US3875851A

US3875851A - Hydraulic motors

Info

Publication number: US3875851A
Application number: US304838A
Authority: US
Inventors: Kenneth William Samuel Foster
Original assignee: Renold Ltd
Current assignee: Renold Ltd
Priority date: 1971-11-09
Filing date: 1972-11-08
Publication date: 1975-04-08
Anticipated expiration: 1992-04-08
Also published as: JPS4861837A; CA971423A; BR7207845D0; AU469445B2; DE2254585B2; JPS5343623B2; AU4870372A; AR198397A1; IT970361B; ES408389A1; DE2254585A1; GB1413107A; FR2160118A5

Abstract

An hydraulic motor having an assemblage of pistons and cylinders rotatable on a stationary pintle presenting a ring of fluid supply and exhaust ports is provided with an intervening porting ring which rotates with the piston and cylinder assemblage. The porting ring is accurately machined to a precise running fit on the pintle. Flexible sealing means is provided in a gap between the porting ring and the piston and cylinder assemblage. This gap absorbs any build-up of more generous tolerances in other parts of the motor, and simplifies manufacture. The gap also accommodates any angular misalignment between the axes of the pintle and the rotatable assemblage. The ring may be fluid pressure balanced to reduce distortion of the ring.

Description

I United States Patent 1191 1111 3,875,851

Foster Apr. 8, 1975 [541 HYDRAULIC MOTORS 3.426.695 2/1969 Klaus 91/498 Inventor: Kenna William Samuel oster 3.696.710 l(l/l972 Ortelll 9I/487 Prestbury. England FOREIGN PATENTS OR APPLICATIONS [73] A i Renold Limieds Manchesten 822.014 10/1959 United Kingdom 91/487 Enghmd 864,023 3/1961 Umted Kmgdom l.233.394 10/]960 France Ml-487 [22] Filed: Nov. 8, 1972 [2 App| 304333 Primary E.\'an1inerWilliam L. Frceh Assistant E.\'aminer-G. P. LaPointe Attorney. Agent. or Firn1-Flynn & Frishauf [30] Foreign Application Priority Data Nov. 9. 1971 United Kingdom 52037/71 [57] ABSTRACT [52l U.S. C1. 91/498 hydraulic "1010f having assemblagfi 0f PismnS I51 I Int. Cl. Ftllb 1/06 and Cylinders rommble Stationary Plmle P 1581 Field of Search 91/491-498, g a g of fluid pp y and exhaust Ports is provided 9 3 7 with an intervening porting ring which rotates with the piston and cylinder assemblage. The porting ring is ac- [56] References Ci curately machined to a precise running fit on the pin- UNITED STATES PATENTS tle. Flexible sealing means is provided in a gap be- 7 7 H6 9H9) d k 91/498 tween the porting ring and the plston and cylmder asgg f 1mg) iss 91/498 scmblage. Th1s gap absorbs any bu1ld-up of more gen- :55 5 91/496 erous tolerances 1n other parts of the motor, and 51m- ]2 g5g Ferris V I I 91/492 plifies manufacture. The gap also accommodates any 31 3 53 5 p Klopp v A r r I H 91/492 angular misalignment between the axes of the plntle 3,0816% 3/l963 Hgnrichggn 91/437 and the rotatable assemblage. The ring may be fluid 3.175.510 3/l965 DAmato r 91/505 pressure balanced to reduce distortion of the ring.

3311711 13 11/1966 Lomnicki ..91/491 3.357.312 12/1967 Rogov et nl 91/498 14 9 Drawing Figures PMENTEDAPR 8I9Y5 SHIT 2 BF 5 PMENTEUAPR 8&975

SHEET u n; 5

Had.

HYDRAULIC MOTORS This invention relates to hydraulic motors and concerns hydraulic motors of the kind. hereinafter referred to as of the kind described, comprising an assemblage of pistons and cylinders rotatable, to provide the output drive of the motor, by the action of fluid under pressure supplied into and exhausted from the cylinders through a ring of ports in a stationary part of the motor casing.

