Spiral bevel gears transmit torque between intersecting or offset shafts in vehicle drivetrains, machine tools and industrial gearboxes. Their tooth flanks are generated on multi-axis CNC cutting machines, where the final accuracy depends on the stiffness, thermal stability and motion fidelity of the direct-drive rotary axes. The C50 is an 11-axis, 6-link machine for batch dry cutting of spiral bevel and hypoid gears with carburized-hardened milling cutters, using continuous indexing or single indexing. Its A, B and C axes are built around Siemens 1FW6230 torque motors and form water-cooled motor spindles. Constant low temperature and long-duration operation are therefore not comfort features but accuracy and reliability requirements.
At one production site, eight C50 machines were arranged symmetrically on four flexible lines. Three machines failed after coolant leakage or blockage burned the motor spindle, with associated 611D or S120 drive faults and servo alarms. Because the three failed machines sat on three different lines, operators had to keep switching fixtures and grippers to maintain production of middle and rear axle bevel gear sets. The result was a system-level availability problem, not merely a single spindle failure.
Machine architecture and accuracy chain
The C50 uses three vertical direct-drive trains: a tool drive train, a workpiece drive train and a base-angle drive train. The A axis rotates the cutter; the X axis provides 610 mm of milling-depth travel, the Y axis provides 820 mm of horizontal tool positioning, the Z axis provides 230 mm of workpiece positioning, and the tool train also includes a Q axis for chamfer-cutter rotation and an X2 positioning axis. The A-axis axial and radial runout are both held to no more than 0.4 μm. The B axis rotates the workpiece and holds axial and radial runout to no more than 0.8 μm. The C axis controls workpiece swing over a base-angle range of ±87°.
| Axis | Function | Drive and geometry | Key accuracy or range | Servicing constraint |
|---|---|---|---|---|
| A | Tool rotation | Siemens 1FW6230 torque motor; vertical; solid shaft with one M16 blind hole | Axial and radial runout ≤0.4 μm | Stator-to-housing radial gap about 1 mm; rotor must be removed before stator |
| B | Workpiece rotation | 1FW6230 torque motor; hollow shaft; 3° mounting tilt | Axial and radial runout ≤0.8 μm | Stator and rotor removed together; end-cover and bearing interference during seating |
| C | Base-angle swing | 1FW6230 torque motor | ±87° | Cannot be placed horizontal; limits B-axis withdrawal geometry |
Repair options and technical constraints
The German OEM proposed module-level replacement of the A and B axis assemblies. For the three affected machines the quotation was nearly RMB 5.58 million, with a total repair cycle of 365 days; the replaced parts were to be returned to Germany free of charge. A domestic repair of a C50 A axis in 2017 had produced chatter marks on the spiral bevel tooth flanks and disabled the automatic dual-manipulator loading and unloading function, leaving manual handling by electric hoist. The B axis had no domestic on-machine repair precedent. Documentation was also limited to external views of the A and B axes. The maintenance approach therefore had to be developed around the machine itself rather than around a complete replacement module.
On-machine disassembly and dedicated tooling
The first problem was extraction without damaging the motor spindle, the machine frame or the operator. The tooling set was built around the actual geometry of each axis.
- B-axis extraction. A gantry-type puller made from 40 mm steel plate was used to draw the complete 1FW6230 torque-motor spindle out of the housing.
- A-axis extraction. Machine space was restricted by side ribs, so a 50 mm plate annular puller was made with two legs and two side ears. The ears seated against the triangular ribs and allowed the spindle to be withdrawn as a unit.
- Safety frames. A B-axis rigid frame was produced by splitting an axle-housing transport frame in two, fitting the narrow one-person maintenance space and preventing the spindle from sliding down. The A-axis frame was made in two welded sections placed under the axis and tied with 80 mm red square-tube cross members, so the machine guard did not have to be removed.
- Stator handling. The 1FW6230 stator coil frame is aluminium and deforms easily under sling pressure. A stator reinforcement ring was bolted to the coil and used with eye bolts for lifting.
- Lifting geometry. Because the C axis cannot be horizontal and the B axis has a 3° installation tilt, a 40 mm wide, 5 mm thick U-shaped flat belt replaced soft rope and a single lifting point. For the A axis, a cross lifting beam was used because the solid shaft has only one M16 blind hole; the rotor is removed first, then the stator is aligned and lifted out through the 1 mm radial gap.
Inside the B axis, 12 DIN 2093 A180 Gr3 disc springs are stacked in alternating orientation; the material is 50CrVA or 50CrV4. They provide a clamping force of 4.0×104 N, and release is assisted by 1.6 MPa air pressure. A small scoop-like extraction tool was used to remove the springs one by one while lightly supporting the stack.
Peripheral access also affected maintainability. Some C50 machines have a wet chip conveyor that must exit from the screen-operation side. When two machines are installed symmetrically on a line, the conveyor has no direct exit and must be withdrawn through the unloading ring gap. The support wheels were therefore modified with an added support plate and changed from fixed casters to swivel casters.
Magnetic gap and controlled seating
The 1FW6230 rotor uses permanent magnets and the stator carries the winding; the air gap is about 0.5 mm. During assembly the magnetic attraction is strong enough to pull the stator and rotor together before the correct position is reached. Three nylon strips, 190 mm wide and 0.5 mm thick, were inserted into the gap by three operators while one operator controlled the stator descent. The strips were withdrawn as the stator lowered, until the lower stator face contacted the spindle end-face datum. Later practice replaced nylon strips with A3 rubber sleeves, which proved easier to handle and less prone to trapping.
