Abstract:
The invention concerns a metering gear pump comprising a set of gears supported and guided solely by the inner peripheral surface of the lobe of the chamber wherein they are housed. They do not comprise any shaft to act as support or guide, thereby enabling to reduce significantly useless spaced difficult to rinse when changing the product to be pumped. For the same purpose, the driving shaft is flexibly connected to the driving gear. Such arrangement provides the advantage of requiring only one single packing seal. Such a pump, connected to a drive motor and an encoder delivering a signal proportional to the number of pump cycles, enables to provide an accurate metering pump for numerous uses.

Description:
BACKGROUND OF THE INVENTION 
   The present invention concerns a metering pump for liquid products. 
   PRIOR ART 
   Notably two types of devices for precision metering of liquid products are known, viz., the motorized gear pump and the flowmeter with flow rate controller. These two types of devices have problems, particularly when there is a need for rather rapid change of the liquid product being metered, as for instance in robotized installations for car body painting. It is necessary, in fact, when the product to be metered is rapidly changed, that the metering device is able to be rapidly and easily rinsed in such a way that during metering of a given product not a particle of the previous product be left in the metering device. The two known types of devices have problems of rinsing caused by difficult access for the rinsing or cleaning product or by idle spaces where the product to be eliminated may persist as a film or lump. On the other hand, rinsing that is not optimized will lead to a larger consumption of the product as well as to a longer duration of acceptable rinsing. 
   It is generally admitted that the gear pump represents the more reliable and more precise metering device, but is also the metering device that is more difficult to rinse in an appropriate manner. A gear pump of the type considered here comprises at least one driving gear and one driven gear, said gears each being held on a shaft mounted on bearings located in the pump body. These bearings give rise to idle spaces difficult to rinse and apt to retain pumped product that is degraded, crystallized or hardened. One way to avoid such idle spaces is that of arranging packing seals on each bearing, which means a minimum of four packings per pump. These packing seals are expensive, generally require some maintenance, and are always susceptible to leak. 
   The document JP 04 041 984 describes a gear pump in which the two gears are guided by the outer peripheral surface of friction teeth sliding on an inner peripheral surface of the pump chamber. While the problems mentioned above are partly eliminated by elimination of the gear shafts, the driving gear of this device is set in rotation by a shaft that is rigidly mounted. This has the particular disadvantage of dictating the lateral position of the driving gear, which necessitates a larger pump chamber and hence additional idle lateral spaces that are difficult to rinse. 
   The patent FR 2 163 935 describes a gear pump in which the driving gear is rotated by a driving shaft fixed in a manner to transmit only a rotation torque, the driven gear comprising no support shaft. The driving shaft of this pump does not comprise a packing seal, the sealing being made by a layer of the pumped liquid. Such disposition does not facilitate the rinsing of the pump and causes difficulties to use in the case of frequent changes of the pumped liquid. 
   The packing described in DE 14 03 912 can under no circumstances be suitable for a pump as proposed here, as it is adapted to a pump having the driving heat rigidly fixed to the driving shaft, being disposed between two bearings supporting the driving shaft. 
   It is a first aim of the invention, therefore, to propose a metering pump for liquid products improved over known metering pumps. 
   It is a further aim of the invention to propose a metering pump having a rinsability distinctly improved over that of known pumps. 
   Still another aim is that of proposing a metering pump that is able to meter a volume of liquid in precise manner. 
   These different aims are attained by a metering pump for liquid products having the characteristics disclosed herein. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferential embodiment of a metering pump for liquid products is described below, the description to be considered while referring to the annexed drawing comprising the figures in which 
       FIG. 1  presents a lateral view along a first section of a metering pump for liquid products according to the invention, 
       FIG. 2  presents the same pump along a section in a plane perpendicular to the plane of the section of the preceding figure, 
       FIG. 3  presents the constituent elements of a metering pump in a schematic way. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows that the body  1  of the pump comprises a bottom plate  10 , an intermediate plate  11  and a front plate  12 , these three plates here having an essentially rectangular external shape and similar dimensions, which are superimposed as can be seen in the figure. Means of attachment schematically shown at  13  hold these three plates assembled. A seal not shown in the figure secures tight sealing of the contacting faces of these three plates. 
