Note: Descriptions are shown in the official language in which they were submitted.
1136~73
The invention concerns a sliding seal for pistons
and piston rods of operating cylinders and flying pistons
of pressure storage devices consisting of a combination of
gaskets and oil rings.
In known seal combinations, the oil rings themselves
are made of a plastically workable material,which in itself
is not self-lubricating, serving as a loose supporting
screen for a high proportion of a solid lubricant of molybde-
num disulfide (MoS2) or similar substances. However, oil
rings of this type become fully effective only when sliding
friction on practically dry sliding surfaces is obtained,
and the oil ring itself or the abraded solid lubricant
particles are stressed with shear forces high enough to
separate suitable layer lattice lamina of single crystal
platelets.
In a known sealing combination of the type mentioned,
the high stress required for the solid lubricant and
the equally important dry operation are made possible by
pressing a high strength, elastic sealing material, pre-
ferably cross-linked polyurethane having about twice the
strength of con~entional natural and synthetic rubber seals,
very strongly against the sliding surface with the aid of
a stepped groove, thus obtaining optimum reductions of the
friction coefficient, wear and frictional heat, by reason of
the smooth and compact layer of the solid lubricant pro-
duced between the seal and the original sliding surface.
1136173
In keeping with the state of the art, however,
permanent pressure on the highly elastic seals of a suf-
ficient degree can be produced only with materials hav-
ing a narrowly limited field of application and, these
S materials specifically are not applicable at elevated
operating temperatures, in the presence of traces of
water in the pressurizing agent and in the presence of
synthetic, non-flammable or organic hydraulic liquids.
Beyond this, the need for a stepped sealing groove elimin-
ates the use of this combination with existing rectangular
sealing grooves.
In another known sealing combination, the elastic
gasket is supported against the side of the groove facing
away from the pressure with a so-called back-up ring
lS (washer) with additional lips pointing in the direction of
the gasket. The lips have the intended function of counter-
acting the destruction and "nibbling" effect of the gasket.
Backup rings of this type, because of their shape, cannot
be cut from simple pieces of tubing and thus their produc-
tion is relatively expensive. Beyond this, while such
backup rings can be combined with gaskets made of different
materials, they themselves cannot be made of materials
forming loose, plastic supporting screens for the embedding
of solid lubricants. Furthermore, such a sealing combina-
tion, even with additional lips on the backup rings, does
not satisfy any of the conditions (absolute tightness,
unusually high compressive and shear forces) necessary
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1136173
for the ormation of a homoyeneous, smooth solid lubricant ~~ r
layer without detectable laminar boundaries on the surface
to be sealed.
Another known sealing arrangement contains,between
the inner gasket and the outer hard backup rings,a relatively -
soft intermediate ring having good adaptability through
plastic flow to the adjacent structural parts. Because of the
parallel shape given and the so-called "cold flow" of the
intermediate ring, the extrusion gap caused by high operat-
ing pressures and also produced by manufacturing inaccuracies,
is completely sealed, thus also protecting the gasket itself
against destruction by "nibbling".
Here ayain, however, practically absolute tightness
cannot be obtained, especially at higher piston velocities
and/or with rough sliding surfaces, nor can adequate shearing
force be permanently applied on the solid lubricant which
may be incorporated in the intermediate ring. The slight
proportion of the solid lubricant potentially present,
in addition, is solidly bonded to the base material of
the intermediate ring. Even with respect to mass, the amount
present would not be sufficient to form a compact inter-
mediate layer between the original sliding surface and the
gasket. Generally known sealing combinations with compact
cross-sections of the gaskets are not always satisfactory
in practical applications. This is also true of the so-
called sleeve packings, using sealing materials such as
fabric-reinforced grades of synthetic rubber, including
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polymers or copolymers of butadiene, acrylonitrile, `~
chloroprene or vinylidene and hexafluoropropene fluorides.
Beside the fact that these sealing combinations
can be used in highly machined tubular cylinders only,
fabric reinforcements are used primarily ~o that the
fabric pattern of the sliding part will retain traces of
the liquid pressurizing agent in order to provide a so-
called "liquid emergency lubrication" under critical
operating conditions and thus to prevent a sudden collapse
of the seals.
It is of disadvantage in this instance that the
amount of oil embedded in the fabric matrix is entirely
inadequate for the establishment of a hydrodynamic lubricat-
ing film on the gaskets themselves, and on the guiding
surfaces associated with it. In order to obtain the satura-
tion of the fabric matrix with liquid pressurizing agents,
the sealing action at the gasket edges themselves or at the
gasket must be reduced to the extent that if a higher
functional quality of such sealing combinations is required,
in most cases several gasketsmust be applied to one sealing
location or the location must be greatly extended and de-
signed with a plurality of gasketing edges.
