Patent Publication Number: US-11396872-B2

Title: Axial piston-type hydraulic rotary machine

Description:
TECHNICAL FIELD 
     The present invention relates to an axial piston-type hydraulic rotary machine that is used as a hydraulic pump, a hydraulic motor in, for example, civil engineering machinery, construction machinery and other general machinery. 
     BACKGROUND ART 
     In general, a hydraulic rotary machine (for example, a fixed displacement type or variable displacement type axial piston-type hydraulic rotary machine) that is used as the hydraulic pump or the hydraulic motor in the construction machinery such as a hydraulic excavator and the general machinery is known. The axial piston-type hydraulic rotary machine of this kind according to conventional art is configured by including a casing, a rotational shaft that is rotatably provided in the casing, a cylinder block that is rotatably provided in the aforementioned casing so as to rotate together with the rotary shaft and in which a plurality of cylinder holes that are separated from one another in a circumferential direction and extend in an axial direction are formed and a plurality of pistons that are inserted and fitted into the respective cylinder holes in the cylinder block to be slidable and reciprocate in the respective cylinder holes with rotation of the cylinder block. 
     Here, the cylinder block that tapered chamfering is performed on the opening end (so-called entrance or inlet) side of each cylinder hole is known. That is, a tapered-state chamfered part is formed on the inlet side of each cylinder hole so as to restrain a piston that reciprocates in the cylinder hole from coming into friction contact with the inlet side of the cylinder hole with the aid of the aforementioned chamfered part and thereby sliding resistance of the both can be reduced (Patent Document 1). 
     According to another conventional art, the cylinder block that a base material of which is formed by using a cast, a steel material is known. A nitriding layer that is made by performing, for example, nitride-based heat treatment is formed on the front surface side of this base material. That is, the nitriding layer is formed on each cylinder hole in the cylinder block and its opening side end surface. Such a nitriding layer is configured by a diffusion layer that is formed on the front surface side of the base material and a compound layer that covers the front surface side of the diffusion layer and is formed as a layer that is harder than the diffusion layer (Patent Document 2). 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open No. 2008-106608 A 
     Patent Document 2: Japanese Patent Application Laid-Open No. 2012-7509 A 
     SUMMARY OF THE INVENTION 
     Incidentally, in the conventional art according to the above-mentioned Patent Document 2, honing is performed on each cylinder hole in the cylinder block thereby to remove a compound layer on a cylinder hole inner circumferential surface (that is, a piston sliding surface). However, there are cases where the high-hardness compound layer remains on the opening end (the inlet) side of the cylinder hole. Consequently, there is a problem that the piston that reciprocates in each cylinder hole is worn down and damaged by the compound layer that remains on the inlet side. 
     In addition, the conventional art according to the aforementioned Patent Document 1 forms the tapered-state chamfered part on the inlet side of each cylinder hole. It becomes possible to suppress friction contact of the piston that reciprocates in the cylinder hole with the inlet side of the cylinder hole with the aid of this chamfered part. However, this conventional art simply performs chamfering. 
     The present invention has been made in view of the above-described problems of the conventional art and an object of the present invention is to provide an axial piston-type hydraulic rotary machine configured to suppress wear and damage on a contact part between each cylinder hole of the cylinder block and the piston and thereby to make it possible to improve durability and life thereof. 
     In order to solve the above-described problems, the present invention is applied to an axial piston-type hydraulic rotary machine comprising: a tubular casing; a rotational shaft that is rotatably provided in the casing; a cylinder block that is provided in the casing so as to rotate together with the rotational shaft and has a plurality of cylinder holes that are separated from one another in a circumferential direction and extend in an axial direction; a plurality of pistons that are inserted and fitted into the respective cylinder holes in the cylinder block to be reciprocally movable; and a valve plate that is provided between the casing and the cylinder block and in which one pair of supply and exhaust ports that communicate with the respective cylinder holes are formed, wherein a cylinder inlet side tapered surface is formed on each of the cylinder holes in the cylinder block by performing cylinder inlet chamfering from an opening side end surface toward a piston sliding surface of the cylinder hole, and a nitriding layer on which nitride-based treatment is performed at least including the piston sliding surface, the opening side end surface of each of the cylinder holes and the cylinder inlet side tapered surface is formed on the cylinder block. 
