US2640433A

US2640433A - Swash plate pump

Info

Publication number: US2640433A
Application number: US757249A
Authority: US
Inventors: Max A Mathys
Original assignee: Ex-Cell-O Corp
Current assignee: Ex-Cell-O Corp
Priority date: 1947-06-26
Filing date: 1947-06-26
Publication date: 1953-06-02
Anticipated expiration: 1970-06-02

Description

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Sm v\ w v Y P z M L .P A uh 2 M. p v $3 June 2, 1953 Filed June 26) 1947 June 2, 1953 M. A. MATHYS 2,540,433

SWASH PLATE PUMP Filed June 26, 1947 2 Sheets-Sheet 2 58 Figs PR assume Patented June 2, 195 3 SWASH PLATE PUMP Max A. Mathys, Detroit, Mich., assignor to Ex- Cell-O Corporation, Detroit, Mich., a corporation of Michigan Application June 26, 1947, Serial No. 757,249

'7 Claims. 1

The present invention relates generally to improvements in fluid pumps of the so-called swash plate type, and more specifically to a pump of the character described in Patent No. 2,299,233 to Hoifer. Although the invention is particularly well suited for use in a small, self-contained hydraulic power unit it possesses, in addition, a relatively broad general utility.

One of the objects of the invention is to provide a small, compact, swash plate pump which will be susceptible of satisfactory operation at substantially higher speeds than prior pumps of a similar type. A related object is to provide a pump of the foregoing class in which throttling of fluid at the outlets of the pump cylinders is completely eliminated,

Another object is to provide a swash plate pump in which the swash plate is rockable about an axis fixed with respect to the rotational axis of the pump, the location of such axis being selected with a view to eliminating the sensitivity of the swash plate angle to changes in delivery pressure and also to eliminating certain pulsations in delivery pressure.

A further object is to provide a pump of the type set forth and including means for balancing the working pressures acting upon the valve faces thereof so as to produce a resultant acting along the rotational axis of the pump.

Other objects and advantages of the invention will become apparent from the following detailed description and from the accompanying drawings in which:

Figure 1 is a front elevational view of a hydraulic power unit in which a pump constituting an exemplary embodiment of the invention is utilized. This figure also includes a fragmentary vertical sectional view of the pump and the fluid delivery control means.

Fig. 2 is an enlarged fragmentary sectional view through the trunnions of the swash plate assembly, taken in the plane of the horizontal 2 pump cylinder body taken in the plane of line 6-6 of Fig. 3.

Fig. '7 is a diagrammatic view showing the relative location of the pressure and suction areas in the plane of bearing engagement between the stationary and rotary valve faces.

Fig. 8 is a fragmentary sectional view through a portion of the valve manifold and taken along the line 8-43 of Fig. 5.1

While the invention is susceptible of various modifications and alternative constructions, a preferred embodiment has been shown in the drawings and will be herein described in some detail, but it is to be understood that there is no intention to limit the inventionrto the specific form disclosed, the intention being, on the contrary to cover all modifications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.

Referring to'the drawings, a pump constituting an exemplary embodiment of the invention is shown comprising a stationary manifold block l0 having a number of fluid passages and which rotatably supports a pump drive shaft ll. Drivingly mounted adjacent one end of the shaft H is a rotatable cylinder body l2 which carries a plurality of reciprocable plungers or pistons it. The entry and discharge of fiuid with respect to the cylinder body [2 are governed by means of a pair of radial valve faces arranged in mutual bearing engagement between the manifold block and the cylinder body so as to form a running pressure seal. For the purpose of regulating the stroke of the pistons I4 and thereby regulating the fluid delivery rate of the pump, a swash plate 15 is rockably mounted on an axis transversely disposed to that of the drive shaft ll. v i

The manifold block Ill (Figs. 1, 3 and 5 serves as a convenient mounting means for the shaft H and the cylinder body 12. On opposite sides it is formed with a pair of alined bosses i6, i8 and running axially through the latter is a relatively large cylindrical bore IS in which the pump drive shaft H is journaled. The boss it, which is of substantially shorter axial length than the boss l6, defines at. its outer end a stationary radial valve face 20 having a plurality of inlet and discharge ports communicating respectively with an inlet passage 2i and a discharge passage 22 (not shown) in the block IE.