Sealing combinations of the type described are
very expensive and their trouble-free use is generally
possible only,if first an acceptable compromise between
permissible leakage and durability is obtained through
extensive experimentation under operating conditions
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1136173
that must be defined as closely as possible. -
The object of the invention is to provide a slid-
ing seal, while avoiding the above-mentioned disadvantages
for the pistons and piston rods of operating cylinders
and the flying pistons of pressure storage devices, which
would safely prevent the formation of a film of a liquid r
originating in the operating medium, both in the high
pressure range and under very slight pressures of the work-
ing medium and at very different stroke velocities on the
side of the sealing combination or the oil ring facing
away from the pressure. This is to be accomplished with-
out suffering appreciable piston and piston rod wear in
operation and additionally,permitting the use of extra-
ordinarily rough sliding surfaces in the tubular cylinders
or piston rods.
The primary purpose to be attained is to provide
a seal capable of working practically without wear and
with low friction even when materials of lesser strength
(e.g. various grades of natural or synthetic rubber)
are used, and even when pressuring agents based on water
emulsions, non-flammable or organic hydraulic fluids,
glycol, etc., are used. Furthermore, it should provide a
pre~sure-resistant, closed and firmly fixed, solid
lubricant layer between the original metallic sliding sur-
face of the tubular cylinder and the piston rod and the
sealing combination.
1136173
Further, the element of the sealing combination
directly suppor~ing the gasket itself should safely pro-
tect the soft and highly elastic gasket, even under the
highest operating pressures and even in the case of coarse
manufacturing tolerances, against extrusion and the
"nibbling" effect. The gasket-supporting element should
be of the simplest possible design and capable of economical
manufacture and capable of satisfying the strictest require-
ments with respect to effective life (even in relation to
the formation of a layer of the solid lubricant). Beyond
this, the sealing combination should be applicable to con-
ventional rectangular gasket grooves (eliminating the need for
stepped or chambered grooves) and be protected safely
against deficient lubrication and fluid mixed friction.
It should also be capable of protecting guide surfaces
associated with it. Even in the case of the most stringent
requirements concerning tightness of seal, life and stroke
velocity, the sealing combination should necessitate only
a single gasket of conventional length in the axial direc-
tion. Through the intentional elimination of a liquid
lubricant phase on the sealing and guide surfaces (pro-
viding improved tightness) and the formation of an
extremely smooth, pressure-resistant lubricant layer between
the sliding parts (providing reduction of frictional
wear and heat), this sealing combination enables the
achievement of safely predictable operating properties
in use, without the need for suitable test runs to determine
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1136173
individual sealing parameters. - h
The objectsof the invention are, therefore,
achieved first by pressure translation within the sealing
combination,by axial deformation with radial expansion,
by radial deformation and/or special fabric reinforcement
of the gasket itself, all of which generate high partial
pressures and second,by providing between the gasket and
the flank of the groove facing away from the pressurized
space, a ring made of a non~self-lubricating material
capable of plastic deformation and containing a high pro-
portion of a dry mineral lubricant (e.g. 60% MoS2).
The object of the invention is characterized
specifically by the fact that a gasket made of a material
selected without consideration of the strength of its base
substance ( the material is thus chosen only with respect
to its elasticity and suitability for use in an effective
combination with the lubricant selected) is in contact
with the running surface to be sealed with an additional
radial prestress. In the alternative, the gasket may be
said to be pressuring a suitable oil ring with the same
pressure against the sliding surface of the tubular
cylinder or the piston rod. In this manner, dry operation
is guaranteed within all customary pressure ranges even with
rapid stroke movements and,the pressure is high enough to
let the dry lubricant used attach itself solidly to the
original sliding surfaces without any interference from
the detergent or dispersant additives in the fuel.
