Gear type fluid motor or pump having fluid pressure compensating means
United States Patent 1 1 Dixon et a1.  
 1451 Apr. 22, 1975 Related US. Application Data [63] Continuation-impart of Scr. No. 219.814. Jan. 21.  
 1972. abandoned. which is a continuation of Ser. No. 768.820. Oct. 18. 1968. Pat. No. 3.637.333.  
 130] Foreign Application Priority Data Oct. 23. 1967 Australia 28809/67 [52] US. Cl. 418/71; 418/125; 418/156; 418/196 [51] Int. Cl...... F01c 21/00; FOlc 19/00; F01c 1/08 [581 Field of Search 418/71. 125. 129. 140. 418/156. 196. 104  
 [561 References Cited UNITED STATES PATENTS 1.287.118 12/1918 Shore 418/75 2.918.877 12/1959 Woodcock 418/71 2.923.249 2/1960 Lorenz 418/104 3.067.691 12/1962 Wiggcrmann 418/196 3.292.550 12/1966 Gordon 418/130 3.306.225 2/1967 Smith 418/131 3.444.819 5/1969 Darwent 418/196 3.637.333 1/1972 Dixon et a1. 418/182 FOREIGN PATENTS OR APPLICATIONS 450.436 10/1927 Germany 418/71 Prinmry Examiner-John J. Vrablik Attorney. Agent. or Firm-Stevens. Davis. Miller &amp; Mosher [57] ABSTRACT A gear-type fluid pump having a two-part housing with axially aligned major and minor bores formed in each housing part and gears mounted in each bore; the pump also includes pressure compensating means comprising a continuous band located around the housing so as to overlie the line of separation between the housing parts. the band engaging each housing part in fluid sealing relationship. and also including a fluid receiving chamber defined between the band and the housing parts with a passage connecting the fluid receiving chamber with fluid manifold means formed in the housing and communicating with the minor bore.  
 5 Claims, 13 Drawing Figures PATENTEUAPR22IQ75 SHEET 2 If 5 GEAR TYPE FLUID MOTOR OR PUMP HAVING FLUID PRESSURE COMPENSATING MEANS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of Ser. No. 2l9.8 l4. filed Jan. 2 l I972. now abandoned. which is a continuing application of Ser. No. 768.820. filed Oct. l8. I968. now US. Pat. No. 3.637.333. issued Jan. 25. 1972.  
  This invention relates to a fluid motor or pump of the gear type that is. of the type utilizing two or more meshing gears which function as the driving or pumping members in a known manner.  
  Motors or pumps of the above type having a relatively simple construction are generally satisfactory for low pressure work. but problems have been encountered in designing units suitable for high pressure applications. For example. high pressure units so far available are usually of extremely complex and/or bulky construction thereby severely limiting their application.  
  A common construction of motors or pumps of the above type. includes a housing composed of several parts and which encloses the gears in relatively close fitting relationship so as to minimize fluid leakage or slippage between the gears and the adjacent surfaces of those housing parts. In operation however. such motors or pumps are subjected to internal pressures which are unevenly distributed within the housing so that the housing parts which should closely fit the gears tend to deflect away therefrom and allow leakage of fluid under pressure with consequent loss of efficiency.  
 It is a principal object of the present invention to provide a motor or pump of the above type which is of relatively simple construction and which is suitable for high and low pressure applications. A further object of the invention is to provide relatively simple means for overcoming the aforementioned deflection without causing any significant increase in the weight or cost of the motor or pump.  
  The following description refers in more detail to these essential features and further optional features of the invention. To facilitate an understanding of the invention. reference is made to the accompanying drawings where these features are illustrated in preferred form. It is to be understood however. that the essential and optional features of the invention are not limited to the specific forms of these features as shown in the drawings.  
