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

The motorized spindle on high-speed CNC machine tools has the advantages of compact structure, light weight, small inertia, complete functions and good performance.

Its disadvantages are difficult to manufacture and maintain and high cost.

Taking the 6-axis CNC broach grinder CORVUS1700 CNC6 motorized spindle as an example, this paper introduces its main structure, analyzes the cause of the fault according to the structure, and finally eliminates the fault.

1. Preface

When the spindle of the special electric spindle of the NC broach grinder stops running, the equipment alarms.

Cause analysis: the shaft of the main shaft rotor has an axial movement of 0.5mm, which makes the main shaft rotor run unbalanced, resulting in sharp current increase, large load, fast temperature rise and abnormal noise.

Contact the manufacturer, who will not provide technical support temporarily, and need to purchase new spare parts. The cycle is 16 weeks, and the cost is 300000 yuan.

In order to break the monopoly of foreign technology and shorten the repair cycle and procurement costs, a technical team was established to independently formulate an improved repair plan without maintenance information.

The technical difficulties faced by the independent improvement are: the balance control measures for the high-speed operation of the spindle (processing speed more than 18000r/min) have no reference basis;

There is no technical support for the pre tightening torque value and method of the shaft;

The assembly and debugging scheme of special motorized spindle and special adjustment tools are missing;

The machining drawing of the damaged parts and the materials of the parts have no reference basis.

Therefore, we try to decompose the motorized spindle and analyze and study its internal structure.

Focus on analyzing the mechanical structure of the motorized spindle, analyze the causes of the failure, master the assembly and adjustment methods, and restore the processing function of the equipment.

2. Electric spindle structure

(1) Appearance of electric spindle

The electric spindle (see Fig. 1) is a machine tool part that combines the spindle and the motor. After the spindle part of the spindle part is properly extended, the motor rotor is directly installed in the extension section to make the motor coaxial with the spindle, and the stator package is installed outside the rotor to form a coaxial motor.

When the motor works, it directly drives the spindle.

electric spindle

Fig. 1 electric spindle

The motor is installed coaxially with the main shaft, with compact overall structure, good rigidity, high transmission efficiency and rotation accuracy, and can quickly realize the speed adjustment between zero and tens of thousands of revolutions.

Motorized spindle is a set of spindle components, rather than an isolated spindle, including motor stator coil, rotor, high-speed bearing, bearing lubrication device and spindle cooling device.

Motorized spindle is a device that integrates 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 shown in Fig. 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 is composed of shaft I and shaft II (see Fig. 3).

Shaft I (see Fig. 4) includes 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.

Two sets of angular contact ball bearings are installed back-to-back.

The shaft sleeve can ensure the synchronous rotation of the two shafts.

In addition, it can play a role of safety protection for the two shafts, so that the accuracy of the main shaft does not decline.

There are two sets of sliding bearings, supporting shaft I, which can reduce the radial circular jump of shaft I.

When the grinding wheel spindle collides, a small amount of deformation occurs instantly, and the disc spring can effectively protect the spindle.

Electric Spindle of CNC Grinding Machine: Typical Fault Analysis and Troubleshooting 1

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

The hollow shaft is made of silicon steel with low hardness;

The gasket material has high hardness.

In the long-term use process, the electric spindle rotates at high speed, and the mechanical vibration loosens the hexagon socket head cap screw, resulting in the increase of the axial movement between shaft I and shaft II, resulting in the asynchronous operation of the hollow shaft and the gasket, which intensifies the wear at the inner shoulder of the hollow shaft.

3. Typical faults and handling methods of motorized spindle

The typical fault phenomenon of the special electric spindle of NC broach grinder is that the electric spindle stops running and the equipment alarms.

3.1 Fault inspection and analysis

(1) Fault check

In the process of checking the non rotation fault of the main shaft, electrical aspects:

It is measured that the coil of the motorized spindle is burnt out, and the coil is wound and repaired by an outsourcing unit, which can meet its performance requirements.

Mechanical aspects:

By disassembling the motorized spindle, it is found that its rotor shaft is composed of shaft I and shaft II.

The fault points are as follows:

① The bearing is not preloaded.

② The hexagon socket head cap screw at the end of shaft I is loose.

③ The inner wall of hollow shaft is worn.

④ The shaft sleeve is broken.

⑤ The adjusting nut is grind dead.

(2) Fault analysis

The main shaft does not rotate, and the hexagon socket head cap screw in the rotor shaft is loose. In the process of high-speed rotation of the main shaft, the operation of shaft II and the gasket is not synchronous, and the gasket will rotate inside shaft II.

The material of the gasket is different from that of shaft II, and the hardness of the gasket is higher than that of shaft II.

