Patent Description:
As is well known, geared rotary positive displacement pumps normally comprise two gear wheels (or gears) of which one, called driving wheel, is connected to a driving shaft and causes the rotation of the other wheel, called the driven wheel. Each gear wheel comprises a plurality of teeth, straight or helical, which are configured to mesh with each other according to successive meshing configurations during the rotation of the wheels.

According to a known solution, designed in the past by the Applicant and illustrated in <FIG> and <FIG>, a geared rotary positive displacement pump is equipped with a pair of gear wheels <NUM> whose teeth <NUM> have a profile characterized by a round or arched shape at the top.

A gear wheel of this type allows a continuous contact between the profiles of the two meshing wheels, in whatever angular position they are, so that there is no fluid encapsulation between the crest and the bottom of two meshing teeth. This happens for example when the shape of the tooth at the top thereof is defined by a function describing an arc of a circle or ellipse, wherein the center thereof or the foci thereof are located inside the tooth itself between the outer diameter and the inner diameter of the gear wheel <NUM>.

Although this known solution has numerous advantages, among which noiselessness, no encapsulation of fluid and very low pulsations on the delivery port, it does not ensure a correct interaction of the gear wheels <NUM> with the body or casing <NUM> of the pump in which they are housed.

In particular, the biggest critical issue for this type of pumps occurs during the running-in phase, where the gear wheels <NUM>, under the load due to the pressure created on the delivery port, create their own seat in the body <NUM> of the pump. In fact, in the running-in phase, the gear wheels <NUM> contact the internal walls of the body <NUM> of the pump in the area close to the low pressure port and, since the wheels themselves are made of a material having a hardness greater than the material of the body <NUM>, the geometry of the latter is altered.

In other words, under the load that is generated during the running-in phase, elastic yieldings of the gear wheels and/or different radial positioning of the said wheels with respect to the axis of rotation occur, which lead to plastic wear of the pump body <NUM> with an often unpredictable and potentially harmful outcome.

Still more in particular, with reference to a schematic example shown in <FIG>, in the solution illustrated above it happens that, under the action of the force due to the pressure of the fluid transported by the gear wheels, there is an elastic yielding of the plain bearings housed in the bearing bushes of said wheels, this yielding being proportional to the operating pressure, to the length of the gear wheels and depending on the materials used. Under certain operating conditions, a mechanical interference occurs between the internal wall of the hole of the body <NUM> in which the wheel <NUM> is housed and the wheel <NUM> itself.

Due to the particular shape of the tooth with rounded tip, this interference therefore results in a wear phenomenon caused by plastic deformation of the material (for example aluminium, but also other materials such as cast iron are not excluded, which are ductile and malleable compared to the material of the gear wheels, normally of hardened and tempered steel) and a subsequent and consequent "spreading" of material removed as a thin layer on the free walls of the holes in the body <NUM>.

Furthermore, during running-in, excessive frictions create energy dissipation (heat) and the aforementioned wear phenomena are even more accentuated and difficult to keep under control.

The spreading effect of the material removed and deposited in the free areas of the walls of the holes, due to the aforementioned round shape of the teeth, mainly depends on the tolerances defined during the design phase. The greater the interpenetration between the gear wheels and the body of the hydraulic system, the greater the heat generated and the volume of material removed; therefore the greater the thermal expansion and the greater the volume of material removed ("torn") and subsequently deposited ("spread").

In certain cases, the seat created by the gear wheel, once back to the starting temperature, has a deformed geometry such as to interfere with the rotation of the wheel itself. The extent of the deformation is also difficult to predict.

This phenomenon is measurable and is also found experimentally: when trying to rotate the driving wheel, the rolling is difficult and takes place in jerks, or even it is rather completely blocked. This can lead to production waste due to quality control failure.

All this has very dangerous consequences for the pump, where in cold starting there is often damage to the pump body with consequent loss of volumetric efficiency. It is therefore desirable that, during the running-in of a pump in which the gear wheels are housed for the first time, this inconvenience does not occur.

