SE541164C2 - Pump with integrated overpressure valve - Google Patents

Abstract

A motor-driven air pump assembly is provided with an integral overpressure release path (569) formed in, or between, contacting faces of the top seal (524) and the valve plate (523).

Description

Pump with integrated overpressure valve Background of the Invention The present invention relates to an air pump assembly, in particular a motor-driven pump assembly for mounting in, or in proximity to, a vehicle seat. Such motor-driven pump assemblies allow the occupant of the seat to adjust the shape and feel of the seat or to provide massage functions or to provide forced air ventilation to the seat. For example, in order to be able to add air to and remove air from an inflatable bladder in the seat, the seat is normally provided with a pump module which includes a motor-driven pump unit pump connected by an airline to one or more valves. The valves are electrically operated and connected to switches which can be operated by the seat occupant to open and close the valve(s) and to actuate the motor of the motor-driven pump. The motor is operated to drive the pump to provide air via an airline to the bladder and a solenoid valve in the airline ensures that the air is retained in the bladder when the pump is switched off. Air can be released from the bladder by opening the solenoid valve.

The bladders need to be protected from excess pressure which might otherwise cause them to burst. This has been achieved in the past by providing a separate over-pressure release valve in the system. The use of such an extra component increases manufacturing costs.

Brief Description of the Invention It is an object of the present invention to provide an air pump suitable for use in a motordriven pump assembly which overcomes the problems of prior art air pumps.

In one embodiment of the invention this is achieved by providing an air pump with an integral over-pressure release valve.

In an embodiment of the invention the integral over-pressure release valve is provided with external biasing means to keep it closed.

In a further embodiment of the invention the integral over-pressure release valve is provided with internal biasing means to keep it closed.

Brief Description of the Drawing Figure 1 shows schematically a lateral view of a typical prior art motor-drive pump assembly.

Figure 2 shows schematically a lateral view of a motor-driven pump assembly according to one embodiment of the invention.

Figure 3 shows schematically a lateral centerline section of the portion enclosed in a dashed square of the valve plate and valve actuator of the pump assembly of figure 2 with the solenoid valve closed.

Figure 4 shows schematically a lateral centerline section of the portion enclosed in a dashed square of the valve plate and valve actuator of the pump assembly of figure 2 showing an embodiment of a solenoid valve with the solenoid valve open.

Figure 5 shows an perspective angled section though a part of a motor-driven pump assembly comprising a solenoid valve according to a second embodiment of the invention.

Detailed Description of the Invention A prior art motor-driven pump assembly 1 for providing air to a vehicle seat is shown schematically in figure 1. The assembly comprises an electric motor 3 which drives a swash plate air pump 5. Swash plate air pumps are well-known in the art and will only be described briefly in the following. Certain types of swash plate pumps include a stationary piston block 7 containing a number of parallel pistons 9, each arranged to slide up and down a respective cylinder 11. The cylinders are spaced radially around, and with their longitudinal axes parallel to, the central longitudinal axis of the block. The proximal end 13 of each piston is forced against a rotating swash plate 15 by springs (not shown). The upper surface of the swash plate is formed as a cam and as it rotates it causes the pistons to reciprocate at a fixed stroke. The interior of the piston block is partly hollow and is open to the air via an inlet spigot 17. The pistons each draw in air into their respective cylinders via respective one-way (also known as “check” or “non-return” valves) inlet valves 19 from the interior of the piston block during half of a revolution of the swash plate and drive fluid out of their respective cylinders during the other half of the revolution via respective one-way outlet valves 21 at the rear of the pump. The output air is collected in an outlet manifold 23 in fluid connection to an outlet spigot 25. The spigot is connected by a flexible tube 27 to a valve block 29. Valve block 29 contains one or more electrically operated valves 31 and is connectable by a further flexible tube 33 to one or more components of the seat (not shown) such as air bladders. The one or more valves are able to control the flow of air to and from the one or more seat components when operated by a user. In order to, for example, inflate a seat bladder the operator actuates the pump motor and this causes compressed air to be generated in the individual cylinders as the swash plate rotates. This compressed air passes though each individual cylinder check valve into the outlet manifold and then is fed to the valve block. The one or more valves in the valve block are set in the appropriate position to allow the compressed to reach the desired seat bladder which then expands and becomes firmer as the compressed air enters it. Once the desired level of firmness is achieved the motor is stopped and no further air is supplied to the seat bladder. If it is desired to decrease the firmness of the seat bladder the operator actuates a valve in the valve block which valve allows air from the seat bladder to escape to atmosphere. The valve is a solenoid valve which produces a clicking sound which originates from the movement of the valve plunger whenever the solenoid valve is activated and deactivated. In particular noise is generated by contact of the valve plunger with other components of the solenoid valve.

