DESCRIPTION

The present invention is in the field of servomechanisms for the hydraulic driving of directional control valves used in the construction of mobile vehicles and, in particular, an object of the present finding is a hydraulic servomechanism that allows reducing the thrust work by the operator, avoiding the oscillation thereof.

The actuation of the servomechanism is carried out by the operator who is driving a mobile vehicle by a mechanic kinematic mechanism controlled by a lever or a pedal.

STATE OF THE ART

A first prior-art example is described in the patent EP1777419, where a spring and a force generated by the application of a reduced pressure on the whole diameter of the spool itself act on the spool and oppose the actuation carried out by the operator. Said force generated by the reduced pressure also acts by opposing an inner spring to implement the adjustment of the spool itself.

In the case that said servomechanism has to be used on large-sized machines, for a proper transmission of the control signals it is necessary to use spools having a larger diameter, which allow dispensing a higher flow rate. While keeping the controlling pressure constant, if the area onto which such pressure acts is increased, the work that the operator has to transmit to the servomechanism in order to achieve the same translation is higher.

Many manufacturers obviated such a problem by using differential area servomechanisms, an example of which is the document EP1031780.

In this type of mechanisms, the reduced pressure no longer acts on the whole diameter of the spool but, due to the difference of the diameters of the spool itself, such a pressure is applied on a circular crown. Even by increasing the spool diameters, the effect of the thrust due to the reduced pressure that acts by opposing the operator is reduced.

The drawbacks of the prior art are related to the swinging phenomena on the control kinematic mechanism of said servomechanisms. Such oscillatory phenomena, by generating such a swinging of the kinematic mechanism as to actuate in an uncontrolled manner the servomechanism, are not only a serious operational defect, but also a problem for the operators' safety, since it causes involuntary and incontrollable operations of the vehicle on which the servomechanism is mounted.

In order that, with the differential area solutions, the servomechanism works properly, the chamber opposite the pusher has to be at the same pressure of the draining. In fact, if this does not happen and some pressure remains trapped, the oscillations described above can start. In order to be able to implement such connection, the prior art hypothesizes three The first solution is to insert along the servomechanism cavity a chamber which intercepts the spool below the supply pressure chamber P and the reduced pressure and is related either to the use of holes or not to intercept the draining line within the body of the servomechanism itself; this involves inserting an additional cavity in the same servomechanism body, increasing the number of the conduits thereof and/or the passages obtained in the core box or by processing which would overlap, and the height of the body itself;

The second solution provides for inserting in the lower part of the servomechanism a direct draining connection between the chamber opposite the pusher and the exterior of the body of the servomechanism, making the insertion of additional outward holes or a lid to convey the chambers towards the only drain of the body necessary, with the drawbacks of the previous solution;

The third solution introduces a through hole that crosses the whole spool, which is capable of connecting the pusher side draining chamber with such a chamber opposite thereto, such as for example described in US 4 566 492; the limit is to obtain a hole having a diameter large enough through to spool to allow discharging all the pressure applied in such a chamber towards the drain, while keeping a sufficient thickness of the spool core in order to ensure a maximum tensional state, which is limited under operative conditions, so as to avoid a fatigue breakage of the core diameter itself below the number of the life cycles provided for said application. Furthermore, solutions similar to those of the aforementioned patent may be subject to an additional drawback, represented by the fact that the reduced pressure chamber that intercepts the spool within the cavity thereof is located between the level of the draining pressure and that of the supply pressure P.

In essence, in a differential servomechanism according to the prior art the recesses connecting the draining and the reduced pressure areas are arranged on a larger collar than that of the recesses connecting the supply pressure to the reduced pressure itself. For this reason, it is necessary to use, in the connection between supply pressure and reduced pressure, deeper recesses in order to maintain the same pressure delta used in a non- differential area servomechanism, but with the construction limit that, if there is a through hole having the core diameter, it is not always possible to bring them to a desired depth, due to the structural limits described above.

OBJECTS AND ADVANTAGES OF THE INVENTION

Object of the present finding is to minimize the actuation work of a hydraulic servomechanism by a differential area compensation system and an appropriate positioning of the reduced pressure and supply pressure chambers intercepting the cavities of the spools that are present along the servomechanism.

In particular, an embodiment of the present invention provides for obtaining the differential area through a change in the area that is obtained along spool within the chamber connected to the driving reduced line.

Another object of the invention is to eliminate the swinging phenomena; this occurs with a channel crossing the whole spool and which joins the chamber opposite the pusher to the draining chamber on the side of the pusher itself.

Another object of the invention is to obtain a differential area downstream the intersection between the supply pressure and the reduced pressure chambers by decreasing the diameter of a second collar of the spool.

By virtue of such solution, the core length connecting the first and the secondo collars of the spool no longer has limitations that require to have a deeper connection recess between supply and reduced pressure to maintain a proper pressure differential, and it is therefore possible to obtain a sufficiently large hole which passes through the entire spool, allowing to discharge the chamber opposite the pusher without affecting the tension limits of the spool itself at the level of the core diameter.

