Patent Publication Number: US-10767523-B2

Title: Auxiliary drive system for a pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 15/851,817 filed Dec. 22, 2017, which claims priority of British Application No. 1621934.7 filed Dec. 22, 2016, the entire disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention is concerned with an auxiliary drive system for a pump, and a pump having an auxiliary drive system. More specifically, the present invention is concerned with an electrically driven oil pump for a vehicle, the pump having an auxiliary or secondary source of power for use during high demand situations. 
     BACKGROUND OF THE INVENTION 
     Internal combustion (IC) engines for vehicles have several moving components which require lubrication. These include rotating shafts, sliding pistons etc. Lubrication occurs by the presence of oil. Oil is usually pumped around the engine by an oil pump. The oil pump will pick up low pressure oil from a sump, and pressurise it before delivery to the engine. Various pressure drops occur as the oil passes through the engine, and the oil eventually returns to the sump for recirculation. 
     The pumping effort required by the oil pump is determined by many factors. Some factors are inherent in the design of the engine (e.g. clearances and the path through which the oil must pass) and some factors vary through the operating cycle of the engine itself. For example, pumping effort decreases with a decrease in the viscosity of the oil, which in turn decreases as the engine (and oil) warms up. Therefore, it is generally much harder to pump oil around a cold engine because the cold oil has a high viscosity. Once the engine has warmed up, the pump does not have to use as much energy to pump the oil. 
     Various pump designs are available. Rotary positive displacement pumps such as gear pumps and gerotor pumps are common in this field, and are generally powered a drive connected to a pump input shaft. In some cases, the drive is the engine crankshaft (connected via a belt and pulley). In other cases, the drive is an electric motor. 
     Electrically driven oil pumps are increasingly common in modern engine design because they offer advanced control. Crankshaft driven pumps are dependent on engine speed, or require a gear train between the crank shaft and pump input shaft. The speed and fluid power output of electrically driven pumps can be varied more easily with electronic control. Electrically driven pumps also have fewer restrictions on placement of the pump (i.e. the input shaft of the pump does not need to be aligned with the crankshaft). 
     A problem with electrically driven oil pumps is the “cold start” condition. Because of the amount of pumping effort required to drive the cold oil through the engine, the electric motor need to produce a significant amount of torque (and therefore power). The intermittent need to produce a large amount of torque can reduce the life of the motor. Further, the cold start condition represents the “maximum power” design point for the electric motor driving the pump. In other words, the motor needs to be designed for this condition, but for most of the operation of the engine (when it is warm), the motor is not operating anywhere near capacity (i.e. it needs to produce less torque than the maximum power condition). Therefore, a much larger motor is usually provided than is necessary for most of the duty cycle. This increases cost and complexity, and takes up space in the engine. 
     SUMMARY OF THE INVENTION 
     It is an aim of the present invention to overcome this problem. 
     According to a first aspect of the invention there is provided a vehicle engine oil pump assembly comprising: 
     a pump subassembly having an inlet and an outlet; 
     an electrical drive arranged to selectively drive the pump subassembly; 
     a mechanical drive comprising a driven member configured to receive a drive torque from the vehicle engine; 
     a clutch in a load path between the driven member and the pump subassembly, the clutch being movable between a first condition in which the driven member drives the pump subassembly and a second condition in which the driven member can rotate freely relative to the pump subassembly; 
     in which the clutch comprises a clutch plate armature defining a friction surface of the clutch and at least partially constructed from a ferromagnetic material, and in which an electromagnet is configured to move the clutch plate armature. 
     Advantageously, this creates a compact and light arrangement. In one embodiment, the clutch plate armature comprises a ferromagnetic material with a friction material layer. The ferromagnetic material forms part of the magnetic circuit with the electromagnet. Preferably in one of the first and second conditions, the position of the clutch plate armature creates a break in the magnetic circuit, and in the other of the first and second conditions the magnetic circuit is made. 
     Preferably the clutch is configured to resile to the first condition upon interruption of electrical power to the clutch and/or the electrical drive. 
     Preferably the clutch is resiliently biased by a spring. 
     Preferably the clutch comprises a clutch plate armature defining a friction surface of the clutch and at least partially constructed from a ferromagnetic material, and in which the electromagnet is configured to move the clutch plate armature. 
     Preferably the electromagnet is positioned within the driven member. 
     Preferably the driven member is at least partially constructed from a ferromagnetic material. 
     Preferably the driven member comprises an outer driven member and an inner driven member defining an annular volume therebetween, in which the electromagnet is positioned within the annular volume. 
     Preferably a lubrication flow path is provided such that at least one of the electrical drive and mechanical drive is at least partially lubricated by fluid from the pump outlet in use. 
     Preferably both the electrical drive and mechanical drive are at least partially lubricated by fluid from the pump outlet in use. 
     Preferably the electrical drive comprises a rotor and a stator, the rotor is supported on an electrical drive bearing, in which a lubrication flow path is provided from the pump outlet to the electrical drive bearing. 
     Preferably the electrical drive bearing is a fluid bearing. 
     Preferably there is provided a sealing structure between the stator and the rotor such that the stator is sealed from the pumped fluid in use. 
