Abstract:
A hydraulic pump includes a cylinder block having a cylinder, a rotor rotatable in the cylinder, and a vane movable in a radial direction of the rotor and biased to the rotor. The vane, the cylinder, and the rotor thereamong define an operation chamber. The rotor draws fluid into the operation chamber and sends the fluid outside the operation chamber. The vane is urged to the rotor according to differential pressure between pressure of fluid at high-pressure in the operation chamber and pressure of fluid at low-pressure in the operation chamber. The vane and the rotor define a hardness ratio being calculated by dividing hardness of the vane by hardness of the rotor, and the hardness ratio is greater than or equal to 1.6.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and incorporates herein by reference Japanese Patent Application No. 2007-91582 filed on Mar. 30, 2007. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a hydraulic pump. In particular, the present invention may relate to a refrigerant pump for pumping refrigerant fluid to a heater in a waste heat recovery system such as a Rankine cycle system. 
       BACKGROUND OF THE INVENTION 
       [0003]    For example, JP-A-63-277883 discloses a rotary compressor configured to be provided to an airconditioner, a refrigerator, or the like. In the present rotary compressor, a vane is provided to a cylinder accommodating a rotor. The vane is movable in a vane groove provided in the cylinder. The vane has a tip end urged to a rotor and the tip end of the vane is formed from a material being excellent in wear resistance compared with that of a body portion of the vane. In the present structure, the tip end of the vane is enhanced in wear resistance, and abrasion caused in a wall surface, which defines the vane groove, is reduced. 
         [0004]    The rotary compressor is configured to compress vapor-phase refrigerant in a refrigeration cycle of, for example, an airconditioner. In the present structure, the vane and the rotor are capable of therebetween sufficiently forming an oil film in a steady operation, thereby maintaining therebetween a state of fluid lubrication. In the present structure, only in a transitional operation at the time of starting or stopping of the operation of the rotary compressor, the vane and the rotor are in a state of boundary lubrication where the vane and the rotor therebetween do not sufficiently form an oil film. Therefore, as indicated by a dashed line in  FIG. 6 , development in abrasion accompanying time progress is not so significant in the rotary compressor. Therefore, in the rotary compressor, wear resistance needs to be considered only in the state of boundary lubrication. 
         [0005]    However, in a hydraulic pump, which pumps low-viscosity fluid, a vane and a rotor are regularly in the state of boundary lubrication at any operations. Therefore, as indicated by a solid line in  FIG. 6 , the vane and the rotor are apt to therebetween significantly develop abrasion accompanying time progress. Accordingly, only the above structure of the rotary compressor, in which the tip end of the vane is formed from the wear-resistive material, may not be practical for reducing abreaction in the hydraulic pump configured to pump low-viscosity fluid. 
         [0006]    As follows, an exemplified premise for calculating a minimum oil film thickness t and an oil film parameter Λ is described with reference to  FIG. 7 . In the present premise, fluid fed by the hydraulic pump has viscosity of 1 [mPa·s], a vane  151  has a tip radius Rv of 20 [mm], the vane  151  exerts load F of 4000 [N/m] per unit length to a rotor  141 , the vane  151  has surface roughness Rzv of 0.8, the rotor  141  has radius Rr of 20 [mm], the rotor  141  slides at sliding speed v of 1 [mm/s], and the rotor  141  has surface roughness Rzr of 0.32. 
         [0007]    According to the premise and the elastohydrodynamic lubrication theory, the minimum oil film thickness t between the vane  151  and the rotor  141  and the oil film parameter Λ are calculated such that the minimum oil film thickness=0.03 [μm] and the oil film parameter Λ&lt;1. According to the calculation, the vane  151  and the rotor  141  are obviously in the state of boundary lubrication in the hydraulic pump. Therefore, abrasion between the vane  151  and the rotor  141  in the hydraulic pump needs to be steadily suppressed so as to enhance product lives of both the vane and the rotor and maintain a performance of the hydraulic pump for a long period. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the foregoing and other problems, it is an object of the present invention to produce a hydraulic pump having a vane and a rotor for pumping liquid-phase fluid, the hydraulic pump being capable of restricting ablation between the vane and the rotor and capable of maintaining a performance thereof. 
