POSITIVE-DISPLACEMENT PUMP AND PUMPING GROUP FOR FLUID PRODUCTS AND METHOD FOR THE USE THEREOF

Abstract

A positive-displacement pump for fluid products, in particular paints, colorants and the like, includes a pump body in which there is formed a pumping chamber, in which a piston is mounted for sliding and is controlled so as to advance and withdraw in order to vary the useful volume of the pumping chamber. The pumping chamber extends in accordance with a longitudinal axis which is inclined, in a non-vertical manner, with respect to a horizontal plane and having an upper region which is positioned at a greater height with respect to a horizontal plane and in the region of which the pumping chamber is placed in communication with at least one intake pipe of a fluid product.

Claims

1. A positive-displacement pump for fluid products, in particular paints, colorants and the like, comprising a pump body in which there is formed a pumping chamber, in which a piston is mounted for sliding and is controlled so as to advance and withdraw in order to vary the useful volume of the pumping chamber, the pumping chamber extending in accordance with a longitudinal axis which is inclined, in a non-vertical manner, with respect to a horizontal plane and having a summit zone which is positioned at the highest level with respect to a horizontal plane and in correspondence with which the pumping chamber is placed in communication with at least one intake pipe of a fluid product.

2. A positive-displacement pump according to claim 1, wherein the pumping chamber comprises a substantially cylindrical portion with a constricted end, the upper region being positioned near or in the region of the constricted end.

3. A positive-displacement pump according to claim 2, wherein the constricted end is substantially frustoconical, the upper region being positioned substantially near or in the region of the connection region or transition between the substantially cylindrical portion and the constricted end which is substantially frustoconical.

4. A positive-displacement pump according to claim 2, wherein the piston has a head end which is constricted to complement the end of the pumping chamber.

5. A positive-displacement pump for fluid products, in particular paints, colorants and the like, comprising a pump body in which there is formed a pumping chamber, in which a piston is mounted for sliding and is controlled so as to advance and withdraw in order to vary the useful volume of the pumping chamber, the pumping chamber being placed in communication with at least one intake pipe and one discharge pipe of a fluid product, which pipes are formed so as to be integral in a head body which is mounted on the pump body.

6. A positive-displacement pump according to claim 1, wherein the pumping chamber communicates with a three-way valve which can be actuated to be in selective communication with the intake pipe and a discharge pipe.

7. A positive-displacement pump according to claim 6, wherein the three-way valve comprises a valve chamber which is arranged above the pumping chamber, the at least one intake pipe being inclined upwards with respect to a horizontal plane from the valve chamber.

8. A positive-displacement pump according to claim 1, wherein an outer skirt of the piston is in the form of a bellows.

9. A positive-displacement pump according to claim 1, wherein there extends from the pumping chamber a single pumping pipe which communicates with the intake pipe and a discharge pipe, the pumping pipe leading into the pumping chamber at the upper region thereof.

10. A positive-displacement pump according to claim 1, comprising interception means or members of the intake pipe which are mounted on a head body which is mounted on the pump body.

11. A positive-displacement pump according to claim 1, comprising interception means or members of a discharge pipe which are mounted on a head body which is mounted on the pump body.

12. A positive-displacement pump according to claim 10, comprising a three-way valve which is mounted on the head body incorporating the interception means or members of the intake pipe.

13. A positive-displacement pump according to claim 12, wherein the three-way valve comprises an actuator member which is mounted on the head body.

14. A positive-displacement pump according to claim 1, wherein the piston is operationally connected to a motor which is mounted on a support which is fixed to the pump body.

15. A positive-displacement pump according to claim 1, wherein inside the pumping chamber there is mounted an adapter for reducing the volume of the pumping chamber and therefore for reducing the cylindrical capacity of the positive-displacement pump.

16. A positive-displacement pump according to claim 15, wherein the pumping chamber comprises a substantially cylindrical portion with a constricted end, the adapter also comprising a cylindrical portion and a constricted end, the geometry and external dimensions of which correspond substantially to the geometry and internal dimensions of the pumping chamber so that the adapter is suitable for covering the internal walls of the pumping chamber in order to produce a smaller pumping chamber.

