Note: Descriptions are shown in the official language in which they were submitted.
~ ~ CA 02237787 1998-05-15
r
FILE, 1~1~ TH~ r~n~
T~ TRANSI A~
P.6714/Gf/Pa
Machine Works Sulzer-Burckhardt AG, Basel (Switzerland)
Piston Ring
The invention relates to a piston ring in accordance with the
preamble of claim 1.
For compressing fluids such as gases or vapours, dry running
compressors are used, with sealing rings of a plastic such
as, for example, PTFE being used at the piston as sealing
elements. Such sealing elements have the disadvantage that
they have a high wear during operation, which leads to an
insufficient lifetime. Especially the free gap cross-sections
in the axial and radial direction, which result from the ring
gaps and the wear of the sealing elements, have a significant
share in the fact that the sealing elements do not attain
their full effectiveness for a desired period of time. A seal
element with a good sealing effect which is constant over a
long period of time is required, in particular when
compressing very light gases such as hydrogen.
The object of the present invention is to propose an
economically more advantageous, in particular a low-wear
piston ring for a dry running piston compressor, in
particular for compressing very light gases.
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This object is satisfied in accordance with the features of
claim 1 or claim 2. The subclaims 3 to 6 refer to further
advantageous developments of the-invention.
The piston ring in accordance with the invention for a dry
running piston compressor consists of two one-piece, ring-
like bodies with ring gaps or butt joints, which bodies are
designated in the following as rings. These rings are
arranged to extend concentric to one another with respect to
an axis C. The first ring has an essentially L-shaped cross-
section with a first limb extending in the direction of the
axis C and a second limb extending outwards radial to the
axis C. The two limbs each have an inner surface, with the
second ring being executed in such a manner that its surfaces
facing the first ring lie in contact with the inner surfaces
of the first ring in a form-fitted manner. The second limb,
extending radially outwards from the axis C, has an inner
surface which is inclined in the radial direction with
respect to the axis C, with the angle of inclination being
less than 90~. The inner surface has a tangent extending in
the radial direction, which tangent intersects the axis C at
an acute angle. In an advantageous embodiment of the
invention this inner surface is executed as a conical
surface.
In dry running piston compressors there is no lubricant
available in order to lubricate the piston rings of a piston
and to additionally seal it. Accordingly metallic piston
rings are unsuitable for such a dry running application. The
dry running frictional pairings function as a result of solid
lubricants which are contained in one of the frictional
partners. The piston ring in accordance with the invention
therefore consists of a plastic specially modified for dry
running with solid lubricants such as PTFE, graphite or
molybdenum disulphide.
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Sealing elements with a very high sealing action are required
above all when compressing very-light gases, such as hydrogen
for example, to very high pressures in order to keep the
leakage as small as possible. A good sealing action can be
achieved for example by combining two sealing rings to a twin
ring in such a manner that no through-going gaps or spaces
result.
The self lubricating action of the dry running frictional
seals has as a consequence that the piston rings which yield
the lubricant gradually abrade away. Known twin rings of
plastic have the disadvantage here that the two ring parts
are displaceable with respect to one another in the radial
direction. Since the pressure profile in the sealing
subsurfaces of the two ring surfaces near the cylinder wall
is not constant, the radial displaceability enables one of
the two ring parts to abrade faster than the other. This
unequal wear has the effect that the two ring parts no longer
fully overlap and hence that gaps arise through which large
amounts of leakage gas flow, in particular for very light
gases under high pressure, which considerably reduces the
amount of the gas forwarded by the dry running compressor.
The piston ring in accordance with the invention is
especially suitable for the dry running compression of very
light gases to high compression end pressure. The piston ring
has the advantage that due to the construction of two ring
parts or two ring-shaped bodies, their mutual slope and the
form-fitting coupling of the two ring parts a uniform,
harmonised wear of the two ring parts takes place during
operation. The two ring parts are pressed against one another
by the pressure of the fluid to be compressed, with no
relative movement or only a slight mutual relative-movement
taking place due to the form-fitting coupling of the two ring
parts, so that the sealing subsurfaces of the two ring parts
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which, for example, face a cylinder wall, experience a
uniform material abrasion. In this manner, no or only slight
local leakage points can form, due to which the sealing
effect of the piston ring remains approximately constant over
a longer operating period in dry running piston compressors.
In the following the invention will be explained with
reference to several embodiments. Shown are:
Fig. 1 a longitudinal section through a piston with a
cylinder;
Fig. 2a a cross-section through a second ring;
Fig. 2b a cross-section through a first ring;
Fig. 3a a plan view of a second ring;
Fig. 3b a plan view of a first ring;
Fig. 4 a piston ring arranged in a one-piece piston;
Fig. 5 two piston rings arranged in an assembled piston;
~ig. 6a a cross-section through a further embodiment of a
second ring;
~ig. 6b a cross-section through a further embodiment of a
first ring.
