MEANS FOR DETECTING FAULTS OR DEFECTS IN MOVING MACHINE PARTS

Abstract: Known are systems or devices wherein the transducer comprises a resonance-frequency-range of 30-40 KHz. Such a system is therefore sensitive for disturbances generated by surrounded elements appearing as 2nd and 3rd harmonics. To overcome these disadvantages the invention introduces a system wherein the transducer (3) mounted on a rolling bear­ ing is connected to an amplifier (4) which first output is con­ nected to a multiplier (5) and its second output connected to an oscillator (6) having a frequency of 455 KHz. The output of oscillator (6) is connected to a low-pass filter (7) of 3 KHz. Consequently the output signals of filter (7) corresponds with a frequency-range of 452-458 KHz. Due to this arrangement 2nd and 3rd harmonic vibrations generated by e.g. running parts outside the bearing cannot appear as disturbances.

Description: The invention relates to means for detecting faults or defects in rotating machine parts comprising a transducer that converts the mechanical vibrations produced by a fault or defect into an electrical signal, and a processing circuit that as a function of this signal emits an output signal which indicates the condition of the machine part. Such a means or detection system is disclosed e.g. in the British patent application GB-A-2104658. In the known systems use is made of the resonance peak of the transducer to improve the selectivity and amplification. In practice, however, the problem arises that when the machine is measured by mechanical vibrations the resonance frequents and the second and third harmonics thereof are also measured. Thus the mechanical amplification of given frequency bands in a machine part on which the measurement is being made may be greater than the mechanical amplification of the transducer. The known means or system is accordingly sensitive to disturbance signals owing to which reliable detection is not really possible. The object of the invention is to procure a means or system of the kind mentioned before wherein these disadvantages are avoided in a simple but nevertheless effective way. Thereto, the means pursuant to the invention is characterised in that the transducer is designed as an acoustic-emission transducer with great width and the processing circuit selects a frequency range in the transducer signal where the half wave-length is smaller than the cross section of the transducer, said processing circuit is provided with a multiplier to the inputs of which are connected an oscillator with a frequency of at least 100 kHz and a band-pass filter with a central frequency corresponding to the oscillation frequency, while the transducer is connected with the input of the band-pass filter and a low-pass filter is connected to the output of the multiplier, whereby the output signals of said low-pass filter are supplied to comparators which outputs are processed in counters, of which the clock inputs are connected to an oscillator unit, the positions of said counters are added to an addition member to which a time clock is connected, the total value of said member is divided by a member which value is converted by a converter and controlled by a monitoring member to which an alarm member is connected. In this way a means is obtained wherein the transducer may without any problems be mounted at any desired location on the machine on which a measurement must be made. In a practical size of the transducer, the frequency range will lie at a frequency at least twice as high as the frequency range of 50 kHz, so that hindrance from the resonance frequencies lying in this last-mentioned frequency range and the second and third harmonics thereof is no longer experienced. The invention is explained in detail below with the aid of the drawing, wherein an exemplified embodiment is schematically represented. A machine 1, indicated schematically, is provided with a bearing 2 which must be checked for faults or defects. For this purpose a transducer 3 with great band width, sensitive to acoustic emission, is mounted on the machine 1, which transducer converts the mechanical vibrations of the machine into an electrical signal. Mechanical vibrations are produced, for example, when a defect in the track of the inner or outer race is contacted by the balls of the bearing 2. The transducer 3 is connected to an amplifier 4 with a band-pass filter. The amplifier 4 is connected to a first input of a multiplier 5, the second input of which receives the output signal of an oscillator 6. The central frequency of the band-pass filter of the amplifier 4 corresponds to the oscillation frequency of the oscillator 6. A suitable commercially available oscillator has an oscillation frequency of 455 kHz. The output of the multiplier 5 is connected with a low-pass filter 7 having a band width of, for example, 3 kHz. The output signal of the low-pass filter 7 then corresponds to the frequency range of 452-458 kHz in the output signal of the transducer 3. The presence of a defect in the bearing 2 then appears in an amplitude increase in the output signal of the low-pass filter 7. The means described has the important advantage that a frequency range in a relatively high frequency is used, in which frequency range disturbing mechanical resonance frequencies of the bearing 2 and second and third harmonics thereof are no longer present. In addition, the wavelength at this frequency is approximately 12 mm, while the transducer 3 has a greater diameter, so that a maximum of a standing wave will always appear in the contact surface of the transducer 3. In the example represented of the means pursuant to the invention the output signal of the low-pass filter 7 is supplied to four comparators 8-11, which compare this output signal with a higher threshold value in each instance. The outputs of the comparators 8-11 are connected to the port input of associated counters 12-15, the clock inputs of which are connected to an oscillator unit 16. The oscillator unit 16 supplies four unlike frequencies 1, 2, 4 and 8 MHz respectively. The counter that receives the clock signal with the lowest frequency is connected by its port input to the comparator with the lowest threshold value, while the counter that receives the clock signal with the highest frequency is connected to the comparator with the highest threshold value. This ensures that the higher amplitudes in the output signal of the low-pass filter 7 are weighed more heavily than the lower amplitudes. The positions of the counters 12-15 that are reached after a fixed time interval are added up by an addition member 17. The time interval is determined by a time clock 18, the time interval being adjustable, for example, to 1, 2, 4 and 6 s. In order to obtain a uniform scale, the total value of the addition member 17 is divided by means of a division member 19 by the time interval selected. The value thus obtained may be converted by means of a digital/analog converter 20 into an analog voltage, which is controlled by means of a monitoring member 21 that prevents a given value from being exceeded. If an impermissible condition is established by the monitoring member 21, the latter emits an output signal to an alarm member 22. It is noted that the band-pass filter in the amplifier 4 is used for override of the multiplier 5 by avoiding low frequencies. The pass band of the filter may, for example, by 355-555 kHz. When samples are taken, very good test results have been obtained with the means described, where it was possible to establish defects in the bearing 2 down to a minimum size of 0.25 mm under a variety of circumstances. It is noted that the method and means described may be used not only for detecting faults or defects in a bearing, but alternatively, for example, for checking cutting tools for fracture or the like. The invention is therefore not limited to the example described above, which may be varied in different ways within the scope of the claims.

