Abstract:
An axial piston hydraulic device. The device has a housing that uses a servo piston in order to control the displacement of a swashplate within the device. By using a servo member disposed within a cavity of an end cap on the housing the axial piston hydraulic device is able to provide three operating positions depending on the angle of the swashplate.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to swashplate axial piston hydraulic devices. More specifically, this invention relates to an apparatus and method that provides three operating displacements in a swashplate type axial piston unit. 
   Present swashplate type axial piston devices comprise a housing having a cylinder block with a plurality of reciprocating pistons therein that are acted upon by a swashplate. The swashplate is connected to a servo piston that is acted upon by a servo spring. Typically, the hydraulic device is a two positioned device in which the swashplate angle is controlled by the servo piston. The maximum angle (displacement) is defined by the point at which the servo piston contacts an end cap of the housing. While the minimum angle is defined by the point at which the servo piston contacts the housing. Thus, the control input is hydraulic pressure and the total movement of the servo piston from minimum to maximum displacement is a predetermined distance. In this embodiment there is no method to control the displacement of the device between maximum or minimum. 
   With hydraulic technology advancing higher speeds of hydraulic units is being achieved. With greater speeds greater torque loses are experienced as a result of the increase in speed range. Additionally, displacement accuracy of the swashplate has also been diminished as higher speeds have been accomplished causing mistracking problems associated with machines such as crawlers and skidsteer loaders. 
   Thus, a principal object of the present invention is to provide an improved swashplate type axial piston unit that provides for improved control of the displacement of the device. 
   Yet another object of the present invention is to provide for multiple functionality of a swashplate type axial piston device. 
   These and other objects, features, or advantages of the present invention will become apparent from the specification and claims. 
   BRIEF SUMMARY OF THE INVENTION 
   An axial piston hydraulic device having a housing with a cylinder block having reciprocating pistons that are acted upon by a swashplate disposed therein. A servo piston is connected to the swashplate adjacent a first end and extends to a second end. Secured to the housing is an end cap having a cavity disposed therein. Disposed within the cavity of the end cap is a servo member that receives the servo piston to provide a plurality of operating conditions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of an axial piston hydraulic device; 
       FIG. 2  is a sectional view of a servo piston in an operating condition; 
       FIG. 3  is a sectional view of a servo piston in an operating condition; 
       FIG. 4  is a sectional view of a servo piston in an operating condition; 
       FIG. 5  is a sectional view of a servo piston in an operating condition; 
       FIG. 6  is a sectional view of a servo piston in an operating condition. 
       FIG. 7  is a sectional view of a servo piston in an operating condition; 
       FIG. 8  is a sectional view of a servo piston in an operating condition; and 
       FIG. 9  is a section view of a servo piston in an operating condition. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1-3  show a first embodiment of an axial piston hydraulic device  10  having a housing  12  that is connected to an end cap  14 . In this embodiment the end cap  14  is shown as detachably secured to the housing  12 , however, in an alternative embodiment the end cap  14  and housing  12  could be of one-piece construction or the like. 
   Within housing  12  is a cylinder block  16  having an input shaft  18  disposed therethrough and having a plurality of reciprocating pistons  20  therein. The reciprocating pistons  20  are acted upon by a swashplate  22  that is connected to a servo piston  24  at a first end  26  of the servo piston  24 . The housing  12  provides a stop  27  for the first end  26  of the servo piston  24 . 
   The servo piston  24  extends from its first end  26  to a second end  28  that terminates in a servo piston head  30 . Surrounding the servo piston  24  is a servo piston spring  32  that biases the servo piston head towards the end cap  14 . 
   End cap  14  has a cavity  34  therein that has an end wall  36  and a sidewall  38  that extends into the housing  12  and terminates at a seat  40  formed within the housing  12 . Additionally disposed through the end cap  14  is a first port  42  that communicates with cavity  34  to pressurize cavity  34 . Additionally, a second port  44  also can provide pressure. 
   Disposed within the cavity  34  of the end cap  14  is a servo member  46 . In the embodiment of  FIGS. 1-3  the servo member  46  is a servo can having a cylindrical shape with an end wall  48  that extends into a sidewall  50  to form a cavity  52 . In a first embodiment sealing rings  54  are disposed around the servo member  46  and contact the end cap  14  within the cavity  34  of the end cap  14 . Specifically, the servo member  46  is moveable from between the end wall  36  of cavity  34  of the end cap  14  to the seat  40  of housing  12  wherein the sidewall  50  of the servo can  46  contacts the seat  40  of housing  12 . Additionally, within sidewall  50  is a servo fill port  56  that allows fluid communication between the cavity  52  of the servo member  46  and the cavity  34  of the end cap  14 . 
