Related Applications

[0001] This application claims the benefit of United States provisional patent application serial nos. 60/622,742 for ROTARY PISTON PUMP filed October 28, 2004, the entire disclosure of which is fully incorporated herein by reference.

Background Of The Invention

[0002] Reciprocating piston pumps having a single reciprocating piston, such a shot meters, are well known. The operation of these pumps consists of an intake stroke and a discharge stroke. During the intake stroke, a piston moves within a cylinder bore to allowing fluid to enter the pump. During the discharge stroke, the piston moves in the opposite direction forcing the fluid out of the cylinder. Typically, check valves are used to ensure fluid only enters the cylinder bore during the intake stroke and only exits the cylinder during the discharge stroke. As such, shot meters do not produce a continuous flow of material but rather a pulsed flow because the piston chamber must be refilled after each discharge stroke.

[0003] A shot meter also tends to have volumetric limitations since it can only discharge the amount of fluid that fits within its cylinder bore. As a result, for large volume dispensing operations, a rather large piston chamber and drive mechanism is needed. Due to the large size, the unit must be remotely located from the application site, thus requiring long hoses which can cause supply hose swelling and surge effects. Should the same large system be used for a smaller volume dispensing operation, the shot meter would have a larger than necessary volume of material. Thus, during low volume dispensing operations, residual material will be left in the piston cylinder. As a result, the first material into the chamber is not necessarily the first material out and some material may reside within the piston chamber longer than desired.

[0004] Gear pumps are a form of continuous flow positive displacement pumps that can be used in some shot meter applications. Gear pumps, however, cannot be used with many materials, especially those materials that can be damaged or otherwise compromised by the crushing nature of the gear pump operation. Gear pumps also do not survive highly abrasive

materials and can experience a limited degree of blow-by, thereby making them less appropriate for high precision metering applications.

Summary Of The Invention

[0005] The invention contemplates a pump concept that provides positive displacement pump operation. In one embodiment, the pump is realized in the form of a rotary pump having a chamber arranged in a body and a displacement member disposed in the chamber for reciprocating therein. A drive member is used to move the displacement member within the chamber in response to relative rotation between the body and the drive member. A valve arrangement may be used to time when the chamber is in fluid communication with inlet and outlet ports on the pump.

Brief Description Of The Drawings

[0006] In the accompanying drawing, which are incorporated in and constitute a part of the specification, embodiments of the invention are illustrated, which, together with a general description of the invention given above, and the detailed description given below, serve to exemplify embodiments of the invention.

[0007] Fig. 1 is an cross-sectional view of an exemplary pump in accordance with the invention with the pump components shown in a generalized manner;

[0008] Fig. 2 is an perspective view of an exemplary pump assembly in accordance with the invention in a typical configuration with a drive motor;

[0009] Fig. 3 is a side view of the pump assembly of Fig. 2;

[0010] Fig. 4 is an inlet end or back end view of the pump assembly of Fig. 2;

[0011] Fig. 5 is a cross-sectional view of the pump of Fig. 1 taken along line 5-5 in Fig. 4;

[0012] Fig. 6 is a perspective view of a valve arrangement for the pump of Fig. 1;

[0013] Fig. 7 is a cross-sectional view of the valve arrangement of Fig. 6 taken along line 7-7 in Fig. 6.;

[0014] Fig. 8 is a cross-sectional view of the pump of Fig. 1 taken along line 8-8 in Fig. 4;

[0015] Fig. 9 is a perspective view of a cylinder block for the pump of Fig. 1 ;

[0016] Fig. 10 is a detailed cross-sectional view of a cylinder block for the pump of Fig. 1;

[0017] Fig. 11 is a perspective view of a piston design for the pump of Fig. 1;

[0018] Fig. 12 is a cross-sectional view of the piston design of Fig. 11 taken along line 12-12 in Fig. 11.;

[0019] Fig. 13 is a cross-sectional view of the pump of Fig. 1 taken along line 5-5 in Fig. 4;

[0020] Fig. 14 is a cross-sectional view of the pump of Fig. 1 taken along line 14-14 in Fig. 3;

[0021] Figs. 15 and 16 illustrate an alternative embodiment of a piston for an exemplary pump in accordance with the invention;

[0022] Fig. 17 is a perspective view of an alterative embodiment of a cylinder block for the pump in accordance with the invention;

[0023] Fig. 18 is a perspective view of a retaining ring for the cylinder block of Fig. 17; and

[0024] Fig. 19 is a partial cross-sectional view of the piston, the cylinder block, and the retaining ring of Figs. 15-18 in an installed configuration.

