Electric Spindle of CNC Grinding Machine: Typical Fault Analysis and Troubleshooting | MachineMFG

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Electric Spindle of CNC Grinding Machine: Typical Fault Analysis and Troubleshooting

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High-speed CNC machine tools equipped with motorized spindles boast of compact structures, light weight, small inertia, complete functions, and excellent performance. However, their downsides include the difficulty of manufacturing and maintaining them, as well as their high cost.

This article takes the 6-axis CNC broach grinder CORVUS1700 CNC6 motorized spindle as a case study to present its primary structure, analyze the cause of the fault based on its design, and ultimately troubleshoot the issue.

1. Preface

When the electric spindle of the NC broach grinder stops running, an alarm is triggered.

Upon analysis, it was found that the axial movement of the main shaft rotor was 0.5mm, causing an unbalanced operation, resulting in increased current, higher load, rapid temperature rise, and abnormal noise.

Contacting the manufacturer yielded no technical support, and the purchase of new spare parts would take 16 weeks and cost 300,000 yuan.

In order to break the monopoly of foreign technology, reduce repair time, and decrease procurement costs, a technical team was established to develop an independent improvement plan without maintenance information.

The independent improvement faced some technical difficulties such as a lack of reference for balance control measures for high-speed spindle operation (processing speed over 18,000 r/min), lack of technical support for pre-tightening torque value and shaft method, and missing assembly and debugging schemes for the special motorized spindle and special adjustment tools. There was also no reference basis for the machining drawing of the damaged parts and their materials.

Therefore, the team decided to decompose the motorized spindle and analyze its internal structure, focusing on the mechanical structure, identifying the causes of the failure, mastering the assembly and adjustment methods, and restoring the equipment’s processing function.

2. Electric spindle structure

(1) Appearance of electric spindle

The electric spindle, as shown in Figure 1, is a machine tool component that integrates the spindle and motor. Once the spindle section is correctly extended, the motor rotor is installed directly into the extension section to ensure that the motor is coaxial with the spindle. The stator package is then installed outside the rotor, resulting in a coaxial motor configuration.

During operation, the motor drives the spindle directly.

electric spindle

Fig. 1 electric spindle

The motorized spindle has a compact overall structure and high rigidity, as the motor is installed coaxially with the main shaft. It offers high transmission efficiency and rotational accuracy, and enables quick speed adjustment between zero and tens of thousands of revolutions.

The motorized spindle is a set of spindle components that includes the motor stator coil, rotor, high-speed bearing, bearing lubrication device, and spindle cooling device. It is not just an isolated spindle.

This device integrates a high-speed motor, high-speed bearing, lubrication system, cooling system, balance technology, and precision manufacturing and assembly technology.

The local mechanical structure of the motorized spindle is depicted in Figure 2.

local mechanical structure of motorized spindle

Fig. 2 local mechanical structure of motorized spindle

  • 1 – shaft sleeve
  • 2 – lock nut
  • 3 – angular contact ball bearing
  • 4 – hollow shaft
  • 5 – sliding bearing
  • 6 – Gasket
  • 7 – small gasket
  • 8 – Hexagon socket head cap screw
  • 9 – adjusting shaft sleeve
  • 10 – hollow adjusting screw
  • 11 – adjusting nut
  • 12 – disc spring

(2) Rotor shaft

The rotor shaft comprises Shaft I and Shaft II, as shown in Figure 3.

Shaft I (refer to Figure 4) consists of two sets of angular contact ball bearings, inner and outer spacer sleeves, lock nuts, shaft sleeves, plain bearings, three disc springs, gaskets, and hexagon socket head screws.

The two sets of angular contact ball bearings are installed back-to-back, and the shaft sleeve ensures the synchronous rotation of the two shafts. Moreover, it provides safety protection for the two shafts, preventing the accuracy of the main shaft from decreasing.

Two sets of sliding bearings support Shaft I, reducing its radial circular jump.

In the event of a collision of the grinding wheel spindle, a small amount of deformation occurs instantly, and the disc spring effectively protects the spindle.

