EP0696683A1 - Molded-in pneumatic circuit - Google Patents

Abstract

The circuit, e.g. for testing pressure instruments in an aircraft, consists of at least one pneumatic chamber, one or more valve (40) and flexible connecting tubes (31). The circuit is encased in an over-moulded resin block (30) which both seals its components and acts as a mechanical support for them. The pneumatic chamber consists of a section of PVC tube which is closed at both ends and has at least one orifice for inserting the end of a supple tube. The resin block is equipped with metal inserts with threaded holes and is made from a bi-component polyurethane resin with a relatively low expansion coefficient and a vitreous transition point above that of the temperature range to which the pneumatic circuit is subjected.

Description

The present invention relates to the field of pneumatic circuits. It relates more particularly to the production of a pneumatic circuit comprising at least one pneumatic capacity, at least one valve, and pipes.

By way of example, FIG. 1 represents the pneumatic diagram of an apparatus 20 comprising a pneumatic circuit 10 of the aforementioned type. This device, marketed by the applicant under the designation "Type 301 pressure generator", makes it possible to test or calibrate on the ground anemobarometric equipment of aircraft, such as anemometers, altimeters, variometers, etc. It has outputs 1, 2, 3, 4 delivering reference pressures Pt or Ps, ranging from a few millibars to several bars, which are applied as input to the pressure taps of an aircraft. Each generator pressure output can be fine-tuned to simulate static pressure (Ps) or total pressure (Pt) values that are normally measured in flight by the aircraft.

The pressure generator 20 essentially comprises, at the inlet to the pneumatic circuit 10, a vacuum pump 5 and a booster pump 6 and, at the outlet of the pneumatic circuit, a system of safety valves 7, a system for displaying the pressures 8 , and various connecting pipes 9.

The pneumatic circuit 10 constitutes the heart of the pressure generator 20. It comprises pneumatic vacuum capacities 11, in which a vacuum is created by means of the pump 5, a pressure boost capacity 12, pressurized by the pump 6, pneumatic switching valves 13-1 to 13-6, one of which receives atmospheric pressure (Pa) as input, various pipes 14, and nodes 15 of pipes, forming the junction of at least three pipes. The pneumatic capacities are tight hollow bodies of a fairly large volume which can reach several hundred cm³. They make it possible to maintain the desired pressure values with good stability at the outlet of the pressure generator, provided that the air flow rate is almost zero.

Essentially two methods are known for producing pneumatic circuits of this type.

The first consists of making the pneumatic capacities with bell-shaped parts, made of stamped metal or plastic, which are screwed to a support by means of a seal. The other elements of the circuit, such as valves, piping, nodes, are commercially available and appear in various catalogs. For example, the nodes are tee-shaped connecting pieces, the pipes are flexible rubber hoses, etc.

The pneumatic circuits produced according to this first method, from individual quality components, have the disadvantage of being costly, because each element and each connection must have its own quality in order to meet the specifications for pressure resistance or depression of the final product. . For example, for the connection of the pipes, one must use screw fittings with O-ring or biconic ring crushing, which increases the cost of the device. Finally, such circuits are bulky, each element having to be fixed on a support, and are susceptible to corrosion.

The second known method, used hitherto by the applicant, is illustrated in FIG. 2. The capacities 11, 12, the pipes 14 and the nodes 15 of the pneumatic circuit 10 are produced in one piece by means of plates 17 of polymethyl methacrylate (Plexiglas®) engraved, stacked one on top of the other. The pipes 14 open onto the surface of the upper plate on which it is possible to fix, by means of metal inserts 16, connection elements or, directly, valves. This technique allows good integration of the pneumatic circuit, but remains very expensive to implement, due to the long labor time required for etching the various plates.

• un bon maintien mécanique des composants,
• une bonne étanchéité au vide et à des spécifications de pression pouvant atteindre quelques bars,
• une bonne résistance mécanique et chimique en environnements sévères (corrosion, vibrations),
• un coût de fabrication et un temps de main d'oeuvre réduits.

Thus, an object of the present invention is to provide a pneumatic circuit providing the following advantages:

• good mechanical maintenance of the components,
• good vacuum tightness and pressure specifications of up to a few bars,
• good mechanical and chemical resistance in harsh environments (corrosion, vibrations),
• reduced manufacturing cost and reduced labor time.

