Abstract:
The invention relates to an aircraft filter device comprising a hydraulic block ( 1 ) adapted to receive a removable filter ( 2 ) comprising a vessel ( 3 ) that receives a cartridge ( 4 ). In accordance with the invention, an insert ( 21 ) is screwed tightly into an internal thread ( 20 ) of the hydraulic block, the insert itself having an internal thread ( 22 ) into which the vessel is screwed, the device also including rotation-preventing means ( 23, 32 ) for preventing the vessel from moving in rotation once it has been screwed into the insert.

Description:
[0001]    The invention relates to a filter device for an aircraft hydraulic circuit. 
       BACKGROUND OF THE INVENTION 
       [0002]    Filter devices for aircraft hydraulic circuits are known that comprise at least one cartridge filter that is separately fitted to a hydraulic block. The filter comprises a vessel that receives a filter cartridge. The vessel is screwed to the hydraulic block and is tightened with torque that is sufficient to ensure that the thread holds together in service, and in particular that avoids any fretting. 
         [0003]    Given the dimensions being proposed for aircraft, such vessels are reaching considerable dimensions, and that the tightening torque increases in proportion, thereby leading to various problems. Firstly, delivering such torque requires torque wrenches to be used that are large in size and heavy, difficult to handle, and above all in zones that are sometimes difficult of access in bays of the aircraft. Such wrenches also require the operator to deliver considerable force. Furthermore, such a tightening torque leads to high levels of stress on the internal thread in the hydraulic block. Given the fatigue cycling that is caused by the vessel being screwed on and off, these high levels of stress constitute a risk of the internal thread in the hydraulic block  1  deteriorating. These risks are made worse by the fact that the hydraulic block is generally made of a material such as aluminum or titanium in which internal threads are known to wear rapidly, even when protective treatments are used. 
       OBJECT OF THE INVENTION 
       [0004]    An object of the invention is to provide a filter device that does not present the above-specified drawbacks. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    According to the invention, there is provided an aircraft filter device comprising a hydraulic block adapted to receive a removable filter comprising a vessel that receives a cartridge. According to the invention, an insert is screwed tight in the internal thread of the hydraulic block and is tightened therein, the insert itself having an internal thread in which an external thread of the vessel is engaged, the hydraulic block also having rotation-preventing means for preventing the vessel from moving in rotation once it has been screwed into the insert. 
         [0006]    The internal thread that is made directly in the hydraulic block and that receives the insert is indeed subjected to a high degree of tightening, however insofar as the insert is not designed to be removed in service, the internal thread is no longer subjected to frequent screwing and unscrewing operations, thereby avoiding it suffering from fatigue degradation. The internal thread in the insert that receives the external thread on the vessel is not subjected to high degrees of tightening, since the vessel is prevented from moving in rotation relative to the insert so there is no risk of the vessel coming unscrewed, and it is therefore not necessary to screw the vessel tight into the insert. As a result frequent removal of the vessel does not run any risk of fatiguing the internal thread in the insert. 
         [0007]    Thus, the use of an insert serves to avoid coupling the effects of torque at a high degree of tightening with the fatigue cycling caused by the filter vessel being screwed and unscrewed. Furthermore, it is possible to select a material for making the insert (e.g. stainless steel) that is much better at withstanding frequent screwing and unscrewing operations than are the materials normally used for fabricating hydraulic blocks (aluminum or titanium). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention can be better understood in the light of the following description given with reference to the figures of the accompanying drawings, in which: 
           [0009]      FIG. 1  is a longitudinal section view of a filter vessel of a filter device in a particular embodiment of the invention; 
           [0010]      FIG. 2  is a view analogous to  FIG. 1  showing the  FIG. 1  vessel with a cartridge inserted therein; 
           [0011]      FIG. 3  is a longitudinal section view of the filter device in which the filter vessel is presented to the hydraulic block while the vessel does not contain any cartridge; 
           [0012]      FIG. 4  is a section view of the filter device of the invention with the filter of  FIG. 3  in place on the hydraulic block; 
           [0013]      FIG. 5  is a section view on line V-V of  FIG. 4 ; 
           [0014]      FIG. 6  is a section view on line VI-VI of  FIG. 4 ; and 
           [0015]      FIG. 7  is a perspective view of a pin fitted to the vessel of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    According to the invention, and with reference to  FIGS. 1 and 2 , the filter device for an aircraft hydraulic circuit includes a filter  2  comprising a vessel  3  that receives a filter cartridge  4 . The filter  2  is designed to be fitted to a hydraulic block  1  as can be seen in  FIGS. 3 and 4 , and that is made out of titanium, in this example. 