~ig. 7a a plan view of a second ring;
~ig. 7b a plan view of a first ring.
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Fig. 1 shows a longitudinal section through a dry running
piston compressor with a cylinder 1 in which a piston 2 is
arranged so as to be upwardly and downwardly movable. The
lower end of the piston 2 merges into a piston rod 3 which is
connected in a known manner, not shown, to a crank drive.
Above the piston 2 is a compression chamber 4 into which a
gas to be compressed is drawn during the downward stroke of
the piston 2 in a known manner, not shown. This gas is
compressed on the following upward stroke and expelled from
the compression chamber. The piston 2 has a rod-like
extension 5 of the piston rod 3 over which a sleeve is placed
bearing seven piston rings 10 placed one above the other. For
an assembled piston 2 the individual parts of the piston 2 as
well as the piston rings 10 are held together by a nut 7
screwed onto the upper end of the extension 5. Likewise shown
is the central axis C of the piston 2.
The section A is shown magnified in Fig. 5, where two
sections A are shown. The assembled piston is fabricated from
individual parts and comprises the sleeve 6 extending in
axial direction, the chamber rings 8a, 8b placed about the
sleeve 6 as well as the piston rings 10 placed in the
grooves. Each of the piston rings 10 consists of a first ring
lOa and a second ring lOb. The two rings lOa, lOb are
executed in a mutually adapted manner such that the mutually
contacting partial areas come to lie against one another in a
form-fitted manner. Each of the rings lOa, lOb has areas lOc,
lOd facing the cylinder wall 1 which are slidingly moved
upwards and downwards along the cylinder wall during the
operation of the piston 2.
Fig. 4 shows a section A of a one-piece piston 2 with chamber
ring 8 and a piston ring 10 consisting of the two rings lOa,
lOb placed in the groove. The first ring lOa is arranged in
the cylinder 1 toward the pressure side, with the gas
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pressure 9 exerting forces on the first ring lOa acting in
the axial direction 9a as well as in the radial direction 9b
so that the entire piston ring 10 is firstly pressed against
the inner wall of the cylinder 1 and secondly pressed in the
axial direction against the boundary surface of the groove
remote from the pressure side. In this way the sealing action
of the piston ring is increased during operation and the two
rings lOa, lOb are held in the groove and mutually form-
fitted without or with only a slight relative movement. This
mutually form-fitted connection has the result that the
subsurfaces lOc, lOd of the two rings lOa, lOb which slide
along and rub against the cylinder wall la are uniformly worn
away so that no local leakage point arises and the two rings
lOa, lOb lie in contact with the cylinder wall la with the
same effect over a longer operating period.
The piston ring 10 shown in Figures 4 and 5, comprising the
rings lOa, lOb, is shown in detail in Figures 3a, 3b in a
plan view and in the Figures 2a, 2b in cross-section. The two
rings lOa, lOb extend circularly about a central axis C. Fig.
3b shows the first ring lOa of the piston ring 10, a ring-
shaped body of L-shape or nearly L-shape with a ring gap or
butt joint lOe. This ring has two limbs lOg, lOh, a first
limb lOh extending parallel to the direction of the axis C
and a second limb lOg extending radially outwardly or
approximately radially outwardly relative to the axis C. The
width of the ring gap lOe is such that the first ring lOa is
capable of a certain resilient movement in the peripheral
direction.
A cross-section through the first ring lOa along the line A-A
is shown in Fig. 2b. The first ring lOa, executed in L-shape,
has a first limb lOh extending parallel to the axis C, which
has an outer surface lOo as well as an inner surface lOn. The
second limb lOg extending radial to the axis C has an outer
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surface lOp extending perpendicular to the axis C. The inner
surface 101 is executed to extend conically and subtends in
the radial direction an angle ~-to the normal to the axis C.
In the present embodiment the second limb lOg is executed in
such a manner that, with respect to the direction determined
by the axis C, it is made relatively narrow at the first limb
lOh and broadens in the direction of the periphery, i.e.
toward the surface lOd. The cross-section of the second ring
lOb, which is shown in Fig. 2a, is shaped in accordance with
the shape of the two inner surfaces 101, lOn. The ring body
lOi has four surfaces, the surface facing the cylinder wall
lOc, the surface lOq facing the chamber ring 8, as well as
the two surfaces lOm, lOk facing the first ring lOa. The two
last named surfaces lOm, lOk are executed to be adapted with
respect to the first ring lOa in such a manner that for rings
lOa, lOb placed one within the other the surfaces lOm, lOk
come to lie in a form-fitted manner on the two inner surfaces
101, lOn. Therefore the surface lOk is likewise made conical,
to extend radially to the axis C at an angle of inclination
~c .
