Abstract:
A hydraulic piston apparatus includes a piston having a piston body movable along an axis, the piston body having a substantially cylindrical shape, a radius, and an outer wall extending substantially parallel to the axis, the outer wall having at least one portion that defines a cavity having a first width and a second width, the second width being greater than the first width and disposed radially inward from the first width. The cavity is configured to apply a retaining force to an attachment during use.

Description:
BACKGROUND 
       [0001]    Certain metal pistons used in hydraulic applications include a polymer based layer applied to an exterior surface of the piston to provide a high-tolerance and low friction seal between the outer surface of the piston body and the interior surface of the hydraulic cylinder. Depending upon the operating conditions and other factors, these polymer layers physically separate from the underlying piston at times. As a result, certain piston bodies have been designed to include one or more annular grooves formed into the exterior surface of the piston prior to applying the polymer material. Though the known annular grooves can decrease some level of movement of the polymer layer, the layer can still slip on, or separate from, the piston depending upon the operation conditions. This slippage or separation can decrease the effectiveness of the seal between the piston and cylinder, increasing the incidence of wear to the piston, and increasing the incidence of piston and seal ring failure. Therefore, there is a need to overcome the disadvantages described above, or otherwise lessen the effects of such disadvantages. 
       SUMMARY 
       [0002]    The present disclosure generally relates to a hydraulic piston apparatus, a method of manufacturing a hydraulic piston apparatus, a piston and cylinder assembly and method of manufacturing same. 
         [0003]    The hydraulic piston apparatus, in one embodiment, includes a cylindrical piston body and a plastic overmold. The cylindrical piston body includes: (a) one or more annular grooves formed into the exterior surface of the piston body; (b) a central interior bore to accommodate a piston rod; (c) a rotation obstructer formed the annular groove; and (d) an annular seal ring groove formed through the plastic overmold and into at least a portion of the metal piston body, where the annular seal ring groove accommodates a seal ring. The plastic overmold is formed about the outer peripheral surface of the piston body and includes an outer cylinder engagement surface. 
         [0004]    The formation of the plastic overmold is accomplished by: (a) placing the piston body in a mold; (b) heating the piston body to a desired temperature; (c) heating the overmold material to a molten or semi-molten state; (d) pumping the molten overmold material into a void space between the mold and the outer peripheral surface of the piston body; and (e) allowing the piston body and the overmold material to cool so that the overmold material solidifies about the piston body and in the annular grooves. As the overmold material is allowed to cool, it contracts in the radial direction so as to form a press-fit connection with the rotation obstructer. 
         [0005]    In one embodiment, the rotation obstructer includes a plurality of different widths. The rotation obstructer effectively minimizes or reduces the plastic overmold from separating from and rotating with respect to the piston body. 
         [0006]    Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0007]      FIG. 1  is a partial cross-sectional view of the piston body and plastic overmold according to an embodiment. 
           [0008]      FIG. 2  is a perspective view of a piston-cylinder assembly according to an embodiment. 
           [0009]      FIGS. 3A and 3B  are perspective and cross-sectional views, respectively, of an embodiment of a cylindrical piston body having a plurality of annular dovetail grooves formed therein. 
           [0010]      FIGS. 4A and 4B  are perspective and cross-sectional views, respectively, of the embodiment shown in  FIGS. 3A and 3B , where a plastic overmold is applied to the outer peripheral surface of the cylindrical piston body. 
           [0011]      FIGS. 5A and 5B  are perspective and cross-sectional views, respectively, of an embodiment of a cylindrical piston body having a plurality of annular dovetail grooves formed therein. 
           [0012]      FIGS. 6A and 6B  are perspective and cross-sectional views, respectively, of the embodiment shown in  FIGS. 5A and 5B , where a plastic overmold is applied to the outer peripheral surface of the cylindrical piston body, and where an annular seal ring groove is formed partially into the plastic overmold. 
