GB2502623A

GB2502623A - Piston device with mechanism to secure piston to slipper

Info

Publication number: GB2502623A
Application number: GB201209838A
Authority: GB
Inventors: Blair Ramsay
Original assignee: Rolls Royce Engine Control Systems Ltd
Current assignee: Rolls Royce Controls and Data Services Ltd
Priority date: 2012-06-01
Filing date: 2012-06-01
Publication date: 2013-12-04
Also published as: GB201209838D0

Abstract

A piston device comprising a piston 14, a part spherical projection 20 extending from the piston and a slipper 16 that includes a recess 24 to receive the projection, where a securing insert 40 comprises a part spherical surface to engage the projection and the piston is held captive within the slipper by securing the securing insert within the recess. The insert may be secured to the slipper by welding, press fitting or deformation of part of the slipper. Preferably the insert is comprised of hard tool steel and is of the same material as the projection. The device may also have a spacer 38 and a load element 34 to ensure correct location of the insert relative to the slipper.

Description

I

Piston Device This invention relates to a piston device and in particthar to a piston device suitable for use in a piston-type pump arrangement Piston operated pumps are weU known and typically comprise a series of p.istons,*each of which is reciprocable within a respective bore formed in a pump housing. An angled swash plate is located adjacent the pump housing. Parts of each piston extend from the housing and are engageable with the swash plate En use, the angled swash plate 1Q is rotated relative to the, pump housing, and the cooperation between he end part of each piston and the swash plate results in each piston being rorced inwardly into the pump housing, displaing fMd frbm tha associated bore.

By adjusting the angl Of the awaSh plate, the stroke through which each piston reciprocates, in use, can be adjusted, resulting ni a change in the output rate of the pump.

Each piston is typically of generaUy cyhndncal form Rather than have the end part thereof bear directly against the swash plate it is known to provide each piston with a so-called slipper the angle of which can change relative to the ciston to accommodate changes in the angle of the swash plate and the relative movement between the piston and the swath plate US6555777, US5704272 and U3200t1/012331 I all describe slipper arrangements. In each case, the piston is formed or provided with a part spherical projection upon which the slipper is mounted, the slipper having a corresponding part spherical recess arranged to receive the part spherical projection so as to allow articulation of the slipper relative to the piston whdst being able to transmit high compressive loadings therebetween.

The sli�pers need to be s*ecureI, attached to the respective pistons. In a known technique, each sUpper is fqrmed with a recess of diameter substantilEy equal to the diameter of the projection on the piston and a blind end of which is of hemispherical form to cooperate with the projecUon.. After tap�ing to ensure a good fit between. the projection on the pistcn and the slipper, the projection is fitted into the recess in the slipper, and an end part of the slipper adjacent the open end of the recess is swaged rolled or otheise deformed so as to result in the projection being held captivewithm the recess, and hence in the slipper being securely attached to the piston. The swaging, rolhng, etc is controlled in such a manner as to allow accurate control over the amount of relative movement between the piston and the slipper, and also to ensure that the sUpper is properly mounted upon the piston The mounting of the slipper on the piston is such that a load of, for example. at least approximately 900lbf must be: applied In order to pull the slippe from the piston.

There is a desIre to use pumps of this genera] type as the high pressure fuel pumps for aircraft engines. In such applications, reliable Oera.tion over extended. periods of time is of paramountJrnportance.

In a typical arrangement, the projection on the piston is of a hard tool steel. The slipper is typically oi a softer, more malleable steel The end part of the recess against wt'iich the projection bears is typically nitrided in order to enhance its ability to rellabiy withstand, over an extended period of time, the loadings which would be appUed thereto if used as a high pressure pump in aerospace apphcations However, treatment of thS parts of ma slipper, which are deformed to securely mount the sUpper onto the piston, to increase their hardness is not posethle as it woulo prevent deformaton of the slipper around the piston Consequently, these parts of the slipper may wear potentially resulting in dislocation of the Sipper from the piston and failure of the pump It is an object of the invention, therefore, to pro\de a piston. deiqe suitable for use in such appUcations, In accordance with the present invention there is provided.a piston device comprising a* piston member a part spherical projection extending from the piston member, and a slippei fitted to the projecton, the slipper including a recess within which the projection is received, wherein a securing insert is located, at least partially, within the recess, the ibsert including a part spherical surface adapted to engage the proJection the insert being secured to the slipper and servirg to hold the projection captive with'n the recess, thereby holding the slipoer captive to the prs4on The insert may be secured to the slipper by being Welded.thereto. Alternatively, it may be secured in positIon by detqrmation of part of the slipper. 3.

