An ultrasonic piezoelectric transducer and a method for measuring and/or monitoring the thickness of a wear member, in-situ, and the amount of wear that has occurred thereto, is disclosed. The transducer includes a sleeve which is received in a blind bore provided in the wear member, a piezoelectric element positioned within the blind bore, and an aligning spacer means interposed between the end of the sleeve and the piezoelectric element. By the application of appropriate voltage pulses to the piezoelectric element, the wall thickness between the bottom of the blind bore and the inner surface of the wear member can be measured.

TECHNICAL FIELD 
The present invention relates to a method for measuring and/or monitoring 
the thickness of a wear member and the amount of wear that has occurred 
thereto, and more particularly to an ultrasonic piezoelectric transducer 
for measuring and/or monitoring such thickness and wear. 
BACKGROUND ART 
Various approaches have been devised for detecting, monitoring and 
measuring the amount of wear which has occurred to a wear member. For 
example, in the area of rotating equipment, a number of electrical devices 
are available to detect and monitor bearing wear. These devices are based 
upon a number of detection techniques. Thus, wear detection might depend 
upon the completion of an electrical circuit through the bearing when 
there is excessive bearing wear, or it might depend upon the generation of 
a voltage if the shaft rotates eccentrically, or it might depend upon the 
detection of an abnormal temperature rise of the bearing. Each of these 
approaches has some inherent disadvantages with respect to accuracy and 
does not measure actual bearing wear or bearing wall thickness, i.e., each 
approach is responsive to bearing wear but does not measure quantitatively 
the amount of wear that has occurred or the wall thickness remaining. 
Other approaches have been devised to measure the thickness of a workpiece 
or wear member, and by measuring such thickness, the amount of wear which 
has occurred can be calculated. These approaches have numerous commercial 
and/or industrial applications, however, their use for measuring the wear 
of a work surface in-situ is cost prohibitive. In addition, these 
approaches typically utilize devices fabricated from materials which limit 
their applications to an operating environment having a temperature of 
normally less than 75.degree. C., and cause the resulting readings to be 
dependent upon the temperature of the operating environment. It has also 
been found that the materials utilized for these devices cannot withstand 
severe operating environments which further limits the applications in 
which they can be used. Thus, these devices and measurement techniques are 
not usable for measuring and/or monitoring the thickness of work surfaces, 
such as a sleeve bearing, in an elevated temperature operating environment 
such as might exist in rotating equipment. This inability to measure 
and/or monitor bearing wear in-situ can result in costly machine downtime 
to inspect the condition of the bearings. Alternatively, this inability 
can result in unnecessary damage to the rotating equipment due to bearing 
failure which was not promptly detected. 
Because of the foregoing, it has become desirable to develop a device which 
can be utilized to measure and/or monitor the thickness of and the amount 
of wear which has occurred to sleeve or thrust bearings, brake discs or 
pads, clutch plates and sealing members, in-situ. 
SUMMARY OF THE INVENTION 
The present invention provides an ultrasonic piezoelectric transducer that 
can be mounted within the wall of a wear member, such as a sleeve or 
thrust bearing, brake disc or pad, clutch plate or sealing device, so that 
measurements of wall thickness can be made in-situ. The transducer 
includes an outer sleeve which is threadedly received in a blind bore 
within the wear member, a piezoelectric element which is positioned within 
the blind bore, and spacer means interposed between the end of the outer 
sleeve and the piezoelectric element. The spacer means and the end of the 
outer sleeve have complementary configurations permitting the spacer means 
to align itself within the end of the outer sleeve and apply a 
substantially uniform compressive force to the piezoelectric element. The 
application of such a substantially uniform compressive force causes a 
firm, electro-acoustical contact to be formed between the piezoelectric 
element and the bottom of the blind bore which insures a highly accurate 
measurement of the wall thickness between the bottom of the blind bore and 
the inner surface of the wear member. For example, it has been found 
experimentally that this transducer can measure the wall thickness (0.100 
inch) of a bronze bearing at 300.degree. F. with a repeatability in the 
sub-micron range utilizing state-of-the-art electronics. The transducers 
can also be located in a pre-determined arrangement around the periphery 
of the wear member so that wear can be measured and/or monitored around 
the periphery thereof. 
In an alternate embodiment of the invention, a mounting ring is provided to 
position one or more transducers against the outer surface of the wear 
member. In this embodiment, the piezoelectric elements contact the outer 
surface of the wear member and the total thickness of the wear member is 
measured. 
