Abstract:
A swash plate type hydraulic pump and a swash plate type hydraulic motor in formed in combination to reduce the amount of oil leakage and to increase efficiency in a hydrostatic continuously variable transmission. A hydrostatic continuously variable transmission including a swash plate type hydraulic pump, a swash plate type hydraulic motor, and a hydraulic closed circuit disposed between the hydraulic pump and the hydraulic motor. The hydraulic pump is integrally combined with the hydraulic motor by an output shaft press-fitted in a cylinder block B in which pump cylinder bores and motor cylinder bores are formed in parallel to the output shaft. The diameters of back portions of the pump cylinder bore and the motor cylinder bore are larger relative to the diameters of entry portions to the pump cylinder bore and the motor cylinder bore. Wherein a press-fitting portion is not provided at the contact portions inside the entry portions with the output shaft.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority of Japanese Patent Application No. 2000-232953 filed on Aug. 1, 2000, the entire contents thereof are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement in a hydrostatic continuously variable transmission including a combination of a swash plate type hydraulic pump and a swash plate type hydraulic motor. 
     2. Description of Background Art 
     A hydrostatic continuously variable transmission including a swash plate type hydraulic pump, a swash plate type hydraulic motor, and a hydraulic closed circuit disposed between the hydraulic pump and the hydraulic motor, wherein the hydraulic pump is integrally combined with the hydraulic motor by an output shaft press-fitted in a cylinder block in which a pump cylinder bore and a motor cylinder bore are made in parallel to the output shaft, is disclosed, for example, in Japanese Unexamined Patent Publication No. 303525/1997. In this kind of conventional hydrostatic continuously variable transmission, the cylinder bore (piston chamber) is deformed at the directly outside portion where the output shaft is press-fitted into the cylinder block by the effect of press-fitting the output shaft to make it impossible to keep its circular cross section As a result, this inevitably enlarges the inner diameter of the cylinder bore and hence increases the gap between a piston and the inner wall of the cylinder bore, which presents a problem of increasing oil leakage and reducing efficiency. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     In order to solve the above-mentioned problem presented by the conventional hydrostatic continuously variable transmission, the present invention proposes a hydrostatic continuously variable transmission including a hydraulic pump, a hydraulic motor, and a hydraulic closed circuit disposed between the hydraulic pump and the hydraulic motor, wherein the hydraulic pump is integrally combined with the hydraulic motor by an output shaft press-fitted in a cylinder block in which pump cylinder bores and motor cylinder bores are made in parallel to the output shaft, and characterized in that the diameters of the back portions of the pump cylinder bore and the motor cylinder bore are larger than the diameters of their entry portions and that a press-fitting portion is not provided at the contact portions with the output shaft inside the entry portions thereof. 
     Since the present invention is constituted as described above and the diameters of the back portions of the pump cylinder bore and the motor cylinder bore are made larger than the diameters of their entry portions, even if the cylinder bore is deformed by press-fitting the output shaft, the interference between the cylinder bore and the piston can therefore be prevented. 
     Since a press-fitting portion is not made at the contact portions with the output shaft inside the entry portions of the cylinder bores, the entry portions of the cylinder bores are not deformed. Therefore, the inner diameters of the entry portions can be determined so that the cylinder bores can maintain suitable clearances with respect to a pump piston and a motor piston. As a result, this can prevent an excessive leakage of oil and a decrease in efficiency. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a longitudinal cross-sectional view to show one preferred embodiment of a hydrostatic continuously variable transmission in accordance with the present invention; and 
     FIG. 2 is an enlarged view to show the vicinity of a cylinder block in FIG.  1  and shows a state in which a piston is removed from a cylinder bore. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in FIG. 1, the left-hand portion shows a fixed displacement swash plate type hydraulic pump P and the right-hand portion shows a variable displacement swash plate type hydraulic motor M. The hydraulic pump P and the hydraulic motor M constitute a continuously variable transmission T. 
     The hydraulic pump P includes an input cylindrical shaft  6  rotatably supported by a casing  1  via a bearing  2 . A pump cylinder  7  is relatively rotatably supported by the input cylindrical shaft  6  via a bearing  3 . A plurality of an odd number of pump pistons  9 , each of which is slidably fitted in each of a plurality of an odd number of cylindrical bores  8  formed in the pump cylinder  7 , are annularly arranged around its rotational axis A. A pump dimple plate  10  includes a front surface for contacting with the outside ends of the pump pistons  9 . A part of the input cylindrical shaft  6  forms a pump swash plate  6   a  and supports the pump dimple plate  10  via a bearing  4  and the like such that the pump dimple plate  10  keeps its position tilted at a predetermined angle with respect to the axis A of the pump cylinder  7 . 
