Mazak VTC20 Spindle Repair
Mazak VTC-20, 8000 RPM, coupling drive, grease lubrication, CT40 taper
Our customer indicated that this spindle would occasionally make a banging noise while in operation. When we broke the spindle down for evaluation, we observed that the grease in the spindle bearings was partially washed out by coolant contamination. But the root cause of failure was loss of preload of the bearings which occurred when the hydraulic nut loosened up on the rear of the shaft. The spindle-side half of the direct-drive coupling actually bolts to the hydraulic nut, so any motor misalignment or other disturbance of that connection will cause this type of failure. In this case, one of the mounting bolts had been broken off, and the coupling pilot OD & shaft mounting ID were both egged & tapered. The loss of preload also allowed the inner bearing spacer to spin on the shaft, damaging the spacer ID. We also observed significant damage in the front of the spindle, including severe egging of the front bearing housing ID, severe wear & runout in the tooling taper and a cracked gripper sleeve.
Our overhaul of this vertical machining center spindle included grind, chrome plate and finish grind of the front bearing housing ID, the shaft CT40 tooling taper, and the inner bearing spacer ID. These grinding operations were accomplished in-house on one of our Kellenberger grinders. We also repaired the hydraulic nut & trued up the inner & outer bearing spacers in our machine shop. We replaced all the bearings, the Belleville washers, the gripper sleeve, the spindle-side drive coupling, and all o-rings and assembly bolts.
While running up the spindle to break in the new Kluber bearing grease, we monitored vibration and thermal measurements. Our customer reinstalled their spindle and now manufactures parts with a spindle that functions at peak efficiency, without making banging noises!