In still another alternate embodiment of the invention, the blind bores 
within the wear member are replaced with through bores to reduce 
production costs. A transducer assembly is received within each of the 
through bores so that its end is flush with the inner surface of the wear 
member. In this embodiment, the end of the transducer assembly is actually 
a part of the wear surface and the thickness of the end of the transducer 
assembly is being measured. 
Regardless of the embodiment utilized, a separate transducer may be placed 
in the same environment as the other transducers for use as a relevant 
time reference.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings where the illustrations are for the purpose 
of describing the preferred embodiment of the present invention and are 
not intended to limit the invention hereto, FIG. 1 is a cross-sectional 
view of the transducer 10 installed in a wear member 12, such as a sleeve 
or thrust bearing, clutch plate, brake disc or pad, sealing member or the 
like, whose thickness is to be measured and/or monitored. The transducer 
10 is comprised of an outer sleeve 14, an aligning and electrically 
insulating spacer 16 received within the end of the outer sleeve, and a 
piezoelectric element 18. 
The outer sleeve 14 is fabricated from round tubing, such as brass tubing 
or the like, which has threads 20 formed adjacent one end thereof. 
Typically, the tubing material has the same or similar thermal expansion 
properties as that of the wear member 12 to maintain a firm contact 
therewith. This firm contact is provided by the threads 20 which engage 
complementary threads provided in the wear member 12, as hereinafter 
described. The threads 20 also permit the adjustment of the outer sleeve 
14 within the wear member 12 to optimize the operation of the transducer 
10 as later discussed. It should be noted that other approaches are 
possible for the adjustable attachment of the outer sleeve 14 to the wear 
member 12, such as a bracket arrangement (not shown) that is adjustable 
with respect to the wear member 12 and which retains the sleeve 14. The 
end 22 of the outer sleeve 14 has an indentation provided therein forming 
a surface 24 connecting the end 22 of the sleeve 14 with the inner 
circumferential wall 26 of the sleeve. This indentation may have a curved 
configuration, such as semispherical or parabolic, or it may have a 
conical configuration which is preferred to permit alignment of the spacer 
16 therein. 
The aligning and electrically insulating spacer 16 is fabricated from a 
ceramic or ceramic-like material that is capable of sustaining high 
temperatures and high pressures. For example, pyrolytic boronitride or 
another ceramic-like material can be used for the spacer 16. The 
particular ceramic or ceramic-like material utilized for the spacer 16 is 
selected to compensate for the thermal expansion properties of the other 
components comprising the transducer 10 and the wear member 12 so that the 
spacer 16 will maintain a substantially uniform compressive force on the 
piezoelectric element 18 over a broad operating temperature range. The 
spacer 16 typically has a conical configuration that is complementary to 
that of the indentation formed in the end 22 of the outer sleeve 14. The 
spacer 16 is received within the indentation so that the outer surface 28 
defining its conical configuration contacts the surface 24 formed by the 
indentation. The use of conical surfaces 24, 28 formed on the outer sleeve 
14 and the spacer 16, respectively permits the alignment of the spacer 16 
within the outer sleeve 14 through elastic deformation of the spacer 16 
and the indentation formed in the end 22 of the outer sleeve 14. In 
contrast, self-alignment of the spacer 16 within the outer sleeve 14 can 
be achieved by using a semi-spherical or parabolic configuration for the 
surfaces 24, 28 formed on the end 22 of the outer sleeve 14 and the spacer 
16, respectively. It should be noted that regardless of the shape of the 
complementary configurations used for the spacer 16 and the indentation in 
the end 22 of the outer sleeve 14, a precise fit of the spacer 16 within 
the indentation is not necessary since any deviations in size or shape 
will be compensated for by the elastic deformation of the spacer 16 and 
the indentation and/or by the self-alignment of the complementary curved 
surfaces. The alignment of the spacer 16 within the outer sleeve 14, 
whether by elastic deformation of the spacer 16 and the indentation in the 
end 22 of the outer sleeve 14 or by self-alignment through complementary 
curved surfaces, is necessary to ensure the application of a uniform 
compressive force on the piezoelectric element 18. Such a substantially 
uniform compressive force also minimizes the possibility of damaging the 
piezoelectric element 18 through the application of a nonuniform 
compressive force thereto. Even though both of the foregoing approaches 
apply a substantially uniform compressive force to the piezoelectric 
element 18, it has been found that the use of an appropriate conical 
configuration for the spacer 16 and the indentation in the end 22 of the 
outer sleeve 14 is easier to implement and may result in a substantially 
higher absorption and obliteration of spurious echoes from the primary 
ultrasonic signal than if complementary curved configurations are used for 
the spacer 16 and the indentation in the end 22 of the outer sleeve 14. 