Figure 2 shows schematically a lateral view of a motor-driven pump assembly 101 in accordance with a first embodiment of the present invention. Enlarged sectional views through the centerline of the valve plate and valve actuator of the assembly are shown in figures 3 and 4. In figure 3 the solenoid valve is closed and in figure 4 the solenoid valve is open. This embodiment of the invention is illustrated by the use of a swash plate pump 105 driven by an electric motor 103 but other types of motor-driven pump are also feasible, for example a diaphragm pump. This example of a pump include a stationary piston block 107 containing a number of parallel pistons 109, each arranged to slide up and down a respective cylinder 111. The cylinders are spaced radially around, and with their longitudinal axes parallel to, the central longitudinal axis of the block. The proximal end 113 of each piston is forced against a rotating swash plate 115 by springs (not shown). The interior of the piston block is partly hollow and is open to the air via an inlet spigot 117. The pistons each draw in air from the interior of the piston block into their respective cylinders via respective one-way inlet valves 119 during half of a revolution and drive fluid out of their respective cylinders during the other half of the revolution though respective outlet ports 121. The ports are covered by a valve plate 123 in which valve components, described below, are formed or positioned. Valve plate 123 is preferably formed of flexible material, such as rubber which is vibration absorbing and thus reduces the amount of sound produced when the valves open and close during operation of the device. The side of the valve plate which does not face the outlet port is covered by a top seal 124, which forms an air-tight cover over the valve plate. Top seal 124 is preferably made of a flexible and resilient material such as rubber. A manifold chamber 125 is formed in or between the facing faces of the valve plate 123 and the top seal 124. The manifold chamber 125 leads to an outlet spigot 129. The valve plate preferably is provided with an individual outlet check valve 127 for each cylinder. These outlet check valves are one-way valves which allow pressurized air to leave each cylinder and enter the manifold chamber but prevent pressurized air in the manifold chamber from entering the cylinders, thereby retaining the pressurized air in the manifold when the pump is not being operated. Preferably these check valves are integrally formed in the valve plate, for example, as flaps which can open into the manifold chamber. Outlet spigot 129 is connectable to a flexible tube 131 which is connectable to one or more components of the seat or other unit (not shown) such as air bladders or ventilation holes. In order to allow the pressurized air in the seat component to be released when desired the top seal is provided with an electrically-operated pressure release valve 133 which is in fluid connection with the manifold chamber via a sealable pressure release port 135 which extends through the top seal 124. Operating the pressure release valve opens the pressure release port and allows pressurized air in the manifold chamber to be release to atmosphere. The pressure release valve is a solenoidactuated valve with a metal valve plunger 137 movable in a bore 141 placed along the longitudinal central axis of a solenoid 138. Preferably solenoid 138 is mounted on the opposite side of the top seal 124 from the valve plate 123. The construction of the solenoid valve, top seal and outlet manifold are shown enlarged schematically in figures 3 and 4 and described below.

Valve plunger 137 comprises an elongated shaft 139 mounted in the central bore 141 in the body 142 of the solenoid. The bore is provided with a stop 140 which limits the movement of the valve plunger - this can be formed as a closure or bridge piece which extends or projects across the distal end of the bore, i.e. the end furthest from the top seal. The bore is open at the opposite, proximal end which is arranged coaxially with the pressure release port. The valve plunger is longitudinally movable in the central bore 141 under the influence of the magnetic field which can be generated in the conventional manner by the coil 142 of the solenoid. The valve plunger is spring-biased by resilient means 143 such as a helical spring into contact with pressure release port 135. One end of resilient means 143 pushes against a reaction shoulder 144 formed on the outside of elongated shaft 139 while the other end is located on a reaction shoulder 146 formed in the wall of central bore 141. The proximal end 145 of the valve plunger is provided with a seal tip 147 made of resilient material such as rubber and is able to form a seal around pressure release port 135 when in spring-biased contact with the top seal. Movement of the elongated shaft and the seal tip away from the pressure release port is generated by applying electrical power to the solenoid. This generates a magnetic field which overcomes the force of the resilient means 143 and pulls the valve plunger away from the pressure release port, which takes the seal tip out of contact with the pressure release port thereby breaking the seal of the pressure release port and allowing pressurized air to escape to atmosphere from the outlet manifold. This causes the pressure to drop in any component in fluid communication with the outlet manifold. When the power to the solenoid is interrupted the resilient means 143 returns the seal tip 147 into sealing contact with the pressure release port. The use of a seal tip and/or a top seal of resilient material prevent noise occurring when the seal tip comes into contact with the top seal.

