US3216355A

US3216355A - Two-cylinder pump

Info

Publication number: US3216355A
Application number: US260512A
Authority: US
Inventors: William F Wanner
Original assignee: Seeger Wanner Corp
Current assignee: Seeger Wanner Corp
Priority date: 1963-02-25
Filing date: 1963-02-25
Publication date: 1965-11-09
Anticipated expiration: 1982-11-09

Description

Nov. 9, 1965 w. F. WANNER Two-CYLINDER PUMP 2 Sheets-Sheet 1 Filed Feb- 25. 1965 mmm@ ONG.

INVENTOR. DV/UAM E MNA/ER A T TOR/v5 Ys 3,216,355' TWG-CYLINDER PUMP Wiiiiam F. Wallner, Minneapolis, Minn., assigner' to Seeger-wanner Corporation, Minneapolis, Minn., a corporation of Minnesota Filed Feb. 25, 1963, Ser. No. 260,512 3 Ciaims. (Cl. 10S-38) This invention relates to improvements in reciprocating pumps.

More particularly, this invention relates to a twocylinder, piston-equipped, reciprocating pump employing a new and very useful combination of casing means and power transmission means. The invention also relates to a means for varying the stroke of opposed piston pairs in such a pump.

It is an object of this invention to provide a high capacity, two-cylinder pump having reciprocating pistons suitable for use in pumping spray solutions and suspensions where high pressure discharge is desired.

It is another object of this invention to provide a compact, efficient, high pressure reciprocating pump capable of long, heavy duty service wherein all wearing parts are easily replaced.

It is another object of this invention to provide a reciprocating pump having a single, central crankcase for the reciprocating mechanism which is separated from the cylinder and fluid passageway so that, for example, such reciprocating mechanism is easily lubricated and not contaminated by luids being pumped.

It is another object of this invention to provide a twocylinder reciprocating pump having replaceable cylinder sleeves and self-cleaning, self-seating, low inertia valves and in which the orientation of associated parts is such that the pump can be serviced without disturbing the plumbing or drive arrangement.

It is another object `of this invention to provide a twocylinder reciprocating pump wherein the casing means further denes openings intermediate the opposed inner ends of the cylinders in said casing and the pistons reciprocatorily moving therein so as to establish communication between said cylinders and the exterior of said casing means.

It is another object of this invention to provide a twocylinder reciprocating pump wherein the O-rings are used between cylinder heads and pump body thereby permitting reliable and eiicient seals to be established and maintained individually about all valves and cylinder heads.

It is another object vof this invention to provide an improved capsule-type check Valve structure adapted for use in high capacity, two-cylinder reciprocating pumps.

Other and further objects of this invention will become apparent to those skilled in the .art from a reading of the present speciiication taken together with the drawings in which:

FIG. 1 is an end elevational view of a pump of this invention;

FIG. 2 is an elevational view of the end opposite to that shown in FIG. 1;

FIG. 3 is a side elevation as seen from right to left 'of FIG. 1, some parts thereof broken away and some parts shown in section;

FIG. 4 is a view in top plan of the structure shown in FIG. 2;

FIG. 5 is a vertical sectional view as seen along the line 5-5 of FIG. 3, portions thereof being shown in elevation with some parts broken away and some parts shown in section;

FIG. 6 is a horizontal sectional view as seen along the line 6 6 of FIG. 2, portions thereof being shown Patent ICC in elevation with some parts broken away and some parts shown in section;

FIG. 7 is an exploded view in elevation of the check valve assembly used in the pump embodiment shown in FIGS. l-6;

FIG. 8 is a plan View of the assembled valve of FIG. 7;

FIG. 9 is an exploded view in perspective of a drive shaft suitable for use in the pump shown in FIGS. 1-6 equipped with means for varying the stroke of opposed piston pairs in such pump; and

FIG. 10 is a plan view of a spider blank for use in the Valve of FIG. 7.

Turning to the drawings, there is seen an embodiment of a reciprocating pump of this invention herein designated in its entirety by the numeral 10. Liquid to be pumped is taken into intake port 11 and is pumped out of output port 12. The body or casing of pump 10 is conveniently formed of a metal, such as aluminum or the like, and is fabricated by casting and/or machining operations. Thus, the body of pump 10 is seen to consist of a block 13 and a bearing housing 14. In block 13 is formed the head and base region for each one of two cylinder portions, a first cylinder portion 16 and a second portion 17. Secured on block 13, one over the head of each

cylinder portion

16 and 17 by means of suitable bolts 18, are, respectively, rst cylinder head 21 and second cylinder head 22.

