Machine tools vibration fault diagnosis.

Parpala, Radu Constantin

1. INTRODUCTION

Vibration in the environment or in industry is caused by particular processes where dynamic forces excite structures. On machines tools, the effect may be wear, reduced performance, faulty operation or any degree of irreversible damage of the machine or the work piece (Gupta, 1997). A vibration signature taken from an appropriate location in a machine can reveal the presence of the following machine defects: Imbalance, misalignment, imperfect foundation, mechanical looseness, rubs, antifriction bearing defects, faults in belt drive, faults in gears, sleeve bearing looseness, oil whirl, blades/vanes defects, local resonances, etc.

2. HARDWARE SET-UP

Data acquisition hardware usual includes three main elements: transducers, signal conditioning and data acquisition board.

2.1 Transducers

Transducers change physical phenomena into electrical signals. The transducer used for this analysis is a Bruel & Kjaer Type 4506 Accelerometer (see Fig. 1)

This transducer is built around a common seismic mass (6). This uni-mass design results in a very compact triaxial accelerometer where all the axes have the same point of reference.

The design also ensures accurate and consistent measurements, even when the accelerometer is exposed to complex vibration patterns. The seismic mass is surrounded by a piezoelectric ring (5) which is surrounded by four individually suspended, curved plates (4). Because of the suspension pins (3), different sections are exposed to shear forces for different directions of acceleration.

By appropriate summation of the signals, the outputs for the X, Y and Z axes are obtained. The assembly is clamped together by the outer ring (7). The preamplifiers (2), suspension pins (3) and Microtech-compatible connector (1) constitute an integral part which is hermetically welded to the titanium housing (8).

Type 4506 is internally insulated from the housing. The risk of ground loops, which can be particularly troublesome in multichannel measurements, is therefore reduced considerably

[FIGURE 1 OMITTED]

2.2 Signal conditioning

The electrical signals generated by the transducers must be optimized for the input range of the DAQ board. Signal conditioning accessories can amplify low-level signals, and then isolate and filter them for more accurate measurements. In addition, some transducers require voltage or current excitation to generate a voltage output.

The most common type of conditioning is amplification. Low-level signals, for example, should be amplified to increase the resolution and reduce noise. For the highest possible accuracy, the signal should be amplified so that the maximum voltage range of the conditioned signal equals the maximum input range of the analog-to-digital converter.

2.3 Data aquisition software

Software transforms the PC and DAQ hardware into a complete DAQ, analysis, and display system. The increasing sophistication of DAQ hardware, computers, and software increases the importance and value of good driver software. Properly developed driver software delivers an optimal combination of flexibility and performance, while also significantly reducing the time required to develop DAQ application. Advanced programing knowledge are needed for using driver software for simple aplications a more adequate solution is using application software. Application software adds analysis and presentation capabilities to the driver software (Fig. 2).

3. MACHINE FAULT DIAGNOSIS

Any malfunction in the operation of a machine element ghive rise to an increase in vibration level. Vibration emanating from a component consists of certain frequencies depending upon its nature of operation. This frequency information does not get changed or lost during transmission of vibration, however, their vibration level may be attenuated.

[TABLE 2 OMITTED]

Because in machine tools diagnosis we usually find a huge range of dominant vibrations it is important to place the accelerometer as close as possible to the test element the easiest way to identify a dominant vibration is to isolate that vibration (Fig. 2)

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

When frequency analyzing machine vibrations, we normally find a number of prominent periodic frequency components which are directly related to the fundamental movements of various parts of the machine. With frequency analysis we are therefore able to track down the source of undesirable vibration (Tranter, 1989)

The diagnostic chart in the Tab 1 & Tab 2 will help isolate the cause of excess vibration when the offending frequencies can be discovered through frequency analysis (Tandon & Choudhury 1999).

4. CONCLUSIONS

A fundamental requirement in all vibration work is the ability to obtain an accurate description of the vibration by measurement and analysis. It is important to identify the faulty element by correlating his dominant frequency with its rotary speed (Fig. 3).

5. REFERENCES

Gupta, K.N.(1997) Vibration--A tool for machine diagnostics and condition monitoring, Sadhana, pp. 393-410. India

Gupta, K.N.(1990) Vibration monitoring--state-of-the-art and future trends. Proceedings of State-of-Art and Future Vision Seminar on Condition Monitoring, New Delhi, pp 1I/1-16

Labview (2000), LabView Data Aquisition Basics Manual, National Instruments Corporation

Tandon, N. & Choudhury, A. (1999) A review of the vibration and acoustic measurement methods for detection of defects in rolling element bearings. Tribology International, 32(8), pp.469-480.

Tranter, J. (1989) The fundamentals of, and the application of computers to condition monitoring and predictive maintenance. Proceedings of. International. Congress on Condition Monitoring and Diagnostic Engineering (COMADEM 1989), pp 372-377

*** (2006) https://ctconline.com/ pdf/pubTechPapers/21Industrial%20Vibration%20Analysis.pdf --Connection Technology Center, Accesed on: 2009-08-10

Tab. 1. Fault identification chart

 Frequency of
 dominant vibration
 Fault Hz = rpm/60 Direction

Rotating members out 1 x RPM Radial
 of balance

 Misalignment & Usually 1 x rpm Radial
 Bent Shaft Often 2 x rpm &
 Sometimes Axial
 3 & 4 x rpm

 Damaged Rolling Depend on affected Radial
 Element Bearings component (Table 2) &
(Ball, Roller, etc.) Axial

 Journal Bearings Sub-harmonics of Primarily
 Loose in Housings shaft rpm, exactly Radial
 1/2 or 1/3 x rpm

 Oil Film Whirl or Slightly less than Primarily
 Whip in Journal half shaft speed Radial
 Bearings (42% to 48%) (high speed)

 Damaged or worn Tooth meshing Radial
 Gears frequencies (shaft &
 rpm Axial
 x number of teeth)
 and harmonics

 Mechanical 2 x rpm
 Looseness

 Faulty Belt Drive 1, 2. 3 & 4 x rpm Radial
 of belt