Patent application <CIT> discloses a pump whose gear wheels comprise a tooth equipped with a cutting edge for removing material from the pump casing, in particular when the latter is made of grey cast iron. The cutting edge is basically a pronounced protrusion of the tooth and defines an acute cutting angle.

Document <CIT> also discloses a gear wheel according to the prior art.

The technical problem of the present invention is to provide a gear wheel, and a related hydraulic apparatus, having structural and functional features such as to allow overcoming the limitations and drawbacks still affecting the known solutions, in particular which does not cause a plastic deformation of the pump body with consequent spreading of material on the surface of the holes housing the gear wheels during the running-in phase.

The solution idea underlying the present invention is to provide a gear wheel whose teeth comprise a cutting element, which is arranged on the top thereof and is capable of removing material from the pump body by removing chips during the running-in phase, so as to limit or even avoid the plastic deformation of the pump body which occurs in the known solutions. The removed material is then expelled by the fluid pumped by the delivery port, with limited or even no spreading of material on the walls of the housing holes.

Based on this solution idea, the aforementioned technical problem is solved by a gear wheel for a hydraulic apparatus according to appended claim <NUM>. Further embodiments are disclosed in the dependent claims <NUM>-<NUM>.

The present disclosure also refers to a not claimed method for manufacturing a gear wheel of the type rotatable about an axis of rotation and comprising a plurality of teeth adapted to mesh with respective teeth of another gear wheel during a rotational motion around the axis of rotation, comprising the steps of:.

According to an aspect of the present disclosure, the method can comprise a step of performing a shape grinding of the tooth profile, by using a single shaped grinding wheel or a screw grinding wheel.

According to another aspect of the present disclosure, the method can comprise a step of grinding the diameter of the gear wheel by means of a round grinding machine.

The characteristics and advantages of the gear wheel, of the apparatus and of the method according to the disclosure will be apparent from the description, made hereinafter, of an embodiment thereof, given by way of indicative and non-limiting example, with reference to the enclosed drawings.

With reference to those figures, a gear wheel according to the present invention is globally and schematically indicated with <NUM>.

It is worth noting that the figures represent schematic views and are not drawn to scale, but instead they are drawn so as to emphasize the important features of the invention. Moreover, in the figures, the different elements are depicted in a schematic manner, their shape varying depending on the application desired. It is also noted that in the figures the same reference numbers refer to elements that are identical in shape or function. Finally, particular features described in relation to an embodiment illustrated in a figure are also applicable to the other embodiments illustrated in the other figures.

The gear wheel <NUM> of the present invention is used in hydraulic apparatuses such as volumetric gear pumps (geared rotary positive displacement pumps) or also hydraulic gear motors. As will be described in greater detail below, the gear wheel <NUM> is used in a hydraulic apparatus comprising a pair of meshing gear wheels, the wheels being mounted so as to be mutually rotatable in a casing or housing body between an inlet side and an outlet side of a fluid having, in use, a flow which is substantially transverse with respect to the axes of rotation of the gear wheels. In their reciprocal rotation, these meshing gear wheels have progressive meshing configurations.

In particular, the present invention will be illustrated below according to a preferred example in which the gear wheel <NUM> is used in a gear pump, even if the disclosed teachings will also be applicable to other hydraulic systems, such as a motor.

With particular reference to <FIG>, the gear wheel <NUM> of the present invention, also called rotor in this field, comprises a main body which is processed so as to have a plurality of teeth <NUM>, which are adapted to mesh with respective and conjugate teeth of another gear wheel during rotation.

More specifically, the gear wheel <NUM> is arranged so as to be rotatable about an axis of rotation, indicated herein as the axis H-H, which is perpendicular to the plane of the figure and configured as an axis passing through the center O of the gear wheel <NUM>, i.e. from the center of the primitive circle. Each tooth <NUM> has a cross-sectional profile P defined by a determined function on a plane orthogonal to this axis of rotation H-H.

In particular, the function that defines the profile P of the teeth <NUM> may describe an arc of a circle at the top of the tooth, said circle being centered in the center O and having a diameter substantially equal to or greater than that of the holes housing the wheel in the pump body, except for processing tolerances. In an embodiment, the present invention is inspired by a profile of this type, which has a rounded shape at the top of the tooth.