         [0009]    According to one aspect of the present invention, a hydraulic pump comprises a cylinder block having a cylinder. The hydraulic pump further comprises a rotor rotatable in the cylinder. The hydraulic pump further comprises a vane movable substantially in a radial direction of the rotor and configured to be biased to the rotor. The vane, the cylinder, and the rotor thereamong define an operation chamber. The rotor is configured to draw fluid into the operation chamber and configured to send the fluid outside the operation chamber. The vane is configured to be urged to the rotor according to differential pressure between pressure of fluid at high-pressure in the operation chamber and pressure of fluid at low-pressure in the operation chamber. The vane and the rotor define a hardness ratio being calculated by dividing hardness of the vane by hardness of the rotor. The hardness ratio is greater than or equal to 1.6. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
           [0011]      FIG. 1  is a sectional view showing a hydraulic pump according to a first embodiment; 
           [0012]      FIG. 2  is a sectional view taken along the line II-II in  FIG. 1 ; 
           [0013]      FIG. 3  is an enlarged sectional view showing a vane, a rotor, and peripheral components of the hydraulic pump; 
           [0014]      FIG. 4  is a schematic view showing an ablation developed in the vane; 
           [0015]      FIG. 5  is a table showing specific wear rates of both the vane and the rotor of the hydraulic pump; 
           [0016]      FIG. 6  is a graph showing a relationship between ablation, which is developed in each of a rotary compressor and a hydraulic pump, and an operation period; and 
           [0017]      FIG. 7  is a schematic view showing a vane and a rotor being in contact with each other. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
       [0018]    In the present embodiment, a refrigerant pump  100  is provided to pump liquid-phase refrigerant as fluid in a Rankine cycle for recovery of waste heat. The Rankine cycle as a waste heat recovery cycle is, for example, provided to a vehicle. Next, a structure of the refrigerant pump  100  is described with reference to  FIGS. 1 ,  2 . The waste-heat recovery Rankine cycle is constructed by combining the refrigerant pump  100 , a heater, an expansion device, and a condenser to define an annual circuit. The condenser condenses therein liquid-phase refrigerant, and the refrigerant pump  100  feeds the condensed refrigerant to the heater. The heater heats the liquid-phase refrigerant using waste heat of an internal combustion engine of the vehicle. The liquid-phase refrigerant in the heater is heated to be superheated steam refrigerant. The superheated steam refrigerant is fed into the expansion device, whereby the expansion device recovers kinetic energy caused by expansion of the superheated steam refrigerant. The refrigerant used in the present waste-heat recovery Rankine cycle is preferably used for a refrigeration cycle of an airconditioner of the vehicle. 
         [0019]    The refrigerant pump  100  is a rotary-vane pump having a cylinder, which accommodates a rotor and a vane. In the present embodiment, the refrigerant pump  100  is a two-cylinder pump having two cylinders  121 , two rotors  141 , and two vanes  151 . 
         [0020]    The refrigerant pump  100  has a body portion, which is constructed by combining a side plate  111 , one cylinder plate  120 , an intermediate plate  112 , another cylinder plate  120 , a side plate  113 , and a bearing holder  114  in order. Each of the components of the body portion of the refrigerant pump  100  is substantially in a flat cylinder shape. 
         [0021]    The side plate  111  has one-end side on the left side in  FIG. 1 , and the one-end side has a large opening being configured to be mounted with an additional device such as an electric motor as a driving source of the refrigerant pump  100 . The opening in the one-end side of the side plate  111  is reduced in diameter stepwise toward the other-end side on the right side in  FIG. 1 , whereby the opening is communicated with the other-end side of the side plate  111 . The other-end side of the side plate  111  has a communication hole having an inner diameter, which is less than an inner diameter of the cylinder  121  of the cylinder plate  120 . The communication hole of the other-end side of the side plate  111  is inserted with a shaft  131 . The other-end side of the side plate  111  has a center portion fixed with a bearing  133 . The bearing  133  rotatably supports one-end of the shaft  131 . 
         [0022]    The intermediate plate  112  and the side plate  113  are disc-shaped members respectively having center portions defining insertion holes. The insertion holes of the intermediate plate  112  and the side plate  113  are substantially the same as the opening of the side plate  111  on the other-end side in inner diameter. The bearing holder  114  is a bottomed cylindrical member having a center portion defining a recess on the one-end side. The recess of the bearing holder  114  is provided with a bearing  134 , which rotatably supports the other-end side of the shaft  131 . 
         [0023]    Each cylinder plate  120  is a cylinder block member being in a disc shape. The cylinder plate  120  has a center portion defining the cylinder  121  being in a circular shape. One of the cylinder plates  120  is interposed between the side plate  111  and the intermediate plate  112 . The other cylinder plate  120  is interposed between the intermediate plate  112  and the side plate  113 . The side plate  111 , the one cylinder plate  120 , the intermediate plate  112 , the other cylinder plate  120 , the side plate  113 , and the bearing holder  114  are communicated with each other through the insertion hole and the cylinders  121 . 