Power may be applied to the pump in any appropriate manner and in the present instance the expedientof'a belt drive is employed. For' the purpose of increasing the ability of the shaft H to withstand force components exerting a lateral thrust thereon, such as belt tension or the camming reaction upon the cylinder body l2, a number of bearings are arranged along the former in axially spaced-apart relationship. These include an oil sealing bearing 24 and a pair of roller bearings 25,- the latter being separated by an enlargement 2 6 on the shaft.- Adjacent its inner end, the shaft H is provided with splines 28 for complementary driving engagement with a splined bore 29 in the cylinder body [2.

Turning to Figs. 3 and 6, it will be noted that the rotatable cylinder body I? is formed with a plurality of unrestricted identical cylindrical bores or pump cylinders 30 equally spaced in an annular pattern concentric with the rotational axis of the shaft H. Slidably mounted and re ciprocable Within the bores 30 are a corresponding number of the plungers 14 which project from the end of the cylinder body away from the manifold block. The bores 30, being of uniform diameter throughout their length, are in unrestricted communication with the opposite end of the cylinder body and which defines a radial valve face 3|.- The latter is adapted to rotate in bearing engagement with the stationary valve face 29 and in cooperation therewith to govern the-admission of fluid to and discharge from the cylinders 38. It will thus be appreciated that the arrangement described makes possible the unrestricted flow of fluid to and from the pumpcyli nders alt-an important ,factor in contributing to the high speed characteristics of the Eachof the plungers i4 oomprisesan elongated body 32 of a diameter slightly smaller than that of the cylinder in which it is housed but large enough to form a good fluid seal, together with a convex head 34 and a neck 35 of reduced diameter whichconnects the head and body. The projecting ends of theplungers are respectively surrounded by coiled loading springs 35 which bias the plungers outwardly by a thrust exerted betweenthe end of the cylinder body I2 and a collar to on each neck 35. 7

As earlier indicated, the stationary valve face 2;} (Fig. which is unitary with the manifold block if] is provided with a number of diametrically opposed inlet and diseharge ports connecting respectively with the inlet and discharge passages in the block l0. Thus clese ly spaced inlet ports 59, 4Q, lll commimicatewith the fluid inlet passage in the manifold block, their spacing producing a generally segmental inlet port area 42 in the valve face 29. Similarly, discharge ports 44, 25, 4G communicate with the discharge passage in the block Ii] defining a segmental dis charge port area 48. Separating the inlet and ischarge port areas42 and 48 are a pair of diametrical ly opposed

land areas

49, 50.

For the purpose of efictinga pressure equalization in each cylinder immediately prior to the start of both the pressure and suction strokes, the port locations have been advanced with respect to the top and bottom dead center positions of plunger movement. Referring more particularly to Fig. 5, the vertical dividing plane between the suction and discharge phases of plunger movement has been designated therein by the line a-it. The center line of the land areas 49 and Eli, designated by the line bb and sometimes referred to as the axis of port separation, has also been advanced from coincidence with the line a-a through a small angle which in the present instance is 9 degrees. As the cylinder body [2 rotates, a plunger crossing the land area 49 will start on its pressure stroke when it crosses the line a-a but the cylinder will not be open to the port 44 until a short time later. Consequently, the pressure in the cylinder will increase to a value approximating the pump discharge pressure before the cylinder is connected to the port 44. This is known as precompression. Similarly, a plunger crossing the land area 50 will commence its suction stroke when it crosses the line 1H. Due to the advancement of the suction port 39, the cylinder will remain closed during the initial part of the suction stroke, and the resulting expansion will lower the pressure approximately to that of the atmosphere before the cylinder and the port 39 are connected. Thus pulsations of pressure due to valve action, together with the noise and other undesirable phenomena associated therewith, are substantially eliminated.

In order to permit the stroke of the phmgers M to be varied between zero and maximum, thereby regulating the fluid delivery rate of the pump, the rockable swash plate [5 is arranged With it race 5! in abutment with the convex heads 34 of the plungers 14. In addition, the swash plate is made adjustable with reference to the angle between its face 51 and the plane of rotation of the cylinder body l2. For example, when the face 5| is parallel with the rotational plane of the cylinder body, the swash plate is ineiiective to reciprocate the plungers i iand the fluid delivery rate of the pump is therefore zero. When the swash plate is rocked to form an angle between its face and the plane of rotation of the cylinder body, the