To achieve these objectives, the present invention provides for making a pneumatic circuit from simplified components connected according to a quick connection method, then for drowning in a resin the pre-assembled circuit, the resin once hardened ensuring all or part of the sealing of the circuit, as well as the maintenance of the various elements and their chemical protection.

Thus, from an imperfect and rudimentary pneumatic circuit, which would not in itself present a sufficient seal if it were mounted in the air, one obtains, thanks to a resin overmolding, a circuit having excellent pneumatic and mechanical properties.

More particularly, the present invention provides a pneumatic circuit comprising at least one pneumatic capacity, at least one valve, and flexible connecting pipes, at least one of these elements or a connection of one of these elements being of a structure very simple, not making it possible to ensure pressure or vacuum resistance in accordance with circuit specifications, circuit coated in a block of resin ensuring all or part of the circuit seal and the mechanical resistance of the elements. If the valve is manually operated, it is of course advisable to leave the valve control member free.

Advantageously, the pneumatic capacity comprises a section of PVC (polyvinyl chloride) tube plugged at its ends, provided with at least one orifice into which the end of a flexible pipe is introduced.

Advantageously, the valve comprises several outlet spouts and / or several inlet spouts to which pipes are connected. Thus, in a simple manner, at the inlet and / or the outlet of the valve, pipeline nodes replacing the conventional connection tees. Pipe nodes can also be formed at the capacity level by introducing the ends of several flexible pipes into the PVC tube. The knot is then formed inside the capacity itself. This method is justified by the simplicity of the connections, which do not require the use of conventional, watertight and expensive connection means.

Advantageously, at least one face of the resin block comprises metal inserts provided with tapped holes. This allows to fix the block on a support.

Preferably, the coating resin is a two-component polyurethane resin (resin and hardener), having a coefficient of expansion fairly low and a glass transition point located beyond the operating temperature of the pneumatic circuit. This type of resin is used in the field of electrical equipment, for example for the coating of transformers. It represents a good compromise between flexibility and rigidity. In addition, polyurethane resins are, unlike epoxy resins, slightly exothermic, so that one does not have to fear, during polymerization, a rise in temperature which could destroy or cause the PVC to creep. Finally, the Applicant has noticed, and this guarantees good sealing of the pneumatic capacities, that the polyurethane resins adhere strongly to PVC.

• les figures 1 et 2 sont relatives à l'art antérieur et ont été décrites précédemment,
• la figure 3 représente un circuit pneumatique selon l'invention, enrobé dans un bloc de résine,
• la figure 4 est une vue en coupe d'un élément du circuit de la figure 3,
• la figure 5 est une vue en coupe d'un autre élément du circuit de la figure 3, et
• la figure 6 illustre un procédé de fabrication d'un circuit pneumatique selon la présente invention.

These objects, characteristics and advantages of the present invention will appear more clearly on reading the following description of an embodiment of the invention, made in relation to the attached figures among which:

• FIGS. 1 and 2 relate to the prior art and have been described previously,
• FIG. 3 represents a pneumatic circuit according to the invention, coated in a block of resin,
• FIG. 4 is a sectional view of an element of the circuit of FIG. 3,
• FIG. 5 is a sectional view of another element of the circuit of FIG. 3, and
• FIG. 6 illustrates a method of manufacturing a pneumatic circuit according to the present invention.

For the sake of simplicity, there is shown in FIG. 3 and in the following figures a pneumatic circuit 60 of less complexity than the circuit shown in FIG. 1, sufficient to describe the characteristics of the present invention. Figures 3 and following therefore represent a pneumatic circuit 60 according to the invention. It comprises two fluid switching valves 40, manually controlled by needles 41, pneumatic capacities 50 (two of which are visible in FIG. 5), and connection pipes 31 made of rubber or Neoprene, forming a pneumatic network.

As can be seen in FIG. 3, the pneumatic circuit 60 seen from the outside is in the form of a block of resin 30, in which the components of the circuit are completely trapped. Only protrude from the block of resin 30 the control pins 41 of the valves 40, and the terminations of certain pipes 31 forming the inputs or outputs of the circuit. For fixing the block 30, there are provided metal inserts 32 having threaded holes.

