Patent Publication Number: US-6987437-B2

Title: Electromagnetic actuator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based upon, claims the benefit of priority of, and incorporates by reference the contents of prior Japanese Patent Application No. 2002-96839 filed Mar. 29, 2002 and No. 2002-370696 filed Dec. 20, 2002. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to electromagnetic actuators, and more specifically, to an electromagnetic actuator of which a housing of the movable core constitutes part of the magnetic circuit. 
   2. Description of the Related Art 
   As disclosed in Japanese Patent Laid-Open Publication No. 2001-332419, a known conventional electromagnetic actuator is equipped with a housing for holding a movable core so that it may freely reciprocate back and forth and a stator having an attraction part that exerts a magnetic attractive force on the movable core in either of the reciprocating directions. The stator is configured together with the movable core to form a magnetic circuit of magnetic flux produced by running electric current in the coil. 
   In the above type electromagnetic actuator however, the housing and the movable core slide directly in contact with each other, and therefore the wear of their sliding faces is a problem. 
   The inventors have found that Ni—P plating or Ni—P plating plus heat treatment on the sliding face of the movable core and gas soft nitriding of the sliding face of the housing, both for improving wear-resistance of the sliding faces, causes problems. Such an electromagnetic actuator equipped with a linear electromagnetic valve mechanism having the above surface-treated sliding faces may be employed in a hydraulic control valve that controls the hydraulic pressure of operation oil supplied to the hydraulic pressure control device of an automatic transmission of a vehicle. Then, although the operation oil pressure controlled by a coil current is within a demanding tolerance, the position of the movable core determined by the same coil current varies depending on the moving direction of the movable core. Additionally, a relatively large hysteresis (attractive force hysteresis) is observed. 
   As a result of an intensive study on the causes for such hysteresis, the inventors have discovered that a 1–2 μm thick porous layer is formed in the surface of the gas soft nitrided sliding face and that this porous layer causes the relatively large hysteresis. 
   In addition, if the electromagnetic actuator is used for a long time, the porous layer peels off, and sliding problems arise. In the electromagnetic valve disclosed in Japanese Patent Laid-Open Publication No. Hei. 4-221810, the movable ferrite core is nitrided (by tufftride treatment) to harden its surface and its surface roughness is raised by wrapping, in order to reduce friction with the guide material. Removal of the porous layer at random, however, will lower productivity. Through further investigation into this problem, the inventors have discovered that the amount of wear decreases significantly if surface roughness is 3.2 Rz or lower, as shown in  FIG. 5 , which describes the relationships between surface roughness and the amount of wear. 
   SUMMARY OF THE INVENTION 
   The present invention has been made with reference to such investigation, and an object of the present invention is to provide an electromagnetic actuator that can extend its life of use by hardening at least either of the sliding faces and to improve productivity by optimizing the level of surface roughness. 
   According to one aspect of the present invention, an electromagnetic actuator includes a movable core, a housing for holding the movable core so that the core reciprocates or shuttles freely, an attraction part for exerting on the movable core a magnetic force pulling the movable core in one of the reciprocating directions, and a stator for forming a magnetic circuit along with the movable core. Further, at least one of the sliding faces of the housing and the movable core in contact with each other is subjected to gas soft nitriding, salt-bat soft nitriding, sulfo-nitriding, or nitriding treatment. Finally, a surface roughness of the treated face is controlled to be within a prescribed range. 
   According to the above configuration, since the sliding face that has been nitrided by gas soft nitriding, salt-bath soft nitriding, sulfo-nitriding, or nitriding treatment is hardened and its surface roughness is controlled to be within a predetermined range, wear of the other sliding face can be reduced. Eventually, the wear of both sliding faces decreases. Then, the hysteresis becomes smaller, and in particular when such a device is adopted in a linear control type electromagnetic valve, the operation performance can be held high. 