Cylinder heads

21 and 22 are, conveniently, of metal construction formed by suitable casting and machining operations.

The

cylinder portions

16 and 17 are each formed to receive metal tubing, such as sleeve inserts 23. These sleeve inserts 23 (two in all per pump) are held round and stopped by simple casting projections 24 integrally formed on the block 13. Each replaceable sleeve insert 23 is pressed into the block 13 through an appropriately formed opening in the opposed outer end of each

respective cylinder portion

16 and 17 and stops against a shoulder 25 formed in the adjacent portion of each of the casting projections 24. The ax-es of the opening in the

cylinder portions

16 and 17 are generally coaxial and normal to the pump axis thereby making the openings and their respective sleeve inserts 23 aligned and axially opposed. The sleeve inserts 23 are each exposed so that each can be removed by merely taking off the

appropriate head

21 or 22 and withdrawing a sleeve insert 23 as with a suitable sleeve insert puller (not shown).

The sleeve inserts 23 taken together with the

adjoining portions

16 and 17 `of the block 13 can be considered to form opposed cylinder portions or cylinder block portions of pump 10 disposed in outwardly spaced relation to the axis of the block 13, which axis is generally normally positioned with respect to the pump axis.

Formed in each

head

21 and 22 and communicating with the interior of each

cylinder portion

16 and 17 is a

valve chamber

27 and 28, respectively. Each

such valve chamber

27 and 28 extends laterally outwardly along a common plane positioned generally normally to the pump axis. integrally formed within block 13 and communicating with an opposite end region of each

valve chamber

27 and 28 are two channels or

passageways

29 and 30. Channel 29 communicates in its mid-region with intake port 11, while channel 30 communicates in its mid-portion with output port 12, both open from block 13 on that end of block 13 opposite that which abuts against bearing housing 14 in the assembled pump 10 (see FIG. 1).

Spaced at a respective end of each

channel

29 and 30 is a check valve (four in all). The check valves at each end of channel 29 are numbered 32 and 33 while those at each end of ychannel 30 are numbered 34 and 35. All check valves are of the capsule type so that each valve can be conveniently mounted in place in block 13 by an appropriately formed bore or enlargement at each end of

channel

29 and 30, respectively (see FIG. 5). Thus, each

check valve

32, 33, 34, and 35 is so located in the block 13 that it can be exposed and removed by merely removing the appropriate adjacent

respective head

21 or 22 and withdrawing a

valve

32, 33, 34, and 35 with a suitable conventional check valve puller (not shown).

The operation and construction of each

check valve

32, 33, 34, and 35 is the same and will now be briefly described by reference to valve 32 as shown in FIGS. 7 and 8. The valve 32 is seen to utilize a cylindrically-sided, cup-shaped housing 37 having an axial fluid opening 38 therethrough. A cup-shaped valve element 39 which is adapted to seat across opening 38 has generally convex external side walls adapted to seat across the iluid Opening 38. Housing 37 and valve element 39 can be formed of any suitable material, such as stainless steel stampings, extruded plastic material (i.e., polytetrauoroethylene, etc.), or the like. A spider 40 is mounted in housing 37. The spider 40 is conveniently formed of a single piece, as a metal stamping operation or extruded plastic shape, or the like, and is formed so as to have the legs 41 extend inward laterally. The spider 4t) is formed here from a blank shown in FIG. lO by turning (bending) blank along fold lines 87 to form three legs 41 which extend transversely to the plane of the spider 40 and which also serves to position and frictionally engage the spider 40 at the marginal portion of the housing 37. The spider 40 is seen in FIG. 8 to include in addition to the legs 41, a hub portion 43 from which three spokes 44 radially outwardly extend at equal angular intervals. It is from a lateral side of each spoke 44 that legs 41 are formed. The spaces between spokes 44 dene ports 45 for liquid passage therethrough, together with a central aperture 42 in hub portion 43.

