Note: Descriptions are shown in the official language in which they were submitted.
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CENTRAL BOLT ULTRi~SONIC ATOMIZER
BACXGROUND OF THE INVENTION
1. Technical Field.
The present invention relates to piezoelectric
ultrasonic atomizers, particularly of the type having
an atomizing surface at a tip of a reduced diameter
amplifying probe at one end of a transducer and a
coaxial fluid delivery channel extending from the other
end of the transducer to the atomizing surface.
2. Background Art.
Piezoelectric ultrasonic atomizers are finding
increasing use in industrial applications where liquid
materials must be delivered in the form of a very fine
spray or mist. The design and construction of such
atomizers is described in U.S. Patent No. 4,337,896 of
BERGER et al. A typical arrangement is to sandwich a
flat electrode between two disks of piezoelectric
material, such as lead zirconate titanate, to form a
drivlng element, and then to clamp the driving element
between a cylindrical front amplifying horn and a
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1 cylindrical rear dummy section. The amplifying horn is
provided with a reduced diameter probe having an
atomizing surface at its tip. The amplification of
vibrational amplitude obtained at the atomizing surface
is approximately equal to the ratio between the
respective cross-sectional areas of the cylindrical
portion of the front horn and of the end of the probe.
In the type of atomizer shown in U.S. Patent No.
4,337,896, the necessary clamping pressure on the
driving element is obtained by providing
circumferential flanges on the adjacent ends of the
front and rear sections and drawin~ the flanges
together with a circle of bolts. The flanges also
provide an annular bearing area for compressing an
elastomeric gasket ring, to prev~nt liquid spray from
contacting the outer peripheries of the piezoelectric
disks. The sealing effectiveness of such a gasket is
an important factor in extending the operating life of
the atomizer.
The clamping flange design has drawbacks, however.
To reduce internal losses, the front and rear horns
should each be made as a single piece. It is wasteful
to have to start with stock having an outer diameter
equal to the flange diameter and then machine as much
as two-thirds of it away. More importantly, the size
of droplets formed by an ultrasonic atomizer varies
inversely with the frequency of the unit. To obtain
very small particles in the micron range, it is
necessary to use verv high frequencies, well over 100
kHz. To avoid significant transverse wave motion in
the transducer, however, the transverse dimensions of
the front and rear sections should be less than one-
quarter wavelength.
As an example, in titanium a quarter wavelength at
freguencies above 100 kHz is less than one centimeter.
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1 It is desirable to have the ratio between cylindrical
section diameter and probe tip diameter be as large as
possible, for increased amplification. At the same
time, the atomizing surface should be large enough to
handle a reasonable flow and the probe must be cturdy
enough to resist breaking in operation. These factors
make it undesirable to use up part of the diametral
dimensions for clamping flanges.
An alternative arrangement for clamping a
cylindrical atomizing transducer and concurrently
protecting the piezoelectric elements from liquid
contamination is disclosed in U.S. Patent No. 3,861,852
of BERGER. In this arrangement, a cylindrical
transducer is inserted into a cup, and the transducer
elements are clamped together by force exerted upon a
flange on the rear dummy section by a cap threaded into
the cup, with the front face of the transducer bearing
against the base of the cup. O-rings at the clamping
surfaces seal the transducer inside the cup from liquid
spray delivered from the tip of a probe extending
through an opening in the base of the cup. It is
difficult to apply and maintain the proper clamping
pressure on the piezoelectric driving element with this
arrangement, however, and the end clamping can
introduce significant damping and thereby reduce
efficiency of the transducer.
Although liquid is fed to the above-described
atomizers through a radial passage that intersects an
axial channel in the front horn of the transducer, it
is also known, for example from U.S. Patent No.
4,352,459 of B~RGER et al., to feed the liquid axially
through the rear section of a flange-clamped
transducer. It is necessary in this design, however,
to provide an annular sealing gasket between the fe~d
5 ~, ~
tube ancl the :inner circllm1.el-ences of the piezoe1ectLi.c 3.isks thuc
reducing the potential cross--sec-tiona:L area of the di.ks and
thereby the avai:Lable vibrational driving power. Ii is also known
tt.o clamp t.he drivincJ e:iemell-t of a piez.oelectric transducer by
means of a solid central bol.t as in United States Patents No.