 In the drawings:  
  FIG. I is an end elevational view of one embodiment of the present invention. which view is partially sectioned for convenience of illustration;  
  FIG. 2 is a cross-sectional view taken along line lI-II of FIG. 1;  
  FIG. 3 is a fragmentary sectional view taken along line III-III of FIG. 2;  
  FIG. 4 is an enlarged sectional view taken along line IV-IV of FIG. 1;  
  FIG. 5 is an enlarged sectional view of a portion of the control valve mechanism shown in FIG. 2;  
  FIG. 6 is a cross-sectional view taken along line VIVI of FIG. 5;  
  FIG. 7 is an end view of a further embodiment of the invention, partially sectioned for convenience of illustration;  
  FIG. 8 is a cross-sectional view taken along line VIII- -VIII of FIG. 7&#39;.  
  FIG. 9 is an enlarged fragmentary sectional view taken along line IXIX of FIG. 8, in which view the main gear has been omitted for convenience of illustratron;  
  FIG. I0 is an enlarged cross-sectional view taken along line XX of FIG. 9;  
  FIG. II is a fragmentary sectional view of a modified version of the embodiment shown in FIG. 8:  
  FIG. 12 is a fragmentary sectional view of a further modification of the embodiment shown in FIG. 8&#39;, and  
  FIG. 13 is a cross-sectional view of yet another embodiment of the invention.  
  In the particular example shown in FIGS. I to 6. the housing includes an intermediate housing part 2 and two end housing parts 3 and 4. The intermediate part 2 has formed therethrough a major bore 6 which con tains a main gear 7 in closely fitting relationship. and one or more minor bores 8 located laterally of the bore 6 but communicating therewith. Preferably. four minor bores 8 are provided in equally spaced relationship about the circumference of the major bore 6. Each minor bore 8 contains a pinion gear 9 in closely fitting relationship. and the lateral spacing of the respective axes of the main gear 7 and each pinion gear 9 is such that the main gear 7 meshes with each pinion gear 9 in a known manner.  
  The housing end parts 3 and 4 are releasably secured by bolts II to opposite ends respectively of the intermediate housing part 2 so as to extend across at least a portion of each of the bores 6 and 8 and thereby limit axial movement of the gears 7 and 9 relative to the housing. For best results. such axial movement. or more correctly the end clearance for the gears 7 and 9. should be kept to a minimum. In one form. the gears 7 and 9 each have a length substantially equal to but no greater than the axial thickness of the intermediate housing part 2, and each end housing part 3 and 4 has a planar surface which engages a respective end face of the intermediate housing part 2 and defines an end wall for each of the gear cavities formed by the bores 6 and 8.  
  Inlet and outlet fluid manifolds I2 and 13 respectively may be provided in the housing end parts 3 and 4 as shown in FIG. 2. Alternatively. the inlet and outlet manifolds l2 and I3 may be formed in the same housing part 3 or 4. It will be appreciated that although manifolds I2 and 13 may act as inlet and outlet respectively when the unit is used as -a motor having a drive in one direction. their roles will be reversed when the direction of drive is reversed. The same may apply when the unit is used as a pump.  
  Each minor bore 8 communicates with the inlet and outlet manifolds through ports I0 and I5 respectively. In this regard. see FIGS. 1. 3 and 6. Ports I0 are provided in the end part 3. whereas ports I5 are provided in endpart 4.  
  A driving or driven member operatively connected to the main gear 7 may be in the form ofa shaft I4 extending through one end of the housing and through an axial bore I6 of the main gear 7. and a quill or hollow shaft 17 may be drivably connected to both the shaft I4 and the main gear 7. Such a drivable connection may include internal and external splines 18 and I9 respectively on the quill 17 which respectively engage with external and internal splines II and 22 on the shaft I4 and main gear 7 respectively. The shaft 14 will be driving or driven according to whether the unit is used as a pump or motor respectively.  