Claims: Means for detecting faults or defects in rotating machine parts comprising a transducer (3) that converts the mechanical vibrations produced by a fault or defect into an electrical signal, and a processing circuit (4-7) that as a function of this signal emits an output signal which indicates the condition of the machine part, characterized in that the transducer (3) is designed as an acoustic-emission transducer with great width and the processing circuit (4-7) selects a frequency range in the transducer signal where the half wavelength is smaller than the cross section of the transducer, said processing circuit is provided with a multiplier (5) to the inputs of which are connected an oscillator (6) with a frequency of at least 100 kHz and a band-pass filter (4) with a central frequency corresponding to the oscillation frequency, while the transducer (3) is connected with the input of the band-pass filter (4) and a low-pass filter (7) is connected to the output of the multiplier (5), whereby the output signals of said low-pass filter (7) are supplied to comparators (8-11) which outputs are processed in counters (12-15), of which the clock inputs are connected to an oscillator unit (16), the positions of said counters (12-15) are added to an addition member (17) to which a time clock (18) is connected, the total value of said member (17) is divided by a division member (19) which value is converted by a converter (20) and controlled by a monitoring member (21) to which an alarm member is connected. Means according to claim 1, characterized in that the frequency range selected lies above 100 kHz, approximately 452-458 kHz.

Inventors: SMULDERS, ADRIANUS JOSEPHUS, VERMEIREN, KAREL NATHALIS
Applicants: SKF INDUSTRIAL TRADING & DEVELOPMENT CO, B.V.
Application year: 1986
Main classification: G01H1
IPC codes: G01H17, G01M13, G07C3, G01H1