     Then, the configuration adopted by the present invention is characterized in that: the piston sliding surface of each of the cylinder holes is formed as a compound layer-removed hole from which a compound layer that is located on the front surface side of the nitriding layer is removed and a compound layer-removed surface from which the compound layer that is located on the front surface side of the nitriding layer is removed is formed on apart where the compound layer-removed hole and the cylinder inlet side tapered surface of each of the cylinder holes intersect. 
     According to the present invention, an inner circumferential surface (the piston sliding surface) of each cylinder hole is formed as the compound layer-removed hole from which the compound layer is removed. Then, the compound layer-removed surface from which the compound layer that is located on the front surface side of the aforementioned nitriding layer is removed is formed on the part where the aforementioned compound layer-removed hole and the cylinder inlet side tapered surface intersect. Compound layer removal machining is performed in a piston sliding range over the inner circumferential surface (the piston sliding surface) and the inlet side of each cylinder hole in this way and thereby the wear when the piston slides can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a longitudinal sectional diagram showing a variable displacement-type inclined shaft-type hydraulic pump according to a first embodiment of the present invention. 
         FIG. 2  is an enlarged sectional diagram showing a piston and a cylinder hole in a cylinder block in  FIG. 1  in an enlarged state. 
         FIG. 3  is a sectional diagram of an inlet part showing a state where the cylinder hole in  FIG. 2  is formed as a compound layer-removed hole in the enlarged state. 
         FIG. 4  is an essential part sectional diagram showing the cylinder hole in a state where a nitriding layer is formed in the enlarged state. 
         FIG. 5  is an essential part sectional diagram showing a state where a compound layer-removed hole and a compound layer-removed surface are formed in and on the nitriding layer in  FIG. 4  in the enlarged state. 
         FIG. 6  is an essential part sectional diagram showing a compound layer-removed hole and a compound layer-removed surface that are formed in and on a cylinder block according to a second embodiment in the enlarged state. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     In the following, an axial piston-type hydraulic rotary machine according to embodiments of the present invention will be described in detail by giving a case of applying it to a variable displacement-type inclined shaft-type hydraulic pump by way of example while referring to the appended drawings. 
     Here,  FIG. 1  to  FIG. 5  show a first embodiment of the present invention. In  FIG. 1 , a hydraulic pump  1  that is configured by a variable displacement-type inclined shaft-type hydraulic rotary machine has a casing  2  that configures an outer shell thereof. This casing  2  is configured by a casing body  3  that exhibits a bent tubular shape and a later described head casing  4 . The hydraulic pump  1  supplies pressurized oil toward various kinds of hydraulic equipment (none of them are shown) that are connected on the downstream side of a hydraulic conduit while sucking hydraulic oil from a hydraulic oil tank. 
     The casing body  3  of the casing  2  is configured by a bearing part  3 A that is located on one side of an axial direction and is formed into an almost cylindrical shape and a cylinder block accommodating part  3 B that incliningly extends from the other end of the bearing part  3 A. The head casing  4  is attached to the other end of this cylinder block accommodating part  3 B. This head casing  4  is provided so as to close the axial-direction other side of the casing body  3 , that is, the cylinder block accommodating part  3 B from the other end side thereof. 
     The head casing  4  has a concave arc shape sliding contact surface  4 B on a one-side surface  4 A that is located on the casing body  3  side. This concave arc shape sliding contact surface  4 B is formed as a concave arc surface that is formed along a rocking radius when a valve plate  10  rocks with a later described center shaft  8  being set as a fulcrum. An opening  4 C for pin that communicates with a later described piston sliding bore  11 A is opened in the concave arc shape sliding contact surface  4 B. This opening  4 C for pin is an opening adapted to allow displacement of a rocking pin  11 C of a later described tilting mechanism  11  and extends along the piston sliding bore  11 A. The piston sliding bore  11 A in the tilting mechanism  11  is formed at a position located on the inner side of the concave arc shape sliding contact surface  4 B of the head casing  4 . Further, a suction flow passage and a delivery flow passage (none of them are shown) that extend from the concave arc shape sliding contact surface  4 B toward mutually opposite sides with the piston sliding bore  11 A being interposed therebetween are provided in the head casing  4 . 
