Liquid delivery system

Abstract

A liquid delivery system 200 includes a reservoir 202 and at least two gear pumps. A first gear pump comprises a pair of gears 230a, 232a and is driven by a motor 210a via a shaft 226a. A second gear pump comprises a pair of gears 230b, 232b driven by a motor 210b via shaft 226b. In use the gear pumps are submerged by the liquid in the reservoir while ball bearings 220a and 220b are arranged above the liquid. Heat generated in the bearings cannot be transferred to the liquid which may comprise a heat-curable ink. The gear pumps are provided with a gear pump cover 208 having holes through which the drive shafts 226a and 226b extend. These holes are dimensioned to pass at least a proportion of the liquid entering the gear pump back into the reservoir 202.

Description

[0001] LIQUID DELIVERY SYSTEM

[0002] The present invention relates to a liquid delivery system having particular application to the pumping of corrosive and / or delicate liquids.

[0003] Inkjet printing requires a controllable flow of liquid ink with minimal fluctuations in pressure to be supplied to the print head and various types of pumps have been applied to this application. Diaphragm pumps have been employed in the past but require expensive attenuators and also require frequent replacement. They further use diaphragm materials and adhesives that can interact with the pumped liquid and suffer from a restriction that the higher the pressure, the lower the flow rate.

[0004] Gear pumps have also been employed and address some of these issues. With gear pumps the pumping capacity is not significantly reduced by increasing output pressure but is partially reduced. Gear pumps often manifest problems arising due to the nature of the ink which is susceptible to curing (particularly in the case of UV-curable inks, for example) when exposed to excessive heat. While gear pumps have the advantage of a smoother output with less in the way of pressure variations, they are also prone to heating due to friction in the bearings. This heating can result in the pumped ink being cured and clogging the gear pump. Even if this does not stop or slow the pump, particles of cured ink can break away and clog other components downstream of the pump, such as print heads.

[0005] This problem has been at least partially addressed in US9738094 by introducing air into the ink upstream of the pump. The resulting dissolved oxygen inhibits the photoinitiators in the ink and so slows the curing process, even when the ink is exposed to elevated temperatures. This approach is somewhat complex, however, as a relatively high flow rate is required and the oxygen has to be removed downstream of the pump in order that the ink can be cured as desired under normal doses of radiation. Furthermore, almost all aqueous inks contain surfactant which is used to lower surface tension of the ink for stable jetting and these inks require degassing in general to avoid excessive foam generation which may trigger an overflow detection and or cause the printhead jetting to be impaired. Clearly, the approach of adding oxygen to the ink is particularly counterproductive in this context. It is also the case that some UV-curable inks would need degassing also, particularly at higher jetting frequencies, and again any oxygen addition is counterproductive in this regard.

[0006] US4792291 also relates to a gear pump for ink but is unsuitable for inkjet printing due to the use of spigots which will cause unacceptable heating of the ink and, since the reservoir is not airtight, the lack of any pressure control.

[0007] Pumping of other liquids, such as corrosive liquids, also presents challenges. In particular, known pumps usually include a number of seals made from materials that will be degraded by contact with the corrosive liquid. This severely reduces the life of such pumps and, because of the corrosive nature of the liquids used, it is often hazardous and time consuming to replace the pumps (or the degraded seals). There is also the problem of pump unavailability which may interrupt an expensive industrial process. Ink recirculation systems are becoming the benchmark for reliable inkjet printing applications and the wear of components is more significant in such pumping systems. Inkjet inks are particularly complex compositions designed to meet the very tight operating windows of inkjet printheads and the jettability of such compositions, as governed by ink rheology, must remain intact when pumps for ink recirculating liquid delivery systems are used.

[0008] It is an object of the present invention to ameliorate the above drawbacks. Furthermore, in the context of inkjet printing, there is a need to be able to position an ink recirculating liquid ink delivery system without the constraints of gravity so that the ink delivery system may be placed above or below the printhead(s) to which it is attached. Currently, commercially available ink delivery systems do not enable such configuration / positioning freedom which limits integration design and increases integration cost, for example, requiring mounting in an elevated position relative to the printhead and requiring the use of header tanks.

