Patent Publication Number: US-2023151848-A1

Title: Sliding component

Description:
TECHNICAL FIELD 
     The present invention relates to sliding components that rotate relative to each other and are used for, for example, a shaft sealing device shaft-sealing a rotary shaft of a rotating machine in an automotive seal field, a general industrial machinery seal field, or another seal field or a bearing of a machine in an automotive bearing field, a general industrial machinery bearing field, or another bearing field. 
     BACKGROUND ART 
     As a shaft sealing device that prevents a leakage of a sealing target fluid, for example, a mechanical seal includes a pair of annular sliding components rotating relative to each other so that sliding surfaces slide on each other. In such a mechanical seal, there has been a recent demand to reduce the energy lost caused by sliding for environmental measures and the like. 
     For example, in a mechanical seal shown in Patent Citation 1, a pair of annular sliding components is configured to rotate relative to each other, a sealing target fluid exists in an outer space, and a low-pressure fluid exists in an inner space. One sliding component is provided with a spiral groove communicating with the inner space in which the low-pressure fluid exists, extending in an arc shape from an inner radial end to an outer radial side while being inclined in a circumferential direction, and having a closed terminating end on the downstream side of a relative rotation direction. Accordingly, since the low-pressure fluid is introduced into the spiral groove of one sliding component during the relative rotation of the pair of sliding components, a positive pressure is generated in the terminating end and the vicinity thereof to slightly separate sliding surfaces of the pair of sliding components from each other. Accordingly, low friction is realized. Further, in the spiral groove, since a negative pressure is generated in the starting end and the vicinity thereof to suck a sealing target fluid flowing from the outer space between the sliding surfaces, it is possible to prevent the sealing target fluid from leaking to the low-pressure inner space from between the pair of sliding components. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Citation 1: JP S62-31775 A (Pages 2 and 3, FIG. 2) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the sliding component shown in Patent Citation 1, since the spiral groove is disposed on the leakage side of one sliding component and extends from the inner radial end toward the outer radial side to introduce the low-pressure fluid, it is possible to reduce wear and suppress leakage. However, since a sufficient dynamic pressure is not generated in the spiral groove until the sliding component reaches a high-speed rotation state of a certain level or more, it takes time to separate the sliding surfaces. As a result, there is a risk that the sliding surface would wear out. 
     The present invention has been made in view of such problems and an object thereof is to provide a sliding component capable of suppressing wear between sliding surfaces from the start of a relative rotation of a pair of sliding components to a high-speed rotation and suppressing a leakage of a sealing target fluid. 
     Solution to Problem 
     In order to solve the foregoing problems, a sliding component according to the present invention is a sliding component formed in an annular shape and disposed at a relatively rotating position of a rotating machine and sliding relative to an opposed sliding component, wherein a sliding surface of the sliding component is provided with a plurality of first dynamic pressure generation grooves disposed on a leakage side, having terminating ends, and generating a positive pressure and a plurality of second dynamic pressure generation grooves disposed on a sealing target fluid side, having terminating ends, and generating a positive pressure, and wherein a depth of the second dynamic pressure generation groove is shallower than a depth of the first dynamic pressure generation groove. According to the aforesaid feature of the present invention, since the depth of the second dynamic pressure generation groove is shallower than the depth of the first dynamic pressure generation groove, a second force caused by the positive pressure generated by the sealing target fluid in the second dynamic pressure generation groove mainly acts to separate the sliding surfaces from each other during the low-speed relative rotation of the sliding component. As the relative rotation speed of the sliding component increases, a first force caused by the positive pressure generated by the leakage side fluid in the first dynamic pressure generation groove suddenly increases. Then, when the relative rotation speed of the sliding component becomes sufficiently high, the first force becomes larger than the second force and the first force mainly acts to separate the sliding surfaces from each other. Accordingly, it is possible to suppress the wear between the sliding surfaces from the low speed to the high speed of the relative rotation of the pair of sliding components. Further, since a gap formed between the sliding surfaces increases during the high-speed relative rotation of the sliding component, the positive pressure is not easily generated in the second dynamic pressure generation groove. Accordingly, the first force caused by the positive pressure generated in the first dynamic pressure generation groove mainly acts to stably separate the sliding surfaces from each other. Thus, it is possible to suppress the wear by separating the sliding surfaces from each other from the start of the relative rotation of the pair of sliding components to the high-speed rotation. Further, since the second dynamic pressure generation groove sucks the sealing target fluid flowing from the sealing target fluid side space between the sliding surfaces, it is possible to prevent the sealing target fluid from leaking to the leakage side space from between the pair of sliding components. 
     It may be preferable that each of the second dynamic pressure generation grooves communicates with a sealing target fluid side space. According to this preferable configuration, it is possible to easily introduce the sealing target fluid into the second dynamic pressure generation groove and to generate a positive pressure at an early time. 
     It may be preferable that an annular land portion having a predetermined radial width or more is continuously provided in a circumferential direction between the terminating ends of the first dynamic pressure generation grooves and the terminating ends of the second dynamic pressure generation grooves. According to this preferable configuration, it is possible to suppress the sealing target fluid between the sliding surfaces from flowing to the leakage side space by the land portion when the sliding surfaces are separated from each other by the second force caused by the positive pressure generated in the second dynamic pressure generation groove. Further, it is possible to suppress the leakage of the sealing target fluid to the leakage side space in a stationary state in which the pair of sliding components does not rotate relative to each other. 