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` `` 113~;173
At the same time, a shearing of lamina of the solid lubricant
of single crystals takes place. In spite of the dry friction
forcibly established in this manner, the frictional force
on all parts in relative motion with respect to each other
is extraordinarily low. This is because the dry lubricant
released by the oil ring produces a running layer on the
corresponding sliding surfaces with a roughness amounting,
as a rule, to values of the order of magnitude of 0.4 to
1.0 micrometer, even when the original roughness of the
metallic sliding surfaces amounted to 12 to 20 micrometers,
prior to a short running-in period. This smoothness
of the sliding surfaces, usually not obtainable in the case
of tubular cylinders by conventional ship-forming machining,
provides in combination with the frictional coefficients of
the layer covering the original sliding surface (e.g. of
pure molybdenum disulfide, having approximately equally
low coefficients of friction in motion and at rest) the
following additional advantage:
The well-known "seizing" of gaskets in cylinders
held for long periods of time at rest under high pressures
and operated with pressure fluids having unfavorable
lubricating properties, is completely eliminated, together
with the "brcaking away" of such cylinders (e.g. mine
pistons) upon their active or passive operation. Beyond
this, the most important condition of the satisfactory
behavior of lubricants in relation to a practically
unlimited life of the gasket is satisfied, that is, the
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113ti173
the achievement of the solid and permanent attachment of
the dry lubricant to the corresponding running surface
through polarization forces. This occurs because during
the slidiny motion the solid lubricant is rubbed onto and
into the running surface and also becomes directionally
oriented. The important fact here is that the attachment
of the solid lubricant by the polarization forces can
take place without interference, because a li~uid film is
prevented from being formed between the running surface
and the gasket and, since the effects of certain components
of the operating medium which otherwise strongly interfere
with this process, can no longer come about. Because of
the very simple forming operation, production and tooling
costs necessarily accruing with sealing combinations of
this type, are substantially reduced. The plastic or
bronze guide rings required,due to the bending forces
acting upon the cylinder,can be entirely eliminted,
because the highly pressure-resistant intermediate layer
safely prevents the seizing of guide parts. One embodiment
of the invention provides that the high pressure and
shear stress desired on the gas]cet is ob-tained by the transla-
tion of pressure within the sealing combination. For this
purpose, a surface area comprised of the sum of the areas
of the oil ring supporting the gasket itself in the axial
direction and the surface area of the chambering part
possibly supporting the oil ring against the bottom of the
groove is kept smaller than the pressurzied surface of the
_g_
1136173
gasket so that in the operating state the partial pressure of the
oil ring amounts to several times the operating pressure. The
permeab~e fabric matrix of the back of the gasket insures, as
usual, that no ~mdesirable pressure of the medium is generated at
the mechanically unsupported back part of the seal.
The present invention,therefore, provides a sliding seal
combination for pistons and piston rods of power cylinders and for
floating pistons of pressure storage units, comprising a combina-
tion of sealing and lubricating rings, enabling a high partial
pressure to be produced between the seal and the running surface
against which it bears thereby to obtain practically complete dry-
running of the seal combination, wherein the lubricating ring is of
a plastically deformable material which is not self-lubricating but
contains a high proportion of dry mineral lubricants and wherein the
sealing ring or at least one of a number of support rings directly
contacting the sealing ring includes reinforcing means which
supports and generates additional pressure at the sealing surface
or edge whenever the sealing ring bears on the running surface.
Additionally, the lubricating ring may be arranged in use so
as to be totally enclosed, and, in use, subject to further addition-
al radially acting outward pressure, urging it against the running
surface, as the other parts of the seal combination deform under
operating pressure.
In an advantageous further embodiment of the invention, e.g. in
the case of an axially deformed sleeve packing conventionally con-
sisting of at least three fabric-reinforced and two fully elastic
gaskets between the required pressure and supporting rings, one of
the fully elastic and two of the fabric-reinforced gaskets can be
,~- 10 -
36J~73
eliminated and replaced by a corresponding oil ring. In the process,
the axial stress is increased to approximately twice the limits
usually observed for reasons of wear. Through this measure, the
specific unit pressure of the remaining gaskets attains a magnitude
leasing to the shearing of solid lubricant lamina of single crystals
and to the formation of a smooth, homogeneous lubricating film even
during the short run-in period. The mechanical properties of the
fully elastic gasket are immaterial, because the gasket is fully
chambered. As is known when MoS2 lubricating rings are used, the
pressure force limits that must be observed in conventional seals
because of friction and wear are sufficient to dislocate MoS2 scales
and loosely adhering MoS2 crystals, i.e. to form relatively smooth
surfaces with permanent identations and clearly marked gain
boundaries. The friction coefficient
,
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113~173
of such surfaces is around 0.1 and remains constant.
The high pressures used in the technology of seal- ~
ing heretofore only in combination with high strength
gasketing materials based on polyurethane and by means of
stepped or chambered grooves, except in the object of the i.
present invention, introduce shear forces in the solid
lubricant sufficient in all cases to shear off an MoS2 layer
of single crystals.