An object ofthis invention is to simplify the manufacture of such a motor by reducing the number of parts which have to be machined to a high degree of accuracy.

This invention provides an hydraulic motor of the kind described wherein a separate porting ring is provided between said assemblage and said casing part. said ring being arranged to rotate with said assemblage and defining openings which communicate said cylin ders with said ports.

The porting ring needs to be machined to a high degree of accuracy with respect to said casing part in relation to which it rotates, but a build-up of more generous tolerances in other parts can be accommodated in the sealing arrangements adopted between the pistonand-cylinder assemblage and the porting ring which rotates therewith.

Preferably. flexible sealing means is provided around each ofsaid openings between the porting ring and said piston-and-cylinder assemblage, and sealing a gap therebetween.

The flexible sealing means can then take up tolerances absorbed in said gap which is not required to remain within precise limits.

According to a feature of this invention, said pistonand-cylinder assemblage and said porting ring are rotatable about said stationary part of the motor casing and the radially outer surface ofsaid porting ring is axially bevelled or cambered so as to reduce the wall thickness of the ring at opposite edge portions of the ring.

By bevelling or cambering the radially outer surface of the ring in this fashion. the degree of angular misalignment which can be accommodated between the axis of rotation of the pistonand-cylinder assemblage and the axis of rotation of the ring. is increased.

According to a further feature of this invention, the porting ring has a pair of circumferential grooves axially spaced on said ring and housing sealing means delimiting an area of the circumferential surface of the ring exposed in said gap. and at least two holes in the ring communicate that circumferential surface area of the ring with pressure fluid leaking along the other circumferential surface of the ring between the ring and said stationary part of the motor casing. said holes being angularly positioned relative to one another such that as the porting ring revolves and the local pressure of fluid leakage between the ring and said stationary part of the motor casing varies between high pressure adjacent the inlet ports and low pressure adjacent the exhaust ports. the pressure acting on said area of said circumferential surface of the ring exposed in said gap maintains an approximately constant. predetermined. intermediate value relative to given high and low pressure values in the inlet and outlet ports respectively.

By approximately pressure balancing the ring on opposite circumferential sides, significant local distortions of the ring, otherwise leading to objectionable consequences. may be avoided.

Specific embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a cross-section on line XX in FIG. 2 showing an hydraulic motor according to this invention FIG. 2 is a section on lines AA and B-B in FIG. I on the two sides of the line C C in FIG. 2 respectively,

FIG. 3 illustrates a detail of construction of the hydraulic motor shown in FIGS. 1 and 2 to a larger size,

FIG. 4 illustrates a further detail of the hydraulic motor shown in FIG. 1 and 2,

FIG. 5 is a cross-section through a further hydraulic motor according to this invention,

FIGS. 6 and 7 illustrate a modification of the motor shown in FIGS. I, 2 and 3, or FIG. 4, FIG. 7 being a cross-section on line DD in FIG. 6 and FIGS. 8 and 9 illustrate an alternative modification of the motor shown in FIGS. 1, 2 and 3 or FIG. 4, FIG. 9 being a cross-section on line EE in FIG. 8.

Referring to the accompanying drawings and first to FIGS. 1, 2 and 3 the motor comprises a stationary casing comprising parts I, 3, 4, 6, 22, 25, 27 and 36 as dc scribed in detail in my concurrently filed patent application Ser. No. 304,840. assigned to the same Assignee as the present application.

The part I, hereinafter referred to asthe pintle 1. presents an exhaust passage 13 leading from four exhaust ports 15 via slots 16 in part 4, to an exhaust outlet 14, and an inlet passage, constituted by the bore of the tube 3, and a passage 8 leading from a fluid pressure inlet 9 to four inlet ports 12.

