Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1299943
PISTON FOR INTERNAL COMBUSTION ENGINES AND LIKE MACHINES
The invention relates to pistons for sliding in
cylinders of reciprocating internal or external combustlon
engines and compressors, i.e., machines in which a working
gas contained in the or each cylinder by a transverse
surface of the piston is at both high pressure and high
temperature, and it preferably, but not exclusively,
concerns pistons for two-stroke or four-stroke reciprocating
internal combustion engines. The invention more
particularly relates to those of these pistons which have,
within an externally cylindrical skirt, a partly spherical
bearing surface for receiving in the manner of a ball joint
the partly spherical head of a connecting rod. For reasons
which will be explained hereinafter, the bearing surface of
the piston and/or the head of the rod may have by
construction a shape which is partly not strictly spherical
but, for simplifying the description, the expression
"spherical" will be used in this context for designating a
shape which is exactly or approximately spherical.
Such pistons are for example disclosed in French patents
965,449 and 1,547,151 and the British patent 293,506. In
these known pistons, the two partly spherical elements of
the piston and rod, which are composed of metal, move in
contact with each other, at least if one ignores the thin
film of lubricating oil maintained therebetween by
; introduction of oil under pressure which comes for example
~ from a passageway extending longitudinally within the rod
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(see in this respect French patent 965,449 and the British
: patent 293,506).
An object of the invention is to enable the articulation
between the piston and the rod to resist large forces
applied to the piston by the maximum pressures cyclically
established in the variable-volume chamber defined by the
piston inside the cylinder.
Hereinafter, when expressions such as "upper", "lower",
"above" or "below" are used, it will be assumed that the
piston is so oriented that its axis is vertical and the rod
: is positioned below the piston. This hypothesis is merely
intended to simplify the description and therefore does not
imply that the piston is in fact oriented in this way when
it is mounted in an internal combustion engine or
compressor.
According to a first aspect of the invention, the piston
of the type defined hereinbefore is essentially
characterized in that it possesses an inner cavity open at
the end opposed to said transverse surface and laterally
defined at least partly by a cylindrical bearing surface of
revolution; the space defined within this cavity of the
piston by the partly spherical head of the rod is filled
with a viscous, pasty or plastically-deformable fluid which
is practically incompressible at the operating pressures and
temperatures of the piston; the diameter of said cylindrical
bearing surface is very slightly larger than the diameter of
; the partly spherical head of the rod so that the operational
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clearance between said cylindrical bearing surface and
partly spherical head is small enough to prevent any
migration of said fluid out of said space at the operating
temperatures and pressures of the piston.
Preferably, the generatrices of said cylindrical bearing
surface are parallel to those of the cylindrical skirt of
the piston, this cylindrical bearing surface being generally
coaxial with this skirt.
In order to avoid any eventual loss of said fluid, a
thin fluid-tight metal shell is advantageously interposed
between the partly spherical head of the rod and said fluid
and is locally maintained in position by an interference fit
with said cylindrical bearing surface, the assembly being
such that the part of the shell creating the interference
with the cylindrical bearing surface is movable in a
direction parallel to the axis of this cylindrical bearing
surface under the effect of forces exceeding a given limit.
According to a second aspect of the invention, a piston
provided for sliding in the cylinders of reciprocating
internal or external combustion engines and compressors,
i.e., machines in which a working gas contained in the or
each cylinder by a transverse surface of the piston is at
both high pressure and high temperature, in particular for
~ur-stroke or preferably two-stroke reciprocating internal
combustion engines, which piston receives in the manner of a
, ball joint the partly spherical head of a connecting rod, is
~;~ essentially characterized in that it has, within an
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externally cylindrical skirt, a cavity which i9 open at the
end opposed to said transverse surface and laterally defined
at least partly by a cylindrical bearing surface of
revolution, preferably coaxial with the piston; there is
placed in said cavity a thin fluidtight metal shell whose
shape is cylindrical in the lower part thereof and is an
interference fit against the cylindrical bearing surface of
revolution of the piston and substantially hemispherical in
the upper part thereof against which the rod head bears; the
metal shell defines, by the outer surface of the
substantially hemispherical part thereof, within the cavity
of the piston a space filled with a viscous, pasty or
plastically-deformable fluid which is practically
incompressible at the operating temperatures and pressures
of the piston; the outside diameter of the partly spherical
head of the rod is slightly less than the diameter of the
cylindrical bearing surface of the piston so that the
cylindrical part of the shell is an interference fit in the
aforementioned manner and any migration of said fluid out of
said space is prevented at the operating temperatures and
pressures of the piston.