  With the above drive arrangement. each housing end part 3 and 4 is preferably provided with an open ended bore 23 and 24 respectively which are in substantial axial alignment with the bore 16 of the main gear 7. An end cap 26 may be attached over the outer end of the bore 24 and locates within that bore to provide a journal bearing for the adjacent end of the shaft 14. An annular bearing member 27 may be secured to the outer end face of the housing end part 3 to rotatably support portion of the shaft 14 extending through that end part.  
  In order to avoid or at least minimize distortion of the housing. a pair of compensating members are located one within the other about the intermediate housing part 2. The outer surface of the intermediate housing part 2 is preferably cylindrical in which case the inner compensating member is preferably in the form of a substantially cylindrical band 28. Nevertheless, the outer periphery of that member may have any other appropriate shape. although a cylindrical shape is generally most convenient to form. The internal diameter of the band 28 is predetermined so that it bears against the outer surface of the intermediate housing part 2. and the band 28 preferably extends across the entire width of the outer surface of the intermediate housing part 2.  
  It is further preferred that the band 28 extends beyond the opposite ends of the intermediate housing part 2 so that end portions of its inner surface bear on cylindrical surfaces 29 and 3t formed on the adjacent end portions of the housing end parts 3 and 4 respectively.  
  The outer compensating member may be also in the form ofa cylindrical band 32. although that is not necessary. It is only necessary that the outer member has a bore therethrough which is substantially complementary in cross-sectional shape to the outer peripheral shape of the inner band 28. so as to bear against the outer surface of that inner band. The outer peripheral shape of the outer member may be of any convenient form.  
  By way of example, the outer member may constitute a casing for the motor or pump housing which serves as mounting and sealing means as well as fulfilling its primary function as a compensating member. In such an arrangement (not shown). the casing may have a bore therein for receiving the inner band 28 in close fit ting relationship. and an end wall defining the aforementioned journal bearing for the adjacent end of the shaft 14. An annular wall may be removably mounted at the other end of the casing to surround the shaft 14 in fluid sealing relationship so that the housing parts 2. 3 and 4 are completely enclosed within the casing. With this particular construction it is not necessary to provide the aforementioned end cap 26 and annular bearing member 27. Also. either one or both of the fluid manifolds may be formed in an end wall of the casing rather than in the housing end parts 3 and 4.  
  For convenience of description however. the outer compensating member will be hereinafter referred to as the outer band 32.  
  The outer band 32 preferably extends across substantially the full width of the inner band 28 and engages the outer surface of the inner band 28 at least at the end portions thereof. A fluid chamber 33 is preferably provided between the adjacent surfaces of the bands 28 and 32. and that chamber may be formed by a relatively shallow circumferential groove formed in either or both of those surfaces. Preferably. the groove or chamber 33 is of substantially the same width as the intermediate housinng part 2 and is disposed in substantial alignment therewith. for a purpose hereinafter made clear. Also resilient 0 rings or other appropriate sealing means may be provided at or adjacent each side of the groove or chamber 33 to prevent or minimize fluid leakage laterally from the chamber 33 between the two bands 28 and 32.  
  A fluid supply passage 34 may be formed through the inner band 28 and the intermediate housing part 2 to provide a communication between the fluid manifolds l2 and I3 and the chamber 33. Any appropriate valve mechanism may be employed to control the flow of fluid through the supply passage 34. That is. the valve mechanism may be operable automatically or manually to connect the fluid chamber 33 with a selected one of the manifolds l2 and 13. For example. a shuttle valve 36 may be employed which will automatically connect the fluid chamber 33 to the manifold 12 or 13 which is subjected to the highest fluid pressure. and disconnect the chamber 33 from the manifold subjected to the lower pressure. The valve 36 may connect the chamber 33 to both manifolds l2 and 13 if they are subjected to equal pressures. The shuttle valve 36 is preferably constructed as shown in FIGS. 5 and 6. That is. it is slidably mounted within a lateral passage 37 communicating with passage 34, and has tapered head portions 38 each of which is adapted to close an orifice 39 formed in a respective valve plate 41.  