The

ports

12 and 15 are arranged alternately and equi-spaced, in a ring, on the diameter 18 of the pintle l, and with their centres on that diameter lying in the plane XX in FIG. 2. FIG. 4 is a development of the interface between the tube 3 and the part 4 hereinafter termed the valve block 4. This shows that the ports 15 are offset axially ofthe tube 3 in respect to the ports 12 at their radially inner ends. This is to allow sufficient spacing at I7 between the inner ends of adjacent ports I5 and 12 to maintain a fluid proof seal between them by virtue of a press fit of the tube 3 in the valve block 4.

The pintle 1 carries a porting ring 19 which is a rotating fit. to precise limits, on the diameter 18 of the pintle. The ring 19 defines six equi-spa'ced, circular or other shaped holes 20 having their centres in the plane X-X. The holes 20 are counter bored in this example to provide rebates 21 at their radially outer ends.

An annular assemblage of pistons and cylinders SI. 60 is rotatably mounted in bearings 31 and 34 in the casing, the cylinder block extending outside the easing where it presents a driving boss 42 of the motor. The assemblage of pistons and cylinders surrounds the pintle diameter 18 and has six identical cylinders lying with their cylindrical axes in the plane XXv Each cylinder has a bore 53 see FIG. 3, in one end wall 52 of the cylinder at its radially inner end. the bore opening within the assemblage opposite the ring of

ports

12 and 15. Press fitted in each bore 53 is a tubular port insert 55 which extends into the recess 21 of one of the holes 20 but remains out of contact with the floor of the recess and therefore the ring I9. The insert is reduced in diameter at its inner end as at 56 so as to present a stepped outer surface defining an annular recess extending in part within the bore 53, and flexible sealing means 57 is provided in this annular recess made up of an O-ring seal. backing a sealing ring of plastics material. the O-ring resiliently engaging the sealing ring with a sealing face formed by the floor of the recess 21. With this arrangement the O-ring is confined between the bore 53 and the recess and against the back of the sealing ring. The sealing means 57 seals the gap between the

assemblage

51, 60 and the ring 19 to prevent the escape therebetween of high pressure fluid being supplied to the cylinders 51.

The ring 19 is connected to rotate with the

assemblage

51, 60 by a peg 58 press fitted in the cylinder block 50 and engaging and precisely locating in, a slot 59 in the ring 19. This connection allows for swash movements due to annular misalignment between the axes of rotation of the piston-and-cylinder assemblage S1, 60 and the ring 19, and to this end the slot is elongated axially of the ring.

Rotation of the

assemblage

51, 60 is effected by the action of fluid under pressure supplied into and exhausted from the cylinders 51 through the holes 20 and the bores of the inserts 55 via the inlet ports 12 and the exhaust ports 15 respectively, as described in detail in my concurrently filed patent application Ser. No. 304.748. assigned to the same Assignee as the present application.

The provision of the porting ring 19 simplifies the manufacture of the motor in as far as it reduces the number of parts which have to be machined to a high degree of accuracy. The ring 19 is a precise tit on the diameter 18 of the pintle 1. The gap between the ring 19 and the assemblage 51. 60 can however absorb quite a wide accumulation of tolerances in other parts which would normally affect the closeness of the fit. and the accuracy of the concentricity of the

parts

1 and 50 if the part 50 were mounted for rotation directly on the pintle I.

By axially bevelling or cambering the radially outer surface of the ring 19 so as to reduce the wall thickness of the ring at opposite edge portions of the ring the degree of angular misalignment of the axes of rotation of the assemblage 51. 60 and the ring 19 which can be accomodated. is increased The inserts 55 have a surrounding groove 70 exposed inside the cylinder bores to facilitate the removal ofthe inserts. and the pistons are relieved at their inner ends so as to clear the inserts 55.

Referring now to FIG. 5 the stationary casing of the motor is again as described in detail in my concurrently filed patent application Ser. No. 304.840 and generally as hereinbefore described. Parts corresponding with parts already described are indicated by the same reference numeral suffixed with the letter a.

The pintle It: has a somewhat longer tube 3a and a somewhat longer tubular end portion presenting the diameter 18a.