     A rotational shaft  5  is provided in the bearing part  3 A of the casing body  3  to be rotatable having a rotational axis O 1 -O 1 . This rotational shaft  5  is rotatably supported to the bearing part  3 A via a bearing  6  and its one side that is the projection side is made into a spline part  5 A. On the other hand, a disc-shape drive disc  5 B is formed integrally with the rotational shaft  5 , being located on a leading end on the side of insertion into the casing body  3 , that is, on the axial-direction other end thereof. 
     A cylinder block  7  is rotatably provided in the casing  2  (that is, in the cylinder block accommodating part  3 B of the casing body  3 ). This cylinder block  7  is coupled to the drive disc  5 B via a center shaft  8 , each piston  9  and so forth that will be described later and rotates integrally with the rotational shaft  5 . Here, the cylinder block  7  is formed into a thick cylindrical shape and a center hole  7 A is provided in its center along a rotational axis O 2 -O 2 . In addition, a plurality (only one of them is shown in  FIG. 1 ) of later described cylinder holes  12  are formed in the cylinder block  7 , being located around the center hole  7 A. 
     Here, the cylinder block  7  is, nitride-based treatment is performed on a later described base material  14  that is formed by using, for example, a cast, an iron-based material such as a steel material and so forth as surface treatment. An end surface on the axial-direction one side of the cylinder block  7  is made into an opening side end surface  7 B of each cylinder hole  12  and each cylinder hole  12  is axially pierced in the cylinder block  7  with this opening side end surface  7 B serving as an inlet. The cylinder block  7  is, an end surface on the axial-direction other side that is the side of a later described valve plate  10  is made into a sliding contact end surface  7 C and this sliding contact end surface  7 C is formed into a concave spherical shape to be sliding-contactable with a switching surface  10 A of the valve plate  10 . 
     The center shaft  8  is fully inserted into the center hole  7 A in the cylinder block  7 . This center shaft  8  is adapted to support the cylinder block  7  between the drive disc  5 B of the rotational shaft  5  and the valve plate  10  in such a manner that it freely tilts. The center shaft  8  is coupled to a rotation center position of the drive disc  5 B of the rotational shaft  5  to be rockable on its one end side and is inserted into a shaft hole  10 C in the valve plate  10  on its other end side that projects from the sliding contact end surface  7 C. 
     The plurality of pistons  9  are inserted and fitted into the respective cylinder holes  12  in the cylinder block  7  to be reciprocally movable respectively. These pistons  9  are coupled to the drive disc  5 B of the rotational shaft  5  to be rockable on their one end sides that project from the cylinder holes  12 . The cylinder block  7  that tilts relative to the rotational shaft  5  rotates and thereby each piston  9  repeats reciprocation in the cylinder hole  12 . That is, each piston  9  sequentially repeats a suction stroke and a delivery stroke of hydraulic oil by sliding and displacing the cylinder hole  12 . Incidentally, surface treatment including nitriding that is almost the same as that on the cylinder block  7  or heat treatment other than nitride-based treatment is performed on the piston  9  for the purpose of increasing the surface hardness and thereby to make improvement of wear resistance of the piston  9  possible. 
     The valve plate  10  is provided between the head casing  4  and the cylinder block  7 . This valve plate  10  has a rectangular outer shape that falls within a width dimension (a lateral direction dimension that is vertical to a tilting direction) of the concave arc shape sliding contact surface  4 B. The valve plate  10  is disposed in the concave arc shape sliding contact surface  4 B of the head casing  4  to be tiltable. The convex spherical shape switching surface  10 A that comes into sliding contact with the sliding contact end surface  7 C of the cylinder block  7  in a surface contact state is provided on a one-side surface of the valve plate  10 . On the other hand, an other-side surface of the valve plate  10  that is located on the opposite side of the switching surface  10 A is made into a convex arc shape sliding contact surface  10 B that projects with an arc that corresponds to that of the concave arc shape sliding contact surface  4 B of the head casing  4  and comes into sliding contact with the concave arc shape sliding contact surface  4 B. 
