DESCRIPTION

Field of the invention

The present invention refers to a filter for hydraulic fluids to be used in hydraulic circuits, such as for example a filter to be used in hydraulic transmission, lubrication systems or pressurised filtration systems of the semi-immersed or immersed type.

For example, the filter object of the present invention can be used in industrial plants, in construction machines, agricultural machines, in earthmoving machines, in vehicles for lifting and transport and other similar applications.

For ease of presentation, the present disclosure is made, by way of non-limiting example, with particular reference to a filter for hydraulic fluids used in the hydraulic circuit of earth lifting machines indicated below also only with the name of "oil" for ease of presentation.

Background of the invention

As is known, hydraulic filters are widely used in lubrication, hydraulic transmission and/or collection circuits to substantially define the means for intercepting and blocking the residues present in the working fluids. De facto, the main cause of the anomalies that can occur in the hydraulic systems is often due to the presence of contaminating elements in the fluid: oil contamination can occur due to water infiltration, or due to the presence of solid particles.

It is clear that a good functioning of the hydraulic filters is a fundamental factor to ensure a correct oil filtration, so as to guarantee the proper functioning of the earthmoving machine.

The correct functioning of the hydraulic filters is fundamental as it allows to make the most of the industrial plants and/or the operating machines in terms of performance, energy consumption and pollution.

In fact, it is clear that an adequate action of filtration and elimination of contaminants, which can be carried out by a filter, ensures that the chemical-physical characteristics of the oil are kept constant over time, preventing impurities and the like from damaging parts of the hydraulic circuits.

Of particular interest are then the hydraulic filters positioned along the line of return to the tank of the hydraulic circuit, which also perform the task of safety filters to protect the working devices from any contaminants present in the fluid, so as to keep the tank fluid clean.

On this point, it should be noted that, generally, hydraulic circuits are provided with collection/accumulation tanks connected to motors and/or pumps: the tank essentially defines the oil storage unit that guarantees to the pump and/or to the motor the use of the desired amount of fluid.

The oil level in these tanks is not at all constant, as it depends on the demand of the hydraulic system of the machine. Thus, for example, in the case of an earthmoving machine provided with a working arm, it is evident that, depending on the position of the working arm, in the extending or rather retracted position, the hydraulic circuit must provide the actuators in charge of moving the working arm with a greater or lesser amount of hydraulic working fluid, fluid coming from the aforesaid collection/accumulation tank.

This explains why the oil level in these tanks is highly variable. Furthermore, it must be considered that at each actuation of a hydraulic pump intended to provide hydraulic working fluid to the users, a transient is established in the fluid circuit during which the hydraulic working fluid is moved to actuate an actuator, with consequent variation in the level of the hydraulic fluid inside the tank.

The filters for hydraulic fluids for hydraulic circuits generally work in a condition such that they are semi-immersed in the hydraulic liquid itself and comprise a filter body, generally having an elongated cylindrical shape, defining the external support casing of the filtering element.

Generally, the filter body has a fixing portion, which allows a stable engagement of the filter body to the tank and the partial immersion of the latter in the working fluid. Again at the top of the filter, the filter body has an inlet provided with a supply chamber through which the working fluid is introduced into the filtering element. At the opposing end in proximity to the bottom of the filter body, there is an outlet for the emission of filtered fluid. The filter body defines in its inside a housing compartment adapted to house the filtering element, the so-called filtering cartridge, which is therefore interposed between the entry and the exit of the working fluid in/from the filter to be crossed by the hydraulic fluid and perform the filtering action thereon.

It should be considered that in the hydraulic circuits a certain amount of air bubbles is often present, since:

- a part of gas (air) is naturally present dissolved in the fluid at an equilibrium point, and can locally, following pressure gradients in the transients, give rise to the formation of bubbles;

- in the hydraulic circuit there may be air suction from the outside due to any small leaks in the circuit;

- air can be trapped in the hydraulic fluid as a result of fluctuations in the level of the fluid in the tank that cause a lapping on the free surface of the hydraulic fluid;

- excessive speed of the fluid exiting the filter may generate turbulence in the hydraulic fluid such as to cause air bubbles to be incorporated, - if the oil outlet is positioned above the free surface of the hydraulic fluid in the tank, foaming/bubbling may occur.