In particular, suitably according to the present invention, at least one tooth <NUM> of the gear wheel <NUM> is shaped so as to comprise a cutting edge <NUM> at at least a portion <NUM> thereof, for example at the top of the tooth. The cutting edge <NUM> of the tooth <NUM> is an active end thereof adapted to remove material in the form of chips from a body which is in contact with the cutting edge, for example from the surface of the housing holes of the pump casing, in particular during the rotation of the gear wheel <NUM> in the running-in phase of the apparatus in which it is housed, as will be illustrated in greater detail below.

Following the processing of the tooth <NUM>, the function that defines its profile P is therefore suitably modified, for example by introducing a discontinuity or in general an irregularity.

With reference to the schematic and non-limiting example of <FIG>, the cutting edge <NUM> is obtained as the edge of a longitudinal groove <NUM> which preferably affects the entire length or depth of the tooth <NUM> or of the wheel <NUM>.

In other words, according to the present invention, the teeth <NUM> of the gear wheel <NUM> are provided with at least one cutting surface for removing material during the rotation of the wheel. This configuration entails considerable advantages, especially during the running-in phase of the pump, as will be described in detail below.

As mentioned above, the portion <NUM> of the tooth <NUM> in which the cutting edge <NUM> is formed is a top portion of said tooth <NUM>, where the term "top" means herein the portion of the tooth furthest from the center O of the primitive circle of the gear wheel <NUM>. Furthermore, in a preferred embodiment of the present invention, the cutting edge <NUM> extends over the entire length or depth of the tooth <NUM>, where the term "length of the tooth" means herein the dimension of the tooth <NUM> along a direction parallel to the axis of rotation H-H.

As better illustrated in <FIG>, the cutting edge <NUM> is obtained by removing material from the tooth <NUM>, in particular at the portion <NUM> of the tooth <NUM>, for example at at least one side of said portion <NUM>, forming the aforementioned groove <NUM> extended longitudinally along the development (depth) of the tooth <NUM>. In a preferred embodiment of the present invention, a second groove <NUM> analogous to the groove <NUM> is formed parallel to the previous groove <NUM> (for instance at another side of the portion <NUM>), defining between said grooves a projection <NUM> corresponding, in this embodiment, to the portion <NUM>.

In <FIG>, the reference P' indicates the profile of the tooth of the known solutions, having a typical round or rounded shape without interruption, while the profile P of the tooth <NUM> of the present invention has, in its section, the projection <NUM> obtained by the aforementioned material removal by means of the two parallel grooves <NUM> and <NUM>, with the formation of the cutting edge <NUM> for example on a longitudinal edge of the projection <NUM>.

The processing techniques of the gear wheel <NUM> are many and the present invention is not limited to one of them in particular. For example, in an embodiment of the present invention, the shape of the cutting edge <NUM> is obtained by shape grinding to obtain the groove <NUM> (and consequently the cutting edge <NUM>) by using for example a single shaped grinding wheel, or it is obtained by using a screw grinding wheel.

In an embodiment of the present invention, the grinding of the outer diameter of the gear wheel <NUM> is achieved by means of round grinding machines, so that the function which defines the profile P of the teeth <NUM> describes, at the top of each tooth, an arc of a circle centered on the axis of rotation H-H of the gear wheel <NUM> and having a diameter equal to (or greater than) a diameter of the housing hole, as indicated above.

Again by way of non-limiting example, with respect to the maximum height of the tooth <NUM> (substantially preferably corresponding to the maximum height of the teeth of the known solutions), the modification of the known profile due to the material removal can lead to a decrease in the height of the tooth (at the foot of the cutting edge <NUM>) equal to about <NUM>, this value shrinking more and more as we move away from the cutting edge <NUM>, resulting in a protrusion of the projection <NUM> with respect to the rest of the remaining profile of about <NUM>.

