US3915064A

US3915064A - Fluid drive means

Info

Publication number: US3915064A
Application number: US447171A
Authority: US
Inventors: Tibor Dancs
Original assignee: Joy Manufacturing Co
Current assignee: Denver Equipment Co; Joy Manufacturing Co; Citibank NA
Priority date: 1974-03-01
Filing date: 1974-03-01
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28
Also published as: AU7857575A; GB1499883A; DE2508861C2; CA1032061A; DE2508861A1

Abstract

A fluid drive device for high torque, low speed applications.

Description

US. Patent Oct. 28, 1975 Sheet 1 of3 3,915,064

FLUID DRIVE MEANS In the art of rotating apparatus fluid powered drive mechanisms have been used to drive large rotating equipment such as large diameter thickeners and settlers and like equipment which must receive power at extremely high torque values for rotation at very low speeds. one such fluid drive mechanism comprises a plurality of hydraulically actuated driving arms disposed generally tangentially to a driven ring or wheel portion of the apparatus to be rotated thereby and intermittently engageable therewith in the manner of a ratchet and pawl mechanism. In practice selected ones of such driving arms engage the ratchet wheel for a power stroke to incrementally rotate the driven apparatus while other selected driving arms simultaneously end a previous power stroke thereof, disengage from the ratchet wheel and are repositioned to reengage the ratchet wheel for a subsequent power stroke. The alternating incremental rotations thus produced rotate the driven apparatus in a smooth, continuous manner. Such a drive system may be constructed so as to provide very low rotational speeds on the order of 0.01 to 10 revolutions per hour at very high torque capacities, for example up to and well in excess of one million foot pounds.

The present invention is an improved fluid drive of the type specified having such advantages over similar known mechanisms as for example a simplified elastomeric mounting which affixes a driving arm assembly in the operational position thereof and renders such assembly laterally movable with respect to the driven ratchet wheel. The improved mounting is operable without recourse to other conventional supports such as pin connections, ball and socket joints, springs and like elements customarily used in the prior art. Additionally, the improved mounting is highly wear resistant andrequires no lubrication.

The present invention additionally includes an improved'driving arm having cam means adjacent respective inner and outer sides thereof which are cooperable with the driven ratchet wheel and with a control valve means to control operation of the drive device.

These and other objects and advantages of the present invention are more completely specified in the following description and illustrations in which:

FIG. 1 is a top plan view, partly in section and partly schematic of a fluid powered drive mechanism constructed in accordance with the principles of the present invention;

FIG. 2 is a side view, partly in section, of the fluid drive of FIG. I substantially as seen from line 2-2 of FIG. 1;

FIG. 3 is an enlarged fragmentary portion of FIG. 1, partly in section, illustrating an actuating cylinder and driving arm assembly and a mounting therefor constructed in accordance with the principles of the present invention;

FIG. 4 is a side view of the driving arm of FIG. 2 substantially as seen from line 4--4 of FIG. 3; and

FIG. 5 is an enlarged fragmentary portion of FIG. 1 illustrating a driving arm and a control valve assembly of the present invention including selected operational positions thereof illustrated diagrammatically.

There is generally indicated at in FIGS. 1 and 2 a fluid powered drive mechanism constructed in accordance with the principles of the present invention and presumed for purposes of illustration to be of a type utilized to power such large rotating apparatus as a large diameter thickener or clarifier. Such apparatus is well known to those versed in the art and therefore is not shown as exhaustively described herein inasmuch as such description is not necessary for an understanding of the present invention. Sufiice it to note in this regard that in practice the drive 10 is disposed atop a thickener tank (not shown) and adapted to drivingly engage one end of a rotatable shaft 8 extending vertically therewithin and having rake arms or the like (not shown) affixed adjacent its opposing end within such a thickener tank whereby drive 10 powers the rake arms in rotation in a well known manner and for such customary purposes as for example the separation of solid components from a liquid mixture. It is of course to be understood that the drive 10 of the present invention may be utilized to rotate various other types of rotating equipment about vertically or horizontally disposed axes, for example a center pier type thickener or clarifier or a large horizontally disposed rotating drum type diffuser, and therefore should not be construed as being limited to the illustrative application described herein.

