Shock absorber for quartz crystal enclosures using multiple contact points to distribute stress

A shock absorbing mechanism for protecting a fragile quartz crystal includes a Teflon coating on the internal surface of the crystal enclosure. The enclosure is provided with a plurality of detents arranged so that when the crystal is shocked in its most fragile plane a plurality of these Teflon coated detents are struck simultaneously to distribute the shock across the surface of the crystal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of shock protection for 
fragile quartz crystals and the like. More particularly, this invention 
relates to a shock absorbing mechanism for protecting quartz crystals from 
impact with the walls of its enclosure. 
2. Background 
Quartz crystals are frequently used in electronic radio equipment such as 
two-way radios, pagers and the like as high stability frequency 
determining elements. They are also used in such radio equipment as 
filtering elements due to their very high "Q" and therefore high 
selectivity. 
Unfortunately, it has long been recognized that such crystal devices are 
usually the most fragile components in the radio equipment. This presents 
special problems when the radio equipment is used in an environment which 
makes it especially susceptible to high mechanical shock such as police 
radios or pagers which may be subject to frequent drops. In these 
environments it is not unusual for crystal devices to shatter or crack 
when presented with excess mechanical shock. The problem is compounded by 
the rapid miniaturization of such equipment making it subject to higher 
impact velocities when carelessly tossed about. 
In order to enhance the reliability of such electronic equipment, it is 
clearly necessary to provide better mechanical shock protection for such 
crystal devices. Crystal devices which can consistently withstand shocks 
of approximately 20,000 to 30,000 times the force of gravity (20,000 to 
30,000 G's) for approximately 0.3 milliseconds are needed to insure the 
reliability of such electronic devices at present and in the future even 
greater shock performance will be necessary. At present, shocks in excess 
of this limit are absorbed by deformation and damage to the plastic 
enclosures typically used on such equipment. 
A number of solutions to this problem have been proposed and have met with 
varying degrees of success. Unfortunately, none of these proposals have 
been able to reliably and consistently enable such crystal devices to 
withstand mechanical shocks in excess of approximately 15,000 G's. Such 
proposals have included simply coating the inner surface of a crystal 
enclosure with plastic to absorb shock and inserting short sections of 
plastic tubing or plastic strips inside the crystal enclosure to absorb 
shock. While such techniques provide improvement of a factor of perhaps 
two to four times the shock levels crystal devices can withstand without 
them (typically approximately 2000 to 3000 G's unprotected), further 
improvement is required to achieve acceptable levels of reliability. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved shock 
absorber for fragile crystal devices. 
It is another object of the present invention to provide a crystal shock 
absorber which will allow crystal devices to survive shocks in excess of 
30,000 G's. 
It is another object of the present invention to provide a crystal shock 
absorber which overcomes the deficiencies of other crystal shock 
absorbers. 
It is another object of the present invention to provide an integral shock 
absorber for a crystal enclosure. 
It is a further object of the present invention to provide a shock 
absorbing system for crystal devices which does not interfere with the 
normal operation of the crystal. 
These and other objects of the present invention will become apparent to 
those skilled in the art upon consideration of the following description 
of the invention. 
According to one embodiment of the present invention, a shock absorbing 
mechanism for protection of a planar crystal device against impact with a 
crystal enclosure wall, wherein the crystal device has opposed major 
surfaces and an edge, includes a shock absorbing device having a plurality 
of contact points for contacting one of the major surfaces when the 
crystal device is subjected to mechanical shock. A carrier holds the shock 
absorbing device between one of the major surfaces and the enclosure wall 
so that the plurality of contact points contact one of the opposed major 
surfaces when the crystal device is shocked by a force exceeding a 
predetermined magnitude in a direction toward the shock absorbing device 
and substantially perpendicular to the opposed major surfaces. 
The features of the invention believed to be novel are set forth with 
particularity in the appended claims. The invention itself, however, both 
as to organization and method of operation, together with further objects 
and advantages thereof, may be best understood by reference to the 
following description taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to FIG. 1, a quartz crystal device 10 is shown attached to a 
base 12 of a crystal enclosure. The crystal device 10 is normally a planar 
quartz structure with opposed major surfaces and may be connected to the 
base by spring type connectors 14 and 16 or by other known techniques as 
is well known in the art. These connectors also serve to electrically 
couple the crystal device 10 to electrical leads 18 and 20. Referring 
momentarily to FIG. 3, insulators 19 and 21 are used to insulate the leads 
from the metal base. The crystal device 10 is enclosed by an enclosure 
cover 22 for protection against the elements and physical damage. Cover 22 
is typically made of metal and has a hard inner wall surrounding the 
crystal. Cover 22 may be attached to base 12 by conventional techniques 
such as soldering as is known in the art. 
