Technical field

The present invention generally relates to a positive displacement pump and to a method of operating same.

More particularly, the present invention relates to a rotary positive displacement vacuum pump equipped with devices arranged to modify the gear ratio between the drive members and the pump.

More preferably, but not exclusively, the present invention is applied in vacuum pumps operated by an internal combustion engine of a motor vehicle.

Prior Art

In the automotive field there are known positive displacement pumps, e. g. vacuum pumps arranged to generate and maintain, in a tank or booster, e. g. of a power brake, a vacuum or depression mainly serving for operating brakes and other devices requiring a vacuum in order to work.

For instance, document WO 2013054263, in the name of the Applicant, discloses a vacuum pump connected to a shaft of the engine and rotating at a speed equal to the speed of the shaft driving it.

Document WO 2004092588 discloses an oil pump and a vacuum pump integrated with each other and structurally independent from the engine. The pump is to be located at a predetermined position, in correspondence of a driving wheel fixedly connected with the driving shaft. The prior art pump has a rotor made of metal with gear wheels made of metal.

Document DE 102004016237 discloses a vacuum pump having two rotors and two corresponding shafts externally equipped with two gear wheels; one of the gear wheels is driven by an internally toothed driving wheel.

A problem with the first prior art is that the rotation speed of the vacuum pump is the same as the speed of the driving shaft.

A problem with the second prior art is that the vacuum pump is constrained to occupy a predetermined space, inside or outside the engine, corresponding to a predetermined angle defined between the engine and the oil pump. Moreover such a prior art, having a rotor and a gear wheel made of metal, has a high moment of inertia.

The Applicant has realised that generally the prior art is not capable of limiting the rotation speed of the positive displacement pumps and, consequently, of limiting the power dissipated by the same pumps.

Moreover the Applicant has realised that generally the prior art is not capable of adequately solving the problem of the pump positioning relative to the engine.

Lastly, the Applicant has realised that some prior art documents propose solutions where the pump rotor has a high moment of inertia, resulting in a high dissipated power. Description of the Invention

It is an object of the present invention to solve the technical problems described above.

The object is achieved by the positive displacement pump as claimed.

The present invention also concerns a method of operating a positive displacement pump.

The claims are integral part of the technical teaching provided herein in respect of the invention.

The following synthetic description of the invention is provided in order to allow a basic understanding of some aspects of the invention. Such a synthetic description is not a thorough description and, as such, it is not to be intended as being suitable for identifying key or critical elements of the invention or for defining the scope of the invention. It is only aimed at setting forth some concepts of the invention in simplified form, as an anticipation of the detailed description below.

In accordance with a feature of a preferred embodiment, the positive displacement pump includes a driven gear wheel and a driving gear wheel, coupled so as to form a gear train in which the gear wheels are placed one inside the other.

In accordance with another feature of the present invention, the driven and driving wheels have parallel axes and are cylindrical wheels with helical or straight teeth or bevel wheels.

In accordance with a further feature of the present invention, provided in another embodiment, the driven and driving wheels are bevel wheels with crossed axes (skew bevel wheels).

In accordance with another feature of the present invention, one of the driven and driving wheels has teeth formed on an outer surface and one of the driven and driving wheels has teeth formed on an inner surface, respectively. Advantageously, this allows making gear trains with a reducing or multiplying gear ratio.

In accordance with yet another feature of the present invention, the pump can be arbitrarily located along a circumference centred on the rotation axis of the driving wheel. Brief Description of the Figures

The above and other features and advantages of the invention will become apparent from the following description of preferred embodiments made by way of non limiting example with reference to the accompanying drawings, in which elements denoted by a same or similar reference numeral correspond to components having the same or similar function and construction, and in which:

- Fig. 1 is an isometric axonometric view of a rotary vacuum pump, as seen from the drive side;

- Fig. 2 is a front view, from the drive side, of the vacuum pump shown in Fig. 1;

- Fig. 3 is an axial cross sectional view of the pump, taken according to a plane passing through line AA in Fig. 2;

- Fig. 4 is an isometric axonometric view of the vacuum pump, as seen from the side opposite to the drive side, without the cover;

- Fig. 5 is a front view of the vacuum pump, from the side opposite to the drive side, without the cover; and

- Fig. 6 shows a detail of the drive side of the pump.

Description of Preferred Embodiments

Referring to Fig. 1, a positive displacement pump 13 according to the invention is, for instance, a rotary vacuum pump.

Positive displacement pump 13, in the exemplary embodiment, has a body 15 defining a chamber 16 with substantially elliptical cross-section, of known type, which in the example is a vacuum chamber, and a drive side 18 (Figs. 1 and 2).