Preferably, said fluid is formed by a fluorine polymer
and more particularly by PTFE (polytetrafluoroethylene) or
"TEFLON" although other materials may be profitably employed
which are chemically and physically stable at the operating
temperatures and pressures of the piston, sufficiently
deformable under compression to be capable of filling said
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space as completely as possible (bearing in mind the
possible presence of said shell) but sufficiently viscous to
; render impossible in operation any escape of said fluid in
the operational clearance between the partly spherical rod
head or shell and said cylindrical bearing surface. It may
in particular concern an oil having high viscosity, of the
type employed for example in viscous dampers (or vibration
shock absorbers). Thus, it can in particular be seen that
said definition of the fluid excludes in particular oils
having low or even medium viscosity.
In this way, irrespective of the position of the rod
relative to the piston, the forces exerted on the latter are
uniformly transmitted to the partly spherical head of the
rod through the cushion formed by said plastically-
deformable fluid, which is prevented from flowing out of the
space provided therefor. Owing to the fact that the shell
~in the preferred case in which this shell exists) is
movable in a direction parallel of the axis of the
cylindrical bearing surface, this shell is subjected to
practically only compression in a direction perpendicular to
its surface and it is therefore protected against risks of
fracture or piercing, the axial movement of the shell
enabling clearance to be taken up automatically.
Note that the aforementioned French patent 1,547,151
proposes disposing a layer of plastics material, such as
PTFE, directly between the partly spherical seat of a piston
and the partly spherical head of a connecting rod, but on
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the side opposed to that on which it is provided in
accordance with the present invention and for a completely
different purpose. It should also be noted that it is known
to provide with synthetic resin, such as PTFE, both the male
and female parts of a ball joint, for example for vehicle
steering mechanisms (see U.S. patent 3,342,513) so that the
elements moving in contact with each other are both composed
of synthetic resin.
Preferably, the metal shell according to the invention
is made from bronze, in particular beryllium bronze, or
other metal or alloy having a good coefficient of friction
in limit lubrication and a high ductility and its
thickness is generally between 1% and 3% of the diameter of
the partly spherical head of the rod.
In the case of the second aspect of the invention
defined hereinbefore, the shell is advantageously so
arranged that its substantially hemispherical part bears
against the spherical head of the rod along a closed,
circular line or band of contact located in, or defined in
its upper part by, a plane passing above the centre of the
rod head and preferably perpendicular to the axis of the
piston; unidirectional means for supplying lubricating oil
under pressure communicating on the upstream side through a
check-valve with an oil passage provided in the rod, opening
onto an interstice defined, above said line or band of
contact, by the surface of the partly spherical head and by
the inner surface of the partly hemispherical part of the
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metal shell, the pressure of the lubricating oil, on the
upstream side of said check-valve being higher, at least
during a part of the operating cycle of the engine, than the
pressure prevailing in said interstice; and the space
located below said line or band of contact, between the
partly spherical head and the shell, communicates with a
zone of low pressure which is lower than the maximum
pressure prevailing in said interstice.
This permits incorporating the metal shell in means
supplying with lubricating oil under pressure the surface of
the partly spherical head of the rod which must be capable
of freely pivoting with respect to the inner surface of said
shell. Various embodiments of said interstice and of said
lubricating oil supply means will be described in detail
hereinafter.
Preferably, in the preferred case where the metal shell
is provided, the partly spherical head of the rod or the
metal shell is provided, on the side thereof facing toward
the rod head, with grooves or oil ways for distributing the
lubricating oil introduced, in the known manner or as
indicated hereInbefore,between this shell and the rod head.
It must be stressed that these oil ways avoid the risk of
being clogged up by the PTFE or other like substance or
fluid since they are provided on surfaces which are not in
contact with this substance or fluid.