     It may be preferable that a radial center of the land portion is disposed on a side of the sealing target fluid with respect to a radial center of the sliding surface. According to this preferable configuration, since the land portion is disposed on the sealing target fluid side in the radial direction of the sliding surface, it is possible to ensure the long extension length of the first dynamic pressure generation groove and to arrange a large number of the first dynamic pressure generation grooves side by side. Accordingly, since the first dynamic pressure generation groove serves as a main dynamic pressure generation source compared to the second dynamic pressure generation groove, it is possible to suppress the leakage of the sealing target fluid to the leakage side space. 
     It may be preferable that each of the terminating ends of the second dynamic pressure generation grooves is provided with a wall portion which extends from a bottom surface of each of the second dynamic generation grooves toward the sliding surface. According to this preferable configuration, since the sealing target fluid concentrates on the wall portion of the terminating end of the second dynamic pressure generation groove during the relative rotation of the sliding component, it is possible to reliably generate a positive pressure in the vicinity of the terminating end. 
     It may be preferable that each of the second dynamic pressure generation grooves has an extension length than an extension length of each of the first dynamic pressure generation grooves. According to this preferable configuration, it is possible to generate a high positive pressure in the first dynamic pressure generation groove during the high-speed relative rotation of the pair of sliding components and to generate a positive pressure at an early time in the second dynamic pressure generation groove during the low-speed relative rotation. 
     It may be preferable that each of the second dynamic pressure generation grooves extends from the sealing target fluid side to the leakage side to be inclined in a circumferential direction. According to this preferable configuration, it is possible to easily introduce the sealing target fluid into the second dynamic pressure generation groove and to generate a positive pressure at an early time during the relative rotation of the pair of sliding components. 
     It may be preferable that each of the first dynamic pressure generation grooves extends from the leakage side to the sealing target fluid side to be inclined in the circumferential direction and each of the second dynamic pressure generation grooves is inclined along the circumferential direction compared to each of the first dynamic pressure generation grooves. According to this preferable configuration, since it is possible to easily introduce the sealing target fluid into the second dynamic pressure generation groove when the relative rotation of the sliding component starts, it is possible to generate a positive pressure in the second dynamic pressure generation groove at an early time. 
     It may be preferable that each of the second dynamic pressure generation grooves is disposed on an outer radial side of the sliding surface. According to this preferable configuration, since the second dynamic pressure generation groove is disposed at a position in which the circumferential speed of the relative rotation of the sliding component is fast, it is possible to easily introduce the sealing target fluid into the second dynamic pressure generation groove when the relative rotation of the sliding component starts. 
     In addition, the sealing target fluid may be a gas or a liquid or may be a mist in which a liquid and a gas are mixed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a longitudinal sectional view showing an example of a mechanical seal according to a first embodiment of the present invention. 
         FIG.  2    is a view showing a sliding surface of a stationary seal ring from the axial direction in the first embodiment. 
         FIG.  3    is an enlarged view showing the sliding surface of the stationary seal ring from the axial direction in the first embodiment. 
         FIG.  4    is a cross-sectional view taken along the line A-A of  FIG.  2   . 
         FIG.  5    is a cross-sectional view schematically showing a first dynamic pressure generation groove and a second dynamic pressure generation groove in the first embodiment. 
         FIG.  6    is an explanatory diagram showing the movement of a fluid in the first dynamic pressure generation groove and the second dynamic pressure generation groove from the axial direction in the first embodiment. 
         FIGS.  7 A to  7 C  are cross-sectional views schematically showing a separation between sliding surfaces at each relative rotation speed of a pair of sliding components. 
         FIG.  8    is an explanatory diagram schematically showing an example of a mechanical seal according to a second embodiment of the present invention. 
         FIG.  9    is an explanatory diagram schematically showing an example of a mechanical seal according to a third embodiment of the present invention. 
         FIG.  10    is an explanatory diagram schematically showing an example of a mechanical seal according to a fourth embodiment of the present invention. 
         FIG.  11    is an explanatory diagram schematically showing an example of a mechanical seal according to a fifth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Modes for carrying out a sliding component according to the present invention will be described below on the basis of the embodiments. 
     First Embodiment 
     A sliding component according to a first embodiment of the present invention will be described with reference to  FIGS.  1  to  7   . Additionally, in this embodiment, an embodiment in which a sliding component is a mechanical seal will be described as an example. Further, a description will be made such that a sealing target fluid exists in an outer space of the mechanical seal, an atmosphere exists in an inner space, an outer radial side of the sliding component constituting the mechanical seal is a sealing target fluid side (high pressure side), and an inner radial side is a leakage side (low pressure side). Further, for convenience of description, in the drawings, dots may be added to a groove and the like formed on a sliding surface. 
     A mechanical seal for general industrial machines shown in  FIG.  1    is of an inside type that seals a sealing target fluid F tending to leak from the outer radial side toward the inner radial side of the sliding surface and allows an inner space S 1  to communicate with an atmosphere A. Additionally, in this embodiment, an embodiment in which the sealing target fluid F is a high-pressure liquid and the atmosphere A is a gas having a pressure lower than that of the sealing target fluid F is illustrated. 
