Abstract:
A positive-displacement rotary pump comprising a pair of meshing gears ( 13, 14 ) or rotors, consisting of a driving gear and a driven gear, contained in a shell ( 10 ) having an output opening and an intake opening for a fluid. The gears ( 13, 14 ) include shafts ( 23, 24 ) which are supported by bushings ( 15   a ,  15   b ) having two faces ( 17   a ,  17   b ) which are subjected, in use, to pressures which bring about an axial load on the bushing itself, wherein the resultant of the axial loads (S′, S″) on the two bushings has a predetermined direction so as to move the bushings ( 15   a ,  15   b ) and the gears ( 13, 14 ) as a whole into close abutment with a predetermined reference plane (VI—VI).

Description:
FIELD OF THE INVENTION 
   The present invention relates to positive-displacement rotary pumps and specifically gear pumps. 
   BACKGROUND OF THE INVENTION 
   Gear pumps generally comprise two gears, one of which, known as the driving gear, is connected to a drive shaft and causes the other wheel, known as the driven gear, to rotate. The pumps of this type for high pressures are generally produced with a so-called “balanced” or “equilibrated” configuration, in which the two opposing faces of the bushings for supporting the gears are subjected to pressures over areas which, although they are large in absolute terms, are not very different from each other in order to generate a moderate differential force which tends to keep each bushing in contact with the gears. 
     FIGS. 1 and 2  illustrate an example of a gear pump of known type. In particular,  FIG. 1  is a longitudinal section along a plane which extends through the axes of rotation of the two gears, and  FIG. 2  is a section taken on line II—II in FIG.  1 . The driving gear  13  and the driven gear  14 , whose shafts are each supported by two bushings  15 , are housed inside a shell  10  which is closed by a front cover  11  and a rear cover  12 . Omega-shaped (Ω) seals  16  which separate the intake zone (A), at lower pressure, from the output zone (M), at higher pressure, are housed on the outer face  17   a  of the bushings  15 . During use, the bushings  15  are subjected to a pressure both on the outer faces  17   a  thereof and on the inner faces  17   b  thereof. The omega-like configuration of the seals  16  is such that the portion of outer face  17   a  of each bushing  15  on which the output pressure, which is greater than the intake pressure, acts is greater than the portion of inner face  17   b  of the bushing which is subjected to the same output pressure. Since the area on which the output pressure acts in the region of the inner face  17   b  of each bushing cannot be determined with accuracy, the optimum configuration of the omega-like seal  16  is usually identified by trial and error. 
   Owing to the difference between the pressures which act on the two faces, the outer face  17   a  and inner face  17   b , the bushings  15  are urged with a force which is moderate, and controlled, against the gears  13  and  14  so as to minimize the leakages over the faces of the gears themselves as a result of the difference in pressure between the intake and output. In the prior art, therefore, the two bushings are floating in an axial sense. 
   Obtaining good leak-tightness between the intake and output is one of the principal objectives in the production of gear pumps. In fact, the efficiency of pumps of this type declines rapidly if the leak-tightness is not total. Another problem which the manufacturers of pumps have to deal with is the noise of the pumps themselves, owing to irregular phenomena, or “ripples”, in the transfer of the fluid. A study of the above-mentioned problems linked to the design of gear pumps is set out in “C. Bonacini, Sulla portata delle pompe ad ingranaggi (On the efficiency of gear pumps), L&#39;ingegnere, 1961 n. 9”. 
   The above-mentioned solutions of the prior art have the common problem consisting in the noise of operation caused by the instantaneous oscillations of the output over time, better known as ripple noise. The above-mentioned oscillations generate a pulsating wave which, by way of the fluid, is transmitted to the surroundings and, in particular, to the walls of the pump, to the pipes and to the output ducts. The noise produced can reach levels which are also unpredictable where the above-mentioned members begin to resonate with the frequency of oscillation or ripple. 
   In addition to this, the rotation of the gears causes a periodic variation in the area of the inner face  17   b  of the bushings  15  that is exposed to the output pressure. This variation determines oscillations in the axial loads on the bushings, which contributes to an increase in the noise of the pump, besides reducing the total efficiency thereof. This oscillation of the axial loads, which is normally of small magnitude in gear pumps having straight teeth, becomes significantly greater in gear pumps having helical teeth, in which the meshing between the gears is the cause of both mechanical and hydraulic axial loads such that the balance and the taking-up of clearances on the bushings illustrated in  FIGS. 1 and 2  is not completely satisfactory, since the hydraulic axial loads have perceptible pulses. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide a positive-displacement rotary pump which overcomes the disadvantages of the prior art and, in particular, which substantially reduces the noise without resulting in a substantial increase in the cost and complexity of production in comparison with pumps of known type. A further object of the invention is to provide a pump which has good leak-tightness characteristics between the intake and output, which is simple and economic to produce and maintain and which has good reliability over time. 
