Abstract:
A method for assembling a piston on a rod, and resultant piston and rod assembly, uses one or more lock rings that are mechanically deformed radially inwardly into respective locking grooves in the rod, thereby axially locking the lock ring to the rod. The lock ring can function as an axial stop for the piston or can be formed integrally with the piston. The method of assembly minimize unwanted strains and stresses in the piston and rod, while providing a secure and durable joint between the piston and rod.

Description:
RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/588,132 filed Jul. 15, 2004, which is hereby incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to piston and rod assemblies used in hydraulic and pneumatic cylinders, accumulators and the like. 
   BACKGROUND OF THE INVENTION 
   As is well known in the art, cylinder products use piston and rod components that are attached to each other by various methods. One such method is peening. Peening requires forces axial to the rod in order to deform material from the rod to interfere with a chamfered area of the back of the piston. During the attachment process these forces have to be countered. The force can be frictionally countered on the outer diameter (OD) of the rod, but this may result in marring of the rod surface. If the rod surface is marred, the operation of the piston/rod assembly can be negatively affected. Alternatively, the force can be opposed at the opposite end of the rod, which can result in column loading of the rod and the potential for bending, particularly when subjected to forces necessary to cause adequate deformation for high strength applications. 
   Another prior art assembly method for attaching the piston to the rod uses a threaded joint. Assembly is effected by simple rotation of the piston and rod relative to one another. During assembly of the piston on the rod, the unthreaded portion of the rod typically acts as a pilot for centering the piston on the rod. The piston and rod may be torqued relative to one another to provide an assembled joint of a desired strength, and a set screw or pin may be installed tangentially to the threaded area of the joint to lock the piston and rod against relative rotation. In order to provide a pressure tight seal between the piston and the rod, and further to add strength to the assembled joint to prevent the parts from separating during operation, an adhesive heretofore has been used at the threaded joint. This, however, usually requires the mating surfaces of the piston and rod to be clean to obtain proper adhesion between the piston and rod. The cleaning process is costly, primarily because of the time needed to clean the parts. In addition, further time is needed to allow the adhesive to properly cure before the assembly can be further assembled with other components of a pneumatic or hydraulic cylinder. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method for assembling a piston on a rod that overcomes one or more drawbacks associated with the prior art methods of assembly. The method and resultant joint uses one or more lock rings that are mechanically deformed into respective locking grooves in the rod, thereby axially locking the lock ring to the rod. The lock ring can function as an axial stop for the piston or can be formed integrally with the piston. This method of assembly minimizes unwanted strains and stresses in the piston and rod, while also providing a secure and durable joint between the piston and rod. 
   Accordingly, the invention provides a method for assembling a piston on a rod that comprises telescopically positioning a lock ring on the rod at a radially outwardly opening locking groove, mechanically deformed the lock ring into the locking groove axially to lock the lock ring to the rod, and using the lock ring to prevent the piston from moving axially on the rod in at least one direction. 
   The invention also provides a piston and rod assembly for use in hydraulic or pneumatic cylinders, comprising a piston having opposite axial ends, a rod having at least one radially outwardly opening locking groove, and at least one annular lock ring telescoped onto the rod and mechanically deformed into the locking groove axially to lock the lock ring to the rod, the lock ring being located adjacent an axial end of the piston to prevent the piston from moving axially on the rod in at least one direction. 
   The lock ring may be unitary with the piston, with the ring extending axially from the adjacent axial end of the piston. Alternatively, the lock ring and piston may be formed as separate components, with an axial end of the piston abutting the lock ring to prevent axial movement of the piston in the direction of the lock ring. The piston may be axially trapped between a pair of lock rings that have been mechanically deformed into respective grooves in the rod, or between a lock ring and a radial shoulder on the rod. 
   As is preferred, the locking groove extends annularly around the rod and has at least one or both side walls provided with a rounded and/or angled portion. The piston may be sealed to the rod by suitable means, such as by an annular seal retained in a sealing groove axially spaced from the locking groove, or by use of an adhesive. 
