Abstract:
The present invention relates to a membrane for secondary air suspension of a land or rail vehicle, in particular of heavy truck or passenger wagon type, this secondary suspension incorporating it, such a vehicle which is provided therewith, and the method for manufacturing this membrane. 
     This substantially toroidal membrane ( 103 ) comprises two attachment areas ( 110 ) and at least one corded layer ( 106 ) of fabric and/or metallic threads which is situated between two radially internal ( 104 ) and external ( 105 ) elastomer layers and which extends between these attachment areas. 
     According to the invention, said at least one corded layer has its or each of its thread(s) individually sheathed by an elastomer sheath compatible with the internal and external layers.

Description:
RELATED APPLICATIONS 
     The present application is based on and claims the benefit of French Patent Application No. 1152452 (filed Mar. 24, 2011) which is hereby incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a membrane for secondary air suspension of a land or rail vehicle, in particular of heavy truck or passenger wagon type, this secondary suspension incorporating it, such a vehicle which is provided therewith, and a method for manufacturing this membrane. 
     BACKGROUND OF THE INVENTION 
     Generally, a passenger car body for a rail vehicle is supported by two bogies respectively situated toward the two ends of the body. Each bogie generally comprises two axles, and each bogie is linked on the one hand to each axle by a primary suspension device with helical springs for example, and on the other hand to the body of the car by two secondary air suspension devices which are each associated, for reasons of safety, to an emergency device suitable for compensating any leak of air in the associated secondary suspension and which could possibly provoke a derailing of the rail vehicle running at high speed. 
     The role of these secondary air suspensions is essentially to improve the comfort of the passengers, and they therefore have to offer a very wide flexibility. Physically, a secondary air suspension consists of a flexible and air-tight membrane, which can take the form of a tire linked to an auxiliary air tank to increase the volume of air stored in the membrane. The document FR-A1-2 896 220 can, for example, be cited for the description of such a secondary air suspension. 
     As is known, these secondary air suspension membranes are reinforced by sheets of woven textile fabrics (inlaid textiles) which are embedded in rubber, as for example described in the document EP-A1-1 918 609 which mentions such textile/rubber composite fabrics and as illustrated in  FIG. 8  appended to the present description which shows a phase of forming, on a mandrel a, of a woven sheet b of such a known membrane. As can be seen in this  FIG. 8  which also shows two rods c provided to reinforce two attachment areas of the membrane and between which the sheet b is intended to extend, the latter comprises warp threads b 1  which extend substantially in the axial direction YY of the mandrel a and which are linked together by weft threads b 2  which extend substantially in the circumferential direction of the mandrel a. It can also be seen in this  FIG. 8  that the duly formed woven sheet b necessarily has an overall axial area of overlap b 3  (known as “duplicature” to those skilled in the art) defining two thicknesses of this sheet b at its connection around the mandrel a. 
     A major drawback with these known membranes with woven fabric sheets lies in their relatively low mechanical strength in operation. In particular, the doubling of the density of threads b 1 , b 2  in this area b 3  constitutes an obstacle to the uniform expansion of the membrane when inflating the membrane preform before vulcanization, which generates a line of mechanical weakness of the membrane in use. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is to propose a substantially toroidal membrane for secondary air suspension of a land or rail vehicle, the membrane comprising two attachment areas and at least one corded layer of fabric and/or metallic threads which is situated between two radially internal and external elastomer layers and which extends between these attachment areas, which in particular remedies this drawback. 
     To this end, a membrane according to the invention is such that said at least one corded layer has its or each of its thread(s) individually sheathed by an elastomer sheath compatible with these internal and external layers. 
     It will be noted that this membrane according to the invention has an asymmetrical geometry, seen in axial section (i.e. in transversal section in a plane perpendicular to the circumferential direction of the membrane), unlike the tires, with, for this membrane, a circumferential bead which extends asymmetrically between its two attachment areas by being inclined axially toward one of these areas. 
     According to another feature of the invention, this membrane is such that said or each corded layer has no discontinuity between said attachment areas, in particular having no area of overlap of this layer by itself extending from one of the attachment areas to the other. 
     It will be noted that this absence of discontinuity from one attachment area to the other, such that the area of overlap or “duplicature” mentioned above in relation to the woven sheet of the prior art, eliminates any line of mechanical weakness for the membrane generated by an expansion defect of such an area during inflation of the membrane, as explained above. 
     According to another aspect of the invention, said or each corded layer has no weft thread, unlike these woven sheet membranes of the prior art. 
     Advantageously, this sheath can be extruded around this/these thread(s) and can have a thickness of between 0.1 mm and 1 mm. 