     Preferably the sealing structure comprises a cylindrical structure spanning a radial gap between the stator and the rotor. 
     Preferably the electric drive rotor is mounted on a common drive shaft with a rotor of the pump subassembly, and in which a return flow path for lubrication flow to the electric drive is provided through the common drive shaft. 
     Preferably the return flow path for lubrication flow passes through the pump to the mechanical drive. 
     Preferably the return flow path for lubrication flow returns to the inlet of the pump subassembly from the mechanical drive. 
     Preferably the common drive shaft extends into the mechanical drive, and in which the lubrication flow from the electrical drive lubricates at least one mechanical drive bearing. 
     Preferably there is a housing, and a mechanical drive bearing between the housing and the driven member of the mechanical drive, in which a lubrication flow path is provided from the pump outlet to the mechanical drive bearing. 
     Preferably the mechanical drive bearing is a fluid bearing. 
     Preferably the electrical drive and the mechanical drive are positioned on opposite sides of the pump subassembly. 
     According a second aspect of the invention there is provided a vehicle engine pump assembly comprising: 
     a pump; and, 
     a clutch having a mechanical input and configured to selectively drive the pump; 
     in which the clutch comprises a clutch plate armature defining a friction surface of the clutch and at least partially constructed from a ferromagnetic material, and in which an electromagnet is configured to move the clutch plate armature. 
     Preferably the clutch is configured to resile to the first condition upon interruption of electrical power to the clutch and/or the electrical drive. 
     Preferably there is provided an electrical drive arranged to selectively drive the pump. 
     Preferably the mechanical input is provided via a driven member, and the electromagnet is positioned within the driven member. 
     Preferably the driven member is at least partially constructed from a ferromagnetic material. 
     Preferably the driven member comprises an outer driven member and an inner driven member defining an annular volume therebetween, in which the electromagnet is positioned within the annular volume. 
     The pump may be a water pump. 
     According to a third aspect there is provided vehicle engine oil pump assembly comprising: 
     a pump subassembly having an inlet and an outlet; 
     an electrical drive arranged to selectively drive the pump subassembly; 
     a mechanical drive comprising a driven member configured to receive a drive torque from the vehicle engine; 
     a clutch in a load path between the driven member and the pump subassembly, the clutch being movable between a first condition in which the driven member drives the pump subassembly and a second condition in which the driven member can rotate freely relative to the pump subassembly; 
     wherein a lubrication flow path is provided such that at least one of the electrical drive and mechanical drive is at least partially lubricated by fluid from the pump outlet in use. 
     Preferably both the electrical drive and mechanical drive are at least partially lubricated by fluid from the pump outlet in use. 
     Preferably the electrical drive comprises a rotor and a stator, the rotor is supported on an electrical drive bearing, in which a lubrication flow path is provided from the pump outlet to the electrical drive bearing. 
     Preferably the electrical drive bearing is a fluid bearing. 
     Preferably there is provided a sealing structure between the stator and the rotor such that the stator is sealed from the pumped fluid in use. 
     Preferably the sealing structure comprises a cylindrical structure spanning a radial gap between the stator and the rotor. 
     Preferably the electric drive rotor is mounted on a common drive shaft with a rotor of the pump subassembly, and in which a return flow path for lubrication flow to the electric drive is provided through the common drive shaft. 
     Preferably the return flow path for lubrication flow passes through the pump to the mechanical drive. 
     Preferably the return flow path for lubrication flow returns to the inlet of the pump subassembly from the mechanical drive. 
     Preferably the common drive shaft extends into the mechanical drive, and in which the lubrication flow from the electrical drive lubricates at least one mechanical drive bearing. 
     Preferably there is provided a housing, and a mechanical drive bearing between the housing and the driven member of the mechanical drive, in which a lubrication flow path is provided from the pump outlet to the mechanical drive bearing. 
     Preferably the mechanical drive bearing is a fluid bearing. 
     Preferably wherein the electrical drive and the mechanical drive are positioned on opposite sides of the pump subassembly. 
     Preferably the pump assembly comprises a positive displacement pump. 
     Preferably which the pump assembly comprises a gerotor pump. 
     Preferably the driven member comprises a pulley. 
     Preferably the driven member comprises a gear formation. 
     There is also provided a vehicle engine comprising a vehicle engine oil pump assembly according to any preceding claim. 
     The invention also provides a method of operation of a vehicle engine oil pump comprising the steps of: 
     providing a vehicle engine pump according to the above aspects; 
     providing a controller configured to selectively power the electrical drive and operate the clutch; 
     receiving an engine parameter with the controller; 
     using the controller to select mechanical and/or electrical power depending on the received engine parameter. 
     Preferably the controller is configured to select electrical power below a predetermined pumping demand, and electrical and mechanical power above the predetermined pumping demand. 
     According to a fourth aspect of the invention there is provided a vehicle engine oil pump assembly comprising: 
     a pump subassembly having an inlet and an outlet; 
     an electrical drive arranged to selectively drive the pump subassembly; 
     a mechanical drive comprising a driven member configured to receive a drive torque from the vehicle engine; 
     a clutch in a load path between the driven member and the pump subassembly, the clutch being movable between a first condition in which the driven member drives the pump subassembly and a second condition in which the driven member can rotate freely relative to the pump subassembly. 