Drive 10 comprises: a generally rectangular housing 20 rigidly carried by a well known truss or bridge 11 spanning the diameter of the thickener tank and rigidly affixed thereto; a ratchet wheel 30 rotatably carried within the housing 20 and drivingly engaging the shaft 8; cooperable

driving assembly pairs

40 and 40a which include driving arms 42 rigidly affixed adjacent the forward ends thereof, the arms 42 extending substantially tangentally to the wheel 30 and adapted to intermittently engage selected peripheral portions thereof; and valve means or control assemblies 50 and 50a rigidly disposed within housing 20 and cooperable with se lected ones of arms 42 to control the

assemblies

40 and 40a through a suitable hydraulic actuating circuit 60.

The housing 20 is a rigid structure shown as comprising a pair of generally rectangular plate-like upper and lower members and 72, respectively, disposed in vertically spaced parallel relation as illustrated in FIG. 2 and retained thereat by a plurality of wall portions 74 extending vertically

intermediate members

70 and 72 and rigidly affixed thereto adjacent outer peripheral portions thereof. The wall portions 74 are rigidly affixed together at respective corners of

members

70 and 72 so as to form an outer peripheral portion of the housing 20. Housing 20 additionally includes a circular inner peripheral portion 76 encompassed by a wall portion 24 extending vertically intermediate and rigidly affixed to

members

70 and 72 and adapted to receive wheel 30 therewithin, and a plurality of driving assembly mounting portions 82 rigidly affixed outwardly adjacent wall portions 74 and adjacent respective comers of housing 20, and adapted. to receive respective mountings 80 (FIG. 3) of

assemblies

40 and 40a.

In FIGS. 1 and 2 wheel 30 is shown as comprising a pair of circular plate-like upper and

lower members

32 and 34 rigidly retained in vertically spaced parallel relation such as by a wall portion 38 extending

intermediate members

32 and 34 adjacent an inner peripheral portion thereof. A plurality of circumferentially spaced cylindrical pins 36 extends axially

intermediate members

32 and 34 and are rigidly afiixed therebetween adjacent an outer peripheral portion thereof.

The wheel 30 is rotatably carried within portion 76 of housing 20 by suitable bearing means (not shown) such as a well known turntable bearing or the like and suitably adapted such as by splines (not shown) to drivingly engage an upper end portion of the shaft 8. The respective arms 42 of

assemblies

40 and 40a intermittently and drivingly engage pins 36 in a ratchet-like manner such that the wheel 30 is powered in rotation thereby.

Referring now to FIG. 3 there is depicted one of driving assemblies 40 shown as comprising a hydraulically actuated cylinder assembly 41 including a piston (not shown) slideably carried therewithin to which is affixed one end of a piston rod 44. One of arms 42 is suitably rigidly affixed as by mating threads 46 adjacent the opposing end of rod 44. Each assembly 40 is retained by a respective mounting 80 adjacent one of assembly mounting portions 82 of housing 20. It is to be understood that assemblies 40a are identical in every respect to assemblies 40. Accordingly, the description hereinabove and hereinbelow regarding structure and operation of the assemblies 40 also applied to assemblies 40a.

Each mounting portion 82 includes an annular flange 94 spaced outwardly from wall portion 74 and rigidly affixed thereto as by

portions

70 and 72 of members 70 and 72 (FIG. 2) extending therebetween and by

support members

75a and 75b also extending therebetween and further extending inwardly of wall portion 74

intermediate members

70 and 72 and rigidly affixed thereto (FIGS. 1 and 3). A space 92 to receive the driving assembly 40 into mounting portion 82 is formed intermediate

adjacent support members

75a and 75b.

Flange 94 carries a generally frustoconical assembly mount retaining member 90 comprising: a transverse annular flange portion 91 which is adapted to be rigidly afiixed adjacent flange 94 such as by bolts 96 or by weldments (not shown); a transverse assembly mounting retaining portion 98 spaced rearwardly from flange 91; and a tapering intermediate portion 100 extending intermediate portions 91 and 98 which receives adjacent a tapering inner surface 97 thereof the mounting 80.

Each mounting 80 includes a generally frustoconical body member 86 formed from a suitably resilient elastomer such as rubber and having forward and rearward faces 81 and 83, respectively. The faces 81 and 83 are bonded to respective forward and rearward

rigid mounting plates

84 and 88 to form the substantially unitary mounting 80.