Detailed Description Qf Exemplary Embodiments

[0025] The invention relates generally to positive displacement fluid pumps. More particularly, the invention relates to a rotary, positive displacement pump that provides an alternative to known shot meters. In an exemplary embodiment, the pump includes one or more displacement members, such as for example pistons, that are disposed in a body and driven by a drive member, such as for example a cam. Timing of the intake and discharge cycles of the pump is controlled by a valve arrangement, such as for example a radial spool valve.

[0026] The pump concepts presented in this application may apply to other pump applications besides a shot meter. The pump design, in the exemplary embodiment, provides a true positive displacement, continuous flow, metering pump; thus, the pump is suitable for a wide variety of pump applications. For example, the pump may be used in a variety of applications in the automotive industry to dispense viscous liquids, which may resist flowing or self leveling, such as adhesives, sealants, or caulks, onto a surface. Examples of this type of application include applying a seam sealant along the seam on lap-jointed and spot- welded underbody sections; applying epoxy around the seam at the rim or perimeter of a door member; and applying Urethane adhesive to bond a windshield to the car body.

[0027] While various aspects and concepts of the invention are described and illustrated herein as embodied in combination in the exemplary embodiments, these various aspects and concepts may be realized in many alternative embodiments, either individually or in various combinations and sub-combinations thereof. Unless expressly excluded herein all such combinations and sub-combinations are intended to be within the scope of the present invention. Still further, while various alternative embodiments as to the various aspects and features of the invention, such as alternative materials, structures, configurations, methods, devices, software, hardware, control logic and so on may be described herein, such descriptions are not intended to be a complete or exhaustive list of available alternative embodiments, whether presently known or later developed. Those skilled in the art may readily adopt one or more of the aspects, concepts or features of the invention into additional embodiments within the scope of the present invention even if such embodiments are not expressly disclosed herein. Additionally, even though some features, concepts or aspects of the invention may be described herein as being a preferred arrangement or method, such description is not intended to suggest that such feature is required or necessary unless expressly so stated. Still further, exemplary or representative values and ranges may be included to assist in understanding the present invention however, such values and ranges are not to be construed in a limiting sense and are intended to be critical values or ranges only if so expressly stated.

[0028] Fig. 1 illustrates a simplified, partial cross-sectional view of an exemplary embodiment of a pump in accordance with the invention with parts shown in a generalized manner. The pump 10 includes a chamber or cylinder 12, a displacement member 14, and a drive member 16. In the exemplary embodiment of Fig. 1, the drive member 16 can be

realized as a cam having a cam profile along its inner surface 23 and the displacement member 14 can be realized as a piston disposed in the chamber 12, which can be realized as a piston cylinder. As illustrated in Fig. 1, the pump 10 will typically have a plurality of piston cylinders 12 arranged radially in a body 18, such as for example a cylinder block 18, each cylinder 12 having a respective piston 14 disposed therein. Fig. 1 illustrates the pump 10 having ten cylinders 12, but a specific number of cylinders is not required. Further, the cross- sectional shape of the pistons 14 and piston cylinders 12 may vary. While the pistons 14 and piston cylinders 12 are illustrated in the exemplary embodiments as having a generally round cross-section, other shapes and configurations may be used, such as for example, oval, square, and triangular. The pump components can be made from a wide variety of materials. Examples of acceptable materials include, but are not limited to, aluminum, steel, stainless steel, plastic, cast material, brass, and sintered material.

[0029] The pump 10 is generally, a rotary pump in which there is relative rotation between the cylinder block 18 and the cam 16. In Fig. 1, the cylinder block 18 is shown rotating in a counter-clockwise direction about a central axis 20. The direction of rotation, however, can be reversed. Reversing the direction of rotation may require reversing the cam profile, as will be apparent from the detailed description of the invention hereinafter.

