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Claims  |
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The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follow:
1. A temperature-compensated cavity filter comprising:
(a) a metallic case defining a resonant cavity, said case having a hole
formed therein; and
(b) a tuning screw mounted in said case hole and protruding into said
cavity, said tuning screw including: an elongate body having a
longitudinal bore; a thermal compensating member positioned at least
partially within said longitudinal bore; at least one compensation
bimetallic washer having top and bottom surfaces, said at least one
compensation bimetallic washer disposed in said longitudinal bore adjacent
said thermal compensating member and being flat at a first temperature,
whereby changes in temperature in a first direction cause said at least
one compensation bimetallic washer to deflect in a direction normal to
said top and bottom surfaces so that said at least one compensation
bimetallic washer moves said thermal compensating member in a first
direction and said at least one compensation bimetallic washer has a
maximum deflection at a second temperature; and a biasing means disposed
in said longitudinal bore adjacent said thermal compensating member for
loading said at least one compensating bimetallic washer and moving said
thermal compensating member in a second direction opposite said first
direction.
2. A tuning screw for use with a cavity filter having a resonant frequency
that drifts in response to changes in temperature, said tuning screw
comprising:
(a) an elongate body having a longitudinal bore;
(b) a thermal compensating member positioned at least partially within said
longitudinal bore;
(c) at least one compensation bimetallic washer having top and bottom
surfaces, said at least one compensation bimetallic washer disposed in a
portion of said longitudinal bore adjacent to a first portion of said
thermal compensating member and disposed around a second portion of said
thermal compensating member for moving said thermal compensating member in
a first direction in response to changes in temperature in a first
direction; and,
(d) a biasing means disposed in said longitudinal bore adjacent said
thermal compensating member for loading said at least one compensating
bimetallic washer and moving said thermal compensating member in a second
direction opposite said first direction.
3. The tuning screw claimed in claim 2, wherein said at least one
compensation bimetallic washer is substantially flat at a first
temperature, said changes in temperature in said first direction causing
said at least one compensation bimetallic washer to deflect in a direction
normal to said top and bottom surfaces, said at least one compensation
bimetallic washer attaining a maximum deflection at a second temperature.
4. The tuning screw claimed in claim 3, wherein a magnitude of said
deflection of said at least one compensation bimetallic washer is linearly
related to said changes in temperature.
5. The tuning screw claimed in claim 4, wherein said maximum deflection of
said at least one compensation bimetallic washer is 0.015 inches.
6. The tuning screw claimed in claim 3, wherein said first temperature is
higher than said second temperature.
7. The tuning screw claimed in claim 6, wherein a magnitude of said
deflection of said at least one compensation bimetallic washer is linearly
related to said changes in temperature.
8. The tuning screw claimed in claim 7, wherein said maximum deflection of
said at least one compensation bimetallic washer is 0.015 inches.
9. The tuning screw claimed in claim 8, wherein said first temperature is
greater than 100.degree. C. and said second temperature is less than
-30.degree. C.
10. The tuning screw claimed in claim 2, wherein said elongate body has
external threads substantially along a length of said elongate body.
11. The tuning screw claimed in claim 2, wherein said longitudinal bore
comprises:
(a) a neck region having a first open end and a second open end opposite
said first open end;
(b) a shaft region having an open end and a shoulder opposite said open
end, said shoulder formed where said shaft region joins said first open
end of said neck region; and,
(c) a head region having an open end and a shoulder opposite said open end,
said shoulder formed where said head region joins said second open end of
said neck region.
12. The tuning screw claimed in claim 11, wherein
(a) said compensation member second portion comprising a shaft having a
first end and a second end, said shaft passing through said neck region
such that said first end lies within said shaft region and said second end
lies within said head region; and,
(b) said compensation member first portion comprising a head having a front
surface and a back surface, said back surface of said head being connected
to said second end of said shaft, said head being positioned substantially
within said head region.
