Process and device for detecting defects in moving parts having a central rotating shaft

An apparatus and method for detecting defects in moving parts having a central rotating shaft, in particular shaft bearings, with the aid of an acoustic-electric transducer converting the mechanical acoustic vibrations received into electric signals to be processed in a signal processing circuit to signals indicative of the status of the parts, the improvement wherein the acoustic-electric transducer is centrally disposed on one end of the shaft so that it rotates therewith and receives vibrations therefrom, and wherein the acoustic-electric transducer comprises a piezoelectric element, and an amplifier having its control terminal coupled to the pass filter.

The invention relates to a process for detecting defects in moving parts 
having a central rotating shaft, in particular shaft bearings, with the 
aid of an acoustic-electric transducer converting the mechanical acoustic 
vibrations received into electrical signals to be processed in a signal 
processing circuit into signals indicating the status of the parts, as 
well as to a device for practicing said process. 
Such a process and device are disclosed in applicant's co-pending 
Netherlands Patent Application 85 03294 now U.S. Pat. No. 4,768,380. The 
acoustic-electric transducer in that case is fixed to a stationary, 
non-moving part, for example the housing of a shaft bearing. In that known 
system, the signals from the transducer, after band filtering and 
amplifying, are passed to a multiplier to the second input whereby a fixed 
frequency is supplied by a frequency generator. The output of the 
multiplier is filtered through a low-pass filter, and the amplitude 
increment of the signal obtained is indicative of a defect in the bearing. 
Such a system is suitable for selective control, whereby the indicating 
signal obtained is referred to a number of standard references in a 
comparator circuit. For accurate absolute measurements, such a system is 
less appropriate, considering that, for example when measuring a ball 
bearing, dampings occur on the outside of the housing due to mutual 
contact of the moving parts found therein. In a ball bearing, for example, 
the points of contact of maximum stress are normally on the inner races of 
such a bearing. Vibrations due to defects occurring therein are damped by 
the respective transitions from inner race to balls, from balls to outer 
race, and from outer race to housing. 
One solution to this would be to fix the acoustic-electric transducer, not 
on the outside but on the inside, that is, on the rotating shaft. However, 
the problem then remains to convey the signals obtained to the outside 
without noise interference. 
According to the invention, it has now been found that it is possible to 
measure a shaft bearing, for example, with an acoustic-electric transducer 
directly on the rotating shaft, provided provision is made for the 
electric charges generated as a result of acoustic vibrations so that they 
are first converted into fluctuations of a direct current. 
For that purpose, the invention provides a process, as outlined in the 
introduction, characterized in that the acoustic-electric transducer is 
fixed centrally co-rotate at one end of the shaft and converts the 
vibrations received into fluctuations in the current from a source, which 
are then coupled out for further processing by way of a rotating electric 
contact. 
The invention further provides a device for practicing the process, 
characterized in that the acoustic-electric transducer consists of a 
piezoelectric element connected in a high-pass filter and coupled to the 
control gate of a current amplifier element, which is connected to a 
direct-current source by way of the rotating contact. 
In the invention, advantageous use may be made of modern rotational 
connectors, connecting rotating conductors to fixed conductors with the 
aid of mercury contacts. An example of a connector suitable for use with 
the invention is the so-called Mercotac.RTM. connector by Sillner 
Maschinenbau GmbH. According to the invention, the direct current is 
passed by way of the rotating contact to the co-rotating circuit of the 
acoustic-electric transducer, where the current is varied by the current 
amplifying element by way of the control gate in accordance with the 
signals received. 
Expediently, the current amplifying element in the case of the invention 
may be a field effect transistor, with its input and output connected to 
the source of current and its control gate to the piezoelectric element. 
With a view to signal processing, the alternating-current component of the 
current must be separated from the direct-current component, and for this 
purpose the invention further provides that the direct-current source be 
connected to the current amplifier by way of a resistance, and the voltage 
fluctuations across this resistance be capacitively coupled out for 
further processing into indicating signals.