Yielding means, in this case a cylindrically-shaped, spirally-coiled spring 49, is interposed between the spider 40 and the valve element 39 to as to yieldingly urge the valve element 39 into seating engagement with fluid opening 38. An inturned peripheral flange on the circumferential edges of aperture 42 in the hub portion 43 of spider 40 serves to seat one end of the spirally wound spring 49, and the cup-shaped valve element 39 has its apex region dished as at 36 so as to seat the other end of the spring 49. The diameter of the valve element 39 is such that during compression and expansion movements of the spring 49, the legs 41 act as guides for valve element 39. Observe that at the spoke end of each leg 41 an inwardly extending shoulder 83 is formed which acts as a limiting stop for valve element 39 during axial reciprocatory movements thereby controlling the extent of travel by valve element 39. It will be appreciated that this capsule-type check valve 32 is a low inertia, self-aligning, selfseating, and even self-cleaning structure.

The

check valves

32 and 33 are each mounted so as to normally prevent the ow of fluid from

valve chambers

27 and 28 into channel 29 but to pass iiuid from the ends of channel 29 into the valve chamber 27. Thus, fluid entering channel 29 from intake port 11 passes from intake port 11 in opposite directions into channel 29

past check valves

32 and 33. Similarly, the

check valves

34 and 35 positioned at either end of channel 30 are so mounted so as to admit iluid from

valve chambers

27 and 28 into channel 30 Ibut to prevent the flow of liquid from channel 30 into

valve chambers

27 and 28. Thus, liquid entering channel 30 from

valve chamber

27 and 28 flows past

check valves

34 and 35 and through channel 30 in opposite directions until output port 12 is reached from whence the iiuid exits from channel 30. Observe that each

cylinder head

21 and 22 is formed so as to circumferentially overlie each

valve

32, 33, 34 and 35 and thereby hold each spider 40 in place with respect to an associated valve housing 37.

As will be appreciated from the following description,

the crankcase 46 which is formed in the central block 13 is physically isolated from the various channels and chambers described.

Reciprocally movable in each insert sleeve 23 are, respectively, first piston 47 and second piston 48. The construction of each piston 47 and 4S is the same and will now be briey described by reference to second piston 48.

A bolt 50, the end region of whose shank is threaded, has slipped (in this order) over its shank a conically tapered follower 51, an O-ring seal 52, a iirst packing cup 53, a spacer washer 54, a second packing cup 55, and a bottom spacer washer 56. The outturned, oppositely extending, respective outer faces of each packing

cup

53 and 55 slidably engage the inside wall of each sleeve insert 23.

The

pistons

47 and 48 are rigidly connected between opposite ends of a piston rod 58 for common reciprocatory movement of the

respective pistons

47 and 48. The method of connecting the ends of a piston rod 58 to the

pistons

47 and 48 is shown in reference to piston 48. Each end region of piston rod 58 is reduced in crosssectional width and equipped with a threaded bore, thereby permitting each end of piston rod 58 to center and position the various parts of each piston, in .this case 48. The threaded bore then receives the shank of ybolt 50, which, when tightened in the bore, rigidly connects the various piston parts 51-56 to the piston rod 58. The mid-portion of the piston rod 58 operatively connects with the reciprocating mechanism 59 which produces the desired to and fro movement of the piston rod 585, thereby causing the common reciprocatory movement of the

pistons

47 and 48. The sides of the piston rod (adjacent the piston rod mid-portion) slidably engage a piston rod guide or bushing 60 (such as a bronze oilite bearing). On that side of each bushing 60 (paired) facing rovi/ardl the head end of each

piston

47 and 48 is positioned a packing 61 (paired), such as a conventional neoprene seal. Each packing 61 encloses the reciprocating mechanism 59 and seals it in crankcase 46 both from the atmosphere and the fluid being pumped. The bushings 60 serve to center and guide the piston rod 58 during the sliding reciprocating movements.

In each

cylinder portion

16 and 17 is formed three grooves, one extending circumferentially about each capsule-

type check valve

32, 33, 34 and 35, respectively, and one extending circumferentially about each sleeve 23 (see FIG. 5). Each such groove is adapted to receive an O-ring seal, numbered 20 in the case df that seal about

valves

32, 33, 34 and 35, and numbered 19 i'r the case of that seal about each sleeve 23. Each O-

ring

19 and 20 serves a dual function in that each not only serves t5 seal its

respective valve capsule

32, 33, 34 and 35 and sleeve 23 with respect to its associated

cylinder portion

16 and 17 but also serves to seal each cylinder'

portion

16 and 17 with respect to its associated

cylinder head

21 and 22, respectively. These O-

ring seals

19 and 20 eliminate the need for gaskets between

cylinder heads

21 and 22 and block 13. Observe that the cylinder heads 21 and 22 are thus each directly in face-to-face engagement with block 13, a desirable situation where, as in the embodiment, the cylinder heads directly bear upon the rim edge of each

check valve structure

32, 33, 34 and 35 whereby to positively hold such check valves against axial movements.