[0030] During relative rotation between the piston cylinder 12 and the cam 16 about the axis 20, the piston 14 reciprocates within the cylinder 12 between a first or inner position and a second or outer position. During the discharge stroke, the piston 14 is moved radially inward, within its piston cylinder 12, by following the profile of the cam 16. During the intake stroke, fluid pressure from fluid entering the cylinder 12 forces the piston 14 radially outward, within its cylinder 12. The generally elliptical profile of the cam surface 23 provides for the movement of the pistons and achieves two complete intake strokes and two complete discharge strokes with 360 degrees of relative rotation between the cam 16 and cylinder block 18.

[0031] The pump 10 includes a valve arrangement 22 for controlling the timing of fluid flow into and out of the cylinders 12. rn the example shown in Fig. 1, the valve arrangement has a pair of inlet openings 24 and a pair of outlet or dispense openings 26, though the number and position of the openings may vary. The valve arrangement 22 and the cam 16 are arranged such that each piston alternates intake and discharge strokes according to the profile of the

cam 16, with the timing controlled by the operation of the valve arrangement 22. Thus, the pump 10 has a timed port concept in which the valve arrangement 22 controls when the piston cylinders 12 are in fluid communication with the inlet openings 24 and the outlet opening 26. As such, the valve arrangement 22 ensures that the inlet and outlet openings 24, 26 are in communication with the proper cylinders 12 at the proper time, and not any improper cylinders. For example, the discharge openings 26 are in communication with cylinders 12 having pistons on the discharge stroke and not in communication with cylinders 12 having pistons on the intake stroke, according to the profile of the cam 16.

[0032] In the example in Fig. 1, the cam 16 is illustrated as a generally annular component centered on axis 20 and having a noncircular, generally elliptical cam profile along its drive surface or inner surface 23. The cylinder block 18 is also illustrated as being a generally annular component disposed radially inward of the cam 16. Further, the valve arrangement 22 is illustrated as being disposed radially inward of the cylinder block 18. The orientation and configuration illustrated in Fig. 1 is exemplary in nature and should not be construed in a limiting sense. A pump according to the present invention may well be realized with alternative orientations or configurations. For example, the profile of the drive or inner surface 23 may be configured in a variety shapes, such as for example, elliptical, oblate, or circular. In addition, the cam may be configured with a profiled drive surface on an outer surface and be positioned radially inward of the cylinder block with a valve arrangement configured radially outward of the block. Also, the cam and valve arrangement may be configured as rotatable and the cylinder block can be stationary. Other configurations and orientations can become apparent to one of ordinary skill in the art upon reading the disclosure herein.

[0033] With reference to Fig. 2, an exemplary configuration of a rotary pump 10 is illustrated in accordance with the invention. The pump 10 may mount on a base 32 via a mounting plate 34 and comiect to a drive mechanism 36, such as for example a motor. The drive mechanism 36 may be any suitable device that generates enough torque to operate the pump 10. The motor 36 may include a variable speed control function or the speed control function can be separately provided. Variable speed operation is not required but in most cases will be used because the output of the pump 10 is a direct function of the rotational speed at which the pump is operating.

[0034] A bracket 38 may mount to the base 32 to support the motor 36. Alternative support configurations, however, may be used for the drive mechanism 36 and pump 10. For example, the motor can mount onto the pump via a C-face mount, as is known in the art.

[0035] The pump 10 has a main housing that includes a hub 40 and a front cover 42 that is assembled to the hub by a series of bolts 44 or other suitable means. The pump has a first or inlet side 46 and a second or outlet side 48. A sensor assembly 50 may be provided for purposes that will be further described hereinafter and may mount to the hub 40 or other convenient location.

[0036] Referring to Fig. 3, a drive shaft assembly 52 can be used to couple the pump 10 to the motor 36 (Fig. 1). The drive shaft assembly 52 includes a drive shaft 54 that extends out of the hub 40 and can be appropriately adapted to connect or operably couple to the drive mechanism 36. A fluid inlet bolt 56 is assembled to the hub 40 on the first or inlet side 46 of the pump 10. A supply hose or line (not shown) may connect to the inlet bolt 56 from a fluid source that is to be pumped (not shown). A cap 58 is assembled on the second or outlet side 48 of the pump 10. The cap 58 includes or communicates with an outlet 60 (Fig. 2) through which the pumped fluid may exit or be discharged from the pump 10. One or more removable plugs 62 are provided in respective bores 64 that extend through the hub 40 wall. Removing the plugs 62 provides access to the pump 10 for draining or adding lubricating oil.