     Advantageously, this configuration allows for electrical power to be used most of the time. When extra pumping effort is required (for example during cold start), mechanical power can be engaged via the clutch to assist the electric motor. The mechanical drive can be driven by e.g. the engine crankshaft. 
     Preferably, a lubrication flow path is provided such that at least one of the electrical drive and mechanical drive is at least partially lubricated by fluid from the pump outlet in use. Preferably both the electrical drive and mechanical drive are at least partially lubricated by fluid from the pump outlet in use. The use of the pumped fluid as lubrication flow provides for simple lubrication in a compact assembly. 
     The electrical drive generally comprises a rotor and a stator, in which the rotor is supported on an electrical drive bearing, and in which a lubrication flow path is provided from the pump outlet to the electrical drive bearing. Preferably the electrical drive bearing is a fluid bearing which is a hydrostatic bearing. This reduced the cost and complexity associated with e.g. rolling element bearings. 
     Preferably there is provided a sealing structure between the stator and the rotor such that the stator is sealed from the pumped fluid in use. Preferably the sealing structure comprises a cylindrical “can” structure spanning a radial gap between the stator and the rotor which separates the motor into a “dry side” and a “wet side”. Preferably the motor is a brushless DC motor, in which case the rotor (which requires no electrical power) is on the “wet side” and the stator (which requires electrical power) is kept on the dry side—i.e. isolated from the pumped fluid. 
     Preferably the electric drive rotor is mounted on a common drive shaft with a rotor of the pump subassembly, and in which a return flow path for lubrication flow to the electric drive is provided through the common drive shaft. The use of the shaft as a fluid path allows for a compact arrangement, and minimises drillings and flow paths in the housing. 
     Preferably the return flow path for lubrication flow passes through the pump to the mechanical drive. More preferably the return flow path for lubrication flow returns to the inlet of the pump subassembly from the mechanical drive. Even more preferably the lubrication flow from the electrical drive lubricates at least one mechanical drive bearing. This makes full use of the pressure of the pumped fluid- to create a lubrication circuit to the electrical drive, through the shaft (past the motor) and to the mechanical drive. This creates a compact and efficient assembly. 
     The assembly comprises a housing, and a mechanical drive bearing is provided between the housing and the driven member of the mechanical drive. Preferably a lubrication flow path is provided from the pump outlet to the mechanical drive bearing. Preferably the mechanical drive bearing is a fluid bearing, which reduces moving parts and cost compared to a rolling element bearing. 
     Preferably the electrical drive and the mechanical drive are positioned on opposite sides of the pump subassembly. 
     Advantageously, placing the pump between the mechanical and electrical drive makes porting for the various lubrication paths more convenient. There is a short path between both drives and the high and low pressure ports of the pump which can be accessed with simple drillings in the housing. This design also places the mechanical and electrical drives at the ends of the assembly, providing easy access without the requirement to take the assembly apart. 
     The pump has a rotor mounted on a pump shaft which can be selectively driven about a pump axis by the electrical and/or mechanical drive to pump fluid through the pump. Preferably the pump shaft extends in to the mechanical drive and the electrical drive, so they can drive it directly. 
     Preferably the clutch comprises a clutch plate moveable along the pump axis between the first and second conditions. The clutch may be a flat plate clutch, or preferably a cone clutch which provides a greater surface area. 
     The clutch may comprise two sub-clutches movable between the first condition and the second condition. Preferably there are two clutch plates which act in opposite directions to balance the axial loads in the assembly and on the shaft to which the clutch is mounted. Preferably the first sub-clutch is a primary clutch, the second sub-clutch is a secondary clutch and the primary clutch is radially outside the secondary clutch. 
     Preferably the clutch is electrically actuated, and the clutch resiles to the first condition in the absence of electrical power. This is a “failsafe” condition, so if electrical power is not available (in which case the electrical drive would stop), the mechanical drive will engage by default to keep the engine lubricated. Preferably the clutch is resiliently biased by a spring. 
     Preferably which the clutch is actuated by an electromagnet. More preferably the clutch comprises a clutch plate armature defining a friction surface of the clutch and at least partially constructed from a ferromagnetic material, and in which the electromagnet is configured to move the clutch plate armature. Combining the armature and the clutch offers a compact design. 
     Preferably the electromagnet is positioned within the driven member, which is a highly compact arrangement. Preferably the driven member is at least partially constructed from a ferromagnetic material, therefore proving dual function by acting as a magnetic field path. 
     Preferably the driven member comprises an outer driven member and an inner driven member defining an annular volume therebetween, in which the electromagnet is positioned within the annular volume. 
     Preferably an electronic control board is mounted to the electrical drive. More preferably the electronic control board is mounted proximate a first surface of housing of the electrical drive, and in which a fluid path from the outlet passes against a second surface of the housing within the electrical drive such that pumped fluid cools the first surface in use. 
     Preferably the pump assembly comprises a positive displacement pump, more preferably a gerotor pump. 