  In use. both the compensating fluid chamber 33 and the main fluid chamber defined by the gear cavities. usually receive fluid from the inlet manifold l2 so that substantially the same pressure may exist in both chambers. The pressure within the main chamber tends to force the intermediate housing part 2 radially outwardly and is therefore transmitted through that housing part to produce a radially outward thrust against the inner surface of the inner band 28. The pressure within the compensating chamber 33 however. produces a radially inwards thrust against the outer surface of the inner band 28, which inward thrust may be substantially equal to the aforementioned outward thrust because of the substantially equal thrust areas involved.  
  As there is usually a pressure gradient within the main fluid chamber. there will be zones at which the inward thrust on the inner band 28 exceeds the outward thrust thereon. The higher inward thrust however. is usually prevented from causing significant distortion or deflection of the inner band 28 because that band bears on the end housing parts 3 and 4 which are able to resist relatively high radial forces because of their relatively large radial thickness.  
  Similarly. the outward thrust may exceed the inward thrust at some zones. but the resultant outward force is generally insufiicient to cause deflection of the inner band 28 such as to disturb the efficiency of the motor or pump. in this regard. the outer band 32 may be of relatively heavy section compared to the inner band 28 thereby providing a solid backing member for the inner band 28.  
  It will be appreciated from the foregoing that the compensating means described will effectively prevent or reduce deflection or distortion of the housing so that fluid loss due to leakage between the housing and the gears is kept to a minimum over a wide range of pressures. The bulk and weight ofa motor or pump utilizing the compensating means described may be considerably less than a motor or pump having a corresponding capacity and which is otherwise rigid enough to resist deflection or distortion by use of previously known means.  
  A motor or pump Incorporating the compensating means described can be pressurized over the outer pe riphery and end faces of the housing and on the inner surface of the main gear, at a pressure which may be less than the pressure operating the motor or pump. being regulated at some optimum proportion of same. Also. a motor or pump so arranged. having a seal on its driven or driving shaft so that leakage of fluid from areas of high pressure and the compensating chamber 33 to areas of low pressure. will automatically result in a pressure within the compensating chamber 33 which is directly proportional to the system pressure and which may be made to bear on all or selected areas of the housing so as to effectively reduce or prevent deflection thereto.  
  A further problem encountered with motors or pumps of the type described. is that they are relatively expensive to manufacture because of their complex construction and the tolerances which must be adhered to in order to obtain efficient operation. One particular cause of this problem is the mounting of the gears. and present means for such mounting involves the use of spindles or stub axles for each gear and which are rotatably mounted in appropriate bearings so as to allow the respective gear to turn freely. Such mounting arrangements require that the gears be confined to precise centres of rotation in order that the operating clearance between the gears and the housing can be maintained sufficiently small and consistent so as to minimize leakage of fluid between the gears and the housing.  
  In addition to the above. such spindles or stub axles and their respective bearings. have been rendered indispensable because of the particular means hitherto adopted for achieving partial balance of the fluid pressure and mechanical loads on the gears. A substantial part of the manufacturing cost of such fluid motors or pumps is incurred in the precise machining operations necessary to locate the spindles or stub axles and bearings within the housing in appropriate relation to the stationary parts of the motor or pump. That location is of course critical so that the respective gears are mounted with sufficient accuracy to achieve the relatively small working clearances necessary for efficient operation.  
  The foregoing problem is particularly evident in relation to mounting ofthe main gear 7. In the construction described. the problem is alleviated by provision of a coupling means for the main gear 7 which allows misalignment of the respective rotational axes of the main gear 7 and the shaft I4. The arrangement is such that the main gear 7 is &#34;floatingly mounted within the major bore 6 so that lateral movement of that gear is restricted only by the cylindrical surface of the bore 6. In use however. a film of fluid is formed between the cylindrical surface of the bore 6 and the crown of the teeth of the main gear 7 thereby preventing metal to metal contact between the gear and that surface.  