[0037] Fig. 4 illustrates the inlet side 46 of the pump 10 and is provided primarily to show the section lines for Figs. 5, 8 and 13 where Fig. 5 and Fig. 13 are taken along the line 5-5 and Fig. 8 is taken on line 8-8. Fig. 5 and Fig. 13, though taken along the same section line, illustrate the pump 10 with the cylinder block 18 at different rotational positions relative to the valve arrangement 22, as will be described hereinafter.

[0038] The drive shaft assembly 52 connects to the inlet side 46 of the pump 10 via a series of bolts 66. Dowel pins or drive keys 68 may be provided on the drive shaft 54 to impart positive drive from the drive mechanism 36. A series of bolt holes 70 may be provided for mounting the pump 10 on the support frame 32, such as by the vertical mounting plate 34 (Fig. 1). Dowel pins 72 may be provided to assure proper alignment of the pump 10 when mounted on the plate 34.

[0039] With reference to Fig. 5, the drive shaft 54 is rotatable about an axis 74 and journals a roller bearing 76. The drive shaft 54 includes a drive gear 78 that meshes with a rotatably mounted driven gear 80. The driven gear 80 includes two counterbores 82 that each retain a first end of a drive pin 84. Screws 86 or other suitable manner of attachment connect the driven gear 80 to the drive pins 84. The drive pins 84 extend forward, towards the outlet side 48 of the pump 10, and are received in respective bushings 88. The bushings 88 are disposed in through holes 90 in the rotatably mounted cylinder block 18. Snap rings 92 or other suitable means are used to retain the drive pins 84 in the bushings 88.

[0040] The driven gear 80 rotatably mounts on a roller bearing assembly 94 that journals a bearing shaft 96. A thrust bearing 98 is provided between the back face 100 of the driven gear 80 and an inner wall bearing surface 102 of the hub 40. The thrust bearing 98 prevents contact between the driven gear 80 and the hub 40 resulting from axial movement of the gear. The mounting of the gears, bearings, and bearing shaft in the manner described, minimizes axial load on the pump 10. Moreover, by separating the gear drive function from the cylinder block 18, via use of the drive pins 84, radial loads on the cylinder block 18 are avoided.

[0041] The hub 40 forms an oil cavity 104 that holds oil to lubricate pump components, such as for example, the roller bearing assembly 94 and the drive and driven gears 78, 80. Seal elements may be provided within the pump at various locations to seal against loss of oil. For example, seals 106 located at the interface between the cylinder block 18 and a valve arrangement 22 prevent against loss of oil through the valve arrangement 22. hi addition, an end cap 108 retains a seal 110 to prevent loss of oil around the drive shaft 54. The various seals within the pump 10 can be made from a variety of seal materials, such as for example, polyethylene and most other polymers.

[0042] The inlet bolt 56 includes an elongated stem 112 having a threaded end 114 that extends into and mates with a threaded hole 116 in the cap 58. Thus, the inlet bolt 56 and the cap 58 axially hold the cylinder block 18, the bearing shaft 96, the roller bearing assembly 94 and the hub 40 together.

[0043] The inlet bolt 56 also includes a fluid passageway 116 formed in the elongated stem 112. At the inlet side 46 of the pump 10, the passageway 116 opens to an inlet port 118 that can receive a coupling or other connection to the fluid supply. Internal to the pump 10, the

fluid passageway 116 opens to a set of cross-bores 118 formed in the stem 112. The cross- bores 118 open to a common annulus 120 that communicates with the valve arrangement 22.

