Patent Publication Number: US-8522822-B2

Title: Hydro-mechanical transmission and valve assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of applicant&#39;s application U.S. Ser. No. 11/488,260, filed 18 Jul. 2006 now U.S. Pat. No. 7,588,510 and titled HYDRO-MECHANICAL TRANSMISSION AND VALVE ASSEMBLY, which application is pending. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The invention relates generally to a hydro-mechanical transmission particularly suited for use in an agricultural tractor. More specifically, the invention relates to an infinitely-variable hydro-mechanical transmission and a valve assembly for controlling the hydro-mechanical transmission. 
     2. Related Technology 
     Hydro-mechanical transmissions are transmissions that combine a mechanical transmission with a hydrostatic unit. Mechanical transmissions generally have simple and reliable designs with relatively high performance efficiency (i.e. low power loss). However, purely mechanical transmissions are not infinitely variable and are thus restricted to particular speeds. In many applications, particularly agricultural tractors, it is desirable for the transmission of the vehicle to be infinitely adjustable. Hydrostatic transmissions typically are infinitely variable. However, hydrostatic transmissions are less efficient than mechanical transmissions. 
     In order to satisfy space limitations, reduce cost, increase efficiency and provide an infinitely variable speed, hydro-mechanical transmissions have been developed to combine the advantages of both types of transmissions. Hydro-mechanical transmissions are typically of a split power input type, where a hydrostatic unit and a mechanical transmission are both driven in parallel by the vehicle&#39;s engine. The hydrostatic output power is combined with the split mechanical power input from the engine to produce hydro-mechanical output power having a variable speed within each of a plurality of gear sets. More specifically, the mechanical transmission includes a plurality of gear sets that are selectively engaged and, within a given gear set, the hydrostatic output power is infinitely variable to vary the output speed. 
     The hydrostatic unit is a closed hydraulic system having a pump that converts rotational motion from the engine input shaft into fluid flow and a motor (hydraulically driven by the pump) that converts the fluid flow back into rotational motion that is delivered to the mechanical transmission. The pump is a variable-displacement pump so that the output speed of the motor is infinitely variable. More specifically, the variable-displacement pump includes an adjustable mechanism, such as a swash plate, that is controlled by a valve assembly. The valve assembly controls the angle of the swash plate to adjust the fluid flow within the hydrostatic unit thereby adjusting the hydrostatic output power. 
     Current hydrostatic units include microcontrollers and other electronic devices for controlling the angular position of the swash plate and to shift between gear ranges. However, these design features may increase the part cost, maintenance costs, and overall complexity of the hydrostatic unit. Furthermore, repair and maintenance of these hydrostatic units may require equipment, such as diagnostic machines, that is expensive and not readily available in some geographic areas. 
     Additionally, current hydrostatic units are controlled by several levers and controls, such as one or more controls for adjusting the operative gear set and one or more levers for controlling the speed and torque within the operative gear set. The multiple controls and levers may be difficult or inconvenient for use by the tractor operator. 
     It is therefore desirous to provide a hydro-mechanical transmission having a simple and reliable valve assembly for shifting between operative gear sets and for controlling output speed within each gear set and to provide simple and convenient controls for the operator of the vehicle. 
     SUMMARY 
     In overcoming the limitations and drawbacks of the prior art, the present invention provides a hydro-mechanical transmission including an input shaft, a hydrostatic unit driven by the input shaft, a mechanical transmission driven by the hydrostatic unit and/or the input shaft, and a valve assembly. The valve assembly includes a pressure control valve assembly to control the output speed of the hydrostatic unit and a shift control valve assembly coupled with the mechanical transmission for controlling first and second clutches and selectively engaging one of the gear sets of the mechanical transmission. 
     The hydro-mechanical transmission further includes a movable control lever coupled with the valve assembly such that the control lever, the shift control valve assembly, and at least one of the first and second clutches are connected with each other by physical working connections. The physical working connections include a mechanical connection between the control lever and the shift control valve assembly and a hydraulic connection between the shift control valve and the first and second clutches. 
     The hydro-mechanical transmission also preferably includes a reverse gear set selectively coupling the driven component with the output shaft when a reverse clutch is engaged. 