The porting ring 1911 is also lengthened to make provision for two rings of holes a serving an assemblage of two rows of pistons and

cylinders

51a, 60a and each cylinder has a bore 53a in the floor 52a in which is press titted an insert 55! which extends into a recess 21:: in one of the holes 2011. and flexible sealing means 57a is provided in association with each insert 551: to seal the gap between the cylinder block 50: and the ring 1912. all as previously described.

As before. the provision of the porting ring, 19a and the flexible sealing means simplifies the manufacture of the motors for the reasons previously described.

The cylinder block 50a is shown to be elongated axially in FIG. 5 to accommodate the two rows of cylinder bores with adjacent bores in the two rows respectively positioned side by side.

A more compact assembly may be achieved by staggering the cylinder bores as described in my concurrently filed patent application Ser. No. 304,840.

In the modification illustrated in FIGS. 6 and 7 parts corresponding with parts already described are indicated by the same reference numeral sufflxed with the letter b.

Each bore 53b receives a port insert which is counter bored at 156 to present a stepped inner surface and a flexible sealing means 157 is located in the annular recess formed by this counter bore and against the floor of the recess 21b which, in this example, is formed by machining a flat sealing face 121 across the porting ring 19b at right angles to its axis. Instead of this the porting ring could simply be spot faced to form the sealing faces.

The flexible sealing means 157 is made up of an O- ring seal 158 lodged in the angle of an L-sectioned plastics sealing ring 159. the sealing ring 159, together with the counter bore 156 enclosing and confining the O ring seal 158.

The flexible sealing means 157 seals the gap between the assemblage 51b. 60b and the ring 1% to prevent the escape therebetween of high pressure fluid being sup plied to the cylinders 51b.

1n the modification illustrated in FIGS. 8 and 9 parts corresponding with parts already described are indi cated by the same reference numeral suffixcd with the letter r.

Each bore 53c is of the same size as the holes 20. Separate grooves 160 in the outer circumferential surface of the porting ring 19( encircle each hole 200. The open tops of the grooves thus face towards the piston and cylinder assemblage. Each groove 160 contains a flexible sealing means 161 consisting of an O-ring seal 162 backing a plastic sealing ring 163 to seal the gap between the assemblage 51c, 60c and the ring 190 to prevent the escape therebetween of high pressure fluid being supplied to the cylinders 510.

Running round the outer circumferential surface of the porting ring 19c are two grooves 165 which are spaced apart axially of the porting ring 190. the grooves 165 being disposed one to each side of the grooves 160. The grooves 165 each contain a flexible sealing means 167 consisting of an O-ring seal 168 backing a plastics sealing ring 169.

The flexible sealing means 167 delimit an area of the outer circumferential surface of the porting ring which is acted upon by fluid under pressure leaking along the diameter of the pintle 1c. and through a plurality ofsmall holes 172. of which there may be two or more.

The holes 172 are so positioned that as the porting ring revolves. and the local pressure of fluid leakage along the diameter 18(' varies from high pressure. adjacent the ports 121'. to low pressure. adjacent the ports 151'. the pressure acting on the outer circumferential surface ofthe porting ring 190 between the sealing means 167 maintains an approximately constant. predetermined, intermediate value The holes 172 have to be positioned such that as they pass over the leakage pressure contours between the pintle and the porting ring, contact is always maintained with both high and low pressure areas of which the mean pressure is a constant.

The axial position of the holes 172 as well as their radial position is important in determining the actual intermediate pressure level which is maintained acting on the outer circumferential surface of the porting ring.

With the modification just described. the pressure of high pressure fluid in the ports 120, when these ports are closed off by the porting ring, and the pressure of leakage fluid leaking along the diameter 18c, is counter-balanced by the pressure of fluid acting on the outer circumferential surface of the porting ring between the flexible sealing means 167, and local distortions of the porting ring which might otherwise occur due to the non-uniform pressure loadings inside the porting ring due to these effects are either reduced or prevented.