The valve plunger is retained in the central bore by appropriate means such as an appropriately shaped retaining ring 149 mounted at the open end 151 of the central bore 141. Preferably the elongated shaft of the valve plunger comprises a central bore 153, which may be a through bore, into which the seal tip 147 can be mounted. Valve plunger centralizing means may be additionally provided to centralize the valve plunger. An example of valve plunger centralizing means is as follows: the closure or bridge at the distal end 155 of central bore of the solenoid comprises a longitudinally extending guide pin 157 which is coaxial with, and a sliding fit inside, the central bore 153 at the distal end 159 of the elongated shaft of the valve plunger. This guide pin extends into the central bore of the valve plunger even when the valve plunger is in contact with the pressure release port and ensures that the distal end of the valve plunger is centralized in the central bore of the solenoid. The guide pin helps to prevent binding of the valve plunger in the central bore (which might otherwise occur if the valve plunger leans to one side) and also prevents the distal end of the valve plunger from wobbling and coming into contact with the sides of the central bore of the solenoid, thus preventing impact noise from being generated.

Preferably the elongated shaft of the valve plunger and the central bore are provided with a valve plunger motion damping means with cooperates with both of these components to dampen and stop the movement of the plunger as it reaches the end of its range of travel when the valve is opened. This valve plunger motion damping means can consist of a circumferential retaining groove 160 on the valve plunger for receiving a resilient ring, a resilient ring 161 and a surface 162 on the inside of the central bore which the resilient ring can contact near the end of the range of travel. As shown in figures 3 and 4 resilient ring preferably has an X-shaped cross-section - and is thus a so-called “X-ring”. Resilient ring has a nominal outer diameter of D mm. Preferably the proximal end of central bore is provided with a surface 162 in the form of a longitudinally extending damping ramp 162’ which ends in a damping ledge 162 ”which extends into the bore and has a minimum diameter d which is less than diameter D, thus providing an obstacle to movement of the X-ring. The damping ledge is positioned in the longitudinal direction of the bore at a distance from the proximal end which ensures that the X-ring is not in contact with it when the solenoid is inactivated but comes into contact with it before the valve plunger reaches the end of its travel when the solenoid is activated as shown in figure 4. The use of an X-ring give progressive damping as the legs of the X-ring which contact the ramp and the ledge can progressively bend in the direction opposite the direction of travel of the valve plunger once it contacts the damping ledge and progressively slow down and stop the movement of the plunger. The position and dimensions of the groove of the plunger, the dimensions of the X-ring and the position of the damping ledge in the central bore, or their equivalents are adapted so that together they form an arrangement for limiting the distance that the valve plunger can move up the bore so that the distal end of the valve plunger is prevented from contacting the closing or bridging piece at the distal end of the central bore - thus preventing contact noise from occurring at the end of the travel of the valve plunger towards the distal end of the bore. The legs of the resilient X-ring provide radial damping of the movement of the valve plunger The damping means also prevent metal-to -metal contact from occurring between the metal valve plunger and the wall of the central bore by helping to centralize the proximal end of the valve plunger in the bore, thereby preventing metal-to -metal contact noise occurring during actuation of the valve. The use of an X-ring is preferred as it can provide more progressive axial and radial damping than a circular-cross-section or square-cross-section ring when used in a cylindrical bore. Other damping means which cooperate with the valve plunger and the central bore are also conceivable, for example the X-ring could be mounted in the central bore and a damping ramp and/or ledge formed on the valve plunger, or an O-ring could be provided on one component and a sloping damping ledge on the other component.