[0044] Referring to Figs. 6-7, in the exemplary embodiment, the valve arrangement 22 is realized in the form of a radial spool valve. The spool valve 22 is generally cylindrical and includes two diametrically opposed inlet openings or slots 24 and two discharge openings or slots 26. The slots 24, 26 are separated, circumferentially, by lands 121. The spool valve 22 further includes a central opening 122 that receives the ported portion of the inlet stem 112 (Fig. 5). The central opening 122 communicates with the diametrically opposed inlet openings 24 formed in the valve arrangement 22 via a connector bore 124 (Fig. 7). Having the two inlet slots diametrically opposed and the two discharge slots diametrically opposed, minimizes the pressure imbalance across the valve arrangement 22.

[0045] Fig. 8 illustrates the pump 10 with the inlet openings 24 of the valve arrangement 22 in fluid communication with the piston cylinders 12. Thus, fluid may enter the pump 10 via the inlet port 118, travel along the passageway 116, and enter the piston cylinders 12 through the cross bores 118, connector bores 124 and inlet openings 24. Pressure from the fluid entering the cylinder 12 moves the pistons 14, within the cylinder 12, radially outward into engagement with the cam 16.

[0046] As shown in Fig. 8 (as well as in Fig. 5), the cam 16 is formed generally as a plate with an inner cam surface 23 adapted to engage the pistons 14. The cam 16 is axially captured between the front cover 42 and the hub 40. O-rings 125 may be used to form a seal between the cam 16 and the front cover 42 and hub 40.

[0047] Figs. 9 and 10 illustrate the cylinder block 18 of the exemplary configuration of pump 10. The cylinder block 18 is generally cylindrical and includes a plurality of radially extending piston cylinders 12. In the exemplary embodiment, the cylinder block 18 includes ten cylinders 12 spaced equally around the circumference of the block. Each cylinder 12 includes an axially extending shoulder 126 and may also include an alignment mechanism 128. hi the exemplary embodiment of the Figs. 9-10, the alignment mechanism 128 includes a slot extending radially from the shoulder 126 to a radial outer surface 130 of the cylinder block 18. The slot 128 and shoulder 126 interact with the piston 14 (Fig. 8), as will be described hereinafter. The cylinder block 18 further includes a central opening 132 that

slideably receives the valve arrangement 22, such that the cylinder block 18 journals the valve arrangement 22.

[0048] Figs, 11 and 12 illustrate an embodiment of the piston 14. The piston 14 includes a piston body 134, a roller 136, a roller pin 138, an alignment pin 140 and an optional sealing element 142. The piston body 134 is generally cylindrical and includes a first portion 144 and a second portion 146 connected by an axially extending shoulder 148.

[0049] The first portion 144 may include a seal post 149 that extends from the body 134 of the piston 14. The post 149 can be used to retain the sealing element 142 to provide a seal with the cylinder 12 during pump operations. The seal 142 can be made from a wide variety of sealing materials, such as for example, polyethylene and most other polymers.

[0050] The second portion 146 includes two radially extending arms 150 adapted to receive the roller 136 between them. The roller 136 rotatably mounts onto the roller pin 138, which mounts to the arms 150 via bores 152 located in the arms. Other methods of rotatably mounting the roller may be realized according to the invention by one of ordinary skill in the art. The roller 136 provides a low friction engagement with the cam 16. Low friction between the pistons 14 and cam surface 16 reduces power consumption of the pump 10 as well as reducing heat generation and likelihood of the pump seizing up.

[0051] The second portion 146 also includes the alignment pin 140 extending generally perpendicular from the second portion 146. The slot 128 of the cylinder block 18 receives the pin 140 to prevent rotation of the piston 14 within the cylinder 12 during operation and ensure proper orientation of the pistons during installation.

[0052] Fig. 13 illustrates a cross-sectional view of the pump 10 with the outlet openings 26 of the valve arrangement 22 in fluid communication with piston cylinders 12. The pistons 14 are disposed in the cylinders 12 with the roller 136 engaging the cam 16. The alignment pin 140 on the piston 14 is shown disposed in slot 128 on the cylinder block 18.

[0053] As a result of the relative rotation between the cylinder block 18 and the cam 16, the cam 16 moves the piston 14 radially inward. The shoulder 126 on the block 18 provides a positive stop for the shoulder 148 on the piston 14 to ensure the post 149 on the piston 14

does not contact the valve arrangement 22. As the piston 14 moves radially inward, fluid in the cylinder 12 is discharged into the outlet opening 26 in the valve arrangement 22.