     The mechanical seal mainly includes a rotating seal ring  20  which is the other annular sliding component provided in a rotary shaft  1  through a sleeve  2  to be rotatable together with the rotary shaft  1  and an annular stationary seal ring  10  which is a sliding component provided in a seal cover  5  fixed to a housing  4  of an attachment target device not to be rotatable and to be movable in the axial direction and when the bellows  7  urges the stationary seal ring  10  in the axial direction, a sliding surface  11  of the stationary seal ring  10  and a sliding surface  21  of the rotating seal ring  20  slide closely with each other. Additionally, the sliding surface  21  of the rotating seal ring  20  is formed as a flat surface and this flat surface is not provided with a concave portion such as a groove. 
     The stationary seal ring  10  and the rotating seal ring  20  are typically formed of SiC (as an example of hard material) or a combination of SiC and carbon (as an example of soft material). However, the present invention is not limited thereto and any sliding material can be applied insofar as it is used as a sliding material for a mechanical seal. It should be noted that the SiC includes a sintered body using boron, aluminum, carbon, or the like as a sintering aid and a material made of two or more types of phases having different components and compositions, examples of which include SiC in which graphite particles are dispersed, reaction-sintered SiC made of SiC and Si, SiC—TiC, and SiC—TiN. As the carbon, resin-molded carbon, sintered carbon, and the like can be used, including carbon in which carbon and graphite are mixed. In addition to the above sliding materials, a metal material, a resin material, a surface modification material (e.g., coating material), a composite material, and the like can also be applied. 
     As shown in  FIGS.  2  and  3   , the rotating seal ring  20  slides relative to the stationary seal ring  10  counterclockwise as indicated by the arrow and in the sliding surface  11  of the stationary seal ring  10 , a plurality of (in the first embodiment, twenty) first dynamic pressure generation grooves  14  are evenly provided in the circumferential direction on the inner radial side and a plurality of (in the first embodiment, twenty) second dynamic pressure generation grooves  9  are evenly provided in the circumferential direction on the outer radial side. 
     Further, a portion other than the first dynamic pressure generation groove  14  and the second dynamic pressure generation groove  9  of the sliding surface  11  is formed as a land  12  which is a flat surface. Specifically, the land  12  includes a portion which is formed between the first dynamic pressure generation grooves  14  adjacent to each other in the circumferential direction, a portion which is formed between the second dynamic pressure generation grooves  9  adjacent to each other in the circumferential direction, and an annular land portion  12   a  which is formed between the first dynamic pressure generation groove  14  and the second dynamic pressure generation groove  9  separated from each other in the radial direction and these portions are arranged on the same plane as the surface (hereinafter, referred to as a flat surface of the land  12 ) of the land  12  on the side of the sliding surface  11 . Additionally, the annular land portion  12   a  will be described later. 
     In the first dynamic pressure generation groove  14 , an inner radial end portion, that is, a relative rotation starting end  14 A communicates with the inner space S 1  and extends in an arc shape from the starting end  14 A toward the outer radial side while being inclined toward the rotation terminating end of the rotating seal ring  20  and an outer radial end portion, that is, a relative rotation terminating end  14 B is closed by a wall portion  14   b  not to communicate with an outer space S 2 . The first dynamic pressure generation groove  14  is formed in an arc shape protruding toward the outer radial side. 
     Specifically, the first dynamic pressure generation groove  14  includes a bottom surface  14   a  which is flat from the starting end  14 A to the terminating end  14 B and is parallel to the flat surface of the land  12 , a wall portion  14   b  which extends vertically toward the sliding surface  11  from the end edge of the terminating end  14 B of the bottom surface  14   a , and side wall portions  14   c  and  14   d  which extend vertically toward the sliding surface  11  from both side edges of the bottom surface  14   a . Additionally, an angle formed by the wall portion  14   b  and the side wall portion  14   c  is an obtuse angle, an angle formed by the wall portion  14   b  and the side wall portion  14   d  is an acute angle, and an acute angle portion  14   f  on the side of the side wall portion  14   d  of the wall portion  14   b  is located on the rotation terminating end side of the rotating seal ring  20  in relation to an obtuse angle portion  14   e  on the side of the side wall portion  14   c  of the wall portion  14   b.    
     The first dynamic pressure generation grooves  14  are arranged so that the plurality of (in the first embodiment, six) first dynamic pressure generation grooves  14  are arranged to overlap each other in the radial direction when viewed from the axial direction. 
     Further, in the second dynamic pressure generation groove  9 , an outer radial end portion, that is, a relative rotation starting end  9 A communicates with the outer space S 2  and extends in an arc shape from the starting end  9 A toward the inner radial side while being inclined toward the rotation terminating end of the rotating seal ring  20  and an inner radial end portion, that is, a relative rotation terminating end  9 B is closed by a wall portion  9   b  not to communicate with the inner space S 1 . The second dynamic pressure generation groove  9  is formed in an arc shape protruding toward the outer radial side. 
     Specifically, the second dynamic pressure generation groove  9  includes a bottom surface  9   a  which is flat from the starting end  9 A to the terminating end  9 B and is parallel to the flat surface of the land  12 , the wall portion  9   b  which extends vertically toward the sliding surface  11  from the end edge of the terminating end  9 B of the bottom surface  9   a , and side wall portions  9   c  and  9   d  which extend vertically toward the sliding surface  11  from both side edges of the bottom surface  9   a . Additionally, an angle formed by the wall portion  9   b  and the side wall portion  9   c  is an obtuse angle, an angle formed by the wall portion  9   b  and the side wall portion  9   d  is an acute angle, and an acute angle portion  9   f  on the side of the side wall portion  9   d  of the wall portion  9   b  is located on the rotation terminating end side of the rotating seal ring  20  in relation to an obtuse angle portion  9   e  on the side of the side wall portion  9   c  of the wall portion  9   b.    
