US3770214A

US3770214A - Fine grinding device

Info

Publication number: US3770214A
Application number: US00197273A
Authority: US
Inventors: K Gabor
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-11-10
Filing date: 1971-11-10
Publication date: 1973-11-06
Anticipated expiration: 1990-11-06
Also published as: JPS4844852A; BE775091A; CA948170A; IT942811B; ES204736Y; ES204736U

Abstract

A comminuting device for materials in a dry or wet phase comprises a housing with a rotatable actuator therein which is in the form of a feed screw having at least two oppositely directed feed sections on a common shaft. Balls are freely displaceable in the housing to assist or promote the comminution process. Housings of various cross-sectional shapes are described which can be used for devices for intermittent and/or continuous operation.

Description

United States Patent 1191 Gabor v Nov. 6, 1973 FINE GRINDING DEVICE 1,242,445 10 1917 lttner 241/46 R ux 2,336,798 12/1943 Nash [76] lnvemol- Kama" F 9 des 2,221,348 11/1940 Hershey 51/263 x Dardanelles, Paris, France 0 G A O F REI N P TENTS R APPLICATIONS [22] 1971 1,359,589 3/1964 France 241/172 [21] Appl. No.: 197,273

[30] Foreign Application Priority Data Nov. 10, 1970 France 7040365 Nov. 13, 1970 France 7040704 [52] US. Cl 241/42 241/43, 241/46.17,

241/153, 241/172 [51] Int. Cl. B02c 17/16 [58] Field of Search 241/42, 43, 46 R, 241/4602, 46.17, 153, 172

[56] References Cited UNITED STATES PATENTS 1,524,605 l/l925 White 241/4617 X Primary Examiner-Donald G. Kelly Attorney-Ernest G. Montague 57 ABSTRACT A comminuting device for materialsin a dry or wet .phase comprises a housing with a rotatable actuator 23 Claims, 10 Drawing Figures PAIENTEUNUV E 'fioiziza SHEET 3 0F 4 1 FINE GRINDING DEVICE The present invention relates to a comminuting device for grinding or milling materials and for mixing them in a fluid state, the device permitting particles of extremely small dimensions to be obtained with a relatively low expenditure of energy.

It is known that the preparation of paints, printing inks, various colouring materials, pharmaceutical products, cosmetic products and other materials requires a comminuting process. to be performed to obtain of powder materials generally in mixture, in which the solid granulae have been reduced to dimensions of the order of ten microns.

In order to obtain these fine grades of comminution grinders or mills are already known which comprise casings in which hard balls intended for crushing the solid particles are contained and in which these balls are set in motion either by the rotation of the said casing itself, or by means of an actuator disposed therein.

The result obtained is on the whole satisfactory, but the energy transmitted to the grinding or milling device is dissipated by impact or by wear-of the pieces in motion.

The present invention has an object a comminuting device the energy consumption of which is reduced owing to the fact that impact shocks are avoided and that displacement of the, material to be comminuted is effected at a small angle in relation to the wall of the device and to the elements in motion.

In a fixed housing containing balls or pellets and the particular material to be comminuted, there is disposed according to the invention a rotary actuator in the form of a shaft supportingat least two successive portions of an Archimedian screw or feed screw, the two portions being ,of opposite sense.

The two feed screws are preferably, but not necessarily of the same diameter and the same pitch. Moreover,

except at their extremities, their diameter may be uniform. Y

Preferably the shaft is vertical and is arranged in such manner that the screw portions are completely immersed in the mixture of balls and the material to be comminuted or ground.

Moreover, the housing isin the case also vertical. Nevertheless its horizontal cross-section is advantageously non-circular in such manner that the walls of this housing are not located at a constant distance from the cylindrical envelope enclosing the screws.

The cross-section is advantageously in the form of a polygon the sides of which may be rectilinear or curved.

In each part of the vertically disposed housing which is occupied by one of the screw portions there is produced an axial current in the mass or mixture of material in contact with the respective screw portion, which current is ascending or descending according to the direction of the sense of the screw, and around the one screw portion there is produced a current which comprises an axial component of opposite sense to the current around the other screw portion; there is also produced a horizontal rotation component which corresponds to the direction of rotation of the screw. All

these currents encounter the wall or walls of the housing and the surface of the screws at small angles'and slide in contact with these different surfaces.

