Note: Descriptions are shown in the official language in which they were submitted.
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GAS SPRING
FIELD OF THE INVENTION
This invention relates to a gas compression spring
and, in particular, to a spring having an improved
packing associated with the slidable piston rod for
permitting the spring to operate for a large number of
cycles, such as at least 1,000,000 cycles, while utiliz-
ing relatively high internal pressures, such as
2,000 psi.
BACKGROUND OF THE INVENTION
Gas springs are utilized in many different types of
equipment for absorbing and cushioning forces. These
gas springs have been extensively utilized in the
automobile industry in association with presses to
cushion the large pressing forces. Such springs, one
example of which is illustrated in U.S. Patent No.
3 101 194 (W. E. Hennells, Sr.), have typically utilized
rather low pressure to permit the spring to perform a
large number of working cycles. This low internal
pressure`has required the use of larger cross-sectional
areas, and the use of a substantial number of air
springs disposed in parallel relationship~ in order to
effectively cushion large pressing forces. Use of such
gas springs is hence often more complex than desired,
and in some instances such usage is impossible due to
space or other operational limitations.
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3~
Because of the desired performance which can be
achieved by gas springs, specifically on presses,
numerous attempts have been made to increase the
internal pressure of the gas spring so as to substan-
tially increase the force-cushioning capacity and/or
reduce the overall spring size. Thus, numerous manufac-
turers have attempted to provide a gas spring having a
normal internal gas pressure in the range of 2,000 to
3,000 psi. However, most of these springs have proven
wholly unsuccessful since the springs have been observed
to rapidly permit leakage of the pressurized gas past
the stuffing box associated with the piston rod, and
hence the spring fails after a relatively small number
of operational cycles.
In an attempt to provide a high-pressure gas spring
which would permit successful operatlon over a larger
number of cycles, such as at least 1,000,000 cycles,
Applicant has extensively studied the performance of gas
springs, and specifically the typical failure point,
namely the packing between the housing and the piston
rod. It has been observed that, when the spring pres-
sure e~ceeds about 1,500 psi, the rubber sealing ring as
typically associated with the packing undergoes rapid
failure since sealing rings constructed of rubber and
similar products are unable to successfully perform a
large number of cyclic operations under pressure levels
in excess of about 1,500 psi. Such rubber sealing rings
have been observed to rapidly wear and deteriorate when
operated under high pressures, apparently due to the gas
permeating the porosity of the rubber and becoming
temporarily trapped therein.
It has also been observed that the typical stuffing
box bushings as provided between the housing and piston
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rod per~orm unsatisfactorily. It is common to utilize
bronze bushings for slidably supporting the piston rod.
However, this bushing is unable to provide sufficient
lubrication between the elastomeric sealing ring and the
piston rod, and hence the sealing ring wears rapidly due
to lack of lubrication.
Experimental evaluation o~ high-pressure gas
springs employing packings formed by bronze bushings and
conventional rubber or elastomeric sealing rings has
indicated the inability of such gas springs to success-
fully operate for a large number of cycles due primarily
to excessive wear of the sealing ring.
In apparent recognition of the failure of the
packing structure, there has been developed one high
pressure gas spring which appears to have the capability
of operating for a large number of cycles. This spring,
known as the "Kaller" spring and manufactured by
Stroemsholmens Mekaniska Verkstad AB, uses an internal
gas (nitrogen) maintained at a relatively high pressure
while allegedly permitting a large number of operational
cycles. In this Kaller spring, there is provided a
special overpressure seal associated with the packing,
which seal has a higher pressure than the nitrogen
pressure so as to prevent gas leakage from the unit.
This special packing involves a pair of opposed portions
defining thereon lips for slidably engaging the piston
rod, which packing is axially compressed by the nitrogen
within the unit through a piston arrangement which
magnifies the pressure so that the packing ring has a
higher pressure imposed thereon in an attempt to create
a long-wearing sliding seal with the piston rod. The
overall arrangement of this Kaller spring, however, and
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specifically the packing structure therefor, i5 complex
and is believed less dependable than desired.