   By use of the servo member  46  the axial piston hydraulic device  10  provides a plurality of operating conditions. Specifically, as shown in  FIG. 1  a first operating condition is provided when the axial piston hydraulic device  10  is in a maximum angle condition. During a maximum angle condition the servo piston  24  and servo member  46  are biased against the end cap  14  by the servo spring  32 . 
   When in the first operating condition, as seen in  FIG. 1 , and the axial piston hydraulic device  10  is commanded to a “mid stroke” (between maximum and minimum angle) condition first port  42  is pressurized. As the first port  42  is pressurized, force builds between the end wall  48  of the servo member  46  and the end wall  36  of the end cap  14 . Once a first threshold pressure is reached servo piston  24  and servo member  46  overcome the force of the spring  32  and move toward the swashplate  22  until the servo member  46  engages the housing  12  at seat  40 . Thus, the servo member  46  moves a first distance X in response to the pressurization of the first port  42 . At this time the axial piston hydraulic device  10  is considered in a second operating condition at a point between the maximum and minimum angle for the axial piston hydraulic device and is shown in  FIG. 2 . 
   To command minimum angle condition, the second port  44  of the end cap  14  is pressurized. This generates a force within the cavity  52  of the servo member  46  against the head  30  of the servo piston  24 . When a second threshold pressure is reached the servo piston  24  moves until the servo piston  24  comes into contact with housing stop  27 . ( FIG. 3 ). The distance moved between the end wall  48  and piston head  30  is a second distance Y. At this point in time the axial piston hydraulic device  10  is considered in a third operating condition. Therefore, when the first distance X is less than the second distance Y the axial piston hydraulic device  10  operates in three distinct displacements or angles. 
   A second embodiment of the hydraulic device  10  is shown in  FIGS. 4-6 . In  FIGS. 4-6  the second port  44  is eliminated and the servo member  46  is represented by a second piston instead of a servo can. Additionally, in this embodiment the housing  12  adjacent the end cap  14  creates a second seat  58  and the head of the first servo piston  24  has an axial flange  60  that provides a servo seating surface  62  for the servo member  46 . Specifically, in an embodiment where the servo member  46  is a second piston, unlike the servo can, end wall  48  is eliminated thus allowing direct communication between the pressurized fluid port  42  and the first servo piston  24 . 
   In operation, in a first operating condition there is nominal pressure being applied through the first port  42  such that the servo spring  32  biases the first servo piston  24  and servo member  46  against the end wall  36  of the cavity  34  of the end cap  14 . Because of the flange  60  engaging the servo member  46  both the head  30  of the first servo piston  24  and the servo member  46  engage end wall  36 . Once a first threshold pressure arises via pressurized fluid flowing through the first port  42  the spring force from the servo spring is overcome and the first servo piston  24  and servo member  46  move toward the swashplate  22  until the servo member  46  engages the second seat  58 . At the time the servo member  46  engages the second seat  58  the hydraulic unit  10  is in a second operating condition. Then, pressure continues to build against the head  30  of the first servo piston  24  until a second threshold pressure is reached. At this time the first servo piston moves toward the swashplate  22  while the servo member  46  remains at rest against the second seat  58 . The first servo piston  24  continues to move toward swashplate  22  until engaging the first seat  40 . Upon engaging the first seat  40  the hydraulic unit  10  is in a third operating condition. 
   In the embodiment of  FIGS. 4-6  each operating condition represents a different swashplate angle. Specifically, when there is no pressure or nominal pressure a first speed is presented wherein the swashplate is at a maximum angle condition. In the second condition when a first threshold pressure is met and the servo member  46  engages the second seat  58  a second speed is accomplished at a swashplate displacement between the swashplate minimum and maximum. At a minimum angle condition wherein the second threshold pressure has been reached a third speed is provided. Therefore, three separate and unique operating conditions are present. 
   Additionally, in this embodiment the first and second threshold pressures may be adjusted depending upon the surface areas of the first and second servo pistons. Similarly, the port  42  may be connected to an external pressure source such as a proportional pressure reducing cartridge or 3-position valve referencing three different pressure sources to provide the needed pressure within the unit  10 . 
