If the servo is working fine by itself and it fails when assembled into the clamp, I can think of two things:
– The zero position of the servo is not the zero position of the clamp. The zero position of the servomotor should be when the gripper is fully open. If the zero position causes the gripper to try to open more than is mechanically possible, the motor will vibrate.
– The pick-up position of the servomotor is too high. As in the previous case, if we force the servomotor to go to an unreachable position for it, it will continuously try to reach it, which will cause vibrations and, sometimes, it could even burn the motor.
Let me know if you have any of these two cases!