     In one aspect of the present invention, the hydro-mechanical transmission includes a valve assembly having first and second actuating valves for respectively engaging the first and second clutch, a shift control valve that is movable to actuate the first and second actuating valves and shift the operative gear set, and a pressure control valve assembly coupled with the hydrostatic unit to control an output speed of the hydrostatic unit. 
     In another aspect of the present invention, the hydro-mechanical transmission includes a shift control valve positioned within a shift cavity of the valve assembly housing and a pressure control valve assembly having a valve spool positioned coaxially with the shift control valve. The shift control valve is movable within the shift cavity to selectively engage one of the first and second gear sets and the pressure control valve assembly is coupled with the hydrostatic unit to control an output speed of the hydrostatic unit. 
     In yet another aspect of the present invention, a valve assembly for a hydro-mechanical transmission is provided. The valve assembly includes a first control valve movable to control first and second actuating valves and a second control valve movable to control the volume of a pressure control cavity. For example, the first control valve is able to translate within a valve support cavity and the second control valve is able to rotate within the valve support cavity. 
     The valve assembly preferably also includes a pressure control valve cooperating with a valve housing to define the volume of the pressure control cavity. The pressure control valve engages the second control valve such that rotation of the second control valve causes translation of the pressure control valve. More specifically, the second control valve preferably includes a base portion having a varying radius and engaging the pressure control valve so that rotation of the valve spool controls the position of the pressure control valve. 
     Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a hydro-mechanical transmission embodying the principles of the present invention, having a hydrostatic unit, a valve assembly, and a mechanical transmission; 
         FIG. 2  is a cross-sectional view of the valve assembly employed in  FIG. 1 ; and 
         FIG. 3  is a cross-sectional view, generally taken along line  3 - 3 , of the valve assembly seen in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, a hydro-mechanical transmission  10  for an agricultural vehicle, such as a tractor is schematically shown in  FIG. 1 . The hydro-mechanical transmission  10  generally includes an input shaft  12  driven by an engine  14 , a hydrostatic unit  16  (having a pump  18  and a motor  20 ) driven by the input shaft  12 , a mechanical transmission  13  driven by the input shaft  12  (via the engine  14 ) and the hydrostatic unit  16 , a valve assembly  15  controlling the output speed of the hydrostatic unit  16  and the operative gear set of the mechanical transmission  13 , and a control lever assembly  17  (accessible to the tractor operator) for controlling the valve assembly  15 . 
     The hydrostatic unit  16  is a closed hydraulic system including a variable displacement pump  18  driven by the input shaft  12  and a fixed displacement motor  20  driving the mechanical transmission  13 . More specifically, the pump  18  includes an input shaft  24  rotatably coupled with the engine input shaft  12  by a pair of gears  26 ,  27 . The pump  18  converts the rotational motion of the input shaft  24  into fluid flow between the pump  18  and the motor  20  via hydraulic lines  22 . The pump  18  is a variable-displacement pump having a variable coupling mechanism, such as a swash plate (not shown), that is controlled by the valve assembly  15 . Accordingly, the valve assembly  15  controls the angle of the swash plate to adjust the fluid flow within the hydrostatic lines  22 . The motor  20  is rotated by the fluid flow within the lines  22  so as to convert the fluid flow into rotational motion of a hydrostatic unit output shaft  28 . Therefore, the angle of the swash plate controls the rotational speed of the hydrostatic unit output shaft  28 . Those skilled in the art will recognize that either or both of the pump  18  and the motor  20  may be variable displacement components. 
     The hydrostatic unit output shaft  28  is coupled with the mechanical transmission  13  by a driving gear  29  connected to the output shaft  28  and a driven gear  30 , as will be discussed in more detail below. 
     As seen above, the mechanical transmission  13  is driven by both a hydrostatic power input element and a mechanical power input element in parallel with each other. The mechanical power input element causes the output of the mechanical transmission  13  to rotate at a set “base” speed, which depends solely on the operative gear set, while the hydrostatic power input element adjusts the base speed via the hydrostatic unit  16 . In this manner, the hydro-mechanical transmission  10  utilizes the efficient nature of the mechanical transmission  13  to achieve a base speed and utilizes the infinitely variable nature of the hydrostatic unit  13  to adjust the output speed as desired. 
