Abstract:
A method of manufacturing a sleeve mechanism without the use of machines by using a hydroforming process. The method includes the steps of providing an inner sleeve, an intermediate sleeve, and an outer sleeve concentrically within one another. The concentric sleeves are then expanded by using a hydroforming process such that an external thread is formed on the inner sleeve and an internal thread is formed on the outer sleeve. After the hydroforming process, the intermediate sleeve is removed by using any desired manner, such as melting or chemical dissolution. The relatively small space remaining between the inner and outer sleeves allow them to freely rotate about each other to ensure smooth operation of the screw and sleeve mechanism. Alternatively, the inner and outer sleeves may be hydroformed in separate operations.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of United States Provisional Application Ser. No. 60/151,780, filed Aug. 31, 1999, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of manufacturing a cooperating threaded lead screw and sleeve mechanism, such as for use in a linear actuator, using a hydroforming process. 
     A linear actuator is a well known device that is adapted to effect linear movement, typically reciprocating linear movement, of a workpiece along a desired path of movement. A typical linear actuator includes an electric motor having a rotatable output shaft that is connected through a gear train to a lead screw and nut mechanism. When the electric motor is actuated, rotation of the output shaft causes corresponding rotation of the lead screw. The lead screw is typically formed from an elongated shaft having an external helical thread provided on the outer surface thereof. The nut is typically formed from a block of material having an opening formed therethrough, with an internal helical thread formed on the inner surface thereof. The nut is mounted on the lead screw in such a manner as to be restrained from rotating with the lead screw when the electric motor is actuated. The helical threads of the lead screw and the nut cooperate with one another such that rotation of the lead screw causes linear movement of the nut axially along the lead screw. The direction of such axial movement of the nut (and of the workpiece connected thereto) is dependent upon the direction of rotation of the lead screw. 
     A variety of methods are known for forming lead screw and nut mechanisms of the general type described above. Typically, the lead screw and the nut are each formed from solid pieces of material to desired rough shapes, then machined to precise final desired shapes. Although known methods have been effective, it would be desirable to provide an improved method for manufacturing same. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method of manufacturing a externally threaded screw and internally threaded sleeve mechanism without the use of machines by using a hydroforming process. Initially, an inner sleeve, an intermediate sleeve, and an outer sleeve are disposed concentrically within one another. The inner and outer sleeves are preferably formed from a rigid metallic material, while the intermediate sleeve may be formed from any desirable spacer material. The concentric sleeve assembly is then disposed within die halves of a hydroforming apparatus and hydraulically expanded such that an external thread is formed on the inner sleeve and an internal thread is formed on the outer sleeve. The intermediate sleeve is then removed in any desired manner, such as by melting or chemical dissolution. The relatively small space remaining between the inner and outer sleeves allow for relative rotation therebetween. Alternatively, the inner and outer sleeves can be hydroformed in separate operations for subsequent use. 
     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional elevational view of an apparatus for manufacturing a lead screw and sleeve mechanism in accordance with this invention, wherein the workpiece is shown prior to the performance of a hydroforming operation. 
     FIG. 2 is a sectional elevational view similar to FIG. 1, wherein the workpiece is shown after the performance of the hydroforming operation. 
     FIG. 3 is a perspective view of the lead screw and sleeve mechanism formed in accordance with the method of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in FIGS. 1 and 2 a workpiece assembly, indicated generally at  10 , that is adapted to be formed into a lead screw and sleeve mechanism in accordance with the method of this invention. The workpiece assembly  10  includes an outer sleeve or tubular member  12 , an intermediate sleeve or tubular member  14 , and an inner sleeve or tubular member  16 . The outer and inner tubular members  12 ,  16  are preferably formed from a material that can be permanently deformed by a hydroforming process, such as metal and the like. The intermediate tubular member  14  is preferably formed from a material that can be deformed by the hydroforming process and subsequently removed from between the outer and inner tubular members  12  and  16 , as described below. However, it will be appreciated that the invention is not intended to be limited by the types of material used for the outer tubular member  12 , the intermediate tubular member  14 , and the inner tubular member  16 . 
     As shown in FIG. 2, the outer tubular member  12  preferably has an inner diameter that is slightly larger than the outer diameter of the intermediate member  14 . Similarly, the intermediate tubular member  14  preferably has an inner diameter that is slightly larger than the outer diameter of the inner tubular member  16 . Thus, the inner tubular member  16  can be disposed concentrically within the intermediate tubular member  14 , and the intermediate tubular member  14  can be disposed concentrically within the outer tubular member  12  with relative ease. The concentrically disposed tubular members  12 ,  14 , and  16  are then disposed between upper and lower die sections  18  and  20  of a hydroforming apparatus, indicated generally at  22 . A typical hydroforming apparatus  22  includes a frame (not shown) supporting the die sections  18  and  20  thereon for relative movement between opened and closed positions. The die sections  18  and  20  have cooperating recesses formed therein that together define a die cavity having a shape corresponding to a desired final shape for the workpiece  10 . When moved to the opened position, the die sections  18  and  20  are spaced apart from one another to allow workpiece  10  to be inserted within or removed from the die cavity. When moved to the closed position, the die sections  18  and  20  are disposed adjacent to one another so as to enclose the workpiece  10  within the die cavity. Although the die cavity is usually somewhat larger than the workpiece  10  to be hydroformed, movement of the two die sections  18  and  20  from the opened position to the closed position may, in some instances, cause some mechanical deformation of the workpiece  10 . In any event, the workpiece  10  is then filled with a fluid, typically a relatively incompressible liquid such as water. The pressure of the fluid within the workpiece  10  is increased to such a magnitude that the workpiece  10  is expanded outwardly into conformance with the die cavity. As a result, the workpiece  10  is deformed or expanded into the desired final shape. Hydroforming is an advantageous process because it can quickly deform a workpiece into a desired complex shape. 