Thus, the use of a conical configuration for the spacer 16 and the end 22 
of the outer sleeve 14 results in a higher signal to noise ratio than if 
complementary curved configurations are used for same. In summary, the 
spacer 16 is necessary in this structure in order to provide a 
substantially uniform compressive force to the piezoelectric element 18 
and to absorb and obliterate spurious echoes. Regardless of the 
configuration utilized for the spacer 16, an aperture 30 is formed 
therethrough. This aperture is sufficiently large to permit the passage of 
an electric conductor therethrough. 
The wear member 12 whose thickness is to be measured and/or monitored is 
provided with a blind bore 36 therein. The blind bore 36 is located so as 
to be substantially perpendicular to the outer and inner surfaces 38, 40, 
respectively of the member 12. If the member 12 is a sleeve bearing, the 
blind bore 36 is directed radially inwardly so as to be perpendicular to 
tangents which intersect its centerline on the outer and inner surfaces 
38, 40 of the member 12. The blind bore 36 is of a predetermined depth and 
has a substantially flat surface 42 at the bottom thereof. The distance 
between the flat surface 42 and the inner surface 40 of the member 12 is 
the distance to be measured and/or monitored. The blind bore 36 may also 
have threads 44 formed therein which terminates adjacent the bottom 
thereof. 
The piezoelectric element 18 is a standard state-of-the-art device and 
typically has a round disc-like shape. The element 18 can be formed from 
commercially available piezoelectric transducer material, such as PZT-5H 
available from Vernitron, Inc. of Bedford, Ohio. The size of the element 
18 is a function of the overall size of the transducer 10, however, an 
element having a diameter of 0.080 inch and a thickness of 0.003 inch has 
been tested experimentally with excellent results. The diameter of the 
element 18 is slightly less than the diameter of the blind bore 36 
provided in the wear member 12. The element 18 is responsive to a short 
voltage pulse, such as a 200 volt DC pulse of 10 nanosecond duration, and 
converts the voltage pulse into a pressure pulse which is applied to the 
surface of the material whose thickness is to be measured and/or 
monitored. Similarly, the piezoelectric element 18 converts the "echo" 
return pressure pulse from the opposite surface of the material whose 
thickness is being monitored into a voltage pulse for measurement 
purposes. The substantially uniform compressive force applied to the 
piezoelectric element 18 by the spacer 16 ensures that the element 18 is 
firmly "seated" within the blind bore 36 for the proper transmission of 
the voltage pulse into the element 18 and the reception of the reflected 
"echo" pulse by the element. 
In order to assemble the transducer 10, the piezoelectric element 18 is 
received within the blind bore 36 and positioned so that one side 46 
thereof contacts the flat surface 42 at the bottom of the blind bore 36. 
Inasmuch as the diameter of the element 18 is only slightly less than the 
diameter of the blind bore 36, the center of the element 18 and the center 
of the flat surface 42 at the bottom of the blind bore 36 will 
substantially coincide, however, such coincidence is not necessary for the 
proper operation of the transducer 10. The other side 48 of the 
piezoelectric element 18 may be soldered to an electrical conductor 50. 
The electrical conductor 50 is received through the aperture 30 provided 
in the spacer 16, and the spacer 16 is received in the blind bore 36 so 
that its base 32 contacts the side 48 of the piezoelectric element 18 
which is mechanically and electrically connected to the electrical 
conductor 50. The threads 20 on the outer sleeve 14 are coated with an 
adhesive, such as Loctite, and the sleeve 14 is threadedly advanced into 
the wear member 12 until the conical surface 24 provided on its end 22 
engages the outer surface 28 of the spacer 16. Further advancement of the 
outer sleeve 14 into the wear member 12 causes the elastic deformation of 
the spacer 16 and the indentation in the end 22 of the outer sleeve 14, 
and the application of a substantially uniform compressive force by the 
base 32 of the spacer 16 to the side 48 of the piezoelectric element 18. 