     The driven member may comprises a pulley or gear driven by the engine crankshaft. 
     The invention also comprises a vehicle engine having a vehicle engine oil pump assembly according to the first aspect. 
     According to a fifth aspect of the invention there is provided a method of operation of a vehicle engine oil pump comprising the steps of: 
     providing a vehicle engine oil pump according to the first aspect; 
     providing a controller configured to selectively power the electrical drive and operate the clutch; 
     receiving an engine parameter with the controller; 
     using the controller to select mechanical and/or electrical power depending on the received engine parameter. 
     Preferably the controller is configured to select electrical power below a predetermined pumping demand, and electrical and mechanical power above the predetermined pumping demand. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
       Various example pump drive systems in accordance with the present invention will now be described with reference to the accompanying Figures, in which: 
         FIG. 1  is a perspective view of a pump having a first drive system in accordance with the invention; 
         FIG. 2  is a section view of the pump of  FIG. 1  taken in the plane of  FIG. 1 ; 
         FIG. 3  is a section view of the pump of  FIG. 1  taken along line III-III in  FIG. 1 ; 
         FIG. 4  is a perspective section view of a pump having a second drive system in accordance with the invention; 
         FIG. 5  is a section view of the pump of  FIG. 4  taken in the plane of  FIG. 4 ; 
         FIG. 6  is a section view of the pump of  FIG. 1  taken along line VI-VI in  FIG. 4 ; 
         FIG. 7  is side view of a pump having a third drive system in accordance with the invention; 
         FIG. 8  is a side section view of the pump of  FIG. 7  along line IV-IV; and, 
         FIG. 9  is a detail view of a part of the pump of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The First Embodiment—Configuration 
     Referring to  FIGS. 1 to 3 , there is shown an oil pump assembly  100 . The pump assembly  100  generally comprises a housing  101 , a pump  102 , an electric drive  104 , a mechanical drive  106  and a control board  107 . The pump assembly defines a main axis X. 
     The housing  101  comprises a first housing part  108 , a second housing part  109  and an end part  134 . The first housing part  108  comprises a pump housing portion  138  defining a rotor cavity  112  eccentric with respect to the main axis X. The pump cavity  112  is in fluid communication with an oil inlet  204  and an oil outlet  206 . The oil inlet  204  is configured to receive low pressure oil and to deliver it to both axial sides of the rotor cavity  112  at a first circumferential position. As well as being fed low pressure oil from the engine, the oil inlet is also in fluid communication with a return channel  208 . The first housing part  108  further defines an annular first housing extension  140  projecting axially opposite to the rotor cavity  112 . The first housing extension  140  has a central shaft bore  141  which is in fluid communication with the return channel  208 . 
     The second housing part  109  defines an annular pump sealing flange  142  having a housing extension  126  extending axially proximate its outer rim. The second housing part further defines an annular second housing extension  144  projecting from its hub, having a radially outwardly facing shoulder  146 . 
     The end part  134  is generally circular having an annular end extension  145  extending proximate its hub defining a radially outwardly facing shoulder  148 . 
     The first and second housing parts  108 ,  109  are fastened together with a series of mechanical fasteners  111 . 
     The pump  102  comprises a rotor assembly  110 . The rotor assembly  110  comprises an outer rotor  114  and an inner rotor  116 . The outer rotor  114  is generally annular having a cylindrical radial outer surface and a radial inner surface having N+1 radially projecting lobes formed thereon. The outer surface of the outer rotor  114  is engaged with the rotor cavity for rotation about an axis offset from X. The inner rotor  116  has a radial outer surface having N radially extending lobes engaged with the recesses between the lobes of the outer rotor. The rotor assembly  110  is positioned within the rotor cavity  112  of the first housing part  108  and enclosed by the second housing part  109 . 
     Rotation of the inner rotor  116  about the axis X rotates the outer rotor  114  and acts to create a pumping effect. As such, the rotor assembly is that of a gerotor pump, which can pump fluid from a first circumferential position of the rotor cavity (where the oil inlet is located) to a second circumferential position (where the oil outlet is located). The general operation of gerotor pumps is well understood in the art and will not be described further here. 
     The inner rotor  116  is driven by a pump input shaft  120  mounted for rotation about axis X. The pump input shaft  120  extends either side of the inner rotor  116  to define a first shaft extension  122  and a second shaft extension  124 , on the opposite side of the pump  102  to the first shaft extension  122 . The shaft  120  defines a central axial fluid channel  121 , which is sealed at the end of the second shaft extension  124  by a seal  125 . The second shaft extension  124  defines a plurality of axially extending openings  127  which place the channel  121  in fluid communication with the central shaft bore  141 , thus facilitating a return flow via return channel  208  to the low pressure inlet  204  of the pump  102 . 
     The first shaft extension  122  is engaged in a plain bearing with the second housing part  109 , and the second shaft extension  124  is engaged in a plain bearing with the first housing part  108 . As such, as the pump  120  pressurises the oil in the cavity  112 , there is provided a hydrodynamic lubricating flow between the shaft  120  and the housing part  109 . This is discussed further below. 