     In a typical hydroforming apparatus  22 , the die sections  18  and  20  are arranged such that the upper die section  18  is supported on a ram of the apparatus  22 , while the lower die section  20  is supported on a bed of the apparatus  22 . A mechanical or hydraulic actuator is provided for raising the ram and the upper die section  18  upwardly to the opened position relative to the lower die section  20 , allowing the previously deformed workpiece  10  to be removed from and a new workpiece to be inserted within the die cavity. The actuator also lowers the ram and the upper die section  18  downwardly to the closed position relative to the lower die section  20 , allowing the hydroforming process to be performed. To maintain the die sections  18  and  20  together during the hydroforming process, a mechanical clamping device is usually provided. The mechanical clamping device mechanically engages the die sections  18  and  20  (or, alternatively, the ram and the base upon which the die sections  18  and  20  are supported) to prevent them from moving apart from one another during the hydroforming process. Such movement would obviously be undesirable because the shape of the die cavity would become distorted, resulting in unacceptable variations in the final shape of the workpiece  10 . The die sections  18  and  20  form a die cavity preferably shaped to have a desired final shape of the tubular members  12 ,  14 , and  16 . In the preferred method, the die cavity is generally circular in shape, having a helical thread or other desired shape formed on the inner surface thereof. The length of the die cavity may be of any length to sufficiently form the screw and sleeve mechanism  10 . 
     The hydroforming apparatus  22  further includes a pair of end feed cylinders  24  and  26  that are positioned at opposite ends of the die sections  18  and  20 . The end feed cylinders  24  and  26  are conventional in the art and are adapted to sealingly engage the ends of at least the inner tubular member  16 . The end feed cylinders  24  and  26  having respective passageways  28  and  30  formed therein that to fill the inner tubular member  16  with pressurized fluid, typically a relatively incompressible liquid such as water, from a source of pressurized fluid (not shown) in a manner well known in the art. The pressure of the fluid within the inner tubular member  16  is then increased to such a magnitude that the tubular members  12 ,  14  and  16  are all expanded outwardly into conformance with the die cavity defined by the die sections  18  and  20 , as shown in FIG.  2 . As a result, an external helical thread is formed on the inner tubular member  16  and a cooperating internal helical thread is formed on the outer tubular member  12 . 
     After the hydroforming process is completed, the intermediate tubular member  14  is removed from between the outer tubular member  12  and the inner tubular member  16 . The tubular members  12 ,  14 , and  16  may remain within the die cavity or may be removed from the die cavity during the removal of the intermediate tubular member  14 . Removal of the intermediate tubular member  14  can be accomplished using several different methods depending on the type of material used to form the intermediate tubular member  14 . For example, if the intermediate tubular member  14  is formed from a plastic having a relatively low melting temperature, then the screw and sleeve mechanism  10  may be heated to a temperature sufficient to melt the intermediate tubular member  14 , allowing it to drain in liquid form from between the outer tubular member  12  and the inner tubular member  16 . Alternatively, the intermediate tubular member  14  may be formed from a material that can be readily dissolved using chemicals, such as a milar or plastic material. 
     Thus, it can be seen that the purpose of the intermediate tubular member  14  is to provide a gap between the outer and inner tubular members  12  and  16  during the initial forming process. Thereafter, the intermediate tubular member  14  is removed, allowing the outer and inner tubular member  12  and  16  to be freely rotatable relative to one another. FIG. 3 illustrates the lead screw and sleeve mechanism  10  after the intermediate sleeve  14  has been removed. The resultant outer and inner tubular members  12  and  16  are generally hollow and cylindrical in shape, but are corrugated to have respective helical threads formed therein. In the preferred method, the outer tubular member  12  and the inner tubular member  16  are hydroformed simultaneously within the same die cavity of the hydroforming apparatus  22 . However, it should be understood that the outer tubular member  12  can be hydroformed separately from the intermediate tubular member  14  and the inner tubular member  16  if desired. 
     The formation of the outer and inner tubular members  12  and  16  has been described and illustrated in the context of the illustrated hydroforming apparatus  22 . However, the method of this invention may be practiced using forming methods other than hydroforming. For example, the outer and inner tubular members  12  and  16  may be deformed using magnetic pulse forming techniques. To accomplish this, an internal magnetic pulse welding inductor assembly is inserted within the inner tubular member  16  and actuated to generate an intense electromagnetic field. The presence of this electromagnetic field causes the tubular members  12 ,  14  and  16  to be expanded outwardly into conformance with the die cavity defined by the die sections  18  and  20 , as shown in FIG.  2 . As a result, an external helical thread is formed on the inner tubular member  16  and a cooperating internal helical thread is formed on the outer tubular member  12 . The intermediate tubular member  14  can then be removed in the same manner as described above. It should be understood that the outer tubular member  12  can be expanded separately from the intermediate tubular member  14  and the inner tubular member  16  if desired. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.