   The intermediate plate  11  seen in elevation in  FIG. 2  essentially consists of a plate, rectangular in this embodiment, which comprises a chamber  110  consisting of at least two lobes each able to receive a gear  30 ,  31  of a set of gears  3 . One can see in  FIG. 1  that in the embodiment of the pump represented, chamber  110  reaches entirely across the intermediate plate  1 . The intermediate plate  11  additionally comprises an entry channel  111  for liquid that ends in the chamber  110 , as well as an exit channel  112  for liquid that leaves said chamber. The openings of these channels can each be arranged on one of the lateral walls of the intermediate plate  11 , as shown here, or channels  111  and/or  112  may be extended into either the bottom plate  10  or the front plate  12  while their openings would be located on a periphery of these plates. 
   The pump additionally comprises a set of gears  3  comprising a driving gear  30  and at least one driven gear  31  as well as drive organs  4  and packing seal  5 . 
   The drive organ  4  comprises a motorized organ not represented in this figure which ends in a drive shaft  40  that goes through a bearing  120  set in the front plate  12 . The bearing  120  can be a sliding bearing as shown here or a roller bearing. The end  41  of shaft  40  has a shape such that it can be introduced into a recess  300  of the gear  30  of corresponding shape so that it will be able to rotate said gear. In the case represented here, the end  41  and the recess  300  are both hexagonal, but any other shape allowing a rotating drive could be suitable. 
   One notices in the figure that the gears  30  and  31  are not supported by any shaft, they are guided while rotating, solely by the outer peripheral surface of the teeth in sliding contact on the inner periphery of the lobes of chamber  10 . The lubrication of the surfaces moving relative to each other is assured by the product being pumped. The shaft  40  that is flexibly connected with the driving gear  30  at its end  41  only serves to drive said gear while being without any support or guiding function. The shaft  40  or its end  41  can hence only transmit a torque to the driving gear  30  while any other force in whatever direction is excluded. Since the gears  30  and  31  are no longer guided laterally by a shaft, they can now settle laterally under the effect of pressure of the product being pumped, which acts on their sides. It is thus possible to have a lateral space to both sides of each gear that is as small as feasible, provided a thin sheet of the pressurized product is present that will secure lateral centering of the gear in its lobe. In this way the hollow spaces which would have to be rinsed between two products being pumped are strongly diminished, and only a single set of packing seals  5  has to be fixed on the shaft  40 . 
   In known manner, the packing seal  5  can consist of packings, of lip seals or, as shown here, of a mechanical lining. The mechanical lining has the advantage over other known types of packing seals, of exhibiting the smallest frictional force in rotation. Therefore, preferably a mechanical packing is selected which consists of a sealing ring  50  freely mounted around the shaft  40  and able to move axially in a recess  121  in bearing  120  while being pressed against the front side of gear  30  by an elastic organ, for instance a spring  51 , in order to mechanically secure a tight seal between the chamber  110  and the shaft  40 . An O-ring  52  serves as a static sealing barrier in the rear of the sealing ring  50 . Means represented in the figure at  55  allow rotation of the sealing ring to be prevented. Relative to known designs, that of the packing seal  5  here described allows the sealing barrier to be as close as possible to the end  41  of shaft  40 , which substantially contributes to a reduction of the hollow spaces that must be rinsed between two products being pumped. It has been mentioned before that the gear  30  was positioned laterally by the equilibrium of the pressures being exerted on its two sides. Since the side of gear  30  that is linked as described to the shaft  40  is lower than the opposite side, gear  30  would tend to be pressed against the inside of the front plate  12 . The elastic organs  51  will therefore be of a size such that they exert a force able to compensate the difference between the opposing forces being exerted on the two opposite sides of gear  30 . 
   Optionally, the packing seal  5  described above is made more complete by a supply  53  of a packing liquid coming from an external reservoir  54 . The packing liquid fills the part of recess  121  on the side of O-ring  52  that is opposite to that in contact with the liquid being pumped, and thus exerts a counterpressure on this seal so that its sealing will be improved. A leak of packing liquid in the direction of chamber  110  or a leak of pumped liquid across the seal  52  would lead to a change in level of the packing liquid in the reservoir  54 . By monitoring this level it is therefore possible to detect liquid leaks at the packing seal in any one direction. The presence of a packing liquid in the hollow parts of recess  121  also prevents a condensation and crystallization of the pump ed liquid in these hollow parts. 