In assembly, the mounting 80 extends intermediate member 90 of portion 82 and the rearward end of hydraulic cylinder 41, the forward plate 84 thereof being rigidly and releasably affixed adjacent the rearward end of cylinder 41 as for example by a suitable adaptor plate 43 rigidly secured therebetween such as by bolts 45a and 45b or the like, and the rearward plate 88 thereof being rigidly and releasably affixed adjacent portion 98 of member 90 as by screws 99. In this con- 7 figurationi configuration body 86 is captively retained 1 within member 90 adjacent the mating tapered surface through a respective opening 78 in wall portion 24 and substantially tangentally to the wheel 30. By virtue of the inherent resiliency of body 86 assembly 40 is laterally movable with respect to the axis XX for purposes to be described hereinbelow.

By reference to FIGS. 3 and 4 it is seen that the arm 42 comprises: a rigid forward end or jaw portion 47; a rigid elongated support portion49 spaced rearwardly therefrom; and'a relatively flexible intermediate portion 48 extending longitudinally between

portions

47 and 49 and rigidly affixed thereto. The

arm portions

47, 48 and 49 have inward and

outward side surfaces

62 and 64, respectively, extending longitudinally thereon intermediate the longitudinal ends of arm 42.

Jaw portion 47 includes: a jaw 51 adjacent the forward end thereof suitably formed to be engageable with pins 36 and having a lip 73 formed adjacent inward side 62; a bifurcated end portion 57 which extends forwardly of jaw 51 adjacent outward side 64; a bifurcated guiding or cam portion 52 rigidly affixed intermediate the longitudinal ends of portion 47 adjacent outward side 64 and extending outwardly therefrom; and a camming or guiding roller 53 rotatably carried intermediate the legs of portion 52. Jaw portion 47 additionally includes a bearing or support surface portion 17 disposed intermediate the longitudinal ends of portion 47 adjacent a lower side thereof and adapted to support the forwardmost end of the assembly 40 by bearing in sliding engagement upon a peripheral portion of wheel member 34 outwardly adjacent pins 36 (FIG. 2).

Support portion 49 includes: an outward offset or bend 66 intermediate the longitudinal ends thereof; a notch 54 formed in inward side portion 62 inwardly adjacent offset 66; and an inwardly slanting portion 71 of inward side 62 extending intermediate notch 54 and a point 68 spaced forwardly therefrom.

Intermediate portion 48 as shown includes a longitudinal web portion 50 disposed substantially in a horizontal plane, and transverse flanges 59 rigidly affixed to the longitudinal ends of web 50 and suitably secured to respective adjacent ends of

portions

47 and 49 such as by screws 61 and lockwires 63 or the like. The portion 48 is designed to be relatively more flexible than

arm portions

47 and 49, it being formed from any suitable high strength material having a relatively low modulus of elasticity, for example aluminum. Additionally, the relatively thin section presented by web 50 provides a degree of vertical bending capacity not oflered by comparatively

rigid arm portions

47 and 49 whereby excessive vertical bending moments and side loads resulting for example from angular misalignment between jaw 51 and a pin 36 will be absorbed by transverse bending of web 50 and will not be transmitted via portion 49 and rod 44 to bushings and seals within the cylinder 41. It is of course to be understood that portion 48 is designed to bend only in response to side loads or bending moments applied thereto, the design thereof being sufficient to sustain the maximum anticipated axial loads to be imposed upon arm 42 without bending or buckling.

A wear plate 55 is rigidly affixed to inward side 62 of portion 49 forwardly adjacent surface 71 and extends longitudinally forwardly therefrom adjacent inward side 62 to terminate rearwardly adjacent a step 56 formed rearwardly adjacent the lip 73. The inwardly exposed side of plate 55 and adjacent surface 71 form a continuous cam or guiding surface 67 having an antifriction surface 69 bonded thereto to minimize friction with pins 36 during engagement therebetween which occurs as described hereinbelow. It is to be noted that plate 55 extends adjacent the inward side 62 of

arm portions

47, 48 and 49 but is rigidly affixed only to portion 49 and thus does not interfere with the hereinabove described bending of portion 48.