swash plate then becomes effective tore'ciprocate the plungers it, the stroke and consequently the fluid delivery rate increasing to a maximum value as the face angle is increased to its maximum value. The adjustable support for the swash plate comprises a swivel frame 52 which is pivotally mounted upon a fixed transverse axis with respect to the rotational axis of the shaft II and the cylinder body [2. The swivel frame carries a guide sleeve or cylinder 54 having a closed end 55 and housing an elongated longitudinal extension of the swash plate IS in the form of a swash plug 56 unitary therewith. The plug 56 is journaled within the sleeve 54 by means of a pair of longitudinally spaced-apart guide bearings 58 thus permitting the swash plate 15 tobe rotated about the longitudinal axis or the swash plug due to contact with the plunger heads 34. Such arrangement precludes deviation of the swash plate 15 from its plane of adjustment regardless of the eccentricity or value of the p'hInger reaction forces inherent in the operation of the pump. It also eliminates substantially all the rubbing friction which would otherwise take place between the plate 15 and the heads 34 if the former were non-rotatable. Various means may be employed for adjusting the swash plate l5 together with its swivel frame 52. In the preferred embodiment this is accomplished by the use of an elongated arm 59 unitary with the swivel frame 52 and extending generally in a direction away from the pivotal axis thereof. A tensile spring 68 is attached to the end of the arm '59, tending to move the latter in a direction which will increase the swash plate angle and therefore the delivery rate of the pump. The spring 8D is opposed by a hydraulic plunger 5] acting upon the arm through a link 62 and which may be subjected to a control pressure derived from the pump discharge pressure for maintaining the proper fluid delivery rate. It should be kept in mind that while the swash plate I5 is rotatable, when it has once been adjusted the plane of its rotation remains fixed with respect to that of the cylinder body until it is readjusted.

A difficulty commonly experienced in prior pumps of the swash plate type is the occurrence of pulsations in deliverypressure with various critical combinations of volume of delivery, speed and pressure. Such pulsations frequently accompany changes in delivery pressure and are primarily-due to a flutter set up in the swash plate as an incident to the shifting of the hydraulic forces reacting through the plungers I4 as the pump cylinders 30 move in succession along the pressure or discharge port area 48. The foregoing forces are sometime spoken of in the art as plunger reactions. An important aspect of the invention includes the provision of means for eliminating the disturbing effect of shifting plunger reactive forces upon the pump delivery pressure. This is accomplished in the present instance by mounting the swash plate I5 upon an axis which is intersected by the locus of resultant plunger reaction, the reactive forces being balanced on either side of such locus. Thus in Fig. 5, the locus of resultant plunger reaction appears as a point B, which is offset from the horizontal medial plane of the drive shaft II due to the precompression feature of the pump, being located on a line -0 which is 90 degrees from the port separation axis bb. Running horizontally through the point R, and at 90 degrees to the vertical dividing line a-a between the suction and discharge phases of I plunger movement, is a line dd representing the pivotal axis of the swash plate I5. In the illustrative embodiment the axis dd happens to fall slightly above the rotational axis of the shaft I I. The actual extent of this distance or offset is immaterial, the important thing being the intersection of the swivel axis and the locus R so that the moments about this axis due to the plunger reactive forces are balanced or cancel out.

In Fig. 2, the details of the above described mounting for the swash platerare shown in plan. The swivel frame 52 is provided with a pair of projecting ears 64 having transverse bores adapted to pivot upon a pair of fixed trunnions 65, the longitudinal axes of the latter coinciding with the axis d-d. With the arrangement. shown, the plunger reactive forces will have no noticeable effect upon the swash plate I regardless of the angular position of the latter. Consequently the discharge pressure of the pump will be free of pulsations due to these forces over the entire range of fluid delivery. This makes it possible.

to dispense with the additional actuator used in certain prior art pumps to neutralize such pulsations.