           [0013]      FIGS. 7A and 7B  are perspective and cross-sectional views, respectively, of an embodiment of a cylindrical piston body having a plurality of annular grooves formed therein, where the side surfaces of each annular groove include rectangular annular recesses that function as the rotation obstructer. 
           [0014]      FIGS. 8A ,  8 B and  8 C are a perspective view, a cross-sectional front view, and a cross-sectional side view, respectively, of an embodiment of a cylindrical piston body having a plurality of annular dovetail grooves formed therein, where bottom surfaces of the grooves include a plurality of conical bores formed therein. 
           [0015]      FIG. 9A  is a perspective view of one embodiment of the piston apparatus, illustrating a cylindrical piston body having a plurality of dovetail grooves formed into the outer peripheral surface of the piston body. 
           [0016]      FIG. 9B  is a cross-sectional view of the embodiment of the piston apparatus shown in  FIG. 9A . 
           [0017]      FIG. 10  is a cross-sectional view of one embodiment of the piston apparatus, illustrating a piston body having a first portion fixed to a second portion. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In one embodiment, the piston of the piston assembly includes a plug or cylinder that is slideable within the inside bore of a cylinder. The piston is operable to either change an enclosed volume inside the cylinder, or to exert a force on a fluid inside the cylinder. The piston is operable in high-pressure hydraulic piston and cylinder apparatuses such as in heavy construction equipment applications. In one example, the piston is functional in a high-pressure hydraulic application, where an excavator has hydraulic cylinders and pistons to actuate movement of a boom, arm, thumb or bucket attached to the body of the excavator. 
         [0019]    1. Piston-Cylinder Assembly 
         [0020]    Referring now to the drawings,  FIG. 2  illustrates an embodiment of a piston-cylinder assembly  100 . In this embodiment, the assembly  100  includes a cylinder  102  having an interior bore  104 , where the interior bore defines an interior surface  106 . In one embodiment, the cylinder  102  material is a metal or metal alloy of the type typically used in high pressure hydraulic applications. However, it should be appreciated that the cylinder may be any other suitable material such as a ceramic material or polymer based material. A piston rod  108  is centrally aligned in the interior bore  104  of the cylinder  102 . The piston rod is inserted through the interior bore of the piston apparatus and further secured to the piston apparatus by a nut  110 . 
         [0021]    In an embodiment, as illustrated in  FIG. 2 , the piston-cylinder assembly includes a piston apparatus  113  (see,  FIG. 4A ) that is slidably engageable with the interior bore of the cylinder  104 . The piston apparatus  113  includes a cylindrical piston body  112  and a plastic overmold  124 . Referring to  FIG. 2 , the cylindrical piston body  112  includes end faces  114  and a central interior bore  118 . The piston body  112  also includes at least one annular groove or channel  120 . The annular groove  120  functions, at least in part, to decrease or regulate lateral movement of the plastic overmold  124  (described in detail below) along an axis of rotation of the piston body  112 . The annular groove further includes a rotation obstructer. In one embodiment, the rotation obstructer includes a dovetail profile peripherally formed into the sidewalls of the annular groove. The dovetail rotation obstructer  122  functions, at least in part, to decrease or regulate rotation of the plastic overmold  124  with respect to the piston body  112  (described in detail below). Accordingly, the annular groove  120  including the dovetail rotation obstructer  122  cooperate to securely fix the plastic overmold  124  to the piston body  112 . 
         [0022]    In an embodiment, the piston body also includes an annular seal ring groove  130 , as shown in  FIGS. 3A and 3B . In general, the annular seal ring groove is located in a central lateral position of the cylindrical piston body  112  and accommodates a sealing ring or other sealing member (not shown). The seal ring includes an outer surface that is in sealing slidable engagement with the interior surface  106  of the cylinder. Accordingly, the sealing ring functions, in cooperation with the plastic overmold  124 , to maintain the hydraulic fluid on one side of the piston apparatus  113  and to guide the piston apparatus  113  along the interior bore of the cylinder. In one embodiment, as illustrated in  FIGS. 4A and 4B , the annular seal ring groove or channel  130  is formed after the formation of the plastic overmold  124  and extends radially through the plastic overmold  124  and into a portion of the metal piston body. 