         [0017]    The vessel  3  has an annular internal groove  5  with a pin  6  inserted therein, which pin can be seen more clearly in  FIG. 7 . The pin is made of spring steel in this example and comprises a belt  7  that engages resiliently in the inner annular groove  5  of the vessel  3 . A key  8  with a curved end  9  projects upwards from the belt  7  so that when the pin  6  is in position on the vessel, its curved end  9  passes over the rim of the vessel  3  and projects radially outside it, as can be seen in  FIG. 1 . A finger  10  also extends from the belt towards the bottom of the vessel, and is curved towards the center of the vessel  3 . 
         [0018]    Returning to  FIGS. 1 and 2 , the cartridge  4  includes a stand  11  that centers the cartridge  4  in the vessel  3 , and a hollow cap  12  with a skirt  13  that fits over the outside of the cartridge, and that terminates in a step  14 . When the cartridge  4  is put into place in the vessel  3 , the finger  10  of the pin  6  is pushed towards the wall of the vessel  3 , thereby having the effect of twisting the belt  7  locally and causing the curved end  9  of the key  8  to retract so that the curved end  9  no longer projects radially from the rim of the vessel  3 , as can clearly be seen in  FIG. 2 . 
         [0019]    The pin  6  thus performs two functions:
       it prevents the filter  2  being mounted on the hydraulic block  1  unless a cartridge is already in place in the vessel  3 . In this situation, as shown in  FIG. 3 , the curved end  9  prevents the vessel  3  being screwed to the hydraulic block  1 ; and   it holds the cartridge  4  in the vessel  3  while the vessel is being removed, by means of the end of the finger  10  co-operating with the step  14  on the cap  12  that forms an obstacle that comes into abutment against the end of the finger  10 , such that during removal of the filter, the cartridge  4  is constrained to move with the vessel  3 . In order to withdraw the cartridge  4  from the vessel  3  once the filter  2  has been removed from the hydraulic block  1 , it is necessary to withdraw the pin  6 , or at least to move the finger  10  away from the step  14 .       
 
         [0022]    According to the invention, and with reference to  FIGS. 3 and 4 , the hydraulic block  1  includes an internal thread  20  receiving an insert  21 , here a stainless steel insert, that is screwed therein and tightened with tightening torque that is sufficient to ensure good mechanical behavior of the threads in service. The insert  21  is normally not removed in service, such that the internal thread  20  is not subjected to the fatigue cycling of screwing and unscrewing operations. 
         [0023]    To lock this screw fastening, a tongue  33  is screwed onto the hydraulic block  1  so as to face a peripheral surface of the insert  21 , which surface is notched. As can be seen in particular in  FIG. 6 , the tongue  33  co-operates with the notched peripheral surface of the insert  21  to prevent any unscrewing of the insert. 
         [0024]    The vessel  3  is not screwed to the hydraulic block  1 , but instead it is screwed into an internal thread  22  of the insert  21 . Here, the vessel  3  is screwed in until a shoulder  23  of the vessel  3  comes into contact against the insert  21 . 
         [0025]    Returning to  FIG. 3 , it can be seen that a spring blade  30  is screwed to the hydraulic block (here using the same screws as hold the tongue  33 ) so as to present a cantilevered portion  31  and a toothed portion  32  that projects towards the shoulder  23  so as to co-operate with peripheral teeth thereof. When the filter is in position on the hydraulic block  1 , as shown in  FIG. 5 , interengagement between the teeth of the shoulder  23  and the teeth of the toothed tab  32  prevents the vessel  3  from turning relative to the hydraulic block  1 . This interengagement is shown in  FIG. 5 . The spring blade  30  is stiff enough to ensure that the various levels of vibration and impacts, in service, cannot cause the toothed tab  32  to separate from the shoulder  23 , and thus to ensure that the vessel cannot become unscrewed from the insert  21 . In order to enable the vessel  3  to be screwed on or off, it suffices to move the toothed tab  32  resiliently away from the shoulder  23 , as represented by the arrow in  FIG. 5 . 