Means is provided for minimizing the leakage of fluid from the pressure seal between the stationary and rotating valve faces 20 and 3|, through the application of an external axial thrust tending to press them together in opposition to the pressure forces tending to separate them. Turning once more to Fig. 3, it will be seen that the swash plug 56 is formed with an annular piston 5-6 which, like the plug itself, is slidable within the sleeve 54. The space enclosed by the piston 65, the sleeve 54, and the closed end 55 thereof comprises a piston chamber 68 which is adapted to receive pressure fluid for exerting a. thrust on the swash plug. Fluid from the discharge line of the pump may be conducted to thechamber 68 in any desired manner and in the embodiment shown this is done by means of a flexible conduit 69. The latter connects with a passage 10 in the swivel frame which in turn opens into an annular chamber defined by a peripheral groove II in the sleeve 54. The groove II communicates with the piston chamber 68 via one or more radial passages 72 in the wall of the sleeve. When pressure fluid is supplied to the chamber 68, a substantial axial force is brought to bear upon the swash plug 56 and such force will have a sizeable component along the rotational axis of the shaft II. The thrust produced by this component, which varies inversely with the angle of tilt of the swash plate, is transmitted axially to the shaft I I through a capsuleshaped compression link 14 having spherical seating surfaces in both the swash plug and the end of the shaft. Since the latter has no thrust bearing, the thrust thereon will be applied axially to the cylinder body I2 by means of a collar or snap ring 15, pressing the valve faces 3| and 20 tightly together. I

In prior pumps the fluid pressure tending to separate the valve faces is substantially greater on the pressure or discharge side than on the inlet side, and the resultant force of such pressure acts upon the valve faces at a point eccentric to the rotational axis of the shaft. Since this resultant is generally opposed by an axial force of some kind, setting up a force couple acting upon the cylinder body, there is a definite tendency toward excessive wear and friction on the valve faces and particularly so at high speeds. Thi problem is overcome in the pump embodying the invention by the use of a pressure counterbalancing means between the valve faces 20 and 3|. Thus the inlet side of the stationary valve face 20 is provided with a relieved area which in the embodiment illustrated comprises a shallow, circular pressure groove I6 (Figs. 5 and 8) located radially outward of the

inlet ports

39, 40, and 4|. The ends of this groove connect with the pressure or discharge side of the pump via a pair of passages 18, I9 in the manifold block III. The effect of the groove I5 is to create a marginalarea of pressure adjacent to but outside the suction or low pressure area on the left hand side of the valve face 20, as indicated schematically in Fig. 7, thus applying a component of force to the cylinder body I2 tending to separate the valve faces. In like manner, the discharge side of the valve face 20 has a similar relieved area in the form of a groove located radially outward of the discharge ports 44, 45, II; and which may communicate with the inlet side of the pump via a passage 8| in the manifold block, diverting high pressure and producing a marginal area of low pressure adjacent to but outside the high pressure area on the right hand side of the valve face 20. This reduces the value of the component of separating force in the region of the discharge ports. The net effect of the distribution of pressure areas described upon the components of separating force is to bring into coincidence with the axis of the shaft II the resultant pressure force tending to separate the valve faces, thus applying such resultant squarely against the external thrust exerted by the swash plug.

While the operation of the pump has already beenbrought out to a certain extent above, a

brief summary thereof might be appropriate at this point. Assuming the control pressure on the plunger 6| to be negligible, the tensile spring I50 will be effective to hold the arm 59 of the swivel frame 52 inc. position corresponding to the maximum angle between the face of the swash plate and the rotational plane of the cylinder body 1'2. This causes the pump to operate at the maximum fluid delivery rate until the discharge pressure builds up to a selected predetermined value, the control pressure on the plunger 6! building up at the same time. Discharge pressure in excess of such predetermined value is accompanied by an increase in control pressure acting upon the plunger 6! causing the latter to overcome the force of the spring 66 and to rock the swivel frame in adirection which decreases the angle between the face of the swash plate and the plane of cylinder body rotation. This decreases the fluid delivery rate of the pump and consequently lowers the discharge pressure to the predetermined value. Should the discharge pressure fall below such value, the spring 60 takes control away from the plunger 6!, increasing the swash plate angle until the pressure is restored to the desired figure. The connection of the spring 60 and the plunger 6! adjacent the end of the arm 59 provides a long moment arm for positioning the swivel frame 52 and the swash plate 15. Due to such arrangement, and also to the elimination of the factors tending to'disturh the swash plate, a slight displacement of the plunger 6| becomes effective immediately to adjust the swash plate angle, thus permitting the regulation of the pump discharge pressure within extremely close limits.