The invention may be applied to motors having an assemblage of more than two rows of pistons and cylinders as will be appreciated and also to motors in which there is a different number of pistons and cylinders per TOW.

The motors described may be driven in the opposite direction by supplying fluid under pressure to the ports and allowing it to exhaust from the ports 12.

Parts shown in the accompanying drawings which have not been described herein or which have not been described herein in detail. are fully and clearly described in one or other of my concurrently filed patent applications referred to herein, and the disclosure of these concurrently filed patent applications are to be regarded as incorporated by reference in the present specification.

The O-ring seals are of rubber or synthetic rubber or equivalent elastomeric material. The plastic sealing rings are composed of a plastic type material with a resistance to hydrostatic extrusion along narrow clearance gaps greater than that normally available with elastomeric seals. Furthermore, the plastic material must also be chemically compatible with the fluid being sealed and not be attacked by it. The plastic must retain a useful proportion of its elastic properties over the temperature range to be encountered in operation of the motor. By way of example, only, suitable materials would be those based on acetal. polyamides, polytetrafluoroethylene in various commercially available forms, the choice depending on the fluid being sealed, the duty and the temperature range. This list is not exhaustive and many similar plastics in many various forms would also be suitable.

I claim:

1. An hydraulic motor comprising:

a motor casing having a stationary part;

an assemblage of pistons-and-cylinders rotatable about the stationary part of the motor casing to provide the output drive of the motor, by the action of fluid under pressure supplied into and exhausted from the cylinders through a ring of ports in the stationary part of the motor casing, each cylinder of said piston-and-cylinder assemblage having a bore in one end wall thereof, said bores opening in said assemblage opposite said ring of ports;

a separate porting ring provided on said stationary casing part and located between said assemblage and said stationary casing part, said porting ring being arranged to rotate with said assemblage and defining openings which communicate said cylinders with said ports;

a tubular insert in each of said bores extending towards but remaining out of contact with said porting ring, said inserts having an annular recess therein; and

flexible sealing means provided around each of said openings of said porting ring, between said porting ring and said piston-and-cylinder assemblage, the flexible sealing means being respectively mounted in said recesses of said tubular inserts and surrounding the bores of said tubular inserts and flexibly engaging said porting ring for sealing the gap between said porting ring and said piston-andcylinder assemblage an compliantly mounting the porting ring with respect to said assemblage.

2. An hydraulic motor as claimed in claim I wherein the radially outer surface of said porting ring is axially bevelled or cambered so as to reduce the wall thickness of the ring at opposite edge portions of the ring.

3. An hydraulic motor as claimed in claim 2 wherein the porting ring is connected to rotate with the pistonand-cylinder assemblage by means of a peg on the cylinder block engaging, and precisely locating in, a slot in the ring, elongated axially of the ring.

4. An hydraulic motor as claimed in claim I wherein the flexible sealing means comprises an O-ring seal backing a sealing ring, the O-ring seal resiliently engaging the sealing ring with a sealing face on the porting ring and surrounding one of said openings therein.

5. An hydraulic motor as claimed in claim 4 wherein the sealing ring is comprised of plastic material.

6. An hydraulic motor as claimed in claim 5 wherein said tubular insert has a stepped outer surface defining said annular recess, said annular recess extends in part within the bore in the cylinder end wall and the O-ring seal is confined between that bore, the recess, and the sealing ring.

7. An hydraulic motor as claimed in claim 4 wherein said tubular insert has a stepped inner surface defining said annular recess, the sealing ring is L-shaped in cross-section and the O-ring seal is lodged in the angle of the sealing ring and is confined between the sealing ring and the annular recess.