The insert ks conveniently of a hard tool steL preferably the s ama material as the *prqjection.

II will be. appreciated that such an arrangement is advantageous in that, as the insert does not need to be deformed, Ft can be treated, fbi example nltrided, to ensure that it is of sufficient hardness that wear is reduced to an accepfable level.

Preferably, location means are provided to ensure the correct location of the Insert relative to the remainder of the slipper For example, a spacer may be provided within the slipper to limit thC distance b'I' which the insert can be inserted. into the recess of the slipper and theröby control the position thereof. Alternatively, the insert may Indude a shoulder which engages against part of the slipper to limit insertion thereof.

The recess may contain a load transmitting insert defining a part sphencal recess within which the frojection engages, The invention will further be described, by Way of example, with reference to the accompanying drawings, in which: Figure 1 isa diagrammatic view illustrating a piston-type pump:; Figure 2 is a diagrammatic section view illustrating a piston device in accordance wth one embodiment of the invention and suitable for use iii the pump of Figure t and Figure 3 is a diagrammatic view illustrating a piston device in accordance With a second embodiment of the invention.

Referring firstly to Figure 1, a piston-type pump is shown which comprises a pump housing 10 within which a series of bores 12 are formed. Reciprocable. within each bore 12 is a respective, piston t4. Each bore is connected to sri inlet and an outlet (not shown), appropriate valVes (nOt shown) controlling the flow or fluid into and/or out of each bore 12. It wilF be appredated that retracUon of each piston 14 from' the associated bore 12 results in fluid being drawn into the respective bore 12 from the inlet. Subsequent return movemertof the piston 14 into the bore. expels at least some of the fluid from the bore 12 via the outlet.

Each piston 14 includes an end part which projects from: the. housing 10. A slipper 16 is mounted upon the end part of each piston 14 and bears against an angled swath plate 18. In use, the angled swash piate 18 rotates relative the pump. housing 10, and it will be appreciated thatthis relatWe rotation, in combination with the engagenient between the shppers 16 and the swasi plate 18 results in the pistons 14 being driven for reciprocating movement and hence in pumping offluid. occurrihg, Turning to Figure 2, a piston device in accordance with one embodiment of the invehtion and suitable for use in the. pump of Figure 1 is illustrated. The piston device of Figure 2 comprises a piston 14, to an end of which is fitted a member 20 defining a part spherical projection or head 22. The rnember 20 defining the head 22 is, in this example, of cPM1OV teeI and so isrelatively hard.

The slipper 16 which is fitted to the head 22 comprises a slipper housing 24 of generally cylindrical form and, for examp'e, of SlOG steel Conveniently, the surfaces of the slipper housing 24, apart from at an end part 44 thereof, are nitrided or otherwise treated to enhance wear resistance. The sUpper housing 24 includes an integral outwardly extending flange 28 which:serves to carry a carbon material pad 30 which, in use, bears against the swath plate 18.

The slipper housing 24 defines a generally cylindnoat recess 32 within which is located a load transmitting insert. 34. The load transmitting insert.4 is conveniently press fitted for example freeze fitted, into place and s of the same material as the member and head 22. It is shaped to define a part ápherical recess 36 01 sUbStantiaii/ the.

same dimetet as the head 22 sUch that compressive loads can be transmitted from the slipper 16 to the. head 22 substantially evenly over fl relatively large area of the head 22.

Also press fitted into the recess 32 is.a spacer rihg 38 which bears against the load transmitting insert 34 and serves to limit the distance by which a securing insert 40 can be press fitted into the recess 32.

The securing insert 40 is of annular form having an inner diameter smaller than the diameter of the head 22, and is preferably of the same material as the head 22. it inokides a part spherical recss or surface 42 havIng substantially the same radius of curvature as the head 22, the surface 42 bearing against a. rear pad 22a. of the head 22..