17. A positive-displacement pump according to claim 15, wherein an external covering of the piston is of bellows-like form, the adapter having at a bottom an annular step which at an outer side moves into abutment with an abutment member at the end of the pumping chamber and at an inner side acts as an abutment for an annular portion having a greater diameter of a sleeve of a piston which has a total diameter which is smaller with respect to a piston which can be used in the pumping chamber without an adapter.

18. A positive-displacement pump according to claim 15, wherein the pumping chamber has a pumping opening which communicates with the at least one intake pipe of a fluid product, the adapter comprising an opening which is placed in correspondence with the pumping opening of the pumping chamber.

19. A positive-displacement pump according to claim 18, wherein the pumping opening and the opening of the adapter are positioned in an upper region of the pumping chamber and the reduced pumping chamber, respectively, which are positioned at a greater height with respect to a horizontal plane.

20. A pumping group comprising at least two positive-displacement pumps according to claim 1, which are operationally connected in such a manner that the withdrawal of the piston of another of the at least two positive-displacement pumps corresponds to the advance of the piston of one of the at least two positive-displacement pumps.

21. A pumping group according to claim 20, wherein the at least two positive-displacement pumps are connected by means of a gear mechanism controlled by a single motor.

22. A method for using a positive-displacement pump according to claim 1, the intake pipe being connected to a tank of fluid product, the pump comprising a discharge pipe and interception members which can be selectively controlled in order to open and close the intake pipe and the discharge pipe, comprising the steps of: controlling the interception members in order to open the intake pipe and to close the discharge pipe; controlling the piston so as to withdraw in order to transfer a quantity of fluid product from the tank to the pumping chamber; controlling the piston so as to advance by keeping the intake pipe open and the discharge pipe closed; opening the discharge pipe and closing the intake pipe during the advance movement of the piston in order to discharge a quantity of fluid product.

23. A positive displacement pump according to claim 11, comprising a three-way valve mounted on the head body incorporating the interception means or members of the discharge pipe.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0042] Additional features and advantages will be appreciated from the following detailed description of some preferred embodiments of the invention, given purely by way of non-limiting example and with reference to the appended drawings, in which:

[0043] FIG. 1 is a longitudinal section of a positive-displacement pump according to the present invention;

[0044] FIG. 2 is a plan view of the positive-displacement pump in accordance with the arrow II of FIG. 1;

[0045] FIG. 3 is a cross-section, drawn to an enlarged scale, of a three-way valve of the positive-displacement pump of FIG. 1;

[0046] FIG. 4 is a cross-section, drawn to an enlarged scale, of a variant of the valve group and discharge group of the positive-displacement pump of the present invention;

[0047] FIG. 5 is a cross-section, drawn to an enlarged scale, of another variant of the valve group and discharge group of the positive-displacement pump of the present invention;

[0048] FIG. 6a is a cross-section, drawn to an enlarged scale, of another variant of the valve group and discharge group of the positive-displacement pump of the present invention in an intake condition of the positive-displacement pump;

[0049] FIG. 6b is a cross-section similar to FIG. 6a, in a discharge condition of the positive-displacement pump;

[0050] FIGS. 7a and 7b are cross-sections of a variant of the pumping chamber of the positive-displacement pump according to the present invention which is without and with a reduction adapter of the cylindrical capacity, respectively;

[0051] FIG. 8 is a schematic view of a pumping group comprising two positive-displacement pumps according to the invention which are connected in a non-parallel manner.