While cover 22 effectively shields the crystal device 10 from humidity, 
debris and other hazards of direct contact with the environment, the cover 
also is usually in close proximity to the crystal device in order to 
maximize volumetric efficiency of the package. In the event of a sudden 
mechanical shock, particularly in a direction perpendicular to the plane 
of the crystal device, the crystal device 10 may impact the inner wall of 
the enclosure cover 22 causing the crystal to shatter or crack resulting 
in failure of the crystal device. 
According to one embodiment of the present invention, a Teflon (Dupont 
trademark for polytetrafluorethylene) insert 24 is utilized to absorb the 
shock of sudden impact. The details of insert 24 may be more readily 
visible in FIGS. 2, 3 and 4 showing top, front and side cutaway views of 
the insert respectively. These Figures should be referred to in 
conjunction with FIG. 1 for a full understanding of the present 
embodiment. This insert is made by heated vacuum forming of a sheet of 
0.004 to 0.005 inch thick Teflon film such as Dupont PFA 350 film in this 
embodiment. This particular dimension and type of film has been found 
suitable for the present embodiment for use with the industry standard 
HC-18 crystal enclosure. Other materials and dimensions may be suitable 
for this and other embodiments without deviation from the spirit of the 
invention. Since insert 24 is made of Teflon film, it may be made very 
thin compared with the tubing or strips of the prior art. The film may 
also be molded to produce controllable springs as will be discussed. 
Insert 24 includes sloping walls 26 and 28 which serve as a carrier for a 
number of concave dimple springs 30, 32, 34, 36, 38, 40, 42, 44, 46, and 
48. These dimple springs are specially formed and shaped to act as springs 
having appropriate spring rates to act as shock absorbers during 
mechanical shock. When inserted inside cover 22, edges 50, 52, 54 and 56 
compress against the inner walls of the cover to form a self aligning 
insert. In addition, the sloping wall 26 is suspended and cushioned by the 
spring action of the thin Teflon springs formed at the interface of the 
edges 50, 52, 54 and 56 and sloping wall 26. A similar spring cushion 
mounting system is preferrably present on both sides of the insert. 
Flat projections 27 and 29 serve to facilitate insertion of insert into 
cover 22. Additionally, flat projections 27 and 29 stiffen the lower 
portion of the insert so that springs 38 and 48 do not buckle and 
permanently deform when the assembly is shocked. The opening at the lower 
portion of the insert serves as a target for the crystal and base during 
assembly. The insert is then inserted in the cover 22 and is held in place 
by spring tension. The length of the insert is selected to assure 
self-alignment when the base assembly is installed. 
The spring structure above provides shock isolation when the crystal device 
is shocked by a force applied substantially in a direction perpendicular 
to the plane of the crystal. This is the most destructive plane of shock 
for the crystal. A pair of side ribs 60 and 62 are provided to provide 
shock isolation when the crystal device is shocked in a direction parallel 
to the plane of the crystal. This is a much less destructive plane of 
shock for the crystal and requires a less elaborate shock isolation 
system. 
The spring rates of the dimple springs are determined by a number of 
factors including the thickness of the material and the radius and height 
of the dimples above the carrier wall. For the present embodiment of the 
invention for use with the HC-18 crystal enclosure, the dimensions of 
Table 1 have been found suitable to consistantly provide protection 
against shocks well beyond 30,000 G's. Of course it is understood that 
these specific dimensions and specifications are presented only by way of 
example and should not limit the scope of the present invention. 
TABLE 1 
______________________________________ 
Parameter Specification 
______________________________________ 
Radius of dimple springs. 
.0...0.37 inches 
Height of springs 32 and 42. 
.0...0.22 inches 
Height of springs 3.0., 34, 36, 
.0...0.27 inches 
4.0., 44 and 46. 
Height of springs 38 and 48. 
.0...0.4.0. inches 
Center to center separation of 
.0...0.95 inches 
bubble springs 32 and 34, and 
42 and 44. 