   In the present invention, the surface roughness is preferably 3.2 Rz or lower. To keep the roughness level at 3.2 Rz or lower, the porous layer is removed after the surface has been subjected to gas soft nitriding, salt-bath soft nitriding, sulfo-nitriding, or nitriding treatment. Otherwise, the surface roughness is made 3.2 Rz or lower in advance before the gas soft nitriding, salt-bath soft nitriding, sulfo-nitriding, or nitriding treatment. The latter method is advantageous in that there is no need to remove any surface porous layer after gas soft nitriding, salt-bath soft nitriding, sulfo-nitriding, or nitriding treatment. Furthermore, since the surface roughness of the nitrided sliding face is optimized, the electromagnetic actuator can be manufactured with a minimum number of steps, and thereby productivity can be raised. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a cross-sectional view of a flow control device equipped with an electromagnetic actuator according to an embodiment of the invention; 
       FIG. 2  is an enlarged cross-sectional view of the major part of a movable core and a stator core; 
       FIG. 3  is an enlarged cross-sectional view of a housing; 
       FIG. 4  is a graph showing the experimental data of the relationship between wear of the counterpart material and hysteresis with respect to the surface roughness of the sliding face hardened by gas soft nitriding; and 
       FIG. 5  is another graph showing the experimental data of the relationship between wear of the counterpart material and hysteresis with respect to the surface roughness of the sliding face hardened by gas soft nitriding. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   Now the preferred embodiments of the invention will be described with reference to the accompanying drawings.  FIG. 1  is a cross-sectional view of a flow control device equipped with an electromagnetic actuator according to an embodiment of the invention. This flow control device is, for example, a spool type hydraulic pressure control valve that controls the hydraulic pressure of operation oil supplied to the hydraulic pressure control device of an automatic transmission of a vehicle or the like. 
   Referring now to  FIG. 1 , the flow control device includes an electromagnetic actuator  100  and a valve unit  200 . 
   (1) Electromagnetic Actuator  100   
   The electromagnetic actuator  100  constitutes a linear solenoid, equipped with a stator  10  and a cylindrical movable core (plunger)  30 . 
   The stator  10  has a hollow stator core  11  that is made of magnetic material and is cylindrically shaped with a protruding portion at one end, much like a derby hat. The stator core  11  has a housing  12  that holds a movable core  30  so that the core  30  reciprocates freely in the lateral direction in  FIG. 1 , and an attraction part  13 . This attraction part  13  extending from the housing  12  toward the valve unit  200  has an inner diameter smaller than the housing  12  and exerts a magnetic attractive force to the movable core  30 . 
   Referring now to  FIG. 2 , a non-magnetic layer  12   a  is formed in the surface of the housing  12 . Referring to  FIG. 3 , the non-magnetic layer  12   a  is formed by subjecting a raw material of the stator core  11 , for example, a ferrite core  12   b  having a hardness of about 1000 Hv to gas soft nitriding treatment (put the stator core  11  in a furnace of a nitrogen or ammonia atmosphere, and hold therein for a predetermined time, for example, 85 minutes, at a predetermined temperature, for example, 580° C. or lower) to form about a 7–20 μm thick nitride layer  12   d  of a hardness of about 1000 Hv in the surface of the ferrite core  12   b,  and then by removing the top surface of 1–2 μm thick porous layer  12   c  (layer above the chain double-dashed line in  FIG. 3 ). Its surface roughness is controlled to be 3.2 Rz or lower. 
   The boundary between the housing  12  and the attraction part  13  is made thin, forming a magneto-resistance part  14  that ensures a magnetic attractive force of the attraction part  13  by limiting the amount of magnetic flux directed from the attraction part  13  to the housing  12 . 
   A resin-molded component  15  is fastened by insertion molding to a concave portion  11   a  in the outer face of the stator core  11 . A coil  16  is buried in this resin-molded component  15  to receive electric power from the outside via a connector (not shown). The resin-molded component  15  surrounds the attraction part  13 , while its portion facing the movable core  30  constitutes a stopper  17  that restricts the movement of the movable core  30  in the direction toward the valve unit  200 . 
   The stator core  11  and the resin-molded component  15  are housed in a yoke  18  that is made of magnetic material and is cylindrically shaped with a bottom. The open-end  18   a  of the yoke  18  is swaged, with the end face  15   a  of the resin-molded component  15  on the valve side being mated with the end face  50   a  of the housing (sleeve)  50  of the valve unit  200  on the resin-molded component side. The electromagnetic actuator  100  is thereby integrated with the valve unit  200 . 
   A non-magnetic layer  30   a  is formed in the surface of the movable core  30 , as shown in  FIG. 2 . The non-magnetic layer  30   a  is formed by subjecting a raw material of the magnetic movable core  30 , for example, pure iron  30   b  to Ni—P plating, and a heat treatment to raise its surface hardness up to around 900 Hv. This heat treatment is not necessary. 