As noted hereinabove the drive includes control assemblies 50 and 50a which are cooperable with selected ones of arms 42 to actuate

respective assemblies

40 and 40a. Control assembly 50, which it is to be understood is identical to assembly 500, is shown in FIG. 5 as including a flat plate-like base 102 rigidly affixed within housing and carrying thereupon a well known pilot valve 106 and a generally angular control arm assembly 108 pivoted thereto as at 114. Control arm assembly 108 has one end portion 110 thereof pivotally affixed as at 116 to an actuating stem 118 of valve 106 and an opposing end portion 120 engageable with pins 36 and with one of the arms 42 carried by assemblies 40. A biasing force on arm assembly 108 is provided by a suitable helical spring mechanism 122 affixed to base 102 and pivoted to arm portion 110 as at 111 intermediate pivots 114 and 116 to continuously urge the arm 108 in a clockwise direction about pivot 1 14 (as viewed in FIG. 5) toward a neutral position thereof which is adjustable as by an adjusting screw 124 carried by arm portion 120 and cooperable with adjacent wall portion 24.

Arm portion 120 includes an end portion 126 which is rigidly secured to arm portion 110 adjacent pivot 1 14 and an elongated cam follower portion 130 which in practice is disposed intermediate the legs of bifurcated cam portion 52 of arm 42 and intermittently engageable by cam roller 53. A cam roller 128 is disposed

intermediate arm portions

126 and 130 and spaced inwardly therefrom as by a bracket 134 whereby in practice roller 128 is intermittently engageable by pins 36. The hereinabove mentioned interactions of cooperable pins 36, driving arm 42 and control arm assembly 108 serve to control operation of the drive 10 through actuation of the circuit 60 as follows.

In FIG. 1 the circuit 60 is shown as comprising: a suitable flow source such as a pump 134;

fluid flow conduits

138 and 140 which communicate between the pump 134 and respective rear and front portions of cylinder assemblies 41; and a main valve 136 which is cooperable with pilot valve 106 to direct fluid flow from pump 134 to the rear or front portion of cylinders 41 through

respective conduits

138 or 140 for advancing and retracting of rods 44. Circuit 60 additionally includes a well known regenerative or feedback conduit 142 communicating between conduit 140 and pump 134 and having a suitable one way check valve 144 therein; a pressure relief valve 146 in communication with conduit 140; and a suitable flow source such as a pump 39 which supplies fluid flow to the pilot valve 106.

It is to be understood that inasmuch as the

conduits

138 and 140 communicate with both cylinders 41 of assemblies 40, the assemblies 40 are operable in unison at all times. The cylinders 41 of assemblies 40a are likewise operable in unison at all times by an actuating circuit (not shown) which is functionally equivalent to the circuit 60 described hereinabove. It is to be further understood that the pins 36 are so spaced and the control assemblies 50 and 50a so positioned with respect thereto that the

assemblies

40 and 40a alternately engage selected pins 36 of the wheel 30 to provide a sequence of incremental rotations thereof.

As illustrated in FIG. 1 the circuit 60 is supplying 5 fluid flow from pump 134 through valve 136 and conduit 138 to the rearward portion of cylinders 41 for a power stroke whereby the arms 42 are being driven forward to incrementally rotate wheel 30. In the course of such a power stroke the arms 42 and wheel 30 will ultimately reach a position near the end of the power stroke as shown by the solid lines in FIG. 5, at which position the pin 36 is about to disengage the jaw 51 and substantially simultaneously engage cam roller 128 which is disposed adjacent the pin 36 intermediate the legs of arm portion 57 and a pin 36a spaced circumferentially clockwise from pin 36 has advanced into the clearance provided by the notch 54 adjacent cam surface 67. At this point the assemblies 40a engage selected pins 36 to begin a power stroke thereof. For a short ensuing period all of

assemblies

40 and 40a are engaging pins 36 and rotating wheel 30 in what is known as an overlap stroke.

As the power stroke of assemblies 40 ends the overlap stroke ends and wheel 30 continues rotating under the impetus of the power stroke of assemblies 40a. As wheel 30 continues rotating the pin 36 disengages from jaw 51 and advances to the position indicated at 36' whereat it is engaging cam roller 128 at 128', and pin 36a concurrently advances to the position indicated at 36a whereat it is engaging the cam surface 67. It is seen that by engaging cam roller 128', pin 36 has displaced arm 108 counterclockwise about pivot 114 to the position 108' against the biasing force of spring 122 whereby pilot valve 106 has been actuated in response thereto to shift main valve 136 in a well known manner and thus direct fluid flow from pump 134 through conduit 140 to the front of the affected cylinders 41 for the powered retraction of respective arms 42. It will be understood that such powered retraction of the arms 42 is completed in substantially less time than the power stroke of assemblies 40a inasmuch as the pump 134 is supplying fluid to a considerably reduced volume including only that portion of the volume of cylinder 41 in front of the piston and not occupied by rod 44.