Vacuum chamber 16, in use, is closed by a cover 17 (Fig. 3).

A rotor 12, for instance a single-vane rotor, is housed within vacuum chamber 16. Of course, in other embodiments, the rotor could be a multi-vane rotor, a so-called

"pendulum" rotor or a rotor of any other kind envisaged in the art.

Rotor 12 employed herein is preferably made of a lightweight material, for instance a plastic or thermoplastic material.

The movement of rotor 12 is preferably constrained by a guide 14 formed in pump body 15.

In known manner, at least one outlet for a discharge duct 20 and at least one inlet for a suction duct 19 are formed in pump body 15 and communicate with chamber 16.

In the preferred embodiment the rotor has, on drive side 18, teeth 11a arranged to form a gear wheel 11 that in the example is a driven wheel. Wheel 11, referred to herein as driven wheel, is for instance a cylindrical wheel with helical teeth.

A shaft 10 forms a control (or drive) member for the pump. In the preferred embodiment, drive member 10 is for instance a shaft of the engine, a cam shaft, a shaft of the alternator, an output shaft of an oil pump, and so on.

A gear wheel 10a, for instance a cylindrical wheel with helical teeth, can be preferably formed at or connected to one end of shaft 10. Preferably said wheel is made of metal and its teeth are designed for meshing with the teeth of driven wheel 11 thereby forming a gear train.

Wheel 10a may be for instance integrally formed with shaft 10 or it may be interference-keyed on shaft 10. Wheel 10a is therefore fixedly connected with shaft 10 and is defined herein as driving wheel.

In the preferred embodiment, driving wheel 10a, fixedly connected with shaft 10, and driven wheel 11, fixedly connected with rotor 12, form a gear train with parallel axes in which the driving wheel is placed inside the driven wheel (Figs. 1 to 3).

In accordance with other embodiments, in which the driving wheel is placed inside the driven wheel, the wheels could also be bevel wheels arranged to form a gear train with skew axes (in this case the whole of the two wheels is referred to as a bevel gear pair).

In the preferred embodiment, driven wheel 11 has greater size than driving wheel 10a, that is, driven wheel 11 has a radius r ₂ greater than radius ri of driving wheel 10a.

In case of gear trains formed by gear wheels, this corresponds to saying that driven wheel 11 has a number of teeth z ₂ greater than the number of teeth zi of driving wheel 10a.

The pair of driving wheel 10a and driven wheel 11 is characterised by a gear ratio R, defined as the mathematical ratio between the angular speeds of the wheels, i.e. R = coi/co ₂ , where coi is the angular speed (measured in revolutions per second) at which the driving wheel can rotate, and co ₂ is the angular speed at which the driven wheel can rotate.

In particular, in case the wheel pair is formed by gear wheels, gear ratio R can also be defined as the mathematical ratio between the numbers of teeth of the gear wheels, i.e.

ω _Λ z ₇

R =— =— where zi is the number of teeth of the driving wheel and z ₂ is the number of teeth of the driven wheel.

In the preferred embodiment, gear ratio R is therefore a reducing ratio (R > 1). This means that rotor 12 of positive displacement pump 13 rotates at a speed co ₂ lower than speed coi of shaft 10, this allowing a reduction of the power dissipated by pump 13.

In case of a reducing gear ratio, in accordance with other embodiments, the driving wheel could be external to driven wheel 11 associated with the pump rotor. In this case the driven wheel will have external teeth in place of internal teeth.

In accordance with other embodiments, the driving wheel has a greater size than the driven wheel. In this case, gear ratio R, as it can be readily understood by a person skilled in the art, is a multiplying ratio (R < 1) and allows the pump rotor to rotate at higher speed than the drive shaft.

Use of a structure consisting of gear trains with parallel or skew axes, in which one wheel is placed inside the other, advantageously allows locating driven wheel 11 in a plurality of possible positions relative to driving wheel 10a.

More specifically, considering the exemplary embodiment described herein, in which driving wheel 10a and driven wheel 11 are cylindrical gear wheels with parallel axes (Fig. 6), rotation axis 0 ₂ of driven wheel 11 (and hence of rotor 12) can be arbitrarily located along a circumference C with radius r centred on rotation axis Oi of driving wheel 10a.

Radius r of circumference C identifying the points where it is possible to locate axis 0 ₂ of driven wheel 11, is equal to the difference between radius r ₂ of driven wheel 11 and radius ri of driving wheel 10a, i.e. r = r ₂ - ri. Otherwise stated, radius r of circumference C is equal to the distance between rotation axis 0 ₂ of driven wheel 11 and rotation axis Oi of driving wheel 10a.