   The lock ring preferably is mechanically deformed by crimping. The lock ring may be mechanically deformed into the locking groove sufficiently to provide a radial as well as an axial interference that prevents rotation of the piston relative to the rod while also preventing axial movement of the piston relative to the rod. 
   Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the annexed drawings 
       FIG. 1  is a perspective view of an exemplary piston and rod assembly according to the invention; 
       FIG. 2  is a perspective view of the rod of  FIG. 1  prior to assembly with the piston; 
       FIG. 3  is a perspective view of the piston with an integral lock ring prior to crimping of the lock ring into a groove on the rod; 
       FIG. 4  is a cross-sectional view of the piston and rod assembly, showing the joint formed between the piston and rod; 
       FIG. 5  is a cross-sectional view of a crimping die used to crimp the lock ring into the groove on the rod; 
       FIG. 6  is an end view of the crimping die of  FIG. 5 ; 
       FIG. 7  is an exploded perspective view of another piston and rod assembly according to the invention; and 
       FIG. 8  is an exploded perspective view of yet another piston and rod assembly according to the invention. 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings in detail and initially to  FIG. 1 , an exemplary piston and rod assembly constructed in accordance with the invention is indicated generally at  10 . Piston and rod assemblies according to the invention may be used hydraulic and pneumatic cylinders, and other products as well. 
   The piston and rod assembly  10  comprises a piston  12  which is permanently attached to a rod  14  by a crimped lock ring  16 . The lock ring  16  can be formed integrally (as one piece) with the piston  12  as shown in  FIGS. 1 and 3 , or as a separate piece as described below in conjunction with the piston and cylinder assemblies shown in  FIGS. 7 and 8 . 
   In  FIGS. 2 and 3 , the rod  14  and piston  12  are shown prior to assembly. As seen in  FIG. 2 , the rod has a radially outwardly opening locking groove  20  disposed inwardly of the axial end face  22  of the rod. The shape of the groove  20  can vary. For example, the shape may be fully radial, or include a flattened area, or have one side shaped and flattened to a squared shoulder. More particularly, the groove may have a generally flat bottom and sidewalls that extend from the flat bottom to the outer surface of the rod. In the illustrated embodiment, the sidewalls are both rounded or curved. A smooth rounded or curved shape may contribute to a permanent and fluid-tight seal when the lock ring  16  is mechanically deformed into the groove as further discussed below. 
   As seen in  FIG. 3 , the lock ring  16  is formed as an integral axial extension of the functional portion of the piston  12 . More particularly, the ring  16  extends from an axial end face  30  of the piston and coaxially with the piston. If desired, the ring  16  may be incorporated into (or formed by) other functional elements of the piston and rod assembly, such as cushion sleeves/spears of the assembly that are located axially beyond the functional portions of the piston. Although the ring that is mechanically deformed into the locking groove is annularly continuous, for some applications it may be desirable for the ring to be composed of a plurality of circumferentially disposed segments that are mechanically permanently deformed into the locking groove need not be circumferentially continuous. However, the ring or segments thereof preferably are squeezed or swaged into the locking groove until it takes the shape of the locking groove. This preferred form of crimping is in contrast to prior art assembly techniques involving the bending of tabs. 
   The inner diameter surface of the lock ring  16  forms a continuation of the inner diameter surface of the piston  12 , and both inner diameter surfaces are sized to be telescoped onto the rod with a close fit as shown in  FIG. 1 . In the illustrated embodiment and as best seen in  FIG. 4 , the piston is telescoped onto the rod until it abuts a radial shoulder  40  on the rod. During assembly, the radial shoulder may function as a positioning stop for properly aligning the lock ring with the groove  20  into which the ring is permanently deformed. After assembly, the shoulder may coact with the ring/groove interlock to hold the piston in fixed position on the rod. 
   After the piston  12  with the lock ring  16  has been assembled onto the rod and the lock ring aligned with the groove  20 , the lock ring is permanently mechanically deformed radially inwardly into the groove as shown in  FIG. 4 , thereby to create a mechanical interference between the ring and the rod. This will prevent the piston from moving axially or becoming detached from the rod. In addition, the mechanical interference is effected in a manner that minimizes stress concentrations in the functional portions of the mating components. In particular, the stress concentration caused by deformation of the lock ring is isolated from the piston proper. Overall, this arrangement provides better fatigue properties than prior art designs, as well as better strength and ease of assembly. 
