I am replacing the servos motor on a vertical milling center. The lead screws are connected directly to the motors so there is no reduction. The original motor was rated at 7 nm of torque for the Z-axis (7 nm is equal to 991 in oz.). I mounted a KL34H295-43-8A 906 oz In. Hybrid Motor on the z-axis connected in bipolar series configuration and I am using a KL-11080 Microstepping Driver. I started off with the drive setup for 3.5 amps when I started to open the valve on the bottle jack I had supporting the spindle motor the started to fall. When the spindle motor was resting on the jack and I jogged the axis up the axis would move with no problems but when it would stop the axis would fall. I made sure that the half current setting on the drive was disabled. Then I tried setting the drive up for higher amps to see what would happen. This time the motor would hold but it would fall occasionally. I checked the motor and it was slightly warm but I do not want to set the amps higher than the rating for the motor.
Why does the motor move the axis at the 3.5 amp setting but does not hold. The specification on the motor is given in holding torque.
To fix my problem I was planning to get a 1200 oz motor but could I possibly have the same problem?
Any suggestion are appreciated.