This application is a National Stage of International Application No. PCT/IB2012/055467 filed Oct. 10, 2012, claiming priority based on Italian Patent Application No. TO2011A000912 filed Oct. 13, 2011, the contents of all of which are incorporated herein by reference in their entirety.
The present invention relates to a rotary vacuum pump and to a method of lubricating such a pump.
Preferably, but not exclusively, the present invention is applied in the automotive field, in particular for intaking air from the brake booster.
Vacuum pumps commonly used in brake boosters of motor vehicles are rotary pumps having a rotor with one or more vanes which, during the rotation of the rotor, give rise to chambers with variable volume. The rotor is made to rotate about an axis, e.g. by the shaft of the vehicle engine, by means of a suitable drive joint, and is housed in a rotor seat or guide that, in most cases, is lubricated, typically with engine oil supplied through a supply channel. Lubrication is aimed at preventing wear of the pump and at creating a seal between the inside and the outside of the pump. Generally, one or more axial grooves are also provided on the rotor guide, in order to improve the transportation of the lubricant towards the pump inside in order to lubricate the components within the pump.
Air from outside the pump (typically at atmospheric pressure) can leak towards the inside of the pump (under negative pressure) through the clearance between the rotor and the guide. Such an air leak towards the inside of the pump increases the power absorbed by the pump and lowers its performance.
In order to reduce such a leak, it has already been proposed to provide an annular groove, filled with lubricant, between the rotor guide and the rotor. The groove may be formed on the rotor guide surface or on the rotor surface or may be defined by steps of such surfaces, and it extends over the whole circumference of the concerned surface(s). Examples of pumps with such an annular groove are disclosed in WO 2009/046810 and FR 2640699. The annular groove improves sealing by providing an oil barrier between the inside and the outside of the pump, thereby preventing air from entering again the pump through the rotor—rotor guide clearances. Yet, such an arrangement gives rise to a problem.
Pressures inside the pump chamber vary with time and are different in the different chambers. Such pressure differences generate forces that radially push the rotor against the guide. Such forces are balanced by the pressure originating in the hydrodynamic bearing provided by the oil between the rotor, which is rotating, and the guide. The provision of an annular groove extending over 360° reduces the area in which the rotor is in contact with the rotor guide and breaks the hydrodynamic bearing in the region of the annular guide and in adjacent areas, thereby reducing the balancing effect on the forces inside the pump. This causes a worsening of the wear with respect to the conventional configurations without annular groove, especially in the mutually contacting areas of the rotor and the guide.
The higher wear, during the pump operating life, produces greater guide-rotor clearances, which in turn cause:
It is the object of the invention to provide a vacuum pump and a method of lubricating same that obviate the drawbacks of the prior art.
According to the invention, this is achieved in that the pump includes at least one partial annular groove (or circumferential groove), which has an angular extension of less than 360° and has at least one interruption arranged to enable creating a hydrodynamic fluid bearing in a region opposite a discharge region of the pump, over a whole axial extension of facing side surfaces of the rotor and the rotor guide.
Advantageously, the at least one circumferential groove has an extension ranging from 150° to 300° and preferably from 180° to 220°.
The at least one circumferential groove may be arranged orthogonally to the rotation axis of the rotor or it may be inclined with respect to said axis. The second solution improves axial lubrication.
The at least one circumferential groove may be formed in the side surface of the rotor or of the guide or it may be defined by steps of said side surfaces. In case of a groove formed in the surface of a vane rotor, the or each groove consists of at least one pair of arcs separated by an equal number of interruptions. In particular, an arc and an interruption are provided for each discharge phase at each revolution of the pump rotor, the arcs and the interruptions being arranged so that, during the discharge phases, the interruptions between the arcs pass in the region opposite the discharge region.
In a second aspect of the invention, a method of lubricating a rotary vacuum pump comprises forming, between facing side surfaces of the rotor and of the rotor guide, at least one circumferential sealing barrier, which has an angular extension of less than 360° and has at least one interruption arranged to enable creating a hydrodynamic fluid bearing in a region opposite the discharge region of the pump, over the whole axial extension of said side surfaces.
Further features and advantages of the invention will become apparent from the following description of preferred embodiments, given by way of non limiting example with reference to the accompanying drawings, in which:
Referring to
Such a pump structure and its general operation are wholly conventional and they do not need a more detailed description.
Guide 3 has formed therein a supply channel 5 for a lubricant, typically the engine oil, intended also to create a seal between the inside 1A and the outside 1B of the pump. Channel 5 ends into a circumferential groove 6 that, in such an embodiment, is formed in guide 3 and lies in a plane perpendicular to the axis of rotor 2. Contrary to the prior art, according to the invention groove 6 does not extend over the whole circumference of guide 3, but only over an arc extending between the two points 6A. There is therefore a region of guide 3 where groove 6 is interrupted.
Groove 6 is to be interrupted where it is necessary or important to provide the hydrodynamic bearing opposing the pressures arising during the discharge phases of the pump (two at each revolution, in the case of the rotor shown in
The extension of groove 6 will be therefore a trade off between the two opposite requirements of not excessively interfering with the formation of the hydrodynamic bearing, and of still having an effective barrier against air leak from the outside. Tests performed by the Applicant have shown that a satisfactory trade off is obtained with an angular extension of groove 6 ranging from about 150° to about 300°. Values at present considered as preferable are in range of about 180° to 220°.
Once the requirement of having the hydrodynamic bearing in the region opposite discharge duct 10 has been met, there are no particular constraints about the position of groove 6. By way of example,
Groove 6 can have any cross-sectional shape (rectangular, trapezoidal, arc of circumference, etc.).
An axial groove 7, extending from circumferential groove 6 towards the inside of the pump or, preferably, extending at both sides of circumferential groove 6, as shown in
The invention solves the problems mentioned above. Indeed, since the circumferential groove does not extend over the whole circumference of the rotor and/or of the guide, an increase of the useful contact area between the rotor and the guide occurs, and the negative phenomenon of the break of the hydrodynamic bearing is avoided. In turn, this entails:
In the variant shown in
In
In
If the pump has a number of discharge phases different from two at each revolution of rotor 102, the circumferential groove formed on the rotor will include an arc and an interruption for each discharge phase, and the arcs and the interruptions will be so arranged that one interruption passes in the region opposite the discharge region at each discharge phase. In the variant shown in
Lastly,
Of course, a plurality of circumferential grooves may be provided also in the embodiment of
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 in the appended claims.
In particular, even if a pump with a vane rotor has been referred to, the invention can be applied also to other types of rotary vacuum pumps. Moreover, is it is self-evident that the invention can be applied whatever to rotation direction of the rotor may be.
Number | Date | Country | Kind |
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TO2011A0912 | Oct 2011 | IT | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/IB2012/055467 | 10/10/2012 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
---|---|---|---|
WO2013/054263 | 4/18/2013 | WO | A |
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Number | Date | Country | |
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20140341767 A1 | Nov 2014 | US |