Abstract:
A crosshead-piston rod assembly for a reciprocating compressor comprises a piston rod; a crosshead having a bore; a connection element attached to the piston rod and at least partially inserted into the bore for connecting the crosshead to a piston.

Description:
BACKGROUND 
       [0001]    Embodiments of the present invention relate to a crosshead-piston rod assembly, specifically in a reciprocating compressor. 
         [0002]    In the technical field of reciprocating machines (i.e. machines having a piston moving inside a cylinder such as, for example, internal combustion engines) the piston is usually connected to a crankshaft directly via a connecting rod. Specifically, the connecting rod is linked to the crankshaft via a crankpin. In this arrangement, however, the sideways forces from the connecting rod are transmitted directly to the piston, as the crankpin (and thus the direction the force is applied) moves from side to side with the rotary motion of the crankshaft. 
         [0003]    These transverse forces are tolerable in a smaller reciprocating machine. However, in a larger one, such as for example a large reciprocating compressor, this would result in much greater forces, consequently causing an intolerable degree of wear on the piston and cylinder, as well as increasing overall friction. 
         [0004]    Therefore, it becomes necessary to decouple the transverse movement of the crankpin from the axial movement of the piston. To perform this function, a crosshead links a piston rod (which is part of the piston) with a connecting rod (which is linked to the crankshaft via the crankpin). The sideways forces transmitted by the connecting rod are absorbed by the crosshead itself, thus allowing the piston rod to move along its axis with negligible transverse load. 
         [0005]    A crosshead-piston rod assembly is known in the state of the art, comprising a piston and a crosshead. The piston comprises a head, which is inserted into a cylinder, and a rod attached to the head. The crosshead comprises a main body having a first end linked to the crankshaft and a second end attached to the piston rod. Additionally, the crosshead is provided with bearings attached to the main body and slidably engaged on an internal wall of the cylinder. The bearings themselves absorb the above mentioned sideway forces, allowing the crosshead to maintain the alignment with respect to the longitudinal axis of the piston. 
         [0006]    In greater detail, the known crosshead comprises a flange which is attached to the main body, specifically onto the second end. The rod has a shoulder and a neck part. The flange has a through bore into which the rod, specifically the neck part, can fit, while the shoulder of the rod is placed in contact with a surface of the flange. In other words, the rod is partially inserted into the bore of the flange, so that a threaded end emerges from the other side. 
         [0007]    To assemble the piston to the crosshead, a nut is screwed onto the threaded end so that the flange is locked between the nut and the shoulder of the rod. The flange is then bolted to the main body of the crosshead. During normal operations of the piston, the loads are mainly of the axial kind. Indeed, this causes a concentration of stress in the zone of the rod between the neck and the shoulder. 
         [0008]    The main disadvantage of the known crosshead-piston rod assembly is that such a concentration of stress in a very small area is often enough to bring the material of the rod to plastic deformation, thus compromising the structural integrity of the rod. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0009]    A first aspect of the invention is therefore directed to a crosshead-piston rod assembly for a reciprocating compressor. This assembly comprises a piston rod and a crosshead having a bore. A connection element is attached to the rod and at least partially inserted into the bore in order to connect the crosshead to the piston. 
         [0010]    This achieves the advantage of improving the overall resistance of the assembly, since the connection element is distinct from the piston rod, and therefore it can be made from a stronger material. 
         [0011]    Another aspect of the invention relates to the fact that the rod has a contact surface for receiving the connection element, while the connection element itself has a gripping surface for engaging the contact surface of the rod. Indeed, in this way the contact between the rod and the connection element is distributed over a relatively large area, further lowering the mechanical stress in that area. 
         [0012]    A third aspect of the invention relates to the fact that the connection element has a first and a second end. The rod has a seat for receiving the first end of the connection element. The above mentioned contact surface is located inside the seat, while the gripping surface is located on the first end of the connection element. This arrangement further improves the stress distribution inside the connection element and the rod, as the stress does not concentrate in a very small area like in the prior art rod. 
         [0013]    A fourth aspect of the invention relates to a female thread onto the contact surface of the rod. The connection element comprises a male thread onto the gripping surface, so that it can engage the female thread. This allows an easy assembly of the rod with the connection element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Further details and specific embodiments will refer to the attached drawings, in which: 
           [0015]      FIG. 1  is a perspective view of a crosshead-piston rod assembly according to an embodiment of the present invention; 
           [0016]      FIG. 2  is a longitudinal sectional view of the crosshead-piston rod assembly of  FIG. 1 ; and 
           [0017]      FIG. 3  is a schematic longitudinal sectional view of a detail of the crosshead-piston rod assembly of  FIGS. 1 and 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The following description of exemplary embodiments refer to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
         [0019]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0020]    Referring to the attached figures, with the number  1  is indicated a crosshead-piston rod assembly according to an embodiment of the present invention. The crosshead-piston rod assembly  1  comprises a piston rod  4  and a crosshead  3 . 