   In order to achieve the above-mentioned objects, the subject-matter of the invention is a positive-displacement pump which comprises the features indicated in the claims appended to the present description. 
   One advantage of the present invention consists in that the axial position of the rotors is unambiguously defined even in the event that they are subjected to axial loads or pressures owing to mechanical contact with the shell or portions thereof. In fact, it is known that, in the running-in stages of pumps of known type, it is accepted and desirable for there to be contact of portions of the rotors with the shell so that the rotors remove an extremely small layer of material until an individual seat has been “scooped out”, in such a manner that, when the pump is used after the running-in operation, the clearance between the teeth of the rotors and the shell has minimal dimensions. This slight interference between the helical teeth of the rotors and the shell of the pump produces additional axial loads on the rotors, which mainly have an unknown value. The present invention, by providing a fixed plane of reference, also ensures the correct positioning of the rotors in the initial running-in stage of the pump, and even in the event of interference between the rotors and the shell, when unknown axial forces resulting from the above-mentioned mechanical contact are added to the axial forces expected in normal operation of the pump. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other characteristics and advantages of the invention will become clear from the following detailed description which is given with reference to the appended drawings which are provided purely by way of non-limiting example and in which: 
       FIGS. 1 and 2  illustrate, as described above, a gear pump of known type, 
       FIG. 3  is a longitudinal section of a gear pump according to the present invention, substantially similar to  FIG. 1 , in which identical reference numerals identify corresponding elements, 
       FIG. 4  is a perspective view of a bushing set of the pump in  FIG. 3 , 
       FIG. 5  is a plan view of an intermediate plate of the pump, taken in the plane of the line V—V in  FIG. 3 , 
       FIG. 6  is a plan view of the front cover of the pump, taken in the plane of the line VI—VI in  FIG. 3 , 
       FIG. 7  is a top view of a variant of one of the two components of a bushing set, configured to promote the hydrodynamic lubrication thereof, and 
       FIG. 8  is a cross-section, drawn to an enlarged scale, taken on line VII—VII in  FIG. 7 , in which the depth of the depressions or channels of the bushing has been greatly increased for clarity of description. 
   

   DETAILED DESCRIPTION 
   Now with reference to  FIG. 3 , a gear pump  20  comprises, as already described above with reference to the known pump in  FIGS. 1 and 2 , a shell  10  having an output opening and an intake opening for a fluid, inside which are housed the driving gear  13  and the driven gear  14 . In the example in the Figure, the gears  13  and  14  are of the cylindrical type having helical teeth, but naturally the invention can also be used with different gears, for example, a pair of straight spur gears, similar to that in the prior art illustrated in FIG.  1 . 
   The shell  10  is closed at the two ends by the front cover  11  and the rear cover  12 . The end  21  of the shaft  23  of the driving gear  13  protrudes from the front cover  11 . For ease of illustration, the seals between the shaft  23  and the front cover  11  have been omitted in FIG.  3 . Inside the shell  10 , the shafts  23  and  24  of the gears  13  and  14 , respectively, are supported by two bushing sets, a front bushing set  15   a  and rear bushing set  15   b . Each of the bushing sets  15   a  and  15   b  can be produced in one piece, as in the case of the known pump in  FIG. 1 , or preferably in two separate pieces  22   a ,  22   b , as illustrated in detail in FIG.  4 . This latter preferred solution is more economical and precise from the point of view of production since it is easier to obtain very high levels of working precision without any necessity for resorting to special machine tools. Furthermore, such a configuration of bushing sets minimizes the axial output passages for fluid, in the region of the central zones  27  of the bushing sets  15   a ,  15   b  which correspond to the meshing zone of the gears  13 ,  14 . Longitudinal channels  25  are preferably, but not in a limiting manner, provided on the two flanks of each bushing set  15   a ,  15   b  and promote the distribution of the output pressure over the flanks of the bushing sets so as to keep the two separate pieces  22   a ,  22   b  close together. 