   The gear metering pump as described above is thus optimized so as to substantially improve its rinsability, by eliminating the hollow spaces to be rinsed between two different p products being pumped d, which leads to savings of both the rinsing product and rinsing time. The simplified pump design which uses a smaller number of pump components and requires just a single packing seal reduces by as much the manufacturing cost as well as the risk of leaks, and improves its reliability. 
   The above pump can advantageously be used to meter a product being pumped, the volume of product pumped being essentially proportional to the number of revolutions of gear  30  or  31 . By monitoring this number of revolutions it is possible, therefore, to obtain a precision metering pump. Such a pump is presented schematically in FIG.  3 . 
   One recognizes in this figure the pump body  1  with the driving gear  30  being driven by the shaft  40  as described previously. The other end of shaft  40  is driven by a motor  41 , which preferably is an electric motor but can also be a pneumatic or hydraulic motor or a motor of any other known type able to drive the shaft  40 . A reducing gear or gear box  42  can be arranged on the shaft  40  between the motor  41  and the pump. The rotating speed of gears  30  and  31  of the pump or the volume of liquid pumped are thus equal or proportional to the number of revolutions of shaft  40  as well as of motor  41 . An encoder  43  able to record this number of revolutions can thus send a control signal to a control unit  44 , for instance an electronic unit containing or not containing a programmed microprocessor and able to record this signal and regulate the pumping process, e.g., by cutting the power supply to the motor  41  when the desired quantity of product has been pumped. The encoder can be arranged at the end of the shaft on motor  41 , as shown schematically by the encoder  43  in position A, on shaft  40  in front of or behind the reducing gear  42 , if present, as shown schematically in positions B and C, or in the pump itself, as shown schematically at position D. The encoder  43  is of any known type, optical, inductive, capacitive or other, that is able to record the number of revolutions of the motor  41 , of the shaft  40  or of one of the gears  30  or  31 , depending on the position (A, B, C, D) where it has been installed. 
   The design of the metering pump can be compact, with the motor  41  being directly glued to the pump body  1 , or more distributed, with a shaft  40  that consists of a flexible drive shaft. Such a pump is advantageously mounted on a robotic painting arm, for instance in the painting of car bodies, where the pump together with its drive motor can be located in the mobile part of the robotic arm or, if one wants to minimize the moving masses, the motor  41  can be housed in a part of the base of the robotic arm while the pump body can be located in the mobile end of the arm, while the two elements are linked by a flexible shaft  40 . A distributed design with flexible or rigid drive shaft  40  can also be used in order to obtain an explosion-proof pump where the motor  41  that might produce sparks can be remote from the pump body I that could be located in an explosive atmosphere. 
   In each of the possible applications of such a metering pump, it will be determined by the application considered whether or not a reducing gear or gear box  42  are incorporated, and where the encoder  43  will be placed among any of the positions described above. 
   The modular design described allows such a metering pump to be employed in numerous applications, painting, metering of chemical products, food products, pharmaceuticals, etc. 
   The constituent elements of the pump body  1  as well as the gears  3  and the packing seal  5  are made of materials which essentially are compatible with the products being pumped, and which can be metals or alloys, for instance stainless steel, synthetic materials, or ceramics, and these materials may be uncoated or coated with a protecting layer. It is not necessary that the different constituent elements of the pump be made of the same material. 
   A variety of different versions can be envisaged for the design of a gear metering pump according to the invention. The pump has been described as having one driving gear and one driven gear; it could just as well comprise a number of driven gears arranged along the periphery of a driving gear. The chamber of the pump body would then have the number of lobes required to receive one gear each. The pump body has been described as consisting of three assembled plates, so as to facilitate machining of the chamber  110  of the intermediate plate. It would also be possible for the intermediate plate  11  to be made as a single piece together either with the bottom plate  10  or the front plate  12 . Also, the pump body  1  has been described as being of rectangular shape, but it actually could have whatever shape able to accommodate a pumping chamber such as that described. On the other hand, the metering pump has been described as comprising an electric motor and an encoder, in particular. These two elements could be replaced by a step motor, where the number of steps to be executed is determined by the volume of product to be pumped.