Power for the reciprocating mechanism 59 is supplied through a drive shaft 63 which is journalled in the bearing housing 14 of the casing of pump 10. A cylindricallyshaped eccentric cam element 64 is mounted on the drive shaft 63 where the latter ends within the crankcase 46. The mid-portion of the piston rod 53 is formed into a yoke 57 adapted to t over the end of the eccentric cam element'64. The portion across the yoke 57 formed in the piston rod 53 is such as to accommodate an outerbearing race 66 and roller bearings 67, the roller bearings 67 and the outer bearing race 66 being positioned over the end of and circumferentially around the ec,-

centric cam element 64. The circumference of the eccentric cam element 64 functions as the inner bearing race for the roller bearings 67. The outer bearing race 66 is contained within the yoke region 57 of the piston rod 5S and bears against the opposed wall portions 62 of the yoke 57 during orbital movements of the reciprocatory mechanism 59 within the crankcase 46. Thus, as the drive shaft 63 rotates and the axis of the eccentric cam element 64 orbits about the axis of drive shaft 63, there is imparted to the piston rod 58 the desired reciprocatory movement.

Since the crankcase 46 is isolated, it can be lled with a lubricant, such as oil (not shown). To provide access to the crankcase 46 in the assembled pump 10, there is provided a till plug 68 which lits into a suitably formed bore which enters the crankcase 46 through the hub 69 formed in the block 13 along the pump axis. Fill plug 68 is also used as the level control for oil in the crankcase. Conveniently, fill plug 68 is equipped with a vent 70 to account for expansion of the oil during pump operation.

In operation, the bushings 60 absorb the side thrust of piston rod 58 and thereby remove substantially all side thrust from the packing 61. These bushings 6) are constantly lubricated, cutting frictional losses to a minimum. Separation of the cylinders from the crankcase 46 eliminates contamination of the reciprocating mechanism 59 by leakage past the packings 61 into the

chambers

27, 28 or, alternately, prevents leakage of fluid

past pistons

53 and 55 into the crankcase 46 (and allows determination of the source of any leakage by direct visual observation). Operating the reciprocating mechanism 59 in an oil bath results in a lubricant cushion between the surfaces of roller bearings 67 and the plane of the yoke formed in the piston rod 5S. The arrangement substantially eliminates fatiguing of the surface of the yoke.

The embodiment shown uses a pair of tapered roller bearings 71 and 72, respectively, which are seated in the bearing housing 14. The use of an oil lled crankcase permits the use of such tapered roller bearings 71 and 72. Such tapered roller bearings have the characteristic of spreading the load centers allowing a maximum load carrying stability with a minimum length of the bearing housing 14. Preferably, the tapered roller bearings 71 and 72 have small annular widths thereby permitting the use of a drive shaft 63 having a large diameter. The oil filled crankcase 46 assures complete and proper lubrication of the parts and requires minimum maintenance attention. The large reserve of lubricant provide for in the embodiment shown extends the time period in which make-up lubricant is required to compensate for normal losses.

The embodiment shown employs means for varying the stroke of the

pistons

47 and 48, Formed in the end of piston rod 53 in crankcase 46 is a first bore 73. This bore 73 is parallel to but spaced from the axis of the drive shaft 63 by a distance equal to but one-half the amount of cam offset required for maximum piston stroke. Similarly, a second bore 74 is formed in the eccentric cam element 64 which is parallel to but spaced from the axis of said cam element 64 by a distance equal to one-half the amount of cam offset required for maximum piston stroke.

Means is positioned both in said rst bore 73 and in said second bore 74 for operatively mounting the cam element 64 on the drive shaft 63 so as to make the axis of the second bore 74 coaxial with the axis of the first bore 73. In this case, such means comprises a cap screw 76. The second bore 74 is so-formed as to receive the head of the cap screw 76. The cap screw 76 uses a Phillips head wrench, and, for this purpose, the head of cap screw 76 is formed with an appropriate Phillips head recess 77. The first bore 73 is threaded to receive the cap screw 76. Thus, the cam element 64 is attached to the drive shaft 63 by cap screw 76 which passes through second bore 74 and into the center of first bore 73 on drive shaft 63.

