Monitoring Of Machine Tool Spindle Assembly Using Vibration Analysis

Vijay Rampal.S1 & Syath Abuthakeer.S2

PG Student, PSG College of technology, Coimbatore 641004, Tamil Nadu, India,rampalmech@gmail.com

Abstract

In order to increase productivity, enhance quality and reduce cost, machine tool have to work free of any failure. The idea of the research is to investigate the response of the spindle bearing system with and without defect. Hear an attempt has been made by modeling the turning centre spindle assembly using finite element software to find out the dynamic response of the system defective bearing, defective spindle and unbalance forces. Model analysis was carried out to define the systems natural characteristics which are the natural frequency and mode shape. Harmonic response analysis was carried out to determine the dynamic response due to unbalanced forces. Transient response analysis was carried out to determine the vibration level of the system at various nodes. The FEM results were compared with the experimental values. Thus the present work shows that the FEM is a valuable tool in finding the vibration for various defects.

Keywords: spindle, bearing, dynamic analysis, vibration, machine tool.

1.0 Introduction

Vibration signal measured contains the hart beat of the machine tool, which, if properly interpreted, can reveal the running condition. The resulting surface marking on the finished work piece are related to the amplitude and frequency content of the vibration present. In the present study, from the failure data, spindle bearing assembly has been identified as the critical subsystem. This project is dedicated to the experimental and FEM dynamic analysis of the spindle bearing assembly with self exited vibration.

1.1 Literature review

Mahboubkhah et al, highlights knowledge on natural frequency of a structure is required to avoid resonance, which leads to breakdown. Claudiu et al, self exited vibration domain is obtained through spectra of two accelerometers, one three axis accelerometer at the tool and the other accelerometer at the front bearing for displacement analysis of self exited vibration in turning. The cutting process of elastic machining system causes work piece tool displacement that causes vibration, so resulting in modified chip size which reflects the dynamic instability. Six component Dynamometer is used to measure the dynamic cutting force. The parameter which has most effect on the amplitude response is the depth of cut, so depth of cut was increased by every excrement and the vibrations were studied. The self exited frequency studied between 120-200 HZ. The tooltip displacement is ellipse and as the depth of cut increases the ellipse gets larger. Nobuaki Kamimoto et al, difference in vibration pattern across various position of the link causes vibration stresses which lead to material fatigue.

1

Mohammed et al. The study reveals, the larger the initial preload applied, the less vibration amplitudes are generated, and consequently less variation in the grinding depth of cut. As the initial preload increases, i.e., the stiffness of the bearing increases, the dominant frequencies of the system shift to higher values. As the preload increases up to a certain value, the peak to-peak amplitude decreases. Beyond this value the reduction in vibration amplitude is insignificant which indicates that larger values of preloading will not further reduce the vibration levels of the machine spindle. Therefore, this analysis can be used to calculate the optimum initial axial preload in order to obtain high accuracy for surface finish. The vibration levels of grinding machine spindle system increase significantly for grinding wheel wear rate percentage greater than 2, and decrease as the work piece material hardness decreases. S. Saravanan et al. The different component defects have their unique frequency, the amplitude denotes severity of the defect and the frequency reflects the source of the defect. Modern techniques for bearing diagnosis are directly applicable for machine tool. Gradual deterioration type of failure is suitable for machine condition monitoring, bearing failure is one example of the above mentioned failure type.

K N Guptha, illustrates how machine troubles/ failures are diagnosed with the help of vibration signature. Deterioration in the operation of a machine component gives rise to increasing in vibration level, mixing of vibration signals does not cause any loss of individual’s frequency information. Vibration signature taken from appropriate location in machine tool can reveal the following defects: imbalance, misalignment, imperfect foundation, rubs, bearing defects, fault in belt drive etc. Piezoelectric transducer is lighter and has better frequency range for application, so the accelerometers are the best choice. The measures of the time based vibration analysis includes rms measurement, peak level (amount of impulse and bearing defect detection), crest factor, shock pulse, kurtosis (statistical movement of the probability density function of the vibration signal, phase). Trend analysis exhibits the rate of deterioration of vibration level in machine tool. The vibration reference standard is VDI 2056. Frequency based vibration signal includes spectrum, waterfall plot and cepstrum.