     According to another feature of the invention, said or each sheathed thread can form a winding of adjacent turns in said at least one corded layer. In other words, in the case where the membrane comprises a single corded layer, the latter can have adjacent turns and, in the case where the membrane comprises a number of radially superposed corded layers, at least one of these layers can have adjacent turns. However, according to a variant of the invention, the corded layer(s) may not have adjacent turns, which means that it is possible to have no corded layer with adjacent turns in the membrane. 
     It will be noted that this individual sheath of thread which coats the turns of said or each thread is not the same as the coating of rubber in which the two sides of a woven textile fabric are embedded according to the prior art. 
     It will also be noted that this sheath makes it possible, on the one hand, to improve the “crude bonding” of the wound thread when forming the non-cross-linked membrane preform on a mandrel with enhanced control of the trajectory or inclination of this thread and, on the other hand, to minimize the wear-resistance in operation of the cross-linked membrane despite the preferably adjacent nature of the turns of the corded winding. 
     Advantageously, the membrane may comprise a number, and preferably 2n (n being an integer ≧1) said corded layers radially superposed, each of which comprises a sheathed textile cord forming said or each thread and preferably at least partly made of a polyamide such as aramid and/or of a polyester such as PET. These corded layers may be formed by a continuous winding of a single said sheathed textile cord according to angles +α, −α with the axis of revolution of the membrane which balance out overall with one and the same number of turns of angle +α and of angle −α, with a preferably being between 0° and 90°. 
     It will be noted that it is possible to use a number of materials for the sheathed thread(s) of the or each corded layer (e.g., a number of textile materials within one and the same corded layer, such as, for example, PET and aramid), said corded layer then having a hybrid structure. 
     Furthermore, and optionally in the case where at least one of the corded layers has this hybrid structure, it is possible to use different thread materials from one corded layer to another one topping it directly or not. 
     Even more advantageously, said corded layers may mutually overlap so that the sheathed thread of one of these layers entirely covers that of the underlying layer, or else cross so that the respective sheathed threads of these layers are interlaced at regular intervals in the manner of a woven fabric. 
     According to another feature of the invention, said or each thread may be covered by an adhesion activator at its interface with said sheath that it includes, this activator preferably being a resorcinol formaldehyde resin (RFL for short) or a methylol donor (e.g. hexamethoxymelamine or hexamethylenetetramine). 
     Advantageously, said sheath may consist of a cross-linked elastomer composition which is based on at least one rubber or thermoplastic elastomer (TPE), just like the internal and external layers, but which is different from that of these two layers, this sheath preferably having a thickness of between 0.1 mm and 1 mm. 
     It will be noted that this sheath makes it possible to improve the bond between the two internal and external layers, but that it should not be too thick so as not to penalize the desired density of threads which should be as high as possible. This sheath thickness is chosen according to the nature of the thread (in particular its mechanical strength) and the density of threads necessary for the good reinforcement of the membrane. 
     Also advantageously, said compositions of sheath and of internal and external layers may be based on at least one halogenated or non-halogenated dienic elastomer, preferably chosen from the group consisting of polychloroprenes (CR), polybutadienes (BR), styrene-butadiene copolymers (SBR), polyisoprenes (IR), natural rubber (NR), ethylene-propylene-diene terpolymers (EPDM) and their mixtures, these compositions also comprising a reinforcing filler preferably based on carbon black and/or a mineral filler such as a silicon oxide. As a variant, these compositions of sheath and of internal and external layers may be based on at least one thermoplastic vulcanizate (TPV) as thermoplastic elastomer, it being understood that other TPE can be used. 
     According to another feature of the invention, said attachment areas of the membrane may be respectively reinforced by two rods which may consist of said thread(s) circumferentially in extension of said at least one corded layer, these rods then being able to advantageously consist of the abovementioned textile cord, thus being non-metallic unlike the rods of the existing air membranes. 
     A secondary air suspension of a land or rail vehicle according to the invention, the vehicle comprising a body supported by at least one wheel train, comprises:
         an auxiliary air tank mounted under the body, and   a substantially toroidal membrane as defined above which is mounted under this tank by communicating with it and which comprises two upper and lower attachment areas mounted against two upper and lower support plates respectively intended to be mounted under this tank and on an emergency suspension attached to the wheel train.       

     A land or rail vehicle according to the invention, in particular a heavy truck or a passenger wagon, is supported by at least one wheel train and is equipped with at least one secondary air suspension as defined above. 
     A manufacturing method according to the invention for manufacturing a membrane as mentioned above comprises the following steps: 
     a) winding around a forming mandrel of said at least one thread, previously sheathed, preferably by extrusion, radially on the outside of said internal layer to form said at least one corded layer, 
     b) deposition of said external layer radially on the outside of said at least one thus wound corded layer to obtain a preform of a cross-linkable membrane gripping this mandrel, and 
     c) molding and cross-linking under pressure of this preform in a press. 