     The mechanical transmission  13  includes a planetary system  32  having three planetary gear sets  34 ,  36 , and  38 . The planetary gear sets  34 ,  36 , and  38  have a common planet gear carrier  40  that carries the integral planet gears P 1  and P 2  of planetary gear sets  34  and  36 , respectively. The gear carrier  40  also carries the planet gears P 3  and P 4  of the reversing planetary gear set  38 . The planet gears P 1  and P 2  are integrally formed and thus rotate together. The planet gears P 2  mesh with a ring gear R 2 . 
     The ring gear R 2  is formed integrally with the driven gear  30 , in a position coaxial with the input shaft  12  of the engine  14 . As mentioned above, the driven gear  30  is rotated by the driving gear  29  of the hydrostatic unit  16  so that the ring gear R 2  serves as the hydrostatic power input element to the mechanical transmission  13 . 
     The transmission input shaft  14  also directly drives a sun gear S 1  of the first planetary gear set  34 . Therefore, the sun gear S 1  is the mechanical power input element to the mechanical transmission  13 . The sun gear S 1  meshes with the planet gear P 1 , that are integrally formed with planet gears P 2  as mentioned above. The planet gears P 2  mesh with a sun gear S 2  and both are part of the planetary gear set  36 . 
     Two clutches, a low range clutch CL and a high range clutch CH, selectively couple elements of the planetary system  32  to the mechanical transmission output shaft  46 . The shaft  46  is a sleeve shaft that surrounds the input shaft  12 , which extends through the entire mechanical transmission  13  to drive a power take off, not shown, and/or to drive other vehicle components, such as a hydraulic pump, in a known manner for an agricultural tractor. The low range clutch CL is engageable to couple the carrier  40  to the output shaft  46  for a low speed forward range. The high range clutch CH is engageable to couple the sun gear S 2  to the output shaft  46  for a high speed forward range. 
     The output shaft  46  is fixed to a sun gear S 3  of the reversing planetary gear set  38 . Ring gear R 3  is selectively grounded by a reverse clutch in the form of a reverse brake  48 . This stops the rotation of the ring gear R 3  and causes the sun gear S 3  to rotate in the reverse direction for a reverse speed range. When the reverse brake  48  is applied, both the low and high range clutches CL and CH are disengaged, whereby the sun gear S 3  drives the output shaft  46 . 
     The output shaft  46  of the mechanical transmission is integrally formed with a gear  50  that in turn meshes with a gear  52  on the offset shaft  54 . The offset shaft  54  is coupled to the differential drive shaft  56  of the tractor to couple the hydro-mechanical transmission  10  to a load. 
     The hydro-mechanical transmission  10  thus operates in three ranges, a reverse range, a low speed forward range and a high speed forward range. Each range uses a separate path through the mechanical transmission to the output shaft  46  resulting in unique gear ratios for each range. 
     The valve assembly  15  and the control lever assembly  17 , shown in  FIG. 1 , cooperate to serve two main functions: first, selectively actuating the low range clutch CL, the high range clutch CH, and the reverse brake  48  to shift between operative gear sets; and, second, adjusting the variable-displacement pump  18  to control the hydrostatic power input element to mechanical transmission  13  and control the speed of the tractor. 
     The control lever assembly  17 , the valve assembly  15 , and each of the respective clutches CL, CH, and reverse brake  48  are connected with each other by physical working connections  60 . As used herein, the term “physical working connections” refers to any type of connection between respective components that utilizes physical forces, such as a mechanical connection, a hydraulic connection, or a pneumatic connection. For example, the physical working connections  60  shown in the figures include a mechanical connection  62  between the control lever assembly  17  and the valve assembly  15  and a hydraulic connection  64  between the valve assembly and each of the respective clutches CL, CH, and the reverse brake  48 . 
     The control lever assembly  17  includes a control lever  66  that is movable along a track  68  and that is accessible to the tractor operator. The control lever  66  extends downward below the exposed surfaces of the tractor and is mechanically connected to the valve assembly  17  by a shift linkage  70  and a pressure linkage  72 , as will be discussed further below. 