[0054] The outlet opening 26 opens to a pair of outlet passageways 154 formed in the cap 58. The outlet passageways 154 communicate with the outlet 60 via cross-bores 156 allowing fluid to be discharged from the pump 10. Thus, the two dispense slots 26 discharge into a common outlet 60. If desired, each dispense slot 26 maybe in fluid communication with its own outlet so that the pump can feed two dispensing systems. Fluid pressure in the two outlet lines, however, may need to be kept equal in some applications in order to avoid radial loads on the spool valve 22.

[0055] Fig. 14 is a cross-sectional view taken along line 14-14 of Fig. 3 (and Fig. 5). hi the exemplary pump of Fig. 14, the drive and driven gears 78, 80 rotate the cylinder block 18, relative to the cam 16 and valve arrangement 22, by virtue of the driven gear's coupling therewith via the drive pins 84. In the view of Fig. 14, the cylinder block 18 rotates in a clockwise direction. Rotation of the cylinder block 18 causes each of the plurality of pistons 14 (in the exemplary embodiment there are ten pistons) to move radially within its piston cylinders 12 in accordance with the radial profile of the cam 16. The pistons 14 are forced radially outward (intake stroke), within the cylinder 12, under the influence of the inlet pressure of the pumped liquid, and are forced radially inward (discharge stroke), within the cylinder 12, by the profile of the cam 16.

[0056] During the intake stroke, the fluid travels from the passageway 116, through the cross bores 118 and into the inlet slots 24. Four cross-bores 118 are provided to assure free flow from the fluid passageway 116 into the inlet slots 24. This eliminates alignment issues between the bores 118 and the slots 24 when the stem 112 is screwed into the hole 116 (see Fig. 5). Fluid pressure from the inlet slots 24 pushes the pistons 14 radially outward such that the roller 136 follows the profile of the cam surface 28 during rotation of the cylinder block 18. The cam 16 is profiled to produce a desired discharge stroke of the pistons 14. During the discharge stroke, the pistons 14 displace liquid from their respective cylinders 12 into the dispense slots 26 which are in fluid communication with the outlet port 60 (Fig. 13). Each piston 14 thus alternates intake and discharge strokes according to the profile of the cam 16, with the timing controlled by the operation of the valve arrangement 22.

[0057] The piston 14, at the end of the discharge stroke, is positioned substantially at the radially innermost edge of the cylinder 12. Thus, substantially all of the fluid is discharged from the piston cylinder 12 after the completion of a discharge stroke. In this way, the pump 10 generally achieves a first-in-first-out (FIFO) operation since little or no fluid in the cylinder will carry over to from the discharge stroke to the next intake stroke.

[0058] The spool valve 22 thus controls the inlet and discharge timing of fluid flow into and from the piston cylinders 12 without the use of check valves. The cam 16 controls the speed and timing of the intake and discharge strokes of the pistons 14 as the cylinder block 18 rotates. The cam 16 is matched to the geometry of the spool valve 22 so that the inlet slots 24 are open to the cylinders 12 during the intake stroke portions of the cam profile and the dispense slots 26 are open to the cylinders during the dispense or discharge portions of the cam profile. Thus, the pump has a timed port concept.

[0059] The spool valve 22 serves to completely isolate the inlet and outlet flow paths during operation of the pump 10. The lands 121 of the spool valve 22 are wider than the width of each cylinder 14. Thus, each cylinder 14 will not be exposed to both the inlet and outlet openings 24, 26 at the same time.

[0060] In this manner the pump 10 operates as a true positive displacement pump in which the dispense flow is independent of the inlet pressure and is thus a function of the speed with which the pistons 14 move during the discharge stroke. The speed of the pistons 14 during the discharge stroke is determined by the selected profile of the cam 16 and the speed that the cylinder block 18 is rotated by the drive mechanism 36. Accordingly, very precise flow rates can be achieved even at very low flow rates.