     It will be noted that the deposition of the internal and external layers can be carried out in the form of sheets or of a winding of extruded strips, as nonlimiting examples, and that it is possible to deposit a number of internal and/or external layers, replacing a single internal and/or external layer. 
     Advantageously, said winding of the step a) can be obtained by a deposition of a single continuous and sheathed said thread, deposited on several, and preferably 2n (n≧1) said corded layers and consisting of a textile cord treated by an adhesion activator and sheathed by a composition preferably based on a halogenated or non-halogenated dienic elastomer, such as those mentioned above. 
     According to another feature of the invention, this method may comprise, in the step a), the winding of said single continuous sheathed thread on said corded layers according to angles +α, −α with the axial direction of the mandrel and which balance out overall with one and the same number of turns of angle +α and −α preferably between ±0° and 90°. 
     Advantageously, it is possible to implement the step a) by displacing said mandrel optionally in translation and rotation relative to its axis of symmetry to produce said winding, this mandrel having, for example, two cylindrical axial end sections linked together by an intermediate section that is, for example, tapered, such that said attachment areas of the preform of the membrane are formed at the respective ends of the mandrel. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Other advantages and features and details of the invention will emerge from the following further description, with reference to the appended drawings, given solely as examples and in which: 
         FIG. 1  is an axial cross-sectional view of an exemplary secondary air suspension for a rail vehicle according to the invention, 
         FIG. 2  is a schematic view in axial section of another exemplary membrane of the invention that can be used in the secondary suspension of  FIG. 1 , 
         FIG. 3  is a schematic view of an installation for forming a membrane preform according to the invention, 
         FIG. 4  is a perspective view with partial cutaway of a first exemplary membrane reinforcement with superposed layers of sheathed threads entirely overlapping, 
         FIG. 5  is a perspective view of second exemplary membrane reinforcement with superposed layers of sheathed threads which are interlaced between these layers in the manner of a woven fabric, 
         FIG. 6  is a schematic view of a bake mold in the open position receiving this preform, shown in half axial section, 
         FIG. 7  is a view of the mold of  FIG. 6  in the closed position with the membrane being baked, in half axial section, and 
         FIG. 8  is a perspective schematic view of a membrane according to the prior art of the type with woven sheets while being formed on a mandrel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The secondary air suspension  1  as illustrated in  FIG. 1  consists of a flexible, deformable and airtight membrane  3  that has an overall toroidal form of a tire (i.e., an axial section substantially “Ω” shaped, like a tire, although asymmetrical unlike said tire), and an auxiliary air tank  7  connected to the membrane  3 . The secondary suspension  1  is associated with an emergency suspension  9  capable of compensating any air leak in the membrane  3 . The secondary suspension  1  and the emergency suspension  9  are superposed one on top of the other by being aligned on a substantially vertical axis of revolution X-X, and have a symmetry of revolution about this axis X-X. 
     The membrane  3  has two attachment areas  10  which, in the example of  FIG. 1 , have substantially the same diameter and which are respectively mounted on the peripheral rim of two circular upper  7   a  and lower  7   b  metal support plates situated facing one another. The auxiliary tank  7  is fixed above the membrane  3  on the upper support plate  7   a  and communicates with the interior of the membrane  3  through a passage  12 . The emergency suspension  9  is fixed under the membrane  3 , and consists of two groups of springs  14  separated by a metal plate  16 . The auxiliary tank  7  and the emergency suspension  9  are respectively fixed to the body C of the vehicle and to the bogie B which supports this vehicle. 
     It will be noted that the invention applies to different forms of membranes, such as, for example, the membrane  103  of  FIG. 2  with attachment areas  110  of different diameters, a membrane  103  on which are identified the radially internal  104  and external  105  layers, both based on an elastomer, such as a dienic elastomer (e.g. a CR) or a TPV. As for the cord reinforcement  106  according to the invention, it is represented schematically in  FIG. 2  (its corded layers not being visible) and will be described in detail below. 
     As illustrated in  FIG. 3 , it is possible to form a preform (not cross-linked) of the membrane  103  according to the invention, for example by means of an installation comprising: 
     a forming lathe  20  comprising a forming mandrel  21  which is for example partly tapered (its exact form is illustrated in  FIGS. 4 and 5 ), which is mounted to move optionally in translation and in rotation about its axis of symmetry YY by control means  22 ,  23  and which is intended to receive the internal  104  and external  105  elastomer layers and, radially between these, the corded layers  106  based on sheathed thread(s)  106   a,    
     a first elastomer strip extruder  30  which is arranged in relation to the mandrel  21  so as to deposit thereon in succession windings of these strips forming the or each internal layer  104  then the or each external layer  105 , and
         a second extruder  40  for individually sheathing the or each thread  106   b  that it receives and which is coupled to a tensioning device  50  (via rollers  51 ,  52 ,  53 ) and for depositing the sheathed thread  106   a , the tension and the deposition in the form of a winding on the mandrel  21  being optionally driven for example by a winding device  60 .       