A dowel pin 77 is positioned in the end ofl eccentric cam element 64 which abuts against the end of drive shaft 63. This dowel pin 77 is adapted to project from the thickened portion of that end of the cam element 64 which is adjacent the drive shaft 63. For this purpose the dowel pin 77 is set in a suitably formed third bore 78 formed in the eccentric cam element 64.

A plurality of pin bores 79 are formed in the drive shaft 63 on the end thereof whereon the eccentric cam element 64 abuts. Each pin bore 79 is radially located from the axis of the bore 73 and parallel thereto. The distance of each pin bore 79 from the axis of rst bore 73 is equal to that from the axis of the second bore 74 in cam element 64 to the location of the third bore 78 in cam element 64 wherein the dowel pin 77 ts. The diameter of the pin bore 79 is such as to receive slidably the dowel pin 77. Thus, there is provided a variety of positions for the eccentric cam element 64 on the end of the drive shaft 63 whereby to provide a predetermined number of xed positions for mounting the cam element 64 on the drive shaft 63. Thus, the cam element 64 can be attached to the drive shaft 63 with the dowel pin 77 engaged in an appropriate pin bore 79 in the drive shaft 63 such that the two offsets add to get a maximum offset between the cam element 64 and the drive shaft 63; or the cam element 64 can be rotated 180 to where the offsets cancel giving a zero offset and a zero stroke. Location of the cam element 64 with the dowel pin 77 engaged in the pin bore 79 intermediate to the positions described yields strokes respectively intermediate to maximum and zero. The number and position of the pin bores 79 on the end of the drive shaft 63 can be predetermined to give any desired number of predetermined strokes and capacities between minimum stroke and capacity and zero.

The operation of the pump 10 is as follows. Fluid enters pump 10 through intake port 11 and flows therefrom in opposite directions through channel 29 through

valves

32 and 33. The suction produced by the reciprocatory motion of the

pistons

47 and 48 draws the liquid being pumped past the

respective valves

32 and 33 into the

respective valve chambers

27 and 28. After passing over the

pistons

47 and 48, the fluid is pushed from the

respective valve chambers

27 and 28 through the

respective valves

34 and 35 into the passage 30, the fluid under pressure entering passage 30 from opposite ends. From the passage 30 the `fluid exits through output port 12 under pressure.

The method of assembling pump 10 can be as follows: Sleeves 23 are inserted into the

cylinder portions

16 and 17 of a block 13. The piston rod 58 is positioned in the block 13 and the bushings 60 and packings 61 inserted. Next, a piston 47 is positioned in a sleeve 23 and the piston rod 58 rigidly secured thereto by means of a bolt 50. Next, the second piston 48 is positioned in the other sleeve 23 and secured to the opposite end of the piston rod 58 by means of the second bolt 5t). Then it is convenient to insert the

valves

32, 33, 34 and 35 into the block 13, mount the

gaskets

19 and 20 in their respective grooves, and finally mount

respective cylinder heads

21 and 22 in place upon the ends of the block 13.

In a separate operation, the tapered roller bearings 71 and 72 are positioned in the bearing housing 14 together with a bushing 36 and a packing 81 for the drive shaft 63. From the interior end of the bearing housing 14, the drive shaft 63 is positioned upon the tapered roller bearing 71 and 72. Observe that the interior end of the drive shaft 63 is flanged outwardly and that the tapered roller bearings 71 rests against the shoulder of the flange. Also note that the reciprocating mechanism 59, including the race 66 and roller bearings 67 are positioned on the eccentric cam element 64 before the bearing housing assembly is mounted against the block 13. As a next operation, the eccentric cam element 64 is positioned on the end of the drive shaft 63 in the manner desired for the particular stroke to be used. Finally, a retaining collar 82 is positioned over the exterior end of drive shaft 63 and secured thereto by means of set screw 83. Note that the drive shaft 63 of pump 10 is provided with a bore 65 for reception of a power shaft (not shown) from a source of power. This power shaft can be rigidly coupled to drive shaft 63 by the set screw 83. Finally, the bearing housing 14 is mounted against the block 13 and secured thereto by means of the four bolts 84 which extend longitudinally through the wall of the bearing housing 14 and screw into sockets provided for them in the block 13. The degree of pressure exerted by drive shaft 63 upon tapered roller bearing 71 is adjusted by means of set screws (three in all) 85.