This inventi.on relates to a gas spring which
incorporates an improved packing structure capable of
providing a lubricated guide and seal for slidable
engagement with the piston rod so as to permit the gas
spring to contain therein a high-pressure gas while at
the same time enabling the spring unit to successfully
function for a large number of operational cycles. The
packing structure includes a pair of lubricating-type
bushings disposed for slidable engagement with the
piston rod, which bushings are axially spaced and
confine a seal ring therebetween, the latter preferably
being of polyurethane. The bushings are preerably
impregnated with a lubricant such as graphite to effect
lubrication of not only the bushings, but also of the
slidable contact area between the piston rod and seal
ring to minimize wear and hence enable the unit to
operate for a large number of cycles. The sealing ring,
by being constructed of a stiff material such as
urethane, can withstand exposure to the high-pressure
gas within the unit and still successfully operate for a
large number of cycles. The innermost bushing pref-
erably abuts against a shoulder on the housing, and not
against the seal ring, to avoid imposition thereon of
excessive compressive forces, whereby the seal ring
remains in its desired configuration so as to effec-
tively sealingly engage the piston rod.
It is thus an object of this invention to provide
an improved gas spring which employs a packing ring
capable of operating with a high-pressure gas, such as
in the order of 2,000 to 3,000 psi, and capable of
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operating for a large number of cycles, such as
1,000,000 cycles.
A further object is to provide an improved gas
spring, as aforesaid, which utilizes a packing structure
which is of substantial simplicity, such as by employing
a pair of bushings which are axially spaced and have a
seal ring therebetween, whereby the packing structure is
relatively economical to manufacture and assemble.
A further object is to provide an improved gas
spring, as aforesaid, which is capable of absorbing or
cushioning large external forces while at the same time
enabling the spring unit to be of minimal size and
complexity, which spring unit can be either self-
contained or connected through conduits to a surge tank
~ontaining pressurized gas so as to positively maintain
a constant base pressure within the spring unit.
Other objects and purposes of the invention will be
apparent to persons familiar with structures of this
general type upon reading the following specification
and inspecting the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a central sectional view of the gas
spring unit, the lower half illustrating the unit in the
extended position assumed when a load is to be applied
thereto, and the upper half illustrating the unit in its
collapsed position after having had an external load
imposed thereon.
Figure 2 is an enlarged sectional view illustrating
the packing structure associated with the unit.
Certain terminology will be used in the following
description for convenience in reference only, and will
not be limiting~ For example, the words "upwardly",
"downwardly", "leftwardly" and "rightwardly" will refer
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to directions in the drawings to which reference is
made. The words "inwardly" and "outwardly" will refer
to directions toward and away from, respectively, the
geometric center of the unit and designated parts
thereof. Said terminology will include the words
specifically mentioned, derivatives thereof, and words
of similar import.
DETAILED DESCRIPTION
Figure 1 illustrates a gas spring unit 10 according
to the present invention. This unit includes a substan-
tially hollow housing 11 defining therein a chamber 12.
A piston 13 is disposed within the chamber 12 and is
secured to the inner end of an elongated piston rod 14
which projects outwardly through one end of the housing,
whereby the piston is hence supported for slidable
movement along the longitudinal central axis 15 of the
unlt .
The housing 11 includes a main body 16 which is of
a tubular or sleevelike construction. Body 16 is closed
at the one end thereof by an end cap 17 which is fixed
thereto, as by welding. The other end of body 16 has a
further end cap 18 secured thereto. This end cap 18
includes a sleeve part 19 which is received within the
end of body 16 and is rigidly joined thereto, as by a
threaded connection 21. A conventional elastomeric
O-ring 22 is seated in a groove formed adjacent the
inner end of sleeve part 19 to create a sealed engage-
ment with the inner wall of body 16. The inner end of
sleeve part 19 is positioned closely adjacent, and
effectively abuts, an annular stop ring 23 which is
stationarily positioned within the body 16 in surround-
ing relationship to the piston rod. This stop ring 23
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in turn abuts against an annular shoulder 24 formed on
the body 16.
The outer end of the cap 19 has a flange portion
which projects radially outwardly so as to e~fectively
radially overlap the body 16, which flange portion has
an annular undercut recess 25 formed thereon. This
recess in turn accommodates therein an annular rim 26
which projects axially from the end of the body 16. In
this manner, the end cap effectively surrounds the end
of the sleevelike body 16 to create a hoop effect to
prevent expansion of the body 16 due to the high-
pressure gas contained therein.
The piston 13 is fixedly secured, such as by
welding, to the inner end of the piston rod 1~. The
piston 13 has an annular guide ring 27 mounted thereon
in surrounding relationship there~o, which guide ring is
disposed in slidable engagement with the surrounding
inner annular wall 28 formed on the body 16. This guide
ring 27 maintains the piston ring in centered and
slidably supported relationship with respect to the body
16. Guide ring 27 is preferably constructed of a
glass-filled nylon to permit the piston to successfully
undergo a large number of cycles.