13. The tuning screw claimed in claim 2, wherein said shaft of said thermal
compensating member passes through said at least one compensation
bimetallic washer such that said at least one compensation bimetallic
washer is positioned between said back surface of said head and said
shoulder of said longitudinal bore head region.
14. The tuning screw claimed in claim 13, wherein said at least one
compensation bimetallic washer is substantially flat at a first
temperature, said changes in temperature in said first direction causing
said at least one compensation bimetallic washer to deflect in a direction
normal to said top and bottom surfaces, said at least one compensation
bimetallic washer attaining a maximum deflection at a second temperature.
15. The tuning screw claimed in claim 14, wherein a magnitude of said
deflection of said at least one compensation bimetallic washer is linearly
related to said changes in temperature.
16. The tuning screw claimed in claim 15, wherein said maximum deflection
of said at least one compensation bimetallic washer is 0.015 inches.
17. The tuning screw claimed in claim 14, wherein said first temperature is
higher than said second temperature.
18. The tuning screw claimed in claim 17, wherein a magnitude of said
deflection of said at least one compensation bimetallic washer is linearly
related to changes in temperature.
19. The tuning screw claimed in claim 18, wherein said maximum deflection
of said at least one compensation bimetallic washer is 0.015 inches.
20. The tuning screw claimed in claim 19, wherein said first temperature is
greater than 100.degree. C. and said second temperature is less than
-30.degree. C.
21. The tuning screw claimed in claim 2, wherein said biasing means is held
in compression between said first end of said shaft and said shoulder of
said shaft region.
22. The tuning screw claimed in claim 21, wherein said biasing means is a
coil spring and said shaft passes through said coil spring.
23. The tuning screw claimed in claim 2, wherein said biasing means
comprises at least one loading bimetallic washer disposed in said
longitudinal bore shaft region adjacent said thermal compensating member
for moving said thermal compensating member in said second direction in
response to changes in temperature opposite said first direction
temperature changes.
24. The tuning screw claimed in claim 23, wherein said at least one loading
bimetallic washer has a top surface and a bottom surface.
25. The tuning screw claimed in claim 24, wherein said at least one loading
bimetallic washer is substantially flat at a second temperature, said
changes in temperature in said second direction causing said at least one
loading bimetallic washer to deflect in a direction normal to said top and
bottom surfaces, said at least one loading bimetallic washer attaining a
maximum deflection at a first temperature.
26. The tuning screw claimed in claims 22 or 25, wherein said at least one
compensation bimetallic washer is substantially flat at said first
temperature, said changes in temperature in said first direction causing
said at least one compensation bimetallic washer to deflect in a direction
normal to said top and bottom surfaces, said at least one compensation
bimetallic washer attaining a maximum deflection at said second
temperature.
27. The tuning screw claimed in claim 26, wherein a magnitude of said
deflection of said at least one compensation bimetallic washer and a
magnitude of said deflection of said at least one loading bimetallic
washer are linearly related to said changes in temperature.
28. The tuning screw claimed in claim 27, wherein said first temperature is
higher than said second temperature.
29. The tuning screw claimed in claim 28, wherein said first temperature is
greater than 100.degree. C. and said second temperature is less than
-30.degree. C.
30. The tuning screw of claim 29, wherein said maximum deflection of said
at least one compensation bimetallic washer and of said at least one
loading bimetallic washer is 0.015 inches.
31. The tuning screw claimed in claim 2, wherein said elongate body and
said thermal compensating member are made of a conductive, nonmagnetic
material.
32. The tuning screw claimed in claim 1, wherein said conductive,
nonmagnetic material is brass.
33. The temperature-compensated cavity filter comprising:
a) a metallic case defining a resonant cavity, said case having a hole
formed therein; and
b) a tuning screw mounted in said case hole and protruding into said
cavity, said tuning screw including: an elongate body having a
longitudinal bore; a thermal compensating member positioned at least
partially within said longitudinal bore; at least one compensation
bimetallic washer having top and bottom surfaces, said compensation
bimetallic washer disposed in said longitudinal bore adjacent to a first
portion of said thermal compensating member and disposed around a second
portion of said thermal compensating member for moving said thermal
compensating member in a first direction in response to changes in
temperature in a first direction; and a biasing means disposed in said
longitudinal bore adjacent said thermal compensating member for loading
said at least one bimetallic washer and moving said thermal compensating
member in a second direction opposite said first direction.