     The second dynamic pressure generation grooves  9  are arranged so that the second dynamic pressure generation grooves  9  which are adjacent to each other in the axial direction overlap each other in the radial direction. 
     Further, the terminating end  9 B of the second dynamic pressure generation groove  9  is disposed to be separated toward the outer radial side in relation to the terminating end  14 B of the first dynamic pressure generation groove  14 . That is, the annular land portion  12   a  which is a land portion having a constant radial width is continuously formed in the circumferential direction between the terminating end  14 B of the first dynamic pressure generation groove  14  and the terminating end  9 B of the second dynamic pressure generation groove  9 . 
     Further, the length of the second dynamic pressure generation groove  9  from the starting end  9 A to the terminating end  9 B, that is, the extension length of the second dynamic pressure generation groove  9  is shorter than the length of the first dynamic pressure generation groove  14  from the starting end  9 A to the terminating end  9 B, that is, the extension length of the first dynamic pressure generation groove  14 . 
     Further, the second dynamic pressure generation groove  9  is inclined along the circumferential direction compared to the first dynamic pressure generation groove  14 . The radial center of the annular land portion  12   a  is provided on the outer radial side in relation to the radial center of the sliding surface  11 . 
     As shown in  FIGS.  4  and  5   , the first dynamic pressure generation groove  14  has a constant depth D 1  from the starting end  14 A to the terminating end  14 B. The depth D 1  of this embodiment is 10 μm. 
     The second dynamic pressure generation groove  9  has a constant depth D 2  from the starting end  9 A to the terminating end  9 B. The depth D 2  of this embodiment is 0.5 μm. 
     The depth D 2  of the second dynamic pressure generation groove  9  is shallower than the depth D 1  of the first dynamic pressure generation groove  14  (D 2 &lt;D 1 ) and the depth D 2  is preferably ½ to 1/20 times the depth D 1 . 
     In addition,  FIG.  5    is a schematic cross-sectional view assuming a state in which each of one set of the first dynamic pressure generation grooves  14  and one set of the second dynamic pressure generation grooves  9  is cut in the longitudinal direction. 
     Next, an operation during the relative rotation between the stationary seal ring  10  and the rotating seal ring  20  will be described with reference to  FIGS.  6  and  7   . First, the sealing target fluid F flows into the second dynamic pressure generation groove  9  during the non-operation of the general industrial machine in which the rotating seal ring  20  is not rotating. Additionally, since the bellows  7  urges the stationary seal ring  10  toward the rotating seal ring  20 , the sliding surfaces  11  and  21  are in the contact state and there is almost no leakage amount of the sealing target fluid F between the sliding surfaces  11  and  21  to the inner space S 1 . 
     At a low speed immediately after the rotating seal ring  20  starts to rotate relative to the stationary seal ring  10 , as shown in  FIGS.  6  and  7 A , the sealing target fluid F in the second dynamic pressure generation groove  9  moves along the rotation direction of the rotating seal ring  20  due to the friction with the sliding surface  21  and the sealing target fluid F of the outer space S 2  is sucked to the second dynamic pressure generation groove  9 . That is, in the second dynamic pressure generation groove  9 , the sealing target fluid F moves from the starting end  9 A toward the terminating end  9 B as indicated by the arrow H 1 . Additionally, the flow of the sealing target fluid F or the atmosphere A of  FIG.  6    is schematically shown without specifying the relative rotation speed of the rotating seal ring  20 . 
     The sealing target fluid F moving toward the terminating end  9 B can increase the pressure in the acute angle portion  9   f  of the wall portion  9   b  of the second dynamic pressure generation groove  9  and the vicinity thereof. That is, a positive pressure is generated in the acute angle portion  9   f  and the vicinity thereof. 
     Since the depth D 2  of the second dynamic pressure generation groove  9  is shallow, a positive pressure is generated in the acute angle portion  9   f  of the wall portion  9   b  of the second dynamic pressure generation groove  9  and the vicinity thereof even when the movement amount of the sealing target fluid F is small at the low rotation speed of the rotating seal ring  20 . 
     The sliding surfaces  11  and  21  are slightly separated from each other by a second force F 2  due to the positive pressure generated in the acute angle portion  9   f  and the vicinity thereof. Accordingly, the sealing target fluid F in the second dynamic pressure generation groove  9  mainly indicated by the arrow H 2  flows between the sliding surfaces  11  and  21 . In this way, since the sealing target fluid F is interposed between the sliding surfaces  11  and  21 , lubricity is improved even at the low-speed rotation and the wear between the sliding surfaces  11  and  21  can be suppressed. Additionally, since the levitation distance between the sliding surfaces  11  and  21  is small, the amount of the sealing target fluid F leaking into the inner space S 1  is small. 
     On the other hand, since the depth D 1  of the first dynamic pressure generation groove  14  is deeper than the depth D 2  of the second dynamic pressure generation groove  9 , the atmosphere A is not sufficiently sealed in the second dynamic pressure generation groove  9  during the low-speed relative rotation of the rotating seal ring  20  and the stationary seal ring  10  and hence a high positive pressure is not generated. As a result, a first force F 1  (not shown in  FIG.  7 A ) caused by the positive pressure generated in the first dynamic pressure generation groove  14  is smaller than a second force F 2 . Thus, the second force F 2  mainly acts to separate the sliding surfaces  11  and  21  from each other during the low-speed rotation of the rotating seal ring  20 . 