Piston 13 divides the interior compartment 12 into
a pair of chambers, namely a main pressure chamber 29
and a balancing chamber or reservoir 31~ The main
pressure chamber 29 is defined rearwardly (that is,
leftwardly) of the piston, whereas the relief chamber 31
is defined forwardly ~that is, rightwardly) of the
piston and comprises the annular region between body 16
and piston rod 14, which annular region extends from the
piston 13 to the stop ring 23. The piston 13 has one or
more ports or orifices 32 extending axially therethrough
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to provide for unrestricted and continuous communication
between the chambers 29 and 31 so as to balance the
pressure on opposite sides of the piston .
The piston rod 14 projects outward:Ly of the housing
through a central opening 33 formed in the end cap 19.
This opening 33 has a packing structure 36 associated
therewith for creating a slidable but sealed supportive
engagement with the external surface 34 of the piston
rod 14.
The packing structure 36, as illustrated by Figure
2, includes a first bushing ring 37 which is station-
arily seated on the annular wall 38 defined by an
internal bore formed in the end cap 18, the bushing ring
37 having an extremely close fit with the wall 38. The
bushing ring 37 effectively has the inner axial end
thereof disposed against the stop ring 23, whereas the
outer axial end is effectively seated against an annular
shoulder 39 defined on the end cap. This first bushing
ring 37 has an inner annular wall or bore 41 which is o~
only slightly larger diameter than the piston rod 14 so
as to effectively slidably support the latter.
The packing structure 36 includes a second bushing
ring 42 which is also stationarily seated in the end cap
18, This bushing ring 42 has the outer annular surface
thereof disposed in engagement with an inner annular
wall 43 defined by the internal bore of the end cap,
this engagement being effectively a light press fit.
Bushing ring 42 also has the outer axial end thereof
disposed in abutting engagement with an annular shoulder
44 formed on the end cap. This bushing ring 42 also has
an inner annular wall 46 which is only of slightly
larger diameter than the rod surface 34 so as to main- -
tain a slidable supportive engagement therewith.
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g
Each of the bushing rings 37 and 42 are of substan
tially the same structure in that they each comprise a
base ring member 47 which is constructed in the manner
of a standard bushing, such as a bronze bushing,
although more preferably of Amco bronze (bron~e contain-
ing steel therein), with the base ring member having
concentric inner and outer diameters and having an axial
length substantially greater than the radial thickness
thereof. The base ring member 47 has a plurality of
small openings 48 extending radially therethrough, which
openings are spaced circumferentially around the com-
plete extent of the ring member, and each of these
openings is filled with a dry lubricant, preferably
yraphite. Guide bushings of similar structure are known
and are manufactured by Oiles Industry Co., Ltd.
The packing structure 36 also includes an e:Lasto-
meric seal ring 51 which i5 confined within the end cap
18 axially between the bushing rings 37 and 42 for
creating a slidable sealed engagement with the rod
surface 34. This seal ring 51 is of the cup-type when
viewed in cross section so as to have inner and outer
annular lipS 52 separated by an annular groove 53, which
groove opens inwardly for communication with the chamber
31. This seal ring has the base end 54 thereof disposed
so as to abut against the inner axial end 56 of the
outer bushing ring 42. The other end of the seal ring,
namely the end defined by the annular lips 52, is
normally maintained in small axially spaced relationship
from the inner bushing ring 37 so that the latter is
prevented from axially compressing the seal ring.
There is also preferably provided a lip seal 58
which is secured within the end cap 18 and creates a
slidable engagement with the rod surface 34. This lip
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seal 58 is disposed axially outwardly from the packing
34 and is provided primarily for preventing contaminants
on the piston rod from entering into the gas spring
unit~
The piston rod 14 is preferabl~ provided with an
enlarged bore or opening 59 extending over a majority of
the length thereoE, which bore opens through the inner
end thereof for open communication with the main pres-
sure chamber 29, whereby this bore 59 in effect com-
prises a part of the main pressure chamber 29 so as toincrease the volume of the chamber and hence the volume
of the high-pressure gas stored therein.
The rear end cap 17 has, in the illustrated embodi-
ment, a passage 61 therethrough for communication with
the main pressure chamber 29. This passage 61, in the
illustrated embodiment, has a spring-urged check valve
62 associated therewith and normally maintained in a
closed position for preventing escape of gas from the
spring unit. This check valve 62 enables the spring
unit to be charged, or recharged when necessary, with
high-pressure gas, preferably nitrogen. If desired, the
passage 61 can be connected via a suitable conduit to an
external surge tank, which surge tank can be of greater
volume and can maintain therein a quantity of high-
pressure gas. A check valve 62 would again be provided,
either on the spring unit or associated with the conduit
connected between the spring unit and the surge tank, so
as to permit the main chamber 29 in the spring unit to
be automatically replenished if the pressure therein
falls below a predetermined magnitude. By using an
external surge tank, a single surge tank can be con-
nected to a plurality of spring units so as to ensure
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that a proper minimum pressure is maintained in the
spring units at all times.