     In addition, the shaft hole  10 C that is located at the center of the switching surface  10 A and is pierced through the valve plate  10  in its plate thickness direction (an axial direction) is provided in the valve plate  10 . The other end side of the center shaft  8  is inserted into this shaft hole  10 C. Further, one pair of supply and exhaust ports, that is, a suction port and a delivery port (none of them are shown) that communicates with each cylinder hole  12  in the cylinder block  7  are provided in the valve plate  10 . These ports are opened in the switching surface  10 A on their one sides and are opened in the convex arc shape sliding contact surface  10 B on their other sides. 
     The tilting mechanism  11  is provided in the head casing  4 . This tilting mechanism  11  is adapted to tilt the valve plate  10  together with the cylinder block  7 . The tilting mechanism  11  is configured by including a piston sliding bore  11 A that is located on the side that is more inward than the innermost part of the concave arc shape sliding contact surface  4 B and linearly extends along a tilting direction of the valve plate  10 , a servo piston  11 B that is inserted and fitted into the piston sliding bore  11 A to be slidable, a rocking pin  11 C that is provided on a length-direction intermediate part of the servo piston  11 B and projects and extends from the servo piston  11 B in a radial direction, and oil passage holes  11 D,  11 E that are provided on the both end sides of the aforementioned piston sliding bore  11 A. The aforementioned rocking pin  11 C is fully inserted into the opening  4 C for pin in the head casing  4  and a leading end thereof is inserted into the shaft hole  10 C in the valve plate  10 . 
     Here, pressurized oil (a tilting control pressure) is supplied into the piston sliding bore  11 A through the oil passage hole  11 D or the oil passage hole  11 E and thereby the servo piston  11 B moves along this piston sliding bore  11 A. When the servo piston  11 B moves in this way, it becomes possible to tilt the valve plate  10  together with the cylinder block  7  via the rocking pin  11 C. Thereby, the tilting mechanism  11  is able to adjust a tilt angle θ between the cylinder block  7  and the valve plate  10  relative to the rotational shaft  5  between a minimum tilt position and a maximum tilt position. 
     For example, five, seven or nine (in general, an odd number of) cylinder holes  12  are provided in the cylinder block  7 . These cylinder holes  12  are separated from one another at fixed intervals in a circumferential direction around the center hole  7 A and are formed so as to extend in the axial direction of the cylinder block  7 . Each cylinder hole  12  has a piston sliding surface  12 A along which the piston  9  is inserted and fitted thereinto to be slidable and a cylinder inlet side tapered surface  12 B that is located on the inlet side thereof as shown in  FIG. 2 . Each cylinder hole  12  has a center axis O 3 -O 3  as shown in  FIG. 2 . 
     The cylinder inlet side tapered surface  12 B of each cylinder hole  12  is formed by performing cylinder inlet chamfering from the opening side end surface  7 B of the cylinder block  7  toward an inner circumferential surface (that is, the piston sliding surface  12 A) of the cylinder hole  12 . The cylinder inlet side tapered surface  12 B is formed so as to expand with a taper angle β relative to the center axis O 3 -O 3  of the cylinder hole  12 . This taper angle β is set to an angle of, for example, 10 to 45 degrees. 
     A nitriding layer  13  is formed on the front surface side of the cylinder block  7  by performing nitride-based heat treatment thereon as shown in  FIG. 4 . This nitriding layer  13  is formed so as to entirely cover the front surface side of the cylinder block  7 , including the center hole  7 A, the opening side end surface  7 B, the sliding contact end surface  7 C and the plurality of cylinder holes  12 . That is, the nitriding layer  13  is configured by performing the nitride-based heat treatment on the base material  14  of the cylinder block  7  that is formed by using, for example, the cast, the iron-based material such as the steel material and so forth from the front surface side thereof. 