With reference to the presence of air bubbles inside the hydraulic fluid, albeit small in size, it should be pointed out that this presence is completely undesired, as it is harmful to the hydraulic systems and circuits as it:

- may give rise to cavitation phenomena in the hydraulic fluid;

- leads to an increase in the noise level of the hydraulic system;

- poses problems of greater compressibility of the hydraulic fluid;

- reduces the thermal conductivity of the hydraulic fluid and

- may result in a deterioration of the characteristics of the hydraulic fluid over time.

On this point, it should be pointed out that the currently known filters are not always able to avoid or reduce these problems, so that the need to have a filter capable of eliminating or reducing the presence of air inside the hydraulic fluid, in particular after the initial step of the working transients of an earthmoving machine, is very much felt today.

Moreover, it has been experimentally possible to verify that the introduction of hydraulic fluid from the filter to the collection/accumulation tank, particularly when the oil level in the collection/accumulation tank is not high, causes bubbling phenomena in the hydraulic fluid and, above all, incorporation into the fluid of air from the collection/accumulation tank.

Furthermore, these undesired phenomena were found to be all the more accentuated:

- the lower the level of the working fluid inside the filter is and, consequently, the filter itself is filled with air and - the greater the speed with which the hydraulic fluid outflows from the filter to be reintroduced into the collection/accumulation tank.

According to a further aspect, the structure of the filter, in addition to being structurally simple and not very bulky, must allow easy maintenance of its spare parts, avoiding the presence of elements that are particularly complicated to be replaced or that may give rise to problems such as, for example, the difficulty of having to screw parts positioned in proximity to the bottom which, moreover, can cause problems of loosening of parts during normal use in the presence of vibrations and impacts.

In view of the above, it is therefore evident that today the need to have available a filter for hydraulic fluids to be used for hydraulic circuits which allows to perform an effective filtration and elimination of contaminating elements, also reducing the occurrence and the presence of air bubbles inside the liquid contained in the accumulation/collection tank is very much felt. At the same time, the need to be met is also to have a filter that can be easily implemented and positioned inside a tank, and that is easy and simple to maintain, even for usual filtering cartridge replacement activities.

Summary of the invention

The problem underlying the present invention is that of devising a filter for hydraulic fluids to be used in hydraulic circuits, such as for example a filter to be used in hydraulic transmission or lubrication systems or in pressurised filtration systems of the semi-immersed or immersed type, said filter having structural and functional characteristics such as to meet the aforesaid needs, while obviating the drawbacks mentioned with reference to filters for hydraulic fluids of the prior art.

This problem is solved by a filter for hydraulic fluids to be used in hydraulic circuits, such as for example a filter to be used in lubrication systems or in filtration systems of the semi-immersed or immersed type in accordance with claim 1. The remaining claims detail further preferred features that enable further benefits to be achieved.

Brief description of the figures

Further features and advantages of the present invention will emerge from the description given below of some preferred embodiments thereof, given by way of nonlimiting example, with reference to the accompanying figures, in which:

- figure 1 represents a side plan view of a filter for hydraulic fluids according to the invention housed in an oil collection tank;

- figure 2 represents the view of figure 1 in section;

- figure 3 represents a perspective view of the only hydraulic fluid filter of figure 1;

- figure 4 represents a perspective partially broken view of the only filter for hydraulic fluids of figure 1 ;

- figure 5 represents a plane view in longitudinal section of the only hydraulic fluid filter of figure 1 ;

- figure 6 represents a plane view in longitudinal semi-section of the only hydraulic fluid filter of figure 1 ;

- figure 7 represents a view according to section line VII- VII of figure 6;

- figure 8 represents a partially exploded view of the only filter for hydraulic fluids of figure 1 , showing an upper part of the filter body, a chamber body that identifies a collection chamber, a support structure of an anti-emptying structural element of the collection chamber, an annular body that identifies a by-pass passage, a filtering cartridge and a tubular body of the filter body provided with outlet openings;

- figure 9 represents a perspective view of the support structure of the antiemptying structural element of the collection chamber; - figure 10 represents a perspective partially broken view of figure 9;

- figure 11 represents an exploded perspective view of figure 9;

- figure 12 represents a perspective view of the annular body that identifies a bypass passage;

- figure 13 represents a perspective partially broken view of figure 12;