Center to center separation of 
.0...0.83 inches 
bubble springs 34 and 38, and 
44 and 48. 
Center to center separation of 
.0..149 inches 
bubble springs 3.0. and 34, 34 and 
36, 4.0. and 44, and 44 and 46. 
Slope of carrier wall measured 
4..0. degrees 
from vertical after restraint in 
cover 22 (angle 8.0.) 
Slope of dimple springs measured 
1.0...0. degrees 
from vertical (angle 71) 
Location of dimple springs 34 
Axis of center of 
and 44. gravity of crystal 
Radius of innermost radius of 
.0...0.16 inches 
rib spring after restraint 
Center to center separation of 
.0..325 inches 
rib springs after restraint 
Thickness of film after forming 
.0...0..0.3 inches 
excluding dimple springs 
Thickness of dimple springs 
.0...0..0..0.6 inches 
at point closest to crystal 
after forming 
______________________________________ 
The operation of the present invention may be more fully appreciated with 
reference to the free-body diagram of FIG. 5. Assuming the crystal device 
is shocked by a force applied perpendicular to the major surfaces of the 
crystal, the crystal deflects at an angle as shown due to the high center 
of gravity and the effects of the spring connectors depicted as spring 69 
of FIG. 5. The crystal blank impacts each spring of the array of dimple 
springs substantially simultaneously distributing the force over the 
entire surface of the crystal. The shock force is then absorbed and 
dissipated by the dimple springs represented as springs 70, 72 and 74 in 
conjunction with the spring cushion mechanism which supports carrier wall 
26. This spring cushion mechanism is depicted as springs 80 and 82 in FIG. 
5. 
If the crystal device is shocked at an angle which is not exactly 
perpendicular to the plane of the crystal, it may initially strike only 
one of the dimple springs. The spring cushion mechanism represented by 
springs 80 and 82 is designed to have a lower spring rate than those of 
any of the dimple springs and in the present embodiment a spring rate of 
approximately 0.88 pounds per inch has been found acceptable. This lower 
spring rate allows the carrier wall 26 to pivot so that another of the 
dimple springs will contact the crystal surface. When three of the dimple 
springs have been contacted, the center of gravity of the crystal applies 
pressure to spring 34 and the stress of the shock is effectively 
distributed throughout the surface of the crystal. At this point the 
deflection of both the dimple springs and the spring cushion work together 
to absorb the shock of impact. 
It has been found especially advantageous to utilize five dimple springs in 
this embodiment with one near the center of the crystal blank and one 
adjacent each of the four crystal quadrants. The advantages of this may be 
better understood by modeling the crystal as a vibrating circular plate 
having simply supported edges. In addition to the conventional vibration 
modes of a quartz crystal, the quartz crystal may take on any of a number 
of high stress vibration modes such as those of the conventional vibrating 
plate model when the crystal device is mechanically shocked. Such models 
are well known and may be found for example in "Mechanical Vibration and 
Shock Measurement", by Prof. Jenstrampe Broch, 1980, Bruel and Kjaer, ISBN 
Number 87 87355 36 1. In each of the highest stress modes of vibration an 
antinode of vibration is located either at the center of the plate or near 
the center of one of the four quadrants. By providing a damped spring 
force in each of the four quadrants and the center, both the level and 
duration of vibrating stresses are reduced. 
The dimple springs are preferably designed to have a nonlinear spring rate. 
This is readily accomplished by making the thickness of the Teflon film 
gradually decrease as the portion of the spring closest to the crystal is 
approached. In the present embodiment, this thickness starts out at 
approximately 0.003 inches at the carrier wall and decreases to about 
0.0006 inches at the innermost point. The change in thickness is not shown 
in the Figures. This results in an initially low spring rate of 
approximately 0.1 pounds/inch which increases to an almost linear spring 
rate (listed below) as the spring is compressed. 
