CHAPTER 6 FAULT DIAGNOSIS OF UNBALANCED CNC MACHINE SPINDLE USING VIBRATION SIGNATURES-A CASE STUDY

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1 81 CHAPTER 6 FAULT DIAGNOSIS OF UNBALANCED CNC MACHINE SPINDLE USING VIBRATION SIGNATURES-A CASE STUDY 6.1 INTRODUCTION For obtaining products of good quality in the manufacturing industry, it is absolutely essential to ensure reliability and accuracy of rotating machinery. This can be achieved by good balancing and keeping vibration level of machine spindle at minimum at higher speeds. Unbalanced rotating machinery results in the loss of production and excessive energy consumption. This study involves dynamic balancing of Computerized Numerical Control (CNC) machine spindle through vibration signatures as a main concept. It aims to reduce the unbalanced CNC machine spindle vibration to minimum possible level. 6.2 INSTRUMENTATION AND EXPERIMENTAL PROCEDURE Figure 6.1 shows the CNC machining center drive assembly and instrumentation. It consists of CNC machine drive spindle assembly and a dual channel vibration analyzer, the accelerometer for measuring vibration amplitude, and tacho sensor with laser light for measuring the phase shift of the unbalance mass. The signal is acquired by fitting the accelerometer

2 82 directly over the bearing housing. Accelerometer output is connected to one of the channels of the vibration analyzer and the other channel of the vibration analyzer is connected with the tacho sensor. Balancing mode is selected in the dual channel anayser for balancing. 400 number of spectral lines, maximum frequency of 60,000 cycles per minute (cpm) and Hanning window are used for measuring the vibration signals. 4 number of signal averaging is considered for better accuracy. Balancing signals are acquired to the computer through RS 232 interface. (a) (b) Figure 6.1 a) Block diagram of a CNC machine drive assembly (b) Experimental setup with instrumentation

3 VIBRATION SPECTRA OF THE CNC SPINDLE BEFORE BALANCING A tool room CNC machine used for producing components of textile machinery is considered for the present case study. The machine is operated at 2500 rpm and it runs two shifts per day. The main problem identified is that the components manufactured are less accurate in dimension and they are not in exact circular shape. Not only that, some unwanted noise is also observed. To trace the fault, the CNC machine spindle bearing vibrations are measured at the operating speed of 2500 rpm. Figure 6.2 displays the vibration spectra of the spindle and Table 6.1 lists ten peak amplitudes with their corresponding frequency of the vibration spectra. From Figure 6.2 the maximum vibration amplitude of mm/s is observed nearer to running frequency (1X) 2550 cpm. This indicates the unbalance of the CNC machine spindle. So, it is desirable to conduct the dynamic balancing test for the CNC machine spindle. Figure 6.2 Vibration spectrum of spindle before balancing

4 84 Table 6.1 Vibration amplitudes of ten peaks before balancing Before Balancing Sl. No Frequency, cpm RMS Velocity, mm/s STEPS BY STEP PROCEDURE FOR FAULT IDENTIFICATION Before carrying out the dynamic balancing it is necessary to conduct three- level tests, one by one, to identify the balancing speed at which the unbalance occur. The drive assembly of the CNC machine is divided into three levels and vibration test is conducted separately for each level Level -1: Motor and Its Pulley In level -1 test drive end motor and pulley alone (refer Figure 6.1) are considered for balancing and the accelerometer, vibration analyzer are used for measuring the vibration amplitude of the drive end assembly. The test is carried out at different speeds of the motor and the maximum vibration

5 85 amplitude is observed at 2500 rpm. Table 6.2 shows the vibration amplitudes of the level-1 test and it is evident that the vibration amplitudes are very small. So it is construed that the drive motor and its pulley are not contributing to the unbalance of the system. Table 6.2 Vibration amplitudes of spindle in level-1 test Speed, rpm NDE RMS Velocity, mm/s DE H V H V Level -2: Motor, Pulley Connected to the Spindle, Pulley and Flange In level -2, drive end motor and pulley, driven end pulley, flange and spindle are considered for the test. In this test, the vibrations are measured in terms of wide band value on both non-drive end (NDE) and drive end (DE) at different speeds both in the horizontal (H) and the vertical (V) direction and are listed in Table 6.3. From Table 6.3, it is noticed that at 2500 rpm (1X), vibration amplitude is very high in both the horizontal and vertical directions at both DE and NDE. So, level-2 test is contributing the unbalance at 2500 rpm when the system assembly with spindle, pulley and flange.