     Here, the nitriding layer  13  is configured by a diffusion layer  15  that is formed by performing nitriding on the front surface side of the base material  14  and a compound layer  16  that is formed so as to cover the front surface side of the diffusion layer  15  as shown in  FIG. 4 . The compound layer  16  is formed as a layer that is harder than the diffusion layer  15  in them and a thickness of the compound layer  16  is, for example, about 10 to 20 μm. In contrast, the diffusion layer  15  is formed on the lower layer side (or the inner side) of the compound layer  16  having a thickness of, for example, about 0.5 to 1.0 mm. 
     A compound layer-removed hole  17  is formed in the piston sliding surface  12 A of the cylinder hole  12 . This compound layer-removed hole  17  is formed by removing the compound layer  16  that is located on the front surface side of the nitriding layer  13  that is formed on the piston sliding surface  12 A by using polishing means such as, for example, honing and so forth. That is, the compound layer-removed hole  17  is, the compound layer  16  (shown by virtual lines in  FIG. 3 ,  FIG. 5 ) that is located on the front surface side of the piston sliding surface  12 A is removed by the polishing means over the entire circumference. 
     A compound layer-removed surface  18  is formed on a part A (that is, a piston contact point A that is shown by a virtual line in  FIG. 5 ) where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B of each cylinder hole  12  intersect and the compound layer  16  that is located on the front surface side is obliquely removed on this part A. That is, the compound layer-removed surface  18  is machined into a tapered state by the polishing means such as, for example, the honing and so forth in such a manner that the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect is made into an inclined surface of an angle α. The part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect is obliquely scraped off by the compound layer-removed surface  18  and is made into the inclined surface of the angle α. 
     Here, when a taper angle of the cylinder inlet side tapered surface  12 B is β and a maximum inclination angle of the piston  9  is γ, the angle α of the compound layer-removed surface  18  is set to satisfy a relation in the following formula 1. That is, the aforementioned angle α is set to an angle that is larger than the maximum inclination angle γ and is not more than the taper angle β. The maximum inclination angle γ means a maximum inclination angle that a dimensional tolerance on the basis of which the piston  9  is able to obliquely incline in the cylinder hole  12  is taken into consideration as shown in  FIG. 2 .
 
γ&lt;α≤β  [Formula 1]
 
     Here, the maximum inclination angle γ is set to an angle of about 0.1 to 2 degrees. The taper angle β of the cylinder inlet side tapered surface  12 B is set to an angle of, for example, about 10 to 45 degrees. Therefore, the angle α of the compound layer-removed surface  18  is in an angle range of 1 to 45 degrees and is preferably set to an angle of 2 to 30 degrees. 
     The inclined shaft-type hydraulic pump  1  according to the first embodiment has such a configuration as mentioned above and, in the following, the operation thereof will be described. 
     First, the pressurized oil for tilting control is supplied from a pilot pump (not shown) into the piston sliding bore  11 A in the tilting mechanism  11  via either one of the oil passage holes  11 D,  11 E. Thereby, the servo piston  11 B slides and displaces in the piston sliding bore  11 A and the valve plate  10  is moved to a desired tilt position together with the cylinder block  7 . At this time, the tilt angel θ between the cylinder block  7  and the valve plate  10  that is a crossing angle between the rotational axis O 1 -O 1  of the rotational shaft  5  and the rotational axis O 2 -O 2  of the cylinder block  7  is variably controlled between the minimum tilt position and the maximum tilt position by the tilting mechanism  11 . 
     A delivery amount (a flow rate) of the pressurized oil by the hydraulic pump  1  is determined depending on the tilt angle θ between the cylinder block  7  and the valve plate  10  relative to the rotational shaft  5 . That is, the delivery amount of the hydraulic pump  1  is minimized at the minimum tilt position where the tilt angle θ is minimized and the delivery amount of the hydraulic pump  1  is maximized at the maximum tilt position where the tilt angle θ is maximized. 