- figure 14 represents an exploded perspective view of figure 12;

- figures 15 and 16 represent plan views in semi-section of the annular body that identifies a by-pass passage with closed and, respectively, open by-pass passage;

- figure 16a represents a perspective view of the annular body that identifies a by-pass passage and of the filtering cartridge before being engaged with each other;

- figure 17 represents a perspective view of the support structure of the antiemptying structural element of the collection chamber associated with the annular body that identifies a by-pass passage;

- figure 18 represents a side plan view of figure 17;

- figure 19 represents a plan view in longitudinal section of figure 17;

- figure 20 represents a perspective view of the only filtering cartridge of the filter of figure 1 ;

- figure 21 represents a perspective partially broken view of the filtering cartridge of figure 20;

- figure 22 represents a partially exploded perspective view of the filtering cartridge of figure 20 and

- figures 23 and 24 represent two exploded views of two distinct components of the filtering cartridge of figure 20.

Detailed description of the invention

With reference to the attached figures, 1 globally indicates a filter for hydraulic fluids according to the invention to be used in hydraulic circuits, such as for example a filter to be used in hydraulic transmission or lubrication systems or pressurised filtration systems of the semi-immersed, or immersed type.

For ease of presentation, reference will be made below to a filter 1 for hydraulic fluids, specifically oil, intended to be housed integrally within a tank S for the storage/collection of oil of an earthmoving machine.

In accordance with a preferred embodiment, the filter 1 has an elongated cylindrical shape extending in a prevailing longitudinal direction X-X and is housed in the aforesaid tank S so as to be inserted from above into the tank, according to a so- called "tank-top" arrangement, in a condition of semi-immersion in the oil contained in the tank S over a prevailing longitudinal section starting from a lower end thereof.

With particular reference to the illustrated embodiment, the filter 1 according to the invention comprises a closed filter body 2 and at least one filtering cartridge 6.

In particular:

- the filter body 2 extends along a predefined axial direction X-X, from a first head end 2a to a second distal head end 2b;

- at the first head end 2a the filter body 2 comprises a chamber body 29 inside which a supply chamber 10 is identified;

- the chamber body 29 is provided with at least one hydraulic fluid inlet 4 in communication with the aforesaid supply chamber 10;

- the filter body 2 also comprises a tubular body 3, delimited by a side wall 3a, which extends longitudinally past said chamber body 29 up to said second distal head end 2b;

- the filter body 2 comprises at least one hydraulic fluid outlet 5 positioned at a substantially median portion of the aforesaid side wall 3a of the tubular body 3; - a compartment 9 is identified in the tubular body 3, which is in fluid communication with the supply chamber 10 and is intended to removably house the aforesaid filtering cartridge 6;

- the filtering cartridge 6 comprises a tubular body extending longitudinally along the aforesaid predefined axial direction X-X from a first head end 6a, which is connected in fluid-tight communication with said supply chamber 10, to a second head end 6b which is closed by a bottom 7;

- said tubular body of said filtering cartridge 6 comprises a continuous tubular layer 8 of filtering material suitable for being crossed by a pressurised hydraulic fluid for performing a filtering action on the pressurised hydraulic fluid;

- the filtering cartridge 6 is housed and supported removably in the aforesaid compartment 9 of the filter body 2.

In a per se conventional manner, the continuous tubular layer 8 may comprise, for example, one or more layers of paper, microfibre, polyester, a sintered material, a non-woven fabric and/or other material suitable for this type of use, as well as it is possible to provide for a net.

Preferably:

- the non-woven fabric materials are based on glass fibres, cellulose fibres, synthetic or polymeric fibres, metal fibres or mixtures thereof;

- fabric materials comprise metallic and/or synthetic threads or filaments and

- the sintered materials comprise synthetic or metal particles or a mixture thereof.

The continuous tubular layer of filtering material 8 subdivides said housing compartment 9:

• into a first chamber 9a (see figures 7, 20, 21) which is internal to the continuous tubular layer of filtering material 8 and is in fluid-tight communication with the supply chamber 10 and

• into a remaining external portion comprised between the tubular layer of filtering material 8 and the side wall 3 a of the tubular body 3.

In accordance with the embodiment considered herein, it has been said that the filter 1 is of the "tank-top" type, i.e. inserted from above into the tank, so that the first head end 2a and the second distal head end 2b of the filter body 2 are the upper end thereof and, respectively, the lower end thereof.