     FIGS. 7-9  show a third embodiment of an axial piston hydraulic device  10 . In this embodiment the servo member  46  is an annular ring disposed within the cavity  34  of end cap  14 . In this embodiment the head  30  of servo piston  24  has an annular flange  64  that extends from the head  30  toward the first end  26  of the servo piston  24 . Additionally, in this embodiment the end cap  14  has an end cap seat  66  upon which the servo member  46  is biased against by an intermediate position spring  68 . In an embodiment as seen in  FIG. 9  the servo member  46  may further comprise an annular flange  70  extending from the annular ring  66  for engagement with a stop  58  of housing  12 . The annular flange  64  of head  30  of servo piston  24  and a servo member  46  surround the servo spring  32 . 
   In operation in a first condition as best shown in  FIG. 9  the head  30  of servo piston  24  is biased against the end wall  36  of cavity  34  and end cap  14 . Similarly, servo member  46  is biased against the end cap seat  66  with the intermediate position spring  68 . As shown a first distance X exists between the annular flange  64  of servo piston  24  and the servo member  46 . 
   When commanded to move from the first condition as shown in  FIG. 9  the axial piston hydraulic device  10  provides pressure within fluid port  42  that overcomes servo piston spring  32  to move the piston head  30  away from the end wall  36  of cavity  34 . When the piston head  30  moves the first distance X the servo member  46  receives and is engaged by the annular flange  64  of the piston head  30  as best shown in  FIG. 7 . At this time the pressure from port  42  is not enough to overcome the combination of the spring biasing force of servo spring  32  and the spring biasing force of intermediate position spring  68 . As a result the movement of the servo piston  24  and consequently swashplate  22  is stopped at an angle between the maximum angle and minimum angle of the swashplate  22 . At this point in time the axial piston hydraulic device is considered in a second condition. 
   When an axial piston hydraulic device  10  needs to be placed in a third condition additional pressure is provided through pressure port  42  into cavity  34  until the biasing force from servo spring  32  and intermediate position spring  68  is overcome. At this time the piston head  30  and servo member  46  begin to move toward the first end  26  of servo piston  24  such that the servo member  46  disengages from the end cap seat  66  and the piston head  30  moves a second distance Y from the end wall  36  of cavity  34 . The piston head  30  and servo member  46  continue to move until either the first end  26  of servo piston  24  engages the stop  27  of housing  12  (see  FIG. 1 ) or the annular flange  70  of servo member  46  engages the second stop  58  of housing  12 . (See  FIG. 9 ). In each embodiment when movement of the servo piston head  30  ceases at a position furthest away from end wall  36  of cavity  34  the axial piston hydraulic device is considered in a third condition. The displacement representing the total movement of the servo piston from minimum to maximum displacement from the end wall  36  of cavity  34  is defined as a second distance Y shown in the figures. 
   While the servo member  46  has been described as an annular ring in one embodiment, the servo member  46  can be any intermediate stop. Specifically, the intermediate stop is preloaded by the intermediate position spring  68  against the end cap  14  with a known preload. Thus, under a first pressure, sufficient force builds between the servo piston  24  and end cap  12  to move the servo piston  24  until the servo piston seats against this intermediate stop. While the force generated by the first pressure is sufficient to overcome the preload provided by servo spring  32 , the first pressure is not sufficient to overcome the preload applied by the intermediate position spring  68  and thus the servo piston head seats against the intermediate stop. 
   Once the shift pressure has been increased to a second greater pressure there is sufficient force to overcome the intermediate position spring  68  biasing force and the servo piston  24  moves in reaction to this force until contacting a stop  27  or  58  of the motor housing  12 . Consequently, another embodiment is provided wherein a servo member  46  provides for three position motor functionality in three operating conditions. 
   Thus, disclosed is an axial piston hydraulic device  10  that uses a servo member  46  to provide a plurality of operating conditions. Specifically, in both a first embodiment when a servo can is used, in a second embodiment when a second servo piston is used and in a third embodiment when an intermediate servo stop is used three operating conditions are present. Therefore, the moveable servo member  46  provides a design to achieve three speed functionality for a swashplate type axial piston unit. By having 3 speeds torque loss is offset by minimizing the high torque travel speeds. Further, with increased displacement accuracy of the swashplate by adding a third position mistracking problems associated with machines such as crawlers and skidsteer loaders is avoided. Consequently, at the very least, all of the stated objectives have been met. 
   It will be appreciated by those skilled in the art that other various modifications could be made to the device without the parting from the spirit in scope of this invention. All such modifications and changes fall within the scope of the claims and are intended to be covered thereby.