DETAILED DESCRIPTION

[0052] With reference now to FIGS. 1 and 2, a positive-displacement pump 1 comprises a pump body 2 which defines a pumping chamber 3 which is elongate in accordance with a longitudinal axis X-X, which is slightly inclined with respect to a horizontal plane. The pumping chamber 3 comprises a cylindrical jacket 3a with a constricted end 4 which is preferably substantially frustoconical. Inside the pumping chamber 3, there is received in a sliding manner a piston 5 having a body 7 which is generally cylindrical and a constricted head 6, with a substantially frustoconical formation, which complements the end 4 of the pumping chamber 3. In greater detail, the piston 5 comprises a cylindrical central shaft 8, on which a sleeve 9 which is of bellows-like or zigzag form is fitted so as to provide a series of expansible circumferential corrugations or notches 10 at the external covering 10 thereof, acting as a seal and guide on the wall of the cylindrical jacket 3a of the pumping chamber 3. The sleeve 9 has a head end 10 which contributes to forming the head 6 of the piston 5. The head end 10 has an external covering 10a which is substantially frustoconical and a threaded axial hole 11 which is screwed onto the threaded end 8a of the central shaft 8. At the other bottom end 12 thereof, the sleeve 9 has an annular portion 13 which has a greater diameter and which is clamped between an abutment member 14 which is formed on the pump body 2 and a clamping flange 15 of a support group 16 of a motor 22, preferably a stepping motor. The motor 22 is fixed to the support group by means of screws 23 or other similar clamping means. The support group 16 is mounted in turn on the pump body 2 by means of screws 17 or other similar clamping means. There is formed on the external wall of the annular portion 13 an annular groove, in which an O-ring 18 or similar fluid-tight seal is received.

[0053] At the opposite side to the pumping chamber 3, the central shaft 8 of the piston 5 extends into a handling shank 19 with a longitudinal guiding fin 20 which is inserted in a slot 21 of the support group 16 of the motor 22. An extension piece 24 projects radially from the handling shank 19 in order to interact with a sensor 25 which is mounted on a support plate 26 which is fixedly joined to the support group 16. The handling shank 19 is connected to the stepping motor 22 by means of a handling screw 31 which allows a conversion of the rotational movement of the stepping motor 22 into a translation movement of the handling shank 19 and therefore of the piston 5 inside the pumping chamber 3.

[0054] There is formed in the pump body 2 an intake pipe 30 which comprises an intake coupling 32 for a pipe which communicates with a tank of fluid product which is not illustrated and which is positioned at a greater height with respect to the positive-displacement pump 1 so as to produce a hydraulic shutter and a fluid travel with a constant descent from the tank, for reasons of elimination of the air, in accordance with the methods which will become clearer below. From the intake coupling 32, the intake pipe 30 comprises a slightly inclined portion 33 which descends with respect to a horizontal plane. The portion 33 leads into a chamber 34 of a three-way valve 35 which is illustrated more clearly in the enlarged illustration of FIG. 3.

[0055] The three-way valve 35 is controlled by an actuator 36 which is mounted on the pump body 2, preferably a solenoid valve actuator. The actuator 36 is connected in known manner to a valve closure member 37. There also leads into the chamber 34 a pumping pipe 38 which communicates with the pumping chamber 3 and a discharge pipe 39 which communicates with a discharge nozzle 40 which is carried by a discharge head 41 which is mounted on the pump body 2. The valve closure member 37 can be moved in the chamber 34 between two positions: a lowered intake position, in which the valve closure member 37 closes the discharge pipe 39 and allows the fluid communication between the intake pipe 30 and the pumping pipe 38, and a raised discharge position, in which the valve closure member 37 closes the intake pipe 30 and allows fluid communication between the pumping pipe 38 and the discharge pipe 39.

[0056] The pumping pipe 38 leads into the pumping chamber 3 in an upper region 42 which is placed in the region of the connection or transition between the cylindrical jacket 3a and the substantially frustoconical end 4. The relationship between the inclination of the longitudinal axis X-X with respect to a horizontal plane and the inclination of the lateral wall of the substantially frustoconical end 4 is selected in such a manner that the upper region 42 into which the pumping pipe 38 leads is located at the highest location of the pumping chamber 3. In this manner, any air contained in the fluid product which is intended to be pumped and which is contained in the pumping chamber 3 spontaneously ascends upwards in order to accumulate in the upper region 42 and to ascend from there into the pumping pipe 38 as far as the chamber 34 of the three-way valve 35 which is placed in an upper position with respect to the upper region 42. When the valve closure member 37 is in the intake position, the air is then capable of migrating towards the inclined portion 33 of the intake pipe 30 and of returning at that location to the tank connected thereto. This configuration allows the spontaneous elimination of any air which may be contained in the fluid product inside the pumping chamber 3, without any need for costly recirculation operations or additional forced extraction systems.