         [0026]    In a variant, the teeth of the toothed tab  32  may present a shape suitable for enabling the toothed tab to be moved away when the vessel is turned so as to unscrew it. 
         [0027]    Thus, there is no point in screwing the vessel  3  tight into the internal thread of the insert  21 , since the vessel cannot be come unscrewed in service. The internal thread  22  receiving the vessel  3  is thus indeed subjected to frequent screwing and unscrewing operations, however it is not subjected to tightening so that these screwing and unscrewing operations do not run any risk of subjecting the internal thread to fatigue damage. 
         [0028]    Thus, the internal thread  20  in the hydraulic block  1 , which is made of titanium, is indeed subjected to a high level of force, but it is not subjected to any repeated screwing and unscrewing, and therefore does not run any risk of damage, providing it is properly dimensioned. The internal thread  22  made in the insert  21  is indeed subjected to repeated screwing and unscrewing, but not to any significant tightening force, such that it does not run the risk of deteriorating either. Making the insert out of stainless steel provides the opportunity of presenting an internal thread that is relatively insensitive to wear due to screwing and unscrewing operations. 
         [0029]    In a particular disposition, the insert  21  forms a seat  28  for a valve member  24  that is movable axially inside the hydraulic block  1 . As can be seen in  FIG. 3 , when the filter is not yet in place on the hydraulic block  1 , the valve member  24  co-operates with the seat  28  to close off communication between a first port  100  in the hydraulic block  1  and the inside of the insert  21 . In contrast, once the filter is in place, as shown in  FIG. 4 , the bottom end of the valve member  24  engages a cone  18  on the cap  12  of the cartridge  4  and is pushed back against a spring  25  by the cap  12  so as to open a passage between the port  100  and the outside of the cartridge  4 . It should be observed that a gasket  19  extends between the cone  18  and the valve member  24 , which gasket tends to retain the cartridge on the valve member  24 , while the filter is being removed. The finger  10  of the pin  6  serves specifically to counter this retention and force the cartridge to come away with the vessel while the vessel is being unscrewed. 
         [0030]    In the same figure, it can be seen that the valve member  24  is hollow and forms a channel towards a second port  101  of the hydraulic block  1 , thereby putting the inside of the cartridge  4  into communication with the second port  101 . Hydraulic fluid can thus flow from the first port  100  towards the second port  101  while passing through the cartridge  4 . In order to avoid any fluid returning towards the first port  100 , the valve member  24  is fitted internally with a check valve  40  that comprises a check valve member  41  urged by a spring  43  towards a seat  42  formed in the recess in the valve member  24 . The check valve member  41  is pushed back against the spring  43  by the fluid coming from inside the cartridge. 
         [0031]    It should be observed that when the filter  2  is in position on the hydraulic block  1 , a dead volume V of fluid (represented by dots in  FIG. 4 ) extends between the top end of the vessel  3  and the seat  28  of the insert  21 . 
         [0032]    When the filter  2  is removed from the hydraulic block  1 , all of the hydraulic fluid contained in the vessel comes with the filter  2 , whereas all of the fluid contained in the hydraulic block  1  above the seat  28  remains in the hydraulic block, being retained by the valve member  24  closing. However, the fluid contained in the dead volume V that extends between the seat  28  and the top end of the vessel  3  is neither contained in the vessel nor retained by the valve member  24 . 
         [0033]    To prevent the dead volume V spilling to the outside when the filter is removed, and in accordance with the invention, the volume occupied by the valve member  24  in the vessel  3  when the filter  2  is in position on the hydraulic block  1  is designed to be greater than the dead volume V. In this way, while the vessel  3  is being unscrewed, the fluid contained in the dead volume V progressively takes up position in the vessel  3  as the valve member  24  moves out from the vessel  3 , thereby releasing volume in the vessel  3  that is available for receiving this fluid. By ensuring that the volume that is released is greater than the dead volume V, it is possible to avoid any fluid being spilt to the outside when removing the filter  2 , other than a few drops. 
         [0034]    The invention is not limited to the above description, but on the contrary covers any variant coming within the ambit defined by the claims. 
         [0035]    In particular, although a member for preventing the vessel from turning is described as comprising a toothed tab co-operating with a toothed shoulder of the vessel, any other equivalent member could be provided, e.g. a retractable finger entering into a recess formed in the wall of the vessel.