I claim as my invention:

l. A variable delivery swash plate pump comprising, in combination, a pump manifold having a stationary radial valve face, a drive shaft journaled within a bore in said manifold, a cylinder body keyed to said shaft for rotation therewith and mounted adjacent said stationary valve face, said body having a plurality of equally spaced cylindrical bores arranged in an annular pattern concentric with the axis of said shaft and parallel thereto, a rotatable radial valve face defined by the end of said cylinder body adjacent said manifold and adapted for bearing engagement with said stationary valve face, reciprocable plun'gers slidably housed within the cylindrical bores of said cylinder body with their ends projecting axially from the end of the latter away from said valve faces, a rockable swash plate with its face disposed in abutment with the projecting ends-of said plungers to vary their stroke and thus to vary the pump discharge pressure, said swash plate having a longitudinal extension unitary therewith, a swivel frame, a guide sleeve carried by said'swivel frame and adapted to rota tionally house said longitudinal extension, a pair of ears projecting from said swivel frame, trun nions adapted to pivotally engage said ears and located on an axis about which the moments due to the plunger reactive forces acting upon said swash plate cancel out, said axis being disposed in transverse but non-intersecting relation with the axis of said drive shaft, an elongated arm unitary with said swivel frame for rendering a relatively small control force effective to rock said swash plate, the length of said arm and the balance of the moments of said reaction forces on said swash plate permitting accurate regulation of the pump discharge pressure.

2. In a high speed variable delivery swash plate pump, the combination of a stationary valve face having inlet and discharge ports therein, a 10- tatable cylinder body having a plurality of un' restricted, identical, cylindrical bores extend mg axially tiierethrcugn and equally spaced in a generally annular pattern concentric with the rotational axis of said cylinder body, a rotary valve face defined by one end of said cylinder body and to which the bores have unrestricted access, said rotary valve face being in bearing engagement with said stationary valve face to gover'fi the entry and discharge of fluid into and out of said cylinder body, a series of plungers reciprocably disposed within the cylindrical bores of said body and projecting outward from the end thereof away from said rotary valve face, a swash plate having a face in abutment with the ends of said plungers and rockable to vary their stroke as the angle between the plane of said face and the rotational plane of said cylinder body is adjusted, such variation in stroke serving to vary the delivery rate of the pump, a trunnion mounted swivel frame for rockably sup-porting said swash plate on an axis transverse to the rotational axis of said cylinder body, a guide "sleeve carried by said swivel frame, a pair of longitudinally spaced-apart bearings housed within said sleeve, and a plug extension defining "a piston unitary with said swash plate and journaled in said bearings for rotation about the longitudinal axis of said sleeve without deviation of said swash plate from the plane of its adjustment.

3. In a high speed variable delivery pump of the swash plate type,- the combination comprising a stationary valve face having inlet and discharge ports therein, a drive shaft projecting through said stationary valve face, a pump cylinder bod-y mounted on said shaft for rotation therewith and partially housing a series of re cip'ro'c'ab'le pistons within a corresponding series of cylindrical bores therein, the ends of said pis tons projecting from one end of said cylinder body, a rotary valve face defined by the opposite end of said cylinder body and disposed in bearing engagement with said stationary valve face to" form a running pressure seal for the inlet and discharge ports thereof, a swash plate mounted with its face in abutment with the projecting ends of said pistons for varying their stroke, a piston integral with said swash plate, a trockably mounted cylinder adapted to house said piston, and means for conducting pressure fluid to said cylinder to produce an axial thrust upon said piston and said swash plate in the proper direction to maintain the pressure seal between said valve faces.

4. In a high speed variable delivery pum of the swash: plate type, the combination compris ing a manifold, a stationary valve face unitary with said manifold and having inlet and discharge ports therein, a drive shaft rotatably supported in said manifold and projecting through said stationary valve face, a pump cylinder body drivingly mounted upon said shaft and carrying a series of reciprocable pistons projecting from one end thereof, a rotary valve face defined by the opposite end of said cylinder body and disposed in sealing engagement with said stationary valve face, a swash plate tiltable about a fixed axis transverse to the rotational axi of the cylinder body and mounted with its face in abutment with the projecting ends of said pistons for varying their stroke, a swash plug unitary with said swash plate and having a piston formed thereon, a closed sleeve for slidably housing said swash plug and forming a piston chamber' between the piston of said plug and the closed end of said sleeve, and conduit means for applying fluid under pressure to the piston chamber so as to exert an axial thrust on said swash plug, such thrust being transmitted by the latter to said cylinder body for maintaining the pressure seal between said valve faces by urging them together.