The solenoid 139 comprises a bobbin 163 connected to electrical supply wires (not shown). The bobbin is mounted on a U-shaped metal frame 165 which mounted on its side with one of its arms 167 in proximity to the distal face 173 of the top seal 124.

Preferably an overpressure release path 169 is formed in or between the top seal and the outlet manifold. As an illustrative example, this overpressure release path could be in the form of a normally closed air channel 169 which extends from the manifold chamber towards the proximal face 171 of the top seal. The distal face 173 of the top seal 124 opposite the air channel can be resiliently biased towards the air channel to keep it closed by biasing means 175, for example, a compression spring extending from a locating projection 177 formed on the distal face of the top seal to a reaction arm 179 projecting laterally from the longitudinally extending portion 181 of the U-shaped metal frame 165. The compression force of the biasing means and any internal resilient force provided by the top seal can be adapted such that in normal use the air channel is closed, but in the event of an overpressure in the outlet manifold the force exerted by the pressurized air in the air channel overcomes the forces holding the top seal in contact with the valve plate at that position, thereby deforming the resilient top seal and allowing a gap to form between the valve plate and top seal and the over-pressurized air to escape - either directly to atmosphere or, as shown here, first to the interior of the housing 183 and then to atmosphere via a though hole 185 in the housing wall.

Figure 5 shows a section through the valve plate, valve actuator and the cylinders of the wobble pump of a second embodiment of a motor-driven pump assembly according to the invention.

This embodiment of a motor-driven pump assembly 501 comprises a wobble pump 505 which comprises a cylinder block 508 made of resilient material. The block comprises a plurality of cylinders 510 with flexible side walls 509. The flexible base 511 of each cylinder 510 is attached to a wobble plate 515 which as it is rotated in a circle by a motor (not shown) pulls down on the base of each cylinder, thereby filling the cylinder with air and then pushes up on the base of each cylinder. The base of each cylinder acts as a plunger, thereby driving the air out of the cylinder though respective outlet ports 521. The ports are covered by a valve plate 523 in which valve components, described below, are formed or positioned. Valve plate 523 is preferably formed of flexible material, such as rubber which is vibration absorbing and thus reduces the amount of sound produced when the valves open and close during operation of the device. The side of the valve plate which does not face the outlet port is covered by a top seal 524, preferably made of a flexible and resilient material such as rubber, which forms an air-tight cover over the valve plate. A manifold chamber 525 is formed in or between the facing faces of the valve plate 523 and the top seal 524. The manifold chamber 525 leads to an outlet spigot 529. The valve plate preferably is provided with a single check valve, shown here as an umbrella valve 527. This check valve allows pressurized air to leave the cylinders and enter the manifold but prevents pressurized air in the manifold from re-entering the cylinders, thereby retaining the pressurized air in the manifold when the pump is not being operated. Outlet spigot 529 is connectable to a flexible tube 530 which is connectable to one or more components of the seat or other unit (not shown) such as air bladders or ventilation holes. In order to allow the pressurized air in the seat component to be released the top seal is provided with an electrically-operated valve 533 which is in fluid connection with the outlet manifold via a pressure release port 535 which extends through the top seal. The valve is a solenoid -actuated valve with a metal valve plunger 537 movable along the longitudinal central axis of a solenoid 538. Preferably solenoid 538 is mounted on the opposite side of the top seal 524 from the valve plate 523. The construction of the solenoid valve, top seal and outlet manifold are shown schematically in figure 5.

Valve plunger 537 comprises an elongated shaft 539 mounted in a central bore 541 in the body 542 of the solenoid. The bore is provided with a stop 540 which limits the movement of the valve plunger - this can be formed as a closure or bridging piece which extends across the distal end of the central bore, i.e. the end furthest from the top seal. The bore is open at the opposite, proximal end which is arranged coaxially with the pressure release port. The valve plunger is longitudinally movable in the central bore 541 under the influence of the magnetic field which can be generated in the conventional manner by the coil 542 of the solenoid. The valve plunger is spring biased by resilient means 543 such as a helical spring towards pressure release port 535. One end of resilient means 543 pushes against a reaction shoulder 544 formed on the outside of the valve plunger while the other end is located on a reaction shoulder 546 formed in the wall of central bore 541. The proximal end 545 of the valve plunger is provided with a seal tip 547 made of resilient material and is able to form a seal around pressure release port 535 when in spring-biased contact with the top seal 524.