~ 368 0$5 of llel~last.er et. al. r No. 3 396 285 of ~linchenko No.
3 689 783 of Williams, and No. 3 5~4 675 of Loveday. The
transducers of these patents are not fluicl feed atomizers
however.
SUMI~IA~Y OF_THE INVEN~ION
It is an ob~ect of the present invention to provide a
piezoelee-trie atomizer design having a maximum praetieal
amplifieation and adapt;ed for high frequency operation above 100
kHz
It is another object of the present invention to provide
an axial feed piezoelectric atomizer that provides effective
internal sealing wit:hout reducing the cross-sectional area
available for the piezoelectric elements.
Another object of -the invention i.s to provide ex-ternal
sealing of the piezoelectrie elements in an atomizer as
characterized above w:ithout axially :loadincJ t.he t.:rallsc1ucer
element.
rhe ahove and ot.her objects a:re aeh:ieved in an
ultrasonic liquid atomizincl transdueer assembly comprising:
a clriviny element includinq a pair of annul.ar
piezoelectric disks and an annular eleetrode coaxially positioned
therebetween;
terminal means for feeding ultrasonic frequeney
22583-373
electrical energy ~o said electrode;
a ~ylindrical rear dummy section having a front end
con~acting one piezoelec-tric disk of the driving element, a rear
end , and a constant outside diameter from the front end to the
rear end;
a front section having a cylindrical porkion, the
cylindrical portion having a rear end contacting the other
piezoelectric disk of the driving element and a front end, and an
amplifying portion extending from the front end of the cylindrical
portion, the amplifying portion comprising a probe having a tip
that forms an atomizing surface, an axial passage being provided
through the length of the transducer assembly from the rear end of
the rear dummy section to the atomizing surface, and a portion of
the passage adjacent the driving element in both the front
atomizing section and -the rear dummy section being enlarged and
internally threaded;
a tubular central bolt having an externally threaded
portion engaging said internally threaded portion of the passage
in both the front atomizing section and the rear dummy sectio!l
with sufficient torque to connect the front atomizing section and
the rear dummy section under a tension that provides all of a
predetermined total compressive preload on the driving element,
the ex.ternally threaded portion extending from a front end portion
of the bolt located in the front atomizing section and formed with
a smooth cylindrical sealing surface to a rear feed tube portion
of the bolt located in the rear dummy section and extendiny
axially beyond the rear end of the dummy section; and
means for sealing the piezoelectric disks from contact
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~iith -the li~uid bei.ng atomized, saicl meanC comprlsing a resi.li.ent
annular sealing member cl~sposed between said sealing surfa(~l and
the a~ial passacle in the front section to prevent liquicl flow1.ng
in ~he passage from reaching the inner c:ircumferentia:L s.urfaces of
the piezoelectric disks.
The above and other objee~.s, features ancl advantages of
the present lnvention ~ril.:l be more readily apparent from the
following description of the preferred embodiments when considered
with the accompanying dra~rings and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example
and not limltation in the figures of the aecompanying drawing in
which li.ke numerals indicate the same or similc,r parts and in
which:
Fig. 1 is a partially cut away perspective view of an
ultrasonic atomizing transdueer assembly according to the
inven~ion, and
Fig. 2 is a view in longitudinal cross-seetion of the
transducer assemhly of Fig. 1.
DES _I TION OF THF. PREFERRED_EIBODIMENT
With reference to the ficlures, a cul-rently preferrecl
embodiment of an ultrasorlic atomizinq transclucer assembl.y l.L
includes a -transdueer 1~ having a driv:ing element 13, a rear dummy
section 14, and a front atomizing section 15.
The driving element 13 is assembled from a ~rasher-shaped
metal electrode L6 sandwiehed be-tween a pair of annular
piezoelectric disks 17 and 18. The electrode may he made of
copper or ary other suitahle metal
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1 having high electrical conductivity, and it is provided
with a terminal 19 for attachment to a source of
electrical energy at the resonant frequency of the
transducer. The piezoelectric disks are made of any
material conventionally used for ~uch service, such as
barium titanate or lead zirconate titanate.