     Turning to the electric drive  104 , this is disposed within the extension  126  of the second housing part  109  of the pump assembly  100 . The electric drive comprises a rotor  128  attached to the first shaft extension  122  and a stator  130  surrounding the rotor  128 . The rotor  128  comprises a plurality of circumferentially spaced permanent magnets  132 . The stator  130  comprises a plurality of electromagnets  134  comprising coils  136  which are attached to the interior surface of the second housing extension  126 . The rotor  128  and stator  130  together form a brushless DC motor (BLDC) capable of driving the shaft  120  in rotation upon application of DC electrical power. 
     A can  150  is positioned between the rotor  128  and stator  130 . The can  150  is a cylindrical component which is sealed against the spaced apart shoulders  146 ,  148  of the second housing part  109  and end part  134  respectively with o-ring seals  152 ,  154 . The can  150  provides a seal between the “wet” rotor and “dry” stator. As discussed above, there a lubricating oil flow from the pump  110  enters the electric drive  104  along the shaft  120 , and the presence of the can prevents the oil from contacting the stator  130 . 
     The shaft extension  122  is engaged via a plain bearing in the extension  145  of the housing end part  134 . 
     Turning to the mechanical drive  106 , there is provided a shaft bearing  156 , a shaft seal  158 , a pulley bearing  159 , a clutch plate boss  160 , a clutch plate mount  162 , a clutch plate armature  164 , a solenoid  166  and a pulley  168 . 
     The shaft seal  158  sits within the first housing extension  140  and bears against the outer periphery of the shaft  120  (in particular the shaft extension  122 ). The shaft bearing  156  facilitates rotation of the shaft  120  within the first housing extension  140 . The shaft bearing  156  is a ball bearing and therefore configured to react any radial load applied to the shaft  120 . 
     The clutch plate boss  160  comprises a shaft portion  170  and a flange  172 . The shaft portion  170  is splined to the shaft  120  for rotation therewith. The boss  160  can therefore slide on the shaft  120  along the axis X. The clutch plate mount  162  is an annular disc which is attached to the flange of the clutch plate boss for rotation therewith. The clutch plate armature  164  is an annular component attached to the clutch plate mount  162 . The clutch plate armature  164  is constructed from a ferrous material and has an annular friction surface  174 . A clutch spring  200  is provided to resiliently urge the clutch plate armature away from the pulley  168 . 
     The solenoid  166  comprises a solenoid mount  176  and an electromagnet  178  comprising a coil which can be selectively charged to produce a magnetic field. The solenoid mount  176  is positioned on the radially inner surface of the electromagnet  178 , leaving the radially outer surface of the electromagnet  178  exposed. The solenoid  166  is attached to the first housing part  108  and is static relative thereto. 
     The pulley  168  comprises a pulley inner  180  and a pulley outer  182 . The pulley inner  180  comprises a hollow shaft which is mounted for rotation about the first housing extension  140  of the first housing part  108  on the pulley bearing  159 . The pulley bearing  159  is a double angular contact ball bearing arrangement which is configured to resist axial loads between the first housing part  108  and the pulley  168 . 
     The pulley outer  182  is attached to the pulley inner  180  for rotation therewith via a press fit (although it is possible to construct them as a unitary component). The pulley outer  182  defines a series of external grooves  184  configured to receive a toothed belt (driven by a crankshaft). The pulley outer  182  is constructed from a ferrous material, and in conjunction with the solenoid mount  176  sandwiches the electromagnet there between. 
     The pulley inner  180  defines an axially facing clutch surface  186  which faces the clutch plate armature  164 . 
     The control board  107  is mounted to the end of the housing end part  134 . The control board is a circular board on which control electronics for the pump assembly  100  are mounted. The solenoid  166  is operated like an additional phase from the motor controller via the vehicle CAN bus from an engine control unit (ECU). Upon receipt of a command from the ECU, the control board can selectively provide power to the electromagnet  134  and/or the electromagnet  178  as will be discussed below. 
     The First Embodiment—Use 
     The pump assembly  100  has three main modes, which will be described below. 
     (i) Electric Only Mode 
     In this mode, the control board  107  receives a pump demand signal from the ECU and provides power to the electromagnets  134  to drive the motor and thereby pump oil through the pump  102 . The input power may be varied to provide the desired pumping effort. 
     (ii) Mechanical Only Mode 
     In this mode, the control board  107  receives a demand which exceeds a predetermined pumping power available from the motor  102  alone. The electromagnets  134  are not energised, and instead the electromagnet  178  in the solenoid  166  is energised. The resulting magnetic field draws the clutch plate armature  164  into contact with the axial end of the pulley inner  180 . This forms a load path from the pulley  168 , through the clutch plate armature  164 , through the clutch plate mount  162  to the clutch plate boss  160  and to the shaft  120  to power the pump  102 . In this way, the pump  102  can be driven by the engine crankshaft. 
     (iii) Hybrid Mode 
     In this mode, the electric drive  104  and mechanical drive  106  are simultaneously activated by the control board  107  to provide extra power to the pump  102 . 