         [0023]    As mentioned above, the piston apparatus  113  includes a plastic overmold  124  formed into the annular groove or channel  120 . In one embodiment, the plastic overmold is composed of a glass-filled nylon material. In general, it should be appreciated that the overmold material should allow for an adequate tolerance, low friction and low wear seal between the piston body  112  and the interior surface  106  of the cylinder. The overmold  124  may function as a guide ring for the piston body. It should be appreciated that the overmold  124  material may include any suitable plastic, glass or carbon filled polymer, or combination thereof suitable for use as a bearing material in a hydraulic or pneumatic application. 
         [0024]    In an example process for forming the plastic overmold  124 , the cylindrical piston body  112  is first cleaned with an appropriate degreasing material and then placed concentrically within a mold cavity (not shown). Optionally, one or more surfaces of the piston body  112  may have a surface roughness or knurling applied thereto in order to increase friction between the piston body and the plastic overmold. After being placed in the mold cavity, the piston body  112  is heated to a temperature of about 175° C. to about 250° C. In general, it should be appreciated that the metal cylindrical piston body should be heated to a temperature sufficient to reduce or minimize immediate cooling and hardening of the liquid plastic overmold material. Although a temperature range of about 175° C. to about 250° C. is described above, it should be appreciated that the piston body may be heated to a sufficiently higher or lower temperature depending on the melting temperature of the selected plastic or polymer overmold material. After the cylindrical piston body  112  is heated, the plastic overmold material is heated to a liquid or semi-liquid state. The plastic overmold material is then pumped into the mold cavity (not shown) to flow into and fill the void space defined between the interior surface of the mold cavity and the outer peripheral surface  116  and annular groove or channel  120  of the cylindrical piston body  112 . Any air contained with the void space is appropriately expelled through a venting means in the mold (not shown). After the overmold material has completely filled the void space, the piston body  112  and the plastic overmold  124  are allowed to cool. The outer surface of the plastic overmold is machined with a lathe to a desired tolerance. Although the plastic overmold material is a glass-filled nylon material in the above-described example, it should be appreciated that the overmold material may be any suitable material that exhibits a adequate tolerance and low friction seal between the outer cylinder engagement surface  126  of the plastic overmold  124  and the interior surface  106  of the cylinder  102 . 
         [0025]    In an embodiment, the plastic overmold material has a coefficient of thermal contraction/expansion that is greater than the coefficient of thermal contraction/expansion of the cylindrical piston body  112 . In one example, where the cylindrical piston body  112  is a metal such as steel and the plastic overmold  124  is a glass-filled nylon material, the glass-filled nylon material has a larger coefficient of thermal contraction/expansion than the steel. Therefore, when the piston body  112  and overmold material  124  are allowed to cool, the plastic overmold  124  and the piston body  112  contract radially inward to a certain degree. In addition, the thickness of the plastic overmold decreased upon cooling. However, as the plastic overmold  124  material has a larger coefficient of thermal contraction, the radial contraction will be greater than the radial contraction of the cylindrical piston body  112 . Accordingly, the plastic overmold  124  contracts in upon the piston body  112  to form a frictional connection. However, as discussed above, this frictional connection may not be sufficient to reduce or minimize separation and rotation of the plastic overmold  124  with respect to the cylindrical piston body  112 . 
