The overload sensor will trip out on your Z servo if the motor draws too much current for too long a period of time. This overload sensor can trip if the motor is working too hard to either move the Z axis or hold it in position.
Many vertical machines have counter-balance mechanisms on the Z axis that takes the static load off the motor. If the counter-balance is working properly, the motor shouldn't have to work very hard just to support the weight of the Z axis while the machine is stationery. The motor should work the hardest when the axis is acellerating or decellerating, or when you're taking a heavy cut. Z servos are most likely to trip while pushing a large diameter drill into tough materials, because the thrust loads for a drill are quite high compared to other types of tools.
Check to see if you have a mechanical (counterweight with chain & sprocket) or possibly an air cylinder as a Z axis counter-ballance mechanism. I've seen broken or disconnected chains on mechanical counterballances, and it's possible that an air cylinder counterballance isn't getting enough pressure.
Another thing to watch for is lubrication. If the machine's been sitting for a while, be sure that the Z ways are getting some lube, and that the Z ballscrew is also well lubricated. It may seem that way lube wouldn't matter when the machine is not moving, but it does. The servo must "hunt" to keep the axis in position, and an unlubricated slide or ballscrew can place a large current load on the motor.  |