         [0024]    Each cylinder plate  120  has an inlet port  122 , an outlet port  123 , a vane groove  124 , and a backpressure feed port  125 , in addition to the cylinder  121 . In the present structure, the inlet port  122  as one communicating portion is provided for communicating an exterior of the cylinder plate  120  with an interior of the cylinder  121 . The outlet port  123  as another communicating portion is provided for communicating the interior of the cylinder  121  with the exterior of the cylinder plate  120 . The outlet port  123  is adjacent to the inlet port  122  with respect to a circumferential direction of the cylinder plate  120 . The vane groove  124  extends from the cylinder  121  radially outward in the cylinder plate  120 . The vane groove  124  is located circumferentially between the inlet port  122  and the outlet port  123 . The backpressure feed port  125  as another communicating portion is provided for communicating an interior of the vane groove  124  with a high-pressure chamber in an operation chamber V. 
         [0025]    The shaft  131  has two portions correspondingly to the two cylinders  121 , and each of the two portions of the shaft  131  is provided with a circular cam portion  132 . The circular cam portion  132  is eccentric relative to an axis of the shaft  131 . 
         [0026]    The rotor  141  is a flat cylindrical member and rotatably equipped to an outer circumferential periphery of the cam portion  132  via a bearing (not shown) for feeding liquid-phase refrigerant. An outer diameter of the rotor  141  is smaller than an inner diameter of the cylinder  121 . The rotor  141  is inserted into the cylinder  121  such that the rotor  141  is configured to revolve around the cam portion  132  in the cylinder  121 . 
         [0027]    The vane  151  is a plate-shaped member and accommodated in the vane groove  124  such that the vane  151  is movable in the vane groove  124 . A spring  152  is interposed between a recess in a bottom of the vane groove  124  and the vane  151 . The spring  152  biases the vane  151  toward the rotor  141  such that a tip end of the vane  151  is in contact with an outer circumferential periphery of the rotor  141  mainly in a condition where the refrigerant pump  100  stops. The rotor  141  and the vane  151  define the operation chamber V in the cylinder  121 . 
         [0028]    Here, hardness of a material of each of the vane  151  and the rotor  141  is predetermined by selecting a material of each of the vane  151  and the rotor  141  and selecting heat treatment applied to the material. More specifically, a hardness ratio Hr between hardness Hvv of the vane  151  and hardness Hvr of the rotor  141  is predetermined to be greater than or equal to 1.6. Here, the hardness ratio Hr is defined by dividing the hardness Hvv of the vane  151  by the hardness Hvr of the rotor  141 . 
         [0029]    The refrigerant pump  100  with the structure described above has the inlet port  122  and the outlet port  123 . The inlet port  122  is connected with an outlet of the condenser, and the outlet port  123  is connected with an inlet side of the heater. When an electric motor (not shown) as a driving source drives the shaft  131 , each rotor  141  revolves around the cam portion  132  in the cylinder  121 , whereby the rotor  141  draws liquid-phase refrigerant from the condenser into the operation chamber V and pumps the liquid-phase refrigerant to the heater. 
         [0030]    In this operation, the high-pressure chamber in the operation chamber V is communicated with the vane groove  124  through the backpressure feed port  125 , so that urging force is exerted to the vane  151  to urge the vane  151  onto the rotor  141  correspondingly to differential pressure between pressure in the low-pressure chamber and pressure in the high-pressure chamber. The urging force is steadily applied to the vane  151  to bias the vane  151  toward the rotor  141 , so that a tip end of the vane  151  is steadily maintained in contact with the outer circumferential periphery of the rotor  141 . In the present structure, the vane  151  is maintained in contact with the rotor  141 , whereby liquid-phase refrigerant in the high-pressure chamber can be restricted from leaking into the low-pressure chamber. 
         [0031]    Next, the urging force exerted to the vane  151  is further specifically described with reference to  FIG. 3 . Here, the spring  152  is provided in the vane groove  124 . The spring  152  exerts biasing force to the vane  151  to maintain the state where the vane  151  is in contact with the rotor  141  mainly in a state where the refrigerant pump  100  stops. In this structure, the spring  152  biases the vane  151  to restrict liquid-phase refrigerant in the high-pressure chamber from leaking into the low-pressure chamber when the refrigerant pump  100  starts operation. The biasing force of the spring  152  is significantly small and negligible compared with the urging force, which correlates with the differential pressure between the high-pressure chamber and the low-pressure chamber. Therefore, in the following description, the biasing force of the spring  152  is disregarded. 