Movement of the elongated shaft and the seal tip away from the pressure release port is generated by applying electrical power to the solenoid as described above in connection with the first embodiment of the invention. When the power to the solenoid is interrupted the resilient means 543 returns the seal tip into sealing contact with the top seal 524. The use of a seal tip and/or a top seal of resilient material prevent noise occurring when the seal tip comes into contact with the top seal.

The valve plunger is retained in the central bore, for example, by an appropriately shaped retaining ring 549 mounted at the open end 551 of the central bore.

The elongated shaft of the valve plunger comprises a central bore 553 which may be a through bore, into which the seal tip 547 can be mounted. Valve plunger centralizing means may be additionally provided to centralize the valve plunger. An example of valve plunger centralizing means is as follows: the closure or bridging piece at the distal end 555 of central bore of the solenoid comprises a longitudinally extending guide pin 557 which is coaxial with, and a sliding fit inside, the central bore 553 at the distal end 559 of the elongated shaft of the valve plunger. This guide pin extends into the central bore of the valve plunger even when the valve plunger is in contact with the pressure release port and ensures that the distal end of the valve plunger is centralized in the central bore of the solenoid. The guide pin helps to prevent binding of the valve plunger in the central bore (which might otherwise occur if the valve plunger leans to one side) and also prevents the distal end of the valve plunger from wobbling and coming into contact with the sides of the central bore of the solenoid, thus preventing impact noise from being generated.

Preferably the elongated shaft of the valve plunger and the central bore are provided with valve plunger motion damping means with cooperates with both of these components to dampen and stop the movement of the plunger as it reaches the end of its range of travel when the valve is opened. This damping means can be in the form of a retaining groove 560 on the valve plunger for a resilient ring, a resilient ring 561 and a surface 562 which it can contact near the end of the range of travel. As shown in figure 5 resilient ring preferably has an X-shaped cross-section. Resilient ring has a nominal outer diameter of D mm. Preferably the proximal end of the central bore is provided with a damping surface in the form of damping ledge562 which extends into the bore and has a diameter d which is less than diameter D. The damping ledge is positioned in the longitudinal direction of the bore at a distance from the proximal end of the bore which ensures that the X-ring is not in contact with it when the solenoid is inactivated but comes into contact with it before the valve plunger reaches the end of its travel when the solenoid is activated and moves the plunger up from the position shown in figure 5. The use of an X-ring give progressive damping as the legs of the X-ring which contacts the damping shoulder can progressively bend in the direction opposite the direction of travel of the valve plunger once it contacts the damping shoulder and progressively brake the movement of the plunger. Preferably the diameter Db of the central bore below the damping shoulder is greater than the diameter of the X-ring so that when the valve is closed the legs of the X-ring are not in contact with the walls of the central bore. This minimizes friction between the valve plunger and the wall of the central bore when the valve plunger initially moves from the closed position. The position and dimensions of the groove of the plunger, the dimensions of the X-ring and the position of the damping ledge in the central bore, and/or their equivalents, are adapted so that together they form an arrangement for limiting the distance that the plunger can move up the bore so that the distal end of the plunger is prevented from contacting the surface of the distal end of the central bore - this preventing noise from occurring at the end of the travel of the plunger towards the distal end of the bore. The legs of the resilient X-ring are able to provide radial damping of the movement of the valve plunger and also prevent metal-to-metal contact of the plunger with the central bore if the valve plunger should wobble from side to side as it ascends and descends. This eliminates the noise that this metal-to-metal contact would otherwise produce. Other progressive damping means are also conceivable, for example the X-ring could be mounted in the blind central bore and the damping ledge formed on the valve plunger, or an O-ring could be provided on one component and a sloping damping ledge on the other component.

The solenoid comprises a bobbin 563 connected to electrical supply wires (not shown). The bobbin is mounted on a U-shaped metal frame 565 which mounted on its side with one of its arms 567 in proximity to the distal face 573 of the top seal 524.