   The lock ring  16  should be small enough in radial dimension to allow for adequate deformation of the lock ring into the groove  20  and yet large enough to provide adequate resistance to forces imposed on the piston and rod assembly when in use. Preferably, the lock ring is sufficiently deformed to fill the groove. The lock ring also may be mechanically deformed into the locking groove sufficiently to provide a radial as well as an axial interference that prevents rotation of the piston relative to the rod while also preventing axial movement of the piston relative to the rod. 
   As above mentioned, a permanent and fluid-tight seal may be formed when the lock ring  16  is mechanically deformed into the groove  20 . This seal may be sufficient for various applications. For higher pressure applications, for example, additional sealing may be desired. To this end, the piston alternatively or additionally may be sealed to the rod by other means, such as by an annular seal  50  retained in a sealing groove  52  axially spaced from the locking groove  20 , or by use of a suitable adhesive which may also contribute to the permanent mechanical connection between the piston and rod. An anaerobic adhesive may be used as it will set up in the absence of oxygen. 
   During assembly, the rod and the piston with the ring may relatively positioned in any suitable manner prior to crimping the ring to the rod. For instance, the rod and piston may be positioned in a die provided with appropriate physical stops for locating the parts in proper position relative to one another during crimping. 
   An exemplary crimping die is shown in  FIGS. 5 and 6 . The crimping die  60  includes a plurality of radially movable segments  62  that each have a radially inner operating surface  64  with a geometry intended to deform a respective portion of the lock ring into the groove in the rod. The axially inner surfaces  66  of the die segments function as a locating stop for an end face of the piston when located in a larger diameter chamber of the die  60 . The rod will also be properly located in the die, with the groove therein radially aligned with the die segments  62 , by engagement of the shoulder  40  with the end face of the piston. 
   While crimping is one preferred means for deforming the lock ring into the groove, it should be appreciated that other mechanical forming methods could alternatively be used. These other methods include, but are not limited to, cold-forming, magnaforming and staking, and generally any other method which achieves a permanent deforming of the lock ring into the grooves. 
   For some applications, the mechanical interference between the crimped ring and groove in the rod alone may by sufficient for securing the piston to the rod. Hence, the rod need not necessarily be provided with the shoulder  40 . The piston may also be provided with two lock rings integrally joined therewith, one projecting from each axial end face of the piston. The rod would then be provided with corresponding grooves into which the lock rings are mechanically permanently deformed to secure the piston to the rod. 
     FIG. 7  shows another piston and rod assembly  70  according to the invention. This assembly  70  is identical to that shown in  FIGS. 1–3 , except the lock ring  72  is formed as a separate piece. After the piston  73  is telescoped onto the rod  74  and butted against the radial shoulder  76  on the rod, the lock ring  72  is slipped onto the rod and aligned with the groove  78  in the rod. The lock ring is then mechanically deformed into the groove as above described to lock the piston to the rod. 
     FIG. 8  shows yet another piston and rod assembly  80  according to the invention. In the assembly  80 , two lock rings  82  are used to secure the piston  84  to the rod  86 . The rod includes two annular locking grooves  88  axially spaced apart such that the piston will be axially restrained against movement relative to the rod by the lock rings after the lock rings have been mechanically deformed into the grooves as above described. A crimping die can be suitably configured with locating means for properly positioning the rod, rings and piston prior to crimping of the rings in the locking grooves. 
   In view of the foregoing, it can now be appreciated that features and advantages of afforded by the invention include, but are not limited to, a permanent mechanical joint, reduction of required machining (no threads common to connections of this type), and reduction in the time and labor content required for manufacture of this assembly. Due to reduced machining and the mechanical interference nature of the connection, machining time may be reduced. The need for special cleaning (although desired when an anaerobic or other adhesive is used as above mentioned) may be eliminated. A mechanical joint formed in accordance with the invention may also provide superior fatigue resistance relative to a welded joint. 
   Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.