If complementary curved configurations, such as semispherical or 
parabolic, are used for the spacer 16 and the indentation in the end 22 of 
the outer sleeve 14, the spacer 16 will self-align itself within the 
indentation in the end 22 of the outer sleeve 14 so that its base 32 will 
apply a substantially uniform compressive force to the side 48 of the 
piezoelectric element 18. Regardless of the shape of the spacer 16 and the 
indentation in the end 22 of the outer sleeve 14, the outer sleeve 14 is 
threadedly advanced into the wear member 12 by manually rotating the outer 
sleeve 14 until a snug fit exists between the indentation provided in its 
end 22 and the outer surface 28 of the spacer 16, and between the base 32 
of the spacer 16 and the side 48 of the piezoelectric element 18. In order 
to ensure that such a snug fit exists, the foregoing advancement of the 
outer sleeve 14 into the wear member 12 is monitored by a pulser-receiver 
device and an oscilloscope (all not shown). With this apparatus a series 
of short voltage pulses is applied by the pulser to the transducer 10 
while the outer sleeve 14 is being threadedly advanced into the wear 
member 12 so that the sleeve 14 can be rotationally adjusted until the 
optimum return "echo" pulse, shown on the oscilloscope, is received by the 
receiver. In this manner, a snug fit between the foregoing components is 
assured and the transducer 10 and the wear member 12 are "tuned" to 
provide the optimum return "echo" pulse. 
Since the piezoelectric element 18 is somewhat formable under a compressive 
force, the application of a substantially uniform compressive force 
thereto results in a firm, optimum electro-acoustical contact between the 
side 46 of the element 18 and the flat surface 42 at the bottom of the 
blind bore 36. By providing such a firm, optimum electro-acoustical 
contact with the flat surface 42 of the blind bore 36, any signals 
emanating from the piezoelectric element 18 will be properly directed 
toward the inner surface 40 of the wear member 12 to be measured and/or 
monitored, and the wear member 12 will provide the proper electrical 
ground for the system. Thus, the surfaces 24, 28 compensate for deviations 
in manufacturing tolerances in the components involved, and the 
possibility that the blind bore 36 may not be positioned exactly normal to 
the inner surface 40 of the wear member 12. Both of these conditions could 
result in the piezoelectric element 18 not firmly contacting the flat 
surface 42 of the blind bore 36 which, in turn, could result in inaccurate 
measurements and/or system malfunctions. After the transducer 10 has been 
assembled and installed in the wear member 12, the area 52 enclosed by the 
inner circumferential wall 26 of the outer sleeve 14 and containing the 
electrical conductor 50 may be filled with a dense insulating and 
dampening material such as epoxy, e.g., Duro epoxy, loaded with tungsten 
for application temperatures less than 400.degree. F. or a loaded ceramic 
adhesive for temperatures in excess of 400.degree. F. This electric 
insulation material and the spacer 16 preferably match the acoustical 
impedance of the piezoelectric element 18 and help suppress spurious 
echoes, which are later in time and thus much weaker than the primary 
ultrasonic pressure pulse, from interferring with the primary pulse. After 
the installation of the transducer 10 in the wear member 12 has been 
completed, an epoxy may be placed on the outer surface of the sleeve 14 
adjacent the top surface of the wear member 12 to prevent any contaminants 
from entering the transducer. 
The wear member 12 may have a configuration that is either flat, such as a 
brake disc, clutch plate, face type seal or thrust type bearing, or 
circular, such as a sleeve bearing or ring type seal. In any case, a 
plurality of transducers can be utilized to measure and/or monitor wear at 
various locations on the wear member 12. If a sleeve bearing is utilized, 
the plurality of transducers 10 can be placed within the outer bearing 
wall and around the periphery of the bearing, as shown in FIG. 2. In this 
manner, bearing wear can be measured and/or monitored around the periphery 
thereof. Thus, by placing the transducer 10 within one or more blind bores 
36 within the bearing, wear can be measured and/or monitored in-situ, 
eliminating costly periodic machine downtime to inspect the condition of 
the bearing. Machine downtime would only occur when a transducer indicates 
that sufficient wear has occurred to justify the replacement of the 
bearing. 