     When the relative rotation speed of the rotating seal ring  20  becomes higher, as shown in  FIGS.  6  and  7 B , the atmosphere A in the first dynamic pressure generation groove  14  moves along the rotation direction of the rotating seal ring  20  due to the friction with the sliding surface  21  and the atmosphere A of the inner space S 1  is sucked into the first dynamic pressure generation groove  14 . That is, in the first dynamic pressure generation groove  14 , a large amount of the atmosphere A moves from the starting end  14 A toward the terminating end  14 B as indicated by the arrow L 1 . 
     The atmosphere A moving toward the terminating end  14 B can increase the pressure in the acute angle portion  14   f  of the wall portion  14   b  of the first dynamic pressure generation groove  14  and the vicinity thereof. That is, a positive pressure is generated in the acute angle portion  14   f  and the vicinity thereof. 
     Since the first force F 1  caused by the positive pressure generated in the acute angle portion  14   f  and the vicinity thereof is added, the sliding surfaces  11  and  21  are further separated from each other compared to  FIG.  7 A . Accordingly, the atmosphere A in the first dynamic pressure generation groove  14  mainly indicated by the arrow L 2  flows between the sliding surfaces  11  and  21 . 
     Since the atmosphere A in the first dynamic pressure generation groove  14  indicated by the arrow L 2  acts to push back the sealing target fluid F in the vicinity of the terminating end  14 B of the first dynamic pressure generation groove  14  toward the outer space S 2 , the amount of the sealing target fluid F leaking into the first dynamic pressure generation groove  14  or the inner space S 1  is small. 
     Further, since the sliding surfaces  11  and  21  are further separated from each other compared to  FIG.  7 A , the sealing target fluid F in the second dynamic pressure generation groove  9  is likely to escape between the sliding surfaces  11  and  21  and hence a second force F 2 ′ becomes smaller than that of  FIG.  7 A . 
     Further, at this time, the sealing target fluid F in the periphery of the portion other than the acute angle portion  14   f  of the first dynamic pressure generation groove  14  is sucked into the first dynamic pressure generation groove  14  by the negative pressure generated in the first dynamic pressure generation groove  14  as indicated by the arrow H 3  and this tendency becomes remarkable in the vicinity of the starting end  14 A. The sealing target fluid F sucked into the first dynamic pressure generation groove  14  is returned from the terminating end  14 B of the first dynamic pressure generation groove  14  to the gap between the sliding surfaces  11  and  21 . 
     On the other hand, since the sealing target fluid F in the vicinity of the acute angle portion  14   f  of the first dynamic pressure generation groove  14  has a high pressure as described above, the sealing target fluid does not almost enter the first dynamic pressure generation groove  14  while being located at the land  12  as indicated by the arrow H 4 . 
     As described above, since the first dynamic pressure generation grooves  14  are arranged so that the plurality of first dynamic pressure generation grooves  14  overlap each other in the radial direction, the sealing target fluid F moving to the land  12  from the acute angle portion  14   f  of the first dynamic pressure generation groove  14  adjacent to a certain first dynamic pressure generation groove  14  at the rotation starting end side of the rotating seal ring  20  is sucked by the negative pressure generated in the first dynamic pressure generation groove  14 . Accordingly, it is possible to prevent the sealing target fluid F from leaking to the inner space S 1 . 
     When the relative rotation speed of the rotating seal ring  20  further increases and reaches a high-speed rotation (that is, a steady operation state), as shown in  FIGS.  6  and  7 C , the inflow amount (see the arrow L 1 ′ of  FIG.  7 C ) of the atmosphere A sucked into the first dynamic pressure generation groove  14  further increases so that a high positive pressure is generated and the first force F 1 ′ increases. As a result, the sliding surfaces  11  and  21  are separated from each other by a long levitation distance Y compared to  FIG.  7 B . Accordingly, a larger amount of the atmosphere A in the first dynamic pressure generation groove  14  indicated by the arrow L 2 ′ flows between the sliding surfaces  11  and  21  compared to  FIG.  7 B . 
     Since the atmosphere A in the first dynamic pressure generation groove  14  indicated by the arrow L 2 ′ acts to push back the sealing target fluid F in the vicinity of the terminating end  14 B of the first dynamic pressure generation groove  14  toward the outer space S 2 , the amount of the sealing target fluid F leaking into the first dynamic pressure generation groove  14  or the inner space S 1  is small. 
     In this embodiment, when the levitation distance Y increases due to the high-speed rotation of the rotating seal ring  20 , the sealing target fluid F in the second dynamic pressure generation groove  9  is likely to escape between the sliding surfaces  11  and  21  and the positive pressure generated in the second dynamic pressure generation groove  9  becomes negligibly small. Thus, the first force F 1  mainly acts to separate the sliding surfaces  11  and  21  from each other at the high-speed rotation of the rotating seal ring  20 . 