[0057] During use, the positive-displacement pump 1 is connected to a tank which is located above fluid product, such as a colorant or paint or the like, by means of a pipe which is connected to the intake coupling 32. For the reasons set out above with regard to the spontaneous elimination of the air, it is desirable for the connection pipe of the tank to the positive-displacement pump 1 to be inclined downwards from the tank to the positive-displacement pump 1. In a dispensing machine for fluid products, there are provided a plurality of positive-displacement pumps 1 for discharging fluid products in accordance with a discharge programme which is controlled by an electronic logic unit which controls the stepping motor 22 which moves the piston 5, and the actuator 36 which controls the opening and closing of the three-way valve 35. The electronic logical unit receives feedback information from the sensor 25 in order to control the positioning of the piston 5.

[0058] During operation of the positive-displacement pump 1, the piston 5 can be moved in translation for intake operation, by controlling the stepping motor 5 so as to rotate in a first direction and thereby actuating the control screw 31 which withdraws the piston 5, moving the head 6 thereof away from the end 4 of the pumping chamber 3, so as to increase the useful internal volume thereof. During the withdrawal of the piston 5, the corrugations 10 of the sleeve 9 move together. Furthermore, the three-way valve 35 is controlled into the intake position so as to place in communication the intake pipe 30, and therefore the tank of fluid product, with the pumping pipe 38. The fluid contained in the tank can thereby fill the additional volume produced in the pumping chamber 3. The shutter pressure which is established by the depth of the tank, the intake speed of the piston 5 and the speed of the fluid product processed, as well as the diameters of the various pipes, are parameters which are considered in the projection of the dispensing system and the electronic logic unit in order to ensure correct priming of the positive-displacement pump 1 at the first actuation thereof, with an empty pumping chamber 3.

[0059] When the pumping chamber 3 is full of fluid product and the piston 5 is in the maximum withdrawal position thereof, the piston 5 can be actuated in the opposite direction, that is to say, in the advance direction, as a result of a reversal of the rotation direction of the stepping motor 22. In this case, the positive-displacement pump 1 can operate with recirculation or discharge of the fluid product, in accordance with the position taken up by the three-way valve 25. If the three-way valve 25 is maintained in the intake position, in which the pumping pipe 38 is in communication with the intake pipe 30 and the discharge pipe 39 is closed, then the fluid product contained in the pumping chamber 3 is urged again towards the tank which is connected to the intake pipe 30. That is a recirculation or movement condition of the fluid product which allows it to be kept moving in order to prevent sediments or dry matter in the pipes which extend from the tank to the pumping chamber 3. Furthermore, the movement of the piston 5 in an advance direction allows recovery of the mechanical play in order to bring the system into a zero predetermined condition, which is important for the precise and repeatable adjustment of the discharge of fluid product. Finally, the advance movement of the piston 5 with recirculation promotes the discharge of any air which may be contained in the fluid product and which has been accumulated naturally in the upper region 42 or in the pumping pipe 38 and which is urged back towards the tank through the intake pipe 30.