         [0026]    In addition to the radial contraction upon cooling, the thickness of the plastic overmold  124  material also decreases, as mentioned above. Therefore, without a rotation obstruction structure such as the dovetail rotation obstructer  122  described above (i.e., as in a simply rectangular annular groove), the plastic overmold  124  could separate from the annular channel or groove  120  formed into the piston body  112 . Therefore, without a rotation obstructer  122  the frictional connection between the plastic overmold and the piston body could become compromised. However, according to this embodiment, the dovetail rotation obstructer  122  of the piston body  112  has an inwardly sloping surface  122  that provides a normal force upon the cooling operation to oppose slippage of the plastic overmold  124  material with respect to the inclined surface. Therefore, upon cooling, the dovetail rotation obstructer  122  effects an improved press-fit frictional seal between the plastic overmold  124  and the piston body  112 . Accordingly, rotational movement and separation of the plastic overmold  124  with respect to the piston body  112  is effectively reduced or minimized. 
         [0027]      FIG. 1  illustrates the forces acting on an element  28  of the overmold material  24 , where the overmold  24  has been formed in the rotation obstructer of the piston body  12 . In this embodiment, the rotation obstructer has an inner surface  20  contactable with a portion of the piston body  12  and an outer surface  26  that is a cylinder engagement surface. In this embodiment, the rotation obstructer is a dovetail shaped groove having a slanted surface  22 . Force elements  30  and  36  are force components acting on the element  28  from the slanted surface  22 . Force component  34  acts on the element  28  from the piston body  12 . Force element  32  acts on the element from the corresponding slanted surface (not shown) of the dovetail groove (see also, reference numeral  122  in  FIG. 3B ). 
         [0028]    2. Annular Seal Ring Groove Extending Partially Through Overmold 
         [0029]    Referring to  FIGS. 5A and 5B , in one embodiment, the piston apparatus  213  includes a piston body  212  having end faces  214 , an outer peripheral surface  216  and a central interior bore  218 . In this embodiment, the piston body  212  includes an annular groove or channel  220  having a dovetail-type rotation obstructer  222 , as described above with reference to  FIGS. 1 ,  2 A,  2 B,  3 A and  3 B. Referring to  FIGS. 6A and 6B , the piston apparatus  213  also includes a plastic overmold  224  having an outer cylinder engagement surface  226 . In this embodiment, the plastic overmold  224  has a thickness T defined by the distance between the outer peripheral surface  216  of the piston body and the outer cylindrical engagement surface  226  of the plastic overmold (see,  FIG. 5B ). This thickness T is greater than the depth of the annular seal ring groove  230  formed into the outer cylinder engagement surface  226  of the plastic overmold  224 . Therefore, the bottom surface  232  of the annular seal ring groove does not extend into the metal cylindrical piston body  212 . Accordingly, when the annular seal ring groove  230  is formed, the plastic overmold  224  is not split into two pieces as in the embodiment described above with respect to  FIG. 4B . Also, the bottom surface  232  that engages the seal ring (not shown) is the plastic overmold material rather than the metal material of the piston body. 
         [0030]    3. Rotation Obstructer Including a Rectangular Recess 
         [0031]    Referring to  FIGS. 7A and 7B , in one embodiment, the piston apparatus  313  includes a piston body  312  having end faces  314 , an outer peripheral surface  316  and a central interior bore  318 . In this embodiment, the piston body  312  includes an annular groove or channel  320  having an annular rectangular recess  332  that functions as the rotation obstructer  320 . As mentioned above, the plastic overmold  324  tends to contract in upon the piston body  312  upon cooling to form a frictional connection. Also, the thickness of the plastic overmold  324  material decreases to a certain degree. In this embodiment, the rectangular recess  332  rotation obstructer of the piston body  312  has an upper surface  328  that opposes thermal contraction of the plastic overmold  324  material to effect an improved press-fit or shrink-fit frictional seal between the plastic overmold  324  and the piston body  312 . Therefore, the press-fit or shrink-fit seal reduces or minimizes separation of the plastic overmold  324  with respect to the piston body. Accordingly, any rotation of the plastic overmold  324  with respect to the cylindrical piston body  312  is effectively obstructed, reduced or minimized. 