8. An hydraulic motor comprising:

a motor casing having a stationary part;

a separate porting ring provided on said stationary casing part and located between said assemblage and said stationary casing part, said porting ring being arranged to rotate with said assemblage and defining openings which communicate said cylin ders with said ports, said porting ring further having grooves therein respectively surrounding each of said openings, the open top of each groove facing towards said piston-and-cylinder assemblage; and

flexible sealing means provided in said groove around each of said openings of said porting ring for flexibly engaging said piston-and-cylinder assemblage around one of the cylinder end wall bores and for sealing a gap between said porting ring and said piston-and-cylinder assemblage, the flexible sealing compliantly mounting the porting ring with respect to said assemblagev 9. An hydraulic motor as claimed in claim 8 wherein the flexible sealing means comprises an O-ring seal backing a sealing ring, the sealing ring confining the O- ring seal in said groove.

10. An hydraulic motor as claimed in claim 8 wherein the sealing ring is comprised of plastic material.

1]. An hydraulic motor comprising:

a motor casing having a stationary part;

an assemblage of pistons-and-cylinders rotatable about the stationary part of the motor casing to provide the output drive of the motor. by the action of fluid under pressure supplied into and exhausted from the cylinders through a ring of ports in the stationary part of the motor casing;

flexible sealing means provided around each of said openings of said porting ring. between said porting ring and said piston-and-cylinder assemblage for sealing a gap therebetween, the flexible sealing means compliantly mounting the porting ring with respect to said assemblage; and

said porting ring further including: a pair of circum ferential grooves axially spaced on said porting ring and housing sealing means delimiting an area of the circumferential surface of the porting ring exposed in said gap. and at least two holes in the porting ring communicating said circumferential surface area of the porting ring with pressure fluid leaking along the other circumferential surface of the porting ring between the porting ring and said station ary part of the motor casing, said holes being angularly positioned relative to one another such that as the porting ring revolves and the local pressure of fluid leakage between the porting ring and stationary part of the motor casing varies between high pressure adjacent the inlet ports and low pressure adjacent the exhaust ports, the pressure acting on said area of said circumferential surface of the porting ring exposed in said gap maintains an approximately constant, predetermined, intermediate value relative to given high and low pressure values in the inlet and outlet ports respectively.

12. An hydraulic motor as claimed in claim 11 wherein each cylinder of said piston-and-cylinder as semblage has a bore in one end wall thereof, the bore opening in the assemblage opposite said ring of ports, and a groove is formed in the porting ring surrounding each of said openings therein, the open top of the groove facing towards said piston-and-cylinder assem blage, the groove housing said flexible sealing means flexibly engaging said assemblage around one of the cylinder end wall bores.

13. An hydraulic motor as claimed in claim 4 wherein the radially outer surface of said porting ring is axially bevelled or cambered so as to reduce the wall thickness of the ring at opposite edge portions of the ring.

14. An hydraulic motor as claimed in claim l3 wherein the porting ring is connected to rotate with the piston-and-cylinder assemblage by means of a peg on the cylinder block engaging, and precisely locating in, a slot in the ring, elongated axially of the ring.

Claims

1. An hydraulic motor comprising: a motor casing having a stationary part; an assemblage of pistons-and-cylinders rotatable about the stationary part of the motor casing to provide the output drive of the motor, by the action of fluid under pressure supplied into and exhausted from the cylinders through a ring of ports in the stationary part of the motor casing, each cylinder of said piston-and-cylinder assemblage having a bore in one end wall thereof, said bores opening in said assemblage opposite said ring of ports; a separate porting ring provided on said stationary casing part and located between said assemblage and said stationary casing part, said porting ring being arranged to rotate with said assemblage and defining openings which communicate said cylinders with said ports; a tubular insert in each of said bores extending towards but remaining out of contact with said porting ring, said inserts having an annular recess therein; and flexible sealing means provided around each of said openings of said porting ring, between said porting ring and said piston-and-cylinder assemblage, the flexible sealing means being respectively mounted in said recesses of said tubular inserts and surrounding the bores of said tubular inserts and flexibly engaging said porting ring for sealing the gap between said porting ring and said piston-and-cylinder assemblage an compliantly mounting the porting ring with respect to said assemblage.