Use of gear trains with skew axes provides an additional degree of freedom with respect to the embodiment using gear trains with parallel axes.

Wheels 10a and 11, described above as cylindrical wheels with helical teeth and parallel axes, may have any shape known in the art, for instance they can be two straight- toothed cylindrical wheels with parallel axes, two bevel wheels with parallel or skew axes, etc.

In case of vacuum pumps, the pressure difference between the environment outside the pump (at atmospheric pressure) and chamber 16 of pump 13 (at sub-atmospheric pressure) causes an axial thrust that acts upon rotor 12 of pump 13 and is proportional to the pressure difference multiplied by the area of the surface onto which it acts, said thrust being directed from drive side 18 towards pump cover 17. Such a thrust entails a greater friction during the movement of rotor 12 and hence power dissipation.

Use of a drive gear pair formed by wheels with helical teeth also generates an axial thrust.

The choice of helical gear trains with a suitably defined and oriented angle of the helix allows obtaining an axial thrust directed from the inside of pump 13 towards the drive side and aimed at balancing, or at least reducing, the axial thrust in the opposite direction due to the pressure difference, thereby reducing the greater friction generated by the latter.

In accordance with other embodiments, a further positive displacement pump, for instance an oil pump, can also be connected downstream vacuum pump 13, which further pump, by exploiting the gear ratio, for instance a reducing gear ratio, dissipates less power.

Other embodiments provide for connecting, downstream vacuum pump 13 a further positive displacement pump, for instance an oil pump, through a pair of gear wheels, as disclosed in the present invention, which are characterised for instance by a multiplying gear ratio.

Configurations of the above type, in which two pumps are arranged downstream each other, are usually referred to as tandem pumps in the present technical field.

Lastly, even though the invention has been disclosed in detail with reference to a rotary vacuum pump, it can be applied to any positive displacement pump for conveying a fluid from a first to a second working environment (for instance an oil pump, a water pump, etc.),

Hereinafter the operation of the pump described above is disclosed with reference to

Figs. 1 to 6, showing an exemplary preferred embodiment.

When drive shaft 10 (and hence wheel 10a) rotates at a given angular speed coi, it transmits the rotary motion or torque to rotor 12 of pump 13, through the pair of driving wheel 10a and driven wheel 11.

Rotor 12 of pump 13, when actuated by the pair of driving wheel 10a and driven wheel 11, rotates at a given angular speed co ₂ .

Rotation speed co ₂ depends on rotation speed coi of drive shaft 10 and on gear ratio

R characterising the pair of driving wheel 10a and driven wheel 1 1, according to relation

In the example shown in the Figures, the direction of rotation of single-vane rotor 12, when viewed from the drive side, is the counterclockwise direction. The operation of rotary vacuum pump 13 is shortly described hereinafter.

Vane 12a of rotor 12 is forced against wall 16a of vacuum chamber 16 so as to ensure tightness with the same wall 16a.

Four steps, occurring twice at each revolution, correspond to each complete revolution of rotor 12:

1) a suction step, in which part of a gas present in at least one environment where vacuum is to be created is sucked into vacuum chamber 16 through at least one suction duct 19;

2) an isolation step, in which the gas sucked during the suction step is isolated in vacuum chamber 16 (the gas being not in communication with suction ducts and discharge ducts);

3) a compression step, in which the gas isolated during the isolation step is compressed;

4) a discharge step, in which the gas compressed during the compression step is discharged through at least one discharge duct 20.

The structure of the positive displacement pump and the corresponding operation method as described herein have several advantages.

The reducing gear ratio of the pair of driving wheel 10a and driven wheel 11 allows reducing in predetermined manner the rotation speed of rotor 12 of pump 13 relative to the speed of driving shaft 10.

In order pump 13, connected with a reducing gear ratio, has a pumping efficiency similar to that of a pump connected with a unit ("impartial") gear ratio - i.e. a ratio where the rotation speeds of the rotor and the driving shaft are the same - it is preferably useful to increase the displacement of pump 13.

The Applicant has experimentally realised that a reducing gear ratio, and hence a reduction in the rotation speed of the rotor of a vacuum pump, allows reducing the power dissipated because of the friction, and hence the emission of exhaust gases (for instance

C0 ₂ ) produced by an engine operating the pump drive shaft, even in case the pump displacement is increased.