6 86 Table 6.3 Vibration amplitudes of spindle in level-2 test Speed, rpm NDE RMS Velocity, mm/s DE H V H V Level - 3: Motor, Pulley Connected to the Spindle, Pulley, Flange, Cylinder and Chuck Since the spindle has unbalanced pulley and flange with respect to motor pulley, it is decided to assemble the cylinder and the chuck on either side of the spindle for further vibration test. These vibrations are measured on both the NDE and the DE at different speeds by keeping the accelerometer in both horizontal and vertical directions. Table 6.4 conveys this information. Table 6.4 Vibration amplitudes of spindle in level -3 test Speed, rpm NDE RMS Velocity, mm/s DE H V H V

7 87 The measured frequency spectrum before dynamic balancing is shown in Figure 6.2 and it reveals clearly that the unbalance component corresponding to the rotational speed of 2550 is very high. Also, from Table 6.4 the maximum amplitude of 0.56 mm/s at 2500 rpm is noticed in level -3 balancing and this unbalance is responsible for the level-2 test. From the frequency spectrum shown in Figure 6.2, the contribution of vibration amplitude due to unbalance alone of mm/s is observed at 2550 rpm. This is very close to running frequency (1X). From level-2 and level-3 test, the maximum vibration amplitude is noticed at 2500 rpm. So it is decided to carry out the dynamic balancing at 2500 rpm. 6.5 DYNAMIC BALANCING OF CNC MACHINE SPINDLE Dynamic balancing is carried out to the whole of the drive assembly as follows: (i) (ii) The vibration signal analyzer is turned on the balancing mode which fixes the accelerometer in one channel and tacho sensor on the other channel. Other settings like sensor details and measurement range are also fixed in the analyzer. Run the spindle at a speed of 2500 cpm which will be held constant throughout the experiment. With the unbalanced rotor, record the vibration amplitudes and phases from both the channel of the analyzer. The recorded readings are given by RMS velocity mm/s at 54 (iii) A trial mass of 6.30 gram is chosen arbitrarily and attached with the spindle at some position. This position is considered as the reference point and the angle is considered as 0.

8 88 (iv) After adding the trial mass of 6.30 g, the balancing is carried out at 2500 rpm and check the vibration amplitude and the phase shift from the vibration analyzer. The vibration amplitude is now reduced and the new trial mass and angular position is noted. RMS velocity of mm/s at 68 and corrected mass showed by the analyzer is 9.36 g at (v) New mass of 9.36 g is added at +224 from the reference point in the opposite direction of the rotation. After the corrected mass is added at the correct location, the tests are repeated and note the vibration amplitude, corrected mass and its position. RMS velocity of mm/s at 38 and recommended corrected mass of 3.5 g at -48 are observed. (vi) This time the amplitude is reduced to mm/s. Again the new mass is added at -48 from the reference point in the direction of rotation. The same procedure is repeated until the minimum level of amplitude is achieved. Now the overall vibration values are brought down to mm/s from 0.56 mm/s. Here further balancing is stopped because the vibration amplitude obtained is reduced by 4.09 times. Figure 6.3 Vibration spectrum of combined assembly after balancing

9 89 Figure 6.3 shows the frequency spectrum after balancing and it reveals clearly the unbalance component corresponding to the rotational speed of 2550 rpm. This is being brought down to a minimum possible extent of mm/s as given in Table 6.5. Table 6.5 Vibration amplitudes of top ten peaks after balancing After Balancing Sl. No Frequency, cpm RMS Velocity, mm/s Table 6.6 shows the comparison of the vibration amplitude of the spindle before and after balancing. Table 6.6 Comparison of amplitudes before and after balancing LEVEL -3 NDE DE Before Balancing H mm/s H mm/s After Balancing H mm/s H mm/s

10 90 After doing dynamic balancing, the vibration amplitudes are measured again at different speeds at both NDE and DE, in horizontal and vertical position. After balancing the results are given in Table 6.7. After dynamic balancing, the reduced overall vibrations of the spindle result in reducing the noise levels, generated by the machine spindle. Table 6.7 Vibration amplitudes of spindle after balancing Speed, rpm NDE RMS Velocity, mm/s DE H V H V Figures 6.4 and 6.5 provide a comparison of the vibration amplitudes at NDE and DE respectively with various spindle speed before and after dynamic balancing. This clearly indicates that after balancing both vertical and horizontal direction the vibrations are drastically reduced by dynamic balancing. Hence the CNC machine spindle is balanced. After balancing the components machined from the CNC machine are tested for their accuracy. The test prevails that the dimensional accuracy and the geometrical accuracy have been maintained well. Rejection of components is not observed.

11 91 Figure 6.4 Comparison of vibration amplitude (NDE) (BB Before Balancing, AB- After Balancing) Figure 6.5 Comparison of vibration amplitude (DE) (BB Before Balancing, AB- After Balancing) 6.6 CONCLUDING REMARKS It has been proved that fault diagnosis of the CNC machine spindle through bearing vibration signatures leads to the following conclusion. (i) From both level-2 and 3 balancing high vibration amplitude has been observed at 2500 rpm. This is very close to 2550 rpm (1X). Before balancing, the maximum amplitude is 0.531

12 92 mm/s at 2550 rpm. Hence dynamic balancing has been carried out at 2500 rpm. (ii) Balancing of the CNC spindle is carried out step by step and the vibration amplitude is drastically reduced to minimum possible level. (iii) It is observed that the diagnosis of dynamic balancing reduce the maintenance work simple and this in turn makes it possible to obtain products of good quality.