Moreover, at the periphery of each screw there exist by reason of the cross-section of the housing, zones in which the wall approaches the cylindrical envelope surrounding the screw in accordance with the direction of rotation of the screw and where compression of the mass or the mixture occurs, and there exist other zones in which the wall recedesfrom the said cylindrical envelope and where in contrast expansion of the mass or mixture occurs.

Preferably the screw portions are disposed at adjacent locations on the shaft which carries them in such manner that the mass or mixture engaged by the screws is divided-into currents which meet without mutual impact, but are pressed against each other at the level at which the feed direction changes.

in fact, between two screw. portions of opposite sense the currents induced by each of the screws have the same sense, either centripetal or'centrifugal, and the streams of fluid current engaged by the two consecutive screw portions are pressed against 'each other by reason of the screwportions approaching each other.

In a preferredconstructional form of the invention, the housing has a cross-section in the form of acircle sector, or in other words of a triangle the base of which is a circular arc; this construction permits several housings to be juxtaposed in a complete cylinder and facilitates the realisation of a complex grinder or mill with a plurality of compartments. I

Generally, for continuous operation, several grinder or mill compartments are associated one with the other and are fed in series with the material to be ground or milled. This operation causes the materials to be mixed and ground down to pass through a succession of continuously operating mixer grinders having vertically disposed axes and similar capacities, but different mechanical characteristics.

This process is preferably performed, as has been mentioned above, with a continuous operating mixing and grinding apparatus constituted by a single housing which is compartmented by vertical walls permitting material to be comminuted to pass in series from one compartment to the other, eachv compartment being equipped with a vertical rotary actuator If the material to be comminuted is associated in mixture with a liquid vehicle and the grinding or milling compartments are watertight, the material is-injected under pressure into this series of compartments, the connection of each compartment with the following one comprising a restrictor device for controlling the passage of the mixture.

Thus the inlet pressure ensures not only the circulation of the mixture from one compartment to the other, but moreover a pressure can be maintained in one or more successive compartments on account of the pressure at which the mixture is injected, which can be close to the inlet pressure and which is favourable for the grinding process, whereas the circulation of'the material to be comminuted is being slowed down.

- Preferably the pressure in each of the compartments is controlled by a measuring device whereby the pres- .sure drop in the compartments can be adjusted in accordance with the desired result.

Preferably the various compartments are actuated as described above in that the pressure control valves are located in external pipe sections which connect the top of one compartment to the bottom of an adjacent compartment, all the compartments being produced by radial partitions in one vertical cylinder.

In one preferred constructional form of the invention, the actuators, that is to say sections of Archimedian screw of opposite sense carried by the same vertical shaft as described above, are driven by hydraulic motors which are fed from a common source which supplies liquid under pressure and which feeds moreover a pump by which the mixture is injected under pressure into the first of the grinding or milling compartments.

Thus a series of actuators with a wide rotary'speed range, for example from zero to 1500 revs. minute can be obtained without a speed reducing device or a variable speed device, and at the same time there is extreme safety if inflammable or explosive materials are being treated.

The common source may be a common hydraulic device, such as a single hydraulic pump possibly associated with a hydraulic accumulator, which is arranged to feed each of the hydraulic motors by way of a pressure regulator with a calibrated valve and an outlet control device.

Each hydraulic motor may be associated with a pump suitable for the power of this motor, and it is possible to operate all these driving fluid delivering pumps by a common motor.

Several embodiments of the invention are described below by way of example with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic vertical section through a grinder or mill according to the invention;

FIG. 2 is a diagrammatic illustration of a preferred constructional form of the grinder or mill according to the invention,

FIG. 3 is an elevation, partly broken away, of a practical embodiment of the invention,

FIG. 4 is a plan view of this latter constructional form,

FIG. 5 is a plan view of a grinder or mill with four compartments,

FIG. 6 is a perspective view of the grinder or mill according to FIG. 5,

FIG. 7 illustrates diagrammatically an assembly of grinders or mills according to the invention,

FIGS. 8 and 9 illustrate in diagrammatic form an elevation and a plan view, respectively, of a grinder or mill arrangement, and

FIG. 10 is a complete connection diagram of a grinder or mill system.