34. The temperature-compensating cavity of claim 33 wherein said case
defines an opening and a base member is attached to said case over said
opening so as to define said resonant cavity.
35. The temperature-compensated cavity filter claimed in claim 33, wherein
said longitudinal bore in said elongate body comprises:
(a) a neck region having a first open end and a second open end opposite
said first open end;
(b) a shaft region having an open end and a shoulder opposite said open
end, said shoulder formed where said shaft region joins said first open
end of said neck region; and,
(c) a head region having an open end and a shoulder opposite said open end,
said shoulder formed where said head region joins said second open end of
said neck region.
36. The temperature-compensated cavity filter claimed in claim 35, wherein:
(a) said compensation member second portion comprising a shaft having a
first end and a second end, said shaft passing through said neck region
such that said first end lies within said shaft region and said second end
lies within said head region; and,
(b) said compensation member first portion comprising a head having a front
surface and a back surface, said back surface of said head being connected
to said second end of said shaft, said head being positioned substantially
within said head region.
37. The temperature-compensated cavity firlter claimed in claim 36, wherein
said shaft of said thermal compensating member passes through said at
least one compensation bimetallic washer and said at least one
compensation bimetallic washer is positioned between said back surface of
said head and said longitudinal bore head region.
38. The temperature-compensated cavity filter claimed in claim 37, wherein
said at least one compensation bimetallic washer is substantially flat at
a first temperature, said changes in temperature in said first direction
causing said at least one compensation bimetallic washer to deflect in a
direction normal to said top and bottom surfaces, said at least one
compensation bimetallic washer attaining a maximum deflection at a second
temperature.
39. The temperature-compensated cavity filter claimed in claim 38, wherein
said biasing means comprises at least one loading bimetallic washer
responsive to said changes in temperature for moving said compensation
member in said second direction in response to changes in temperature in a
second direction opposite said first direction, said at least one loading
bimetallic washer is held in position between said first end of said shaft
and said shoulder of said shaft region, said at least one loading
bimetallic washer having a top and bottom surface.
40. The temperature-compensated cavity filter claimed in claim 39, wherein
said at least one loading bimetallic washer is substantially flat at said
second temperature, said changes in temperature in said second direction
causing said at least one loading bimetallic washer to deflect in a
direction normal to said top and bottom surfaces, said at least one
loading bimetallic washer attaining a maximum deflection at said first
temperature.
41. The temperature-compensated cavity filter claimed in claim 38, wherein
said biasing means is a coil spring, wherein said shaft passes through
said coil spring, said coil spring is held in compression between said
first end of said shaft and said shoulder of said shaft region.
42. The temperature-compensated cavity filter claimed in claims 41 or 39,
wherein said first temperature is higher than said second temperature.
43. The temperature-compensated cavity filter claimed in claim 42, wherein
said maximum deflection of each of said at least one compensation
bimetallic washers and each of said at least one loading bimetallic
washers is 0.015 inches.
44. The temperature-compensated cavity filter claimed in claim 43, wherein
said body and said compensating member are made of a conductive,
nonmagnetic material.
45. The temperature-compensated cavity filter claimed in claim 44, wherein
said conductive, nonmagnetic material is brass.