Another undesired effect of a servomechanism turns out to be the excessive rapidity in the dynamic response; in order to ensure a more gradual return of the spool of the servomechanism, which can be annoying if it is too quick, it is important to be able to narrow the connection between the reduced pressure chamber of the servomechanism and the discharge, and this happens by one or more transversal holes obtained on the spool collar and suitable to intercept an inner conduit.

Said objects and advantages are all achieved by the hydraulic servomechanism with an oscillation damping system, which is the subject matter of the present finding, which is characterized by what is provided in the claims set forth below.

This and other features will be more detailed by the following description of some embodiments that are illustrated, by way of example only and not by way of limitation, in the attached drawing tables, in which :

Fig . 1 illustrates a section of a non-differential area hydraulic servomechanism according to the prior art;

Fig . 2 illustrates a section of a differential area servomechanism, according to the prior art, with the chamber opposing the pusher directly connected to a transversal draining conduit that intercepts the chamber itself; said conduit is arranged along the cavity located below the supply line chamber and the reduced pressure chamber, which intercept the spool;

Fig . 3 illustrates a further section of a differential area servomechanism, according to the prior art, with a through hole through the spool joining the chamber opposite the pusher with the drain;

Figs. 4 and 4A illustrate a section of the differential area servomechanism which is the subject matter of the invention and a detail thereof.

DETAILED DESCRIPTION

Referring to Fig. 1, the operation modes of a hydraulic servomechanism according to the most widespread prior art are illustrated.

It consists in a body la within which pressure reducing valves operate.

The pressure reducing valve consists in a spool 8, springs 2a and 2b, and a plate 7, and it is actuated by a pusher 4 which in turn is actuated by a cam 5 integral to the driving kinematic mechanism 20.

Three overlapping chambers intercepting the cavity 3 of the spool 8 are located in the body la; one in a central position 9, connected to the pressure line P, and another one in the upper position 6, connected to the discharge line T; finally, a chamber is located in the lower position, in which the reduced pressure generated between the two chambers (12) is present.

Both chambers are connected to the ports P and T, for the connection with the pump and the discharge, respectively, located in the lower part of the servomechanism.

Specifically, the chamber 6 is connected to the discharge port T via a hole 10 located in the lower part of the chamber itself.

When the cam 5 pushes the pusher 4 downwards, the latter acts on the plate 7 and thereby the spring 2b is compressed, which, by acting on the spool 8, pushes the latter downwards to the operative position.

A first drawback is due to the fact that, during the above- mentioned actuation, the modulated reduced pressure that is present in the chamber 12 opposes the pusher on the spool face 13 : the larger the diameter thereof is, the higher the actuation required by the user is.

The prior art already provides hydraulic servomechanisms capable of reducing the actuation work also when the spool 8 requires a diameter 13 having a considerable diameter to allow the use of higher oil flow rates along the draining line; with reference to Fig. 2, the characteristics as a differential area servomechanism according to the prior art are illustrated. In the body lb the three overlapping chambers are arranged in the following order: the centrally-positioned chamber is connected to the reduced pressure 12, the upper one is connected to the draining 6, and the lower one is connected to the supply pressure P(9). Unlike the previous solution, a fourth chamber 21 is also present, which is created along the cavity 3 of the spool 108 at the end opposite the pusher.

The spool 108 is divided into two collars (14,18) separated by a core 17. The draining 6 is connected to the reduced pressure chamber 12 through one or more recesses 15 and the supply pressure chamber 9 is connected to the reduced pressure chamber through one or more recesses 19.

The upper collar 14 has a larger diameter than the lower collar 18; thus, the reduced pressure acting on the upper collar area 16 and the lower collar area 25 is not balanced and generates an upward thrust opposing the pusher 4 when it is actuated . Since the recess(es) 19 are obtained on a collar 18 having a smaller diameter, to properly adjust the differential pressure between the supply chamber and the reduced pressure chamber, they have to be deeper, with the construction limit of said processing being dictated by the dimension of the diameter of the core 17 of the spool itself.

A further drawback of the configuration of the servomechanism according to the prior art described above is due to the fact that, in the chamber 21, the creation of undesired forces on the spool 108, which are generated through the intervention of the pressure on the lower face 103 of the spool, leads to lose the control of the servomechanism. It is necessary that the chamber 21 is connected to the draining line 10 via a transversal conduit 23 or via an outward draining hole 22 (just one of the two measures described above is sufficient). Such a solution involves a higher implementation complexity level within the body lb of the servomechanism as regards the number of conduits, corresponding intersections within the body thereof, outward connections and/or cavities integrally obtained.

In Fig. 3, a differential area servomechanism is shown, in which the chamber 21 is connected via a through hole 24, obtained within the spool 208, and the successive transversal hole 26 transversal to the chamber 6 connected to the draining line 10. The construction limits of said variation as shown in Fig. 3 are of a mechanical-structural nature; in fact, the hole 24 has to be large enough to discharge to the draining 6 all the pressure of the chamber 21, the diameter of the core 17 of the spool 208 has to be dimensioned so as to be large enough to house a properly- sized hole 24 and ensure a maximum tensional state, which is limited under operative conditions, in order to avoid a fatigue breakage below the number of the life cycles provided for above- mentioned application. As illustrated above, the upper limit of said core diameter is given by the depth of the recess(es) 19 obtained on the lower collar 18 of the spool 208.