The end part 44 of the slipper housing 24, which, as mentioned hereinbefore, is not nitrided is deformed for example by being rolled over the end of the securing insert 40, after insertion �t the securing insert 40. This provides a secondary means of retaining the securing insert 40 within the slipper housing 24 in case the press fitted securing thereat works loose., It will be appreciated that Such retention of the securing insert 40 within the slipper housing 24 serves to hold the head 22 captive within the shpper housing 24, and thus secures the slipper 15 to the head As the securing insert is of the same material as the head 22: and may be treated in the same way, if desired, the interface therebetween is compliant and of good wear resistance. The presence of the spacer ring 38 Units the distance by wnich the securing insert 40 can be inserted into the recess 32, and so prevents the operation of press fitting the securng insert 40,n posEtion from causing the head 22 to be clamped between the load transmitting insert 34 and the securing, insert 40, instead ensuring that a desired minimal clearance IS maintained therebetween. The size of the spacer ring 38 can be adjusted to precisely set this minimal clearance between the head 22 and both the load transmitting insert 34 and the securing insert 40.

As the inner diameter of the securing insert 40 is smaller than the diameter of the head 22 ii will be appreciated that the secunng insert 40 must be located around the member 20 prior to attachment, for example by bond:ng of the member 20 to the piston 14.

As shown., the head 2.2 may include a restricted fluid flow passage 22b whereby fluid may pan for lubrication, purposes.

in use, the pump operates. subStantially as described hereinbefore. A,s mentioned above the slippers 16 are able to articulate relative to the pistons 14 whilst being securely attached thereto by the cooperation between the securing inserts 40 and the rear pails 2a of the heads 22. Wear between these components can be reduced to an acceptable level by appropriata selection of materials, and the manner in which the device is *assernbed ensures that the required clearance between the various components is maintained to allow articulation to occur.

S Figure 3 illustrates an 8ltomative. to the arrangement of Figure. 2. in the arrangement pf Figure 3 the securing insert 40 is welded.,. for example by electron beam welding; to the slipper housing 24 rather than being secured thereto by press fitting and rolling over of theend part of the slipper housing 24. Fudhermcre, in this airangemeht.. limitation of the distance by which the securing insert 40 is introduced into the slipper housing 24 s controlled by abutment of a surface 40a of the secunng insert 40 against an end surface of the slipper hoUsing 24.

Furthermore, in this arrangement, the load transmitting insert 34 is omitted and instead the head 22 bears directly against an appropriateiy shaped, and gruund end of the recess 36 the slipper housing 24 being, in this embodiment, of the same material as the head 22.

A distinct difference between the two Embodiments descnbed hereinbefore is in the number of component parts that make up the piston and slipper assembly. In the embodiment of Figure 2, there are five key parts: the piston head 22; slipper housing 24; loading transmitting insert 34 spacer ring 38 and securing insert 40; whereas in embodiment of Figure 3 comprises three key parts the piston head 22 slipper housing 24; and the securing insert 40. In this embodiment, the slipper housing 24 and söcuring insert 40 effectiveJy form two interlocking sections of the s upper 16.

it will be appreciated that whilst specific example embodiments of the invention are set out herein, number of modificatiOns and alterations may be made thereto without departing from the scope of the ci aims

Claims

CLAIM& 1 A piston device cornprisihg a piston member, a part sphetical projection e*tancihg from the piston merr.ber, and a sUpper fitted to the projecUon, the s.flpper including a recess within which the projection is received wherein a securing insert is located, at least partially within the recess, the insert inciuding a part spherical surface adapted to engage the projection, the insert being secured to the slipper and serving to hold the projection captive within the recess, thereby holding the slipper captive to the piston.
2. A piston device according to Claim 1, wherein the insert, is secured to the sUpper by beipg welded ther.
3. A piston device' according to Claim 1, wherein the mean is secured in positioh by being press fitted to the slipper.
4.. .A piston device according to Claim $ w.h rein the inaert is fyrther secured in position by deformation of part of the slipper..
5. A piston device accorcin,g to any of the preceding claims, whereTh the insert is of.a hard tool steel.
6. A piston device accoidinO to any of the prebeding claims, wherein the insert is of the same material as the prection.
7. A piston device according to any of the preceding claims, furthet comprisin location means to ensure the correct location of the insert relative to the remainder of the slipper.
8 A piston devce according to Clam 7, wherein the location means comprises a spacer located ithih the slipper to limit the. distance by which, the insert can be inserted into the slipper and thereby qntro.l the positiqn thereof.
9. A piston device according to Claim T wherein the location means comprtses a shouider provided on the insert which engages against part of the slipper to Umit insertior thereof.
10. A piston device according to any of the preceding claims, wherein the recess contains a icad transmitting insert d.efinin a prt spherical recess Within which the prttjection engaes.