In the absence of said shell, lubricating oil under
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pressure may nonetheless be supplied to an annular chamber
partly defined by the rod head, as described hereinafter~ so
as to promote by entrainment the formation of a film of
lubricating oil on this head.
The invention will now be described in more detail with
reference to the accompanying drawings.
In the drawings:
Fig. 1 of these drawings partly represents, in axial
section, the assembly of a piston according to a first
embodiment of the invention and the corresponding connecting
rod.
Fig. 2 represents the partly spherical head of the rod
as viewed in the direction of arro~ F of Fig. 1.
Fig. 3 represents the details of the parts forming a
ball joint between the piston and the rod before the
assembly thereof.
Figs. 4 to 7 are comparative diagrams illustrating
certain advantages of the invention.
Fig. 8 represents, in a semi-axial section, the assembly
of a piston according to a second embodiment of the
invention and of tne corresponding connecting rod.
Fig. 9 partly represents, in axial section, the assembly
of a piston according to a third embodiment of the invention
and of the corresponding connecting-,rod.
Fig. 10 partly represents, in axial section, the
assembly of a piston according to a fourth embodiment of the
invention and of the-corresponding connecting rod.
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Figs. 11 and 12 illustrate lubricating means arranged in
two different ways, respectively.
Figs. 13 and 14 illustrate a first modification of the
embodiment of Fig. 12, respectively by an axial sectional
view and by a top plan view of the rod head.
Fig. lS illustrates a second modification of the
embodiment of Fig. 12.
The piston according to a first embodiment of the
invention (Figs. 1 and 2), generally designated by the
reference character 1, is adapted to slide in a cylinder 2.
The working gas is contained in this cylinder 2 by the
transverse surface 10 of the piston 1.
Within an externally cylindrical skirt 3 (the term
"cylindrical" of course does not exclude the presence of
grooves for piston rings or the like), the piston 1 has a
partly spherical bearing surface 4 adapted to receive, in
the manner of a ball joint, the partly spherical head 5 of a
connecting rod 6.
According to the invention, the piston has an inner
cavity 7 (see Fig. 3) at least partly defined by a
cylindrical bearing surface of revolution 8 which is
preferably (but not necessarily) coaxial with said skirt 3
~the common axis being designated by X-X), the diameter D of
this cylindrical bearing surface being slightly larger than
the diameter d of the head 5. The cavity 7 is open at the
end adjacent to the bottom of Fig. 1, i.e., at the end
opposed to the transverse surface 10. The bearing surface 4
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is constituted in the upper part thereof (according to Figs.
1 and 3) by a surface 9a of the approximately hemispherical
surfaces of a thin metal shell 9 which has a concavity
facing away from said transverse surface 10 of the piston 1,
i.e., downwardly in Figs. 1 and 3, and is maintained in
position by an interference fit in the cylindrical bearing
surface 8. The space 12 defined within the cavity 7 by the
other surface 9b of the surfaces of the shell 9 is filled
with a viscous, pasty or plastically-deformable fluid 13
which is practically incompressible at the operating
temperatures and pressures of the piston 1.
As mentioned hereinbefore, the fluid or substance 13 is
advantageously polytetrafluoroethylene or an equivalent
paste.
The advantage of the cylindrical shape of revolution of
the bearing surface 8 cooperative with the partly spherical
shape of the rod head 5 and of the magnitude of the diameter
D thereof is revealed by the comparative diagrams of Figs. 4
to 7 in which the metal shell 9 has been omitted either to
render the drawing more clear when this shell exists, or to
take into account the embodiment described hereinafter (Fig.
9) in which this shell 9 is eliminated.
Indeed, if the diameter ~ of the cylindrical bearing
surface 8 is too small (Fig. 4), there is a metal-to-metal
contact between the head 5 and the edge of the cylindrical
bearing surface 8 and there is no hydrostatic pressure in
the fluid or substance 13.
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26361-53
If the diameter of the cylindrical bearing surface 8 is
too large ~Fig. 5), lateral leakages of the substance 13
will occur, as shown diagrammatically by the arrows in Fig.
5.
5If the entrance diameter of the cavity 7 is correct but
if the lateral bearing surface 8 of this cavity is not
cylindrical ~Fig. 6), there is no taking up of the clearance
which may result from wear, from a differential deformation
(elastic or non-elastic), or from a differential expansion,
and lateral leakages will in the end occur, as in the case
of Fig. 5.
