Abstract:
A hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units, an input shaft for coupling to a variable speed power source and an output shaft for coupling to a constant speed load, comprises: a wobbler and a port plate for the variable displacement hydraulic unit coupled to the stationary housing; an axial block and piston set for the variable displacement hydraulic unit, a port plate for the fixed displacement hydraulic unit and a wobbler for the fixed displacement hydraulic unit coupled to the input shaft; and an axial block and piston set for the fixed displacement hydraulic unit coupled to the output shaft; wherein the variable displacement hydraulic unit port plate and the fixed displacement hydraulic unit port plate couple together through a circumferential interface to minimise hydraulic thrust forces developed by the hydraulic unit.

Description:
FIELD OF THE INVENTION 
   The invention relates to a hydraulic differential, and more particularly to a hydraulic differential that eliminates high hydraulic thrust loads that act on bearings or housing structure. 
   BACKGROUND OF THE INVENTION 
   A common type of hydraulic differential comprises a first axial piston hydraulic machine of variable displacement coupled to a second axial piston hydraulic machine of fixed displacement in such a fashion that driving one of the hydraulic machines at a certain rotational speed with a source of mechanical power whilst changing the phase and amplitude of displacement of the variable displacement machine relative to its porting causes a change in rotational speed of the undriven hydraulic machine. This operation is particularly useful for converting a source of mechanical power that has a variable rotational speed to a constant rotational speed. By changing the displacement of the variable hydraulic machine in proportion to the mechanical source speed changes, the output of the undriven hydraulic machine may remain constant. 
   It is common to configure the mechanical and hydraulic coupling of the hydraulic machines or units in such a hydraulic differential so that the variable and fixed displacement units have common rotational axes. Such an end-to-end arrangement is very compact and permits direct hydraulic coupling between the blocks of the hydraulic units with suitable port and valve plates. However, such an end-to-end arrangement also tends to generate high hydraulic thrust forces between the interfaces of the hydraulic units since they generate significant working pressure in operation. Consequently, the hydraulic differential requires large bearings to withstand these hydraulic thrust forces, adding to cost, weight and size of the hydraulic differential. 
   One design of hydraulic differential has a configuration that contains these hydraulic thrust forces in such a way that the differential does not require large bearings is described in Iseman, U.S. Pat. No. 4,794,756. This differential has a structure that clamps the wobbler for the fixed displacement hydraulic unit to the block and piston set for the variable displacement hydraulic unit, thereby containing all the hydraulic thrust forces so that it does not require large bearings. One disadvantage is that the variable and fixed port plates needs to be shimmed close together to minimize leakage between the variable and fixed blocks port plates or the piece parts that clamp around the port plate need to be machined to impractical tolerances. 
   SUMMARY OF THE INVENTION 
   The invention comprises a hydraulic differential that has radial dynamic porting between the variable and fixed displacement hydraulic units to avoid large hydraulic thrust forces between them and allows for reasonable manufacturing tolerances in the porting method. 
   Generally, the invention comprises a hydraulic differential with a variable displacement hydraulic unit of the axial piston type, a fixed displacement hydraulic unit of the axial piston type, a stationary housing for mounting the hydraulic units, an input shaft for coupling to a variable speed power source and an output shaft for coupling to a constant speed load, comprising: a wobbler and a port plate for the variable displacement hydraulic unit coupled to the stationary housing; an axial block and piston set for the variable displacement hydraulic unit, a port plate for the fixed displacement hydraulic unit and a wobbler for the fixed displacement hydraulic unit coupled to the input shaft; and an axial block and piston set for the fixed displacement hydraulic unit coupled to the output shaft; wherein the variable displacement hydraulic unit port plate and the fixed displacement hydraulic unit port plate couple together through a circumferential interface to minimise hydraulic thrust forces developed by the hydraulic units. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a hydraulic differential that is suitable for incorporating the invention. 
       FIG. 2  is a cut-away side view of a hydraulic differential according to a possible embodiment of the invention. 
       FIG. 3  is a sectional cut-away view of high pressure porting for the hydraulic differential shown in  FIG. 2 . 
       FIG. 4  is a sectional cut-away view of low pressure porting for the hydraulic differential shown in  FIG. 2 . 
       FIG. 5  is a sectional cut-away view of oil transfer passages for the hydraulic differential shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A hydraulic differential that is suitable for incorporating the invention comprises a variable and fixed axial piston machine combination. This combination works to maintain a constant output rotational speed for a predetermined input rotational speed range. In this way, it converts a variable speed power source, such as an engine, to a constant speed that is suitable for powering a constant speed device, such as a fixed frequency alternating current generator. 
     FIG. 1  is a block diagram of a hydraulic differential  2  that is suitable for incorporating the invention. The hydraulic differential  2  comprises a variable displacement hydraulic unit  4 , a fixed displacement hydraulic unit  6 , an input power shaft  8  and an output power shaft  10 . The variable displacement hydraulic unit  4  comprises a variable unit block and piston set  12 , a variable unit port plate  14  and a variable unit wobbler  16 . The fixed displacement hydraulic unit  6  comprises a fixed unit block and piston set  18 , a fixed unit port plate  20  and a fixed unit wobbler  22 . 