In a preferred embodiment, illustrated in <FIG>, each tooth <NUM> of the gear wheel <NUM> comprises the cutting edge <NUM>, even if other solutions are within the scope of the present invention; it is in fact possible to provide for a configuration in which the cutting edge <NUM> is present only on some teeth of the gear wheel, for example a configuration in which a tooth comprising the cutting edge alternates with a tooth that does not include any cutting edge.

Furthermore, it is also possible to provide an embodiment in which at least one tooth <NUM> comprises a plurality of cutting edges <NUM> spaced from each other along the profile P, for example equally spaced, such as for example the teeth of a saw. In other words, in this embodiment, the tooth <NUM> comprises a plurality of portions <NUM> which have the cutting edge <NUM>, said portions being spatially spaced from each other (for example, equally spaced) along the profile P of the tooth <NUM>.

As mentioned above, the cutting edge <NUM> introduces a discontinuity in the function that defines the profile P of the tooth <NUM>, for example a discontinuity of the first kind (for example in the form of a step), in which at the left of the cutting edge <NUM> the function that describes this profile P approaches the cutting point with a value that is different from the one it has when approaching from the right.

In another embodiment of the present invention, the cutting edge <NUM> introduces a point of non-differentiability in the function that defines the profile P of the tooth <NUM>. For example, the cutting edge <NUM> can introduce a corner point or a cusp.

All said different profiles are illustrated schematically and not exhaustively in <FIG>, which shows that the principles of the invention are applicable to a large number of possible shapes for the profile P of the tooth <NUM>. It is observed in fact that the present invention is not limited to a particular shape of the teeth <NUM>, in particular of the tooth portion <NUM> comprising the cutting edge <NUM>, which can vary according to the needs and/or circumstances.

More specifically, <FIG> shows various alternative solutions indicated with the letters from (a) to (p) which illustrate a section of the tooth <NUM> at portion <NUM> (i.e. a top portion). The solution (a) substantially corresponds to the example embodiment of <FIG> discussed above, while the other solutions can be considered as alternative embodiment.

Consequently, the tooth portion <NUM> comprising the cutting edge <NUM> can be considered a projection <NUM> formed in the top profile, as for example in the embodiment of figures from <NUM> to <NUM> and of <FIG>, or in general obtained by means of at least one longitudinal groove <NUM>, preferably extended over the entire length of the tooth.

In the context of the present invention, the projection <NUM> of the embodiment of <FIG> and of <FIG> is to be understood in a relative way with respect to the depression (dip) at the flanks of said projection; in other words, the projection <NUM> is a discontinuity made by means of at least one groove <NUM> extended over the entire length or depth of the tooth, and it does not protrude much beyond the rounded line of the effective profile P of tooth <NUM> (it protrudes in fact preferably by about <NUM>, in a range from <NUM> to <NUM>), and defines the cutting edge <NUM> in at least one edge where a change of level with respect to the center O of this tooth <NUM> occurs. In any case, the maximum height at the projection <NUM> is preferably substantially equal to the maximum height of the profile of the tooth of the known solutions, from which material is then removed as illustrated above, obtaining said projection <NUM>.

According to the claimed invention, the projection <NUM> has a width (intended as a transverse dimension with respect to the length of the tooth) between <NUM> and <NUM>.

In other words, the cutting edge <NUM> is made by means of at least one groove <NUM> which is lowered by an amount from <NUM>% to <NUM>%, preferably from <NUM>% to <NUM>%, still more preferably <NUM>%, of the height H' of the tooth <NUM>, the groove <NUM> decreasing away from the cutting edge <NUM> along the profile P of said tooth <NUM>. For example, the cutting edge <NUM> introduces, at the top of the tooth <NUM>, a discontinuity in the function that defines the profile P of said tooth <NUM> made by means of such lowered groove <NUM>. In the context of the present invention, the term height H' means the distance between the top of the tooth and the base at the primitive circle, for example measured along a direction orthogonal to the length of the tooth.

For example, for a tooth <NUM> having a height of <NUM>, the groove <NUM> can vary from <NUM> to <NUM>, preferably <NUM>. The maximum values of the range indicated above are more suitable for large-sized teeth, for example having a height greater than <NUM>.