[0009] According to a first aspect of the present invention there is provided a liquid delivery system comprising: a reservoir arranged to contain a volume of liquid and including an output line for pumped liquid, a first gear pump arranged within the reservoir, the first gear pump comprising a pump body, an inlet, an outlet, first and second pumping gears arranged within the pump body and an aperture in the pump body adjacent the first pumping gear, a first driveshaft connected towards a first end to a first rotary drive source, the rotary drive source arranged, in use, to be above a level of liquid in the reservoir, the first driveshaft passing through the aperture in the pump body and being connected towards a second end to the first pumping gear, a second gear pump arranged within the reservoir, the second gear pump arranged to be driven by a second rotary drive source via a second drive shaft passing through a second aperture in the pump body, the second rotary drive source arranged, in use, to be above a level of liquid in the reservoir, wherein the outlet of the first gear pump is coupled to the outlet line from the reservoir, whereby, in use, the first gear pump is submerged by the liquid in the reservoir and is operable to pump liquid between the inlet and the outlet, and whereby, in use, a proportion of the liquid pumped through the inlet is arranged to flow around the gears and leak from the first and second gear pumps via at least the apertures in the pump body to be returned to the reservoir.

[0010] By submerging the gear pumps in the reservoir but placing the bearings of the drive mechanism above the gear pump and higher than the maximum level of liquid in the reservoir, frictional heating of the pumped liquid is minimised or eliminated. Flow of liquid via the aperture (and possibly other paths) carries any slight heat generated in the gear pumps back into the reservoir. By eliminating the need for seals that come into contact with the liquid, liquids that might react with such seals may readily be pumped from the reservoir for extended periods. The liquid delivery system is also designed to be cost-effective to manufacture and integrate, and because the reservoir and pumps are already combined, requires less integration effort on behalf of the user. In a preferred embodiment, the compact nature of the system is enabled by having the pumps motors directly above the reservoir so as the reduce the width of the liquid delivery system and hence the cabinet in which it may sit.

[0011] In some embodiments, the second gear pump is provided to fill (and, if driven in reverse, to empty) the reservoir. In other embodiments, the second gear pump is used in a push-pull configuration.

[0012] Preferably the reservoir has a capacity to maintain, in use, a head of liquid of at least 10mm above the gear pump and preferably 15mm above the gear pump. This ensures minimal pressure variations in the output of the pump while maintaining the liquid delivery system at a manageable size. Too low a level in the reservoir can also result in an undesirable vortex at the inlet to the pump. The reservoir also preferably has a volume of negative pressure gas (air) above the liquid in the reservoir and this gas volume provides damping on the return to the reservoir. Preferably the proportion of liquid arranged to leak via the aperture (and possibly further paths back to the reservoir) in the pump body is less than 25% and more preferably less than 15%. While a greater leakage has advantages of increased cooling and / or lubrication, this reduces the efficiency of the pump. This leakage is more preferably less than 5% and still more preferably less than 1%. Note that these are proportions determined at full pump output. If there is no output pressure then the pump is doing little or no work and little or no leakage occurs. A minimum level of leakage of 0.5% is also preferred.

[0013] The rotary drive source is preferably a magnetic drive source comprising a motor and a first magnetic coupling arranged outside of the reservoir and a second magnetic coupling arranged inside the reservoir. This provides mechanical isolation between the motor and the gear pump, meaning that the reservoir can be maintained at a positive or negative pressure. This has the advantage of maintaining a constant pressure which is important in various applications, such as ink delivery. It also has the advantage that solvents in the liquid to be pumped will not evaporate. In the case of ink delivery, the ink will not thicken so quickly, thus reducing the possibility of printhead nozzles becoming blocked.

[0014] The reservoir is thus preferably a sealed (airtight) reservoir. In order to achieve a positive or negative pressure, the system preferably further comprises a gas pump coupled to the reservoir, the gas pump arranged, at least, to reduce the air pressure within the reservoir. More preferably the gas pump comprises an air pump and still more preferably the air pump comprises a peristaltic pump.

[0015] The first gear pump is preferably arranged substantially at the lowest point of the reservoir. This minimises the lowest level of liquid in the reservoir and reduces the risk of vortices forming.

[0016] While the rotary drive sources for the gear pumps may be operable in only one direction, the rotary drive source is preferably arranged to drive the gear pumps in either direction. This allows for pipelines to be cleared and the reservoir to be emptied.