The continuous tubular layer of filtering material 8 of the filtering cartridge 6 is configured and positioned inside the filter body 2 to be interposed in a hydraulic fluid path between the inlet opening, or openings, 4 and the outlet opening 5 so as to be crossed by the hydraulic fluid and to carry out the filtering action thereon, causing the filtration of the hydraulic fluid entering the filter body 2 through the inlet opening, or openings, 4 and exiting from said filter body 2 through the at least one outlet opening 5.

Preferably, the filter body 2 of the filter 1 comprises a fixing portion, which is preferably positioned at the aforesaid chamber body 29 (see figures 1 and 2), to enable a stable and firm fixing of the filter body 2 inside the tank S for the storage/collection of hydraulic fluid.

Preferably, the aforesaid first head end 6a of the filtering cartridge 6 comprises a quick-fit coupling means with a bayonet coupling 21 to ensure a secure and removable fixing of the filtering cartridge 6 inside the filter body 2 in a manner that is better described below in the present description.

Preferably, the aforesaid first chamber 9a internal to the filtering cartridge 6 is connected in hydraulic fluid communication with the supply chamber 10 of the filter body 2 by the interposition of hydraulic fluid interception means provided to create resistance to the passage of hydraulic fluid in order to contrast, slow down and/or prevent the emptying of hydraulic fluid from the supply chamber 10 towards the first chamber 9a internal to said filtering cartridge 6.

Preferably, the aforesaid hydraulic fluid interception means provided in the filter body 2 to slow down, or prevent, the emptying of hydraulic fluid from said supply chamber 10 towards said first chamber 9a internal to said filtering cartridge 6 comprises:

I) a structural element 11 positioned, preferably sealingly, between said supply chamber 10 and said first chamber 9a and comprising:

• a layer provided with a plurality of small calibrated passages suitable for slowing down, or stopping, the passage of hydraulic fluid in particular until a predefined maximum pressure value, in particular when said hydraulic fluid is in a static or rest condition in said supply chamber 10;

• a layer of material permeable to hydraulic fluid;

• a layer of non-soluble granular material of small particle size;

• a microporous continuous layer;

• a filtering net with small meshes;

• an open cell foam of predefined porosity;

• a layer of non-soluble granular material; for example a sintered filter which can be made of synthetic or metal particles and/or a mixture thereof;

• a layer with labyrinth passages;

• a compact layer formed by interwoven threads; and/or

• a layer of fabric, mesh or non-woven fabric material, for example based on glass fibres, cellulose fibres, synthetic or polymeric fibres, metal fibres or mixtures thereof, or

II) valve means reversibly movable from and towards a total or partially closed condition of the passage gap between said supply chamber 10 and said first chamber 9a internal to said filtering cartridge 6, wherein said valve means:

• can be actuated by an actuator or

• may provide a shutter/membrane pushed elastically to close against a shutter seat by elastic means (in accordance with two non-illustrated embodiments).

With reference to the aforesaid shutter/membrane pushed elastically to close against the seat of the shutter by elastic means, it should be noted that the opening of the shutter from the closed position takes place by means of the hydraulic fluid flowing in the filter from the inlet opening towards the at least one outlet, said opening intervening as soon as the pressure of said hydraulic fluid is such that it exceeds the elastic load exerted by the elastic means. Thus, the aforesaid shutter/membrane acts as an antiemptying valve when the hydraulic fluid is at rest, while allowing the passage of the hydraulic fluid as the pressure increases with respect to the pressure value when the fluid is at rest.

Similarly, the aforesaid structural element 11 positioned sealingly between the supply chamber 10 and the first chamber 9a (see figures 4 and 19) allows to act as an anti-emptying device to contrast, slow down and/or prevent the passage of hydraulic fluid when said hydraulic fluid is in a rest condition inside the supply chamber 10, while allowing to the hydraulic fluid to pass, from the collection chamber 10 to the chamber 9a internal to the filtering cartridge 6, as the pressure increases with respect to the pressure value of the hydraulic fluid when said hydraulic fluid is at rest in the collection chamber 10, i.e. when the hydraulic fluid is pressurised by a hydraulic motor/pump.