  The quill I7 functions as the coupling means and is able to tilt relative to the rotational axis of either the shaft I4 or the gear 7 thereby allowing the aforementioned misalignment to occur. The splines 18 and I9 of the quill I7 may be manufactured with a backlash to facilitate tilting of the quill axis as required. It will be appreciated that other coupling means such as a Hookes joint or a universal joint may be used with equal effectiveness.  
  Turning now to the embodiment of FIGS. 7 to 10, that embodiment is of particularly simple construction thereby enabling comparatively economic manufacture and assembly. One of the principal differences between this embodiment and that of FIGS. I to 6. is the fact that the housing is constructed in two main parts. The housing parts 103 and I04 abut in a plane extending normal to the rotational axis of the main gear 107, and are retained in assembly by studs III.  
  Major and minor bores I06 and I08 communicate as previously described. and in the preferred arrangement there are five minor bores I08. A pinion I09 is provided in each bore 108 so as to mesh with the main gear I07. Each minor bore 108 communicates with inlet and outlet manifolds H2 and H3 through ports 110 and IIS respectively. The manifolds H2 and H3 are both formed in the end part I04. as are the ports H0 and IIS (see FIGS. 8 and 9).  
  The ports I10 and preferably communicate with the minor bores 108 through respective shallow grooves 151. A narrow channel 152 may be formed at the end of each groove I51 remote from the respective port as shown in FIG. I0. Such channels I52 have been found useful in preventing or minimizing cavitation.  
  A further important distinction over the previous embodiment is the mounting of the pinion gears I09. As shown in FIG. 8. each minor bore I08 extends completely through the housing parts 103 and 104. although it may extend through only one end face of the housing if desired. The opposite ends of each pinion 109 are secured against relative movement to respective cylindrical bearing members I53. In the embodiment shown. the bearing members 153 are secured to the pinion 109 by screw threads. but any other suitable means may be used. The bearing members 153 are coaxial with the pinion 109 and have the same outer diameter. and the complete pinion assembly is rotatably and slidably located within its respective bore I08.  
  It will be appreciated that the bores 108 are simple to form. and there is no problem in assembling the pinions I09 within the housing. Each pinion 109 is retained in its correct axial position by teeth of the main gear 107 locating between and being engageable with adjacent faces of the bearing members I53. Seal caps 154 may be removably retained in each end of each bore 108 by circlips 156. or any other appropriate means.  
  Pressure compensating means may be provided in this embodiment. In one form, such means includes at least one. but preferably two cavities I57 provided in the housing parts I03 and I04 radially outwardly of each minor bore 108. Preferably. each cavity I57 is elongated in the axial direction of the associated pinion I09. and extends for substantially the same length as the pinion I09 and is in substantially radial alignment therewith. Although the cavities 157 may have any suitable shape in transverse cross section. they are preferably areuate and generally follow the contour of the adjacent bore 108 as shown in FIGS. 7 and 9. A passage 134 (see FIG. 8) connects each cavity I57 with the adjacent bore 108.  
  The embodiment of FIGS. 7 to I also preferably includes a coupling means as described in relation to the previous embodiment. That is. a quill H7 has an outer spline I19 at one end drivably engaging with an inner spline I22 of the main gear I07. An inner spline I18 at the opposite end of the quill ll7 engages a spline I2I formed on a shaft 114. The quill I17 and the main gear 107 function as previously described. In particular. the gear 107 is floatingly mounted.  
  In a modification of the embodiment of FIGS. 7 to I0. as illustrated in FIG. I]. a shaft 114 is not provided but the quill 117 is arranged to accept the spline for a shaft or any other driving or driven member. Suitable seals I58 and 159 are provided to prevent or minimize fluid leakage. In all other respects. the arrangement is substantially as previously described. Such a construction is particularly applicable to mounting direct on a wheel or a winch for example.  