A comminuting device, grinder or mill illustrated in FIG. 1 comprises a housing 1 with vertical walls and any kind of horizontal cross-section, and a vertical shaft 2 arranged to be driven by a motor 3. The shaft carries two portions of Archimedian screws or

feed screws

4 and 5 which are of the same pitch but opposite sense; moreover, the screws have the same envelope diameter D.

The housing contains a material to be comminuted or ground which may be a powder or a mixture of powders, in the dry state or also incorporated in a liquid vehicle in such manner that it forms a paste as well as balls of steel, ceramic, glass, steatite, or any other hard elements the balls being either all of the same diameter or of different diameters.

The material to be ground may be introduced through a lateral inlet opening 6 of the housing and may be discharged through an outlet opening which is provided with an appropriate closure and comprises a filter 8 adapted to retain the balls, but permitting the comminuted material to leave. a

When the shaft 2 is set in rotation, each of the

screw portions

4 and 5 produces according to the sense of rotation and independently of the all-over rotary movement effected in the mass by the rotation of the screw, an axial current around the shaft 2, and an axial current of opposite direction combined with the said rotary movement in the space comprised between the cylindrical envelope of the screw and the wall of the housmg. Preferably, as illustrated in FIG. 2, the axial currents adjacent to the shaft are descending in the lower portion of the screw 5 and ascending in the upper portion, the corresponding currents close to the walls being in the opposite direction.

Thus, at a level N at which the sense of the screws is reversed, the currents C3 and C3 of the two circulations are pressed against each other and an intensive rolling effect is produced between the balls which are located there. This rolling effect, moreover, increased in intensity by the fact that the currents C3 and C3 are centripetal, that is to say, because the material which is conveyed by them is progressively pressed into a more reduced space.

Experience has shown that in order to obtain in the engaged mass a direct axial current which is reasonably uniform and which is produced with the lowest possible expenditure of energy, it is preferable to use a screw portion the development of which is substantially equal to 2% turns.

A practical constructional example is illustrated in FIGS. 3 and 4.

The housing 1 has in cross-section the form of circle sector occupying a quarter circumference. Its two flat walls are sheathed by an angled plate 9 which is fixed thereto by means of sealing devices 11 in order to define a cooling water jacket connected to feed pipes and outlet pipes 10.

The motor 3 drives the shaft 2 by means of

pulleys

12 and 13 and a belt 14.

The motor is connected to a timing device 15 having a control range of for example from 1 to 60 minutes. The whole assembly is enclosed in a casing 16 which is perforated and louvred for ventilation and which is mounted overlying a base 17. The openings 6 and 7 are controlled by valves 18 in the form of sectors.

This apparatus may be utilised with discontinuous operation, that is to say by charging it through the upper opening 6 with a limited quantity of material and setting it to rotation for a period of time determined by the timing device 15.

Alternatively it may be fed continuously, but in this case through the lower orifice 7, the continuous flow escaping as overflow through the upper opening 6. In this case, the continuous feed may be obtained, when the apparatus operates in liquid phase, from a charge reservoir or also by means of a feed pump.

When, the apparatus operates dry, the feed from a hopper may be effected by means of an Archimedian screw or feed screw. The continuous flow of dry particles may be extracted from the outlet by a suction device and separated from the air current by filtering.

It will be noted that the lower opening 7 is placed, as shown in FIG. 2, at the level of the lower helical portion and that a suction effect for the material supplied is produced because of the vicinity of the level N at which the. helices change direction, in such manner that for operating direction indicated in FIG. 2, the lower part of the apparatus operates like a suction pump.

When, for continuous operation, a plurality of devices are mounted in series; it is therefore advisable to connect the upper overflow of one preceding housing to the lower opening of the following housing, which latter opening is located a little below the level N.

The assembly may be realised as illustrated in FIGS. 5 and 6. 1

Four

housings

1A, 1B, 1C and ID are realised.by.di-. viding a cylinder 20 by means of two crossed

partition walls

21 and 22. Each of the housings accommodates an actuator 2 fitted with two (or more than two) screw portions of opposite pitch directions, the whole being actuated in such manner that at the level at which the sense between the two screws .changes direction and nearest to the bottom, the converging material currents are centripetal.

In the wall of the cylinder 20 are provided two windows 23 for each housing one at the top, the other at the bottom.