Under the continuing impetus of the assemblies 40a pin 36a advances to a position 36a" and cams the retracting arm 42 laterally outwardly by engagement with surface 67, and pin 36 concurrently advances to the position 36". In response to the outward displacement of arm 42 roller 53 engages the follower portion 130 to cam arm 108 outwardly thereby maintaining circuit 60 in the retract mode of operation.

During the outward displacement of arm 42 the entire assembly 40 is pivoted outwardly about mount body 86, for example from an axis XX (FIG. 5) to an outwardly displaced axis XX', thereby deforming the mount body 86. The inherent resiliency of body 86 thereupon exerts an inwardly directed restoring force upon the assembly 40 whereby assembly 40 is maintained in forceful engagement with pin 36a throughout the retract portion of the cycle. It is of course to be understood that body 86 provides the capability for deflection of assemblies 40 in any plane containing the axis XX in the manner of a universal pivot.

When arm 42 has been completely retracted to the position 42' it is retained thereat during the dwell portion of the cycle as pump 134 maintains a nominal pressure in the conduit 140 and dumps excess flow to a suitable reservoir through relief valve 146. It is to be noted that during dwell the arm at 42 maintains circuit 60 in the retract mode by means of roller 53' engaging cam portion 130 of arm 108 as at 130.

The continuing power stroke of assemblies 40a further advances pin 36a to a position (not shown) forwardly adjacent the lip 73 of jaw 51 whereat the inward bias provided by mounting body 36 urges arm 42 inwardly such that pin 36a engaged the jaw end portion 57 forwardly of jaw 51 and the cam roller 53 disengages cam follower portion 130 thereby permitting spring 122 to pivot arm 108 in a clockwise direction to the neutral position thereof. In response thereto circuit 60 shifts to the power stroke mode illustrated in FIG. 1.

During the initial stages of the power stroke mode the arm 42 advances at a relatively rapid rate in an overtake stroke thereof by virtue of the additional fluid supplied to conduit 138 from the front portion of cylinder 41 through

conduits

140 and 142, and valve 144. Arm 42 thus quickly overtakes and engages the pin 36a just prior to the end of the power stroke by assemblies 40a whereupon an overlap stroke is begun. Subsequently, assemblies 40 begin a new power stroke as assemblies 40a disengage respective pins 36 and are actuated through sequential retract, dwell and overtake strokes as hereinabove described, and a full cycle of operation of drive 10 is thus completed.

By virtue of the structure defined hereinabove a greatly simplified and streamlined drive mechanism is provided. The drive mechanism of the present invention provides a plurality of driving assemblies each supported by an elastomeric mounting affixed adjacent one longitudinal end thereof and captively retained within a socket-like member, and by a support portion disposed adjacent the other longitudinal end thereof and adapted to engage the driven member. Such driving assemblies are thus supported solely from their respective axial extremities with sufficient axial stiffness to withstand maximum anticipated axial loads and in the manner of a universal pivot which permits angular deflection of the driving assembly. The simplified mounting additionally provide an inherent restoring bias to oppose all such angular deflections whereby the driving assembly mounting scheme is operable without recourse to customary universal or pivot joints or spring biasing means.

Furthermore, the driving arms hereof are greatly streamlined by the use of camming means carried only by such arms to control all phases of the operating cycle except the beginning of the retract stroke which is controlled by the ratchet pins. It is to be noted that the drive hereof is operational in response to the relative positions of ratchet pins and driving assemblies; therefore, a retract stroke cannot begin until a pin has been advanced to a predetermined position and a power stroke cannot begin until the driving arm has assumed a predetermined position behind the pin to be engaged.

Finally, it is to be noted that wear on driving cylinder seals and bushings is greatly alleviated by the flexible driving arm portion which bends in response to bending moments and side loads applied thereto.

