Power transmission

Abstract: An electrohydraulic servovalve comprising a torque motor (10) which receives an electrical signal and positions a flapper (11) between a pair of opposed nozzles (12) to control a spool valve (16) and a feedback spring (14) connected to the flapper (11) and to the spool (15) of the spool valve (16). The flapper (11) is connected to an armature (20) of the torque motor (10). The joint between the armature (20) and the flapper (11) comprises a one part, heat cured thermosetting adhesive (A).

Description: This invention relates to an electrohydraulic servo valve comprising the features of the preamble to claim 1. One common type of electrohydraulic servovalve comprises a torque motor as a first stage which receives an electrical signal and positions a flapper between a pair of opposed nozzles to control a spool valve as the second stage. A feedback spring is connected to the flapper and to the spool of the spool valve. In some designs, the mechanical output motion is very small and can be as small as 0,5 µ (20 millionths of an inch). Since repeatability of better than 0,5 % is required, it is apparent that the mechanical rigidity of the components which convert electrically generated forces to physical motion must be high. A servo valve of the type mentioned hereinabove and according to the preamble of claim 1 is disclosed e.g. in FR-A- 2 573 503. Application of current to the coils of the torque motor polarizes the armature which reacts with the field in the pole piece air gaps. This results in a moment on the armature and the armature/flapper assembly rotates around the virtual pivot point. Resisting the moment applied to the armature is the force required to bend the spring tube as a cantilever beam and a pressure unbalance in the two nozzless facing the flapper. In the FR-A 2 573 503 the armature is directly attached to the flapper, e.g. by clamping, soft soldering, hard soldering or press fitting. Directly attaching the flapper to the armature provides for a metal to metal interface with the necessary rigidity, freedom from friction, stability and long life required by the armature/flapper joint. However, all the methods of directly attaching the flapper to the armature as mentioned hereinabove have manufacturing problems which result in added cost, loss of integrity or loss of mechanical or magnetic properties. An ideal attachment method would introduce no undesirable materials such as soldering flux, provide no mechanical stress on the armature to degrade the magnetic properties and not expose the armature/flapper/spring tube assembly to temperatures which may alter the mechanical or magnetic properties of the components. The problem of the present invention is to provide a servo valve which overcomes the problems of the prior art mentioned above; wherein the armature/flapper joint is stress free; wherein the armature and flapper are precisely positioned related to one another; which does not require the use of soldering flux and corrosive problems associated therewith; which has no creep movement under long term stress conditions; which can be readily made in commercial production; and which can be repeatedly and accurately provided in commercial production. This problem is solved by a servo valve with an armature, a flapper, a spring tube and a feedback spring, in that said flapper and said spring tube form a subassembly which is fixed to said armature by a one part thermosetting adhesive (A). Preferably the joint between the armature and the flapper comprises a heat cured one part thermosetting structural adhesive. Description of the Drawings FIG. 1 is a cross sectional view of a servovalve embodying the invention. FIG. 2 is a fragmentary sectional view of a portion of the servovalve shown in FIG. 1 on an enlarged scale. FIG. 3 is a sectional view showing one method forming the joint between the armature and flapper. FIG. 4 is a sectional view showing another method of forming the joint between the armature and flapper. Description Referring to fig. 1, the invention relates to servovalves of the type comprising a first stage torque motor 10 which receives an electrical signal and positions a flapper 11 between a pair of opposed nozzles 12 to control a spool valve and includes a feedback spring 14 connected to the flapper 11 and to the spool 15 of a spool valve 16. Specifically in such servovalve, the torque motor comprises a motor that includes pole pieces 17, permanent magnets 18, and coils 19 having openings therein. An elongated armature 20 is positioned with its ends projecting between the pole pieces and being driven thereby. The return force is develloped by a spring tube 21, the first end thereof being fixed to the flapper 11 and armature 20 and the second end thereof connected to a stationary housing part. As shown in fig. 2, the first end extends in an opening 31 of the armature 20 and is fixed thereto. The upper end of the flapper 11 is fixed to the upper end of the tube 21 as at 32 and the lower end of the flapper projects between two nozzles 12 in a nozzle block. The torque motor 10 is mounted on a housing 22 of the spool valve 16 which is shown as of the four-way closed center type, the spool 15 thereof sliding in a bore 23 and adapted to uncover openings 24, in a sleeve 25 in the bore 23 to meter flow to control ports. Positioning of the spool 15 relative to the metering slots provides precision controlled flow. The feedback spring 14 is mounted on the lower end of the flapper and includes a ball 26 that extends into an opening 27 in an insert 28 in the spool 15. When an input signal is applied to the coils 19, the armature 20 ends are polarized creating a rotational torque on the armature 20. The tube 21 acts as a spring centering the flapper motion between the two nozzle openings 12. As the flapper 11 moves toward one nozzle or another, a pilot flow (pressure differential) is supplied which is applied through passages 30 to one end or the other of the spool 15 to position the spool 15. As the spool moves, the feedback spring 14 bends and applies a force to the flapper 11 which tends to recenter the flapper 11 between the nozzles 12. Positioning of spool 15 occurs at the point in which the spring feedback force equals the torque motor force induced by the input current. The spool 15 stops at this position and the flapper 11 is essentially centered until the input current changes to a different level. With constant supply pressure and flow of the servovalve, output control flow is infinitely proportional to the input current. Such construction is old and well known. In accordance with the invention as shown in fig. 2, the flapper 11 is fixed to the spring tube 21 and such subassembly 11/21 is mounted in the armature opening 31 by utilizing a one part, heat curing, thermosetting plastic adhesive A which is applied between the surfaces, namely, the inner surface of opening 31 on the armature 20 and the outer surface on the upper end of the flapper 11 and spring tube 21. It has been found that a preferred composition that produces satisfactory results is a one part epoxy adhesive. Satisfactory results have been achieved by utilizing an adhesive made by 3M of St. Paul, Minnesota and sold under the product specification 2214. In a typical construction, the clearance between the tube 21 and the armature opening is about 0,05 mm (.002 inches). The adhesive may be applied by hand to the two surfaces and the surfaces brought together producing satisfactory results. Alternatively, as shown in FIG. 3, the adhesive may be forced through an injection nozzle 35 axially into opening 36 of the spring tube 21 and through diametrically opposed radial openings 37 in the upper end of the spring tube 21 to the space between the tube 21 and the opening of the armature 20a. In the form shown in FIG. 4, an injection nozzle 40 is brought adjacent to a radial opening 41 in the armature 20b and the adhesive A is forced into the space between tube 21 and armature 20b and permitted to extrude through an opposed radial opening 42 in the armature. The adhesive, after being applied is cured at a temperature of 121°C (250°F). It has been found that the above arrangement of torque motor armature/flapper subassembly produces the following advantages: 1. Solidifies in a stress free state and thereby leaves the armature/flapper subassembly exactly as fixtured. 2. Has a shear strength greater than the class of solders generically called "soft". 3. Permits use of wider tolerance bands on the mating parts for ease of assembly and cost reduction. 4. Does not require soldering flux (acid) which may attack the thin walled 0,045 mm (.0018) spring tube and which must be neutralized to prevent long term corrosion and failure of the tube. 5. Has no appreciable "creep" or movement under long term stressed conditions. 6. Cures at a low temperature 121°C (250°F) which is within the normal operating temperature range of commercial torque motors. 7. Provides easily controlled filling of the joint by controlled volume injection. 8. By proper location of injection ports, all air or voids are eliminated in the joint for uniform joint quality. It can thus be seen that there has been provided such a servovalve which overcomes the problems of the prior art; wherein the armature/flapper joint is stress free; wherein the armature and flapper are precisely positioned related to one another; which does not require the use of soldering flux and corrosive problems associated therewith; which has no creep movement under long term stress condition; which can be readily made in commercial production; and which can be repeatedly and accurately provided in commercial production.

