Abstract:
A fluid-actuated cylinder includes a piston rod with a circumferential groove. A split ring, with predetermined linear expansion properties, is cooled to a temperature below the temperature of the rod, and the ring halves are located in the groove. The ring is then equalized with the temperature of the rod, and expands into compression against the sidewalls of the groove. An interference fit is provided with the groove, which prevents axial and rotational movement of the ring with respect to the piston rod. The expansion also prestresses the piston rod between the groove sidewalls. The split ring is then connected to an annular flange on a ring-shaped piston to fix the piston with respect to the piston rod. The piston rod assembly is then located in a cylinder housing, with the piston providing a fluid seal with the cylinder walls as the piston rod reciprocates.

Description:
CROSS-REFERENCE TO RELATED CASES  
       [0001]    The present application claims the benefit of the filing date of U.S. Provisional Application Serial No. 60/353,795; filed Feb. 1, 2002, the disclosure of which is expressly incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to fluid-operated cylinder assemblies, and more particularly to fluid-operated cylinder assemblies utilizing a split ring coupling.  
         BACKGROUND OF THE INVENTION  
         [0003]    Fluid-actuated cylinders are known which have an elongated piston rod located in a cylinder housing and supported for longitudinal movement. In certain of these cylinders, typically referred to as “double rod end cylinders”, the rod is supported such that the ends of the rod project outwardly from opposite ends of the housing. A piston assembly is attached to the rod along the length of the rod, and includes a ring-shaped piston which is fluidly sealed to the housing as the rod reciprocates, to define first and second fluidly-separated chamber portions at opposite ends of the housing. The controlled introduction/removal of fluid in the chamber portion(s) moves the piston rod in the desired direction in the housing.  
           [0004]    Various techniques have been developed to attach the piston assembly to the rod. One known technique is to have a pair of piston rod portions, and to capture the ring-shaped piston between the ends of the rod portions as the rod pistons are connected together (such as by a threaded connection). Another known technique is to turn and grind a unitary (one piece) piston rod, and to thread the piston rod into a piston. In this technique, the rod can have an enlarged, radially-projecting annular shoulder which is received within a counterbore in the piston, to properly locate the piston on the piston rod.  
           [0005]    A still further technique is to provide two piston halves, screw threads on a piston rod, and to locate a wire in the slots defined by the threads. The piston halves are then screwed onto the piston rod against one another, with the wire retaining the piston halves on the rod.  
           [0006]    The above techniques are useful in many applications; however, they can require special, higher-cost material for the piston rod; have concentricity issues (between the piston and rod; as well as between two piston portions); require tight tolerances between the inter-engaging threads; and can require complicated machining and/or assembly, which make many of these techniques expensive to manufacture and repair.  
           [0007]    It is further known to form a groove in the piston rod and to locate a split ring in the groove, and then to connect the split ring by some means (such as screws or bolts) to the piston. Some applications of this technique use radial or axial pressure to retain the split ring within the groove, such as by threaded locking nuts bearing against one side of the split ring (U.S. Pat. No. 3,426,657); a two-piece piston ring clamped around a split ring (U.S. Pat. No. 4,004,499); or slip-fit locking nuts or bearing bands outwardly surrounding the split ring (U.S. Pat. No. 3,457,842). U.S. Pat. No. 4,180,274, as a further example, uses multiple tongue and grooves formed in the split ring and piston rod, and then an additional snap ring to retain the snap ring on the piston. However, as can be appreciated, these applications also require additional components, multiple machining steps and/or close tolerances with the piston and the split ring. As such, it is believed that, regardless of the technique used, several complex components and/or machining steps are necessary in the known prior art to hold the piston on the piston rod.  
           [0008]    It is therefor believed there is a demand for an improved technique for attaching a piston to a piston rod, and particularly an improved technique for retaining a piston to a piston rod which employs a split ring, where the technique is simple and straightforward, does not require additional components or difficult machining steps, and thereby reduces the manufacturing and repair expenses associated with the fluid cylinder.  
         SUMMARY OF THE PRESENT INVENTION  
         [0009]    The present invention provides a novel and unique attachment of a piston to a piston rod which is simple and straightforward, does not require additional components or difficult machining steps, and thereby reduces the manufacturing and repair expenses associated with the fluid cylinder.  