Inasmuch as the present invention comprises a fluid drive system having a plurality of driving assemblies affixed by elastomeric mountings to a housing member and including pusher arms therefor adapted to engage a driven member and further adapted to control the operational cycle thereof, various modifications thereto are possible without departing from the broad spirit and scope thereof. For example mounting body 86 may take any of numerous forms consistent with the requirements of axial shock absorbing capacity and lateral motion such as a frustrum of a pyramid or a rectangular solid, camming means carried by arms 42 could take various forms, flexible arm portion 48 could be of a cylindrical or other suitable cross section to allow for bending in various planes and the like.

These and other modifications having been envisioned and anticipated it is requested that this invention be interpreted broadly and limited only by the scope of the claims appended hereto.

What is claimed is:

1. A hydraulic driving system for imparting a rotary motion to a rotary load comprising: a body member; a driving ring adapted to be drivingly connected to such a rotary load; said driving ring including a plurality of circumferentially spaced driving portions thereon; a plurality of elongated intermittently acting hydraulically actuated piston assemblies carried by said body member and each having reciprocable extensible portions which are drivingly engageable with selective ones of said driving portions throughout at least a portion of the extension thereof; valve means adapted to communicate with a hydraulic fluid pressure source and operative to control the flow of fluid from such a source to said piston assemblies to provide sequential extension and retraction of said extensible portions; and at least one of said piston assemblies including camming means thereon which are cooperable with selective ones of said driving portions to control the operation of said valve means.

2. A hydraulic driving system as specified in claim 1 wherein said camming means are carried solely by said extensible portion.

3. A hydraulic driving system as specified in claim 1 wherein each of said piston assemblies is pivotally mounted to said body member adjacent one end thereof opposite said extensible portions and said extensible portions extend generally tangentially with respect to the periphery of said driving ring.

4. A hydraulic driving system as specified in claim 3 wherein during at least a major portion of the retraction stroke of said extensible portion of said at least one of said piston assemblies said selective ones of said driving portions engages a portion of said camming means on the inner longitudinal side of said extensible portion to cause said at least one of said piston assemblies to pivot outwardly from the periphery of said driving ring about said one end thereof.

5. A hydraulic driving system as specified in claim 4 wherein during at least a major portion of the retraction of said extensible portion of said at least one of said piston assemblies a portion of said camming means on the other longitudinal side of said extensible portion engages said valve means to cause said valve means to maintain pressure fluid flowing to the retract side of said piston assemblies.

6. A hydraulic driving system as specified in claim 1 wherein at the end of the extension stroke of said extensible portion of said at least one of said piston assemblies, the driving portion being driven by said extensible portion operatively engages said valve means to cause said valve means to channel pressure fluid to the retract side of said piston assemblies and initiate a retraction stroke of said extensible portion.

7. A hydraulic driving system as specified in claim 3 wherein said piston assemblies are pivotally connected to said body member by respective pivot assemblies each comprising: an elastomeric member rigidly secured to said one end; a cup-shaped retaining member rigidly secured to said body member and said elastomeric member; and the inner periphery of said retaining member conforms to the outer periphery of said elastomeric member in a manner that at least the major portion of said outer periphery engages said inner periphery when said elastomeric member is rigidly secured to said body member.

8. A method of operating a piston assembly which has a reciprocable extensible portion thereof selectively drivingly engaging circumferentially spaced engageable portions of a driving ring which is drivingly connected to a rotary load to apply a rotary motion thereto comprising the steps of: extending said extensible portion by supplying a hydraulic pressure fluid to the extension side of said piston assembly; simultaneously with said extending, drivingly engaging a selective one of said engageable portions with the free end of said extensible portion throughout at least the major portion of said extending; at the termination of said extending, said extensible portion engaging a valve means in cooperation with said selective one of said engageable portions to cause the supply of hydraulic pressure fluid to be supplied to the retraction side of said piston assembly to retract said extensible portion.

9. A method of operating a piston assembly as specified in claim 8 including the additional step of: retracting said extensible portion from the extended position thereof by supplying hydraulic pressure fluid to the retraction side of said piston assembly.

10. A method of operating a piston assembly as specified in claim 9 including the additional step of: simultaneously with said retracting engaging said valve means with said extensible portion to maintain the flow of hydraulic fluid to said retract side throughout said retracting.

11. A method of operating a piston assembly as specified in claim 10 wherein said last mentioned engaging is maintained by biasing said extensible portion outwardly with respect to the periphery of said driving ring.