[0061] Fig. 14 illustrates that the cam profile may include a number of different portions and functions. A first portion 160 of the cam 16 is the portion closest to the spool valve 22 and thus, corresponds to the ending portion of the discharge strokes. A second portion 162 is characterized by a steep angled surface profile (meaning the radius of the cam surface 28 increases significantly per unit arc length). This is provided so that the pistons 14 can rapidly move radially outward during the intake stroke. A third portion 164 is characterized by a somewhat shallower angled cam surface profile. This provides a somewhat slower movement of the piston during discharge than during the intake stroke. Furthermore, this provides the length necessary to achieve a controlled overlap of the acceleration and

deceleration of the pistons, as will be described hereinafter. The rate of change of the radius of the cam profile for the discharge stroke can be designed so that the discharge rate of fluid from the piston cylinders is or nearly constant. This is an optional feature for applications needing smooth flow of liquid from the pump. Still a fourth portion 166 of the cam profile can be used between the various intake and discharge stroke portions and is characterized by constant radius of the cam surface (appearing "flat" in cross-section). These portions can be understood as dwell times when the pistons are stationary.

[0062] As shown in Figs. 7 and 14, the outlet or dispense slots 26 are larger that the inlet slots 24. In other words, the dispense slots 26 subtend an arc that is greater than the arc of the inlet slots 24. hi this example, the inlet slots 24 are dimensioned so that during an intake stroke, at most, two piston cylinders 12 are open to each inlet slot 24; thus, at most, a total of four cylinders are filling at a time during the intake stroke portions of the cam profile. The dispense or discharge slots 26, however, are large enough so that three cylinders can rotate into fluid communication with each dispense slot 26 during the discharge portions of the cam profile. The profile of the cam 16, however, balances the piston speeds such that the pump 10 can dispense fluid volume equivalent to two piston cylinders into each dispense slot for each discharge stroke, for a total volume of four cylinders.

[0063] In particular, in reference to pistons A-C in Fig. 14, the two outboard pistons A and C work in a complementary manner according to the cam profile, meaning that as one of the outboard pistons (for example piston C in Fig. 14) is decelerating {i.e. completing its discharge stroke), the other outboard piston A is accelerating {i.e. beginning its discharge stroke). The rate of deceleration of piston C and acceleration of piston A are balanced such that they cumulatively provide the same piston speed and discharge rate as the middle piston B. The middle piston B at this time is generally in the center alignment with the discharge slot 26. The profile of the cam 16 is such that during the majority of the discharge stroke, the pistons 12 are moving at a constant speed. Thus, the cam profile provides generally constant piston speed during most of the discharge stroke and overlaps acceleration and deceleration of the pistons to keep the discharge rate from the pump generally constant when the rotational speed of the pump is generally constant.

[0064] The exemplary embodiment in Fig. 14 achieves a continuous flow of liquid from the pump because the cam profile assures that the equivalent of four cylinders are in a dispense

stroke sequence at any given time. Thus, liquid is constantly being pumped out. Further, the pump does not create a pulsed flow because the cumulative velocity of the dispensing cylinders is generally constant. If a pulsed flow is desired, however, the cam profile can be modified so that the pistons discharge without overlapping operation.

[0065] The pistons 14 can be designed with a close fit within the cylinders 12 to prevent oil seepage into the discharge slots 26. In addition, the lands 121 of the valve arrangement 22 can have a close fit with the interior surface of the opening 132 in the cylinder head 18 so as to prevent or minimize cross-over of fluid from an inlet slot 24 to a discharge slot 26. Due to the close machining tolerances and clearances between moving metal parts, it is expected that oil lubrication alone may not be enough to reduce the coefficient of sliding friction between closely spaced metal parts, such as for example between the pistons 14 and cylinders 12 and between the spool valve 22 and cylinder block 18. Therefore, the surfaces that are exposed to potentially high frictional contact with other surfaces may be treated as required to reduce the coefficient of friction. For example, a solid surface treatment such as, for example, Amorphous Diamond Like Coating (ADLC) may be used. This process involves the application of a coating to the surface by a plasma assisted chemical vapor deposition process and is known to those skilled in the art and is a commercially available process. Other processes or coatings may be used as required, such as for example a MOST™ process available from Ion Bond. Some pump designs and applications, however, may be able to rely on oil lubrication alone.