46. A tuning screw for use with a cavity filter having a resonant frequency
that drifts in response to changes in temperature, said tuning screw
comprising:
(a) an elongate body having a longitudinal bore;
(b) a thermal compensating member positioned at least partially within said
longitudinal bore;
(c) at least one compensation bimetallic washer having top and bottom
surfaces, said at least one compensation bimetallic washer disposed in
said longitudinal bore adjacent said thermal compensating member and being
flat at a first temperature, whereby changes in temperature in a first
direction cause said at least one compensation bimetallic washer to
deflect in a direction normal to said top and bottom surfaces so that said
at least one compensation bimetallic washer moves said thermal
compensating member in a first direction and said at least one
compensation bimetallic washer has a maximum deflection at a second
temperature; and
(d) a biasing means disposed in said longitudinal bore adjacent said
thermal compensating member for loading said at least one compensating
bimetallic washer and moving said thermal compensating member in a second
direction opposite said first direction. |
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Claims |
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Description |
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FIELD OF THE INVENTION
This invention relates to cavity filters and, more particularly, to tuning
screws for cavity filters.
BACKGROUND OF THE INVENTION
Cavity filters and their uses are well known in the electrical filter art.
A cavity filter is basically a tuned electrical filter having a resonant
frequency that is determined, in part, by the geometry of a cavity. The
cavity may house component(s), such as a helical coil or rod, for example.
Openings in the case surrounding the cavity may be used to affect the
resonant frequency of the cavity filter. The cavity and the component(s)
housed in the cavity (if any) create capacitance and inductance values
that determine the resonant frequency of the cavity filter.
Cavity filters normally include a tuning apparatus that permits a user to
precisely tune the filter to a nominal resonant frequency. Such an
adjustment may be required to compensate for manufacturing tolerances
and/or variations in materials used to build the cavity filter, which can
cause the actual resonant frequency to vary from the nominal resonant
frequency. One form of tuning apparatus is a tuning screw. Tuning screws
are usually in the form of a threaded slug that is screwed into the cavity
of the filter through a threaded hole. The amount that the tuning screw
penetrates into the cavity controls the capacitance or the inductance
values of the filter. Consequently, the resonant frequency of the cavity
filter can be changed by changing the penetration of the tuning screw.
One problem associated with cavity filters is their sensitivity to
temperature changes. Changes in temperature produce physical changes in
cavity geometry that, in turn, produce changes in the electrical
characteristics of the cavity filter. These capacitance and/or inductance
changes cause the actual resonant frequency of the cavity filter to
"drift" from the nominal resonant frequency. In many applications, cavity
filters are required to operate over a wide temperature range. In such
applications, temperature-induced frequency drift can be significant
enough to make a cavity filter ineffective. For example, a significant
lowering of the resonant frequency due to increasing temperatures will
cause a corresponding lowering of the stopband cutoff frequency of a
cavity filter. As a result, important signal information may be
inadvertently filtered out by the cavity filter.
One approach that has been adopted by cavity filter manufacturers to
correct the temperature-induced frequency drift problem is the use of
temperature-stable materials in the construction of cavity filters. Cavity
filters built with materials having low thermal expansion characteristics
are less sensitive to temperature change than cavity filters built from
other materials. However, when exposed to large temperature changes, even
these cavity filters are subject to frequency drift problems, albeit to a
lesser degree. In applications where a cavity filter forms a part of
signal transmitting and receiving equipment operating with high frequency
signals, even reduced frequency drift can adversely affect the sensitivity
of the equipment or render the equipment totally useless.
Unfortunately, the tuning screws previously used in cavity filters are not
effective in correcting temperature-induced frequency drift because prior
art tuning screws react similarly to temperature changes as do the
components of a cavity filter, i.e., prior art tuning screws expand and
contract in response to increasing and decreasing temperatures in the same
way that other cavity filter components respond to increasing and
decreasing temperatures. As a result, prior art tuning screws may
contribute to the temperature-induced frequency drift problem, rather than
provide a solution to the problem.
As will be appreciated from the foregoing discussion, there has developed a
need in the electrical filtering art for a cavity filter whose performance
is less sensitive to temperature change over a wide range of temperatures.
The present invention provides a temperature-compensated tuning screw
that, when used with prior art cavity filters, actively compensates for
temperature-induced changes in the cavity filter and thereby reduces the
amount of temperature-induced drift in the resonant frequency of the
cavity filter.