   In the electromagnetic actuator  100  above, if a current runs in the coil  16 , a magnetic flux runs in the magnetic circuit composed of the yoke  18 , the stator core  11  and the movable core  30  and pulls the movable core  30  leftward in  FIG. 1  by a magnetic attractive force of the attraction part  13  of the stator core  11 . The leftward movement of the movable core  30  is limited by the stopper  17 . If the current to the coil  16  is shut down, the magnetic attractive force disappears, and the movable core  30  moves rightward in  FIG. 1  due to a spring  60 . This aspect will be described later. 
   When the movable core  30  reciprocates, the non-magnetic layer  30   a  of the movable core  30  and the non-magnetic layer  12   a  of the housing  12  slide in contact with each other. 
   (2) Valve Unit  200   
   The valve unit  200  includes a spool  40  whose axis lies in the line extending from the axial line of the movable core  30 , a housing  50  that holds the spool  40  so that the spool  40  freely reciprocates in the lateral direction in  FIG. 1 , and a spring  60  that is installed in the end of the housing  50  and constantly pushes (biases) the spool  40  toward the movable core  30 . The spool  40  disposed between the movable core  30  and the spring  60  has a rod  41  that projects into the electromagnetic actuator  100  and constantly contacts an end face of the movable core  30 , a small land  42  axially extending from the rod  41 , a small junction  43  whose diameter is smaller than that of the small land  42  for forming a feedback area (room), an input side large land  44  axially extending from the small junction  43 , an output side small junction  45  axially extending from the large land  44  for forming an output area (room), a drain side large land  46  axially extending from the small junction  45 , and a spring seat  47  axially extending from the large land  46 . 
   The housing  50  has a feedback port  51  that opens up beside the outer face of the small junction  43  for forming the feedback room, an input port  52  that opens up beside the outer face of the input side large land  44 , an output port  53  that opens up beside the outer face of the small junction  45  for forming the output room, and a drain port  54  that opens up beside the outer face of the drain side large land  46 . The input port  52  is a port into which operation oil supplied from a tank (not shown) flows. The output port  53  is a port from which operation oil is supplied to an engaging device of the automatic transmission (not shown). The feedback port  51  is linked with the output port  53  in a certain place (not shown), and serves as a port through which part of the operation oil flowing from the output port  53  is introduced. The drain port  54  is a port through which operation oil is sent to the tank. 
   In the above configured valve unit  200 , it is possible that no magnetic attractive force acts on the movable core  30 , or, that is, the spool  40  does not receive a force from the movable core  30  when there is no current running in the coil  16  of the electromagnetic actuator  100 . Instead, the spool  40  receives a force toward the movable core  30  applied by the spring  60  and a force toward the spring  60  applied by the feedback operation oil of the feedback port  51 , based on the difference in area between the end of the input side large land  44  and that of the small land  42 . Then the spool  40  is situated in the position where the two forces balance. The axial length of the housing wall  55  facing the input side large land  44  between the input port  52  and the output port  53 , or the seal length, is shorter than a seal length provided when a current runs in the coil and the hydraulic pressures of the feedback operation oil are equal to each other. Thus the amount of operation oil flowing from the input port  52  to the output port  53  is large. Meanwhile, the axial length of the housing wall  56  facing the drain side large land  46  between the output port  53  and the drain port  54 , or the seal length, is longer than that provided when a current runs in the coil and the hydraulic pressures of the feedback operation oil are equal to each other; and the amount of operation oil flowing from the output port  53  to the drain port  54  is small. 
   Since a magnetic attractive force works on the movable core  30  while a current is running in the coil  16 , the spool  40  receives a force from the movable core  30  in addition to the forces of the spring  60  and the feedback operation oil. The spool  40  is situated in a position where the force of the spring  60  becomes equal to the sum of the force of the feedback operation oil and the force of the movable core  30 . Then the axial length of the housing wall  55  facing the input side large land  44  between the input port  52  and the output port  53 , or the seal length, is longer than that provided when no current runs in the coil and the hydraulic pressures of feedback operation oil are equal to each other; and the amount of operation oil flowing from the input port  52  to the output port  53  is small. 
   At the same time, the axial length of the housing wall  56  facing the drain side large land  46  between the output port  53  and the drain port  54 , or the seal length, is shorter than that provided when no current runs in the coil and the hydraulic pressures of the feedback operation oil are equal to each other; and the amount of operation oil flowing from the output port  53  to the drain port  54  is large. 
   Meanwhile, when a current is running in the coil  16 , the magnitude of magnetic attractive force acting on the movable core  30  is proportional to the magnitude of the current. Thus, when the hydraulic pressures of feedback operation oil are the same, the current is larger, the spool  40  is closer to the spring  60 , the operation oil flowing from the input port  52  to the output port  53  is less, and the operation oil flowing from the output port  53  to the drain port  54  is greater. 