Claims: An electrohydraulic servovalve comprising: a spool valve (16) including a spool (15) and a pair of opposed nozzles (12), each of which is connected to a respective end of said spool (15); a torque motor (10) including an armature (20), a spring tube (21) and a flapper (11), said spring tube (21) and said flapper (11) being fixedly connected to said armature (20); said flapper (11) being arranged between said pair of opposed nozzles (12), and a feedback spring (14) connected to said flapper (11) and said spool (15), characterized in that said flapper (11) and said spring tube (21) form a subassembly which is fixed to said armature (20) by a one part thermosetting adhesive (A). The servovalve set forth in claim 1 wherein said subassembly of spring tube (21) and flapper (11) also includes said feedback spring (14) which subassembly is fixed to said armature (20) by said adhesive (A). The servovalve set forth in claim 1 or 2 wherein said adhesive (A) comprises an epoxy resin. The servovalve set forth in claim 2 or 3 wherein said flapper (11) and said spring tube (21), fixed to one another, include an axial opening (36) and radial openings (37) through which the adhesive (A) extends. The servovalve set forth in any of claims 1 to 3 wherein said flapper (11) and said spring tube (21), fixed to one another, are fixedly connected to an armature portion (20b) which includes radial openings (41,42) through which said adhesive (A) extends. The method of assembling the armature (20) and a subassembly of spring tube (21) and flapper (11) set forth in claim 4 or 5 including the steps of applying a one part, thermosetting plastic adhesive (A) to the adjacent surfaces of an opening (31) in the armature (20) and an adjacent portion of the subassembly (11,21) and thereafter permanently assembling the armature (20) and the subassembly (11,21). The method set forth in claim 6 wherein said adhesive (A) is applied by injecting the adhesive between the surfaces of the armature opening (31) and the outer surface of the adjacent subassembly portion. The method set forth in claim 7 wherein said subasssembly (11,21) has said axial opening (36) and radial openings (37) of claim 4 and through which said adhesive (A) is injected, which enters axially into said axial opening (36) and flows radially outwardly through said radial openings (37) into the space between said subassembly (11,21) and said armature (20a) eliminating voids in the adhesive (A). The method set forth in claim 7 wherein said subassembly (11,21) has said radial openings (41,42) in said armature portion (20b) of claims 5, the adhesive (A) is injected through one (41) of the radial openings into the space between said subassembly (11,21) and said armature portion (20b) and the adhesive (A) is permitted to extrude out of the other (42) of said openings eliminating voids in the adhesive (A).

Inventors: BLATTER ALBERT, DAVIS ROBERT E, BLATTER, ALBERT, DAVIS, ROBERT E.
Applicants: VICKERS INC, VICKERS INCORPORATED
Application year: 1991
Main classification: F15B13
IPC codes: F15B13