     Next, when the rotational shaft  5  is rotationally driven by a motor (not shown) such as an engine and so forth, the cylinder block  7  rotates together with the drive disc  5 B of the rotational shaft  5 . The pistons  9  reciprocate respectively in the respective cylinder holes  12  with rotation of the cylinder block  7 . Here, an oily liquid is sucked into the cylinder hole  12  via the aforementioned suction passage of the head casing  4 , the aforementioned suction port of the valve plate  10  in the suction stroke of each piston  9  that reciprocates. The pressurized oil is delivered out of the cylinder hole  12  and this pressurized oil can be supplied toward the hydraulic equipment via the aforementioned delivery port of the valve plate  10 , the aforementioned delivery passage of the head casing  4  in the delivery stroke of each piston  9 . 
     Next, a manufacturing process of the cylinder block  7  will be described. 
     First, the cylinder block  7  is molded by using means such as casting and so forth from the base material  14  that is configured by, for example, the cast, the iron-based material such as the steel material and so forth. Cutting work for rough finishing is performed on the base material  14  of the cylinder block  7  as required. Next, the nitriding layer  13  that is made by performing, for example, the nitride-based heat treatment is formed on the front surface side of the base material  14 . This nitriding layer  13  is formed as a surface treatment layer so as to entirely cover the front surface side of the cylinder block  7 , including the center hole  7 A, the opening side end surface  7 B, the sliding contact end surface  7 C and the plurality of cylinder holes  12 . 
     Then, polishing for removing the compound layer  16  that is located on the front surface side of the nitriding layer  13  is performed on the piston sliding surface  12 A of each cylinder hole  12  by using the polishing means such as, for example, the honing and so forth. Thereby, the piston sliding surface  12 A of each cylinder hole  12  is formed as the compound layer-removed hole  17 . 
     Further, the polishing for removing the compound layer  16  that is located on the front surface side of the nitriding layer  13  is performed on the part A (that is, the piston contact point A shown by the virtual line in  FIG. 5 ) where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B of each cylinder hole  12  intersect similarly by using the polishing means such as the honing and so forth. Thereby, the tapered-state inclined surface of the angle α is formed on the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect as the compound layer-removed surface  18 . 
     The part where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect is polished into the tapered-state inclined surface of the angle α as the compound layer-removed surface  18  in this way in the first embodiment. Thereby, the wear when the piston  9  comes into contact with the inlet side (that is, the part where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect) of the cylinder hole  12  is reduced to make it possible to improve the durability and the life thereof. 
     Incidentally, in a case where the compound layer-removed surface  18  is not formed in the vicinity of the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect, there is the possibility that such a problem as described below would occur. 
     That is, when the rotational shaft  5  of the hydraulic pump  1  is rotationally driven by the engine, this rotation is transmitted from the drive disc  5 B to the cylinder block  7  via the plurality of pistons  9 . The plurality of pistons  9  come into contact with the inlet sides of the respective cylinder holes  12  and transmit loads thereto in this rotation transmission. At this time, the piston  9  inclines relative to each cylinder hole  12  in a range of, for example, the maximum inclination angle γ shown in  FIG. 2 . In addition, the load that is rotationally transmitted from each piston  9  to the cylinder block  7  is determined depending on the load that is needed to drive a hydraulic actuator (not shown) that is connected to the delivery side of the hydraulic pump  1 . 
     However, in a case where the inlet side of the piston sliding surface  12 A of each cylinder hole  12  is in the form of an edge shape, an area when the piston  9  comes into contact with this part results in contact of a small area. Therefore, a contact part of the small area reaches a high contact surface pressure and there is concern about the wear of the piston  9  surface. Further, in a case where compound layer removing is performed on the piston sliding surface  12 A of the cylinder hole  12  by the honing and so forth after nitriding, the high-hardness compound layer  16  remains on the inlet side (that is, the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect) of each cylinder hole  12 . Therefore, when the piston  9  comes into contact with the inlet side of the cylinder hole  12 , for example, the piston  9  is, the wear becomes liable to occur on its contact part. 