Alternatively, rather than placing a plurality of transducers 10 within the 
blind bores provided in the outer bearing wall, a mounting attachment 54, 
such as a ring as shown in FIG. 3, could be used to retain the transducers 
10 in a radially spaced apart relationship. In such an arrangement, the 
mounting attachment 54 would be slipped over the sleeve bearing 56 and the 
piezoelectric elements 18 would firmly contact the outer surface of the 
bearing wall. Thus, no blind bores would be required in the bearing wall. 
In this arrangement, since the radius of the curvature of the bearing 56 
is substantially greater than the diameter of each piezoelectric element 
18 and inasmuch as a substantial compressive force is being applied to 
each element 18 by its associated spacer 16, it has been found that 
sufficient surface contact exists between each element 18 and the outer 
surface of the bearing 56 to produce very accurate distortionless 
measurements of wall thickness. Thus, by using this apparatus, the total 
bearing wall thickness can be measured and/or monitored at various 
locations on the bearing. 
In addition to being able to measure wear in-situ, the construction of the 
transducer 10 provides another advantage in that no buffer element is 
required between the piezoelectric element 18 and the wall whose thickness 
is being measured and/or monitored, i.e., the distance between the flat 
surface 42 of the blind bore 36 and the inner surface 40 of the wear 
member 12. Typically, in prior art devices such a buffer element is 
required for mechanical support, impedance matching and sealing of the 
transducer, however, its use greatly attenuates and degrades the primary 
pulses produced by the transducer and the reflected "echo" pulses received 
by the transducer. Inasmuch as the transducer 10 requires no buffer 
element, such signal attenuation and degradation does not occur. In 
addition, because of the absence of a buffer element, a firm electrical 
and acoustical contact can be made by the piezoelectric element 18 
directly to the wall thickness being measured and/or monitored, and the 
resulting measurements have a much higher degree of accuracy than those 
resulting from prior art devices. For example, measurements with a 
repeatability in the sub-micron range utilizing state-of-the-art 
electronics have been achieved. And lastly, due to the inherent simplicity 
of the structure of the transducer, it is substantially less costly to 
produce than the prior art devices. 
In an alternate embodiment of the invention, as shown in FIG. 4, the blind 
bore 36 in the wear member 12 is replaced with a through bore 60 
connecting the outer and inner surfaces 38, 40 of the member 12. The 
through bore 60 may have threads 62 formed therein. A transducer 64 
comprising an outer sleeve 14, a spacer 16, and a piezoelectric element 18 
is received within a blind bore 66 in a wear reference member 68 which may 
have threads 70 formed on the outer surface thereof. The wear reference 
member 68 is received within the through bore 60 so that its end 72 is 
substantially flush with the inner surface 40 of the wear member 12. The 
inner surface 40 of the wear member 12 is then machined to ensure that the 
end 72 of the wear reference member 68 is flush with the inner surface 40. 
It should be noted that the material utilized for the wear reference 
member 68 may be the same as or may be different from the material 
comprising the wear member 12 inasmuch as only the thickness of the end of 
the reference member 68 is being monitored and/or measured. The operation 
of this embodiment is similar to the previous embodiment utilizing a blind 
bore, however, it is easier and less costly to produce. 
With any of the foregoing embodiments, it might be desired to compensate 
for the temperature and pressure of the environment, the density of the 
material utilized for the transducer, and the strains existing on the 
transducer. Such compensation can be accomplished by using a time 
reference transducer 80, as shown in FIG. 5. The structure of this 
transducer 80 is similar to transducer 10, in that it is comprised of an 
outer sleeve 14, a spacer 16, and a piezoelectric element 18, however, the 
foregoing components are received in a blind bore 82 provided in a 
reference member 84, which is similar to wear reference member 68. The 
material utilized for the reference member 84 is the same as or similar to 
the material for the wear member 12 if a blind bore 36 is utilized in the 
member 12, or the same as or similar to the material for the wear 
reference member 68 if a through bore 60 is provided in the wear member 
12. The assembly of the transducer 80 and the reference member 84 is 
placed within the same temperature, pressure or material environment as 
the other transducers 10, though not necessarily contacting the wear 
member 12. By monitoring the measurements of the reference distance, 
produced by the transducer 80, the measurements produced by the 
transducers 10 can be adjusted to compensate for possible measurement 
variations caused by operating environment changes. 
Certain modifications and improvements will occur to those skilled in the 
art upon reading the foregoing. It should be understood that all such 
modifications and improvements have been deleted herein for the sake of 
conciseness and readability, but are properly within the scope of the 
following claims.