Note: Descriptions are shown in the official language in which they were submitted.
2106~1~
ENCAPSULATION OF PIEZOELECTRIC HIGH VOLTAGE ELEMENT
TO PREVENT FRACTURE THEREOF
TECHNICAL FIELD
The present invention relates, in general, to piezoelectric high voltage
impact
devices and, more particularly, to an impact device structure which prevents
or
minimizes fracturing of the piezoelectric element contained therein.
BACKGROUND ART
High voltage impact devices of the piezoelectric type typically include a
piezoelectric element, an anvil member on which the piezoelectric element is
seated, an
impact member oppositely disposed to the anvil member and contacting the
piezoelectric
element, and a hammer member for striking the impact member causing the
piezoelectric
element to produce a voltage. All of the foregoing elements are contained
within a
housing. The piezoelectric element can be molded within the housing or can be
loosely
assembled therein. A portion of the space between the piezoelectric element
and the
housing is usually filled with an encapsulating material such as silicone oil,
epoxy resin,
silicone grease or rubber. Numerous instances of piezoelectric element
fracturing have
occurred with this structure. The frequency of such fracturing has been
reduced by
replacing the foregoing encapsulating material with a viscid encapsulating
material, such
as that disclosed in U.S. Patent No. 4,051,396 (Berlincourt). In addition to
reducing the
frequency of piezoelectric element fracturing, the utilization of such a
viscid encapsulat-
ing material minimizes dielectric breakdown across the surface of the
piezoelectric
element and the terminals of the impact device, and permits utilization of a
piezoelectric
element that does not have smooth and/or parallel impact surfaces.
Even though fracturing of the piezoelectric element occurs less frequently as
a
result of using the viscid encapsulating material, fracturing of the
piezoelectric element
is still a problem particularly at temperatures in excess of about 150°
F at which point
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the viscid encapsulating material becomes essentially non-viscid. In view of
the
foregoing, it has become desirable to develop an impact device structure
wherein the ~
frequency of piezoelectric element fracturing is further reduced thus
increasing the life
expectancy of the device particularly at elevated temperatures.
r.
SUMMARY OF THE INVENTION
The present invention solves the problems associated with the prior art and
other
problems by providing an impact device structure which further reduces the
frequency
of piezoelectric element fracturing and which provides support to the element
if
fracturing has occurred, thus permitting continued use of the piezoelectric
high voltage
impact device. The foregoing is accomplished by the use of tape which is
applied to the
surface of the piezoelectric element and wrapped therearound. The opposite
ends of the
piezoelectric element which are adjacent the impact member and the anvil
member
remain uncovered ensuring firm electrical contact between the piezoelectric
element and
these members. By the proper selection of the tape material and the adhesive
applied to
the surface thereof that grippingly engages the piezoelectric element, the
frequency of
piezoelectric element fracturing is significantly reduced. In addition, if the
piezoelectric
element becomes fractured during use, the tape provides support to the element
. permitting continued use of the impact device. If a viscid encapsulating
material, such
as that disclosed in U.S. Patent No. 4,051,296, is used to fill a significant
portion of the
space between the surface defining the bore in which the piezoelectric element
is received
and the outer surface of the tape, the incidence of piezoelectric element
fracturing is
further reduced. The viscid material also provides a very thin cushioning film
between
the ends of the piezoelectric element and the impact and anvil members. The
encapsulat-
ing material acts in conjunction with the tape to further prevent fracturing
of the
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210G~15
3
piezoelectric element and to support same permitting the continued use of the
high
voltage impact device after the element has fractured.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional view of a prior art high voltage piezoelectric
impact
S device.
Figure 2 is an enlarged partial cross-sectional view of a piezoelectric impact
device that incorporates the present invention.
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 described herein, Figure 1 is a cross-sectional view of a
prior art
piezoelectric high voltage impact device 10. The impact device 10 is of the
axially
actuated type and includes a body member 12 having a plurality of stepped
bores therein.
A piezoelectric element 14 is received in one of the stepped bores and is
interposed , ,
between an anvil member 16 and an impact member 18. A spring 20 holds a hammer
member 22 in a spaced apart relationship with respect to the impact member 18.
An
oppositely disposed spring 24, having a spring constant substantially greater
than the
spring constant of spring 20, contacts the hammer member 22 and is positioned
such that
its free end is received within a bore 26 provided within an actuating member
28. A cap
30 is received over a flange portion 32 provided on body member 12 and
grippingly
engages same to maintain the foregoing elements in an assembled relationship.
The
electrical output of the impact device 10 is provided across an electrical
contact 34 which
is attached to impact member 18 and tip 36 on the end of anvil member 16.