         [0032]    It should be appreciated that, although the structure of a rotation obstructer  320  has been described above with respect to a dovetail profile and a rectangular recess  322  formed into the sidewalls  334  of the annular groove  320  formed into the piston body, the rotation obstructer  320  included in the piston body  312  can include any suitable recess formed into the surface of the piston body, where the recess includes at least one surface oriented in such a manner as to: (a) oppose contraction of the plastic overmold material upon cooling; or (b) oppose the operating forces acting on piston apparatus  313  in operation. With regard to opposing contraction upon cooling, the overmold  324  volumetrically contracts to a greater degree that the piston body  312  such that the surface of the piston body obstructs at least a portion of the possible contraction. In operation, the overmold  324  expands slightly due to an increase in temperature. Because the overmold  324  has a radial thermal expansion associated with an increase in temperature the surface opposes said expansion. In one example, where the piston body  312  includes one or more annular groove or channels as described above, the rotation obstructer may be a circular recess formed into the sidewall of the annular groove, a circular nodule extending from the side walls of the annular groove, a triangular or notched structure extending into or out the side walls, or any other suitable structure or profile that includes at least one surface that opposes the contraction of the plastic overmold. The surface may be inwardly slanted as in the examples of the dovetail profile or triangular notches structure. The opposing surface may be curved as in the example of the circular or ovular notch. Moreover, the opposing surface may be substantially coplanar with respect to the outer cylindrical engagement surface  326  of the plastic overmold as in the example of the rectangular recess. Therefore, at least one opposing surface of the rotation obstructer provides an opposing force to the plastic overmold  324  upon cooling to effect a press-fit seal. Accordingly, rotation of the plastic overmold  324  with respect to the piston body  312  can be effectively reduced or minimized. 
         [0033]    4. Rotation Obstructer Including Conical Bores 
         [0034]    Referring to  FIGS. 8A ,  8 B and  8 C, in one embodiment the piston apparatus  413  includes a cylindrical piston body  412  and a plastic overmold  424 . The cylindrical piston body  412  includes end faces  414  and a central interior bore  418 . The piston body  412  also includes at least one annular groove or channel  420 . As described above, the annular groove  420  functions, at least in part, to reduce or minimize lateral movement of the plastic overmold  424  with respect to an axis of rotation of the piston body  412 . The annular groove further includes a rotation obstructer  422  including a dovetail profile peripherally formed into annular groove or channel  420 . The piston body  412  also includes an annular seal ring groove  430  having a bottom surface  432 . As described above, the annular seal ring groove  430  accommodates a seal ring (not shown). In addition to the dovetail rotation obstructer described above, a plurality of conical bores  434  are formed into the bottom surface of the annular grooves or channels  420 . The conical bores  434  cooperate with the dovetail rotation obstructer  422  to reduce or minimize rotation of the plastic overmold  424  with respect to the piston body  412 . 
         [0035]    When the liquid plastic overmold material is introduced into the mold cavity (not shown), the overmold material fills the annular grooves and also fills the conical bores. The slanted surface of the conical bores further reduces or minimizes the tendency of the cured or cooled plastic overmold to rotate with respect to the piston body (i.e., they modify the smooth cylindrical profile of the annular grooves). 
         [0036]    It should be appreciated that the conical bores could alternatively be any suitable geometry such as a rectangular recess, a square recess, a cylindrical bore or any other suitable shape. It should also be appreciated that the additional rotation obstruction structures may be protrusions that extend radially away from the bottom surface of the annular groove  420 , or may be any combination of protrusions and recesses or bores. Similar to the recesses or bores described above, a suitable protrusion would also cooperate with the dovetail rotation obstructer  422  to reduce or minimize rotation of the plastic overmold with respect to the piston body. It should also be appreciated that the above described recesses, bores  434  and/or protrusions may be utilized with other suitable primary rotation prevention structures other than the dovetail rotation obstructer  422 , such as in the embodiment described above having rectangular recesses  322  formed in the sidewalls of the annular groove  320  of the piston body  312  (see,  FIGS. 7A and 7B ). 