Fig. 3 composition of rotor shaft

components of shaft I

Fig. 4 components of shaft I

(3) Composition and wear analysis of shaft II

Shaft II consists of a hollow shaft, a gasket, an adjusting nut, a hollow adjusting screw, an adjusting shaft sleeve, two sets of angular contact ball bearings (installed back to back), inner and outer spacer sleeves, and a bearing gland.

The hollow shaft is made of low-hardness silicon steel, while the gasket is made of a high-hardness material.

During prolonged use, the electric spindle rotates at high speeds, causing mechanical vibrations that can loosen the hexagon socket head cap screw. This results in increased axial movement between shaft I and shaft II, leading to asynchronous operation of the hollow shaft and gasket and intensifying wear at the inner shoulder of the hollow shaft.

3. Typical faults and handling methods of motorized spindle

The typical fault observed in the special electric spindle of an NC broach grinder is the sudden cessation of its rotation accompanied by an equipment alarm.

3.1 Fault inspection and analysis

(1) Fault check

During the examination of the non-rotation fault in the main shaft, both electrical and mechanical aspects were inspected.

Regarding the electrical aspect, it was discovered that the coil of the motorized spindle was burnt out. An external unit was tasked with winding and repairing the coil, ensuring that it meets the necessary performance requirements.

In terms of the mechanical aspect, after disassembling the motorized spindle, it was determined that the rotor shaft consisted of two shafts, shaft I and shaft II. The following fault points were identified:

  • The bearing was not preloaded.
  • The hexagon socket head cap screw at the end of shaft I was loose.
  • The inner wall of the hollow shaft was worn.
  • The shaft sleeve was broken.
  • The adjusting nut was worn down.

(2) Fault analysis

The main shaft is not rotating, and the hexagon socket head cap screw in the rotor shaft is loose. During the high-speed rotation of the main shaft, the operation of Shaft II and the gasket is not synchronized, causing the gasket to rotate inside Shaft II.

The gasket material is different from that of Shaft II, and it has a higher hardness. This often leads to wear of the shoulder in Shaft II, shortening the distance between the shoulder in Shaft II and the end face of the shaft sleeve. However, the distance between the sleeve on Shaft I and the left end face of the gasket remains unchanged, resulting in the phenomenon of “shaft lengthening,” which prevents the rotor shaft from pre-tightening and causes an axial movement of 0.5mm.

Additionally, the coaxiality of Shaft I and Shaft II is out of tolerance. While Shaft I is closely matched with the shaft sleeve, the fracture of the shaft sleeve causes it to rotate around Shaft I. Consequently, when the main shaft rotates at high speed, the operation of Shaft I and Shaft II becomes out of sync, causing an excessive load on the electric spindle and increased current.

If the electric spindle continues to operate under these conditions, the coil will burn out, and the electrical components will age, leading to a shortened service life.

3.2 Troubleshooting methods

Please follow the troubleshooting methods below:

  • Pre-tighten the bearings on both ends of the spindle.
  • Assemble the shaft sleeve onto shaft I (refer to Figure 5) and heat-install it onto shaft I.
  • Prepare the gasket for shaft II, increase its thickness by 5mm, and mill a 2mm-deep groove near the sliding bearing end (during the trial assembly, the groove depth can be adjusted to 0.5mm, 1mm, 1.5mm, or 2mm).

Make an adjusting nut for shaft II (see Figure 6).

  • Investigate the assembly method for the special motorized spindle and create special adjustment tools.

Fig. 5 Shaft sleeve parts drawing

Fig. 6 adjusting nut

3.3 Installing the power distribution spindle

The two sets of bearings on Shaft I are installed back-to-back, with the outer ring of each bearing fixed in the step hole of the coil stator. A spacer ring is placed between the two sets of bearings, and the inner ring is set 0.02mm lower than the outer ring.

Using the hollow shaft and gasket (as shown in Fig. 2), tighten the hexagon socket head cap screw to allow Shaft I to bear the pull to the right and stretch to the right. At the same time, lock the nut, push the inner ring of the bearing in the opposite direction, and use the height difference between the inner and outer rings to eliminate the bearing clearance of Shaft I, thus achieving the axial preload of Shaft I.