     Generally, the valve assembly  15  is fluidly connected to the pump  18  by a pair of hydraulic lines  74  so as to define a closed hydraulic system for controlling the position of the swash plate and by another pair of hydraulic lines  75  so as to define a second closed hydraulic system for adjusting the operative gear set. Also, the valve assembly  15  is fluidly connected with each of the respective clutches CL, CH, and reverse brake  48  by a plurality of hydraulic paths  76 ,  78 ,  80 . Although the hydraulic paths  76 ,  78 ,  80  are illustrated as being outside of the valve assembly in  FIG. 1 , the paths  76 ,  78 ,  80  are actually defined by the housing of the valve assembly, as will be discussed further below. 
     Referring to  FIG. 2 , the valve assembly  15  is shown in more detail. Particularly, the valve assembly  15  has a housing  82  supporting a shift control valve assembly  84  for shifting the operative gear set and a pressure control valve assembly  86  for controlling the hydrostatic power input element to mechanical transmission  13 . The housing  82  also supports a low gear actuating valve  88  for engaging the low clutch CL, a high gear actuating valve  90  for engaging the high clutch CH, and a reverse gear actuating valve  92  for engaging the reverse brake  48 . Moreover, the housing defines: the low hydraulic path  76 , which extends between the shift control valve assembly  84  and the low gear actuating valve  88 ; the high hydraulic path  78 , which extends between the shift control valve assembly  84  and the high gear actuating valve  90 ; and the reverse hydraulic path  80 , which extends between the shift control valve assembly  84  and the reverse gear actuating valve  92 . Although the respective paths  76 ,  78 ,  80  are not depicted in  FIG. 2 , the paths are defined by the pathways formed within the housing  82 , each pathway having an appropriate size and shape for receiving a working fluid. 
     Generally, the shift control valve assembly  84  includes an inner valve  94  positioned within a shift cavity  96  of the housing  82 . The pressure control valve assembly  86  includes an outer valve  98  coaxially positioned around the inner valve  94  within the shift cavity and a pressure control valve  100  engaging a base portion  102  of the outer valve  98  and extending generally perpendicularly away therefrom. Each of the valves  94 ,  98 ,  100  is movable within the housing  82 . Namely, the inner valve  94  is movable with respect to the outer valve  98  along a central longitudinal axis  104  of the inner and outer valves  94 ,  98 ; the outer valve is rotatable about the longitudinal axis  104  within the shift cavity  96 ; and the pressure control valve  100  is movable with respect to the housing  82  along a transverse axis  106  that is generally perpendicular to longitudinal axis  104 . 
     The inner valve  94  is mechanically connected to the control lever  66  (seen in  FIG. 1  and shown in phantom in  FIG. 3 ) by the shift linkage  70 . The shift linkage  70  shown in the figures is a bore extending through a top portion of the inner valve  94  for receiving an engagement pin of the control lever  66 , but any other appropriate connection may be used. For example, one or more connection arms and other linkages may be utilized to mechanically link the control lever  66  to the inner valve  94 . The inner valve  94  is releasably secured in one of four positions by a pair of support pins  108  that extend through the outer valve  98  and engage a groove  110  formed in the surface of the inner valve  94 . The support pins  108  are both biased inward towards the inner valve  94  by respective springs  112  so that the inner valve  94  is held in place within the outer valve  98 , unless a sufficiently strong force acts on the inner valve  94  via the shift linkage  70 . 
     As mentioned above, the inner valve has four positions, each corresponding to an operative gear set  32 ,  34 ,  36 . For example, the position shown in  FIGS. 2 and 3  is a first position, where the support pins  108  are aligned with the top groove  110  and the inner valve  94  is in its lowermost position within the outer valve  98 , such that the reverse gear set  38  is engaged. As the inner valve  94  is moved upwards into each of the remaining three positions, the mechanical transmission  13  will progressively engage the low gear set  34 , the high gear set  36 , and a neutral position where none of the gear sets  34 ,  36 ,  38  are engaged. 