           [0010]    According to the present invention, a piston rod with a circumferential groove is provided, and a split ring, with predetermined linear expansion properties, is first cooled to a temperature below the temperature of the piston rod. The ring normally has an axial length somewhat longer than the axial length of the groove in the piston rod, but when cooled, has a length somewhat smaller than the groove, such that the split ring coupling can be easily located in the groove. The ring is then equalized with the temperature of the piston rod, and in so doing, expends into compression against the sidewalls of the groove. This expansion creates an interference fit with the groove, and prevents axial and rotational movement of the ring with respect to the piston rod. The expansion also prestresses the piston rod between the groove sidewalls, which increases the fatigue resistance of the piston rod.  
           [0011]    After the ring is located and fixed to the rod as above described, the ring halves are then connected such as with bolts to an internal, radially-inward projecting annular flange on the piston. The bolts are received through axial openings in the split ring coupling, and into corresponding axial threaded openings in the annular flange of the piston to securely and rigidly fix the piston with respect to the piston rod.  
           [0012]    The piston rod assembly is then located in a cylinder housing, with the piston providing a fluid seal with the cylinder walls as the piston rod reciprocates within the housing.  
           [0013]    In some applications, a further circumferential groove, spaced from the first groove, can be provided on the outer surface of the piston rod, and an annular seal can located in the further groove in sealing engagement with piston, if necessary or desirable.  
           [0014]    The technique as described above is particularly useful for double rod end cylinders, but also finds applicability to single rod end cylinders, as well as to other applications where it is necessary or desirable to use a split ring to couple a piston to a piston rod.  
           [0015]    The present invention thereby provides an improved technique for attaching a piston to a piston rod, and particularly an improved technique for retaining a piston to a piston rod which employs a split ring, where the technique is simple and straightforward, does not require additional components or difficult machining steps, and thereby reduces the manufacturing and repair expenses associated with the fluid cylinder.  
           [0016]    Further features of the present invention will become apparent to those skilled in the art upon reviewing the following specification and attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a cross-sectional side view of a cylinder constructed according to the principles of the present invention;  
         [0018]    [0018]FIG. 2 is an enlarged view of a portion of the piston rod and piston assembly of FIG. 1;  
         [0019]    [0019]FIG. 3 is an exploded view of the piston rod and piston assembly for the cylinder of FIG. 1;  
         [0020]    [0020]FIG. 4 is an assembled view of the piston rod and piston assembly of FIG. 3;  
         [0021]    [0021]FIG. 5 is an end view of the piston rod and piston assembly; and  
         [0022]    [0022]FIG. 6 is an enlarged view of a portion of the piston rod and piston assembly of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    Referring to the drawings, and initially to FIG. 1, a fluid-actuated cylinder constructed according to the principles of the present invention is indicated generally at  10 . The cylinder  10  includes an elongated, hollow, preferably tubular housing  12  having cylinder heads  14 ,  16  enclosing opposite ends of the housing. The housing  12  and cylinder heads  14 ,  16  define an internal piston chamber  17 . An elongated, preferably unitary (one piece), piston rod  18  is slideably supported within the piston chamber, and moveable in both axial directions through circular openings formed in the cylinder heads. Inlet and outlet ports  19 ,  20  are provided in cylinder heads  14 ,  16 , respectively, to direct fluid into and out of opposite ends of the piston chamber  17  to reciprocate the piston rod, as will be described in more detail below.  
         [0024]    Cylinder heads  14 ,  16  are preferably removably connected to the housing  12  in any appropriate manner, as should be well within the knowledge of those skilled in the art. As illustrated, elongated tie rods  22  project through the housing from one end to the other, and nuts  23  are provided on the ends of the tie rods to retain the cylinder heads on the housing, although again, other techniques could be used. Seals  24  can be provided at the ends of the housing where the housing butts up against the cylinder heads to fluidly seal the housing to the cylinder heads.  
         [0025]    Bushings  25 ,  26  are bolted on the cylinder heads  14 ,  16 , respectively, and carry inner and outer seals/wipers  27  to provide a fluid seal against the piston rod  18  as the piston rod reciprocates within the housing. The seals/wipers  27  can take many shapes, and are retained on the bushings in any appropriate manner.  