SUMMARY OF THE INVENTION
In accordance with the present invention, a temperature-compensated tuning
screw for use with a cavity filter having a tuned frequency that drifts
with changes in temperature is provided. The temperature-compensated
tuning screw comprises: an elongate body having a longitudinal bore; a
compensating member; and, first and second biasing elements. The
compensating member is positioned substantially within the bore. The first
biasing element, which is responsive to changes in temperature, is coupled
to the compensating member so as to move the compensating member when the
first biasing element responds to changes in temperature. The second
biasing element loads the first biasing element. More specifically, the
first biasing element moves the compensating member in a first direction
in response to changes in temperature, and the second biasing element
moves the compensating member in a second direction, which is opposite to
the first direction.
When coupled to a cavity filter, the compensating member penetrates into
the cavity of the cavity filter. The amount of penetration is controlled
by the state of the biasing elements. Since the state of the first biasing
element is temperature dependent, the amount of compensating member
penetration is temperature dependent. In accordance with this invention,
the amount of temperature dependent penetrating change is chosen to
compensate for cavity geometry changes caused by temperature changes.
In accordance with further aspects of this invention, the first biasing
element comprises at least one compensation bimetallic washer that is
coupled to the compensating member and the bore such that the deflection
of the washers moves the compensating member in the first direction. The
second biasing element is a coil spring held in compression by the
compensating member and the bore.
In accordance with alternative aspects of this invention, the second
biasing element comprises at least one loading bimetallic washer that is
coupled to the compensating member and the bore such that deflection of
the washers moves the compensating member in the second direction. The
loading and compensation bimetallic washers are oriented such that changes
in temperature in one direction, e.g., an increase, cause movement of the
compensating member is one direction and changes in temperature in the
opposite direction, e.g., a decrease, cause movement in the other
direction.
In accordance with other aspects of this invention, the bore comprises a
head region, a shaft region, and a neck region that lies between the head
region and the shaft region. The compensating member comprises a head
coupled to one end of a shaft. The shaft passes through the neck region
and lies substantially within the shaft region. The head lies
substantially within the head region. The shaft passes through the
compensation bimetallic washers, which are held in place by a back surface
of the head region and a back surface of the head. The spring encircles
the shaft and is held in place by an end of the shaft opposite the head,
and a back surface of the shaft cavity.
As can be appreciated from the foregoing summary, the
temperature-compensated tuning screw of the present invention changes its
overall length so as to compensate for temperature-induced changes in a
cavity filter. As a result, the temperature-compensated tuning screw
substantially corrects for temperature-induced cavity geometry changes
that cause the resonant frequency of the cavity filter to drift.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other advantages of the present invention will become
better understood by reference to the following detailed description when
taken in conjunction with the accompanying drawings wherein:
FIG. 1 is a simplified elevation of a cavity filter and a prior art tuning
screw;
FIG. 2 is a sectional view of a preferred embodiment of a
temperature-compensated tuning screw formed in accordance with the present
invention;
FIG. 3 is a sectional view of an alternative embodiment of the
temperature-compensated tuning screw illustrated in FIG. 2;
FIGS. 4A and 4B are elevations of a compensation bimetallic washer suitable
for use in the temperature-compensated tuning screw depicted in FIG. 2;
and,
FIG. 5 is a simplified sectional view of a temperature-compensated tuning
screw formed in accordance with the present invention mounted in a helical
resonator-type cavity filter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
There has developed a need in the electrical filter art for a cavity filter
that is less sensitive to temperature changes over a wide range of
temperatures. The present invention is a temperature-compensated tuning
screw that can be used with prior art cavity filters to provide this
result.
FIG. 1 is illustrative of a cavity filter 10 having a prior art tuning
screw 14 coupled thereto. The cavity filter 10 comprises: a case 12; a
cavity 18 formed by walls of the case 12; and a threaded hole 13 that
passes through one wall of the case 12. The prior art tuning screw 14 is
an externally threaded slug that is screwed into the hole 13. The
rotatable tuning screw 14 penetrates into the cavity 18 a distance, P and
is secured by a lockdown nut 16.