[0060] The switching of the three-way valve 35 during the operation of the piston 5 in an advance direction allows a change from the above-described recirculation condition to the discharge condition, in which the fluid product contained in the pumping chamber 3 is urged through the pumping pipe 38 and redirected into the discharge pipe 39 as far as the nozzle 40, while the intake pipe 30 is closed by the valve closure member 37. In the positive-displacement pump 1 of the present invention, it is particularly advantageous that the change to the discharge condition does not involve the interruption of the pressure applied by the piston 5 to the fluid product. This allows the fluid product to be maintained under pressure in such a manner that the influence of the air which may still be contained in the fluid product on the volume of the system is minimized, thereby reducing to a minimum the volumetric variations of the fluid product which would otherwise cause a low level of repeatability of the discharge in subsequent pumping operations. The first step of the advance travel of the piston 5, in the recirculation condition of the three-way valve 35, contributes to the elimination of any air contained in the fluid product. Subsequently, in the discharge condition, the pressure of the piston 5 on the fluid product during the advance travel may typically reach from 10 to 15 bar and at this pressure the influence of any residual air enclosed in the fluid product becomes almost insignificant from the point of view of volume. The result is that the performance levels in terms of volume and the precision of discharge of the positive-displacement pump 1 of the present invention are predictable, measurable and repeatable with a very high level of precision. In other words, the first step of the advance travel of the piston compresses the air enclosed in the fluid product and makes the system particularly immune to the influence thereof on the discharge performance levels of the positive-displacement pump. In the first advance step of the piston, therefore, the air present at the upper portion of the pump is discharged from the pumping chamber 3; the air remaining in the fluid product which is not discharged in this manner is compressed to a pressure which is in accordance with the shutter, the connections and the ambient pressure but which is still far greater than ambient pressure.

[0061] Furthermore, the fact that the piston 5 remains under compression during the switching to the discharge condition of the fluid product prevents any relaxation of the mechanical play which is taken up during the first step of the advance travel thereof, in the recirculation condition, substantially thereby also improving the precision and the repeatability of discharge of the fluid product.

[0062] The positive-displacement pump 1 described above can be produced with construction variants which make the production, maintenance and use thereof advantageous.

[0063] FIG. 4 illustrates a variant of the positive-displacement pump of the present invention which comprises a head group 2′ which is constructed to be separate from the pump body 2 and which is connected thereto using screws, bolts or the like (not illustrated). In greater detail, there is constructed in the head group 2′ an intake pipe 30′ with an integral intake opening 32′ for connection to a tank of fluid product (not illustrated). The pumping chamber 3 having the sliding piston 5 therein is inclined in such a manner that the upper region 42′ thereof, that is to say, the highest location thereof with respect to a horizontal plane, is substantially at the most advanced region of the substantially frustoconical end 4. The upper region 42′ is pierced so as to form an extremely short pumping pipe 38′ in communication with the valve chamber 34′ of the three-way valve 35′ which is actuated by the actuator 36′ which, in the configuration illustrated in FIG. 4, is advantageously inclined with respect to the vertical, with a reduction of the dimensions in that direction. There extends from the valve chamber 34′ the discharge pipe 39′ which is advantageously arranged in a position near the pumping chamber 3 in order to further reduce the dimensions of the positive-displacement pump in the horizontal direction. At the end 50 of the discharge pipe 39′, there is mounted a discharge nozzle 51 which is separate from the head group 2′ in order to allow easier replacement thereof in the event, for example, of blockages as a result of sediments.

[0064] FIG. 5 schematically illustrates another variant of the positive-displacement pump of the present invention which comprises a head group 2″ which is constructed to be separate from the pump body 2 and which is connected thereto using screws, bolts or the like (not illustrated), with the interposition of a seal 52. Unlike the embodiments described above, the head group 2″ does not comprise a three-way valve but instead a non-return intake valve 68 which is mounted on the intake pipe 30″, and a non-return discharge valve 69 which is mounted on the discharge pipe 39″. The switching between the intake condition and the discharge condition is brought about automatically by means of the unidirectional behaviour of the non-return valves 68 and 69. When the piston 5 moves back, the non-return discharge valve 69 closes while the non-return intake valve 68 is free to open so as to cause the fluid product to flow from the tank connected to the intake pipe 30″ to the pumping chamber 3 through the pumping pipe 38″. When the piston 5 advances, however, the non-return intake valve 68 closes while the non-return discharge valve 69 is free to open so as to cause the fluid product to flow from the pumping chamber 3 to the discharge nozzle connected to the discharge pipe 39′. It is possible to provide for an external controlled valve, which is preferably a three-way valve, in order to actuate the recirculation of the fluid product towards the tank, as will become clearer below with reference to the example of FIG. 8.