         [0026]    The aforementioned components of the cylinder housing are conventional and should be well-known to those of ordinary skill in the art. The components are made out of material appropriate for the particular application, preferably using conventional manufacturing techniques. It should be appreciated that the above is only one embodiment of the present invention, and that the present invention is appropriate for many embodiments of a fluid-actuated cylinder. In particular, it should be appreciated that the illustrated embodiment shows a double rod end cylinder, that is, a cylinder where the piston rod projects outwardly from both ends of the cylinder. This type of rod is appropriate for many applications, and the present invention is particularly useful with this type of cylinder. However, it should be appreciated that the present invention is also useful with other cylinder types, such as single rod end cylinders. Thus, the description of the present invention being used with a double ended cylinder is meant to be exemplary in nature, and is not meant to be limiting to the illustrated embodiment.  
         [0027]    Likewise, the piston rod is illustrated as being formed from a single piece, i.e., having a unitary form. This is preferred, as it allows the piston rod to be manufactured in a minimum number of steps, and maintains straightness and concentricity (e.g., as compared to a two-piece piston rod); although again, it should be appreciated that the present invention could be just as useful with multiple piece piston rods, if the application so required.  
         [0028]    In any case, referring now to FIGS.  1 - 3 , the piston rod  18  includes a groove  28  formed (machined) circumferentially around the outer surface of the rod. The groove can be formed at any location appropriate for the particular application, and is preferably formed toward the middle for a double rod end cylinder. As shown particularly in FIG. 3, the groove  28  includes a recessed sidewall portion  29  and a pair of opposing sidewalls  30 ,  31  at opposite ends of the sidewall portion  29 . The sidewalls  30 ,  31  project radially outward preferably substantially normal/transverse to the axis of the piston rod.  
         [0029]    A piston assembly, indicated generally at  34 , is fixed to the piston rod  18 , and provides a fluid-tight seal between the piston rod and the housing  12  as the piston rod reciprocates. The piston assembly  34  divides the piston chamber  17  into fluidly-separated chamber portions  35 ,  36 , each of which is in communication with a respective port  19 ,  20 . As should be appreciated, the introduction of fluid into one of ports  19 ,  20  and the removal of fluid from the other of ports  19 ,  20 , increases the fluid pressure against one side of the piston assembly, which moves the piston assembly, and piston rod, axially within the cylinder. Of course, the introduction and removal of fluid from the opposite port arrangement causes the piston rod to move in the other direction. Thus, controlling the introduction and removal of fluid in the cylinder causes the piston rod to reciprocate within the cylinder housing  12 .  
         [0030]    The piston assembly  34  includes i) a ring-shaped piston  40 ; ii) a split ring coupling, indicated generally at  42 ; and iii) fasteners, indicated generally at  44 , for fastening the split ring coupling  42  to the piston  40 . As shown in FIGS.  1 - 4 , the piston  40  has a smooth exterior surface  45  and flat annular end faces  46 ,  47 , and is dimensioned to be closely received within the housing  12 . One or more exterior circumferentially extending channels or grooves  48  can be formed along the length of the piston, and appropriate seals/wipers  49  can be located therein to provide a fluid-tight seal with the housing  12 . The axial length of the piston can vary depending upon the particular application, as should be appreciated by those skilled in the art.  
         [0031]    The piston  40  preferably has a smooth annular inner surface  51 , and includes an internal, radially inward projecting annular flange  52 , located toward one end of the piston. The annular flange  52  also has a smooth annular inner surface  53  with a diameter dimensioned to closely receive the piston rod  18 ; and a forward annular end surface  54 . A series of evenly-spaced, axially extending threaded bolt holes , as at  55 , are provided in flange  52 , the reason for which will be described more fully below.  
         [0032]    The split ring coupling, as shown in FIGS.  1 - 3 , preferably comprises a pair of semi-circular ring portions  42   a ,  42   b , which when assembled together (end-to-end), define a continuous ring. While two ring portions are shown, it should be appreciated that more than two ring portions could be used, although it understandably then becomes more difficult to manufacture and assemble a multi-piece split ring. The split ring coupling  42  has a smooth exterior surface  58  with an outer diameter that fits loosely within the inner surface  51  of the piston  40 ; and a pair of annular end faces  60 ,  61 , each of which extend substantially transversely to the axis of the ring.  
