Diagnostics of rotors supported in sliding bearings 1

Transcription

1 Diagnostics of rotors supported in sliding bearings 1 Diagnostics of rotors with sliding bearings Growing operation speed of machines increases demands on vibration monitoring. For vibro-diagnostics of rotors with sliding bearings it is necessary to use so called relative sensors, i.e. sensors measuring movement of rotor relative to bearings. In contrast to rolling bearings, which have practically no damping, oil film of sliding bearings significantly damp rotor vibrations. Transfer of vibrations to bearing support plates or machine casing is thus very limited and through measuring vibrations by absolute sensors at machine casing it is often impossible to obtain any relevant data. There are known cases of bearing lining fatigue damage due to unacceptable journal excursions, without recording substantially increased vibration level monitored by absolute sensors. It is therefore necessary to observe rotor excursions by relative sensors located in vicinity of both journal bearings. Big rotating machines and machine units (steam, gas and water turbines with generator, compressors with the motor and gearbox) are almost generally operated with permanent monitoring of relative rotor excursions. Smaller machines (turbochargers, fans, high-speed blowers) should have relative rotor vibrations measured at least at test operation. Instability of outer oil film is encountered in some turbochargers, which in most cases use rotating floating rings. Although vibration amplitudes of the rotor achieve practically the whole bearing clearance, measurement with absolute sensor at machine casing do not indicate high vibration level. Due to wear the bearing clearance can change in operation to such extent, that instability is evoked, even if the rotor operated stably at the beginning. As instability is always indicated by marked non-synchronous component, it would be purposeful to observe relative vibrations of rotors with rotating bushings at least by one sensor. In order to ease passage through bending critical speeds, high-speed rotors with rolling bearing often use support of outer rings in compliant elements, which are characterized by certain material damping. The situation is then similar to that with sliding bearings and for vibration monitoring of the rotor is necessary to use relative sensors. Types of relative sensors. From working principle point of view can be relative sensors divided into these groups: 1) inductive 2) capacitive 3) optic (laser) 4) eddy current Inductive sensors: Inductive sensors, which were supplied mostly by the firm Hottinger, were very sensitive to changes of temperature and inhomogeneities of measured subjects. They are suitable to measurement of speed or phase angle, e.g. in balancing. Inductive sensors Hottinger in differential circuit were used to measure excursions of test bearing relative to the shaft and frame at the test stand for identification of sliding bearings dynamic properties in early 80ies [1]. The use of the same Hottinger apparatus also for measurement of harmonic excitation force had advantage in elimination of phase shifts, which would with the most probability occur between channels of different design. Utilization of Hottinger sensors for measurement of dynamic phenomena was on the whole successful and enabled to verify accuracy of calculated stiffness and damping coefficients of journal bearings. However, measurement of static movements, which should determine journal position in the bearing in dependence on load, was due to temperature drift quite unsuccessful.

2 Diagnostics of rotors supported in sliding bearings 2 Capacitive sensors: Main advantage of capacitive sensors is the possibility of their use in very demanding conditions (high temperatures and pressures) and their insensitivity to material properties of measured object. Nowadays they are not much used for common measurement because of temperature drift occurring when using simple sensors consisting of electrode isolated from machine casing. However, capacitive sensors can be used for dynamic measurement with lower demands to accuracy. For measurement of small excursions they can have very low dimensions and that is why they were used in small machines. In 70ies and 80ies capacitive sensors were successfully used to measure relative excursions of helium expansion turbine rotors at speeds up to rpm [2]. Sensors consisting of electrode 2 mm in diameter were located at both rotor ends in vicinity of aerodynamic journal bearings. Sensors wee connected to apparatus DISA 51E05, which secured linearization and amplification of the signal. Output of DISA apparatus was led to two-channel oscilloscope, enabling display of the signal in time domain and approximate reading of vibration amplitude. Sensitivity of sensors was established in the state without rotation by calibration with the help of dial indicator. Utilization of capacitive sensors enabled to operate expansion turbines at far higher speeds than in the past without danger of bearing damage due to excessive vibrations. Measured data were used as the resource of methodology of the 1 st rotor bending mode correction [2], which also utilizes measurement of relative vibrations at rotor both ends. Capacitive sensor was also used for measuring of the dental drill with operating speed in the range from to rpm.. Sensor about 1 mm in diameter was located opposite to dental drill stem 2,5 mm in diameter and resulting vibration signal was led to oscilloscope and frequency counter. At present sensors working on capacitive principle are supplied by the firm Micro-epsilon. The author has no personal experience with these sensors, but it can be assumed, that information about temperature drift is not valid for new generation of sensors. Much better properties of sensors are compensated by somewhat bigger dimensions (the smallest sensor has 6 mm in diameter) and by higher price too. Optical sensors: Optical sensors use in most cases laser principle with reflection of the ray from measured object. The advantage of these sensors is high resolution with relatively big measuring range. Among disadvantages one can name relatively big dimensions, dependence on quality of measured object surface and high price. Optical sensors cannot be used in dirty environment or where the measured object is not visible. In vicinity of hydrodynamic bearings, where oil mist is present, the contamination cannot be avoided. Suitable subjects for usage of optical sensors are therefore the rotors supported in aerodynamic bearings, where no danger of contamination exists, if at least part of the rotor, necessary for measurement, is accessible. In small machines it can be a problem to fasten relatively big sensor to the casing. Sensors working on eddy current principle: This type of relative sensors is with regard to their generally favourable properties the most widespread. Among advantages are acceptable price, sufficient measuring range and resistance to disturbances (electric noise). Excellent resistance to electric disturbances have namely sensors with the oscillator integrated into sensor body, e.g. types IN-081 and IN-085 (Fig. 1) of the firm Brüel Kjær Vibro. These sensors have with active part diameter of 8,5 mm measuring range of 1,5 mm. For their use it is sufficient to have DC supply of 18 to 30 V, because with standard sensitivity of 8 mv/µm they can be connected directly to measuring card input. Sensors have M10x1 thread, length of sensor body and connecting cable are optional.