If the cavity 7 is arranged in accordance with the
invention (Fig. 7), the clearances are automatically taken
up.
15According to a preferred embodiment, the metal shell 9
is made from bronze, in particular beryllium bronze, or
other metal or alloy having a good coefficient of friction
in limit lubrication and a high ductility, it being
understood that conventional means, diagrammatically
represented by the passages 14 and 15 respectively extending
through the rod 6 and the head 5 are provided for
introducing lubricating oil under pressure between the outer
surface of the head S and the surface 9a of the shell 9.
Preferably, the partly spherical head 5 (Fig. 2) or the
shell 9 (Fig. 3), on its surface 9a, is provided with
grooves or oil ways 16.
In this way, there is obtained a piston whose production
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will be clear from the foregoing and in which there is
introduced in the bearing surface 4 the head 5 of the
connecting rod 6, the latter being thereafter maintained in
position, at least on a piston for a four-stroke internal
combustion engine, for example by a rigid ring 17
elastically yieldably biased upwardly in Fig. 1 by an
elastically yieldable ring 18 which bears externally against
the piston 1. On a piston for a two-stroke internal
combustion engine, the maintaining means such as the rings
17, 18 may be dispensed with, since the forces are always
exerted in the same direction on the piston in the course of
the operating cycle of the engine.
In operation, the force developed on the upper surface
of the piston 1 by the pressure of the working gas is
uniformly distributed over the head 5 by the cushion
constituted by the substance 13.
As shown diagrammatically in Figs. 1 and 8, the metal
shell 9 may be maintained in position by an interference fit
in the cylindrical bearing surface of the piston 1, provided
this shell is capable of moving in a direction parallel to
the axis X-X of this cylindrical bearing surface under the
effect of forces exceeding a given limit.
According to the embodiment of Fig. 1, the shell 9,
;~; whose hemispherical part is extended by a cylindrical skirt
11 in contact with the cylindrical bearing surface 8 of the
piston 1, is mounted in the piston 1 with no other fixing
; means than its interference fit between the head S and the
lZ99943
cylindrical bearing surface 8 of the piston, this
interference being due to an appropriate choice of the
thickness (at rest) and/or of the outside diameter of the
shell 9 and of the difference between said dlameters D and
d. This thickness is generally between 1% and 3~ of the
diameter d of the head 5.
According to the embodiment of Fig. 8, the hemispherical
part of the shell 9 is followed by a highly flared skirt 20
which is retained between the base of the skirt 3 of the
piston 1 and a ring 22 which is fixed to this skirt base by
suitable means, such as screws. To enable the shell 9 to
move in a direction parallel to the axis X-X, the base of
the skirt 3 of the piston 1 is provided with a rounded
surface 21 and the dimensions of the shell 9, and in
particular of the skirt 20, are so determined, bearing in
mind the volume of the fluid or substance 13, that a
clearance 23 is provided at rest between the rounded
surface 21 and the confronting part 9c of the shell 9, this
Glearance enabling the part 9c to be deformed without being
subjected to tensile forces when the shell 9 moves axially
with respect to the cylindrical bearing surface 8 of the
piston 1.
The embodiment of Fig. 9 differs from those of Figs. 1-2
and 8 by the absence of the shell 9 between the partly
spherical head 5 of the connecting rod 6 and the cylindrical
bearing surface of the piston 1. In other words, the
deformable substance 13 is in direct contact with the head 5
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and is prevented from escaping from the cavity 7 merely by
the choice of the clearance provided between the head 5 and
the cylindrical bearing surface 8 ~or, which a~ounts to the
same thing, the choice of the difference between said
diameters D and d), bearing in mind the characteristics of
viscosity of the substance 13.
Preferably, the ratio between the area of the
diametrical section of the partly spherical head 5 and the
area of the cross-section (measured in the direction
perpendicular to the axis X-X) of piston 1 is greater than
or equal to 0.5, which advantageously limits the pressure
stresses produced in operation on the substance 13 and the
head 5. This characteristic applies also advantageously in
the presence of a shell 9, although the diagrammatic
representations of Pigs. 1 and 8 do not illustrate this.