With the cylindrical shoulder of the hollow shaft and the inner ring of the bearing, the shaft sleeve of Shaft II can be indirectly tightened by tightening the hollow adjusting screw (which matches the adjusting nut). This eliminates the axial movement of Shaft II and achieves the axial pre-tightening of the bearing on Shaft II.

Shaft I is equipped with a shaft sleeve with a groove, while Shaft II is designed with a boss that matches the groove of the shaft sleeve. During assembly, the hollow shaft is connected with the shaft sleeve, with the inner circle of the gasket matched with Shaft I and the outer circle of the gasket matched with the inner wall of the hollow shaft. Three disc springs are placed on the gasket, and the small gasket, gasket, and disc spring can be fixed on Shaft II by screwing the adjusting nut.

Using the hollow shaft and gasket, the axial movement of Shaft I can be eliminated by tightening the hexagon socket head cap screw. Similarly, the axial movement of Shaft II can be eliminated by tightening the hollow adjusting screw (which matches the adjusting nut) and indirectly tightening the shaft sleeve, based on the outer shaft shoulder and bearing inner ring of the hollow shaft.

At this point, the assembly of the motorized spindle is completed.

4. Assembly relationship and precautions between shaft I and shaft II

(1) The two sets of bearings on Shaft I are installed back-to-back.

The assembly requirements for sliding bearings are mainly to achieve the necessary clearance and good contact between the journal and bearing hole of Shaft I. This allows for smooth operation of Shaft I within the bearing.

(2) The installation and axial preloading method for the two sets of bearings on Shaft II are the same as those for Shaft I.

It should be noted that Shaft II is a hollow shaft and that the fit between the bearing and the hollow shaft should be tight. This is necessary to prevent the shaft from contracting and loosening the fit.

(3) Eliminating the axial clearance of Shaft I and Shaft II:

Shaft I is equipped with a shaft sleeve with grooves, and the assembly of the hollow shaft and shaft sleeve is shown in Fig. 7.

Shaft II is designed with a flange that matches the groove of the shaft sleeve (see Fig. 8).

During assembly, the axial movement of Shaft I can be eliminated by tightening the hexagon socket head cap screw based on the hollow shaft and gasket.

Similarly, based on the outer shaft shoulder and bearing inner ring of the hollow shaft, the axial movement of Shaft II can be eliminated by tightening the hollow adjusting screw and indirectly tightening the shaft sleeve.

Fig. 7 assembly of hollow shaft and shaft sleeve

Fig. 8 flange and groove matched with it

(4) Precautions for on-site installation and commissioning of a motorized spindle:

Before starting the motorized spindle, monitor the working conditions of the cooling system, lubrication system, and compressed air supply.

Once the spindle is running normally, start it again and observe the change in load current.

At the beginning, keep the spindle speed below 3000r/min and let it run for 10-20 minutes before slowly accelerating it to 5000r/min for trial processing.

Do not exceed the rated speed of the electric spindle. Ensure that the spindle starts, runs, accelerates, and decelerates slowly and repeatedly.

Furthermore, the motorized spindle contains three sets of circularly-working cooling water pipes, oil pipes, and gas pipes. These are necessary to ensure the reliable operation of the high-speed motorized spindle.

5. Conclusion

The special electric spindle for CNC broach grinder has a unique structure, consisting of one rotor and two shafts that should rotate simultaneously.

It is essential that the hexagon socket head cap screw used to adjust the clearance between the two shafts is not loosened. If the screw becomes loose, the bearing will not be pre-tightened, which will affect the radial circular runout and axial movement of the main shaft.

If the radial circle runout is too large and the gap between the main shaft rotor and the stator coil is too small (0.25~0.5mm), the rotor and stator are prone to scratching, and the current becomes unstable, causing the coil or frequency converter to burn out.

In one instance, when the frequency converter was outsourced for repair, it was discovered that the parameter setting range was too large, leading to excessive current parameters of the frequency converter and burning out the coil.

Numerical control equipment equipped with high-speed motorized spindle plays a vital role in production. Each piece of equipment is a valuable textbook for maintenance personnel. Only by mastering the structure, functions, and working principles of various components can maintenance personnel accurately, thoroughly, and quickly maintain the equipment.

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