     As described above, since the depth D 2  of the second dynamic pressure generation groove  9  is shallower than the depth D 1  of the first dynamic pressure generation groove  14 , the second force F 2  caused by the positive pressure generated by the sealing target fluid F in the second dynamic pressure generation groove  9  mainly acts to separate the sliding surfaces  11  and  21  from each other during the low-speed relative rotation of the rotating seal ring  20 . As the relative rotation speed of the rotating seal ring  20  increases, the first force F 1  caused by the positive pressure generated by the atmosphere A in the first dynamic pressure generation groove  14  suddenly increases. Then, when the relative rotation speed of the rotating seal ring  20  becomes sufficiently high, the first force F 1  becomes larger than the second force F 2  and the first force F 1  mainly acts to separate the sliding surfaces  11  and  21  from each other. Accordingly, it is possible to suppress the wear between the sliding surfaces  11  and  21  from the low speed to the high speed of the relative rotation of the stationary seal ring  10  and the rotating seal ring  20 . 
     Further, since a gap formed between the sliding surfaces  11  and  21  increases during the high-speed relative rotation of the rotating seal ring  20 , the positive pressure is not easily generated in the second dynamic pressure generation groove  9 . Accordingly, the first force F 1  caused by the positive pressure generated in the first dynamic pressure generation groove  14  mainly acts to stably separate the sliding surfaces  11  and  21  from each other. Thus, it is possible to suppress the wear by separating the sliding surfaces  11  and  21  from each other from the start of the relative rotation of the stationary seal ring  10  and the rotating seal ring  20  to the high-speed rotation. 
     Further, since the second dynamic pressure generation groove  9  communicates with the outer space S 2 , it is possible to easily introduce the sealing target fluid F into the second dynamic pressure generation groove  9  and to generate a positive pressure at an early time. 
     Further, since the annular land portion  12   a  having a constant radial width is continuously provided in the circumferential direction between the terminating end  14 B of the first dynamic pressure generation groove  14  and the terminating end  9 B of the second dynamic pressure generation groove  9 , it is possible to suppress the sealing target fluid F between the sliding surfaces  11  and  21  from flowing to the inner space S 1  by the annular land portion  12   a  when the second force F 2  caused by the positive pressure generated in the second dynamic pressure generation groove  9  separates the sliding surfaces  11  and  21  from each other. Further, it is possible to suppress the leakage of the sealing target fluid F to the inner space S 1  in a stationary state in which the stationary seal ring  10  and the rotating seal ring  20  do not rotate relative to each other. 
     Further, since the radial center of the annular land portion  12   a  is disposed to be closer to the sealing target fluid than the radial center of the sliding surface  11 , it is possible to ensure the long extension length of the first dynamic pressure generation groove  14  and to arrange a large number of the first dynamic pressure generation grooves  14  side by side. Accordingly, since the first dynamic pressure generation groove  14  serves as a main dynamic pressure generation source compared to the second dynamic pressure generation groove  9 , it is possible to suppress the leakage of the sealing target fluid F to the inner space S 1 . Additionally, the radial center of the annular land portion  12   a  is the radial position obtained by adding the outer diameter and the inner diameter of the annular land portion  12   a  and dividing the result by 2 and the radial center of the sliding surface  11  is the radial position obtained by adding the outer diameter and the inner diameter of the sliding surface  11  and dividing the result by 2. 
     Further, since the depth of the second dynamic pressure generation groove  9  is set to a dimension that reliably reduces the influence of the positive pressure generated in the second dynamic pressure generation groove  9  during the high-speed rotation of the rotating seal ring  20 , it is possible to reliably separate the sliding surfaces  11  and  21  by the first force F 1  caused by the positive pressure generated in the first dynamic pressure generation groove  14 . 
     Further, since the terminating end  9 B of the second dynamic pressure generation groove  9  is provided with the wall portion  9   b  which extends from the bottom surface  9   a  toward the sliding surface  11 , the sealing target fluid F concentrates on the acute angle portion  9   f  of the wall portion  9   b  of the terminating end  9 B of the second dynamic pressure generation groove  9  during the relative rotation of the stationary seal ring  10  and the rotating seal ring  20  and hence the positive pressure can be reliably generated in the vicinity of the terminating end  9 B. 
     Further, since the extension length of the second dynamic pressure generation groove  9  is shorter than that of the first dynamic pressure generation groove  14 , it is possible to generate a high positive pressure by the first dynamic pressure generation groove  14  during the high-speed relative rotation of the stationary seal ring  10  and the rotating seal ring  20  and to generate a positive pressure by the second dynamic pressure generation groove  9  at an early time during the low-speed relative rotation. 
     Further, since the second dynamic pressure generation groove  9  extends from the starting end  9 A toward the inner radial side while being inclined toward the rotation terminating end of the rotating seal ring  20 , it is possible to easily introduce the sealing target fluid F into the second dynamic pressure generation groove  9  and to generate a positive pressure at an early time during the relative rotation of the stationary seal ring  10  and the rotating seal ring  20 . 
     Further, since the first dynamic pressure generation groove  14  extends from the starting end  14 A toward the outer radial side while being inclined toward the rotation terminating end of the rotating seal ring  20  and the second dynamic pressure generation groove  9  is inclined along the circumferential direction compared to the first dynamic pressure generation groove  14 , it is possible to easily introduce the sealing target fluid F into the second dynamic pressure generation groove  9  when the relative rotation of the stationary seal ring  10  and the rotating seal ring  20  starts and to generate a positive pressure by the second dynamic pressure generation groove  9  at an early time. 
     Further, since the second dynamic pressure generation groove  9  is disposed on the outer radial side of the sliding surface  11 , the second dynamic pressure generation groove  9  is disposed at a position in which the circumferential speed of the relative rotation of the rotating seal ring  20  is fast and hence the sealing target fluid F can be easily introduced into the second dynamic pressure generation groove  9  when the relative rotation of the stationary seal ring  10  and the rotating seal ring  20  starts. 