[0065] Advantageously, the head group 2″ can be interchanged with a head group which is similar to the one in FIG. 4 and which is also shown in FIGS. 6a and 6b in a slightly different configuration. FIG. 6a in particular shows the positive-displacement pump having the valve 35′ in an intake condition. In this case, the closure member 37 closes the opening of the discharge pipe 39′ and a backward movement of the piston 5 allows the fluid product contained in the tank which is connected to the intake opening 32′ to flow through the intake pipe 30′ in accordance with the direction of the arrows A in order to fill the pumping chamber 3. However, FIG. 6b shows the same positive-displacement pump having the valve 35′ in a discharge condition. In this case, the closure member 37 closes the opening of the intake pipe 30′ and an advance movement of the piston 5 allows the fluid product contained in the pumping chamber 3 to be discharged from the pumping pipe 38′ and then to pass through into the discharge pipe 39′ in accordance with the direction of the arrows E in order to arrive at the discharge nozzle (not shown in the Figure) which is positioned at the end of the discharge pipe 39′.

[0066] The cylindrical capacity of the positive-displacement pump 1 is given by the maximum useful volume of the pumping chamber 3 when the piston 5 is in the zero position, which is found, for example, by the sensor 25, and corresponds to the position in which the three-way valve 35 or 35′ can be switched from the recirculation condition to the discharge condition after the mechanical play has been taken up and the air in the fluid product contained in the pumping chamber 3 has been eliminated or compressed.

[0067] The maximum quantity of fluid product which can be discharged in a single advance travel of the piston 5 is proportional to the cylindrical capacity of the positive-displacement pump 1.

[0068] The resolution of the positive-displacement pump, that is to say, the minimum quantity of fluid product which can be discharged per single step of the stepping motor 22, is instead inversely proportional to the cylindrical capacity of the positive-displacement pump. The optimum cylindrical capacity of a positive-displacement pump depends on the intrinsic characteristics of the fluid product to be discharged, for example, the viscosity thereof, and on subjective characteristics in terms of consumption of the fluid product. For example, in the field of colorant dispensing machines, the dispensing of yellow colorant is normally far greater than the dispensing of the colour viola. For those reasons, it is advantageous to be able to provide a positive-displacement pump which, with little effort and cost, can be produced with a cylindrical capacity which is different in accordance with the use for which it is intended.

[0069] FIGS. 7a and 7b are partial sections of a pumping chamber 3′ of the positive-displacement pump according to the present invention which is without and with an adapter 55 for reducing the cylindrical capacity, respectively. In greater detail, FIG. 7a is a cross-section of a pumping chamber 3 which is similar to the one described above, with a pumping opening 56. As set out previously, the pumping chamber 3 has a constricted end 4, which is preferably substantially frustoconical and at the inner side of which a piston 5 slides with an external sleeve 9 of the bellows type.

[0070] When it is desirable to provide a positive-displacement pump with a smaller cylindrical capacity, for example, in order to change from a cylindrical capacity of 5 cc to a cylindrical capacity of 1.7 cc, it is possible to reduce the volume of the pumping chamber 3 by means of the adapter 55 which has a substantially cylindrical form with a constricted end 4′ and which is preferably substantially frustoconical and which is suitable for covering the internal walls of the pumping chamber 3 so as to produce a reduced pumping chamber 3′. The adapter has an opening 57 which is placed in correspondence and preferably substantially aligned with the pumping opening 56 of the pumping chamber 3. The adapter 55 has at the bottom an annular step 58 which, at the outer side thereof, moves into abutment with the abutment member 14 while, at the inner side thereof, it acts as an abutment for the annular portion 13′ having a greater diameter of a sleeve 9′ of a piston 5′ which has a total diameter which is smaller with respect to the piston 5 in order to adapt to the smaller volume of the pumping chamber 3′.