FIG. 4 represents a switching valve 40. The valve comprises a metal body 42 entirely trapped in the resin, with the exception of the upper face which is flush. Conventionally, the control needle 41 is screwed into a threaded cylindrical cavity 43 of the valve body. It carries a first O-ring 44 which isolates the cavity from the external pressure, and a second O-ring 45 which ensures the closing of the valve when the needle is in the low position. According to the invention, the inlet and the outlet of the valve each comprise two connection nozzles, respectively 46-1, 46-2 and 47-1, 47-2, which open into the internal cavity, onto which come fitting pipes 31. These double inlet or outlet nozzles make it possible to obtain the equivalent of a pipe node upstream and a pipe node downstream of the valve, without resorting to a particular part. As the number of nozzles with which the valve may be provided is not limited, those skilled in the art can adapt the valve to the configuration of the circuit to be produced.

FIG. 5 represents, at the heart of the resin block 30, pneumatic capacities 50 according to the present invention. The pneumatic capacities 50 are obtained by means of sections of PVC (polyvinyl chloride) tubes 51, of an ordinary type commonly available commercially and at very low cost. The tubes are plugged at their ends by conventional end pieces 52 in PVC, fitted and glued to the inner wall of the tubes. For their connection, the tubes are simply drilled and have holes 53 through which the ends of pipes 31 penetrate. Preferably, the diameter of the holes 53 is substantially less than the external diameter of the pipes 31.

It can be seen that this elementary embodiment of pneumatic capacities and connections is not in itself suitable for meeting the pressure or vacuum specifications of the circuit, and would not have sufficient tightness if the circuit were not coated with the resin. In addition, under the action of pressures or depressions, shocks and vibrations, the pipes would come sooner or later to move in the orifices 53, which would be a source of additional failure.

It is therefore easy, thanks to the simplicity of the means of the invention, to produce all types of circuits, whatever the complexity. In addition, as has already been done with the valve 40 of FIG. 4, it is possible to create pipeline nodes without having recourse to additional parts, by introducing several pipes into the tubes 51.

• une résine souple, qui va garantir une bonne étanchéité en conservant son adhérence dans le temps, elle va absorber les contraintes mécaniques ou thermiques, et notamment les diverses dilatations des éléments du circuit,
• une résine rigide, devenant moins adhérente avec le temps mais assurant une bonne tenue mécanique des éléments, une rigidité suffisante du bloc de résine, et capable d'autre part de maintenir les inserts de fixation 32.

Those skilled in the art will note that the choice of coating resin 30 can determine the quality of the sealing of the various connections. First, one must choose a resin capable of adhering to the various materials from which the circuit elements are made. We must then make a compromise between:

• a flexible resin, which will guarantee a good seal while retaining its adhesion over time, it will absorb mechanical or thermal stresses, and in particular the various expansions of the elements of the circuit,
• a rigid resin, becoming less adherent over time but ensuring good mechanical strength of the elements, sufficient rigidity of the resin block, and capable on the other hand of maintaining the fixing inserts 32.

• faible coût,
• mise en oeuvre rapide et aisée,
• polymérisation à la température ambiante en 24 heures,
• pas ou peu d'exothermie durant la polymérisation,
• coefficient de dilatation linéaire faible,
• température de transition vitreuse assez élevée, de l'ordre de 90°C,
• dureté suffisante pour introduire des inserts de fixation dans le bloc de résine.

The choice of coating resin therefore results from a compromise which is within the reach of those skilled in the art. However, it is advisable, by way of nonlimiting example, to use a polyurethane resin of hardness between 50 and 80 points on the shore hardness scale D, chosen by the applicant for the following reasons:

• low cost,
• quick and easy implementation,
• polymerization at room temperature in 24 hours,
• little or no exotherm during polymerization,
• low linear expansion coefficient,
• fairly high glass transition temperature, of the order of 90 ° C.,
• sufficient hardness to introduce fixing inserts into the resin block.

• une adhérence moyenne sur les tubes de caoutchouc, qui peut entraîner des décollements le long des tuyaux. Toutefois, on a remarqué que ces décollements ont une longueur limitée à quelques millimètres, et sont sans conséquences sur l'étanchéité des capacités pneumatiques,
• une adhérence moyenne sur le métal, qui peut entraîner un perte d'étanchéité au niveau des becs de raccordement des vannes, à laquelle on peut remédier de diverses manières, indiquées plus loin.
• une adhérence excellente sur le PVC, qui garantit l'étanchéité en toutes circonstances des capacités pneumatiques, et de façon générale de tout élément en PVC.