M.A.Mannan and B.J. Stone. The tradeoff between preload, stiffness and bearing life is exhibited. As the preload is increased, stiffness increases and the life of bearing decreases and as the preload decreases, stiffness is reduced and the machine tool performance is deteriorated. Vibration measurement serves as an effective tool for setting the preload during the assembly processes. There is no need to have a theoretical model as the required information can be obtain experimentally so that the method would not depend on the accuracy of a model.

2. MODELING AND ANALYSIS

The ultimate aim of a finite element analysis is to recreate mathematically the behavior of an actual engineering system. The spindle assembly was developed using beam and spring

elements. Figure 1 shows the finite element model of the Spindle assembly of a turning centre.

2

Figure1 finite element model of the Spindle assembly

Table 2.1 Natural characteristics of the spindle assembly

Modal analysis is a process whereby

we describe a structure in terms of

its natural characteristics, which are

the frequency and mode shapes. It

can also serve as a starting point for

another, more detailed, dynamic

analysis, such as transient analysis,

harmonic response analysis, or

spectrum analysis. The natural

frequencies and mode shapes are

important parameters in the design

of a structure for dynamic loading

conditions. The understanding and

visualization of mode shapes is

invaluable. It helps to identify areas

of weakness in the design or areas

where improvement is needed. The

development of a modal model is

useful for simulation and design

studies. Six mode shapes and the

natural frequency of the lathe are

tabulated in table 1.

3

Mode Natural

Frequency (HZ)

Mode Shape

1 587.9

2 590.3

3 635.6

4 664.4

5 698.7

6 777.6

Harmonic response analysis was used to determine the response of the system due to

harmonic load. Due to the presence of the rotating members in the structure, there exist

unbalanced forces, which vary harmonically. These unbalanced forces given by various

elements are in the structure was calculated and applied in the structure at the appropriate

locations.

The unbalanced forces is given by

F=m× r ×❑2

Where m= Mass of the element Kg

r= Eccentricity mm

= Angular velocity rad/s

The unbalanced forces for the spindle is calculated as follows

Weight of the spindle =r2L* Density

Volume =1.3383×106 mm3 , Density= 7.8×10-6

Weight of the spindle=102.3964 N

Calculation of Angular Velocity()

Speed of the spindle = 1200 rpm

=1200/60=20 Hz

Therefore = 2×20

=125.66 rad/s

According to the standards, the balanced quality grade for machine tool spindle is G 6.3mm/s (vibration level V).

V =r

Eccentricity r =6.3/125.66

=0.05mm

Spindle unbalanced force = 8.241 N

Similarly, unbalanced forces due to front and rear bearings are 0.101 N and 0.0692 N.

Applying the unbalanced forces at respective points, the analysis was carried out considering

first ten modes and 3% damping. The response along horizontal and vertical direction was

determined at front and rear bearing housing are presented in table 2.

4

Table 2.2 Theoretical Vibration Velocities for Different Spindle Speeds

Sno

Spindle Speed RMS

Vibration Velocity at Front Bearing mm/s

Vibration Velocity at Rear Bearing mm/s

Horizontal

Vertical Horizontal Vertical

1 100 0.019 0.025 0.016 0.0132 150 0.023 0.029 0.020 0.0173 300 0.028 0.040 0.022 0.0224 500 0.038 0.052 0.035 0.0275 700 0.039 0.058 0.037 0.0296 1200 0.072 0.075 0.052 0.0507 1500 0.083 0.081 0.071 0.074

3.EXPERIMENTATION

Experiments were carried out on a cnc turning centre (GALAXY Midas O). The vibration

velocities was measured by using NI -PXI 1402Q and accelerometers. The accelerometers

ware fixed on the horizontal and vertical axis of the spindle at front and rear bearing housing.