The cavity filter 10 is tuned to a nominal resonant frequency, f, by
adjusting the penetration, P, of the tuning screw 14. A change in the
tuning screw penetration, .DELTA.P, will result in a change in the
resonant frequency, .DELTA.f. As is well known in the filter art, cavity
filters, such as the cavity filter 10 depicted in FIG. 1, are sensitive to
temperature. More specifically, temperature changes cause changes in the
geometry of cavity filters that result in changes in the resonant
frequency, .DELTA.f'. Accordingly, the nominal resonant frequency, f,
typically corresponds to a resonant frequency of the cavity filter 10 at
some nominal temperature, T. For example, if the cavity filter 10 is
intended for operation between operating temperatures of -30.degree. C.
and +60.degree. C., the nominal resonant frequency, f, may correspond to a
nominal temperature of 20.degree. C. (i.e., T=20.degree. C.). Accordingly,
the cavity filter 10 will remain tuned to the nominal resonant frequency,
f, as long as the operating temperature remains at, or near, the nominal
temperature, T. As noted above, a change in the operating temperature
results in a change in the nominal resonant frequency, .DELTA.f', i.e., a
change in operating temperature causes a frequency "drift". In general,
.vertline..DELTA.f'.vertline. increases as the operating temperature moves
further away from the nominal temperature, T. As a result, the
temperature-induced frequency drift, of a cavity filter 10 can be large
when the deviation from the nominal temperature is large.
While the prior art tuning screw 14 can be adjusted initially to tune the
cavity filter 10 to the nominal resonant frequency, f, it cannot
compensate for a temperature-induced frequency drift. Rather, the prior
art tuning screw 14 may actually increase the frequency drift problem. For
example, as the operating temperature deviation from the nominal
temperature increases, the geometry of the cavity 18 will change (i.e.,
expand), thereby changing electrical characteristics of the cavity filter
10. The changing electrical characteristics, in turn, cause f to change by
some value of .DELTA.f'. Additionally, the prior art tuning screw 14 will
also expand, i.e., lengthen, as operating temperature deviation increases.
The increased length of the tuning screw 14 causes the penetration, P, to
increase by some distance, .DELTA.P. As noted above, a change in the
tuning screw penetration, .DELTA.P, results in a change in the resonant
frequency, .DELTA.f. In the above example, both the expanding cavity 18
and the expanding tuning screw 14 create temperature induced drift values
that result in lowering of the nominal resonant frequency, f.
FIG. 2 illustrates a preferred embodiment of a temperature-compensated
tuning screw 22 formed in accordance with the present invention. As will
become better understood from the following discussion, this embodiment of
the temperature-compensated tuning screw 22 reduces its overall length (L)
in response to increasing temperature and extends its overall length in
response to decreasing temperature. As a result, unlike the prior art
tuning screw 14 discussed above, the temperature-compensated tuning screw
22 can be used to reduce the temperature-induced frequency drift, of a
cavity filter by compensating for temperature induced cavity geometry
changes by changing the amount of tuning screw penetration in a
compesatory manner.
The temperature-compensated tuning screw 22 comprises: a cylindrical
elongate body 36; a thermal compensating member 40; a coil spring 42; and,
six bimetallic washers 44. The body 36 includes a longitudinal bore 38 and
has an integral hexagonal head 37 located at one end. As an alternative to
the external hexagonal head 37, an internal hexagonal opening 78 could be
employed as illustrated by phantom lines in FIG. 2. Preferably, the body
36, and the compensating member 40 are machined from a conductive,
nonmagnetic material, such as brass, for example. If the tuning screw 22
is going to be used in a corrosive environment, other materials, such as
aluminum or stainless steel, may be preferable over brass.
External threads 46 are preferably located along substantially the entire
length of the body 36. The threads 46 and the cylindrical shape of the
body 36 permit screwing the tuning screw 22 into, for example, the
threaded hole 13 of the cavity filter 10 discussed above and illustrated
in FIG. 1.