5. In a high speed variable delivery pump, the combination of a stationary valve face having inlet and discharge ports therein, a rotatable cylinder body carrying a series of reciprocable pis tons projecting from one end thereof, a rotary valve face defined by the opposite end of said cylinder body and disposed in bearing engagement with said stationary valve face to form a running pressure seal for the inlet and discharge ports, a swash plate rockably mounted on a fixed axis transverse to the rotational axis of said cylinder body and in abutment with the projecting ends of said pistons for varying their stroke, means including a relieved area on said stationary valve face located radially outward of said inlet ports but in communication with the discharge ports for applying a component of force to said cylinder body tending to separate said valve faces and of a magnitude such as to produce, conjointly with a similar separating force in the region of the discharge ports, a resultant separating force acting axially of said cylinder body.

6. In a high speed variable delivery pump, the combination of a manifold, a stationary valve face on said manifold and having diametrically opposed inlet and discharge ports communicating respectively with inlet and discharge passages in said manifold, a drive shaft journaled in said manifold, a rotatable cylinder body drivingly mounted on said shaft and carrying a series of reciprocable pistons projecting from one end thereof, a rotary valve face defined by the opposite end of said cylinder body and disposed in bearing engagement with said stationary valve face to form a running pressure seal for the inlet and discharge ports, a swash plate disposed in abutment with the projecting ends of said pistons for varying their stroke, a piston integral with said swash plate, a guide sleeve having a closed. end and adapted to slidably house said piston, said guide sleeve being mounted for rocking movement about an axis transverse to the rotational axis of said drive shaft, means including a relieved area on said stationary valve face located radially outward of said inlet ports but in communication with the discharge passage for applying a component of force to said cylinder body adjacent the region of the inlet ports and tending to separate said valve faces, said means also including a second relieved area on said stationary valve face located radially outward of said discharge ports but in communication with the inlet passage for diverting pressure thereinto thus reducing the value of a similar separating force in the region of the discharge ports, the separating forces producing conjointly a resultant force acting axially of said cylinder body, and means for conducting pressure fluid to said guide sleeve between said closed end and said piston to produce a thrust on said swash plate axially thereof, such thrust having a component running axially of said cylinder body in opposition to said resultant of said separating forces.

7. A high speed variable delivery pump of the swash plate type comprising, in combination, a pump manifold having a stationary valve face including diametrically opposed inlet and discharge ports, a drive shaft, a rotatable cylinder body drivingly mounted on said shaft adjacent said manifold and having a plurality of unrestricted cylindrical bores extending axially therethrough, a rotary valve face defined by one end of said body and to which the bores have unrestricted access, said rotary valve face being arranged in bearing engagement with said stationary valve face to form a pressure seal therewith, a series of plungers slidably housed within the cylindrical bore of said body and projecting from the end thereof away from said rotary valve face, a swash plate disposed in abutment with the ends of said plungers and adapted to vary their stroke, a piston integral with said swash plate, a guide cylinder adapted to rotatably house said piston, said cylinder having a closed end and a pair of longitudinally spaced bearings therein, said guide cylinder being rockably mounted on an axis running transversely of the rotational axis of said cylinder body, the axis on which said guide cylinder rocks being located in a position where the moments due to the plunger reactive forces acting upon said swash plate are balanced, means including a relieved area on said stationary valve face located radially outward of said inlet ports but in communication with the discharge ports for applying a component of force to said cylinder body adjacent the region of the inlet ports and tending to separate said valve faces, said means also including a second relieved area located radially outward of said discharge ports for diverting pressure adjacent the region of the discharge ports and thus reducing the value of a similar separating force in the region of these ports, the separating forces together exerting a resultant axial force on said cylinder body, and conduit means for applying pressure fluid to said guide cylinder and said piston so as to exert an axial thrust on said swash plate, such thrust having a component running axially of said cylinder body in opposition to the resultant of the forces tending to separate said valve faces.

MAX A. MATHYS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,274,391 Davis Aug. 6, 1918 1,362,040 Pratt Dec. 14, 1920 1,407,047 Trowbridge Feb. 21, 1922 2,298,850 Vickers Oct. 13, 1942 2,379,546 Snader et al. July 3, 1945 2,382,437 Molly Aug. 14, 1945 2,388,644 Roessler Nov. 6, 1945 2,403,371 Ifield et a1 July 2, 1946 2,421,846 Neuland June 10, 1947 FOREIGN PATENTS Number Country Date 23,0 10 Denmark 1918 500,937 Great Britain 1939 527,359 Great Britain 1940 691,199 France 1930