An overpressure release path 569 is formed in or between the top seal and the outlet manifold. As an illustrative example, this overpressure release path could be in the form of a normally closed air channel 569 which extends from the manifold chamber towards the proximal face 571 of the top seal. The distal face 573 of the top seal opposite the air channel can be resiliently biased towards the air channel to keep it closed by external biasing means 575, for example, a compression spring extending from a locating projection 577 formed on the distal face of the top seal to a reaction arm 579 projecting laterally from the longitudinally extending portion 581 of the U-shaped metal frame 565. The compression force of the biasing means and any resilient force provided by the top seal can be adapted such that in normal use the air channel is closed, but in the event of an overpressure in the outlet manifold the force exerted by the pressurized air in the air channel overcomes the forces holding the top seal in contact with the valve plate at that position, thereby deforming the resilient top seal and allowing a gap to form between the valve plate and top seal through which the overpressurized air can escape - either directly to atmosphere or, as shown here, first to the interior of the housing 583 and then to atmosphere via a though hole (not shown) in the housing wall.

Alternatively, or in combination with the external biasing means described above which are external to the top seal, the thickness of the top seal over the air channel may be reduced to provide a region of relative weakness which is biased by resilient internal biasing means (i.e. formed by its own internal resilient biasing force) towards the valve plate. The presence of an overpressure in the manifold would overcome the internal resilient in the top seal holding it against the valve plate and this weak area would bend away from the valve plate, thereby opening the air channel and releasing pressurized air. Once the pressure in the outlet manifold dropped to a sufficiently low level, the internal resilient biasing force in the weak area would spring it back into contact with the valve plate and quietly close off the air channel.

The above description of the present disclosure is provided for the purpose of illustration, and it would be understood by those skilled in the art that various changes and modifications may be made without changing the concept and essential features of the present disclosure. Thus, it is clear that the above-described embodiments are illustrative in all aspects and do not limit the present disclosure.

The scope of the present disclosure is defined by the following claims rather than by the detailed description of the embodiment. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the present disclosure.

Claims (9)

Claims

1. Motor-driven air pump assembly (501) comprising a solenoid (138) adapted to operate a valve in a valve plate (523), characterized in that said valve plate (523) is covered by a resilient top seal (524) having a proximal face (571) and a distal face (573) and an integral overpressure release path (569) is formed in, or between, contacting faces of the top seal (524) and the valve plate (523), wherein said overpressure release path (569) comprises a normallyclosed air channel (569), which air channel (569) extends from a manifold chamber (125) formed in said valve plate (523) and/or said top seal (524) towards the proximal face (571) of the top seal (524), and characterized in that the distal face (573) of the top seal (524) opposite said air channel (569) is resiliently biased into a channel-closing position towards the air channel (569) by external resilient biasing means (575) provided outside of said top seal (524).

2. Motor-driven pump assembly (501) according to claim 1 wherein the solenoid (138) comprises a U-shaped metal frame (565) with a longitudinally extending portion (581), and wherein the external resilient biasing means (575) comprising a compression spring extending from a locating projection (577) formed on the distal face (573) of the top seal (524) to a reaction arm (579) projecting laterally from the longitudinally extending portion (581) of the U-shaped metal frame (565) of the solenoid (138).

3. Motor-driven pump assembly (501) according to claim 1 or claim 2 wherein the distal face (573) of the top seal (524) opposite said air channel (569) is additionally resiliently biased into a channel-closing position towards the air channel (569) by internal biasing means (575) comprising a resilient biasing force generated in said top seal (524).

4. Motor-driven pump assembly (501) according to any of claims 1-3 characterized in that said assembly (501) is further provided with a pressure release valve with a valve plunger (137) able to be operated by said solenoid (138) wherein in the closed position the valve plunger (137) seals a pressure release port (535) and in the open position the valve plunger (137) allows air to flow through the pressure release port (535).

5. Motor-driven pump assembly (501) according to claim 4 characterized in that said valve plunger (137) comprises a resilient tip seal.

6. Motor-driven pump assembly (501) according to claim 4 or 5 characterized in that said valve plunger (137) is spring loaded into the closed position and can be by moved by said solenoid (138) into the open position.

7. Motor-driven pump assembly (501) according to any of the previous claims characterised in that it comprises a wobble pump (505).

8. Motor-driven pump assembly (501) according to claim 7 characterised in that the wobble pump (505) comprises a cylinder block (508) made of resilient material.

9. Motor-driven pump assembly (501) according to claim 8 wherein the cylinder block (508) comprises a plurality of cylinders (510) each with a flexible side wall (509).