     An input gear connected to a crank shaft, not shown, is fixed to the right-hand end of the input cylindrical shaft  6 . 
     When the input cylindrical shaft  6  rotates, the pump swash plate  6   a  reciprocates the respective pump pistons  9  via the pump dimple plate  10  to make them repeat suction strokes and discharge strokes. 
     On the other hand, the hydraulic motor M includes a motor cylinder  17  rotatably supported by a casing  11  and arranged coaxially with the pump cylinder  7  at the right-hand side thereof via a bearing  12 . A plurality of an odd number of motor pistons  19 , each of which is slidably fitted in each of a plurality of an odd number of cylinder bores  18  formed in the motor cylinder  17 , are annularly arranged around a rotational axis A. A motor dimple plate  20  includes a front surface for contacting with the outside ends of the motor pistons  19 . A motor swash plate  21  is provided for supporting the motor dimple plate  20  via a bearing  14  and the like. 
     A part of the casing  11  forms a motor swash plate anchor  11   a  and supports the back surface of the motor swash plate  21 . The back surface of the motor swash plate  21  and the front surface of the motor swash plate anchor  11  a form a spherical surface and are in contact with each other and the tilting angle of the motor swash plate  21  and the motor dimple plate  20  can be set at an arbitrary angle by means of a screw drive mechanism  22 . When the motor cylinder  17  rotates in a state where the motor swash plate  21  is tilted, it reciprocates the motor pistons  19  to make them repeat expansion strokes and contraction strokes. 
     The pump cylinders  7  and the motor cylinders  17  are integrally formed to constitute a cylinder block B and an output shaft  27  is press-fitted through the center portion of the cylinder block B. A spline  28  is made between the cylinder block B and the output shaft  27  to connect them in the rotational direction. In this case, portions designated by reference characters  29 ,  29  at the inner sides of the back portions  8   a ,  18   a  of the cylinder bores  8 ,  18  become press-fitting portions and portions designated by reference characters  30 ,  30  at the inner sides of the entry portions  8   b ,  18   b  of the cylinder bores  8 ,  18  become loose portions (see FIG.  2 ). 
     Further, the output shaft  27  is rotatably supported at the left end by the casing  1  via a bearing  31  and is relatively rotated with respect to the input cylindrical shaft  6  via a bearing  32  disposed between them and is rotatably supported at a portion near the right end by the casing  11  via a bearing  33 . Further, a spline  34  is made at the right end to connect the output shaft  27  to a driving shaft of a vehicle not shown. 
     As shown in FIGS. 1 and 2, between the pump cylinder  7  and the motor cylinder  17 , an annular inside oil passage (low pressure oil passage)  41  is formed between the inner peripheral surface of the cylinder block B and the outer peripheral surface of the press-fitted output shaft  27 , and an annular outside oil passage (high pressure oil passage)  42  is formed between the outer peripheral surface of the cylinder block B and a sleeve  40  fitted over and brazed to the outer peripheral surface of the cylinder block B. Further, a first group of valve bores  43  and a second group of valve bores  44  are made radially through the annular wall between both the oil passages  41  and  42  and the sleeve  40  such that they abut on the group of cylinder bores  8  and the group of cylinder bores  18 . The respective first valve bores  43  communicate with the respective abutting cylinder bores  8  and the respective second valve bores  44  communicate with the respective abutting cylinder bores  18 . A spool type first distributor valve  45  is fitted in each of the first valve bores  43  and a spool type second distributor valve  46  is fitted in each of the second valve bores  44 . 
     The outer periphery of the first distributor valve  45  is connected to the first slip ring  47  fitted in the input cylindrical shaft  6  by means of the first expansion ring  49  concentric with it such that it is always in contact with the first slip ring  47 , and the outer periphery of the second distributor valve  46  is connected to the second slip ring  48  fitted in the casing  11  by means of the second expansion ring  50  such that it is always in contact with the second slip ring  48 . The axes of the first slip ring  47  and the second slip ring  48  are eccentric in the direction crossing the paper with respect to the rotational axis A of the cylinder block B. The amount and direction of eccentricity of the first slip ring  47  are not identical to the amount and direction of eccentricity of the second slip ring  48 . 