It is known that in this manner, a very smooth,
solid and compact MoS2film is formed in which hardly any
stepping of MoS2 layers may be observed, even under strong
magnification. Upon the completion of the slip of the
single crystal layer, the friction coefficient of such a
surface stabilizes -at 0.03, i.e at roughly one-third of
its previous value with only twice the pressure. r
Still another embodiment of the invention provides
for the extension of the fabric reinforcement otherwise
present only at the back and the running surface of the
gasket, to the sealing lip of the gasket to increase
rigidity in comparision with the elastic base material.
The values of the elastic modulus is, -thereby, increased
from the conventional maximum values of 100 kp/cm2 to
values of approximately 250 to 750 kp/cm . The simple
increase in the strength and pressure force of the fabric-
reinforced gasket in combination with the oil ring
supporting the seal on its side facing away from the
pressure leads to the effective process of reducing the
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1136173
the frictional force and of reducing wearand also to the
dry operation of the sealing combination, as described
above.
It is self-evident that it is similarly possible
to embed an oil ring with greater radial or axial dimensions, r
directly into a corresponding recess of the fabric-
reinforced sealing part.
In another embodiment of the invention, the plastic-
ally deformable ring consists of two partial pieces with
dry lubricants differing in their material, with the first
substance having a high bonding force, colloquially: high
affinity of chemical elements with respect to the material
to be sealed, while the second dry lubricant behaves as an
inert substance with respect to the running surface.
The first dry lubricant (e.g. molybdenum disulfide)
is deposited during stroke motions on the surface or the
surface roughnesses of the corresponding running surface
(.e.g. steel) and adheres or is bonded strongly due to
polarization forces or due to the reaction, MoS2 + 2Fe
2FeS + Mo. The relatively rough layer produced of the
first crystalline dry lubricant is filled out and thus
smoothed by the second dry lubricant (e.g. polytetra-
fluoroethylene), which is solidly anchored by mechanical
means in the running surface of the first dry lubricant
after adequate plastic deformation. In this manner, an
extremely dense, smooth surface sliding surface, consisting
of the two lubricants solidly adhered to the running sur-
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1136~73
faces, is formed for the sealing combination. By experi-
ment, it has been shown that after the formation of said
sliding surface usual abrasion of individual particles
of the gasketing practically ceases altoaether. This
additionally again reduces considerably the friction
coefficient of the sliding pair, and insures the
initial movement of the piston without jerking at the be-
ginning of the stroke and substantially reduces the tempera-
ture generated at the surface of the gasket by friction.
A similar effect can be obtained if the plastically
deformable ring consists of different dry lubricants, at
least one of which has a high bonding force to the material
of the running surface, and at least one additional dry
lubricant in a chemically inert manner with respect to the
lS running surface. Both dry lubricants are held by a support-
ing screen of a non-self-lubricating material such as a
modified polyolefin.
It is important in this connection that the material
used for the holding of the dry lubricants should not inter-
fere with reactions of the individual dry lubricants in
the molecular range and,that it should be capable of re-
leasing said dry lubricants in the active form even under
a relatively slightexternal stress. A particularly economical
and advantageous embodiment of the object of the invention
results from the use of one of the sliding seals described
in tubular cylinders with particularly rough sliding surfaces,
such as produced by the simple cold drawing of tubes.
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1136173
Experiments show that when such tubes are used, the original
bonderized surface with a roughness depth of approximately
20 micrometer is covered completely after only a few strokes
with a solid dry lubricant layer with a depth of roughness
of approximate 0.4 micrometer.
Certain preferred embodiments of the invention are
presented in the following with the aid of the drawings,
where:
F~GS. 1 and 2 are schematic axial sectional views
of two piston packings, acting singly or doubly;
FIGS. 3 and 4 show similar sections, but with two
piston rod packings;
FIG. 5 shows an axial section of a so-called
"axially deformed sleeve packing", but with a reduced number
of fully elastic and fabric-reinforced gaskets;
FIGS. 6 to 10 display axial sections of different
embodiments of single and double-acting forms of seals.
According to FIGS. 1 to 4, a piston 1 slides
in a cylinder tube, not shown, and piston rod, not shown,
in an end closure 2, with the prevailing operating medium
exercizing pressure in the direction of the arrows 3 or
alternating in the direction of the arrows 3a and ~b on
the sliding seal combination 4.
The sealing combination 4 is arranged in a con-
ventional sealing groove 5 and here, as an example, con-
sis-ts of the gasket 4a with a fabric reinforcement 4b on its
side facing away from the pressure and a ring 4c made of a
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1136173
plastically deformable, non-self-lubricating material and ~~
containing a high proportion of a mineral dry lubricant.