     It will be noted that as the pump  102  pressurises the oil therein, there is provided a hydrodynamic lubricating flow between the shaft  120  and the housing part  109 . This lubricates the plain bearing between the shaft  120  and the second housing part  109 . The oil passes through the “wet” rotor in the electric drive  104  and to the plain bearing between the shaft  120  and the housing end part  134 . 
     The oil then passes into the end of the shaft  120  and enters the central channel  121  under pressure. As the oil passes into the axial end of the shaft extension  122  within the end part  134 , it also cools the adjacent control board  107 . The lubricating flow then proceeds through the channel  121 , back past the pump  102  and to the mechanical drive  106 . As the channel  121  is sealed by the seal  125 , the oil escapes through the openings  127 . The oil cannot pass the shaft seal  158  and passes though the plain bearing between the shaft extension  121  and first housing extension  140  back to the low pressure pump inlet. 
     The ability to flood the motor rotor is beneficial for lubrication and cooling and permits use of plain, fluid lubricated bearings which offers excellent radial load reaction as well as long life and reliability. 
     The Second Embodiment—Configuration 
     Referring to  FIGS. 4 to 6 , a second embodiment of a pump assembly  1000  is shown. Reference numerals used are common with those in the first embodiment. 
     As with the first embodiment, the pump assembly comprises a housing  101 , a pump  102 , an electric drive  104 , a mechanical drive  106  and a control board  107 . The pump assembly defines a main axis X. 
     The housing  101 , pump  102 , electric drive  104  and control board  107  are physically identical to those in the first embodiment. The mechanical drive  106  differs, as will be described below. 
     The mechanical drive  106  comprises a shaft bearing  156 , a shaft seal  158 , a pulley bearing  159 , a clutch plate boss  160 , a clutch cone armature  164 , a solenoid  166  and a pulley  168 . 
     The shaft bearing  156 , shaft seal  158 , pulley bearing  159  and solenoid  166  are substantially identical to those of the first embodiment. 
     The clutch plate boss  160  comprises a shaft portion  170  and a flange  172 . The shaft portion  170  is keyed to the shaft  120  for rotation therewith. The shaft portion  170  defines an external spline  190  onto which the clutch cone armature  164  is mounted via a corresponding female spline  192 . The clutch cone armature  164  is therefore fixed for rotation with the boss  160  but can slide relative thereto along the axis X. 
     The clutch cone armature  164  is constructed from a ferrous material and defines an external conical friction surface  194  which tapers radially outwardly towards the pump assembly  1000 . The clutch cone is biased in an axial sense by a clutch spring  200 . The clutch spring  200  is a compression spring which bears against the flange  172  of the clutch plate boss  160  and the clutch cone armature  164 . 
     The pulley  168  comprises a pulley inner  180 , a pulley outer  182  and a pulley clutch collar  196 . The pulley inner  180  is identical to that of the first embodiment. The pulley outer  182  is attached to the pulley inner  180  for rotation therewith. The pulley outer  182  defines a series of external grooves  184  configured to receive a toothed belt (driven by a crankshaft). The pulley outer  182  is constructed from a ferrous material, and in conjunction with the solenoid mount  176  sandwiches the electromagnet therebetween. 
     The pulley clutch collar  196  is an annular component which is attached to the pulley outer  182  by mechanical fasteners. The collar  196  has a conical radially inner friction surface  198  which is configured to receive the external conical surface of the clutch cone armature  164 . The clutch spring  200  biases the clutch cone armature into engagement with the pulley clutch collar  196 . 
     The Second Embodiment—Use 
     The second embodiment of the pump assembly  1000  has three main modes, which will be described below. 
     (i) Electric Only Mode 
     In this mode, the control board  107  receives a pump demand signal from the ECU and provides power to the electromagnets  134  to drive the motor and thereby pump oil through the pump  102 . For electric-only operation, the solenoid  166  is energised, which draws the clutch cone armature  164  towards it. This compresses the clutch spring  200  and disengages the clutch cone armature from the pulley clutch collar  196 . In this manner, the load path between the pulley  168  and the shaft  120  is broken. 
     The input power to the electric drive  102  may be varied to provide the desired pumping effort. 
     (ii) Mechanical Only Mode 
     In this mode, the control board  107  receives a demand which exceeds a predetermined pumping power available from the motor  102  alone. The electromagnets  134  are not energised, and instead the electromagnet  178  in the solenoid  166  is de-energised. The action of the spring  200  pushes the clutch cone armature  164  into engagement with the collar  196  which forms a load path from the pulley  168  to the shaft  120  to power the pump  102 . In this way, the pump  102  can be driven by the engine crankshaft. 
     (iii) Hybrid Mode 
     In this mode, the electric drive  104  and mechanical drive  106  are simultaneously engaged by the control board  107  to provide extra power to the pump  102 . It will be noted that to engage the mechanical drive, the solenoid  166  needs to be de-energised. 
     This embodiment provides a “failsafe” condition should electrical power be interrupted. A complete loss of electrical power to the assembly  1000  will result in the mechanical drive  106  being activated with the electric drive dormant. 