To each window is attached with the interposition of a sealing device, a box 24, for example, by means of a compression nut 25 whcih is preferably operable by hand in order to permit quick mounting and dismounting. Each box comprises a connection for a tube 26. It contains, moreover, a filter or grid 30 which retains the balls in each housing.

Also, the material to be treated enters the lower por tion of the housing IA and leaves the upper portion of the latter in order to enter the lower portion of the next following housing 18, and so on, until the product leaves the upper portion of the housing 1D.

It must be understood that the upper outlet windows 23 of each housing are provided with filters, grids or strainers in order to retain in each housing, the grinding balls which it contains.

By dismantling the nuts 25 the connecting tubes between the containers can be easily cleaned when the material to be ground is changed. Moreover it is easy to change the grids 30 for retaining the balls in order to adapt them to the diameter of the latter.

In a device such as illustrated in FIGS. 5 and 6, the

material which is comminuted in a moist or dry phase,

may be treated in the same manner in each housing that is to say with balls of the same diameter and of the same quantity, and with shafts 2 carrying identical helices rotating at the same speed. It is also possible to use balls of different diameters or different speeds in each of the housings.

For example the balls in the four housings may be of decreasing diameters. By way of example, in the four housings the balls may have diameters of 6; 4; 2.5; and 1.5 mm, respectively.

If the balls of all housings have the same diameter and the actuators rotate at te same speed, it is possible which is connected to the following one by a pipe reto establish a correlation betweenthe grinding time for one dimension by means of a simple apparatus such as illustrated in FIGS. 3 and 4 and the continuous hourly output of an apparatus such as illustrated in FIGS. 5 and 6. l

The longer the grinding time of one dimension is, the more must the output be reduced.

spectively a1, a2, a3, in each of which a valve b1, b2, b3, respectively, is disposed afurther valve b4 being located at the output of compartment C4. Each pipe connects the upper portion of preceding compartment to the lower portion of the next following compartment.

The compartments are fed in series under pressure by a pump Pwith the material to be ground in mixture with a liquid. When all valves bl, b2, .b3 and b4 are open, the pressure of the pump is dissipated progressively in the four compartments as far as an outlet S. When progressively the valve bl is closed, the pressure in the compartment C1 increases until it approaches the pressure of the pump P, while the circulation speed of the mixture decreases. Thus, the pressure in the compartments Cl and C2 can be maintained in a similar way by closing the valve b2.

Complementary valves cl and c2 may be provided to permit the mixture to be re-cycled by guiding it back upstream of the pump. Finally, valves d1 and d2 control respectively the inletand the outlet of the mixture.

The compartments C1, C2, C3 and C4 are preferably provided in a cylinder 1 by means of

rectangular partitions

42 and 43. The various pipes a1, a2 and a3 are located externally of this cylinder; they may be disposed as indicated herebefore.

For the drive of each of the actuators e1, e2, e3, e4 (FIG. 10) respectively contained in each of the cornpartments C1, C2, C3 and C4, there are preferably used hydraulic motors M1, M2, M3 and M4. The actuators themselves are preferably constituted by vertical carrier shafts each having at least two' Archimedian screw portions of opposite sense, as also described above. Moreover, the pump P is also driven by a hydraulic motor M.

Each of the hydraulic motors receives fluid under pressure from a driver pump, respectively P0 in the case of the motor M and P1, P2, P3 and P4 in the case of the motors M1, M2, M3 and M4. The five pumps are carried by a common shaft 44 which is driven by an electric motor 45, preferably of the flame-proof type. The pumps draw hydraulic fluid from an oil tank 6 through filters F0, F1, F2, F3 and F4.

The feed pressure is adjusted by means of pressure regulators with calibrated controlled valves g0, g1, g2, g3 and g4, and the quantity of liquid distributed to each motor is determined by a respective control valve h0, hl, h2, h3 and k4. As indicated by arrows, the leakages from the various motors return to the reservoir.

Speed indicators j0, jl, j2, j3 and j4 indicate the rotational speed of the various motors. However, whereas the speed indicators jl, j2, j3 and j4 are normally graduated in revs.per minute, the speed indicator j0 which is associated with the volumetric pump P serves as an output meter and may be graduated in litres per hour.

As before, the valves b1, b2, b3 and b4, permit the control of the pressure in the various compartments. This pressure is indicated by the pressure gauges kl, k2, k3 and k4, whereas the pressure gauge k0 indicates the initial pressure.

