The power screw is driven from the drive gear on top of the motor to the driven gear on top of the power nut. The power nut is an aluminum nut that the power screw passes through. First check drive gear on top of the motor. The drive gear is held to the motor shaft by a setscrew against the motor shaft. The setscrew can loosen and the drive gear will disengage from the driven screw. If this is the case turn the power off, move the drive gear to engage the driven gear and tighten the setscrew. If the gears are properly engaged, the threads in the aluminum power nut can wear out. Thread the power nut off the top end of the power screw and inspect it. On older 94 series and all 93 series the torque bar power screw and torque bar can jump if a large specimen breaks. This may disengage the drive and driven gears. Inspect and engage gears.