GB2257566A

GB2257566A - Proportional stroke wet pin solenoid

Info

Publication number: GB2257566A
Application number: GB9114641A
Authority: GB
Inventors: John Thomas Marsden
Original assignee: Parmeko Ltd
Current assignee: Parmeko Ltd
Priority date: 1991-07-06
Filing date: 1991-07-06
Publication date: 1993-01-13
Also published as: GB9114641D0

Abstract

The solenoid comprises an armature 32 axially movable in a pressure vessel, in a common diameter operating bore which spans a guide tube 20 and a blind recess 23 in one end of a pole piece 22. The shape of the lip of the pole piece 22 around the blind recess creates the proportional stroke characteristic. The accuracy of the common bore spanning the guide tube and pole piece is created by connecting together the guide tube 20 and pole piece 22 in a coaxial but mutually spaced condition by means of a stainless steel connecting tube 24 received in rebated outer cylindrical portions of the guide tube and pole piece, then re-boring the guide tube and blind recess to a very accurate common diameter and coaxial alignment, and finally honing the common bore to achieve even higher accuracy. An end plug 26 closes the guide tube 20 after honing, to close the pressure vessel. <IMAGE>

Description

TITLE: Proportional Stroke Wet Pin Solenoid DESCRIPTION Field of the Invention The invention relates to proportional stroke wet pin solenoids, and to a method for their manufacture.

Background Art Solenoids are well-known for use as actuators for control valves in hydraulic installations. Conventionally, the armature of the solenoid carries or enagages an actuator pin which effects the actuatuation of the hydraulic control valve. Because the actuator pin extends into the hydraulic control valve and therefore into the hydraulic circuit, the high pressure boundary of the hydraulic circuit generally includes both the actuator pin and the armature. Such solenoids are known in the art as wet pin solenoids because the actuator pins are not fluidically sealed from the hydraulic circuit.

Wet pin solenoids are known in which the actuation is simply on-off, and in which the force/stroke characteristic of the armature and actuator pin are proportional over a chosen operating length of stroke of the actuator. The latter are referred to as proportional stroke wet pin solenoids, and their construction requires considerably more technical accuracy than that of simple on/off solenoids.

Early forms of proportional stroke wet pin solenoids mounted the armature on a shaft passing completely through a housing for the solenoid, with shaft bearings providing both a centring guide for the shaft and armature and a seal for the hydraulic circuit. A later proposal of European Patent Application No. 146951A was for a stepped armature to be axially slidable in a tubular guide which, together with the pole piece of the solenoid, defines an armature chamber. The tubular guide has, however, to be differentially heated along its axial length during manufacture of the solenoid, both to join it to the pole piece and to create a non-magnetic portion intermediate its ends. The accuracy with which such a solenoid could be constructed was limited, and as with all prior proposed proportional stroke wet pin solenoids, the unit could not be disassembled for servicing.The coil could not be removed from around the armature chamber, and neither could the armature be removed from within the armature chamber.

It is an object of the invention to provide a proportional stroke wet pin solenoid in which the armature is guided much more accurately than in the prior art, and which can be disassembled for servicing purposes.

Summary of the Invention The invention provides a proportional stroke wet pin solenoid comprising an armature axially movable in a pressure vessel surrounded by a magnetic coil, wherein the pressure vessel comprises a magnetically permeable guide tube held spaced from a magnetically permeable pole piece by a non-magnetic connecting tube received in rebated outer cylindrical portions of the guide tube and pole piece, and a removable end plug in the guide tube, the pole piece being shaped to achieve proportionality of stroke of the solenoid and having a blind recess therein for receiving a leading end of the armature on its operating stroke, and wherein the blind recess has been rendered precisely cylindrically coaxial with the guide tube by removing the end plug, re-boring the guide tube and pole piece and finally honing the guide tube and pole piece.