2. An hydraulic motor as claimed in claim 1 wherein the radially outer surface of said porting ring is axially bevelled or cambered so as to reduce the wall thickness of the ring at opposite edge portions of the ring.

3. An hydraulic motor as claimed in claim 2 wherein the porting ring is connected to rotate with the piston-and-cylinder assemblage by means of a peg on the cylinder block engaging, and precisely locating in, a slot in the ring, elongated axially of the ring.

4. An hydraulic motor as claimed in claim 1 wherein the flexible sealing means comprises an O-ring seal backing a sealing ring, the O-ring seal resiliently engaging the sealing ring with a sealing face on the porting ring and surrounding one of said openings therein.

8. An hydraulic motor comprising: a motor casing having a stationary part; an assemblage of pistons-and-cylinders rotatable about the stationary part of the motor casing to provide the output drive of the motor, by the action of fluid under pressure supplied into and exhausted from the cylinders through a ring of ports in the stationary part of the motor casing, each cylinder of said piston-and-cylinder assemblage having a bore in one end wall thereof, said bores opening in said assemblage opposite said ring of ports; a separate porting ring provided on said stationary casing part and located between said assemblage and said stationary casing part, said porting ring being arranged to rotate with said assemblage and defining openings which communicate said cylinders with said ports, said porting ring further having grooves therein respectively surrounding each of said openings, the open top of each groove facing towards said piston-and-cylinder assemblage; and flexible sealing means provided in said groove around each of said openings of said porting ring for flexibly engaging said piston-and-cylinder assemblage around one of the cylinder end wall bores and for sealing a gap between said porting ring and said piston-and-cylinder assemblage, the flexible sealing compliantly mounting the porting ring with respect to said assemblage.

9. An hydraulic motor as claimed in claim 8 wherein the flexible sealing means comprises an O-ring seal backing a sealing ring, the sealing ring confining the O-ring seal in said groove.

11. An hydraulic motor comprising: a motor casing having a stationary part; an assemblage of pistons-and-cylinders rotatable about the stationary part of the motor casing to provide the output drive of the motor, by the action of fluid under pressure supplied into and exhausted from the cylinders through a ring of ports in the stationary part of the motor casing; a separate porting ring provided on said stationary casing part and located between said assemblage and said stationary casing part, said porting ring being arranged to rotate with said assemblage and defining openings which communicate said cylinders with said ports; flexible sealing means provided around each of said openings of said porting ring, between said porting ring and said piston-and-cylinder assemblage for sealing a gap therebetween, the flexible sealing means compliantly mounting the porting ring with respect to said assemblage; and said porting ring further including: a pair of circumferential grooves axially spaced on said porting ring and housing sealing means delimiting an area of the circumferential surface of the porting ring exposed in said gap, and at least two holes in the porting ring communicating said circumferential surface area of the porting ring with pressure fluid leaking along the other circumferential surface of the porting ring between the porting ring and said stationary part of the motor casing, said holes being angularly positioned relative to one another such that as the porting ring revolves and the local pressure of fluid leakage between the porting ring and stationary part of the motor casing varies between high pressure adjacent the inlet ports and low pressure adjacent the exhaust ports, the pressure acting on said area of said circumferential surface of the porting ring exposed in said gap maintains an approximately constant, predetermined, intermediate value relative to given high and low pressure values in the inlet and outlet ports respectively.

12. An hydraulic motor as claimed in claim 11 wherein each cylinder of said piston-and-cylinder assemblage has a bore in one end wall thereof, the bore opening in the assemblage opposite said ring of ports, and a groove is formed in the porting ring surrounding each of said openings therein, the open top of the groove facing towards said piston-and-cylinder assemblage, the groove housing said flexible sealing means flexibly engaging said assemblage around one of the cylinder end wall bores.

14. An hydraulic motor as claimed in claim 13 wherein the porting ring is connected to rotate with the piston-and-cylinder assemblage by means of a peg on the cylinder block engaging, and precisely locating in, a slot in the ring, elongated axially of the ring.