In Fig. 2, as well as in Fig . 3, the connection between the chamber 6 and 12 is carried out on the upper collar 14 of the spool 108/208 through one or more recesses 15. By varying such narrowed passage, it is possible to control the reduced pressure discharge so as to ensure a more gradual and not excessively quick return of said adjusting spool 108/208, thus reducing the unpleasant effects to the operator of a too rapid dynamics.

In view of the problems in the prior art as described above, the object of the present finding is to provide a new servomechanism capable of solving the problems set forth above and minimizing the processing costs. The means through which the above-mentioned problems are solved will be better explained with the help of Fig . 4, representing the hydraulic differential area servomechanism which is the subject matter of the present finding.

In the body Id, the three overlapping chambers are arranged in the following order: the one in the central position is connected to the supply pressure 9, the upper one is connected to the draining 6, and the lower one is connected to the reduced pressure 12.

A fourth chamber 21 is defined, along the cavity 3, exactly on the end opposite the pusher 4; the chamber 21 is adapted to receive the end portion of the spool 308 in operative configurations. Again, the cavity 21 is also connected to the draining chamber 6. In the example, such a connection is obtained by a through hole 24 and a transversal hole 26.

The spool 308 comprises two collars, the upper one being indicated with 314, while the lower one is indicated with 318, which are separated from the core of the spool itself, indicated with 317. The lower collar 318 is in turn composed of two parts 318a and 318b, of which the first part 318a is adjacent to the core 317 and has the same diameter as the upper collar 314. On the other hand, the core 317 has a lesser diameter than that of the upper collar 314 and, consequently, also of the first part 318a of the lower collar.

It follows that the differential area used to perform the adjustment of the hydraulic servomechanism itself is no longer obtained through the difference of the two areas 316 and 325 placed at the two ends adjacent to the core, because they are the same. It is no longer necessary to create deeper recess(es) 319 to carry out a modulation of the reduced pressure between the supply chamber 9 and the chamber 12, since said recess is obtained on the larger diameter of the spool: the core 317 has a less stringent construction limit in terms of maximum size compared to the previous cases of the prior art.

The hole within the core 317 itself can be made with a sufficient size to discharge the pressure that is created within the chamber 21 with minimum load losses.

The lower portion of the collar 318 (318b) has a smaller diameter than that of the upper portion adjacent thereto 318a of said collar, within the chamber 12 a change of area 28 is created on 318, which forms a narrowing with respect to both the first part 318a and the second part 318b of the collar, onto which the reduced pressure of the servomechanism now acts. This variation of area defines a step that has the same effect as the differential area created in the servomechanisms obtained according to the prior art as per Figs. 2 and 3.

Since the hole 24 can be made of a smaller size than that case of the prior art, then it is possible, through this hole, to connect and modulate the reduced pressure that is present in the chamber 12 with the draining that is present in the chamber 6.

The above-mentioned adjustment is obtained by one or more transversal holes 27 obtained on the lower collar 318 of the spool 308, which intercept the conduit 24. By varying the dimension of said hole(s) 27, it is possible to control the discharge of the reduced pressure so as to ensure a more gradual and not excessively timely return of said adjusting spool 308.

The chambers 6, 9, 12 are suitable to intercept the cavity 3 transversally along the body Id; in more detail, the chamber 9 (receiving the operative hydraulic pressure) turns out to be the intermediate one, while le reduced pressure chamber 12 is the lower one.

With such arrangement of the chambers 6, 9, 12, a hydraulic differential area servomechanism is obtained, which is unresponsive to the oscillations caused by the residual pressure of the chamber 21 in case it is not properly connected to the discharge.

In addition, without the stringent depth limits for the recess(es) 319, the diameter of the core 317 is wide enough to allow the construction of an inner conduit 24 without being excessively stressed from the point of view of the material tensions under operating conditions, and to allowing the adjustment between reduced pressure and draining. INDEX OF REFERENCES TO THE FIGURES:

la, lb, lc, Id body

2 springs (2a and 2b)

3 spool cavity

4 pusher

5 cam

6 draining chamber

7 plate

8 spool

108 spool

208 spool

308 spool

9 supply chamber (P)

10 discharge conduit (T)

12 reduced pressure chamber

13 spool 8 lower area

14 upper collar

15 upper collar recesses

16 upper collar area

17 core spool

18 lower collar lower collar recesses

kinematic mechanism

chamber opposite the pusher

outward draining hole

draining 10 connecting conduit

through hole

lower collar area

spool 108 lower area

spool 208 lower area

spool 308 lower area

upper collar

upper collar area

spool 308 core

lower collar

a upper collar upper part

b upper collar lower part

lower collar recesses

lower collar area

transversal hole intercepting the draining reduced pressure modulation hole area variation