The invention also provides a method for the manufacture of such a solenoid, which comprises machining and assembling the pressure vessel in the following sequence: pressing the magnetically permeable guide tube and the magnetically permeable pole piece onto opposite ends of the non-magnetic connecting tube so that the connecting tube ends are received in rebated outer cylindrical portions of the guide tube and pole piece; re-boring the guide tube and a central cylindrical recess in the pole piece while both are rigidly mounted on the connecting tube; honing the guide tube and pole piece internal bores to a precise common diameter on a common axis; inserting the armature and actuator pin into the guide tube; and securing in place the removable end plug of the guide tube, to seal hydraulically the guide .tube, pole piece and connecting tube into the pressure vessel and to retain the armature and actuator pin for axial movement within that pressure vessel.

Preferably the connection between the connecting tube and the guide tube and pole piece are rendered fluid tight by placing the connecting tube around one or more copper rings recessed into the rebated portion of each of the guide tube and pole piece, and then heating the assembly of connecting tube, guide tube and pole piece uniformly to achieve a copper brazed joint at each end of the connecting tube. By heating the assembly uniformly, thermal distortions are avoided so that even on cooling the axial bores in the guide tube and pole piece have a higher degree of axial alignment than in the finished prior art solenoid of EP 146951A.By making the connecting tube shorter in axial length than the magnetic coil, so that the magnetic flux path passes directly into the guide tube and pole piece respectively at opposite ends of the connecting tube, it can be ensured that the connecting tube does not interfere at all with the magnetic flux path, and the connecting tube and therefore the assembled structure can therefore be made of substantial rigidity. This rigidity permits extremely accurate machining when the axially aligned bores are first re-bored and then honed to a common diameter and a precise common axis.

After honing, the armature assembly is inserted into the guide tube and retained in position by the end plug which is itself retained for example by a circlip internally of the guide tube.

The armature assembly preferably comprises the armature itself onto which is plated a substantial thickness, for example, O.lmm, of electroless nickel. The nickel used can be of a non-magnetic quality, since its function is solely to create a smooth bearing for the armature. The nickel coating is then ground back to about 0.05mm, so as accurately to provide a precise sliding fit of the coated armature in the guide tube. If desired, the nickel coating at the leading end of the armature can be cut back or bevelled to avoid fouling the entrance to the lined recess in the pole piece, which can occur for example when dirt enters the actuator chamber.

The assembly of connecting tube, pole piece, guide tube and end plug forms a pressure vessel in which the armature is axially slidable, and that pressure vessel can if desired be removed intact from the centre of the magnetic coil to permit replacement of either the pressure vessel or the magnetic coil in the case of malfunction. Servicing is also facilitated by the fact that the end plug is removable, so that at any stage in the life of the solenoid the armature assembly can be removed for cleaning.

A non-magnetic shim washer is preferably provided between the armature and the blind end of the recess in the pole piece, for restricting the stroke of the armature to a preselected portion of its total possible stroke, to confine the operating characteristic of the assembled solenoid to a predefined portion only of its total force/stroke curve.

The accurate machining of the solenoid of the invention is greatly facilitated if the pole piece is formed from a first hollow cylindrical portion and a stop member blocking off a portion of the through-bore through that cylindrical portion. The stop member, which like the cylindrical portion should be made of a magnetically permeable material, is preferably axially removable and is not present during the re-boring and honing of the guide tube and pole piece.

Drawinqs Figure 1 is an exploded view of a proportional stroke wet pin solenoid according to the invention; Figure 2 is an axial section through the assembled solenoid of Figure 1; and Figure 3 is an axial section similar to that of Figure 2 but through an alternative embodiment of the invention.

Figure 1 shows the solenoid of the invention divided into three component parts, namely a pressure vessel 2, a solenoid coil assembly 4 and a retaining nut 6. Those three component parts are assembled by inserting the pressure vessel 2 into the central bore in the solenoid coil assembly 4 so that it projects from the left-hand end of the coil assembly as illustrated, and screwing the nut 6 onto the projecting end of the pressure vessel in abutment with a leftwardly facing face of the solenoid coil assembly 4.

Referring next to Figure 2, the solenoid coil assembly 4 comprises a wound coil 10 formed around a coil former 12. The coil 10 and coil former 12 are encased in a two-part metal cover 14, 14A, and the whole is encapsulated in an injection moulded outer shell 16, with electrical connectors 18 protruding from the outer shell to provide electrical connections to the coil 10.