For B-axis installation, the gantry puller was reversed and its flat face used as a reaction plate. An M24×1000 mm screw passed through a φ26 mm centre hole, through the hollow B-axis centre and into an 80 mm square tube. The spindle was drawn gradually toward the end cover. When the outer ring entered the B-axis housing and only the last 10 mm remained, the screw puller was removed and a φ120 mm × 1000 mm copper drift was used to seat the assembly. This final stage is sensitive because the end-cover outer ring has a relatively large interference fit with the housing bore and the NNU double-row cylindrical roller bearing is installed at the same time. Excessive impact force can crack the Z-axis linear-guide slider or damage the encoder readhead, so axial seating must be controlled rather than treated as a simple hammering operation.
Encoder alignment, software recovery and drive optimisation
The A, B and C axis rotary positions are fed back by an encoder drum on the rotor and a readhead on the stationary side. The installation gap is 0.1 ± 0.015 mm. A gap that is too large prevents distance calculation; a gap that is too small can rub the steel band and destroy the readhead. A PWT18 encoder quick-adjustment instrument was used to set the grid amplitude near 1 Vss, within the permitted range of 0.6–1.2 Vss, and to place the black origin-mark ends between the two brackets. The case record notes that measured amplitudes of 0.82–0.97 Vss still coincided with multiple alarms and servo faults, so amplitude alone was not a sufficient acceptance criterion; gap, drum condition and readhead alignment had to be checked together.
On the control side, operators reported rotor-position synchronisation errors and servo faults. Repeated position adjustment and restarting allowed the C50 to enter production, but the adjustment had to be repeated every 8 h. After clearing the NC and PLC data, an EBOOT USB boot disk was used to perform a one-key Ghost restoration of the SINUMERIK 840D PCU50 hard-disk image. The safety integrated functions were then copied into the servo drive. Servo drive optimisation was carried out after the mechanical restoration, so that the feedback and current-loop behaviour matched the reassembled spindle.
Chatter correction and clamping-system change
After spindle repair, chatter on spiral bevel tooth flanks is not necessarily a spindle-balance problem alone. In the 2017 A-axis repair, the tooth flanks showed visible and hand-detectable chatter marks, and the dual-manipulator automation was lost. The later corrective package treated chatter as a system-level stiffness, damping and clamping problem: all fixtures were replaced and reinstalled, the B-axis speed loop was re-optimised, the clamping method was changed from a pneumatic circuit to pure disc-spring tension, and the cutting process parameters were adjusted.
| Parameter | Before | After |
|---|---|---|
| MD1200 | 3 | 4 |
| MD1219 | 510 | 1080 |
| MD1220 | 150 | 500 |
| MD1407 | 12000 | 15000 |
| Clamping force Fz | 4.2 kN | 4.7 kN |
The pneumatic circuit P120.60 was removed and the fixture was changed to pure disc-spring clamping, raising the clamping force from 4.2 kN to 4.7 kN. Together with fixture renewal and speed-loop optimisation, this addressed the interaction between workpiece clamping, axis dynamics and tooth-flank generation rather than isolating a single spindle component.
Safety-chain recovery and diagnostic motion tests
The C50 safety integrated chain can stop the loading door from opening or closing when it is triggered. In the reported condition, signals $A_PLCSIIN[11] and $A_PLCSIIN[03] became abnormal. An eight-wire, one-program procedure was used during troubleshooting: N900–N901, N902–N903, N567–N267 and N667–N376 were shorted in the electrical cabinet to restore loading-door motion. This is a temporary diagnostic intervention, not a permanent substitute for the safety function; permanent operation requires validation of the safety chain and door interlocks.
For multi-axis linked machines, MDA-mode cyclic programs are an efficient way to locate intermittent faults. Running the vertical X axis back and forth in MDA allows the maintenance engineer to listen for abnormal noise and monitor ball-screw load, which helps assess the condition of the screw-nut pair. Running the C swing axis in MDA allows the actual alarm angle to be identified; C-axis parameters can then be corrected, or a paid COP32 program upgrade used as an alternative to repeated mechanical adjustment. In this way, a sudden stop during gear cutting can be traced to a specific axis condition rather than treated as a general servo failure.
Outcome and transferability
The program established an on-machine process for water-cooled motor spindles covering extraction, stator and rotor handling, magnetic-gap control, bearing seating, encoder alignment, drive and software restoration, and system-level chatter correction. The affected spindles were disassembled in place and successfully reassembled, avoiding the OEM module-exchange route quoted at nearly RMB 5.58 million and 365 days for the three machines. The local route reduces dependence on complete-module replacement, but it requires dedicated tooling, dimensional checks, controlled axial seating and safety validation.
The approach is transferable to lathe, machining-centre, grinder and tool-grinder motor spindles when the specific cooling circuit, rotor-stator geometry, encoder system and safety chain are respected. For spiral bevel gear production, the practical benefit is not only repair cost. The accuracy chain must be restored as a whole: spindle runout, encoder feedback, clamping force, fixture condition and cutting parameters all influence tooth-flank quality. On-machine servicing is therefore viable when it is treated as an integrated mechanical, electrical and process-control task rather than as a bearing or winding repair alone.