My invention has been thoroughly tested and found to be completely satisfactory for the accomplishmen-tof the above objects, and while I have shown and described a preferred embodiment, I wish it to be specifically understood that the same is capable of modification without departure from the spirit and scope of the appended claims.

The claims are:

1. Means for varying the stroke of a pair of opposed pistons rigidly interconnected by a piston rod in a piston pump wherein rotary movement of a drive shaft is converted into common reciprocatory movement of said opposed pistons by operatively connecting said piston rod With a cylindrical eccentric cam positioned on the forward end of said shaft, said stroke varying means comprising:

(a) a first bore formed and axially extending said drive shaft,

(b) la second bore axially extending but eccentric to the eccentric element,

(c) a shaft journallling said eccentric element on said drive shaft and extending through the axes of said rst bore and said second bore,

(d) a dowel pin projecting axially outwardly from said eccentric element in the direction of said drive shaft, and

(e) a plurality of dowel pin receiving recesses formed in said drive shafts end and circumferentially spaced about the axis of said rst bore.

2. Means for varying the stroke of a pair of opposed pistons rigidly interconnected by a pist-on rod in a piston pump wherein rotary movement of a drive shaft is converted into common reciprocatory movement of said opposed pistons by operatively connecting said piston rod with a cylindrical eccentric cam positioned `on the forward end of said shaft, said stroke varying means comprising:

in said drive shafts one end `but eccentric to the axis of formed in said eccentric element axis of said (a) a rst bore formed in said drive shafts one end parallel to but spaced from the axis of said drive shaft lby a distance equal to one-half the amount of offset required for maximum piston stroke,

(b) a second bore formed in said eccentricA element which is parallel to but spaced from the axis of said eccentric element by a distance equal to onehalf the amount of offset required for maximum piston stroke,

(c) a shaft j-ournalling said eccentric element on said drive shaft and extending through the axes of said first bore and said second bore,

(d) a dowel pin positioned in and adapted to project from that end of said cam adjacent said shaft in the thickened portion thereof diametrically opposite sa-d second bore, and

(e) a plurality of pin bores defined in that shaft end bearing the rst bore, each pin bore being radially located from the axis of said counter bore by a distance equal to that from the axis of said bore to said dowel pin in said cam, each such pin bore being adapted to receive said dowel pin whereby to provide a predetermined number of xed posi- -tions for mounting said cam on said shaft.

3. The structure defined in claim 2 in which said eccentric element comprises the inner race of a rolling friction bearing including a cylindrical outer race concentric with said eccentric element, and a plurality of circumferentially spaced rolling bearing elements disposed between said eccentric and said outer race, said outer race being disposed generally tangentially between opposed at walls of said piston rod and slidable therebetween, Isaid Iopposed walls being normal to the longitudinal dimension of Vsaid piston rod.

References Cited by the Examiner UNITED STATES PATENTS 272,511 2/83 Angell 74-571 287,697 10/83 Marchand 10S-38 986,123 3/11 Atkinson 74-571 2,257,854 10/41 Peterson 103-38 2,363,119 11/44 CroSSman 10.3-171 2,545,315 3/51 Sproull 103--38 2,771,037 11/56 Johnson 103-171 2,834,223 5/58 Strnad 74-571 2,900,839 8/59 Mackintosh 74-571 LAURENCE V. EFNER, Primary Examiner.

BROUGHTON G. DURHAM, Examiner.

Claims

1. MEANS FOR VARYING THE STROKE OF A PAIR OF OPPOSED PISTONS RIGIDLY INTERCONNECTED BY A PISTON ROD IN A PISTON PUMP WHEREBY ROTARY MOVEMENT OF A DRIVE SHAFT IS CONVERTED INTO COMMON RECIPROCATORY MOVEMENT OF SAID OPPOSED PISTONS BY OPERATIVELY CONNECTING SAID PISTON ROD WITH A CYLINDRICAL ECCENTRIC CAM POSITIONED ON THE FORWARD END OF SAID SHAFT, SAID STROKE VARYING MEANS COMPRISING: (A) A FIRST BORE FORMED IN SAID DRIVE SHAFT''S ONE END AND AXIALLY EXTENDING BUT ECCENTRIC TO THE AXIS OF SAID DRIVE SHAFT, (B) A SECOND BORE FORMED IN SAID ECCENTRIC ELEMENT AXIALLY EXTENDING BUT ECCENTRIC TO THE AXIS OF SAID ECCENTRIC ELEMENT,