[0017] In order to provide an airtight reservoir, it preferably comprises a body constructed from a single piece of material and a removable lid.

[0018] In use, it is important for the gear pump to be maintained submerged under the liquid, so the system preferably comprises a level-sensor arranged to determine the amount of liquid within the reservoir.

[0019] The level-sensor arranged to determine the amount of liquid within the reservoir preferably comprises a float arranged to be constrained by the first and second drive shafts. The rotation of the first and second driveshafts helps to prevent the float becoming stuck at a particular height within the reservoir. Alternatively, a float may be provided with a hole through which the driveshaft passes.

[0020] The liquid delivery system preferably comprises a controller arranged to disable the liquid delivery system in response to a level-sensor output indicating too little liquid within the reservoir. This is a preferred method of ensuring that there is sufficient liquid within the reservoir to reduce the risk of damage to the gear pump (although at priming, the pump will be running dry for a very short time). The controller may also be arranged to prevent overflow of the reservoir.

[0021] The reservoir is preferably constructed from a material that has no chemical reactivity with the liquid to be pumped, in one example at least the body of the reservoir is constructed from acetal. More preferably the liquid path through the system comprises no elastomers. In certain applications, such as inkjet printing, the reservoir preferably comprises a return line for returning pumped liquid to the reservoir.

[0022] Preferably the return line is arranged below the minimum in use reservoir liquid level. This avoids generating froth, foam or bubbles in the liquid.

[0023] Preferably the return line may be fed via at least one bypass valve and may be used to provide more rapid start-up of the system.

[0024] One particular application of the liquid delivery system comprises a pumping arrangement for an inkjet printer, in which case the liquid comprises printer ink.

[0025] To maximise the life of the gear pump, the first and second pumping gears of at least the first gear pump preferably comprise PEEK gears. PEEK stands for poly ether ether ketone and is a hard- wearing and unreactive material. Ink jetting trials with UV-curable inks with the liquid delivery system of these embodiments have resulted in stable pumping and jetting for thousands of hours.

[0026] According to a second aspect of the present invention, there is provided a gear pump arrangement comprising: a gear block having a first and second port and containing a first and second gear, the first and second gears located within respective wells in the gear block, a gear block cover for substantially sealing the first and second gears in the gear block, an aperture in the cover located adjacent the first gear, a drive shaft coupled to the first gear and passing through the aperture in the gear block cover, wherein in use, a proportion of the liquid pumped through the first port is arranged to flow around the gears and leak from the gear pump via at least the aperture in the gear block cover.

[0027] In a preferred embodiment, two gear pumps are provided in the reservoir. One application of the second pump is to fill (and possibly empty) the reservoir and an alternative application is to provide a return line for use with a recirculation printhead. If the second pump is used to provide a return line for a recirculation printhead, then a third pump may be provided to perform reservoir filling or a further pump may be provided outside the reservoir to perform this function.

[0028] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0029] Figure 1 shows a block diagram of an ink delivery system to which embodiments of the present invention may be applied,

[0030] Figure 2 shows a cross section of two gear pumps and their associated drive system,

[0031] Figure 3 shows a plan view of a gear block forming part of the pumps of Figure 2, and

[0032] Figure 4 shows a side sectional view of the gear block of Figure 3.

[0033] Figure 1 shows a block diagram of an ink delivery system to which pumping arrangements according to embodiments of the present invention can be applied.

[0034] Dotted line 1 shows an outer perimeter of a cabinet which contains several of the components of the ink delivery system. External to the cabinet are a printhead 26 which is provided with a sensor manifold 25 which contains sensors that detect temperature and pressure of ink flowing into, and out of, the printhead 26. A bypass valve 28 is provided across the manifold 25. The outputs of the sensors in the manifold are fed to a control system 23 within the cabinet (connections not shown for clarity). While the printhead 26 is shown to be of the recirculating type, that is to say one in which a return flow of ink is returned to a reservoir (at 5 in the figure), the pumping arrangements in accordance with embodiments of the present invention are equally applicable to ink delivery systems that provide a flow only to the printhead. Also outside of the cabinet 1 is a user interface 24 which may comprise a suitably-programmed PC, a liquid supply container 18 and a filter 19. A suitable filter is available from Pall Corporation at w.pall.co.uk. All of the piping comprises stainless steel tubes which are provided with interference fits such that there are no adhesives or silicones in the wetted path. The interference fit is provided by tubes having an external diameter 0.1mm greater than the internal diameter of the feature to which they are fitted. For many applications, it is important not to have adhesives or silicones in the wetted path because they are prone to become swollen, delaminate and even break off, causing blockages. For an ink pumping application, the absence of such materials in the wetted path provides wide ink compatibility.