   The input shaft  8  rotates the variable block and piston set  12 , the fixed unit port plate  20  and the fixed unit wobbler  22 . The fixed unit block and piston set  18  rotates the output shaft  10 . The operation of the hydraulic differential  2  is such that in order for the rotational speed of the fixed unit block and piston set  18  to add or subtract from the rotational speed of the variable block and piston set  12 , the variable unit port plate  14  and the variable unit wobbler  16  must be stationary with respect to the rotational movement of the variable block and piston set  12 , the fixed unit port plate  20  and the fixed unit wobbler  22  that the input shaft  8  drives, such as by fastening them to a stationary frame or housing  24 . Thus, the input shaft  8  rotates the variable unit block and piston set  12 , the fixed unit port plate  20  and the fixed unit wobbler  22  together as a single unit. Likewise, the fixed unit block and piston set  18  rotates the output shaft  10  as a single unit. 
   With this configuration, the porting between the stationary variable unit port plate  14  and the rotating fixed unit port plate  20  is dynamic. Furthermore, if the dynamic porting interface between them comprises planar surfaces that are normal to the axes of the input shaft  8  and the output shaft  10 , high hydrostatic separating forces may develop between them due to the working pressure of the variable displacement hydraulic unit  4  and the fixed displacement hydraulic unit  6 . Therefore, according to the invention, the dynamic porting interface between the stationary variable unit port plate  14  and the rotating fixed unit port plate  20  comprises a generally circumferential radial porting interface and radial porting between generally cylindrical axial surfaces. 
     FIG. 2  is a cut-away side view of a hydraulic differential  2  according to a possible embodiment of the invention. The hydraulic differential  2  contains the variable displacement hydraulic unit  4  and the fixed displacement hydraulic unit  6  within its stationary housing  24 . The stationary variable unit port plate  14  attaches to the stationary housing  24 . The variable unit wobbler  16  attaches to the stationary housing  24  by way of a variable wobbler housing  26 . 
   A variable speed power source, such as a gas turbine engine (not shown) drives the input power shaft  8 , typically by way of an input gear  28 . The input shaft  8  drives the variable unit block and piston set  12 , the fixed unit rotating port plate  20  and the fixed unit wobbler  22  that couple to it. In  FIG. 2 , the fixed unit rotating port plate  20  couples to the input power shaft  8  by way of a spline  30  on the input shaft  8 . The variable unit block and piston set  12  couples to the input shaft  8  by way of a variable block shaft  32  and a spline  34  that couples to the fixed unit rotating port plate  12 . The fixed unit wobbler  22  couples to the input shaft  8  by way of a fixed unit housing  36  that couples to the fixed unit rotating port plate  20  with a plurality of fasteners  38 . A key or pin  40  prevents rotation of the fixed unit wobbler  22  relative to the fixed unit housing  36 . The stationary frame  24  supports the other end of the fixed unit housing  36  by way of a fixed unit housing roller bearing  42 . 
   The fixed block and piston set  18  drives the output shaft  10 . In  FIG. 2 , the fixed block and piston set  18  couples to the output shaft  10  by way of a spline  44 . The output shaft  10  adds or subtracts speed to a shaft (not shown) that drives a constant speed load, such as a generator (not shown), by way of an output gear  46 . The fixed unit housing  36  supports one end of the output shaft  10  by way of an output side roller bearing  48 . The rotating fixed unit port plate  20  supports the other end of the output shaft  10  by way of an output shaft journal bearing  50 . 
   The stationary variable unit port plate  14  supports the rotating fixed unit port plate  20  by way of an interface journal bearing  52  that comprises a generally cylindrical end  54  of the rotating fixed port plate  20  and a generally cylindrical undercut and orifice  56  in the stationary variable unit port plate  14  that receives the cylindrical end  54 . The cylindrical undercut and orifice  56  prevents pressure from building on the cylindrical end  54  and developing a separating force. The input shaft  8  rotates coaxially within the output shaft  10  with additional end support from an input side roller bearing  58 . A variable unit roller bearing  60  that mounts in the variable wobbler housing  26  and variable unit journal bearing  62  that mounts in the stationary variable unit port plate  20  supports the variable block shaft  32 . 
   The stationary variable unit port plate  14  receives low pressure charging fluid from the stationary housing  24  by way of charging fluid supply port  64  for a charging fluid supply manifold  66  within the stationary variable unit port plate  14 . The charging fluid supply manifold  66  distributes charging fluid to the variable unit block and piston set  18  by way of a variable unit charging fluid port  68  in the stationary variable unit port plate  14 . The charging fluid supply manifold  66  also distributes charging fluid to the rotating fixed unit port plate  20  by way of a charging fluid annulus  70  along a circumferential interface between the cylindrical end  54  of the rotating fixed unit port plate  20  and the cylindrical orifice  56  in the stationary variable unit port plate  14  that comprises the interface journal bearing  52 . The fixed unit block and piston set  18  receives the charging fluid from the rotating fixed unit port plate  20  by way of a fixed unit charging fluid passage  72  and fixed unit charging fluid port  74  within the rotating fixed unit port plate  20  that couple to the charging fluid annulus  70 . The supply of charging fluid compensates for fluid that is lost due to leakage caused by charge and working pressure. 