In accordance with a preferred embodiment, the aforesaid structural element 11 has:

- a plurality of calibrated holes or passages having a maximum dimension of the order of 100-1,500 p m and/or

- a porosity comprised between 40% and 60% (measured voids to solid).

Preferably, the aforesaid hydraulic fluid interception means is supported by a support structure 12 coupled and retained in position with internal components of said filter body 2 by the interposition of sealing means 13.

Preferably, the aforesaid support structure 12 extends longitudinally into the supply chamber 10 with a converging pattern, more preferably with a converging truncated cone pattern, towards a first head end 12a starting from a second head end 12b proximal to an end 29b of the chamber body 29 facing the filtering cartridge 6.

Preferably, the aforesaid support structure 12 extends from the first head end 12a to the second head end 12b by means of a plurality of vanes 22 inclined with respect to the longitudinal axis X-X with helical pattern, so that said support structure 12 identifies a static deviator suitable for diverting a flow of hydraulic fluid transiting in said supply chamber 10 imposing thereon a helical displacement component with respect to said longitudinal axis X-X, so as to ensure better mixing of the hydraulic fluid entering the collection chamber 10. This is particularly advantageous if there are multiple inlet openings 4.

The filtering cartridge 6 comprises a first tubular support element 14 circumscribed about said continuous tubular layer of filtering material 8 for exerting on said continuous tubular layer of filtering material 8 a containment/encircling action, in order to preserve the geometry of said continuous tubular layer of filtering material 8 also in the presence of pressurised hydraulic fluid.

The aforesaid first tubular support element 14 comprises a plurality of passages 15 in order to be crossed by a flow of hydraulic fluid transiting through the continuous tubular layer of filtering material 8. Advantageously, the filtering cartridge 6 also comprises a second tubular element 16 (see figures 21, 22) which is:

- circumscribed about the aforesaid first tubular support element 14;

- spaced at a predefined distance from said first tubular support element 14 and

- comprises a plurality of passages 18 in order to allow to be crossed by a flow of hydraulic fluid transiting through the continuous tubular layer of filtering material 8 and through the first tubular support element 14.

The presence of the aforesaid second tubular element 16 allows to subdivide the aforesaid remaining external portion of the compartment 9 comprised between the tubular layer of filtering material 8 and the side wall 3a of the tubular body 3 (see figures 7, 21 and 22):

- into a first intermediate chamber 9b comprised between the tubular layer of filtering material 8 and the second tubular element 16 and

- into an external chamber 9c comprised between the second tubular element 16 and the side wall 3a of the tubular body 3 of the filter body 2, said second tubular element 16 acting in said filter 1 as a diffuser for slowing down the hydraulic fluid and reducing turbulence.

Preferably, the aforesaid passages 15 of the first tubular element 14 are shaped so as to identify oblong slots extending with a circumferential pattern along the wall of said first tubular element 14.

Preferably, the aforesaid passages 18 of the second tubular element 16 are shaped so as to identify oblong slots extending with a circumferential pattern along the wall of said first tubular element 16.

Preferably, the aforesaid passages 18 of the aforesaid second tubular element 16:

- identify overall a larger passage area, preferably at least 25% larger, than the passage area identified overall by the passages of the first tubular element 14 and/or

- have larger dimensions, preferably at least 25% larger, than the dimensions of the passages of the first tubular element 14.

Preferably, the filtering cartridge 6 comprises:

- an end annular support element 19 provided with means for coupling and fixing to a remaining part of the filter 1 and

- an opposing closing bottom 7.

The annular support element 19 and the opposing closing bottom 7 identify the aforesaid first head end 6a and the aforesaid second head end 6b, respectively, of the filtering cartridge 6 between which the first tubular element 14 and the second tubular element 16 extend longitudinally.

Preferably, the aforesaid means for coupling and fixing said annular support element 19 to a remaining part of the filter 1 is a quick-fit coupling means, more preferably it is a quick- fit coupling means with a bayonet coupling 21.

Preferably said first tubular element 14 and said second tubular element 16 have respective head ends firmly coupled, more preferably removably and/or by means of snap-fit coupling means, to said annular end support element 19 and to said closing bottom 7.