[0035] The cabinet 1 contains a reservoir 2 constructed from a single piece of material such as acetal or PEEK, the reservoir containing a first pump 9a and a second pump 9b. These are both gear pumps that will be described in greater detail below. Although two pumps are shown in this embodiment, embodiments of the present invention may comprise three pumps for a push-pull system although only two gear pumps are needed in a basic open to atmosphere liquid delivery system) The first pump 9a is driven by a motor-to-driveshaft-coupling 13a via a shaft 10a. The second pump 9b is driven by a motor-to-driveshaft-coupling 13b via a shaft 10b. Each motor-to-driveshaft coupling 13a, 13b comprises a magnetic linkage between a respective motor 14a, 14b and a respective driveshaft 10a, 10b. The magnetic linkages couple a rotary motion from the respective motors to the respective driveshafts without any direct mechanical connection. The weight of the driveshafts is borne by a bearing in their respective driveshaft couplings 13a, 13b so that there is no mechanical load imposed on components at the lower end of the driveshafts. The lower end of the driveshafts comprises a mechanical coupling that engages with a drive gear of the gear pump at the other end. The drive gear is free to float up and down along the driveshaft so that, in use, the pressure generated by the pump causes some of the pumped liquid to move underneath both the drive gear and the driven gear of the pumps. This provides lubrication and cooling for the motion of the gears. It also ensures that none of the weight of the driveshaft is borne by the gear. The shaft also poses no heating threat to the liquid being pumped. It is important to note that the ability of the gears to float allows them to be free of spigots, since these would be responsible for causing undue heating of the ink. In a preferred embodiment, both the drive gear and the driven gear of the pumps are spigot free.

[0036] The top of the reservoir is provided with an air tight seal to a top plate 12 which permits the air pressure within the reservoir to be controlled. The air-tight seal may be provided by a bead of silicone sealant or an O-ring (which will be well clear of the liquid path). Certain embodiments of the present invention do not require a sealed reservoir, such as a push-pull arrangement for inkjet printing. The air pump may then also be omitted.

[0037] The second pump 9b is connected to transfer liquid from the liquid supply container 18 into the reservoir 2. The pump 9b may also be driven in reverse to substantially empty the reservoir, for example for maintenance. The first pump 9a is connected to pump liquid from the reservoir via a supply line filter 15 and a heater 17 to the manifold 25 and printhead 26. The motors are each controlled by the controller 23. The heater comprises 6 heater pads attached to a machined aluminium block with a total capacity sufficient to raise the ink to jetting temperature. The heater 17 is also controlled by the controller 23 (connections omitted for clarity). Between the supply line filter 15 and the heater 17 is a connection to an optional filter bleed valve 16 which is arranged to provide. Bleed valve 16 is primarily intended for removing an amount of fluid from the supply side of the filter back to the reservoir. Any liquid that flows through the bleed valve 16 is returned via a smaller-diameter pipe to the reservoir at 6. The bleed valve is arranged above the filter. Optionally, liquid impedance elements for control of flow or removal of dissolved gas may be provided in the same path as the filter 15 and heater 17. In addition, optionally, other filters such as in fill line between an external tank and the liquid delivery system and in the supply line external to the liquid delivery system and before a printhead may be used.

[0038] Within the reservoir is a float 11 which is arranged to move up and down with the liquid in the reservoir. The buoyancy and mass of the float must be selected to accurately follow the liquid level. In this embodiment, the float includes a magnet (not shown) whose location is detected by a reservoir level sensor 27 which provides liquid level information to the controller 23. The float is dimensioned to be captive between the drive shafts 10a and 10b. This has the benefit that the rotation of the driveshafts will impart some slight movement to the float 11 and likely prevent it from becoming stuck at a single level. Other suitable level sensing technologies will be apparent to the skilled reader such as an optical sensor (light path is broken by liquid at the correct level) or an ultrasound sensor (detecting round trip travel times from a point above the liquid).