According to the embodiment of Fig. 9, the grooves or
oil ways such as those designated by the reference character
16 in Figs. 1, 2 and 8 are not to be envisaged since they
would become plugged up with the substance 13 and would tend
to encourage its migration out of the space provided
therefor. In this case, the passage 14 for supplying
lubricating oil under pressure provided in the connecting
rod 6 communicates with passages 15a extending through
the connecting rod head 5 and permanently opening out
below the "equatorial" plane P (according to Fig. 9)
; perpendicular to the axis X-X and passing through the centre
C of the partly spherical surface of the head 5, onto an
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annular chamber 25 defined, below said plane P, by a ring
17a which is similar to the ring 17 of Fig. 1 but provided
with outlet orifices such as 26 for the lubricating oil
which reaches the chamber 25 through the passages 14 and
15a. The circulation of oil achieved in this way ensures,
on one hand, an intense cooling of the rod head 5 and
possibly of the piston 1 and, on the other hand, the
maintenance, by the effect of entrainment, of a film of
lubricating oil on the rod head 5.
In any case, the height of the cylindrical bearing
surface 8 must be sufficient to allow accommodating the
axial travel of the head 5 of the rod 6 relative to the
piston 1 which corresponds to:
the differential thermal expansion (due to the
difference between the coefficients of expansion), and/or
the compressibility of the substance 13, and/or
the manufacturing tolerances, and/or
wear phenomena.
In the foregoing, it has been assumed that the fluid 13
was constituted by PTFE. Notwithstanding its obvious
advantages, this substance has certain drawbacks. In
particular, small or very small deformations of the shell 9
~when the latter exists) are liable not to be followed on
the side of the cushion formed by the PTFE. Moreover, the
transverse relaxation time of a deformation is relatively
long.
In some cases, it is advantageous, for overcoming
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1299943
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the defects of PTFE, to replace the latter by a liquid
having high viscosity, such as silicone oils employed in
viscous da~pers, preferably in combination with a shell 9
which is subjected, in operation, to a cyclic deformation.
This is illustrated in Fig. 10 in which the space 12 has
been filled from the bottom (in the filling position) to
avoid any air bubble, with an oil 13 having a high viscosity
through an orifice 27 which was subsequently closed by a
plug or pointed screw 28. In the course of this filling,
the air escapes in the upper part (in the filling position)
of the space 12 through a purge orifice 31 which is
subsequently closed by a plug or pointed screw 32. The
space 12 terminates at a short distance below the equatorial
plane P in a rounded portion 29 (and not by a chamfer).
Indeed, it is necessary to bring down to the equatorial
plane P, the hydrostatic pressure of the cushion constituted
by the viscous liquid 13 by giving thickness to this cushion
down to this plane P. An annular sealing element 30 is
disposed in a groove provided in such manner in the piston 1
that it opens onto the cylindrical bearing surface 8 below
said rounded portion 29 but as close as possible to the
equatorial plane P so that the hydrostatic pressure does not
move the shell 9 away from the cylindrical bearing surface 8
and thereby result in a leakage of the viscous liquid 13.
It is nonetheless necessary to arrange that the space 12
have a relatively small thickness, on the order of for
example 2 mm, so that it is possible to neglect the effect
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26361-53
of the compressibility of the oil 13 of high viscosity and
this oil can consequently be considered to be practically
incompressible at the operating temperatures and pressures
according to the general definition provided at the
beginning of this description.
There will now be described the means for taking
advantage of the presence of the shell 9 for maintaining a
film of lubricating oil between this shell 9 and the rod
head S.
10According to the invention and as illustrated in Fig.
11, for forming said means the shell 9 is so arranged that
its substantially hemispherical part 9a, 9b bears against
the partly spherical head 5 of the connecting rod 6 along a
closed circular line of contact 33 located in a plane Q
extending above (according to the convention explained in
: the preamble) the centre C of the head of the connecting rod
S and preferably perpendicularly to the axis of the piston
1. According to a modification, the shell 9 may bear
against the head 5 along a circular band 34 defined in its
upper part by the line 33 and/or the plane Q.