   As mentioned above, the non-magnetic layer  30   a  of a hardness of about 900 Hv is formed in the surface of the raw material  30   b  for the movable core  30  by applying Ni—P plating and, if necessary, heat treatment. The nitride layer  12   d  of a hardness of about 1000 Hv is formed in the surface of the raw material  12   b  for the housing  12  of the stator core  11  by applying gas soft nitriding. After this, the surface porous layer  12   c  is removed to form the non-magnetic layer  12   a,  and its surface roughness is controlled to be 3.2 Rz or lower. Methods for removing the porous layer include shot blasting in which small steel balls are accelerated onto the face to be hardened and the wrap finishing that polishes the target surface with abrasives. 
     FIG. 4  is a graph demonstrating the experimental data of the relationship between the wear of the counterpart material and hysteresis with respect to surface roughness of the sliding face hardened by gas soft nitriding. This wear of the counterpart material is the wear of the movable core  30  that has reciprocated 4 million times simulating 200 million meters of vehicle travel. 
   Referring to  FIG. 4 , the wear of the counterpart material  30  for the sliding face  12   a  produced by removing part of the porous layer  12   c  is less than that of the counterpart material  30  of the sliding face  12   d  from which the porous layer  12   c  has not yet been removed. However, the sliding face  12   d  still having the porous layer  12   c  meets the prescribed tolerance, for example, 12 μm, with a sufficient margin. When the clearance between the counterpart material  30  and the sliding face  12   d  or  12   a  hardened by gas soft nitriding was 30 μm, the hysteresis was about 6N when the surface roughness was 0.2 Rz and 1 Rz. When the surface roughness was 2 Rz, the hysteresis was about 5N. This indicates that the hysteresis does not become small when the surface roughness is made high. 
   According to the present embodiment, since the housing  12  of the stator core  11  is subjected to gas soft nitriding treatment, the hardness of the sliding face  12   d  is raised and the wear of the sliding face  30   a  of the counterpart material  30  can be reduced. When the surface roughness is made at 3.2 Rz or lower by removing the porous layer  12   c,  the attractive force hysteresis can be made smaller. By removing the porous layer, sliding problems due to peel-off of the porous layer  12   c  can be prevented. 
   In the above embodiment, the housing  12  of the stator core  11  is subjected to gas soft nitriding treatment, and its porous layer is removed. The movable core  30 , instead, may be subjected to the same treatment. The surface roughness is not limited by the method chosen for removing the porous layer. Because the porous layer resulting from soft gas nitriding or sulfo-nitriding treatment is 1–2 μm thick, the roughness of the sliding face can be held at 3.2 Rz or lower by making the roughness of the sliding face at 3.2 Rz or lower prior to such surface hardening and then nitriding. Then, there is no need for removing the porous layer, and thereby productivity improves significantly. 
   Instead of gas soft nitriding treatment, salt-bath soft nitriding, sulfo-nitriding, or nitriding treatment can also provide a sliding face of a high hardness, low friction coefficient and little wear. In the salt-bath soft nitriding treatment, the steel material is immersed in a salt-bath held at about 500–600° C. to incorporate N and C therein for producing a nitride or carbide surface layer of a high hardness and low friction coefficient. In the sulfo-nitriding treatment, the top surface takes in N and C, or N, S and C to form a top surface of a high hardness and low friction coefficient. In the sulfo-nitriding treatment, since an iron sulfide layer of self-lubrication capability is formed in the surface, the resulting surface has a friction coefficient smaller than that of the surface obtained by the soft nitriding process. The nitriding treatment takes several times longer than the gas soft nitriding, salt-bath soft nitriding and sulfo-nitriding treatment. However, it can also produce a nitride surface layer with a high hardness and a low friction coefficient. 
   According to the present invention, one of the sliding faces is subjected to gas soft nitriding, salt-bath soft nitriding, sulfo-nitriding, or nitriding treatment. Then the hardness of the sliding face that has been subjected to such nitriding treatment is raised. In addition, the wear of the other sliding face can be reduced because the surface roughness is controlled to be within a prescribed range, and eventually the wear of both sliding faces can be reduced. As a result, the hysteresis becomes smaller and, in particular, when it is adopted in a linear control type electromagnetic valve, the operational performance can be held high. Because the roughness of a nitrided sliding surface is optimized, the electromagnetic actuator can be manufactured in a minimum number of steps and therefore productivity is improved. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.