     As mentioned above, the housing  82  defines a plurality of hydraulic paths  76 ,  78 ,  80  extending between the shift cavity  96  and the respective actuating valves  88 ,  90 ,  92 . As the inner valve  94  moves between the four positions, each of the respective actuating valves  88 ,  90 ,  92  selectively fluidly connected with the shift cavity  96 , thereby selectively actuating one of the valves  88 ,  90 ,  92  and engaging one of the clutches CL, CH, and reverse brake  48 . More specifically, the inner valve  94  defines a bore  114  extending therethrough and the housing  82  defines a passageway  116  extending between the pump  18  and the shift cavity  96 . Furthermore, the respective hydraulic paths  76 ,  78 ,  80  each connect to the shift cavity  96  at different positions along the vertical longitudinal axis  104  so that the bore  114  is only aligned with one of the respective hydraulic paths  76 ,  78 ,  80  at a time. The outer valve  98  also includes bores (not shown) extending through the outer valve wall and grooves  118  aligned with each of the bores and extending annularly around the outer valve  98  so that fluid can flow through the outer valve  98 . The pump  18  is fluidly connected with the shift cavity  96  by hydraulic lines  75  so that the pump  18  and the shift control valve assembly  84  cooperate to define a closed hydraulic system. 
     During operation, as the inner valve  94  moves between the respective four positions, the respective hydraulic paths  76 ,  78 ,  80  will be selectively fluidly connected with the shift cavity  96  and the pump  18  via the bore  114  in the inner valve  94 . The valve assembly  15  also includes a purge conduit  122  ( FIG. 3 ) connected to the shift cavity  96  so that the working fluid is drained from a given hydraulic path and the respective actuating valve is disengaged when the inner valve  94  moves out of a particular position. Therefore, only one actuating valve  88 ,  90 ,  92  is actuated at a time. 
     As mentioned above, the pressure control valve assembly  86  includes the outer valve  98  coaxially positioned around the inner valve  94  and the pressure control valve  100  that engages the base portion  102  of the outer valve  98 . The outer valve  98  is connected to the control lever  66  (seen in  FIG. 1  and shown in phantom in  FIG. 3 ) by the shift linkage  72 . The shift linkage  72  shown in  FIG. 3  is a radial arm  124  extending away from the outer valve  98  and includes a bore extending through a portion of the arm for receiving an engagement pin of the control lever  66 . While specifically shown, any other appropriate connection may be used. For example, one or more connection arms and linkages may be utilized to mechanically link the control lever  66  to the outer valve  98 . 
     The outer valve  98  is free to rotate within the housing  82  when a sufficiently strong rotational force is applied to the radial arm  124 . As mentioned above, the base portion  102  of the outer valve  98  includes a varying radius  125 . Therefore, as the outer valve  98  is rotated, the pressure control valve spring  127  is caused to compress or relax. The spring force is countered by the pressure in the pressure control cavity  126  of the pressure control valve  100 . As the spring force is adjusted, the control pressure in the cavity  126  is adjusted and the working fluid flows towards or away from the pump  18  via the hydraulic lines  74 , thereby adjusting the angular position of the swash plate in the pump  18  and adjusting the hydrostatic power input element acting on the mechanical transmission  13 . 
     Referring back to  FIG. 1 , during operation of the tractor, the control lever  66  is movable along the track  68  to follow a generally step-shaped path. More specifically, the track  68  includes: a reverse portion  130  controlling the speed of the tractor when the reverse gear set  38  is engaged, a low gear portion  132  controlling the speed of the tractor when the low gear set  36  is engaged, a high gear portion  134  controlling the speed of the tractor when the high gear set  34  is engaged, a first shifting portion  136  for shifting between the reverse and the low gear sets, and a second shifting portion  138  for shifting between the low gear and the high gear sets. The reverse, low, and high gear portions  130 ,  132 ,  134  of the track  68  are each generally parallel with each other so that movement of the control lever  66  along a first axis  140  actuates the pressure linkage  72  and rotates the outer valve  98 , thereby adjusting the hydrostatic power input element acting on the mechanical transmission  13 . Similarly, the first and second shifting portions  136 ,  138  of the track are generally parallel with each other so that movement of the control lever  66  along a second axis  142  actuates the shift linkage  70  and vertically moves the inner valve  94 , thereby shifting the operative gear set  34 ,  36 ,  38 . Therefore, the operator is able to operate the tractor in a relatively simple, convenient manner. 
     It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention. For example, the hydrostatic pump and motor may be replaced with a variable speed friction drive that is controlled hydraulically.