By Charles Corson, February 2010.
There are a great number of things that can cause the linear actuator to fail. I would first decouple it from the linear actuator from the articulation arm, the straight piece which comes down from the gripper face.
This decoupling is very simple: you remove the jam nut on the end of the ball screw at the left end of the actuator. Once done, you command the gripper to open and close.
What you are looking for is a smooth, consistent in/out motion of the balls screw, the linear actuation. Without any mechanical loading, this should be a simple test of the actuator itself. If it appears to be 'sticking', you can apply a drop or two of light lubricant. I've seen these actuators fail because the original lubricant has 'gummed' up, become dry and very sticky, waxy. You also want to look closely at the linear keyway on the ball screw. You will see just at that face of the teflon nut, a teflon key, or tab, that fits directly into that slot. The Tab should be in good shape, not rounded, or deformed. If it is, the ball screw is likely turning with the motor under high load and then there is a complete loss of linear actuation. The ball screw must have no rotation in relation to the motor turns. Of course, it rotates within the nut.
Let us assume that the linear actuator is good with no mechanical load. I would then be highly suspect of the various mechanical adjustments: position of jam nut for correct 'relax' tension on the button, position of the close 'stop', position of the 'open' stop. There is no easy way to explain this fully, adequately. I've found that it is an iterative tuning so that OPEN is at the correct position, N-steps produces the correct relative position for 'relax' state, and the right position for close. To throw one more adjustment in the mix, one then has to have the 'tension' the spring force of the jam nut and spring on the linear actuator at the right state.
Here is YET ANOTHER issue to consider. There is a IC stepper driver on the controller card. Under mechanical load, if this driver is 'bad' or over-heating, the stepper will stall of course.
Don't assume all gripper failures have to do with the motor.