The pressure vessel 2 comprises a mild steel guide tube 20 and a mild steel pole piece 22 connected together in axially spaced relationship by a stainless steel connecting tube 24. The mild steel of the guide tube 20 and pole piece 22 is a magnetically permeable material, whereas the stainless steel of the connecting tube 24 is non-magnetic. Therefore when the coil 10 is activated, the magnetic flux is forced to flow through an armature 32 axially movable in the guide tube 20, causing movement of the armature.

The left-hand end of the pressure vessel is closed by an end plug 26 which is held in position by a circlip 28 in abutment with an internal shoulder of the guide tube 20.

An optional mechanical finger-push actuator 30 is shown in Figure 2, to permit an operator to move the armature physically by depression of the finger-push actuator, as an alternative to electrical actuation.

An armature assembly within the pressure vessel comprises the armature 32 which is made of a magnetically permeable material with low retentivity. The armature 32 mounts an actuator pin 34 which extends through the pole piece 22 into a hydraulic chamber 35 in which is located a hydraulic control valve (not shown) to be actuated by the solenoid. A hydraulic passage to the interior of the presure vessel 2 is provided by a clearance between the interior of the pole piece 22 and the actuator pin 34, and by an array of longitudinally extending bores 33 through the armature 32. The armature 32 and actuator pin 34 thus move in an environment lubricated by the hydraulic fluid of the hydraulic chamber 35.

A non-magnetic shim washer 36 is provided to limit the rightward actuating stroke of the armature 32. The thickness of the shim washer 36 is selected so as to provide the assembled solenoid with an operating characteristic that is confined to a predetermined portion of the force/displacement characteristic of the assembly. The establishment of a portion of that curve in which the actuating current, armature stroke and armature force are all directly related in a linear fashion is a characteristic of the shape of the lip of the pole piece surrounding its blind recess 23 into which the armature 32 extends. The armature 32 in Figure 2 is shown in its fully actuated condition, extending the full distance into the recess 23 until stopped by the shim washer 36.

A smooth bearing for the armature 32 is provided by depositing on the armature 32 a thick electroless nickel plate layer, preferably of a thickness of upto 0. imam.

That nickel layer is then ground back accurately to the precise diameter needed to provide a very accurate sliding fit in the central bore in the guide tube 20, which has been machined as described below in great detail.

The machining and assembly of the pressure vessel 2 of the solenoid of Figures 1 and 2 is as follows. First of all a thin copper ring (not shown in Figure 2) is fitted around a very shallow recess in each of the externally rebated portions of the guide tube 20 and pole piece 22.

The connecting tube 24 is then pushed over those rebated end portions and their associated copper rings, until the ends of the connecting tube 24 abut the shouldered ends of the rebated portions as illustrated in Figure 2.

The assembly of guide tube 20, connecting tube 24 and pole piece 22 is then heat-treated in a furnace to provide a copper brazed joint between the guide tube 20 and connecting tube 24, and between the pole piece 22 and connecting tube 24. During this brazing excess copper sometimes flows to the outside of the assembly, as indicated at 50 in Figure 1. The heating of the assembly is uniform, so that there is minimal heat distortion. After cooling, the guide tube 20, connecting tube 24 and pole piece 22 form a rigid assembly in which a blind recess in the left-hand end of the pole piece 22 is generally axially aligned with the central bore of the guide tube 20.

The above assembly is then re-bored to improve the common diameter and coaxial alignment of the bore of the guide tube 20 and the blind recess of the pole piece 22. For even greater accuracy, the internal bore and blind recess are then honed. The nickel coating 38 on the armature 32 is ultimately ground and polished to provide the best possible sliding fit in this internally honed assembly.

As will be seen in Figure 2, in which the electroless nickel plate is shown of greatly exaggerated thickness for illustrative purposes only, the nickel plating 38 does not extend all the way to the right-hand end of the armature 32. To extend it all the way is unnecessary, and encourages the entrapment of dirt between the leading edge of the armature and the blind recess in the pole piece 22, leading to fouling of the armature 32 against the pole piece 22.

Also shown in Figure 2 is an indication of the maximum total stroke of the armature 32 and actuator pin 34, indicated as "t", and the portion of that stroke in which a good proportional response can be ensured, indicated "p". Typically "t" could be 3.0mm and "p" could be 2.0mm.