In addition, the spring rates of the springs preferrably decrease as the 
base 12 is approached. This is advantageous due to the increased velocity 
of the crystal near the top and relatively lower velocity near the base 
when shocked perpendicular to the crystal's major surfaces. By utilizing 
stiffer springs near the top, the greater velocities of impact are more 
readily absorbed further enhancing reliability. In the present example, 
the spring rate of dimple springs 32 and 42 is approximately 3.3 pounds 
per inch. The spring rate for dimple springs 30, 34, 36, 40, 44 and 46 is 
approximately 2.9 pounds per inch. The spring rate of dimple springs 38 
and 48 is approximately 2.2 pounds per inch. The combined spring rate for 
all five dimple springs on each carrier wall is approximately 14.2 pounds 
per inch. Of course, these spring rates are not to be limiting. These 
combinations of spring rates and dimensions have been found to be 
effective for thin crystal blanks on the order of 0.003 inches thick with 
a mass of roughly 0.01 grams including spring connectors 14 and 16 and 
provides reliable and repeatable protection against mechanical shocks well 
beyond 30,000 G's without interfering with the operation of the crystal in 
any way. As the thickness of the crystal device increases, it becomes less 
fragile and the spring rates may be adjusted accordingly. 
In an alternate embodiment, rib springs may be used in place of the dimple 
springs. This embodiment is illustrated in FIG. 6 in which insert 124 is 
depicted. The overall structure of the insert is very similar to that of 
insert 124 except for the substitution of rib springs 130, 132, 134, 140, 
142 and 144 for the dimple springs. These rib springs are supported by 
carrier walls such as wall 126 which is at a similar angle to wall 26. The 
height of the rib springs tapers inward toward the crystal device similar 
to the way the dimple springs tapered in the previous embodiment. 
Due to slightly different manufacturing technique, it is more difficult and 
therefore more expensive to obtain varying material thickness with the rib 
springs in order to obtain the desired nonlinear spring rate. It is 
however a desirable feature if the added expense is acceptable. Otherwise, 
the thickness may be left at approximately the original film thickness 
with acceptable results. It should be noted that the spring rate of the 
ribs varies continuously over the length of the rib to obtain the desired 
change in spring rates for the varying distance from the base. The dual 
spring system of the present embodiment functions in the same manner as 
that of the previous embodiment wherein the cushion spring system is 
bordered by 152, 154 and 156. 
In another embodiment, sloping dimple or rib or other appropriately shaped 
springs may be formed as shown in FIG. 7. In this embodiment (illustrated 
with dimple springs) a plurality of dimple springs 230, 232, 234, 236, 
238, 240, 242, 244, 246 and 248 are implemented by forming appropriate 
sized detents in a crystal enclosure cover 222. A Teflon or similar 
coating may then be applied to the inner coating of the cover 222 by 
spraying electrostatically charged particles inside the cover or by other 
known methods of obtaining a consistant coating. A smooth coating of 
approximately 0.015 to 0.005 inches in thickness is appropriate to provide 
an acceptable level of protection. The springs are created in a geometry 
which allows the surface of the crystal to impact each of the springs 
adjacent the surface being impacted substantially simultaneously similar 
to the other embodiments. This results in distribution of stress 
throughout the crystal when shocked in its most fragile plane. 
In this embodiment, the shock is absorbed by the compression of the Teflon 
coating whereas the previous embodiments utilized both the compression and 
the flexure of the springs to absorb the impact. In addition, the present 
embodiment uses only a single spring system rather than the cushion spring 
and the dimple (or rib etc.) spring system discussed previously. As a 
result, the present embodiment will not provide protection as effectively 
as the previous embodiments. Improvements in shock performance on the 
order of 7 to 8 times that of an unprotected crystal can be anticipated. 
In some applications this level of protection may be adequate and the use 
of the present embodiment may provide manufacturing advantages by reducing 
the number of parts required for final assembly of the crystal package. 
Slight size reduction may also be possible by using the present 
embodiment. 
While the embodiments presented are specifically directed toward crystal 
enclosures having somewhat oval cross sections, it will be evident to 
those skilled in the art that the present invention may be readily adapted 
to other crystal enclosure geometries such as the cylindrical I.B.P. type 
package by making suitable modifications. Suitable modifications may also 
be made to the spring materials and spring geometries without deviating 
from the scope of this invention. 
Thus it is apparent that in accordance with the present invention an 
apparatus that fully satisfies the objectives, aims and advantages is set 
forth above. While the invention has been described in conjunction with 
specific embodiments, it is evident that many alternatives, modifications, 
and variations will become apparent to those skilled in the art in light 
of the foregoing description. Accordingly, it is intended that the present 
invention embrace all such alternatives, modifications and variations as 
fall within the spirit and broad scope of the appended claims.