Fig. 3a shows a plan view onto the second ring-formed body
lOb with ring gap or butt joint lOf, which body is designated
in the following as a second ring lOb. The ring body lOi has
the ring gap lOf at the one side and a protrusion lOk
projecting toward the axis C at the side opposite this ring
gap lOf. In the assembled piston ring 10 the second ring lOb
is placed over the first ring lOa while retaining the
position shown in Figs. 3a, 3b so that the protrusion lOk
comes to lie in the ring gap lOe of the first ring lOa. By
this means the second ring lOb is secured with respect to the
first ring lOa against mutual rotation since the protrusion
lOk and the limb lOh restrict their relative movement. The
protrusion lOk is advantageously made narrower in the
peripheral direction than the width of the ring gap lOe so
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that the first ring lOa retains a certain mobility in the
peripheral direction at the ring gap lOe. Figs. 7a and 7b
show a further exemplary embodiment of an execution of the
rings lOa, lOb for the mutual securing against a rotation. A
cylindrical fixing pin lOr which extends in the vertical
direction is arranged at the limb lOg on the side of the
first ring lOa lying opposite the ring gap lOe. When
assembling the piston ring 10 the second ring lOb is laid
over the first ring lOa, while maintaining the position
illustrated in Figs. 7a, 7b, so that the fixing pin lOr comes
to lie between the ring gap lOf of the second ring lOb,
whereby the second ring lOb is secured with respect to the
first ring lOa against a mutual rotation. In the assembled
state the rings lOa, lOb can be inserted into the groove of a
one-piece piston 2 by simultaneous spreading apart of the two
ring parts as shown in Fig. 4 or into an assembled piston 2
without spreading apart as shown in Fig. 5.
An advantage of the piston ring 10 in accordance with the
invention is to be seen in the fact that the subsurfaces 101,
lOn, lOk, lOm of the two rings lOa, lOb remain lying one upon
the other in a form-fitted manner over long periods of time,
even during the operation of the piston ring 10. The two
rings lOa, lOb are executed in such a manner that a direct
action of the gas 9 under pressure on the subsurfaces 101,
lOn, lOk, lOm is prevented as far as possible. The gas
pressure 9 acting on the rings lOa, lOb normally acts at the
ring gap lOf and exerts a force acting in the peripheral
direction of the first ring lOb, which, for an angle a= O,
could lead to a lifting off of the first ring lOb from the
second ring lOa so that the gas pressure would act directly
on the subsurfaces 101, lOn, lOk, lOm. Such a direct action
of the gas pressure on the subsurfaces lOm, lOk of the second
ring lOb would result in a relatively rapid wear of the
second ring lOb. In order to avoid this effect the ring in
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accordance with the invention is executed in such a manner
that a lifting off of the first ring lOb from the first ring
lOa is prevented. In accordance-with Fig. 4 the piston ring
10 is pressed in the axial direction against the chamber ring
8 by the action of the gas 9a and in the radial direction
against the wall of the cylinder 1 by the action of the gas
9b. The first ring lOa subjected to this pressure exerts
corresponding forces on the second ring lOb. Due to the
conical execution of the subsurfaces 101, lOk the forces
acting due to the gas pressure cause an increased mutual
wedging together of the two rings lOa, lOb so that the
subsurfaces lOk, 101, lOn, lOm are pressed against one
another more strongly. A direct action of the gas pressure on
the subsurfaces lOk, 101, lOn, lOm is thereby prevented,
which leads to a low wear of the first ring lOb, and a
uniform wear of the two rings lOa, lOb, in particular of the
subsurfaces lOc, lOd, and thus to a long lifetime of the
piston ring 10.
There is a great number of possibilities for the design of
the subsurfaces lOk, 101, lOn, lOm in such a way that a
mutual wedging together occurs between rings lOa, lOb placed
together and loaded by gas pressure with the result that the
subsurfaces lOk, 101, lOn, lOm in mutual contact are
additionally pressed against one another. Figures 6a, 6b show
a cross-section through a first ring lOa as well as a ring
lOb of a further exemplary embodiment. Here the two
subsurfaces lOm, lOn are executed to extend cylindrically,
parallel to the axis C. The further subsurfaces 101, lOk are
executed to extend in curved form in such a manner that they
come to lie against one another in a form-fitted manner in
the assembled piston ring 10.
The inner surface 101, lOk has an inclination in the
direction extending radially to the axis C, with the angle
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between a tangent to the inner surface 101, lOk and the axis
C amounting to about 90~ in the region of the surface lOd and
becoming increasingly acute in the direction of the limb lOh,
i.e. adopting an angle which becomes smaller than 90~.
The rings lOa, lOb are formed of a plastic, in particular of
plastics such as polytetraflouorethylene (PTFE), a modified
high-temperature polymer such as polyetheretherketone ( PEEK),
polyetherketone (PEK), polyimide (PI), polyphenylene sulphide
(PPS), polybenzimidazole (PBI), polyamidimide (PAI) or a
modified epoxy resin. The high-temperature polymers are
plastics which are not capable of dry running in pure form,
so that the above named plastics are usually filled with
additional solid lubricants such as e.g. carbon, graphite,
molybdenum disulphide or PTFE.