     When a relative rotation is produced between the input cylindrical shaft  6  of the hydraulic pump P and the pump cylinder  7  thereof, the respective first distributor valves  45  are reciprocated in the respective first valve bores  43 . In the discharge region of the hydraulic pump P, the first distributor valve  45  moves toward the inner end side of the first valve bore  43  to make the corresponding cylinder bore  8  communicate with the outside oil passage  42  and to shut the communication between the cylinder bore  8  and the inside oil passage  41 , whereby the working oil is pumped from the cylinder bore  8  to the outside oil passage  42  by the pump piston  9  in the discharge stroke. Further, in the suction region, the first distributor valve  45  moves toward the outer end side of the first valve bore  43  to make the corresponding cylinder bore  8  communicate with the inside oil passage  41  and to shut the communication between the cylinder bore  8  and the outside oil passage  42 , whereby the working oil is drawn from the inside oil passage  41  to the cylinder bore  8  by the pump piston  9  in the suction stroke. 
     On the other hand, when the motor cylinder  17  of the hydraulic motor M rotates, the respective distributor valves  46  are reciprocated in the respective valve bores  44 . In the expansion region of the hydraulic motor M, the second distributor valve  46  moves to the inner end side of the second valve bore  44  to make the corresponding cylinder bore  18  communicate with the outside oil passage  42  and to shut the communication between the cylinder bore  18  and the inside oil passage  41 , whereby the high-pressure working oil is supplied from the outside oil passage  42  to the cylinder bore  18  of the motor piston  19  in the expansion stroke. Further, in the contraction region, the second distributor valve  46  moves toward the outer end side of the second valve bore  44  to make the corresponding cylinder bore  18  communicate with the inside oil passage  41  and to shut the communication between the cylinder bore  18  and the outside oil passage  42 , whereby the working oil is returned from the cylinder bore  18  of the motor piston  19  in the contraction stroke to the inside oil passage  41 . 
     In this way, the cylinder block B is rotated by the sum of a reactive torque applied by the pump swash plate  6   a  via the piston pump  9  in the discharge stroke and a reactive torque applied by the motor swash plate  21  via the motor piston  19  in the expansion stroke and its rotational torque is transmitted to the output shaft  27 . 
     By changing the tilting angle of the motor swash plate  21  and the motor dimple plate  20  via the screw drive mechanism  22 , a transmission gear ratio can be continuously variably controlled. 
     In the present preferred embodiment, as shown in FIG. 2, the diameters of the back portions  8   a ,  18   a  of the pump cylinder bore  8  and the motor cylinder bore  18  are larger than the diameters of the entry portions  8   b ,  18   b  by about 0.5 mm to 1.0 mm. 
     Since the back portions  8   a ,  18   a  are positioned directly adjacent to the portions  29 ,  29  where the output shaft  27  is press-fitted in the cylinder block B, as described above, the cylinder bores  8 ,  18  are deformed by the effect of the press-fitting of the output shaft  27  into the cylinder block B and the cylinder bores  8 ,  18  can-not keep their circular cross sections. Therefore, conventionally, the inner diameters of the cylinder bores  8 ,  18  are made larger, that is, the gap between the PUMP piston  9  and the cylinder bore  8  and the gap between the motor piston  19  and the cylinder bore  18  are enlarged, but the present preferred embodiment is intended to prevent interference between tie cylinder bore  8  and the pump piston  9  and interference between the cylinder bore  18  and the motor piston  19  by enlarging only the diameters of the back portions  8   a  and  18   a  of the cylinder bores  8  and  18   a.    
     Since the entry portions  8   b ,  18   b  of the cylinder bores  8 ,  18  are positioned outside the portions  30 ,  30  where the output shaft  27  is loosely inserted into the cylinder block B, the cylinder bores  8 ,  18  are not deformed there and hence their inner diameters can be determined so that the cylinder bores  8 ,  18  can keep suitable gaps with respect to the pump piston  9  and the motor piston  19 . If the gap between the cylinder bore  8  and the pump piston  9  and the gap between the cylinder bore  18  and the motor piston  19  are suitable, they can prevent an excessive leaking of working oil and a decrease in efficiency. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.