         [0032]    In  FIG. 3 , a thickness direction of the vane  151  is defined as a horizontal direction (left and right direction), and a direction perpendicular to the sheet of  FIG. 3  is defined as a depth direction of the vane  151 . The operation chamber V on the right side of the vane  151  is a low-pressure chamber directly communicating with the inlet port  122 . The operation chamber V on the left side of the vane  151  is a high-pressure chamber directly communicating with the outlet port  123 . 
         [0033]    In an initial state, the tip end of the vane  151  is substantially in convex. The tip end has a center portion with respect to the thickness direction, and the center portion has the maximum projected portion relative to the rotor  141 . The tip end of the vane  151  and the rotor  141  therebetween have a contact portion in which the tip end of the vane  151  defines a contact point  151   a . The contact point  151   a  is at a distance Lhigh from an end of the vane  151  on the high-pressure side, i.e., on the side of the high-pressure chamber. The contact point  151   a  is at a distance Llow from an end of the vane  151  on the low-pressure side, i.e., on the side of the low-pressure chamber. Backpressure Pb is applied to the vane  151  toward the rotor  141 . The high-pressure chamber is at pressure Phigh. The low-pressure chamber is at pressure Plow. 
         [0034]    The vane  151  has a depth DP with respect to the depth direction orthogonal to the page of  FIG. 3 . The vane  151  is exerted with back pressure force Fb, which is calculated by the following equation 1. 
         [0000]        Fb=Pb ×( L high+ L low)× DP   (1) 
         [0035]    The vane  151  is also exerted with pushback force Fhigh from the high-pressure chamber. 
         [0000]        F high= P high× L high× DP   (2) 
         [0036]    The vane  151  is further exerted with pushback force Flow from the low-pressure chamber. 
         [0000]        F low= P low× L low× DP   (3) 
         [0037]    Here, the backpressure Pb is equal to the pressure Phigh, and according to the above equations (1) to (3), urging force Fu, with which the vane  151  is exerted toward the rotor  141 , can be calculated by the following equation (4). 
         [0000]        Fu=Fb −( F high+ F low)=( P high− P low)× L low× DP   (4) 
         [0038]    Thus, the urging force Fu is determined correspondingly to the difference between the pressure Phigh in the high-pressure chamber and pressure Plow in the low-pressure chamber. The urging force Fu becomes large as the distance Llow increases. 
         [0039]    As an operation period of the refrigerant pump  100  increases, abrasion between the vane  151  and the rotor  141  develops. Specifically, as shown in  FIG. 4 , the convex portion of the tip end of the vane  151  is scraped substantially to be a flat surface extending along an envelope defined by revolution of the rotor  141 . In the initial state, the tip end of the vane  151  is in contact with the rotor  141  via a contact surface of a contact width A. As the vane  151  is scraped and worn out as the rotor  141  revolves, the contact width A increases to a contact width B. Therefore, at a time point in the operation of the refrigerant pump  100 , the contact point  151   a  largely moves toward the low-pressure chamber, and the distance Llow decreases. As a result, the urging force Fu exerted to the vane  151  decreases. Consequently, liquid-phase refrigerant may leak from the high-pressure chamber into the low-pressure chamber, and such leakage results in decrease in a pump performance of the refrigerant pump  100 . In addition, the vane  151  may fluctuates to cause further ablation, breakage in the components, and abnormal noise between components. 
         [0040]    Therefore, in the present embodiment, the hardness ratio Hr is defined by the value of: (hardness Hvv of the vane  151 )/(the hardness Hvr of the rotor  141 ), and the hardness ratio Hr is determined to be greater than or equal to 1.6. In the present structure, ablation between the vane  151  and the rotor  141  can be steadily reduced. 
         [0041]    Specifically in the present embodiment, as shown in  FIG. 5 , the base material of the vane  151  is selected from four kinds of tool steel including chrome molybdenum steel (SCM415), high-speed tool steel (SKH51), and alloy tool steel (SKD11). The surface of the vane  151  is applied with heat treatment including carburizing, quenching, and tempering (GC), softnitriding (TFG) with quenching and tempering (QT), chromium nitride coating (CrN), and titanium nitride coating (TiN). Surface roughness Rz of the vane  151  is predetermined to 3.2 or 0.3. 
         [0042]    Here, the surface roughness Rz is, for example, a ten points average height as mean roughness depth calculated by measuring distances between highest five peaks and lowest five bottoms, then averaging the distances. 