[0039] A third pump 21 is located outside of the reservoir and has a first port connected via an optional overflow detection system 20 (providing an overflow indication to the controller 23 by connections not shown for clarity) to the reservoir 2 at point 7 in the figure. The connection 7 comprises an interference fit. The pump 21 is an air pump arranged to either pressurise the reservoir or evacuate some of the air within the reservoir to provide the desired pressure within the reservoir. It is possible that the pump 21 will be introducing air into the reservoir but still maintaining a vacuum overall (or vice versa). The other port of the pump 21 is vented to the ambient environment at 22. The pump 21 is a peristaltic pump although alternative pumps will be apparent to the skilled reader. The controller 23 is arranged to operate the air pump 21 so as to maintain an appropriate meniscus pressure at the print head. Embodiments provide for different printheads and printing environments. In many instances this requires the air pump to maintain a vacuum in the reservoir but in some instances, usually when the liquid delivery system is below the printhead(s), a positive pressure may be required. Embodiments of the presently-disclosed ink delivery system will also work with printheads that face upwards. Embodiments of the present invention may thus provide great flexibility in the relative location of the delivery system and the printhead(s), thanks to the ability to control meniscus pressure at the printhead(s)' nozzleplate(s) independently of differential pressure across the printhead(s).

[0040] The capacity of the air pump 21 may need to be greater to address demands during the priming process. As the reservoir is filled, the increase in liquid level will naturally tend to increase the air pressure in the reservoir, i.e. reduce the level of vacuum (and vice versa).

[0041] In some embodiments, the air pump may also be capable of pressurising the reservoir to a pressure in order to assist in purging various lines in the ink delivery system.

[0042] The controller is located in a separate compartment to minimise the risk of exposure to ink.

[0043] At start-up, the controller 23 is arranged to operate the second motor 14b so that the second pump 9b will suck ink from the liquid supply container 18. The reservoir 2 will start to fill and this should be reflected in the output of the level sensor 27. If the liquid level in the reservoir does not increase, the controller will conclude that the liquid supply container 18 is empty, that the filter 19 is blocked or that there is some other problem and de-activate the motor 14b to avoid damage. In this case, the controller will provide a fault message to an operator via the user interface 24, usually explaining the nature of the fault and remediation steps to be taken. While the reservoir is being filled the air pump 21 will be controlled to maintain the desired pressure within the reservoir.

[0044] Once the reservoir contains sufficient liquid, the controller 23 will de-activate the motor 14b and the pump 9b will stop (or at least adopt a level maintenance role). The first pump 9a and heater 17 are now activated (possibly in response to in input from an operator) and ink will flow via the filter 15, heater 17 and manifold 25 back to the reservoir at point 6. The temperature of the ink will thus increase and the controller will monitor the output of the temperature sensors in the manifold to determine whether it is sufficiently hot to commence printing operations. An optional bypass valve 28 may be provided to substantially bypass the manifold 25 to allow for a higher flow rate through the heater and shorten start-up time. Once the correct operating temperature has been attained (user-settable via the user interface), the controller 23 will inform the operator via the user interface 24 and de-activate the heater 17 to prevent the ink from becoming too hot. Ink in the reservoir may be heated to around 40°C. Printing may then commence.

[0045] During operation the controller will then: adjust the output of the heater to ensure that the ink is provided within the correct temperature range to the print head 26 monitor the level of liquid in the reservoir and operate the second gear pump 9b as to ensure that there is sufficient liquid within the reservoir for the correct operation of the delivery system and to avoid damage to the gear pumps monitor the pressure of the ink at the manifold 25 and alter the operation speed of the first pump 9a as necessary, as well as controlling the air pump 21 to ensure that the correct level of vacuum is maintained in the reservoir

[0046] The gear pumps may run at speeds of up to several thousand r.p.m.

[0047] The ink delivery system is suitable for any ink recirculation system including in particular piezo (thin film and bulk)printheads. The system can be used with gravity-fed printheads with appropriate amendments to the piping.

[0048] A combination of pumps can be operated to substantially drain the whole system of ink.