In order to obtain complete chambering of the ring
4c at very high operating pressures, additionally a support-
ing ring 6 made of a wear-resistant material, and potentially
a supporting collar 7, may be arranged at the back of the
seal. The generation of the pressure translation desired
within the seal combination is described for the sake of
simplicity only by the example of FIG. 1, but takes place
accordingly in the seals of FIGS.2, ~ and 4. The pressure
on the seal combination 4 during operation is determined
by the prevailing operating pressure, the dimensions of
the impacted surface area ~/4(Dl -D22), the pre-stressing
at the sealing edge 4d and the magnitude of frictional forces
at the seal during operation, while the seal is supported
merely on a surface area of ~/4(Dl - D3) and the surface
area ~/4(D23 - D22) of the seal is not supported or possibly
pressurized by the reverse pressure of the low pressure side.
The annular space 8 defined among others by the dimensions
of the area ~/4(D3 - D22) cannot be pressurized by the pre-
vailing operating pressure or by the seal itself under said
opera-ting pressure, because the pressure fluid possibly
present in said annular space is able to escape through the
supporting ring 6 which is slit in the conventional manner,
or through the relatively rough front surface, facing away
from the pressure of the ring 4c and the matrix of the
fabric reinforcement 4b.
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1136~73
Due to the process described above, the ring 4c
is pressed much stronger than with conventional seal
combinations against itscorresponding metallic sliding
surface (not shown) which produces immediately an extremely
smooth, pressure-resistant lubricant layer on the metallic
sliding surface.
FIG. 5 shows a seal combination 10 for the sealing
of the running surface of a piston rod (not shown). Between
the fabric reinforced sealing parts, pressure ring 12 gasket
13and supporting ring 14, there is a fully elastic gasket
15. A ring 16 made of a plastically deformable, non-self-
lubricating material and containing a high proportion of a dry
lubricant, is arranged between the front surface facing
away from the pressure of the supporting ring 14and the
flank of the groove 17,again facing away from the pressure.
The entire seal combination 10 is arranged longitudin-
ally so that, after assembly in the sealing groove 17, 18
by way of the projection 19 of the gasket 12,possibly
equipped with pressure equalizing notches, which after the
insertionof the piston rod (not shown) results on the one
hand in complete chambering of the sealing parts15 and 16,
and, on the other hand, in an additional pressure, other-
wise inadmissible, by these sealing parts on the running
surfaces of the piston rod. The magnitude of this additional
pressure is determined in a simple manner by axial deforma-
tion with the aid of the deformation path forced at the
projection l9by the distance of the groove flanks 17and 18,
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113~;173
so that the ring ~,in the case of operational sliding
motions,immediately produces a smooth, pressure resistant
lubricatiny layer on the running surface of the piston
rod, while simultaneously the fully elastic gasket 15
assures dry running at all stroke velocities. Obviously,
a corresponding axial deformation can be produced by r
means other than with projection l9(e.g. by a compression
or plate spring acting in the axial direction, or by
a packing box design).
10 FIGS. 6 and 7 show axial cross sections of a simply
acting piston and piston rod gaskets 1 with a ring 2,
containing a high proportion of a dry lubricant. An
auxiliary line ~ indicates the position of a not otherwise
shownrunning surface of a piston rod and a tube cylinder
with respect to position of the gasket in the fully
assembled position. Fox the sake of clarity, the seal
itself is illustrated in its form prior to the insertion
of the piston rod and the insertion of the associated tube
cylinder. Both seals 1 are designed as a single piece
and consist of a fully elastic section la and a fabric
reinforced section lb, where the fabric reinforced section
lb through its e~tension in the direction of the sealing
edge 4 and a suitable design, reinforces, supports and
stiffens the fully elastic section of the section of the
seal so that even though adequate elasticity is still
available in the fully mounted state, the values of the
pressure between the sealing edge 4 and the "running
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1136173
surface" 3 assume a constant magnitude. The magnitude is ~~
at least twice as high as that in conventional seals, thus
insuring the success desired with respect to dry running
and the formation of a lubricant layer on the running
surface.
In FIGS. 8, 9 and 10, the same designations as
in FIGS. 6 and 7 are used. So-called "compact seals"
are illustrated with the conventional low pressure at
the sealing edge 4 and the sealing part 5. The high pres-
sure required for the safe operating of the ring 2 on the
running surface represented by the auxiliary line 3 is
produced by the ring 2, embedded in the rigid fabric
section lb, by the fact that the radial height of the ring
2 is greater than the corresponding height of the recess
provided for the embe-ding in section lb. For this reason,
the ring 2 is pressured radially and directly between the
corresponding running surface and the bottom of the embed-
ding groo~e. The magnitude of this radial pressure can
be determined specifically by the rigidity of the fabric
section.
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