[0071] If the quantity of fluid product to be discharged is greater than the cylindrical capacity of the positive-displacement pump, the discharge operation which can be actuated with a single positive-displacement pump is necessarily discontinuous, because at the end of the advance travel of the piston 5 it is necessary to switch the three-way valve into the intake condition and to move back the piston 5 until the pumping chamber 3 moves back to fill with fluid product from the tank which is connected to the intake pipe 30, 30′. In some cases, there is a need to improve and make faster the discharge of the complete quantity of the fluid product, avoiding down times as a result of the filling of the pumping chamber 3. In those cases, it is advantageous to mount a pair of positive-displacement pumps which act in a non-parallel manner in such a manner that each of them takes fluid product from the same tank (or from an individual tank containing the same fluid product which is also charged in the tank of the other pump) when the other positive-displacement pump is in the discharge condition.

[0072] This continuous discharge behaviour for a fluid product can be brought about by means of the synchronized control of the pair of positive-displacement pumps by means of the electronic logic control unit, which provides for the synchronization of the movement of the respective stepping motors 22 and the three-way valves 35, 35′ of the two positive-displacement pumps, or also as a result of a configuration of a mechanical connection which will be described below with reference to FIG. 8.

[0073] FIG. 8 is a schematic illustration of a pair of positive-displacement pumps of the present invention which are connected in a non-parallel manner for discharging fluid product in a substantially continuous manner in quantities greater than the cylindrical capacity of each positive-displacement pump taken individually.

[0074] In greater detail, two positive-displacement pumps 1a, 1b each comprise a pumping chamber 3a, 3b which is similar to the pumping chamber 3 of the pump described above, in which there are mounted in a sliding manner respective pistons 5a, 5b which are capable of moving in a non-parallel manner: when the piston 5a advances in the pumping chamber 3a, the piston 3b moves back in the pumping chamber 3b thereof, and vice versa. Each piston 5a, 5b has a respective central shaft 8a, 8b which extends into a respective handling shank 19a, 19b which is connected to a respective handling screw 31a, 31b which is mounted on a transmission casing 60 in which there is received a gear mechanism 61 comprising two toothed wheels 62a, 62b having a gearing ratio of 1. Each toothed wheel 62a, 62b is supported in the transmission casing 60 by bearings 63 and is fitted to a handle 64a, 64b of a respective handling screw 31a, 31b. The handle 64a of one of the two handling screws 31a extends and is fixed to the drive shaft 65 of a single stepping motor 66 which is fixed to the transmission casing 60. The mutual angular position of the toothed wheels 62a, 62b is such that, when the handling shank 19a of one of the two pumps 1a is at the maximum extent thereof, the handling shank 19b of the other pump 1b is in the position of maximum withdrawal thereof. The actuation of the stepping motor 66 brings about the non-parallel movement of the two handling shanks 19a, 19b and therefore of the pistons 5a, 5b of the two pumps 1a, lb.

[0075] FIG. 8 also illustrates an alternative configuration of the intake and discharge pipes of the pumps 1a, 1b, each comprising non-return valves in place of the three-way valves. In greater detail, the intake pipes 30a, 30b of the two positive-displacement pumps 1a, 1b each have a non-return valve 68a, 68b which allows the fluid product from a tank 67 to be introduced into the respective pumping chambers 3a, 3b during the withdrawal movement of the corresponding piston 5a, 5b, and prevents the return of fluid product to the tank 67 through the same intake pipe 30a, 30b. Similarly, there are mounted on the respective discharge pipes 39a, 39b non-return valves 69a, 69b which allow the discharge of fluid product towards a common three-way valve 70, which communicates at one side with the tank 67 and at the other side with a common discharge pipe 71, which terminates with a common nozzle 72. The switching of the common three-way valve 70 allows the fluid product discharged by each positive-displacement pump 1a, 1b during the advance travel of the respective piston 5a, 5b to be conveyed alternately towards the tank 67, in a recirculation condition, or towards the common discharge pipe 71 and the nozzle 72 for the discharge of fluid product.

[0076] Naturally, the principle of the invention remaining the same, the forms of embodiment and construction details may be varied widely with respect to those described and illustrated, without thereby departing from the scope of the present invention.