The check valve 62 is, in the illustrated embodi-
ment, of a conventional structure substantially corre-
sponding to a tire-stem valve. ~owever, numerous other
check valve constructions could be utilized, if desiredO
OPERATION
While the operation of the gas spring unit is
believed obvious from the description set forth above,
nevertheless same will be briefly described to ensure a
complete understanding thereof.
The gas spring unit 10 is initially charged with
gas, particularly nitrogen, so that the gas pressure in
the chambers 29 and 31 is at the same high level, such
as approximately 2,000 to 3,000 psi. When in the
charged condition, and when not subjected to an external
load, the pressure of the gas in main pressure chamber
29 acts against a greater piston area than the gas in
the chamber 31, and hence the piston rod assembly 13-14
automatically moves outwardly (rightwardly) until the
piston abuts the stop ring 23. Hence, the pressurized
gas spring unit is normally maintained in the position
wherein the piston rod 14 is fully extended, as illus-
trated by the lower half of Figure 1.
When an external load, such as a shock force, is
imposed axially against the exposed end of the piston
rod 14, the piston rod assembly 13-14 collapses inwardly
(leftwardly) into the housing 11. This effects a
reduction in the combined volume of the chambers 29 and
31, and causes a compression of the gas within the
chambers and hence an increase in the gas pressure. The
orifices 32 permit the pressure gas to flow into the
balancing chamber 31 so as to equalize the pressure in
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chambers 29 and 31. This hence causes the unbalanced
gas pressure to act only on an area equal to that
defined by the outer diameter of rod 14, thereby pre-
venting creation of an undesirably large cushioning
force. In this manner, the spring unit is able to
absorb and cushion the external load or shock force
which is imposed on the piston rod and hence effects
compression of the piston rod into the housing.
The piston rod assembly 13-14 will remain in the
collapsed position illustrated by the upper half of
Figure 1 until the e~ternal load i5 removed. There-
after, the pressurized gas within the main pressure
chamber 29, since it acts against a bigger piston area
than the gas within the chamber 31, causes the p:Lston
rod assembl~ 13-1~ to automatically move leftwardly
until the piston rod is again fully extended, and is
ready to have an additional shock load imposed thereon.
During the cyclic operation of the spring unit,
namely the cyclic movement of the piston rod 1~, the
spaced bushing rings 37 and 42 slidably support the
piston rod. Further, the graphite 49 associated with
the bushing rings provide for limited lubrication
between the piston rod and the bushing rings so that the
latter will undergo very little wear, and hence will
maintain a close but free slidable engagement with the
piston rod so as to permit the spring unit to operate
over a large number of cycles. In addition, some of the
graphite will contact the rod surface 34 at locations
disposed on axially opposite sides of the annular
sealing ring 51, and this graphite will be carried into
contact with the inner annular lip 52 of the seal ring
so as to also provide at least minimal lubrication
between the piston rod and the inner lip of the sealing
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ring 51. This minimal lubrication is sufficient to
substantially minimize weax of the sealing ring, where-
upon the sealing ring permits the spring unit to undergo
a large number of operational cycles, while at the same
time not interfere with the desirable sealed relation-
ship created between the sealing ring and the piston
rod.
The sealing ring 51 is preferably constructed of
polyurethane since this provides the sealing ring with a
limited amount of elasticity so as to create the desired
sealed engagement with the piston rod, while at the same
time the sealing ring still possesses substantial
stiffness and strength so as to not undergo any
undesired deformation due to its exposure to the high-
pressure gas contained within the spring unit.
The gas spring unit of this invention, substan-
tially as illustrated by the drawings, has already been
experimentally tested with a precharge internal pressure
therein of 2,000 psi (which precharge pressure exists
when the spring unit is in the extended position), and
this experimental spring unit has been subjected to
l,000,000 cycles of operation. The experimental unit
was observed to successfully operate in the desired
manner throughout the 1,000,000 test cycles, and was not
observed to experience any significant leakage of gas or
to experience any significant wear of the packing
structure.
Although a particular preferred embodiment of the
invention has been disclosed in detail for illustrative
purposes, it will be recognized that variations or
modifications of the disclosed apparatus, including the
rearrangement of parts, lie within the scope of the
present invention.