  A further modification of the FIG. 7 to embodiment is illustrated in FIG. 12. In that modification. the compensating means includes a cylindrical band 128 instead of the cavities 157. The band 128 is located within a recess I61. and a fluid receiving space 133 is defined between the inner surface of the band I28 and the base of the recess I61. The inner surface of the band 128 is preferably substantially complementary to the base of the recess so that only a film of fluid is receivable within the space 133. Seals I60 are provided at the corners of the recess I6I to prevent or minimize fluid leakage from the space 133.  
  The space 133 is connectable with manifolds I12 and H3 through passages I62. I63 and I64. A control valve 136 is preferably provided to automatically conneet the space I33 to the manifold subjected to the higher pressure. and to automatically disconnect the space 133 from the manifold subjected to the lower pressure. The valve I36 may be of the same basic construction as the valve 36 described in relation to the first embodiment. and consequently has tapered heads I38 operable to close respective orifices 139. Each orifice 139 is connected to a respective one of the passages I63 and I64.  
  The housing construction and pinon assembly as described in relation to the embodiments of FIGS. 7 to I2. have several important advantages. The housing construction for example is comparatively simple to manufacture and has a high resistance to distortion compared to a three part housing construction as described in relation to the embodiment of FIGS. 1 to 6. Also. manufacturing errors are necessarily reduced because of the small number of parts comprising the housing. For example. the pinion bores 108 can be formed after the housing parts I03 and 104 are assembled. thereby enabling a high degree of accuracy.  
  The pinion assembly enables the use of a two part housing because the bearing members 153 have a common diameter with the pinion body I09. The bearing members 153 carry radial loads applied to the pinion I09 by hydraulic pressure and tooth loads. and they also resist axial hydraulic forces acting between the pinion teeth thereby relieving the housing of those forces. In addition. the members I53 effectively seal the ends of the pinion cavity against loss of fluid and also locate the pinion in its correct axial position by close fitting relationship on opposite sides of the main Ill gear 107. Other advantages of such a pinion assembly will be readily apparent to persons skilled in the art.  
  From the foregoing it is clear that the method of manufacturing the two-part housing would include the steps of forming the housing parts such as by casting and machining the mating surfaces smooth. Pockets would be formed in each half to define the major bore and (preferably a plurality) minor bores when the housing parts are assembled together. In precision casting such as die casting. the machining can be eliminated or minimized and the forming of an opening completely through one housing half could be part of the precision casting or a separate step. As previously stated. the axes of the minor bores are substantially parallel to the axis of the major bore and are located radially outwards just a sufficient distance so that the pinions will mesh with the main gear.  
  The pinions may. of course. be mounted on various types of bearing means such as a cylindrical bearing or a shaft fixed to the housing such as being secured to the end bearing members which are in turn secured to a respective housing part against relative movement. such as by a press fit or by fastening screws. The securing means for the shaft can be a seal cap mounted. preferably removably. in the opening which was formed for the making of the minor bore. With a fixed shaft various types of bearings may be used such as a needle hearing within the pinion if the shaft extends through the pinion or the pinion can be supported on two stub shafts which extend from cylindrical bearings as previously described.  
  Yet another embodiment of the invention is illustrated in FIG. 13. The principal and important distinction of this embodiment is the housing construction. All other components are substantially as described in relation to the first embodiment. and consequently will be given passing reference only.  
  The housing includes an intermediate part 202 and end parts 203 and 204 which are substantially as described in relation to the first embodiment. Those parts however. are contained within a bore 266 of a casing having a skin 232 and an end wall 267 which closes one end of the bore 266. Axial movement of the parts 202. 203 and 204 within the bore 266 is restricted by a retainer plate 268 which is releasably locked within the open end of the bore 266 by an annular nut 269.  
  A plurality of pinions 209 (five for example) are rotatably mounted within the housing and mesh with a main gear 207. Each pinion 209 may have its opposite end portions rotatably mounted in respective end parts 203 and 204 by needle bearings or any other suitable means. A quill 2I7 drivably connects the main gear 207 to a shaft 214 in the manner previously described so that the main gear 207 is &#34;floatingly mounted. Manifolds 212 and 213 may be provided within the casing end wall 267.  