Vibration velocity in mm/s at various spindle in non cutting condition are present in table 3. Lab

View 8.5 software was used to manipulate the extracted vibration signatures. The software

provide greater ease of use and faster performance for your data acquisition (DAQ) devices,

sound and vibration software provides a complete software solution for all audio, noise and

vibration, and machine condition monitoring applications. The LabVIEW Analysis VIs provide

additional LabVIEW functions for power spectra, frequency response functions (FRFs),

fractional-octave analysis, sound level measurements, order spectra, order maps, order

extraction, sensor calibration, human vibration filters, and torsional vibration.

Fig 3.1 Experimentation Setup

5

Figure 3.2 Power spectrum of the experimental model analysis

Table 3 Experimental Vibration Velocities for Different Spindle Speeds

SnoSpindle Speed RMS

Vibration Velocity at Front Bearing mm/s

Vibration Velocity at Rear Bearing mm/s

Horizontal Vertical Horizontal Vertical

1 100 0.024 0.02981 0.01846 0.01567

2 150 0.0251 0.0304 0.02231 0.01871

3 300 0.0304 0.0431 0.0243 0.02412

4 500 0.0401 0.0568 0.03874 0.02931

5 700 0.0405 0.0607 0.03875 0.0312

6 1200 0.07423 0.07716 0.05439 0.0521

7 1500 0.08712 0.08393 0.07968 0.0762

6

4. RESULT AND DISCUSSION

The results from the theoretical model analysis (Table2.1) and experimental model analysis

(Fig. 3.1) obtained were close, it was observed that theoretical values are less compared to the

experimental values. The results from the Structural analysis were shown with the experimental

results for various spindle speed in the table 2.2, it was absorbed that theoretical values

obtained were closely representing the vibration obtained from the experiments (table 3.2). It

was found that theoretical values were less than the experimental values, this may be due to un

balanced forces such as belt tension fluctuation, bearing defects and material defects which are

action during operation.

100300

7001500

0

0.02

0.04

0.06

0.08

0.1

vibration velocity of Front Bearing Horizantal direction

Experemental Vibration veloc-ity (mm/s)Theoretical vibra-tion velocity (mm/s)

Ampl

itude

RM

S

100300

7001500

00.010.020.030.040.050.060.070.080.09

vibration velocity of Front Bearing Virtical direction

Experemental Vibration veloc-ity (mm/s)Theoretical vi-bration velocity (mm/s)

Ampl

itude

RM

S

100300

7001500

00.010.020.030.040.050.060.070.080.09

vibration velocity of Rear Bearing Horizantal direction

Experemental Vibration veloc-ity (mm/s)Theoretical vibra-tion velocity (mm/s)

Ampl

itude

RM

S

100300

7001500

00.010.020.030.040.050.060.070.080.09

vibration velocity of Rear Bearing virtical direction

Experemental Vibration veloc-ity (mm/s)Theoretical vi-bration velocity (mm/s)

Ampl

itude

RM

S

7

5. MAJOR REFERENCE

Claudiu F. Bisu, Philippe Darnis, Alain Gérard and Jean-Yves K’nevez. (2008) Displacements analysis

of self-excited vibrations in turning. Int J Adv Manuf Technol.

K N GUPTA (1997). Vibration - A tool for machine diagnostics and condition Monitoring. Sadhana, Vol.

22, Part 3, pp. 393-410.

M. Mahboubkhah, M. J. Nategh and S. Esmaeilzadeh Khadem. (2008). A comprehensive study on the

free vibration of machine tools’ hexapod table. Int J Adv Manuf Technol.

M.A. Mannan and B.J. Stone (1998). The use of vibratio measurement for quality controle of machine

tool spindle. Int J Adv Manuf Technol 14: 889-893.

Mohammed A. Alfares, Abdallah A and Elsharkawy (2003). Effects of axial preloading of angular

contact ball bearings on the dynamics of a grinding machine spindle system. Journal of Materials

Processing Technology 48–59.

Nobuaki Kamimoto, Yutaka Yamada, Masami Kitamura and Kiyoshi Nishikawa (2005) Evaluation of

vibration in many positions by SOM. Artif Life Robotics 9:7–11.

S. Saravanan, G.S. Yadava and P.V. Rao. (2006) Condition monitoring studies on spindle bearing of a

lathe. Int J Adv Manuf Technol 28: 993-1005.

8