[0049] Although this preferred embodiment is arranged to provide either a vacuum or a positive pressure in the reservoir, variants of the embodiment exist in which it is not necessary to maintain a vacuum in the reservoir. One example is of a push-pull arrangement in which a further pump is provided on the return line from the printhead. In this case the air pump 21 may be omitted, the top plate 12 of the reservoir may be fitted in a manner that is not air-tight and the indirect couplings 13a, 13b may be replaced with a mechanical connection. In some embodiments the overflow sensor 20 may be omitted and the controller arranged to detect an overflow by means of the level sensor 27. In some embodiments the float 11 and level sensor 27 may be replaced by alternative components such as a light beam or an ultrasonic level detection system or even omitted.

[0050] In an alternative embodiment, three gear pumps may be provided in the reservoir for a push-pull arrangement. In another alternative, the two pumps in the reservoir may be used for push-pull and a third pump, such as a peristaltic pump outside the reservoir, is used for maintaining the liquid level in the reservoir. While a single printhead 26 is shown in figure 1, the system may be arranged to supply more than one printhead. While these will generally need to be at the same horizontal level but there is scope for some variation depending upon the specifications of the printheads.

[0051] While the line 1 denotes the extent of the cabinet of the ink delivery system, it will be appreciated that more or fewer components may be included therein.

[0052] The pumping arrangement in the embodiment of Figure 1 works well with the printhead(s) higher, at or lower than the level of the reservoir, providing a significant advantage over prior art arrangements.

[0053] Figure 2 shows a partial cross section 200 of the reservoir and two gear pumps suitable for use with the embodiment of Figure 1. Some of the housing and mounting components have been omitted for clarity.

[0054] The reservoir 202 has a top plate 204 and a gear block 206 sandwiched between a bottom plate 209 and a gear block cover 208 all located at the bottom of the reservoir. The bottom plate is optional in that the gears may be arranged to run directly on the base of the reservoir but the bottom plate is preferred to allow the use of different materials and ensure a good lifetime for the pumps. The remaining components are duplicated for each pump and will be given an "a" suffix for the first pump and a "b" suffix for the second pump. Only the components for the first pump will be described.

[0055] A motor 210a is arranged above the reservoir with its shaft 212a extending vertically downwards. The motor shaft is connected to a magnetic drive cup 214a which carries a pair of drive magnets 216a. When the motor rotates, the magnetic drive cup rotates about the axis of the motor shaft and the pair of magnets generate a rotating magnetic field. Located within the magnetic drive cup are another pair of magnets, driven magnets 218a mounted towards the top of a vertical drive shaft 226a. The driven magnets are responsive to the rotating magnetic field in order to drive the drive shaft 226a when the motor 210a is rotating. The motor 210a is arranged to be driven in either direction so that the gear pump can blow and suck although an alternative embodiment in which the motor only rotates in one direction is also possible.

[0056] The drive shaft 226a rotates around a ball bearing 220a which is supported by a bearing support 224a which in turn is mounted to the top plate 204. As well as supporting the bearing 220a, the support 224a also provides splash protection to the bearing. Between the drive magnets 216a and the driven magnets 218a is a vacuum cap or top hat 222a which is sealed in an air-tight manner to the top plate 204 of the reservoir. The vacuum cap contains the driven magnets 218a and the bearing 220a. The vacuum cap is made from a non-magnetic material and encloses a volume that is in gas communication with the reservoir but has no connection with the ambient environment.

[0057] The combination of components 214a, 216a, 218a and 222a thus provide a rotary drive from the motor 210a to the drive shaft 226a without any mechanical connection between the two. This allows driving of the gear pump while allowing a vacuum to be maintained in the reservoir.

[0058] In an alternative embodiment where there is no vacuum requirement, the magnetically-coupled drive may be replaced by a straightforward mechanical coupling. In other words, the motor shaft 212a is coupled directly to the drive shaft 226a.