   The bushing sets  15   a ,  15   b , both in the one-piece version and in the version produced by means of separate pieces  22   a ,  22   b , can preferably have passages having variable width in order to allow the hydrodynamic lubrication thereof. One embodiment is illustrated in  FIGS. 7 and 8 , wherein the face  17   b  of each piece  22   a ,  22   b  of one or both of the bushing sets  15   a ,  15   b  has depressions or channels  50  which extend in a radial direction and whose profile-section is slightly concave in a direction orthogonal to the radius of the piece  22   a ,  22   b . A slightly deeper slot or channel  51  is preferably provided in a substantially central position in respect of each depression or channel  50  for better distribution of the lubricating fluid. The behaviour in use of a pump with bushing sets configured in this manner allows the performance thereof to be further improved, especially during use at high operating pressures. 
   At the end adjacent the rear cover  12 , the shafts  23 ,  24  react against a pair of check pins or balancing pistons  29 ,  30  which are mounted for axial sliding in a close-fitting manner in respective axial bores  31 ,  32  which are provided in an intermediate plate  26 . The ends of the check pins  29 ,  30  that are remote from the shafts  23 ,  24  are directed towards a common chamber  34  which is provided in the rear cover  12  and which, in use, is preferably in communication with the output of the rotary pump. In this manner, the pressurized fluid which will occupy the chamber  34  acts on the check pins  29 ,  30  so as to oppose the axial load produced by the gears  13 ,  14 . 
   As is visible in  FIG. 5 , a groove  27  for accommodating a seal  16  which is, for example, substantially configured in an “omega”-like manner, is provided in the face of the intermediate plate  26 . An opening  28  is provided at the side of the plate that, when the pump is in use, communicates with the intake A. The configuration of the intermediate plate  26 , which is similar to that of the lower cover of the pumps of the prior art in  FIGS. 1 and 2 , allows, during use of the pump, a distribution of output pressure M to be obtained over the outer face  17   a  of the rear bushing  15   b  which substantially affects the area P MAX  indicated with hatching in FIG.  5 . This area, as is known, is greater than the area of the inner face  17   b  of the rear bushing  15   b  which is subjected to the output pressure, as long as the force differential owing to the pressure provides for the production of an axial load on the rear bushing  15   b  directed towards the gears  13 ,  14 , as indicated by arrow S′ in FIG.  3 . 
   With reference now to  FIG. 6 , the inner face of the front cover  11 , unlike the pumps of known type, does not have the omega-like seal, and is instead provided with a large opening  36  which communicates with the intake A of the pump. Two limbs  37  which communicate with the openings  38 ,  39  for housing the shafts  23 ,  24  of the gears  13 ,  14  extend from the opening  36 . This configuration of the face of the front cover  11  ensures that the outer face  17   a  of the front bushing  15   a  is subjected only to the intake pressure, which affects, by way of indication, the hatched area, which is denoted P MIN  in FIG.  6 . Since, however, a portion of the inner face  17   b  of the same front bushing  15   a  is also subjected to the output pressure, the pressure differential acts on the front bushing  15   a  in a manner counter to that seen previously for the rear bushing  15   b , as long as during use of the pump, the front bushing  15   a , and in particular the two pieces  22   a ,  22   b  thereof, are urged firmly against the face of the front cover  11 , as indicated by arrow S″ in FIG.  3 . 
   At all times during use of the pump, the pressure applied to the shafts  23 ,  24  by the check pins  29 ,  30  is such that the gears  13 ,  14  are in turn thrust axially onto the front bushing set  15   a , as indicated by arrow S′″. Consequently, the axial forces S′, S″ and S′″ which are produced during the operation of the pump all act in the same direction and contribute to keeping the gears  13 ,  14  and the front bushing set  15   a  and rear bushing set  15   b  as a whole in abutment with the reference plane, indicated by line VI—VI in  FIG. 3 , which is constituted by the face of the front cover  11  that is directed towards the inside of the shell  10 . 
   In this manner, there is obtained a taking-up of the axial clearances which is complete and constantly defined in spite of the oscillation of the axial forces produced during the rotation and the meshing of the gears  13 ,  14 . 
   As is visible in  FIG. 3 , the check pins  29 ,  30  have different diameters in order to apply different axial loads to the two gears  13 ,  14 . This is because, in the example of the Figure, the driving gear  13  and driven gear  14  are of a helical type and therefore, during operation, produce per se axial loads whose direction is counter to and in accordance with the direction of the axial load applied by the check pins  29 ,  30 . Naturally, in the case of gears having straight teeth which do not produce per se axial loads, the check pins  29 ,  30  can have substantially corresponding diameters so as to apply an axial load of equal intensity to both of the gears. 
   Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated, without thereby departing from the scope of the present invention.