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Operationally, the length of spring 20 and its spring constant are such so as
to
position hammer member 22 above a ledge 40 formed within body member 12 and in
a
spaced-apart relationship therewith. As the actuating member 28 is depressed,
a
circumferential flange surface 42 on hammer member 22 comes into contact with
ledge
40. Further depression of actuating member 28 causes surface 44 defining the
entrance
to bore 26 within actuating member 28 to engage a conical surface 46 on hammer
member 22 causing the circumferential flange surface 42 thereon to become
disengaged
from ledge 40 permitting hammer member 22 to move rapidly toward impact member
18. Inasmuch as spring 24 has a substantially larger spring constant than
spring 20,
spring 24 causes hammer member 22 to strike impact member 18 which, in turn,
results
in piezoelectric element 14 producing a voltage across electrical contact 34
and tip 36 of
anvil member 16. Repeated striking of impact member 18 by hammer member 22
causes
piezoelectric element 14 to eventually fracture resulting in the reduction or
extinction of
a voltage across electrical contact 34 and tip 36 of anvil member 16 when the
hammer
member 22 strikes the impact member 18.
The present invention is directed to structure for an impact device which
minimizes the incidence of fracturing of the piezoelectric element and, if
fracturing
occurs, for supporting the piezoelectric element permitting the continued use
of the
impact device. Referring now to Figure 2, a partial cross-sectional view of a
portion of
an impact device 50, which can be one of any number of types of impact
devices, such
as an axially or a rotary actuated device, is illustrated. Impact device 50
includes a body
portion S2 having a blind bore 54 therein which receives a piezoelectric
element 56. An
anvil member 58 and an impact member 60 are received within the body portion
52 and
are disposed on opposite sides of piezoelectric element 56. The piezoelectric
element 56 .
is cylindrical in configuration and its diameter is such so that an annular
space 62 exists
between its cylindrical surface and the surface defining the blind bore 54. It
has been
. CA 02106815 1999-10-20
found that the use of a strip of tape 64 having an adhesive on
one side thereof for attachment to the cylindrical surface of the
piezoelectric element 56 substantially reduces the incidence of
fracturing of the element 56 resulting from repeated strikes by
5 a hammer member (not shown) on impact member 60. The tape 64 is
usually 0.5 to 2.0 mils thick and the backing material for same
can be paper, Mylar*, polyester film, Kapton*, etc. The tape 64
should have a sticky adhesive coating applied to the side that
grippingly engages the cylindrical surface of the piezoelectric
element 56, and the properties of the adhesive coating should
remain substantially constant over the operating temperature
range of the impact device 50. Alternatively, the surface of the
tape that grippingly engages the cylindrical surface of the
piezoelectric element 56 can be coated within an adhesive that
hardens as a catalyzed epoxy resin. Regardless of the type of
tape employed and/or the adhesive utilized for same, the width of
the tape 64 in the longitudinal direction should be slightly less
than the longitudinal width of the piezoelectric element 56 so
that the edges of the strip of tape 64 do not enter the spaces
between the piezoelectric element 56 and the impact member 60 and
anvil member 58. Ideally, the piezoelectric element 56 should be
wrapped with 1.2 to 1.9 turns of tape 64, with the optimum number
of turns being approximately 1.33 turns, however, satisfactory
test results have been obtained even when the piezoelectric
element 56 has been wrapped with less than one full turn of tape .
From the foregoing, it is apparent that even if the entire
cylindrical surface of the piezoelectric element 56 is not
covered by the tape 64, the tape provides support to the portion
of the piezoelectric element 56 that is covered which, in turn,
provides support to the entire piezoelectric element 56.
Test conducted with the piezoelectric element 14 wrapped
with 1.3 turns of polyester film tape in combination with a
viscid encapsulating material within the annular space 62
indicate that the impact device can be operated at an elevated
temperature of 300°F. for several thousand actuations without
fracturing the piezoelectric element. The
*trade~mark
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6
foregoing was accomplished with polyester film tape (4113 tape) obtained from
the Great
Tape Company of Newburyport, Massachusetts. The polyester film tape utilized
had a
total thickness of 2.5 mils comprised of a 1.0 mil polyester film used as a
backing
material and an acrylic thermosetting adhesive applied to one side thereof.
The foregoing
polyester film tape is typically used as an outer wrapping on capacitors and
coils and for
holding transformer construction and wire harness bundling. In contrast, tests
in which
only viscid encapsulating material was.utilized within the annular space 62
resulted in the
piezoelectric element fracturing after only 50 to 100 impacts at an elevated
temperature
of 300° F. and fracturing after several thousand impacts at 200°
F.
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.