         [0043]    In the present embodiment, the base material of the rotor  141  is chrome molybdenum steel (SCM415) as case-hardened steel. The surface of the rotor  141  is applied with carburizing, quenching, and tempering (GC). Surface roughness Rz of the rotor  141  is predetermined to 3.2. 
         [0044]    Vickers hardness (Hv) of each material applied with heat-treating is obtained. The hardness ratio Hr is defined by the following equation (5) using hardness Hvv of the vane  151  and hardness Hvr of the rotor  141 . 
         [0000]        Hr=Hvv/Hvr   (5) 
         [0045]    A specific wear rate WR [m2/N] of each of the vane  151  and the rotor  141  is defined by the following equation (6) using abrasion ABR [m3], load L [N], and sliding distance D [m]. 
         [0000]        WR [m2/N]= ABR [m3]/ L [N]× D [m]  (6) 
         [0046]    As shown in  FIG. 5 , a result indicating the specific wear rate WR of each of the vane  151  and the rotor  141  relative to each hardness ratios Hr is obtained by combining relations between the materials, the heat treatment, and the surface roughness Rz of the vane  151  and the rotor  141  at four levels. According to the result, as the hardness ratio Hr increases, the specific wear rate WR of the vane  151  substantially decreases, and the specific wear rate WR of the rotor  141  substantially increases. It suffice to determine the hardness ratio Hr to be greater than or equal to 1.6 so as to restrict the specific wear rate WR of the vane  151  to be less than the specific wear rate WR of the rotor  141 . 
         [0047]    It is confirmed in advance that it suffices to restrict the specific wear rate WR of each of the vane  151  and the rotor  141  to be less than or equal to about 10 to the minus 16th power [m2/N] so as to restrict leakage of liquid-phase refrigerant. To satisfy the present condition of the specific wear rate WR, it suffice to restrict the hardness ratio Hr to be less than or equal to about 2.5. Thus, according to the result shown in  FIG. 5 , both the levels  2 ,  3  are determined to be preferable. 
         [0048]    As described above, the specific wear rate WR of the vane  151  relative to the rotor  141  can be steadily suppressed by determining the hardness ratio Hr between the vane  151  and the rotor  141  to be greater than or equal to 1.6. Therefore, by determining the hardness ratio Hr to be greater than or equal to 1.6, the contact width of the vane  151  relative to the rotor  141  can be restricted from increasing. Thereby, the contact point between the vane  151  and the rotor  141  can be restricted from moving toward the low-pressure chamber as described above with reference to  FIG. 4 . Thus, the pump performance of the refrigerant pump  100  can be maintained for a long period. The specific wear rate WR of, in particular, the rotor  141  can be suppressed by determining the hardness ratio Hr to be less than or equal to 2.5, and the pump performance of the refrigerant pump  100  can be significantly maintained. 
       Second Embodiment 
       [0049]    The specific wear rates WR of the vane  151  and the rotor  141  can be further reduced by appropriately determining the surface roughness Rz of the vane  151  and the rotor  141 , in addition to the determination in the first embodiment. 
         [0050]    According to the first embodiment, when the surface roughness Rz of the rotor  141  is reduced from 3.2 to 0.08 in the level  2  in  FIG. 5 , the specific wear rate WR of the vane  151  can be reduced to a value of 8.63×10 to the minus 19th power [m2/N], and the specific wear rate WR of the rotor  141  can be reduced to 2.22×10 to the minus 18th power [m2/N]. In the present determination of the surface roughness Rz of the rotor  141 , the specific wear rate WR of the vane  151  is reduced by two orders of magnitude, and the specific wear rate WR of the rotor  141  is reduced by one order of magnitude. The specific wear rate WR is considered to be presently reduced, since abrasion between the vane  151  and the rotor  141  is reduced by the determination of the surface roughness Rz of the rotor  141 . 
         [0051]    Therefore, it suffices to determine the surface roughness Rz of the vane  151  to be less than or equal to 0.3 and the surface roughness Rz of the rotor  141  to be less than or equal to 0.1 so as to reduce the specific wear rates WR of both the vane  151  and the rotor  141  to be less than or equal to 10 to the minus 18th power [m2/N]. 
       Other Embodiments 
       [0052]    In the first embodiment, the above structure is applied to the refrigerant pump  100  for the waste-heat recovery Rankine cycle provided to the vehicle. Alternatively, the above structure may be applied to various hydraulic pumps for pumping fluid. For example, the above structure may be applied to a hydraulic pump for a Rankine cycle provided to a stationary power generator or the like. 
         [0053]    Various modifications and alternations may be diversely made to the above embodiments without departing from the spirit of the present invention.