     It will be noted that the successive deposition on the mandrel  21  of the windings of the internal  104 , corded  106  with sheathed thread(s)  106   a  and external  105  layers can be carried out manually or else at least partly automatically by such a winding device. 
     As indicated previously, use is preferably made of a continuous winding with turns, for example adjacent, of a single sheathed textile cord  106   a  (for example made of aramid) as thread, which forms 2n (n being an integer ≧1) radially superposed corded layers and deposited according to angles +α, −α with the axial direction YY which balance out overall with one and the same number of turns of angle +α and of angle −α, with a being between 0° and 90°. To produce this continuous winding, the mandrel  21  is displaced at the same time, optionally axially along the axis YY and necessarily in rotation about this axis. 
     As for the sheath of this textile cord  106   a , it is, for example, made of a dienic elastomer (e.g. CR) or of a thermoplastic elastomer (e.g. a TPV) and it can, for example, have a thickness of a few tenths of millimeters (this thickness being chosen to be relatively small to guarantee a predetermined density of threads  106   a  that is sufficient to reinforce the membrane  103 , but being able to vary according to the mechanical strength of the textile cord  106   b ). For a good adhesion of the sheath to the cord  106   b , the latter is first surface treated by means of an adhesion activator for example based on an “RFL” resin or a methylol donor (e.g. hexamethoxymelamine or hexamethylenetetramine). 
     The example of  FIG. 4  shows, on a mandrel  21  consisting of two cylindrical sections  21   a  and  21   b  linked together by a tapered section  21   c , a membrane reinforcement  106 ′ consisting of two corded layers C 1  and C 2  of the same sheathed textile cord  106   a  mutually overlapping (i.e. “thread to thread”) according to these respective angles +α and −α, and that of  FIG. 5  shows, on this same mandrel  21 , a reinforcement  106 ″ consisting of corded layers interlaced in the manner of a weave (i.e. with “crossed reinforcements”). In either case, and at the end of the winding of the cord  106   a , it is possible to advantageously obtain, at the respective ends  21 A and  21 B of the mandrel  21 , and by the same winding operation, the two rods intended to reinforce the two attachment areas  110  of the membrane  103 , these rods thus consisting of this cord  106   a  (unlike the usual metal rods which are manufactured for the membranes independently of the reinforcement sheets). 
     As indicated above, the sheath the or each cord  106   a  includes makes it possible to optimize the direction of winding of the turns for each corded layer C 1 , C 2 , through the bonding nature that it confers on this cord  106   a . Furthermore, this sheath makes it possible to enhance the bond between the internal  104  and external  105  elastomer layers. 
     It will be noted that it is possible to obtain, by this technique of winding a previously sheathed continuous textile cord  106   a , corded layers C 1  and C 2  arranged mutually according to patterns totally different from those of  FIGS. 4 and 5 , provided that this winding forms 2n corded layers C 1  and C 2  (for example two corded layers, with n=1) with turns that may or may not be adjacent. 
     Once the preform  103 A has been formed on this mandrel  21 , the end rods are brought together so as to form a substantially toroidal bead for this membrane  103 , then the duly obtained folded preform  103   a  is introduced between the two parts  71  and  72  of a mold defining a vulcanization press  70  (see  FIG. 6 ) to cross-link it in appropriate temperature and pressure conditions which are those used in the prior art for the pressurized vulcanization of the secondary suspension air membranes. Following the closure of the mould  70  and the vulcanization of the preform  103 A (see  FIG. 7 ), the membrane  103  is obtained as illustrated in  FIG. 2 . 
     As indicated previously, it will be noted that the sheathing of the or each textile cord  106   b  makes it possible to minimize the wear-resistance of the membrane  103  in operation despite the preferably adjacent nature of the turns of each corded layer C 1 , C 2 , it also being specified that a membrane  103  according to the invention has to be capable of withstanding, in use on a rail vehicle, operating pressures that typically vary between 8 and 12 bar. 
     It will be noted finally that none of these corded layers C 1  and C 2  advantageously has the slightest discontinuity between its two attachment areas  110 , such as an area of overlap likely to form a line of mechanical weakness for the membrane  103  because of insufficient expansion in this line on inflation of the preform  103 A before vulcanization.