Moreover, the installation may comprise cocks ml, m2, m3 and m4 for the removal of samples of the comminuted product.

Finally, a control device n for the temperature of the final product may be mounted at the outlet of the apparatus.

Such installation permits the output of the treated product to be varied by controlling the pump P, varying the speed of each of the actuators varying the internal pressure in the compartments, and by adjusting the temperature in operation. As indicated in the first part of this description, the control may be at least approximately effected by an adjustment of the output as a function of the grinding period, in a single comminuting device, of a defined sample quantity.

It is moreover possible to vary the diameter of the balls and their nature. For example, balls of steel, glass or steatite may be used, the diameters of which vary in steps from 6 mm to 1.5 mm in decreasing order.

It will be noted that a number of compartments lower than four can be used in the example illustrated and if necessary a single compartment only, and the apparatus may even be divided into two groups of two compartments in order to obtain simultaneously comminution of two different materials. It is, however, preferably in this latter case to provide a second feed pump.

The invention is useful as already indicated for fine grinding in a liquid phase as well as in a dry phase, pharmaceutical products, cosmetic products, foodstuffs and so on. Also, paints, printing inks and, in a general way, all colouring materials may be ground and dispersed by apparatus constructed in accordance with the invention. According to the nature of the material to be treated and the desired grinding capacity, the volume of the housings may vary from a few litres to a few cubic metres.

two portions of Archimedean screws of opposite sense, respectively, mounted thereon. 3. A device according to claim 1, wherein said shaft is vertical.

4. A device according to claim 3 wherein said housing is vertical, and defining walls located at different distances from an imaginary cylindrical envelope enclosing said screw means. I

5. A device according to claim 4 wherein the crosssection of the housing is in the form of a sector ofa circle.

6. A device according to claim 1, wherein said Archimedean screwmeans are adjoining.

7. A device according to claim 1, wherein the direction of rotation of the rotary shaft and the disposition of the screws are such that at the lowermost level at which the sense of the screws changes direction, the currents in the mixture are centripetal.

8. A device according to claim 1, wherein the direction of rotation of the rotary shaft and the disposition of the screws are such that at the lowermost level at which the senseof the screws changes direction, the currents of the mixture are centrifugal.

9. A device according to claim 1, wherein a portion of the Archimedean screw comprises 2% screw turns.

10. A device according to claim 3 wherein the vertical shaft carries only two superimposed Archimedean screw portions, the lower portion being the shorter one.

11. A device according to claim 1 comprising a plurality of said housings defining containers, each defining a volume in the shape of a cylindrical sector, and said various containers communicating in series with each other.

12. A device according to claim 10, comprising a plurality of said housings defining containers, each defining a volume in the shape of a cylindrical sector, said various containers communicating in series with each other,

said containers communicate with each other by pipes which are external to a cylindrical volume defined by said plurality of cylindrical sectors, and which connect an upper part of one housing to the lower part of the next following housing in the zone thereof comprising the lower screw portion.

13. A device according to claim 10, comprising a plurality of said housings defining containers, each defining a volume in the shape of a cylindrical sector, said various containers communicating in series with each other,

said housings communicate with each other by pipes which are external to a cylindrical volume defined by said plurality of cylindrical sectors, and which connect a lower part of one housing to the upper part of the next following housing in the zone thereof comprising an upper of said screw means.

l4.'A device according to claim 12, wherein each housing comprises in its cylindrical wall an upper window and a lower window to each of which is attached from the outside a connecting box provided with or adapted to receive a pipe for connecting two housings.

15. A device according to claim 1, wherein said housing constitutes a single housing in the form of a cylindrical sector, the plane walls of which comprise a jacket for the circulation of a cooling fluid, and

a motor means for said actuator means which is controlled by a timing device.

16. A device as claimed in claim 1 adapted more particularly for materials in a liquid phase, comprising a plurality-of compartments mounted in series, wherein the compartments are watertight, wherein the material is injected under pressure into the compartments which are connected in series, andwherein the connection of each compartment to the following compartment and the outlet of the last one compartment comprises each a valve adapted to restrict the passage of the material.

17. A device according to claim 16, wherein upstream of each valve, there is mounted a pressure gauge for indicating the operating pressure, whereas at the inlet of the first compartment, there is mounted a pressure gauge indicating the pressure at which the material is injected.