     Further, since the terminating end  14 B of the first dynamic pressure generation groove  14  and the terminating end  9 B of the second dynamic pressure generation groove  9  are arranged not to overlap each other in the radial direction and the terminating end  14 B and the terminating end  9 B are separated from each other, a positive pressure is generated in the vicinity of the terminating end  9 B of the second dynamic pressure generation groove  9 , the sealing target fluid F moving between the sliding surfaces  11  and  21  does not easily flow into the second dynamic pressure generation groove  9 , and hence the sealing target fluid F does not easily leak to the inner space S 1 . Further, since the first force F 1  of the first dynamic pressure generation groove  14  and the second force F 2  of the second dynamic pressure generation groove  9  are generated at a non-overlapping position in the radial direction, a well-balanced force can be applied between the sliding surfaces  11  and  21  to separate them. 
     In addition, the depth D 1  and the depth D 2  are not limited to those of the first embodiment and may be freely changed if the depth D 2  is shallower than the depth D 1 . 
     Second Embodiment 
     Next, a mechanical seal according to a second embodiment of the present invention will be described with reference to  FIG.  8   . In addition, the description of the configuration that is the same as that of the above-described embodiment and is duplicated will be omitted. Further, in  FIG.  8   , the length from the starting end to the terminating end of the first dynamic pressure generation groove is depicted to be shorter than the actual length. 
     As shown in  FIG.  8   , a bottom surface  140   a  of a first dynamic pressure generation groove  140  of a stationary seal ring  100  of this second embodiment is inclined so that the axial dimension becomes smaller from the starting end  140 A toward the terminating end  140 B. 
     The depth D 2  of the second dynamic pressure generation groove  9  is deeper than the depth in the vicinity of the terminating end  140 B of the first dynamic pressure generation groove  140 , but is shallower than the depth D 3  of the deepest position (deepest portion) of the first dynamic pressure generation groove  140  (D 2 &lt;D 3 ). 
     In this way, since the depth D 2  of the second dynamic pressure generation groove  9  is shallower than the depth D 3  of the deepest portion of the first dynamic pressure generation groove  140 , the second force caused by the positive pressure generated by the sealing target fluid F in the second dynamic pressure generation groove  9  mainly acts to separate the sliding surfaces  11  and  21  from each other during the low-speed relative rotation of the rotating seal ring  20 . Further, the positive pressure is easily generated in the vicinity of the terminating end  140 B of the first dynamic pressure generation groove  140 . 
     Third Embodiment 
     Next, a mechanical seal according to a third embodiment of the present invention will be described with reference to  FIG.  9   . In addition, the description of the configuration overlapping with the same configuration as that of the above-described embodiment will be omitted. Further, in  FIG.  9   , the length from the starting end to the terminating end of the first dynamic pressure generation groove is depicted to be shorter than the actual length. 
     As shown in  FIG.  9   , a bottom surface  240   a  of a first dynamic pressure generation groove  240  of a stationary seal ring  101  of this third embodiment is formed in a step shape from a starting end  240 A to a terminating end  240 B. 
     Specifically, a deep bottom surface  240   c  having a large axial dimension is provided on the side of the starting end  240 A of the bottom surface  240   a  and a shallow bottom surface  240   d  having a small axial dimension is provided on the side of the terminating end  240 B of the bottom surface  240   a  with the longitudinal center of the first dynamic pressure generation groove  240  as a boundary. Further, an intermediate wall portion  240   e  is provided to extend vertically from the end edge of the deep bottom surface  240   c  toward the shallow bottom surface  240   d  and a wall portion  240   b  is provided to extend vertically from the end edge of the shallow bottom surface  240   d  toward the sliding surface  11 . 
     The depth D 2  of the second dynamic pressure generation groove  9  is shallower than the depth D 4  (specifically, the depth of the deepest position (deepest portion) of the first dynamic pressure generation groove  240 ) of the first dynamic pressure generation groove  240 . Additionally, a case in which the bottom surface  240   a  of the first dynamic pressure generation groove  240  is formed in a two-stage step shape has been illustrated, but the present invention is not limited thereto. For example, three stages or more may be used. 
     In addition, in the first embodiment to the third embodiment, the second dynamic pressure generation groove  9  has a constant depth D 2  from the starting end  9 A to the terminating end  9 B, but the present invention is not limited thereto. For example, the bottom surface may be inclined so that the depth gradually becomes shallow from the starting end toward the terminating end or may be formed in a step shape or the like. That is, the depth of the deepest portion of the second dynamic pressure generation groove may be formed to be shallower than the depth of the deepest portion of the first dynamic pressure generation groove. 
     Fourth Embodiment 
     Next, a mechanical seal according to a fourth embodiment of the present invention will be described with reference to  FIG.  10   . In addition, the description of the configuration overlapping with the same configuration as that of the above-described embodiment will be omitted. 
     As shown in  FIG.  10   , a terminating end  340 B of a first dynamic pressure generation groove  340  of a stationary seal ring  102  of this fourth embodiment overlaps the terminating end  9 B of the second dynamic pressure generation groove  9  in the radial direction. 
     Accordingly, since the first force of the first dynamic pressure generation groove  340  and the second force of the second dynamic pressure generation groove  9  are generated at the radially overlapping position, it is possible to largely separate the sliding surfaces  11  and  21  in a short time and hence to quickly exhibit high lubricity between the sliding surfaces  11  and  21 . 