Advantageously, the described configuration allows to obtain an extremely resistant cutting edge <NUM> which is not subject to breakage, since it protrudes very little. Furthermore, the described configuration can be easily made with no need for complicated processing, requiring only a minimal removal of material to form the aforementioned groove <NUM>.

The cutting edge <NUM> is arranged at a rim of the portion <NUM> (i.e. it is the edge of said portion), which therefore can have any shape, for example a square or a wedge shape. In general, the tooth <NUM> has thus a profile P provided with a sharp portion.

It is further observed that the profile P of the tooth <NUM> can be a symmetrical profile (such as the profile of the embodiment of <FIG> and of <FIG>), or the profile P can be an antisymmetric profile.

In an embodiment of the present invention, the teeth <NUM> are helical teeth, and the gear wheel <NUM> has a number of teeth between <NUM> and <NUM>, preferably <NUM>-<NUM>, still more preferably <NUM>.

Furthermore, the gear wheel <NUM> according to the present invention, when it is helical, has a helical pitch Pe (measured in mm) equal to (F*n)/<NUM>>Pe>(F*n)/<NUM>, preferably Pe=(F*n)/<NUM>, where F is the gear wheel band length and n is the number of teeth.

Obviously, the principles of the invention also apply to bi-helical gear wheels, such as for example gear wheels of the type described in patent application number <CIT> in the name of the Applicant, as well as apply to gear wheels with straight teeth.

The above described configuration adopted for the gear wheel <NUM> has numerous advantages and allows to effectively solve the technical problem of the present invention.

In this regard, referring to <FIG>, a hydraulic apparatus <NUM> including a pair of gear wheels <NUM> according to the present invention is disclosed. In their mutual rotation, the gear wheels <NUM> cause meshing between respective cooperating teeth.

As indicated above, the grinding of the outer diameter of the gear wheel <NUM> is such that the function that defines the profile P of the teeth <NUM> describes, at the top of each tooth, an arc of a circle centered on the axis of rotation H-H of the gear wheel <NUM> and having a diameter equal to a diameter of a hole housing the wheel, except for processing tolerances.

In one embodiment, the starting shape of the tooth <NUM> at the top thereof is expressed by a function that describes an arc of a circle or ellipse, whose center or foci are located inside the tooth itself (between the outer diameter and the inner diameter of the gear wheel).

The improved profile comprising the cutting edge allows in any case to obtain gear wheels having a "continuous contact" along almost the whole profile, that is, there is a single theoretical contact point that moves from one flank of the gear wheel to the other one with continuity, without generating areas of entrapment of fluid during meshing (i.e. not encapsulating fluid). The aforesaid contact does not take place only in the portion affected by the cutting edge <NUM>. In this case, a small encapsulation volume is created, but the cutting edge <NUM> is designed and sized in such a way as to make said modification completely imperceptible. In fact, from tests carried out by the Applicant, there are no volumetric losses due to leakage of the pumped fluid and the excellent characteristics of noiselessness of these apparatuses are maintained.

The non-incapsulating profile is thus suitably modified by introducing the cutting edge <NUM> of the present invention.

In another embodiment, the starting profile on which the cutting edge is made is a so-called semi-encapsulating profile, wherein an area of entrapment or encapsulation of fluid is formed between the teeth of two meshing wheels, said area decreasing gradually during the rotation of the wheels, until it is substantially cancelled off when the head of a tooth of a gear wheel touches the bottom of a tooth of the other wheel.

The apparatus <NUM> comprises a casing or body <NUM>, which is provided with housing through holes <NUM> for housing the gear wheels <NUM>.

As mentioned above, the hydraulic apparatus <NUM> can be a volumetric rotary gear pump or a hydraulic motor.

During the operation of the apparatus <NUM>, it is known that, under the action of the force due to the pressure of the fluid transported by the gear wheels <NUM>, there is an elastic yielding of the plain bearings housed in the bushes. This yielding is proportional to the operating pressure, to the length of the rotors and depends also on the materials used. All this causes the occurrence of mechanical interference between the inner wall of the housing holes <NUM> of the casing <NUM> and the gear wheels <NUM>.