     It follows that the plurality of pistons  9  repeat reciprocation (sliding contact) along the inner circumferential surfaces (the piston sliding surfaces  12 A) of the respective cylinder holes  12  while rotationally driving the cylinder block  7  of the hydraulic pump  1  together with the rotational shaft  5  in such a state. Therefore, sliding surfaces of each piston  9  and the cylinder hole  12  become liable to be worn down and improvement thereof is desired. 
     In addition, the conventional art according to aforementioned Patent Document 1 simply performs chamfering without performing nitriding and so forth on the base material of the cylinder block and no consideration is given to removing and so forth of the compound layer. Therefore, it is difficult to improve the durability and the life of the piston. 
     Accordingly, the base material  14  of the cylinder block  7  is formed by using the cast, the steel material and so forth and the nitriding layer  13  that is made by performing, for example, nitride-based heat treatment is formed on the front surface side of the base material  14  in the first embodiment. This nitriding layer  13  is formed to entirely cover the front surface side of the cylinder block  7 , including the center hole  7 A, the opening side end surface  7 B, the sliding contact end surface  7 C and the plurality of cylinder holes  12 . Then, the piston sliding surface  12 A of each cylinder hole  12  is formed as the compound layer-removed hole  17  by removing the compound layer  16  that is located on the front surface side of the nitriding layer  13  by using the polishing means such as, for example, the honing and so forth. 
     Further, the compound layer-removed surface  18  is formed on the part A (that is, the piston contact point A shown by the virtual line in  FIG. 5 ) where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B of each cylinder hole  12  intersect by using the polishing means such as, for example, the honing and so forth. That is, the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect is obliquely scraped off by the compound layer-removed surface  18  and the compound layer-removed surface  18  is formed as the tapered-state inclined surface of the angle α. The angle α of the compound layer-removed surface  18  is set to an angle that is larger than the maximum inclination angle γ and is not more than the taper angle β so as to satisfy the relation in the aforementioned formula 1 relative to the taper angle β of the cylinder inlet side tapered surface  12 B and the maximum inclination angle γ of the piston  9 . 
     The compound layer-removed surface  18  from which the compound layer  16  that is located on the front surface side of the nitriding layer  13  is removed is formed on the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect in this way. Therefore, it is possible to prevent the high-hardness compound layer  16  from remaining on the opening end (inlet) side of each cylinder hole  12  with the aid of the compound layer-removed surface  18 . As a result, it is possible to restrain the piston  9  that reciprocates in each cylinder hole  12  (the compound layer-removed hole  17 ) from being worn down and damaged on its inlet (the cylinder inlet side tapered surface  12 B) side for a long period of time. 
     In other words, the piston sliding surface  12 A of each cylinder hole  12  is made into the compound layer-removed hole  17  and thereafter the compound layer-removed surface  18  is formed in such a manner that the compound layer  16  does not remain in the vicinity of the piston contact point A shown in  FIG. 5  in the first embodiment. Thereby, the wear when the piston  9  comes into contact with the inlet side of the cylinder hole  12  can be reduced. In addition, delamination and so forth of the compound layer  16  on the inlet side of each cylinder hole  12  can be suppressed. 
     Accordingly, the wear when the piston slides can be suppressed and the durability and life thereof can be improved by performing compound layer removal machining in a sliding range of the piston  9  over the inner circumferential surface (the piston sliding surface  12 A) and the inlet side of each cylinder hole  12  according to the first embodiment. In addition, the contact area when the piston  9  comes into contact with the inlet side of the cylinder hole  12  can be made large and the contact surface pressure can be reduced by forming the compound layer-removed surface  18  as the tapered-state inclined surface of the angle α. 
     Next,  FIG. 6  shows a second embodiment of the present invention and the characteristic of the second embodiment lies in a configuration that a compound layer-removed surface is formed with a machined surface that is configured by a curved surface. Incidentally, the same symbol is assigned to the constitutional element that is the same as that in the first embodiment and description thereof is omitted in the present embodiment. 