   High pressure working fluid communicates between the variable unit block and piston set  12  and the fixed unit block and piston set  18  by way of a variable unit working fluid port  76  and a variable unit working fluid passage  78  within the stationary variable unit port plate  14 , a working fluid annulus  80  along the circumferential interface between the cylindrical end  54  of the rotating fixed unit port plate  20  and the  6 stationary variable unit port plate  14  that comprises the interface journal bearing  52 , a fixed unit working fluid passage  82  and a fixed unit working fluid port  84  within the rotating fixed unit port plate  20  that communicates with the working fluid annulus  80 . 
     FIG. 3  is a cut-away side view of the hydraulic differential  2  shown in  FIG. 2  along a section A-A of the stationary variable unit port plate  14  and the rotating fixed unit port plate  20  through the charging fluid annulus  44 .  FIG. 4  is a cut-away side view of the hydraulic differential  2  shown in  FIG. 2  along a section B-B of the stationary variable unit port plate  14  and the rotating fixed unit port plate  20  through the working fluid annulus  80 .  FIG. 5  is a cut-away side view of the hydraulic differential  2  shown in  FIG. 2  along a section C-C of the stationary variable unit port plate  14  through the charging fluid manifold  66  and the variable unit working fluid transfer tube  78 . 
   In operation, the variable speed power source rotates the input gear  28  to rotate the input shaft  8 , which in turn rotates the variable unit block and piston set  12 , the rotating fixed unit port plate  20  and the fixed unit wobbler  22 . The rotational speeds of the input shaft  8 , the variable unit block and piston set  12 , the rotating fixed unit port plate  20  and the fixed unit wobbler  22  are therefore proportional to the rotational speed of the variable speed power source. 
   A control system (not shown) senses any deviation of these rotational speeds from a desired set point and changes the angle of the variable unit  16  wobbler to change the displacement of the variable block and piston set  14 . When the control system senses no deviation of the rotational speeds from the set point, it adjusts the position of the variable unit wobbler  16  to be perpendicular to the centreline of the input shaft  8 , or zero degrees. When the position of the variable unit wobbler  16  is at zero degrees, there is no fluid flow between the variable block and piston set  12  and the fixed block and piston set  18 . In this case, the fixed block and piston set  18  hydraulically locks to the variable block and piston set  12  so that the rotational speed of the fixed block and piston set  18  is the same as the variable block and piston set, a condition of “straight through” speed. The rotational speed of the output shaft  10  is then the same as the rotational speed of the input shaft  8  and the rotational speed of a constant speed load coupled to the output gear  44  is proportional to their rotational speed. 
   If the control system senses that the rotational speeds of the variable speed power source and the input shaft  8  deviate above the set point, the control system adjusts the position of the variable unit wobbler  16  at an angle from normal to the centreline of the input shaft  8  as shown in  FIG. 1 . The fixed unit block and piston set  18  begins to rotate relative to the variable unit block and piston set  12  because fluid starts to flow between them. In this instance, the fixed unit block and piston set  18  acts as a pump and the variable unit block and piston set  12  acts as a motor. The direction of rotation of the fixed unit block and piston set  18  is opposite that of the variable unit block and piston set  12  so that the rotational speed of the output shaft  10  is that of the variable unit block and piston set  12  minus that of the fixed unit block and piston set  18 . 
   Likewise, if the control system senses that the rotational speed of the rotational speeds of the variable speed power source and the input shaft  8  deviate below the set point, the control system adjusts the position of the variable unit wobbler  16  at an angle from normal to the centreline of the input shaft  8  opposite to that shown in  FIG. 1 . The fixed unit block and piston set  18  once again begins to rotate relative to the variable unit block and piston set  12  because fluid starts to flow between them. In this instance, the variable unit block and piston set  12  acts as a pump and the fixed unit block and piston set  18  acts as a motor. The direction of rotation of the fixed unit block and piston set  18  is the same as that of the variable unit block and piston set  12  so that the rotational speed of the output shaft  10  is that of the variable unit block and piston set  12  plus that of the fixed unit block and piston set  18 . 
   Because the charging fluid and the working fluid passes between the stationary variable unit port plate  12  and the rotating fixed unit port plate  20  flows axially through the circumferential interface between the cylindrical end  54  of the rotating fixed unit port plate  20  and the cylindrical orifice  56  in the stationary variable unit port plate  12  that comprises the interface journal bearing  52 , most of the resulting hydraulic thrust force exerts itself on the interface radially instead of axially, thereby containing all the hydraulic thrust forces so that the hydraulic differential  2  does not require large bearings. This reduces size, weight and cost of the hydraulic differential  2 . 
   The described embodiments of the invention are only some illustrative implementations of the invention wherein changes and substitutions of the various parts and arrangement thereof are within the scope of the invention as set forth in the attached claims.