     The Third Embodiment—Configuration 
     Referring to  FIGS. 7 to 9 , there is shown a pump assembly  1100  which is similar to the pump assemblies  100 ,  1000  and like reference numerals will be used to describe similar features. 
     As with the first embodiment  100 , the pump assembly  1100  comprises a housing  101 , a pump  102 , an electric drive  104 , a mechanical drive  106  and a control board  107 . The pump assembly defines a main axis X. 
     The pump  102 , electric drive  104  and control board  107  are physically identical to those in the first embodiment. 
     The housing  101  comprises a first housing part  108 , a second housing part  109  and an end part  134 . The first housing part  108  comprises a pump housing portion  138  defining a rotor cavity  112  eccentric with respect to the main axis X. The first housing part  108  further defines an annular first housing extension  140  projecting axially opposite to the rotor cavity  112 . The first housing extension  140  comprises a central shaft bore  141 . The first housing part  108  defines an oil inlet  204  and an oil outlet  206 . The oil inlet  204  is configured to receive low pressure oil and to deliver it to both axial sides of the rotor cavity  112  at a first circumferential position. As well as being fed low pressure oil from the engine, the oil inlet is also in fluid communication with a return channel  208  in communication with the interior of the first housing extension  140 . 
     The oil outlet  206  is configured to receive high pressure pumped oil from both axial sides of the rotor cavity at a second circumferential position, diametrically opposed to the first. As well as being connected to the engine, the oil outlet  206  is in fluid communication with the rotor of the electric drive  104  via an electric drive oil supply channel  210 . The oil outlet  206  is also in fluid communication with a first mechanical drive oil supply channel  212  and a second mechanical drive oil supply channel  220 . The first mechanical drive oil supply channel  212  splits into a radially extending sub-channel  222  which opens to the exterior circumferential surface of the shaft extension  140  and an axially extending sub-channel  224  which opens to the axial end of the shaft extension  140 . The second mechanical drive oil supply channel  220  extends axially to an annular, axially facing surface of the solenoid mount  176 . 
     The second housing part  109  and end part are similar to those of the first and second embodiments. 
     Turning to the mechanical drive  106 , this operates in a similar manner to the mechanical drive of the second embodiment (i.e. utilises a cone clutch rather than the plate clutch of the first embodiment). 
     As will be described below, the mechanical drive  106  of the pump assembly  1100  has significantly reduced radial load. Therefore there is no need for a shaft bearing. The shaft seal  158  is also omitted as the mechanical drive is run “wet”. 
     The mechanical drive  106  comprises a clutch plate boss  160 , a clutch cone armature  164 , a solenoid  166  and a spur gear  168 . 
     The clutch plate boss  160  comprises a shaft portion  170  and a flange  172 . The shaft portion  170  is keyed to the shaft  120  for rotation therewith. The shaft portion  170  defines an external spline  190  onto which the clutch cone armature  164  is mounted via a corresponding female spline  192 . A fluid thrust bearing  213  is provided between the clutch plate boss  160  and the housing extension  140 . The clutch cone armature  164  is therefore fixed for rotation with the boss  160  but can slide relative thereto along the axis X. The flange  172  extends radially outwardly from the shaft portion  170  and defines a tapered, male frustroconical clutch surface  214  on the radially outer position thereof. The frustroconical clutch surface  214  tapers radially inwardly moving axially towards the pump  102 . 
     The clutch cone armature  164  is constructed from a ferrous material and defines an external conical friction surface  194  which tapers radially outwardly moving axially towards the pump  102 . The clutch come armature further defines an annular abutment surface  218  facing the pump  104 . The clutch cone is biased in an axial sense by a clutch spring  200 . The clutch spring  200  is a compression spring which bears against the flange  172  of the clutch plate boss  160  and the clutch cone armature  164 . 
     The solenoid  166  comprises a series of windings mounted on a solenoid mount  168 , the solenoid mount being constructed from a ferromagnetic material. 
     The spur gear  168  comprises a gear inner  180 , a gear outer  182  and a gear clutch collar  196 . The gear inner  180  is similar to that of the first and second embodiments and is constructed from a ferromagnetic material. The gear inner  180  defines a tapered female frustroconical clutch surface  216 . The gear outer  182  is attached to the gear inner  180  for rotation therewith and defines a series of gear teeth  184  ( FIG. 7 ) configured to mesh with another gear (driven by a crankshaft). The gear outer  182  is constructed from a ferromagnetic material, and in conjunction with the solenoid mount  176  sandwiches the electromagnet therebetween. The spur gear  168  is capable of a small degree of movement (less than 1 mm) along the axis X. 
     The gear clutch collar  196  is an annular component which is attached to the gear outer  182  by mechanical fasteners  202 . The collar  196  has a conical radially inner friction surface  198  which is configured to receive the external conical surface of the clutch cone armature  164 . The clutch spring  200  biases the clutch cone armature into engagement with the gear clutch collar  196 . The gear clutch collar  196  is specifically constructed from a material that is not (or is minimally) ferromagnetic. 