     Here, a compound layer-removed surface  21  is adopted in place of the compound layer-removed surface  18  described in the aforementioned first embodiment. This compound layer-removed surface  21  is configured by forming the machined surface that is configured by a curved surface that is arc-shaped in section on the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect by using the polishing means such as, for example, the honing and so forth. 
     That is, the compound layer-removed surface  21  is formed by abrasively machining the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect into a curved-surface shape in such a manner that its angle δ is gradually widened. The angle δ of the compound layer-removed surface  21  is an angle that is gradually increased in multiple stages of two or more stages and is set so as to satisfy a relation in the following formula 2. That is, the angle δ in this case is set to an angle that is larger than the maximum inclination angle γ and is not more than the taper angle β.
 
γ&lt;δ≤β  [Formula 2]
 
     Thus, the piston sliding surface  12 A of each cylinder hole  12  is formed as the compound layer-removed hole  17  by removing the compound layer  16  that is located on the front surface side of the nitriding layer  13  by using the polishing means such as, for example, the honing and so forth also in the second embodiment that is configured in this way. Then, the compound layer-removed surface  21  is formed on the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B of each cylinder hole  12  intersect by using the polishing means such as, for example, the honing and so forth. 
     The compound layer-removed surface  21  is formed by abrasively machining the part A where the compound layer-removed hole  17  and the cylinder inlet side tapered surface  12 B intersect into the curved-surface shape in such a manner that its angle is gradually widened particularly in the second embodiment. For this reason, remaining of the high-hardness compound layer  16  on the opening end (the inlet) side of each cylinder hole  12  can be surely eliminated with the aid of the compound layer-removed surface  21 . Thereby, it is possible to restrain the piston  9  that reciprocates in each cylinder hole  12  (the compound layer-removed hole  17 ) from being worn down and damaged on its inlet (the cylinder inlet side tapered surface  12 B) side for the long period of time. 
     The contact area across which each piston  9  comes into contact with the inlet side of each cylinder hole  12  can be made large and the contact surface pressure of the piston  9  can be more reduced by forming the compound layer-removed surface  21  as the curved surface in such a manner that the angle thereof gradually changes starting from the inlet side of each cylinder hole  12  in this way. 
     Incidentally, description is made by giving a case of forming the compound layer-removed surface  21  as the curved surface by way of example in the aforementioned second embodiment. However, the present invention is not limited to this and the compound layer-removed surface may be formed as a plural-stage tapered-state inclined surface that is widened in a plurality of stages such as, for example, two to four stages. 
     In addition, description is made by giving the inclined shaft-type variable displacement-type hydraulic pump as an example of the axial piston-type hydraulic rotary machine in each of the aforementioned embodiments. However, the present invention is not limited to this and may be applied to, for example, a fixed displacement-type inclined shaft-type hydraulic pump, a fixed displacement-type or variable displacement-type inclined shaft-type hydraulic motor. Further, it may be also applied to fixed displacement-type or variable displacement-type swash plate-system hydraulic rotary machines (hydraulic pump, hydraulic motor). 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1 : Hydraulic pump (Axial piston-type hydraulic rotary machine) 
           2 : Casing 
           3 : Casing body 
           4 : Head casing 
           5 : Rotational shaft 
           7 : Cylinder block 
           7 A: Center hole 
           7 B: Opening side end surface 
           8 : Center shaft 
           9 : Piston 
           10 : Valve plate 
           11 : Tilting mechanism 
           12 : Cylinder hole 
           12 A: Piston sliding surface 
           12 B: Cylinder inlet side tapered surface 
           13 : Nitriding layer 
           14 : Base material 
           15 : Diffusion layer 
           16 : Compound layer 
           17 : Compound layer-removed hole 
           18 ,  21 : Compound layer-removed surface 
         A: Part where the compound layer-removed hole and the cylinder inlet side tapered surface intersect 
         α: Angle 
         β: Taper angle 
         γ: Maximum inclination angle