The Applicant provides hereinbelow an example of approximate calculation of dissipated powers, assuming a vacuum pump where the reducing gear ratio is 2 (i.e. where the pump rotor rotates at half the speed of the drive shaft) and the displacement is twice the displacement of a reference pump having a unit gear ratio.

In the example, the Applicant has assumed that, in the average use of the motor vehicle, the pump operates for 10% of the operation time in order to restore a vacuum required by the braking system and for 90% of the operation time in order to maintain such a vacuum. In any case, this assumption has been confirmed by the practice.

Experimentally, the Applicant has realised that, due to the greater pump displacement, the power absorbed by the vacuum pump increases, during vacuum restoration, by about 50% relative to the power absorbed by the reference pump.

Moreover, the Applicant has realised that, due to the lower rotation speed, the power absorbed by the vacuum pump during vacuum maintenance decreases by about 50% relative to the power absorbed by the reference pump, since, under such conditions, the power due to the torque, or resistant torque, associated with the pump rotor does not change as the number of revolutions of the pump changes.

Taking as a reference a commonly known formula for calculating the power dissipated by a pump:

W = (torque x 2π x revolutions per minute)/60, where:

W = power absorbed by the pump, in watts;

torque = torque applied to the pump, in N ^' m;

the energy balance observed by the Applicant, based on the experimental data disclosed above, is therefore as follows:

(90% x 0.5) + (10% x 1.5) = (45% + 15%) = 60%.

This means that the power absorbed by a pump dimensioned in accordance with the example is experimentally about 60% of the power absorbed in use by the reference pump, that is, there is a saving by about 40% considering the average use of the motor vehicle.

The structure and the operation method of a pump using gear trains with parallel or skew axes, like the gear trains described above, offers the advantage of making installation of pump 13 easier, depending on the available space in a motor vehicle.

This allows optimising the space occupied by the motor and the positive displacement pump(s).

In accordance with other embodiments the driving wheel can for instance be exchanged with the driven wheel, i.e., the teeth of the driven wheel are formed on the external surface thereof and the teeth of the driving wheel are formed on the internal surface thereof.

In accordance with such embodiments, in case of a low rotation speed of the drive shaft and the driving wheel associated therewith, it is possible to use drive gear trains characterised by a multiplying gear ratio towards the driven wheel. A reducing or multiplying gear ratio is advantageous, for instance, in tandem configurations, in which a first positive displacement pump is arranged downstream a second positive displacement pump, while maintaining all advantages of installation simplicity and energy balance as described above.

Another advantage of the invention is the possibility of using a rotor made of a lightweight material, for instance a plastic or thermoplastic material, with a consequent reduction in the moment of inertia of the same rotor.

A further advantage results from the possibility of limiting, in the case of vacuum pumps, the axial thrust acting on rotor 12 because of the pressure difference between chamber 16 of pump 13 and the outside environment.

This is achieved by using wheels with suitably configured helical teeth, capable of generating an axial thrust in opposite direction to the thrust due to the pressure difference. Lastly, if free spaces exist inside a motor vehicle and actually make locating the pump in a plurality of positions possible, as described in the present invention, it is for instance advantageous to locate the pump in such a manner as to counterbalance or reduce a force, referred to as skewing force, acting on the pump rotor.

Indeed, as experimentally realised by the Applicant, in a positive displacement pump the rotor is acted upon by radial internal forces, due to different pressures operating within the pump chamber. The resultant of such forces, known as "skewing force", causes a thrust of the rotor against its guide 14, thereby generating a friction and hence a higher dissipated power.

Such a skewing force, as experimentally realised by the Applicant, is applied to the pump rotor and consequently to the guide thereof, passes through the centre of rotation of the rotor, is directed from a compression region to a suction region in the vacuum chamber and has an orientation that is rotated by about 20*, in the direction of rotation of the rotor, relative to the straight line passing through the rotor centre and the point of tangency between the rotor and the vacuum chamber.

The Applicant has also experimentally realised that a drive shaft configured in accordance with the present invention generates a radial thrust on the rotor, due to the drive of the same rotor, in such a direction as to space out the teeth of the driven wheel from the teeth of the driving wheel along a straight line passing through the respective centres of rotation.

The Applicant has therefore realised that, under the above-describe circumstances (i.e. no positioning constraint for the pump), it is advantageous to locate the pump so that the radial thrust due to the drive is directed in such a direction as to counterbalance, or at least reduce, the effects of the skewing force, thereby reducing the power dissipation due to the friction.

It is clear that the above description has been given only by way of non-limiting example and that changes and modifications are possible without departing from the scope of the invention as defined by the following claims.