Unidirectional means for supplying lubricating oil under
pressure communicate on the upstream side through a check-
valve 35 with the passage 14 provided in the connecting rod
6 and opening onto an interstice 36 defined by the parts of
the rod head S and the shell 9 which are located above said
line of contact 33 (or band of contact 34). The assembly is
:arranged in such manner that the pressure of the lubricating
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oil on the upstream side of the check-valve 35 exceeds, at
least during a part of the compression-expansion cycle of
the working gas, the pressure prevailing in the
interstice 36. The space 37 between the rod head 5 and the
shell 9 below the line or band of contact 33 or 34,
communicates with a discharge zone 38 at low pressure, i.e.
at a pressure lower than the maximum pressure prevailing in
the interstice 36. In the case where a ring 17a, similar to
the ring of Fig. 9, is provided for maintaining the head 5
inside the piston 1, at least one orifice or passage 26 must
extend through or around this ring 17a in order to
permanently put the space 37 in communication with the
discharge zone 38. Indeed, if the space 37 were fluidtight,
the lubricating oil would stagnate in the interstice 36.
Preferably, and as shown in Figs. 13 and 14, the
; interstice 36 is essentially constituted by the space
between the shell 9 and the rod head 5 and located above
their line or band of contact 33 or 34 and by a network of
grooves, such as 39, which are provided on at least one of
the confronting surfaces of the shell 9 and the rod head 5
and communicate with the outlet of the unidirectional
lubricating oil supply means, i.e. with the downstream side
of the check-valve 35. In order to simplify the drawing,
there has been shown in Fig. 11 only said space between the
shell 9 and the rod head S and the line of contact 33 and
the band of contact 34, the latter being also shown in Fig.
14. Preferably, the width of the grooves 39 and others is
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26361-53
1 9 _
approximately equal to the thickness of the shell 9. In
general, the grooves 39 are meridian grooves passing through
this intersection and leading to said line or band of
contact 33, 34. As for the plane P termed the "equatorial"
plane, the term "meridian" is employed here in likening the
rod head 5 to a globe of the earth, the polar line of
which would constitute the axis of the connecting rod 6 and
would coincide with the axis X-X of the piston 1 when, as
shown for example in Fig. ~3, the rod 6 is placed on the
axis of the piston 1.
As shown in Figs. 14 and 15, said outlet of the
unidirectional means for supplying lubricating oil under
pressure is located at the intersection of the partly
spherical surface of the rod head 5 and the axis X-X of the
piston 1, irrespective of the relative angular position of
the rod 6 and the piston 1 and the grooves 39 pass through
this intersection and lead to said line of contact 33 or the
upper limit of the band of contact 34.
In addition to the meridian grooves 39, at least one
other groove 40 is provided on the rod head S in a meridian
plane perpendicular to the axis X-X of the piston 1 above
the line or band of contact 33, 34 or at the upper limit of
this band of contact 34. At ieast one other groove 41 is
provided on the head 5 in a plane perpendicular to the axis
25 X-X of the piston 1 below the line or band of contact 33, 34
or at the lower limit of this band of contact 34. Other
grooves 42 of these grooves may also be provided below this
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band of contact 34 in meridian planes. As a modification,the grooves 39, 40 and 41 could be placed on the inner
surface 9b of the shell 9, their situation being the same as
the situation of the grooves provided preferably on the rod
~-~ 5 head 5, at least when the rod 6 is aligned with the piston
~; 1 .
According to the arrangement illustrated in Fig. 11,
sufficiently small values are given to the thickness of the
shell 9 and the stiffness of the material from which it is
made so that the quasi-hemispherical upper part 9a, 9b of
the shell 9 perfectly marries up with the spherical outer
shape of the rod head 5 by elastic deformation when the
resultant of the forces applied to the piston 1 is maximum
so as to expel the oil trapped in the interstice 36 through
said line or band of contact 33, 34 while forming a
hydrostatic film ad~acent to said discharge zone 38, and
sufficiently large values to ensure that the shell 9 resumes
its initial shape when the resultant of the forces applied
to the piston 1 is minimum so as to cause the pressure
prevailing in the interstice 36 to drop to a value below
that of the pressure of the oil supplied, through the check-
valve 35, by the unidirectional oil supply means.