     A hydraulically lubricated bearing is formed between the radial outer surface of the first housing extension  140  and the inner surface of the gear inner  180 . Oil is supplied via the radially extending sub-channel  222  of the first mechanical drive oil supply channel  212 . Hydraulically lubricated fluid thrust bearings are formed as follows ( FIG. 9 ): (i) a thrust bearing  213  is formed between the axial end of the first housing extension  140  and the clutch plate boss  160  and (ii) a thrust bearing  215  is formed between the solenoid mount  168  and the gear inner  180 . Oil for the thrust bearing  213  is supplied via the axially extending sub-channel  224  of the first mechanical drive oil supply channel  212 . Oil for the thrust bearing  215  is supplied via the second mechanical drive oil supply channel  220 . The oil from these lubricated bearings returns to the low pressure oil inlet  204  via the shaft bore  141  and return channel  208 . It will be noted that the electric drive lubrication and oil flow is the same as with the first and second embodiments. 
     A difference between the second and third embodiments is the provision of a secondary clutch (formed by surfaces  214 ,  216 ) between the clutch plate boss  160  and the gear inner  180 . This clutch is oppositely oriented to the primary clutch between the clutch cone armature  164  and the collar  196 . 
     The modes of operation of the pump assembly  1100  are the same as those of the pump assembly  1000 . The differences in the modes of operation will be discussed below. 
     (i) Electric Only Mode 
     Referring to  FIG. 9 , the solenoid  166  is energised. It will be noted that the solenoid mount  168 , gear inner  180 , gear outer  182  and the clutch cone armature  164  are all constructed from a ferromagnetic material. The gear clutch collar  196  is constructed from a material which is not (or minimally) ferromagnetic. 
     The magnetic circuit MC created by the energised solenoid  166  is shown in  FIG. 9 . There are four clearance gaps between the various components which the circuit has to bridge, thus creating an electromagnetic force therebetween: 
     Gap 1: (G1) is a pair of annular axially extending gaps between the clutch cone armature  164  and the gear inner  180 . This acts to draw the clutch cone armature  164  towards the gear inner  180 . 
     Gap 2: (G2) is an annular axially extending gap between the solenoid mount  168  and the gear inner  180 . This acts to draw the gear inner  180  towards the solenoid mount  176 . 
     When the solenoid is energised, the attractive force felt by the clutch cone armature  164  is transferred to the clutch spring  200 . This compresses and transfers load to the clutch plate boss  160 . The motion of the clutch plate boss  160  is constrained against the thrust bearing  213 . The attractive force on the gear inner  180  from gap G2 disengages the clutch formed between the gear inner  180  and the clutch plate boss  160 . The gear inner  180  moves axially until it is constrained by the thrust bearing  215 . In this state the thrust bearings  213 ,  215  carry the entire load produced by the solenoid  160 . It will be noted that in this position, neither the clutch cone armature  164  nor the clutch plate boss  160  contacts the gear inner (although they are constantly being pulled in that direction as long as the solenoid is energised). In this way, both primary and secondary clutches are disengaged. 
     (ii) Mechanical Only Mode 
     In this mode, the solenoid is de-energised. The spring  200  separates the clutch cone armature  164  and the clutch plate boss  160 . In doing so, the spring  200  forces both cones  194 ,  214  of the primary and secondary clutches respectively apart. 
     The clutch plate boss  160  is urged towards the pump. Movement of the clutch plate boss  160  is constrained by the thrust bearing  213 . The spring  200  then urges the clutch cone armature  164  away from the pump. As the clutch cone armature  164  contacts the gear clutch collar  196  (engaging the primary clutch), the gear outer  182  (along with the gear inner  180 ) is pulled slightly away from the pump. This also facilitates engagement of the secondary clutch as the gear inner  180  is moved towards the now stationary clutch plate boss  160 . This effectively creates a closed force loop maintained by the clutch spring  200 . Once fully engaged, no further axial load is exerted on the thrust bearings  213 ,  215 . 
     This engages both the primary and secondary clutches to form two drive paths between the gear  168  and the shaft  120 . 
     (iii) Hybrid Mode 
     As above, both drives are engaged. 
     The ability to remove the rolling element bearings from the mechanical drive  106  is afforded as a result of using a gear transmission instead of a belt drive in the second and third embodiments. 
     Variations fall within the scope of the present invention. 
     Although the following embodiments relate to positive displacement oil pumps, it will be understood that the drive systems described herein can be applied to other types of pumps. For example, the technology may be applied to rotordynamic pumps, and/or coolant pumps. 
     The first and second embodiments have a sealed wet and dry side on the mechanical drive (separated by the dynamic shaft seal). In a further embodiment, the seal has been eliminated, where the entire mechanical drive is lubricated. The oil is allowed to leak into the transmission sump. 
     In further embodiments of the present invention, the mechanical drive, and more specifically the clutch could be used without the electrical drive. For example, in situations where the pump needed to be switched on and off by interrupting the mechanical drive, this could be achieved with the above-described clutch arrangement. 
     One such example could be a water pump which does not need to run continuously. The ability to deactivate the water pump would increase the efficiency of the vehicle. 
     Generally, as such pumps are not performance critical (like an oil pump), the failsafe provided by a cone clutch ( FIG. 4  onwards) is not necessary, although may be implemented if desired.