Various tests carried out with this resin have shown:

• medium grip on rubber tubing, which can cause loosening along the hoses. However, it was noted that these detachments have a length limited to a few millimeters, and have no consequences on the sealing of the pneumatic capacities,
• an average adhesion on the metal, which can cause a loss of tightness at the level of the connection spouts of the valves, which can be remedied in various ways, indicated below.
• excellent adhesion to PVC, which guarantees the sealing in all circumstances of the pneumatic capacities, and in general of any PVC element.

The system thus produced successfully withstood pressures of several bars, thermal cycles ranging from -15 ° to + 50 ° C, and storage for several hours at -40 ° C as well as + 70 ° C . Furthermore, its cost price is approximately 10 to 20 times lower than that of an equivalent conventional device.

Of course, depending on the resin chosen, and due to the mechanical work which the resin undergoes, detachments may occur from the emergence points of the pipes 31, or from the outcrop areas of the valves 40. However, these detachments do not spread to the heart of the circuit and are only likely to affect areas close to the surface. Thus, as already indicated, detachments between pipes and resin can occur at the emergence points of the pipes 31, but will only propagate along the pipes over a few millimeters. In the case of the valve 40 shown in FIG. 4, greater detachment can also occur from the surface of the resin (because the adhesion to the metal is less good), and propagate along the valve body 42, at the metal / resin interface, up to the spouts 46, 47, creating a line of flight. We will therefore take certain precautions by strengthening the tightness of connections close to the surface. For example, we can provide the spouts 46, 47 with serrations, as shown in Figure 4, or, if the surface of the spouts is smooth, carefully glue the pipes 31. With the coating resin, we know that these connections will remain waterproof because they will be firmly held, even if possible detachments occur. Another solution, which can be combined with the previous one, consists in carrying out rough machining of the surface of the valve body, to give it a certain roughness, so that the adhesion of the resin will be notably improved. On this occasion, the surface of the valve body can be crenellated to improve its mechanical strength in the resin block. Finally, one last an even more efficient method is simply to make the valve body with a material on which the resin will adhere well, for example PVC if the polyurethane resin proposed above is chosen.

The overmolding process calls on normal knowledge in the matter. It is illustrated in FIG. 6. The elements of the circuit 60 are introduced into a removable box 70. The valves 40 are placed at the bottom of the box and held by means of screws with flat heads 71 screwed in place of the control needles. The PVC tubes 51 are placed on the valves and stacked on top of each other, the rigidity of the pipes 31 being sufficient to maintain the assembly. Preferably, sufficient space will be left between the PVC tubes so that the coating is done well all around the tubes. The resin is prepared by adding the hardener, then unpacked in a vacuum bell. The resin is poured in, allowed to polymerize and then the box is dismantled.

It will be clear to a person skilled in the art that the present invention is capable of numerous variants of embodiments and improvements. Thus, one embodiment that the applicant plans to test, consists in producing the pneumatic capacities with a solvent material, for example polystyrene, dissolved after polymerization by means of a selective solvent injected into the circuit. On the other hand, hollow glass beads can be added to the resin in order to decrease its density. Also, one can use electrically controlled valves fully embedded in the resin. Finally, the present invention can be extended to the realization of all types of pneumatic functions.

Claims (5)

Pneumatic circuit (60), comprising at least one pneumatic capacity (50), at least one valve (40), and flexible pipes (31) for connection, at least one of these elements or a connection for one of these elements being of a very simple structure not making it possible to ensure resistance to pressure or depression in accordance with circuit specifications, characterized in that it is coated in a resin block (30) ensuring all or part of the tightness of the circuit and mechanical maintenance of the elements. Pneumatic circuit according to the preceding claim, characterized in that said pneumatic capacity comprises a section of PVC tube (51) plugged at its ends (52), provided with at least one orifice (53) into which the end d 'a flexible hose (31). Pneumatic circuit according to one of the preceding claims, characterized in that said valve (40) comprises several outlet spouts (46-1, 46-2) and / or several inlet spouts (47-1, 47-2) to which pipes (31) are connected. Pneumatic circuit according to one of the preceding claims, characterized in that the resin block (30) comprises metal inserts (32) provided with tapped holes. Pneumatic circuit according to one of the preceding claims, characterized in that the coating resin (30) is a two-component polyurethane resin having a fairly low coefficient of expansion and a glass transition point situated beyond the operating temperature range of the pneumatic circuit.

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