[0059] The drive shaft 226a includes a mechanical drive member 228a at its lower end and this drive member is coupled to a drive gear 230a of the first gear pump. The drive gear is engaged with a driven gear 232a in known manner and the two gears are located in a hollow section of the gear block 206 (to be described further with reference to Figure 3). The drive shaft 226a passes through a hole in the gear block cover 208 and the two gears 230a, 232a are supported by the bottom plate 209. The hole in the gear block cover is dimensioned to allow some leakage of liquid from the inside of the gear pump back into the reservoir. In the current embodiment, this is arranged to be substantially 5% of the total flow of liquid through the gear pump at maximum output. Higher levels of leakage, such as 15% or even as high as 25% may be provided in alternative embodiments to provide greater cooling but this will, of course, reduce the efficiency of the pump. Further leakage paths (beyond the hole in the gear block cover) may be provided to carry the leaked proportion of the liquid back to the reservoir. The degree of leakage is determined by the degree of lubrication and cooling required by the gear pump. In most applications a minimum leakage of 0.5% of the flow at the inlet to the pump is required. Suitable ranges are therefore 0.5% to 5% and 0.5% to 25%.

[0060] By placing the ball bearing 220a above the level of the liquid to be pumped, any heat generated in the bearing will not be conveyed to the liquid and, in the case of ink, will not cause heating and unintentional curing thereof.

[0061] The flow of liquid around the gears in the gear pump provides sufficient lubrication of the interface between the gears and the static parts of the gear pump. The continuous flow of liquid around the gears and leakage back into the reservoir ensures that any heat transferred to the liquid is quickly transferred to the reservoir and the temperature of the liquid does not increase to any appreciable degree. In the case of ink, this ensures that the ink is not cured and so cannot clog any of the moving parts of the pump or the remainder of the delivery system. The key thing to note is that liquid is not trapped as it would be in prior art gear pumps where it will just get hotter and hotter.

[0062] In an alternative embodiment, the outer magnets may be replaced by coils and the inner magnets by a motor rotor. The coils may then be driven by an alternating current at the desired frequency which in turn drives the rotor and the shaft. In effect, this amounts to a motor built on either side of the vacuum cap / top hat.

[0063] Figure 3 shows a plan view 300 of the two gear blocks 206a, 206b of Figure 2. Each gear block is arranged to contain one gear pump but an alternative embodiment will include only one gear block containing both gear pumps (or three pumps in a further embodiment). As before, the features of the gear block that relate to the first gear pump have the suffix "a" and the features that relate to the second gear pump have the suffix "b". Only the features with the suffix "a" will be described.

[0064] A figure-of-eight well 304a passes completely through the gear block, as do four mounting holes 310 for fitting of the gear block cover (208, Figure 2). A pump inlet 306a is coupled to the figure-of eight well 304a by a passage 312a. A pump outlet 308a is coupled to the figure-of-eight well by a passage 314a. The inlet, outlet and the passages only extend a short way into the gear block as can be seen in Figure 4, which shows a side, sectional view of the gear block on the line 400-400. The inlet for the drive pump is arranged in the top of the gear block and the outlet is arranged to be below the gear block when mounted in the reservoir but the inlet and the outlet could equally be located elsewhere within the pump body. The reverse is true for the fill pump but, again, the inlet and the outlet could be arranged elsewhere.

[0065] The figure-of-eight well is dimensioned to accept the desired sizes of gears and the passages are dimensioned to carry the maximum volume of liquid at the appropriate flow rate. The gear block cover (208, Figure 2) is provided with a hole above the well in the gear block that houses the drive gear. Assuming that the dimensions of this hole (and corresponding drive shaft) does not vary, the gear block cover can be same regardless of the size of the pump gears. The fixings for the gear block cover may also be the same, meaning that only the gear block and the gears differ between pumps of different capacity, with attendant benefits in inventory.

[0066] While the two figure-of-eight wells 304a, 304b in the gear blocks are shown to be the same size in Figure 3, this need not be the case so that the capacity of the first and second gear pump can differ. For the ink delivery system discussed above, this means that the fill pump and the printhead supply pump can be dimensioned as desired.

[0067] While an external gear pump has been disclosed in the present embodiments, the present invention is equally applicable to an internal gear pump, i.e. one in which one of the gears rotates inside the other.