  Compensating means is provided and preferably includes a cylindrical band 228 which contains the housing parts 202, 203 and 204 and engages an outer cylindrical surface of each one of those parts. The band 228 is interposed between the aforementioned housing parts and the skirt 232 so that the skirt functions as the outer band 32 of the first embodiment described. and a fluid receiving space 233 is defined between the band 228 and the skirt 232. The outer surface of the band 228 is preferably substantially complementary to the surface of bore 226 so that only a film of fluid is receivable within the space 233.  
  Fluid from the major and minor bores (not shown) of the housing is conveyed to the space 233 by passing between the adjacent surfaces of plate 268 and end part 203 and the adjacent surfaces of wall 267 and end part 204. The fluid pressure between those surfaces and in the space 233 is usually substantially the mean of the respective pressures acting in the manifolds 2l2 and H3. Thus, during use, the housing parts 202, 203 and 204 are completely encapsulated in pressurized fluid thereby minimizing distortion or deflection of those parts.  
  A shallow groove 271 may be formed in the outer cylindrical surface of the intermediate part 202 so as to allow limited radially inward deflection of the band 228. Preferably, that groove 271 communicates with atmosphere through passages 272, 273 and 274.  
  It will be appreciated that a casing as described in relation to the last embodiment, may be used with any one of the previous embodiments. Furthermore. the compensating means described in relation to the various embodiments has the important advantage of enabling manufacture of a compact unit in which fine clearances may be provided between the gears and the casing. in particular, the compensating means avoids fi&#39;ilitortion of the housing such that the pinions foul with t ir respective surrounding cavity wall.  
  Having now described our invention. what we claim as new and desired to secure by letters patent is:  
  l. A fluid motor or pump of the gear type including a housing having a major bore and at least one minor bore formed therein. the axes of the said bores being substantially parallel and being relatively disposed so that said minor bore communicates with said major bore; said housing being formed as two parts which are assembled in face to face relationship along a plane extending normal to the axes of said bores and located between axially opposite ends of the major bore: said major bore being defined by a pair of axially aligned cylindrical pockets each of which is formed in a respective one of said housing parts; a main gear rotatably mounted within said major bore; a pinion gear rotatably mounted within said minor bore and meshing with the main gear; fluid manifold means formed in said housing and communicating with said minor bore; and presure compensating means located radially outwards of said minor bore so that a portion of said housing is interposed between said pressure compensating means and said minor bore. said pressure compensating means including a continuous band located around said housing so as to overlie the line of separation between said housing parts; said band engaging each said housing part in fluid sealing relationship, and a fluid receiving chamber defined between said band and said housing parts. said pressure compensating means being arranged to receive pressurized fluid and thereby apply a radially inwardly directed force to said housing portion to minimize radially outward deflection thereof.  
  2. A fluid motor or pump according to claim 1 wherein passage means connects said fluid receiving chamber with said fluid manifold means.  
  3. A fluid motor or pump according to claim I, wherein said minor bore extends to at least one of the two axial end faces of said housing. a pair of cylindrical bearing members support respective opposite ends of said pinion within said minor bore so as to rotatably mount said pinion within said housing. and at least that part of said bearing member adjacent the pinion has an outside diameter substantially equal to that of the said pinion.  
  4. A fluid motor or pump according to claim I wherein said band is located within a recess formed within the radially outer surface of said housing parts. said fluid receiving chamber is defined between the radially inner surface of said band and the base of said recess. and the radial thickness of said chamber is small by comparison with the axial length thereof so that only a thin film of fluid is receivable within said chamber.  
  5. A fluid motor or pump according to claim 4. wherein said fluid receiving chamber is connected to said fluid manifold means through a valve mechanism which is operable to automatically connect said chamber with that part of the manifold means which is subjected to the higher pressure and to prevent communication between said chamber and that part of the manifold means which is subjected to the lower pressure.