     Fifth Embodiment 
     Next, a mechanical seal according to a fifth embodiment of the present invention will be described with reference to  FIG.  11   . In addition, the description of the configuration overlapping with the same configuration as that of the above-described embodiment will be omitted. 
     As shown in  FIG.  11   , in a first dynamic pressure generation groove  440  of a stationary seal ring  103  of this fifth embodiment, side wall portions  440   c  and  440   d  extend toward a terminating end  440 B to approach each other and the terminating end  440 B is tapered. Further, in a second dynamic pressure generation groove  190 , side wall portions  190   c  and  190   d  extend toward a terminating end  190 B to approach each other and the terminating end  190 B is tapered. 
     Accordingly, the positive pressure is likely to be generated in the vicinity of the terminating end  440 B of the first dynamic pressure generation groove  440  and the vicinity of the terminating end  190 B of the second dynamic pressure generation groove  190  during the relative rotation of the rotating seal ring  20 . 
     Although the embodiments of the present invention have been described with reference to the drawings, the specific configuration is not limited to these embodiments and is included in the present invention even if there are changes or additions within the scope of the present invention. 
     For example, in the above-described embodiments, as the sliding component, the mechanical seal for general industrial machines has been illustrated, but other mechanical seals for automobiles, water pumps, and the like may be used. Further, the present invention is not limited to the mechanical seal and may be a sliding component other than the mechanical seal such as a slide bearing. 
     Further, in the above-described embodiments, an example in which the first dynamic pressure generation groove and the second dynamic pressure generation groove are provided in the stationary seal ring has been illustrated, but the first dynamic pressure generation groove and the second dynamic pressure generation groove may be provided in the rotating seal ring. 
     Further, the sealing target fluid side has been described as the high pressure side and the leakage side has been described as the low pressure side. However, the sealing target fluid side may be the low pressure side, the leakage side may be the high pressure side, and the sealing target fluid side and the leakage side may have substantially the same pressure. 
     Further, in the above-described embodiments, an embodiment of the inside type that seals the sealing target fluid F tending to leak from the outer radial side of the sliding surface toward the inner radial side thereof has been illustrated, but the present invention is not limited thereto. For example, an outside type that seals the sealing target fluid F tending to leak from the inner radial side of the sliding surface toward the outer radial side thereof may be used. 
     Further, as shown in  FIG.  2   , the same number of the first dynamic pressure generation grooves and the second dynamic pressure generation grooves are provided on the sliding surface  11  of the stationary seal ring  10 , but the present invention is not limited thereto. For example, the number may not be the same. 
     Further, a case has been illustrated in which the second dynamic pressure generation groove has a shorter extension length than that of the first dynamic pressure generation groove and is inclined along the circumferential direction and the radial center of the annular land portion  12   a  is on the outer radial side in relation to the radial center of the sliding surface  11 , but the present invention is not limited thereto. For example, the first dynamic pressure generation groove may have a shorter extension length than that of the second dynamic pressure generation groove and may be inclined along the circumferential direction. 
     Further, a case in which the first dynamic pressure generation groove communicates with the inner space has been illustrated, but the present invention is not limited thereto. For example, the first dynamic pressure generation groove may not communicate therewith if the dynamic pressure can be generated. 
     Further, a case in which the second dynamic pressure generation groove communicates with the outer space has been illustrated, but the present invention is not limited thereto. For example, the second dynamic pressure generation groove may not communicate therewith if the dynamic pressure can be generated. 
     Further, a case has been illustrated in which the annular land portion  12   a  is provided between the first dynamic pressure generation groove and the second dynamic pressure generation groove and the first dynamic pressure generation groove and the second dynamic pressure generation groove are arranged to be separated from each other in the radial direction, but the present invention is not limited thereto. For example, the terminating end of the second dynamic pressure generation groove may be disposed on the inner radial side in relation to the terminating end of the first dynamic pressure generation groove and the terminating end of the first dynamic pressure generation groove and the terminating end of the second dynamic pressure generation groove may overlap each other in the circumferential direction. 
     Further, in this embodiment, the sealing target fluid F has been described as the high-pressure liquid, but the present invention is not limited thereto. For example, the sealing target fluid may be a gas or a low-pressure liquid or may be a mist in which a liquid and a gas are mixed. 
     Further, in this embodiment, the leakage side fluid has been described as the atmosphere A corresponding to the low-pressure gas, but the present invention is not limited thereto. For example, the sealing target fluid may be a liquid or a high-pressure gas or may be a mist in which a liquid and a gas are mixed. 
     Further, in the second and third embodiments, the depth of the dynamic pressure generation groove is defined as the depth of the deepest portion, but the depth of the dynamic pressure generation groove may be any depth as long as the depth substantially contributes to the generation of positive pressure. 
     REFERENCE SIGNS LIST 
     
         
         
           
               9  Second dynamic pressure generation groove 
               10  Stationary seal ring (sliding component) 
               11  Sliding surface 
               12   a  Annular land portion (land portion) 
               14  First dynamic pressure generation groove 
               14   a  Bottom surface 
               14   b  Wall portion 
               20  Rotating seal ring (opposed sliding component) 
               21  Sliding surface 
             A Atmosphere 
             D 1 , D 2  Depth 
             F Sealing target fluid 
             F 1  First force 
             F 2  Second force 
             S 1  Inner space 
             S 2  Outer space 
             Y Levitation distance