The gear wheels <NUM> are made of a material, for example steel, having a hardness that is greater than the material of the casing <NUM>, for example aluminium, so that the aforementioned mechanical interference between the gear wheels <NUM> and the casing <NUM> generates, in the known solutions, a wear phenomenon due to plastic deformation, in particular due to the particular shape of the rotor tooth, which is rounded at the tip.

Advantageously according to the present invention, the cutting edge <NUM> is configured to remove chip material from the inner surface of the housing holes <NUM> of the casing <NUM>, which is contacted by the cutting edge during the rotation of the gear wheels <NUM>.

In this way, during the running-in phase, the cutting edge <NUM> acts by removing material in the form of chips, which is then expelled together with the pumped fluid from the delivery port with limited or even absence of fill/spreading of material on the inner surface of the housing holes <NUM>. The final geometry of the housing hole (which can be expected with modest tolerance dimensions) does not affect the performance of the apparatus.

It is therefore evident that, at the end of the running-in phase, all the problems caused by the deposition/spreading of unwanted material on the internal surface of the housing hole no longer occur, such spreading resulting, in some known solutions, in an unwanted and completely random modification of the mechanical plays that leads to different performances and difficult starting in cold conditions, with consequent loss of volumetric efficiency.

The innovation made to the profile P of the tooth <NUM> of the gear wheel <NUM> therefore allows to improve the performance and the working life of the hydraulic apparatus <NUM> which incorporates said gear wheel <NUM>, by speeding up the running-in times and ensuring an improvement in the overall quality in terms of constancy, reliability and repetitiveness of the parameters characterizing it. The cutting edge <NUM> on the top of the teeth <NUM> in fact eliminates the problems of some known profiles which, due to the exclusively round shape, sometimes creates irreversible damage to the body of the apparatus itself at the end of the running-in phase.

The adopted configuration allows in any case to minimize the losses due to fluid leakage, and in general the pump performance is in no way affected by the presence of the cutting edge <NUM>.

While not altering the performance, such a configuration allows in any case a significant improvement in terms of reliability, repeatability of performance, wear, durability, mechanical and volumetric yield, in particular due to the particular shape of the cutting edge.

Finally, it should be noted that all the advantages of the known solutions with continuous contact (and also of the solutions with semi-encapsulation of fluid) are maintained, since the profile is minimally modified only at the top for forming the cutting edge <NUM> (which does not protrude to much since the depth of the groove is very little), which allows to always guarantee maximum performance even after the running-in phase, said cutting edge <NUM> suitably removing chip material that is subsequently expelled together with the fluid pumped into the apparatus.

Tests carried out by the Applicant have demonstrated that the extremely small dimensions of the cutting edge <NUM> do not entail any drawback in the post-running-in phase of the pump, on the contrary they represent a minimum escape route for micro-encapsulations of fluid which are easily discharged while keeping the noiselessness of the pump extremely reduced.

Claim 1:
A gear wheel (<NUM>) for a hydraulic apparatus, comprising a plurality of teeth (<NUM>) and arranged to be rotatable about an axis of rotation (H-H), said teeth (<NUM>) having a sectional profile (P) and being adapted to mesh with respective teeth of another gear wheel during a rotational motion about the axis of rotation (H-H), wherein at least one tooth (<NUM>) is shaped so as to have at least one cutting edge (<NUM>) configured to remove material from a body which is contacted by said cutting edge during the rotation of said gear wheel (<NUM>), characterized in that said cutting edge (<NUM>) is defined by at least one groove (<NUM>) which is lowered, with respect to a starting rounded profile (P') without the groove (<NUM>), by an amount from <NUM>% to <NUM>% of the height (H') of said tooth (<NUM>), said groove (<NUM>) decreasing away from the cutting edge (<NUM>) along the profile (P) wherein the groove (<NUM>) forms a projection (<NUM>) which comprises the cutting edge (<NUM>) and which protrudes with respect to the rounded profile (P) of the tooth (<NUM>) by an amount from <NUM> to <NUM>, and wherein the projection (<NUM>) has a width from <NUM> to <NUM>, the width being intended as a transverse dimension to the length of the tooth.