In accordance with the embodiment illustrated in figures 23 and 24, the first tubular element 14 and the second tubular element 16 are formed by a plurality of tubular segments (four and two respectively in the example of the figures) coupled and hooked to each other head-on, for example by means of hooking tabs and respective complementary hooking seats. This allows the filtering cartridge 6 to be made as a modular element whose length can easily be varied based on the specific design needs to be met, as it is possible to connect one, two or more tubular segments head-on to vary the length of the filtering cartridge. Obviously, the axial length of the tubular body 3 must also be set based on the axial length of the filtering cartridge.

Preferably, the aforesaid at least one hydraulic fluid outlet 5 comprises a plurality of through openings 5, preferably holes with a circular section, obtained in the side wall 3 a of the tubular body 3 of the filter body 2.

In accordance with one embodiment, the aforesaid through openings 5 are distributed over a predefined longitudinal section of the side wall 3 a and have larger dimensions in proximity to the second distal head end 2b of the filter body 2 and smaller dimensions in proximity to the first head end 2a of the filter body 2.

Preferably, said plurality of through openings 5 has a decreasing pattern proceeding longitudinally along said side wall 3a of the tubular body 3 of said filter body 2 from said second head end 2b of said filter body 2 towards said first head end 2a of said filter body 2, preferably the aforesaid decreasing distribution of said openings 5 is formed by a succession of longitudinal sections of side wall 3 in which:

- inside the same longitudinal section of side wall 3a of the tubular body 3 of said filter body 2, the through openings 5 have substantially equal dimensions and

- a longitudinal section of side wall 3a of the tubular body 3 of said filter body 2 more proximal to said first head end 2a of said filter body 2 has through openings substantially with the same smaller dimension than the dimension of the through openings 5 of a longitudinal section of side wall 3a of the tubular body 3 of said filter body 2 more proximal to said second head end 2b of said filter body 2.

Preferably, the side wall 3a of the tubular body 3 of the filter body 2 comprises:

- a section free from side openings extending over a distance equal to 15-40% of the longitudinal length of said side wall 3 a of the tubular body 3 of said filter body 2 starting from said second head end 2b of said filter body 2 and/or - a section free from side openings extending over a distance equal to 10-25% of the longitudinal length of said side wall 3 a of the tubular body 3 of said filter body 2 starting from said first head end 2a of said filter body 2.

Preferably, the filter 1 comprises a by-pass means for identifying a by-pass path 27 suitable for connecting the supply chamber 10 to the at least one hydraulic fluid outlet 5 by by-passing the filtering cartridge 6.

In accordance with the illustrated embodiment, the aforesaid by-pass means comprising:

- a tubular body 25, extending over a predefined longitudinal section X-X, inside which, in parallel to a central through channel 26 that identifies a section of path for the hydraulic fluid from the supply chamber 10 to the first chamber 9a inside said filtering cartridge 6, there is identified a by-pass path 27 that allows the supply chamber 10 to be put in fluid connection with the at least one outlet 5 without passing through the tubular layer of filtering material 8 of the filtering cartridge 6, a shutter seat 20 being identified in said by-pass path;

- a shutter 23 movably housed and guided in said by-pass path 27 from and towards a fluid-tight advanced closing position with said shutter seat 20, the hydraulic fluid pressure in said supply chamber acting in the sense of moving said shutter 23 away from said shutter seat 20 and restoring the circulation of hydraulic fluid in said by-pass path 27 and

- elastic means 24 positioned to act on said shutter 23 with a predefined elastic load and push it towards said fluid-tight advanced closing position with said shutter seat 20, the action of said spring being in use intended to contrast the action exerted on said shutter 23 by pressurised fluid contained in said supply chamber 10, in use the aforesaid shutter 23 being intended to move backwards from the shutter seat 20 when the pressure of the pressurised hydraulic fluid acting on said shutter 23 is greater than the force exerted by said elastic means 24.

Advantageously, the aforesaid by-pass means is associated with the filter body 2 in proximity to the supply chamber 10.

Preferably, with reference to the hydraulic fluid travelling direction in the filter 1 from the inlet 4 to the at least one outlet 5, said by-pass path is positioned:

- upstream of the filtering cartridge 6,

- downstream of the supply chamber 10 and

- downstream of the aforesaid hydraulic fluid interception means provided to create resistance to the passage of hydraulic fluid in order to contrast, slow down and/or prevent the emptying of hydraulic fluid from said supply chamber 10 towards said first chamber 9a internal to said filtering cartridge 6.