The rear dummy section 14 is a metal cylinder,
preferably titanlum, having a length egual to a quarter
wavelength at the designed operating frequency of the
transducer. A front end 20 of the rear section 14
contacts the rear piezoelectric disk 18, and a rear end
21 of the rear section is free to vibrate as an
antinodal plane. The front atomizing section 15
includes a cylindrical portion 22 having a rear end 23
that contacts the front piezoelectric disk 17 and a
front end 24 that lies in a nodal plane/ the
cylindrical portion 22 being designed to be one-half
wavelength long at the operating frequency of the
transducer. From the front end of the cylindrical
portion 22, a quarter wavelength amplifying probe 25
extends to a frustoconical tip 26 having an atomizing
surface 27~ The front atomizing section preferably is
made of the same material as the rear dummy section,
although a different material could be used if desired,
so long as the appropriate wavelength dimensions were
used to match the operating frequency of the rear
section.
Except for a narrow circumferential flange 28 at
the rear end of the front section, the outer diameter
of the transducer is e~ual to the diameters of the
front and rear sections. These sections are clamped
against the driving element 13 with a predetermined
compressive stress by a central tubular bolt 29 that is
formed as an enlarged threaded stud on the end of a
liquid feed tube 30. The tubular bolt engages an
1 internally threaded enlarged portion 31 of an axial
passage 32 that extends through the transducer from the
rear end of the rear dummy secton 14 to open onto the
atomizing surface 27 at the tip of the probe 25.
To prevent liquid flowing through the delivery
tube 30 into the passage 32 from penetrating past the
threaded portion of the front section and contacting
the internal surfaces of the piezoelectric disks, an 0-
ring seal 33 is provided between a smooth sealing
surface 34 machined on the front end of the central
bolt 29 and the inner surface of the passage 32. As
illustrated, the O-ring is fitted into a
circumferential groove machined into the wall of the
passage to assure that the 0-ring is properly located
with respect to the sealing surface 34. The groove
could equally well be formed on the end of the bolt, or
any other conventional sealing arrangement could be
used between the end of the bolt and the inner surface
of the passage in the front section.
An additional O-ring 35 is provided to seal
between the otlter circumference of the feed tube 30 and
the inner circumference of the axial passage. This
second 0-ring prevent6 ingress of moisture from the
environment surrounding the atomizer.
Because there are no clamping flanges on the
transducer body to provide an annular area for a
compressed ring gasket around the outside of the
driving element, the outer peripheries of the
piezoelectric disks are protected by an enclosed shell
36. This shell is in the form of a cylindrical cup 37
having a screw cap 38. The cup 37 has an end wall 39
provided with an opening 40 which receives the
cylindrical portion 22 of the front section of the
transducer. This opening is sealed by a radially
compressed 0-ring 41 disposed between the outer
1 circumference of the cylindrical portion 22 and a
counterbore 42 in the opening 40. The screw cap 38 has
an end wall 43 with a similar but smaller opening 44.
An O-ring 45 in a counterbore 46 seals this opening in
the same way as O-ring 41 seals the front opening. As
illustrated, O-ring 45 is radially compressed between
the counterbore 46 and a cylindrical collar 47
extending from the end 21 of the rear dummy section.
Alternatively, the dimensions of the collar and
the counterbore could be revised so that the O-ring 45
could seal radially against the outer periphery of the
feed tube 30 and abut against the end of the collar.
It is important, however, that there be no axial
compression force exerted by the shell against the
transducer body via the O-rings 41 and 45. In this
connection, the narrow flange 28 at the rear of the
front section serves merely to locate the O-ring 41 as
close as possible to the nodal plane defined by the
electrode 16. ~here should be no axial force exerted
against this flange by the O-ring since the O-ring 45
at the rear of the shell has room to float axially.
~onsequently, the transducer is supported in the shell
substantially purely radially, with no axial force
exerted between the shell and the transducer.
