Catastrophic bearing failure was the main cause of this spindle malfunction Continue reading to learn how Northland Tool performed this repair and converted a Weiss GHK 176239 spindle from grease-injected to permanent grease lubrication……Read More
Mazak VTC-20 Spindle Repair
The Mazak VTC-20 is a CNC vertical machining center. It comes equipped with a grease lubricated and coupling driven spindle capable of running up to 8,000 rpm.
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 and shaft mounting ID were both egged and tapered. The loss of preload also allowed the inner bearing spacer to spin on the shaft, damaging the spacer ID. We 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 a 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 and trued up the inner and outer bearing spacers in our machine shop. We replaced the bearings, belleville washers, gripper sleeve, spindle-side drive coupling, o-rings, and assembly screws.
After reassembly of all components, adjusting bearing preload, and undergoing break-in and testing procedures, this spindle passed our final quality inspection before being returned to our customer with a 1-year warranty.
DETAILED CAUSE OF FAILURE
Bell mouthed taper from years of heavy use and decreased drawbar force.
A look into the taper showing the poor contact surface.
This is the wear on the spindle shaft from the inner bearing spacer. The nut holding the bearing preload had loosened allowing the inner spacer to move and over time damage the shaft.
View of the fretting done to the interior of the front bearing spacer
Pointed out here are the cracks in the tool retention sleeve. These cracks allowed the sleeve to expand causing a decrease in drawbar force. This let the tooling move in the taper causing the damage.
This is the inner and outer front bearing spacer together. Notice the contaminated grease left after the bearings were removed.
The contaminated grease in the front bearing.
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