Claims

CLAIMS1. A liquid delivery system comprising: a reservoir arranged to contain a volume of liquid and including an output line for pumped liquid, a first gear pump arranged within the reservoir, the first gear pump comprising a pump body, an inlet, an outlet, first and second pumping gears arranged within the pump body and an aperture in the pump body adjacent the first pumping gear, a first driveshaft connected towards a first end to a first rotary drive source, the first rotary drive source arranged, in use, to be above a level of liquid in the reservoir, the first driveshaft passing through the aperture in the pump body and being connected towards a second end to the first pumping gear, a second gear pump arranged within the reservoir, the second gear pump arranged to be driven by a second rotary drive source via a second drive shaft passing through a second aperture in the pump body, the second rotary drive source arranged, in use, to be above a level of liquid in the reservoir, wherein the outlet of the first gear pump is coupled to the outlet line from the reservoir, whereby, in use, the first gear pump is submerged by the liquid in the reservoir and is operable to pump liquid between the inlet and the outlet, and whereby, in use, a proportion of the liquid pumped through the inlet is arranged to flow around the gears and leak from the first and second gear pumps via at least the apertures in the pump body to be returned to the reservoir.

2. A liquid delivery system as claimed in claim 1, wherein the system is arranged, in use, to maintain a liquid level of at least 10mm above the gear pumps.

3. A liquid delivery system as claimed in claim 1 or claim 2, wherein the proportion of liquid arranged to be returned to the reservoir is less than 25%.

4. A liquid delivery system as claimed in claim 3, wherein the proportion of the liquid arranged to be returned to the reservoir is less than 15%.

5. A liquid delivery system as claimed in claim 4, wherein the proportion of liquid arranged to be returned to the reservoir is less than 5%.

6. A liquid delivery system as claimed in claim 5, wherein the proportion of liquid arranged to be returned to the reservoir is less than 1%.

7. A liquid delivery system as claimed in any one of the claims 1 to 6, wherein the first and second rotary drive sources are magnetic drive sources comprising a motor and a first magnetic coupling arranged outside of the reservoir and a second magnetic coupling arranged inside the reservoir.

8. A liquid delivery system as claimed in any one of the claims 1 to 7, wherein the reservoir is a sealed reservoir.

9. A liquid delivery system as claimed in claim 8, further comprising a gas pump coupled to the reservoir, the gas pump arranged, at least, to reduce the air pressure within the reservoir.

10. A liquid delivery system as claimed in claim 9, wherein the gas pump comprises a peristaltic pump.

11. A liquid delivery system as claimed in any one of the claims 1 to 10, wherein the first and second gear pumps are arranged substantially at the lowest point of the reservoir.

12. A liquid delivery system as claimed in any one of the preceding claims, wherein the rotary drive source is arranged to drive the gear pump in either direction.

13. A liquid delivery system as claimed in any one of the preceding claims, wherein the reservoir comprises a body constructed from a single piece of material and a removable lid.

14. A liquid delivery system as claimed in any one of the preceding claims, further comprising a levelsensor arranged to determine the amount of liquid within the reservoir.

15. A liquid delivery system as claimed in claim 14, the level sensor comprising a float arranged to be constrained by the first and second drive shafts.

16. A liquid delivery system as claimed in claim 14 or claim 15, further comprising a controller arranged to disable the liquid delivery system in response to a level-sensor output indicating too little liquid within the reservoir.

17. A liquid delivery system as claimed in any one of the preceding claims, wherein at least the drive gears of the first and second gear pumps are free to float up and down along their driveshafts.

18. A liquid delivery system as claimed in any preceding claim, wherein the gears of the first and second gear pumps are spigot free.

19. A liquid delivery system as claimed in any one of the preceding claims, wherein the liquid path through the system comprises no elastomers.

20. A liquid delivery system as claimed in any one of the preceding claims, further comprising a return line for returning pumped liquid to the reservoir.

21. A liquid delivery system as claimed in claim 20, wherein the return line is arranged below the minimum in use reservoir liquid level.

22. A liquid delivery system as claimed in claim 20 or claim 21, wherein the return line is fed via at least one bypass valve.

23. A liquid delivery system as claimed in any one of the preceding claims, wherein the liquid comprises printer ink.

24. A liquid delivery system as claimed in any one of the preceding claims, wherein the first and second pumping gears of at least the first gear pump comprise PEEK gears.

25. A gear pump arrangement comprising: a gear block having a first and second port and containing a first and second gear, the first and second gears located within respective wells in the gear block, a gear block cover for substantially sealing the first and second gears in the gear block, an aperture in the cover located adjacent the first gear, a drive shaft coupled to the first gear and passing through the aperture in the gear block cover, wherein in use, a proportion of the liquid pumped through the first port is arranged to flow around the gears and leak from the gear pump via at least the aperture in the gear block cover.

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