         [0037]    5. Rotation Obstructer Including a Plurality of Dovetailed Grooves 
         [0038]    Referring to  FIGS. 9A and 9B , in one embodiment the piston apparatus  513  includes a cylindrical piston body  512  and a plastic overmold  524 . The cylindrical piston body  512  includes end faces  514  and a central interior bore  518 . The piston body  512  also includes at least one annular seal ring groove  530 . In addition, the outer peripheral surface  516  of the piston body  512  includes a plurality of grooves or channels  534  formed therein. In an embodiment, the grooves or channels  534  (see,  FIG. 9A ) are rectangular channels including a dovetail rotation obstructer  538  (see,  FIG. 9B ) formed therein. The grooves are spaced radially about the outer peripheral surface  516  of the piston body  512 . In an embodiment, the grooves  534  are oriented an angle relative to the axis or rotation of the cylindrical piston body  512 . In the illustrated embodiment (see,  FIG. 8A ), the grooves are oriented at an approximate  45  degree angle relative to the axis of rotation of the piston body. However, it should be appreciated that any suitable angle may be used. Also in the illustrated embodiment, the grooves are formed at alternating forty-five degree angles. The angled grooves  534  having the dovetail rotation obstructers  522  function to reduce or minimize rotation of the plastic overmold  524  with respect to the piston body  512  and function to reduce or minimize lateral movement of the overmold with respect to the axis of rotation of the piston body. When the liquid plastic overmold material is introduced into the cavity of the mold (not shown), the overmold material fills the angled grooves. Upon a cooling step, the dovetail rotation obstructers  534  resist separation of the plastic overmold  524  from the bottom surfaces  536  of the angular grooves  534 . In addition, the grooves effectively reduce or minimize lateral movement of the plastic overmold  524  when the piston is subject to the rigorous draft forces when sliding in the cylinder. Accordingly, the piston body effectively reduces or minimizes movement of the plastic overmold  524  with respect to the piston body  512 . 
         [0039]    6. Multi-Component Piston Apparatus Including Rotation Obstructer 
         [0040]    Referring to  FIG. 10 , in one embodiment, the piston apparatus  613  includes a first portion  614  and a second portion  615 , wherein the first portion  614  is fixedly connectable to the second portion  615 . After the first portion  614  is connected to the second portion  615  to form a cylindrical piston body, a central interior bore  618  is formed therein. Also, the piston apparatus  613  includes an annular groove or channel  620  formed into the outer peripheral surfaces  616   a,    616   b  of the first portion  614  and second portion  615  of the fixedly connected piston body. In one embodiment, the annular groove  620  includes a dovetail rotation obstructer  622 , as described above. A plastic overmold  624  including an outer cylinder engagement surface  626  is formed in the annular groove. As described above, the dovetail rotation obstructer  622  effects a press-fit or shrink-fit seal that reduces or minimizes rotation of the plastic overmold with respect to the piston body. 
         [0041]    In each of the embodiments described above, the piston body includes one or more rotation prevention structures that restrict rotational movement of the applied plastic overmold material with respect to the piston body. In certain embodiments, the rotation obstructer includes a structure formed into the outer peripheral surface of the piston body, where the structure includes at least one surface that restricts thermal contraction of at least a portion of the plastic overmold material to form a press-fit or shrink-fit connection. In other embodiments, the rotation obstructer includes recessed structures or protruding structures formed into the outer peripheral surface of the piston body. Therefore, the rotation obstructers of the above-described embodiments, alone or in a suitable combination, effectively reduce or minimize the plastic overmold from separating from and moving with respect to the piston body. Accordingly, the piston apparatus minimizes or reduces wear and minimizes or reduces the incidence of seal failure in high-pressure hydraulic cylinder applications, as described above. 
         [0042]    In one embodiment, the piston apparatus includes a suitable combination of one or more components of one or more of the embodiments described above. 
         [0043]    It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.