3 Diagnostics of rotors supported in sliding bearings 3 Fig. 1 Sensors IN-081and IN-085 Brüel Kjær Vibro When measuring vibration of high-frequency electric motor [4] with IN-081 sensors located in close vicinity of winding, the measured signal were not practically disturbed. This property excelled namely in comparison with sensors, which had standard oscillators located outside sensor body. Sensors of the firm Micro-epsilon are suitable for smaller machines; dimensions of sensors (see Fig. 2) enable installation even into very small spaces (Fig. 3). Fig. 2 Dimensions of Micro-epsilon sensors working on eddy current principle The smallest Micro-epsilon sensor EU05 has active diameter of 2,5 mm, thread M3x0,35 and length of 17 mm. Measuring range of the sensor is 0,5 mm and standard sensitivity is 20mV/µm. An example of installation of 4 sensors ES04 for vibration measurement pro of the rotor and floating ring at both journal bearings of the turbocharger is shown in Fig. 3 [5]. Although the sensors were mounted in very cramped space, where it was not possible to observe all recommendations of manufacturer concerning free room around the sensor, and they were exposed to very demanding conditions (high temperature, presence of oil and oil mist), the measured results were very well reproducible and they brought a great number of information about the rotor and floating ring behavior at speeds up to rpm. In turbochargers used in motors of passenger vehicles, which has still substantially smaller dimensions (shaft diameter 6 mm, bearing span 20 mm), we succeeded in installation of two sensors ES04 in location between journal bearings. Vibration of the rotor in two directions were scanned in case of rotating floating ring support, while with stalled bushing (common for both journal bearings) movement the rotor as well as the bushing were traced. With this arrangement it was possible to follow relative vibration up to speeds exceeding rpm.

4 Diagnostics of rotors supported in sliding bearings 4 Fig. 3 Example of Micro-epsilon ES04 sensors installation into turbocharger Relative vibration measurement with up to now highest speed was realized in helium expansion turbines with the speed exceeding rpm. Signal from sensors 5 and 6, located at the ends of rotor in balancing facility (Fig. 4), serves also as a basis for correction of the 1 st bending mode of the rotor. Fig. 4 Facility for in-situ balancing of helium expansion turbine rotors Note: Sensitivity of sensors to inhomogeneity of measured object material can be a problem, when high precision of measurement is required. In case, that excursions of measured object are very small, they can be quite overlaid by defective signal invoked by the passage of material with different electric properties around the circumference of measured surface below sensor [6]. That is why it is necessary to carry out in vibro-diagnostic systems correction of so called run-out, i.e. to subtract from the real vibration signal the one measured with slow roll of the rotor.

5 Diagnostics of rotors supported in sliding bearings 5 Conclusions For diagnostics of high-speed machines with sliding bearings it is necessary to use relative sensors, which follow deviations of the rotor relative to bearings. The same holds for rotors with rolling bearings with outer rings supported in damping elements. In both cases the transfer of vibrations to machine casing is very limited and absolute sensors, fastened to the casing, cannot in most cases record onset of vibrations dangerous for rotor operation. It does not mean, that operation of machines with sliding bearings cannot be monitored by absolute sensors - accelerometers. However, data from these sensors can serve more as a supplemental information, eventually only as a means for shutting off the machine in case of substantially increased vibration level. Ideal solution for machine vibration monitoring are two relative sensors located 90 one from the other at each bearing, because in this way not only vibrations in time domain, but also trajectory of rotor movement can be displayed. In the least favourable case only one sensor on the rotor can be used for cost reduction or due to lack of space. Sensor located at one of the bearings should record significant increase of vibrations, even if it occurs at reverse rotor side. The most suitable relative sensor type for the rotor vibro-diagnostics is apparently the sensor working on eddy current principle, the variant of which with the oscillator located inside sensor body is characterized by excellent resistance against external disturbances. New generation of capacitive sensors is suitable for precise laboratory measurement, because its function is not affected by material properties of measured object. References: [1] Šimek, J.: Experimental research of journal bearings 90 mm in diameter. Research report No. SVÚSS [2] Šimek, J. - Svoboda, R.: Design aerodynamic support and dynamic analysis of helium expansion turbine rotor Research report No.. SVÚSS [3] Šimek, J. - Korec, L.: Development of expansion turbine rotors balancing technology Technical report TECHLAB No [4] Šimek, J.: Results of tests of industrial turboblower aerodynamic bearing support Technical report TECHLAB No [5] Šimek, J.: Measurement of turbocharger rotor relative vibrations Technical report TECHLAB No