[0066] With reference to Fig. 14, the sensor assembly 50 is realized in the form of a proximity sensor 170, such as for example, an inductive proximity sensor which are well known to those skilled in the art. The sensor 170 can install into a hole 172 formed in the cam 16. The hole 172 extends all the way through the cam 16 so that the sensor end 174 is positioned adjacent or generally flush with the inner surface 28 of the cam 16 so that the sensor can detect the presence of the pistons 14, as will be described. The sensor 170 can be electronically coupled, by signal wires for examples, to a sensor mounting arrangement 176 that can mount to the outer perimeter of the hub 40. The sensor arrangement 50 may include an electrical connector 178 (Figs. 2 and 3) to allow the sensor 170 output signal to be coupled to a circuit for analysis.

[0067] The sensor 170 can be used to detect that each piston 14 folly radially extends towards the cam 16 during the intake stroke. When folly extended, the proximity sensor 170 detects the outer distal end of each piston 14 as it rotates past the sensor 170. The sensor 170 signal (typically "counts") can then be compared to the rotational speed of the pump 10, measured by conventional means, such as for example, a tachometer (not shown) or other speed indicator, to detect if any pistons 14 are not functioning properly. Missed "counts" can indicate, for example, that the inlet pressure is insufficient to fill the cylinders 12 or that there is a leak or other anomaly within the pump 10. Alternatively, an inlet pressure sensor (not shown) may be used in combination with the sensor 170 to provide an inlet fluid pressure measurement. Proper pump operation can be confirmed when the "counts" of the sensor 170 are consistent with the pump rotational speed and inlet pressure is verified to be sufficient.

[0068] Figs. 15 and 16 illustrate another embodiment of a piston for an exemplary pump according to the present invention. The piston 14' is generally cylindrical and includes a first end portion 200 and a second end portion 202. The first end portion 200 is substantially similar to the first portion 144 of piston 14 of Figs. 11-12 except that the first portion 200 does not include the optional seal post 149 and sealing element 142 of piston 14. The piston 14', however, maybe configured to include a sealing element if desired.

[0069] The second end portion 202 includes an axially extending slot 204 that forms an alignment lip 206. The alignment lip 206 forms part of an alignment mechanism 207 that will be described hereinafter. The piston 14' also includes a curved "follower" surface 208. This surface 208 is selected to provide a low friction contact with the cam 16, preferably although not necessarily a line contact.

[0070] Fig. 17 illustrates another embodiment of a cylinder block for an exemplary pump according to the present invention. In this embodiment, the cylinder block 18' has the same basic design and features as were described above for the cylinder block 18 of Figs. 9-10. Namely, the cylinder block 18' is generally cylindrical and includes a plurality of circumferentially spaced piston cylinders 200 and a central opening 212 for receiving the valve arrangement 22 (not shown).

[0071] In this example, however, the piston cylinders 200 do not include a shoulder similar to the shoulder 126 of the cylinders 12 of Fig. 10. Instead, the piston cylinders 200 in the cylinder block 18' are substantially straight to match the contour to the pistons 14'. Further,

the cylinder block 18' also includes a step 214 along its outer surface 216. The step 214 forms a notch 218 in each cylinder 200 and an axially extending shoulder 220 on the outer surface 216.

[0072] When each piston 14' is properly inserted into its cylinder 200, the lip 206 must align with the notch 218. A piston retaining ring 222 (Figs. 18) is press fit installed on the shoulder 220 adjacent the notched portion 218 of the cylinders 200. The shoulder 220 is formed into a portion of the cylinders 12 so that when the ring 222 is installed, an inner peripheral portion 224 extends into the piston slots 204 (see Fig. 19). In this manner the ring 222 prevents the pistons 14' from falling out of the cylinder block 18' during assembly of the pump. The ring 222 may include a series of notches 226, each of which aligns with a respective cylinder 200 to permit the flow of oil within the cylinder 200.

[0073] The alignment mechanism 207 of this exemplary embodiment, therefore, can include the piston slot 204, the cylinder notch 218, and the retaining ring 222. This arrangement assures proper alignment of the pistons 14' with the cam 16 during pump operation.

[0074] The invention has been described with reference to the preferred embodiments. Modification and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.