FIG. 1, to an enlarged scale, shows a section of an embodiment of the 
acoustic-electric transducer according to the invention, generally 
designated by 1. This device is mounted on a projecting shaft 2 of a shaft 
bearing (not shown) the vibrations whereof are to be measured for timely 
identification of adverse signals due to incipient defects in the bearing. 
The transducer has a cylindrical housing 3 with a central recess 4 in its 
anterior end, intended to accept the end of the shaft 1, so that when the 
shaft rotates, the housing with the parts contained therein rotates with 
it. 
In the housing 3, a piezoelectric sensor element 5 is accommodated, being 
protected in front by insulation 6. Behind the piezoelectric element there 
is a hollow chamber 7 in which the electronic circuit of the transducer is 
accommodated. Behind this there is a connection socket 8 into which is 
plugged a rotating contact 9. This contact comprises a connector capable 
of establishing a conductive connection between a rotating part and a 
stationary part. A suitable example of such a connector is the so-called 
Mercotac.RTM. connector. 
FIG. 2 schematically shows an alternative embodiment in which the 
electro-acoustic transducer is placed, not on but in the shaft of a 
bearing or other rotating system to be measured. In this last embodiment, 
the shaft 2 is provided at its end with a central axial hole 10, and the 
transducer system, instead of a recess, has a projection 11 intended to be 
placed in the central axial hole 10. This embodiment is intended for 
shafts of larger diameter, the first embodiment being more suitable for 
shafts of small diameter. 
FIG. 3 indicates the electronic circuit used for such transducer systems. 
The piezoelectric sensor 5 is part of an RC high-pass filter, 
schematically indicated by a resistance 12 connected in parallel across 
the piezoelectric element 5. 
The output of the piezo element is coupled to the control gate of a field 
effect transistor 13, the input and output of which are connected by way 
of the rotating contact 9 to a supply voltage V.sub.o coupled to the test 
circuit by way of a resistance 14. So that the alternating-current 
component due to the signals from the piezoelectric element 9 can be 
uncoupled from the voltage connected by way of the resistance R, a 
condenser 15 is provided. 
The acoustic transducer described above operates as follows. If vibrations 
arise owing to rotation of the shaft 1 in the bearing, they are picked up 
by the piezoelectric element 5 and converted into electric pulses which by 
way of the control gate of the field effect transistor 13, cause the 
current passing through it from the source voltage V.sub.o to fluctuate. 
By way of the rotary coupling 9, having a very low resistance, this 
current is carried off across the resistance 14. If this resistance 14 has 
a value R, there will be a voltage V=R(i+.DELTA.i) across it, of which 
V=Ri is the direct-current component and V=R.DELTA.i is a fluctuating 
alternating-current component representative of the signals received by 
the piezoelectric element. This latter component 
V.about.=R.multidot..DELTA.i is capacitively coupled out by way of the 
condenser 15 and may then be further processed, for example for 
registration on an oscilloscope or signal recorder. 
With the device according to the invention, sensitive and accurate 
measurements are possible, with a minimum of noise interference. This is 
because, contrary to known systems, the measurements are not taken from a 
stationary part of a rotating system, but directly on the central rotating 
shaft of the system. In the case of a shaft bearing, as remarked earlier, 
the highest load is in the center of the bearing, and that is where 
incipient defect signals would be expected. For in measuring on the 
outside, a good deal of damping occurs from the inside out, but by 
measuring over the shaft in accordance with the invention, this damping is 
eliminated. Hence a sensitive and reliable measurement is obtained, fully 
representative of the status of the bearing to be measured. 
Characteristic curves of measurements performed on the shaft of a bearing 
with the device according to FIG. 1. These curves were recorded on a time 
recorder and represent voltage-time graphs. One curve, the first, was 
recorded with non-rotating shaft. Two others, a second and third curve, 
were recorded with shaft rotating. 
In the first curve, of course, only very little noise response is 
perceptible. The second curve was a recording at a speed of 1600 
revolutions per minute, and here again there is only very little noise, 
indicating that the bearing is in good condition. 
The third curve was a recording taken at a shaft speed of 2000 rpm, 
likewise with a damaged bearing cage. In this case there is a quite 
appreciable noise amplitude, which is easily interpreted. With such a 
device, then, it is possible in a convenient and simple manner to monitor, 
control or measure up a shaft bearing or other rotating system by way of 
its shaft. An essential feature of the invention is that the measurements 
always be taken during rotation, centrally on the rotating shaft, and 
co-rotating, that the signals received in the rotating part of the 
rotating system be already converted into current variations of a direct 
current, capable of being conveyed without loss or noise to an outside 
stationary system by way of modern rotating contacts, for example mercury 
connectors such as Mercotac.RTM. connectors in order to be processed into 
indicating signals. 
Although the invention has been described above in terms of a single 
embodiment with a variation, it will be apparent that changes and 
modifications are possible. Thus for example the output signals may be 
compared with alarm thresholds, in order to provide for alarm signals, 
possibly signals to stop a rotary drive system in order to avoid an 
emergency, etc. These and other modifications of the device according to 
the invention will be clear to those skilled in the art from the above.