The longitudinal bore 38 is centered along the longitudinal axis 48 of the
tuning screw 22. The bore 38 includes a cylindrical shaft region 50 that
extends from one end 56 of the tuning screw 22 through the hexagonal head
37 and into the body 36, and a cylindrical head region 52 that extends
from an opposite end 58 of the tuning screw 22 into the body 36. The shaft
and head regions 50 and 52 are connected by a cylindrical neck region 54.
The neck region 54 has a diameter substantially smaller than diameters of
the shaft and head regions 50 and 52. In accordance with the preferred
embodiment of the invention, the diameter of the head region 52 is
substantially larger than the diameter of the shaft region 50. When
measured along the longitudinal axis 48, the shaft region 50 is
substantially longer than the head region 52, which is longer than the
neck region 54. Obviously, other size relationships commensurate with
achieving the objectives of the invention can be used if desirable.
The compensating member 40 includes a cylindrical head 62 located at one
end of a shaft 64. The head 62 includes a back surface 63 located adjacent
to the shaft 64 and a front surface 61 located at the outer end of the
head. The diameter of the head 62 is substantially larger than a diameter
of the shaft 64. The diameter of the head 62 is substantially equal to the
diameter of the head region 52, such that the head 62 can be slidably
received in the head region 52. The diameter of the shaft 64 is
substantially equal to the diameter of the neck region 54, such that the
shaft 64, but not the head 62, can be slidably received in the neck region
54. An end 66 of the shaft opposite the head 62 has a small axial bore 68
and a neck 70. An edge 71 of the neck 70 is flared outwardly by a stake 74
that is inserted into the bore 68. A retaining washer 72, mounted on a
shoulder that surrounds the neck 70, is held in place by the flared edge
71.
The diameter of the holes in the six bimetallic washers 44 is slightly
larger than the diameter of the shaft 64 and the diameter of the outer
periphery of the bimetallic washers 44 is slightly less than the diameter
of the head region 52. Further, the coil spring 42 is sized to be slidably
mounted on the shaft 64 of the compensating member 40. The temperature
compensated tuning screw 22 is assembled by mounting the six bimetallic
washers 44 on the shaft 64 of the compensating member 40. Then the head 63
of the compensating member 40 is mounted in the head region 52 such that
the shaft 64 passes through the neck region 54. As a result, the six
bimetallic washers are located between the back surface 63 of the head 62
and a shoulder formed where the head region joins the neck region. Next
the coil spring 42 is mounted on the shaft 64 and the retaining washer is
mounted on the shoulder that surrounds the neck 70. Then the stake 74 is
driven into the bore 68.
In accordance with the preferred embodiment of the invention, and as will
be discussed more fully below, the bimetallic washers 44 are responsive to
temperature changes lying within a particular range, and thus, produce a
force that is temperature dependent. The force controls the position of
the compensating member 40. Thus, the bimetallic washers 44 can be defined
as compensation bimetallic washers. For example, in one particular working
model of the temperature-compensated tuning screw 22, the compensation
bimetallic washers 44 are responsive to temperature changes lying between
-30.degree. C. and 100.degree. C. The compensation bimetallic washers 44,
in the above example, flatten with increasing temperatures and deflect so
as to become conical with decreasing temperatures within the temperature
range (i.e., -30.degree. C.-+100.degree. C.). The compensation bimetallic
washers 44 are stacked on the shaft 64 such that the deflection of one
compensation bimetallic washer 44 is in the opposite direction of the
deflection of the adjacent compensation bimetallic washer(s) 44. This
stacking arrangement is further illustrated in FIG. 2, which depicts the
compensation bimetallic washers 44 in conical states.
As will become better understood from the following discussion, the
deflection of the compensation bimetallic washers 44 causes the overall
length, L, of the temperature-compensated tuning screw 22 to increase with
decreasing temperatures. An increase in temperature causes the
compensation bimetallic washers to flatten, which, in combination with the
expansion of the coil spring 42, causes the overall length, | | |