Figure 3 shows a modification of the solenoid of Figures 1 and 2 and only the constructional differences are described below.

In Figure 3, the pole piece 22 is formed from two elements, an outer sleeve 22A and an inner end stop 22B which defines the end of the blind bore. The two portions 22A and 22B of the pole piece are made from highly permeable mild steel and engage one another with a light interference fit.

The two-part construciton of the pole piece 22 in Figure 3 is of assistance in the re-boring and honing of the pressure vessel assembly, since those machining operations can be carried out with both the end plug 26 and the end stop 22B removed. Also in some installations it might be found preferable to remove the end stop 22B rather than the end plug 26 for servicing.

Claims

CLAIMS:

1. A proportional stroke wet pin solenoid comprising an armature axially movable in a pressure vessel surrounded by a magnetic coil, wherein the pressure vessel comprises a magnetically permeable guide tube held spaced from a magnetically permeable pole piece by a non-magnetic connecting tube received in rebated outer cylindrical portions of the guide tube and pole piece, and a removable end plug in the guide tube, the pole piece being shaped to achieve proportionality of stroke of the solenoid and having a blind recess therein for receiving a leading end of the armature on its operating stroke, and wherein the blind recess has been rendered precisely cylindrically coaxial with the guide tube by removing the end plug, re-boring the guide tube and pole piece and finally honing the guide tube and pole piece.

2. A wet pin solenoid according to claim 1, wherein the magnetic coil is of a greater axial length than the non-magnetic connecting tube and overlies the guide tube on one side of the connecting tube and the pole piece on the other side of the connecting tube to establish a flux path which does not have to pass through the non-magnetic wall of the connecting tube.

3. A wet pin solenoid according to claim 1 or claim 2, wherein the connecting tube is made of stainless steel.

4. A wet pin solenoid according to claim 3, wherein the connecting tube is secured to the guide tube and to the pole piece, and the joint rendered fluid-tight, by copper brazed joints between the connecting tube and the rebated portions of the guide tube and the pole piece separately.

5. A wet pin solenoid according to any preceding claim, wherein the blind recess in the pole piece is formed by a through-bore in the pole piece that is blocked off by a stop member of magnetically permeable material.

6. A wet pin solenoid according to claim 5, wherein the stop member is axially removable, and is not present during the re-boring and honing of the guide tube and pole piece to ensure their precise coaxial alignment.

7. A wet pin solenoid according to any preceding claim, wherein the end plug is made of a magnetically permeable material and carries an O-ring seal to ensure a fluid-tight seal of the pressure vessel.

8. A wet pin solenoid according to claim 7, wherein the end plug is in use retained in position in the guide tube between an internal shoulder of the guide tube and an internal circlip in the guide tube, but is removable to permit removal of the armature for servicing the solenoid.

9. A wet pin solenoid according to any preceding claim, wherein the end plug carries a mechanical actuation plunger aligned with the armature to effect an actuating movement of the armature when the coil is not energised.

10. A wet pin solenoid according to claim 9, wherein an O-ring establishes a fluid seal between the mechanical actuation plunger and the end plug to preserve the pressure integrity of the pressure vessel.

11. A method for the manufacture of a proportional stroke wet pin solenoid according to any preceding claim, which comprises machining and assembling the pressure vessel in the following sequence: pressing the magnetically permeable guide tube and the magnetically permeable pole piece onto opposite ends of the non-magnetic connecting tube so that the connecting tube ends are received in rebated outer cylindrical portions of the guide tube and pole piece; re-boring the guide tube and a central cylindrical recess in the pole piece while both are rigidly mounted on the connecting tube; honing the guide tube and pole piece internal bores to a precise common diameter on a common axis; inserting the armature and actuator pin into the guide tube; and securing in place the removable end plug of the guide tube, to sealhydrauiically the guide tube, pole piece and connecting tube into the pressure vessel and to retain the armature and actuator pin for axial movement within that pressure vessel.

12. A proportional stroke wet pin stroke solenoid constructed and manufactured as described herein with reference to Figures 1 and 2.

13. A proportional stroke wet pin stroke solenoid constructed and manufactured as described herein with reference to Figures 1 and 2 as modified by Figure 3.