In accordance with the illustrated embodiment:

- the by-pass path 27 is identified by an annular peripheral channel 27 placed coaxially outside said central through channel 26;

- the aforesaid shutter seat 20 is an annular seat obtained in said annular peripheral channel 27 of the tubular body 25 which identifies a by-pass passage;

- the shutter 23 is an annular shutter suitable for fitting the shutter seat 20 which is annular and

- the aforesaid elastic means 24 is arranged to act circumferentially along the annular shutter 23.

Preferably, the aforesaid elastic means 24 is helical springs distributed circumferentially along the circumference of the annular shutter 23.

In accordance with non-illustrated embodiments, said elastic means may comprise a wave spring, extending circumferentially below the shutter, or may comprise a layer of elastically compressible material.

Preferably, such helical springs 24 are held in place by a respective internal pin 28 having a smaller length than the stroke of the annular shutter from and towards the shutter seat 20.

Preferably, the shutter 23 is provided with sealing means carried to contact said shutter seat 20.

In particular:

- figure 15 illustrates a "normal" operating condition in which the elastic means 24 keeps the annular shutter 23 in abutment against the annular seat 20 preventing the passage of hydraulic fluid, while

- figure 16 illustrates an operating condition that can occur when the pressure of the hydraulic fluid has increased to the point of exerting on the shutter 23 a force greater than the force exerted on said shutter 23 by the elastic means 24, so that the annular shutter 23 is moved backwards from the annular seat 20 opening the by-pass path 27 so as to allow the passage of a flow of hydraulic fluid (schematically indicated by the arrow P in figure 16) towards the at least one outlet 5 by-passing the tubular layer of filtering material 8 of the filtering cartridge 6.

It is worth pointing out that, even in the aforesaid operating condition with the by-pass path open, the hydraulic fluid must pass through the aforesaid structural element 11 positioned sealingly between the supply chamber 10 and the first chamber 9a, so as to be subjected to at least one coarse filtering action.

In accordance with the illustrated embodiment, the first head end 6a of the cartridge 6 is removably coupled, by means of quick coupling connection means, in the example bayonet coupling means 21, with a head end (in the figures it is the lower end) of the tubular body 25 inside which the by-pass path 27 is identified (see figure 16a). As can be appreciated from what has been described, the filter for hydraulic fluids according to the invention allows to meet the aforesaid needs and at the same time to overcome the drawbacks referred to in the introductory part of the present description with reference to the filters of the prior art.

In particular, it was found that:

- the presence of the aforesaid hydraulic fluid interception means, such as for example a structure with calibrated holes or a net, allows to create a force resistant to the passage of oil allowing the oil to incorporate the air present inside the aforesaid supply chamber in the first start-up cycles, preventing the continuous formation of air bubbles during the use of the filter;

- the structural element 11 positioned sealingly between the supply chamber 10 and the first chamber 9a and its structure with calibrated passages/holes allows it to perform the task of pre-filtering element with respect to the filtering cartridge 6, so as to avoid re-circulating totally unfiltered hydraulic fluid in the event of opening of the bypass valve;

- the presence of the aforesaid hydraulic fluid outlet openings 5 obtained in the side wall 3a of the tubular body 3 of the filter body 2, thanks to the presence of holes with decreasing diameters/dimensions towards the supply chamber 10, allows a high flow rate of hydraulic fluid to be disposed of in the first working step of the filter following a start-up of the hydraulic system of a machine, since the smaller holes, placed closer to the collection chamber 10, allow any air that is displaced, particularly during the initial transient, to be disposed of with the volume of oil;

- the second tubular element 16 of the filtering cartridge 6 identifies a gap with respect to the side wall 3 a of the tubular body 3 of the filter body 2 to allow to the oil escaping from the first tubular support element 14 to reduce its speed in the face of an increase in the passage section of the second tubular element 16 of the filtering cartridge 6 with respect to the first tubular element 14 of said filtering cartridge 6, moreover the aforesaid second tubular element 16 also performs the task of supporting and hooking to the head ends of the filtering cartridge 6. Obviously, a person skilled in the art may make numerous modifications and variations to the filter for hydraulic fluids described above so as to satisfy contingent and specific requirements, all of which are contained in the scope of protection of the invention as defined by the following claims.