The procedure for assembling the transducer is as
follows. After the O-ring 33 is installed into its
groove in the front section 22, the central bolt is
screwed into the front section until it bottoms. The
piezoelectric disks and the center electrode are then
passed over the bolt. If desired, a sleeve of
electrical insulating material (not shown) may be
inserted between the bolt and the inner circumferences
of the disks and electrode. This will help to center
the driving element as well as to prevent a short
35 circuit of the driving element. It also may be
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1 desirable to add a second annular electrode (not shown)
between the rear piezoelectric disk and the rear dummy
section to provide a æecond terminal to facilitate
completing the electrical cîrcuit across the
piezoelectric disks.
After the driving element is assembled onto the
bolt, the 0-ring 35 is fi~ted over the feed tube 30,
and the rear dummy section is then screwed down against
the driving element. The proper compression force is
obtained by applying a torque wrench to two
diametrically spaced detent holes 4~3 drilled in the
rear end 21 of the rear dummy section.
Following assembly of the transducer, the shell
can be mounted by first installing O-ring 45 on the
collar 47 (or on the tube 30 in the above-mentioned
alternative arrangement) and then sliding the threaded
cap 38 ov~r the tube 30 into place over the rear dummy
section. A lead wire 49 attached to a hermetically
sealed coaxial fitting 50 mounted on the end wall of
the cap is then clipped or soldered to terminal ~9 of
the center electrode 16. If a second electrode is
provided, as described above, a second lead wire (not
shown) from a second coaxial fitting (not shown) should
be similarly attached to the second electrode.
Finally, the O-ring 41 is placed on the cylindrical
portion 22 of the front atomizing section, and the cup
37 is slipped onto the cylindrical section 22 and
screwed into the cap 38 until it bottoms. The cap can
be tightened by means of a spanner wrench fitting the
detent holes 51 in the end wall of the cap.
Since the transducer is connected to the shell
only radially through the "axially floating" 0-rings 41
and 45, the transducer can he mounted by clamping or
fastening to the shell in any desired way without
adversely affecting either the compression preload on
1 the driving element 13 or the resonant frequency of the
transducer. Liquid can be delivered to the rear of the
unit via a flexible hose (not shown) connected to the
delivery tube 30 by the standard coupling connectors 52
(see Fig. 1). Alternatively, the assembly can be
supported by a rigid liquid supply pipe coupled to the
delivery tube 30.
To test the above-described design, an atomizing
transducer was built and tested. The dimensions were
chosen for an operating frequency of about 56 kHz. The
front and rear transducer sections were made of
titanium and the central bolt was made of 316 stainless
steel, to provide corrosion resistance for a wide
variety of operating liquids. Due to the relatively
low yield strength of this material, however, it is a
marginal choice particularly for higher frequencies,
because the bolt may have to be torqued beyond its
yield point to obtain the required compression on the
piezoelectric disks. Thus, in applications where
corrosion resistance is not a prime consideration, it
may be preferable to use a stronger steel for the
central bolt material.
In testing the completed assembly, it was found to
be essential to avoid any axial loading on the O-rings
of the shell; otherwise, the electrical impedance of
the unit would vary over a wide range with time, making
it impossible to maintain operation at peak efficiency.
With purely radial compression of the O-rings, however,
stable operation and repeatable results were easily
obtained. The shell was leak-free even when the unit
was operated submerged under water.
Accordingly, the design of the present invention
is adapted to provide an ultrasonic atomizing
transducer that is simple to manufacture and is
completely shielded from damp or hazardous
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1 environments, such as explosive a~mospheres. By
eliminating the clamping flanges of prior designs, it
is possible to obtain a high amplification factor
without having the transverse dimensions of the
transducer body exceed the practical limit for
achieving substantially one-dimensional vibration.
Certain changes and modifications of the disclosed
embodiment will be readily apparent to those skilled in
the art. For example, the central bolt could be
integrally formed as part of the rear or front section
in applications where the material of the section is
strong enough to carry the necessary tensile stress for
preloading the piezoelectric disks. In addition, it is
possible to provide many different sealing arrangements
within the prescribed limitations. It is the
applicants' intention, therefore, to claim all those
changes and modifications which could be made to the
disclosed embodiment without departing from the spirit
and scope of the invention.