The main shaft often works like this, resulting in the wear of the shoulder in shaft II, resulting in the shortening of the distance between the shoulder in shaft II and the end face of the shaft sleeve, but the distance between the sleeve on shaft I and the left end face of the gasket remains unchanged, which will lead to the phenomenon of “shaft lengthening”, resulting in the overall failure of the rotor shaft to pre tighten, and the axial movement of 0.5mm;

At the same time, the coaxiality of shaft I and shaft II is out of tolerance.

The shaft I is closely matched with the shaft sleeve, and the fracture of the shaft sleeve makes the shaft sleeve rotate around the shaft I.

In this way, when the main shaft rotates at high speed, the operation of shaft I and shaft II will be out of sync, resulting in excessive load of the electric spindle and increased current.

If the electric spindle works like this for a long time, the coil will be burnt out, the electrical components will be aged, and the service life will be shortened.

3.2 Troubleshooting methods

Take the following troubleshooting methods.

1) Pre tighten the bearings at both ends of the spindle.

2) Assemble the shaft sleeve on shaft I (see Fig. 5) and heat install it on shaft I.

3) Prepare the gasket on shaft II, increase its thickness by 5mm, and mill the groove near the sliding bearing end, with the groove depth of 2mm (the groove depth is 0.5mm, 1mm, 1.5mm and 2mm respectively during trial assembly).

Make the adjusting nut on shaft II (see Figure 6).

4) Explore the assembly method of special motorized spindle and make special adjustment tools.

Electric Spindle of CNC Grinding Machine: Typical Fault Analysis and Troubleshooting 2

Fig. 5 Shaft sleeve parts drawing

Electric Spindle of CNC Grinding Machine: Typical Fault Analysis and Troubleshooting 3

Fig. 6 adjusting nut

3.3 Installing the power distribution spindle

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

Based on the hollow shaft and gasket (see Fig. 2), tighten the hexagon socket head cap screw to make shaft I 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 to move in the opposite direction, and use the height difference between the inner and outer rings to eliminate the bearing clearance of shaft I to realize the axial preload of shaft I.

Based on 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 (matched with the adjusting nut), which can eliminate the axial movement of shaft II and realize the axial pre tightening of the bearing on shaft II at the same time.

Shaft I is equipped with a shaft sleeve with a groove, and shaft II is designed with a boss matching the groove of the shaft sleeve.

During assembly, the hollow shaft is connected with the shaft sleeve, the inner circle of the gasket is matched with shaft I, and the outer circle of the gasket is 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.

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

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 (matched with the adjusting nut) and indirectly tightening the shaft sleeve.

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 of sliding bearings are mainly to obtain the required clearance and good contact between the journal and bearing hole of shaft I, so that shaft I can run smoothly in the bearing.

2) The installation and axial preloading method of the two sets of bearings on shaft II is the same as that of shaft I.

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

3) Eliminate the axial clearance of shaft I and shaft II.

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

The shaft II is designed with a flange matching 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.

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.

Electric Spindle of CNC Grinding Machine: Typical Fault Analysis and Troubleshooting 4

Fig. 7 assembly of hollow shaft and shaft sleeve

Electric Spindle of CNC Grinding Machine: Typical Fault Analysis and Troubleshooting 5

Fig. 8 flange and groove matched with it

4) Precautions for on-site installation and commissioning of motorized spindle.

Monitor the working conditions of cooling, lubrication and compressed air at the beginning of starting.

When it works normally, start the spindle again and observe the change of load current at the same time.

At the beginning, the spindle speed should be below 3000r/min, run for 10 ~ 20min, and then slowly accelerate to 5000r/min for trial processing.

The working speed of the electric spindle should not exceed the rated speed. Let the spindle start, run, accelerate and decelerate repeatedly, and be sure to accelerate and decelerate slowly.

In addition, there are three groups of cooling water pipes, oil pipes and gas pipes inside the motorized spindle, and the three groups of pipes work circularly to ensure the reliable operation of the high-speed motorized spindle.

5. Conclusion

The special electric spindle for CNC broach grinder has a special structure. It has one rotor and two shafts.

The two shafts should rotate at the same time.

The hexagon socket head cap screw that adjusts the clearance between the two shafts should not be loosened.

If the hexagon socket head cap screw is loosened, the bearing is not 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 scratch and the current is unstable, causing the coil or frequency converter to burn out.

When the frequency converter was outsourced for repair, it was found that the parameter setting range was too large, resulting in excessive current parameters of the frequency converter, and the coil had been burnt out.

Numerical control equipment equipped with high-speed motorized spindle plays an important role in production.

Each equipment is a valuable textbook for equipment maintenance personnel.

Only by mastering its structure, functions and working principles of various components, can we accurately, thoroughly and quickly maintain the equipment.

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