Patent Application
20080098886
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.
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.
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.
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.
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.
Additional features and advantages are described herein, and will be apparent from, the following Detailed Description and the figures.
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.
1. Piston-Cylinder Assembly
Referring now to the drawings,
In an embodiment, as illustrated in
In an embodiment, the piston body also includes an annular seal ring groove 130, as shown in
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.
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.
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.
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.
2. Annular Seal Ring Groove Extending Partially Through Overmold
Referring to
3. Rotation Obstructer Including a Rectangular Recess
Referring to
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.
4. Rotation Obstructer Including Conical Bores
Referring to
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).
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,
5. Rotation Obstructer Including a Plurality of Dovetailed Grooves
Referring to
6. Multi-Component Piston Apparatus Including Rotation Obstructer
Referring to
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.
In one embodiment, the piston apparatus includes a suitable combination of one or more components of one or more of the embodiments described above.
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.