During the reciprocating movements of the piston, the
; quasi-hemispherical part of the shell 9 thus performs the
function of a diaphragm of a diaphragm pump, the intake
strokes of this pump being due to the enlargement of the
interstice 36 and the discharge strokes to its shrinkage by
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elastic deformation of the shell 9.
In this case, the shell 9 may be formed in the following
manner:
a) In the case of a shell having a quasi-hemispherical
shape at rest in its upper part, the shell 9 may be formed
by a press operation in a die which imparts thereto the
required shape, it being then possible to make the rod head
5 perfectly spherical (except, of course, in its lower part
by which it is connected to the rod 6).
tOThe drawback of this solution is that the interstice 36
is fixed relative to the piston 1 and requires grooves for
supplying oil to this interstice from the oil outlet which
is fixed relative to the rod 6.
b) In the case of a shell which is hemispherical at
rest in its upper part, the rod head 5 may be sa machined as
to provide the interstice 36 around the oil outlet. This
machining, for example by a digital control machine, may
be effected by turning a member of revolution. It is then
possible to dispense with the grooves referred to in the
preceding paragraph-
According to a second arrangement illustrated in Figs.
12 to 15, the unidirectional means for supplying oil under
pressure are formed by a piston pump incorporated in the
~assembly of the connecting rod 6 and the rod head 5 which
;~25 draws oil through an inlet valve 43 and discharges the oil
through said check-valve 35.
In an advantageous construction, said pump is formed by
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22 26361-53
a cavity 44 of variable volume located between the inlet valve 43
and the check-valve 35 and defined by the body of the connecting
rod 6 whose upper part 45 is so arranged as to form a piston which
is axially slidable in a bGre 46 provided in the rod head 5. A
spring is interposed between the body of the rod 6 and the rod
head 5.
The compression travel of this spring is sufficient to
permit the pressure of the oil trapped in said cavity 44 between
the two valves 35, 43, to exceed the pressure exerted in said
interstice 36 during at least a part of the compression-expansion
cycle of the working gas, the length of the spring being large
enough to ensure that its relative compression does not exceed the
elastic deformation and fatigue limits of the material from which
the spring is made, the stiffness of the spring being small enough
to enable the spring to be compressed in such manner as to put the
bearing surfaces of the rod head 5 and the body of the rod 6 in
contact with each other while the resultant of the forces applied
against the piston 1 is at the most maximum and large enough to
ensure that the spring can extend while the resultant of the
forces applied against the piston is at least minimum.
: According to the simplest embodiment shown
diagrammatically in Fig. 14, the spring in question is a simple
coil spring 47. Mechanical means (not shown) limit the travel of
the rod head 5 in the direction away from the rod 6.
According to the more elaborate embodiments shown in
' ~
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- 23 -
Figs. 13 to 15, said spring is formed by a metal bar,
preferably composed of titanium.
According to the embodiment of Fig. 13, the metal bar,
here designated by the reference character 48, haa a tubular
shape and surrounds the end part 45 of the body of the
connecting rod 6. This part 45 has a reduced cylindrical
shape relative to the adjacent part of the body of the rod 6
so as to provide an annular shoulder 49 against which bears
one of the axial ends of the tubular bar 48.
According to the variant of Fig. 15, the metal bar, here
designated by the reference character 50, has a solid shape
apart from a longitudinal passage 51 which extends
therethrough in the extension of the passage 14 in the rod
6.
Whatever embodiment is adopted, the relative contraction
of the bar 48 or 50 is on the order of 0.25%, its cross-
section representing preferably about 3% of the cross-
section of the piston 1.
A metal bar such as 46 or 50, which may normally be
considered rigid, can act as a spring if its cross-section
is small enough relative to the compression stresses exerted
thereon, but large enough to remain within the domain of
elastic compression of the metal from which it is made. The
advantage of the metal bar over the spring 47 of Fig. 12
resides in its reduced overall size and in the reduction in
the harmful empty spaces. The choice of titanium in
preference to steel is justified by the fact that its
~gg~3
- 24 -
Young's modulus (11.3 x lOS bars) is about one half of that
of steel ~22 x 10 bars). For a given stress, the
compression travel of the bar 46 or 50 is twice that of a
bar of steel or, for a given compression travel, the length
of a bar of titanium is one half of the length of a bar of
steel.