18. A device according to claim 14, wherein the connecting pipes between said containers are conduits mounted externally of the latter, and control valves being disposed in said conduits.

an output control valve.

21. A device according to claim 19, wherein each of the hydraulic motors is associated with a feed pump at the outlet of which are disposed a pressure regulator 19. A device according to claim 14, wherein the maand an output control valve.

terial to be comminuted is injected under pressure into a first of said containers by a volumetric pump driven by a hydraulic motor, each of the actuator means of the various containers being also driven by a hydraulic motor.

20. A device according to claim 19, wherein the hydraulic motors are fed by a source of fluid under pressure, each through an adjustable pressure regulator and 22. A device according to claim 19, wherein each of the hydraulic motors comprises a revolution counter, the revolution counter of the motor driving the pump being graduated in output units.

23. The comminuting device, as set forth in claim 1, wherein said housing is circular in cross section.

Claims

1. A comminuting device comprising a housing, an actuator means rotatably mounted in said housing, said housing being provided with inlet and outlet openings for a material to be comminuted, and said actuator means including a shaft having mounted thereon at least two Archimedean screw means of opposite sense, respectively.

2. A comminuting device comprising at least one fixed housing containing a mixture of balls and a material to be comminuted, and an agitator for setting the mixture in movement, said agitator comprising a rotary shaft having at least two portions of Archimedean screws of opposite sense, respectively, mounted thereon.

3. A device according to claim 1, wherein said shaft is vertical.

4. A device according to claim 3 wherein said housing is vertical, and defining walls located at different distances from an imaginary cylindrical envelope enclosing said screw means.

5. A device according to claim 4 wherein the cross-section of the housing is in the form of a sector of a circle.

6. A device according to claim 1, wherein said Archimedean screw means are adjoining.

8. A device according to claim 1, wherein the direction of rotation of the rotary shaft and the disposition of the screws are such that at the lowermost level at which the sense of the screws changes direction, the currents of the mixture are centrifugal.

9. A device according to claim 1, wherein a portion of the Archimedean screw comprises 2 1/2 screw turns.

12. A device according to claim 10, comprising a plurality of said housings defining containers, each defining a volume in the shape of a cylindrical sector, said various containers communicating in series with each other, said containers communicate with each other by pipes which are external to a cylindrical volume defined by said plurality of cylindrical sectors, and which connect an upper part of one housing to the lower part of the next following housing in the zone thereof comprising the lower screw portion.

13. A device according to claim 10, comprising a plurality of said housings defining containers, each defining a volume in the shape of a cylindrical sector, said various containers communicating in series with each other, said housings communicate with each other by pipes which are external to a cylindrical volume defined by said plurality of cylindrical sectors, and which connect a lower part of one housing to the upper part of the next following housing in the zone thereof comprising an upper of said screw means.

14. A device according to claim 12, wherein each housing comprises in its cylindrical wall an upper window and a lower window to each of which is attached from the outside a connecting box provided with or adapted to receive a pipe for connecting two housings.

15. A device according to claim 1, wherein said housing constitutes a single housing in the form of a cylindrical sector, the plane walls of which comprise a jacket for the circulation of a cooling fluid, and a motor means for said actuator means which is controlled by a timing device.

16. A device as claimed in claim 1 adapted more particularly for materials in a liquid phase, comprising a plurality of compartments mounted in series, wherein the compartments are watertight, wherein the material is injected under pressure into the compartments which are connected in series, and wherein the connection of each compartment to the following compartment and the outlet of the last one compartment comprises each a valve adapted to restrict the passage of the material.

19. A device according to claim 14, wherein the material to be comminuted is injected under pressure into a first of said containers by a volumetric pump driven by a hydraulic moTor, each of the actuator means of the various containers being also driven by a hydraulic motor.

20. A device according to claim 19, wherein the hydraulic motors are fed by a source of fluid under pressure, each through an adjustable pressure regulator and an output control valve.

21. A device according to claim 19, wherein each of the hydraulic motors is associated with a feed pump at the outlet of which are disposed a pressure regulator and an output control valve.

22. A device according to claim 19, wherein each of the hydraulic motors comprises a revolution counter, the revolution counter of the motor driving the pump being graduated in output units.