         [0033]    The split ring coupling is received within the groove  28  formed along the piston rod  18 . The split ring coupling has a smooth inner surface  63  with an inner diameter which loosely surrounds the recessed sidewall portion  29  of groove  28 , and which is therefore slightly smaller than the outer diameter of the remainder of the piston rod  18 . The depth of the groove and the radial dimensions of the split ring coupling can vary depending upon the application (e.g., the desired load carrying capacity of the piston assembly), and can also be readily calculated by those of ordinary skill in the art. The split ring coupling  42  has an axial length “L” which is slightly longer than the axial distance “D” between the sidewalls  30 ,  31  of the groove, at least when the split ring coupling and piston rod are at the same temperature. To locate the split ring coupling in the groove, the ring portions  42   a ,  42   b  are cooled to a temperature somewhat below the temperature of the piston rod such that the split ring coupling has an axial dimension less than the distance between the sidewalls, and the coupling loosely fits within the groove. The exact temperature necessary for cooling the coupling portions can be easily determined by the axial length of the split ring coupling portions, the axial length of the groove, the coefficient of thermal (linear) expansion of the coupling material, and the desired fit (compression) between the split ring coupling  42  and the sidewalls  30 ,  31  of the groove  28  depending on the load capacity of the cylinder.  
         [0034]    After the split ring coupling is cooled and located in the groove, the split ring coupling is brought back up to the temperature of the piston (i.e., ambient temperature), such that the temperature is equalized between the split ring coupling and the piston rod. When so assembled, the end faces  60 ,  61  of the split ring coupling directly engage the sidewalls  31 ,  30 , respectively, of the groove  28 . Compression occurs between the split ring coupling and the sidewalls of the groove. An interference fit is preferably created between the split ring coupling and the piston rod, such that the coupling is axially and rotationally fixed with respect to the rod. An additional benefit is that the piston rod is put under stress (i.e., “prestressed”), at least in the axial area between the sidewalls of the groove, which is believed prolongs the life of the piston rod during repeated cycling.  
         [0035]    The split ring coupling has a series of evenly-spaced, axially extending through holes as at  70 , and the piston  42  is slid over the piston rod/split ring coupling subassembly, such that the annular end face  61  of the split ring coupling is located in adjacent, surface-to-surface relation with the annular end face  54  of the piston, and through holes  70  in the split ring coupling are aligned with the threaded holes  55  in the inner flange of the piston. A pair of dowel pins  78  can be located in holes  79  in the split ring coupling and in the flange  52  of the piston to properly rotationally align these components.  
         [0036]    The fasteners  44  preferably comprise a series of elongated bolts or mounting screws, which are then received in through holes  70  in the split ring coupling and corresponding threaded holes  55  in the piston ring, to securely and rigidly fix the split ring coupling to the piston, and hence securely and rigidly fix the piston to the piston rod. Appropriate locking compound can be used with the bolts/screws, if necessary or desirable. The number of bolts/screws appropriate for the particular application can vary, and can be easily determined by those of ordinary skill in the art.  
         [0037]    In many applications, it is believed a fluid-tight mechanical seal will be created between the piston  40  and piston rod  18 , by virtue of the attachment technique described above. However, if necessary or desirable, a further circumferential groove  80  (see also, FIG. 6) can be provided in the exterior surface of the piston rod in the area surrounded by the inner surface  53  of the piston, and an O-ring  81  and a pair of back-up rings  82  can be located in the groove, to further provide a seal between the piston and rod.  
         [0038]    Thus, as described above, the present invention thereby provides an improved technique for attaching a piston to a piston rod, and particularly an improved technique for retaining a piston to a piston rod which employs a split ring, where the technique is simple and straightforward, does not require additional components or difficult machining steps, and thereby reduces the manufacturing and repair expenses associated with the fluid cylinder.  
         [0039]    The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein should not, however, be construed as limited to the particular form described as it is to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the scope and spirit of the invention as set forth in the appended claims.