axle", 0.01, rospy.Time(), rospy.Duration(-1))
rospy.sleep(0.05)
apply_effort("axle", -0.01, rospy.Time(), rospy.Duration(-1))  # Cancel 
previous torque.

Is my understanding correct?

I tried to use gazebo/clear_joint_forces instead of applying a -0.01 
effort, but it appears that clear_joint_forces is no longer in ROS, even 
though it is in the Gazebo package documentation in the ROS wiki.

-- 
Jim Rothrock | Wunderkammer Laboratory
jim.rothrock@wunderkammerlab.com