         [0021]    Indeed, the piston rod  4  itself can be connected to a piston head (not shown in the figures) in order to define a piston. The piston rod  4  can thus provide the head with a reciprocating motion so that it can, for example, compress a working fluid in a cylinder (not part of the present invention and not shown in the attached figures). For example, the working fluid can be hydrogen, carbon dioxide, methane, ethylene or mixtures of hydrocarbons. 
         [0022]    Indeed, the piston rod  4  develops along a longitudinal axis “A”. In other words, the piston rod  4  is elongated in the direction of the longitudinal axis “A”, as shown in  FIG. 2 . In operation, the rod slides along the longitudinal axis “A” in order to move the head of the piston. 
         [0023]    For linking to the piston, in particular to the piston rod  4 , the crosshead  3  is provided with a bore  5 . Furthermore, the assembly  1  comprises a connection element  6 . This connection element  6  is linked to the piston rod  4  and at least partially inserted into the bore  5 , so that it connects the crosshead  3  to the piston. 
         [0024]    More particularly, it is therefore possible to make the connection element  6  from a different material with respect to the piston rod  4 . Specifically, it is a benefit to realize the connection element  6  from a stronger material and the piston rod  4  from a lower cost material, since the connection element  6  is subject to a higher mechanical stress as it will be apparent from a following part of the present disclosure. 
         [0025]    With more detail, the piston rod  4  has a contact surface  7  for receiving the connection element  6 . The connection element  6  has a gripping surface  8  that is able to engage the contact surface  7  of the piston rod  4 . Specifically, it is to be noted that the interaction between the gripping surface  8  and the contact surface  7  is distributed over a wide area. This is a benefit, since it favors an overall lower mechanical stress. 
         [0026]    Indeed, the connection element  6  develops along the longitudinal axis “A” of the piston rod  4 . Specifically, the connection element  6  and the piston rod  4  are coaxial. More particularly, the connection element  6  is symmetrical with respect to the longitudinal axis “A”. 
         [0027]    In greater detail, the connection element  6  has a first  6   a  and a second end  6   b.  The piston rod  4  has a seat  4   a  for receiving the first end  6   a  of the connection element  6 . In other words, the first end  6   a  of the connection element  6  fits inside the seat  4   a  of the piston rod  4 . 
         [0028]    The piston rod  4  is provided with an abutment surface  16  which defines an edge of the seat  4   a.  Furthermore, the abutment surface  16  is oriented transversally with respect to the longitudinal axis “A”, more particularly perpendicularly. 
         [0029]    As shown in  FIG. 3 , the above mentioned contact surface  7  is located inside the seat  4   a.  The gripping surface  8  is located on the first end  6   a  of the connection element  6 . 
         [0030]    Furthermore, the seat  4   a  is substantially cylindrical. Consequently, the first end  6   a  of the connection element  6  is also cylindrical, thereby ensuring a precise coupling with the seat  4   a  of the piston rod  4 . 
         [0031]    With further detail, the piston rod  4  comprises a female thread  9  onto the contact surface  7 . The connection element  6  comprises a male thread  10  onto the gripping surface  8 , so as to engage the female thread  9 . 
         [0032]    As shown in  FIG. 2 , the connection element  6  has the shape of a tie bolt, which works primarily under an axial load. To apply this load during assembly, the connection element  6  comprises a further male thread  11  located more particularly on the second end  6   b.  The assembly  1  further comprises a nut  12  attached to the connection element  6 , more particularly onto the second end  6   b.  Specifically, the nut  12  is screwed onto the further male thread  11  of the connection element  6 . 
         [0033]    The crosshead  3  comprises a main body  13 , onto which sliding shoes  15  are attached. The sliding shoes  15  are themselves known to the person skilled in the art, and will therefore not be described in further detail. 
         [0034]    The crosshead  3  also comprises a flange  14 , which is attached to the main body  13 . The function of the flange  14  is to make the assembly easier, as the piston rod  4  can be linked to the flange  14  before the flange  14  itself is connected to the main body  13 . 
         [0035]    As shown in  FIG. 1 , in an embodiment the flange  14  is a plate, more particularly a moderately thick plate. The above mentioned bore  5  is located in a central zone of the flange  14 . Also, the bore  5  extends through the flange  14 , more particularly along the longitudinal axis “A” of the piston rod  4 . In other words, in operating conditions the bore  5  is aligned to the longitudinal axis “A” of the piston rod  4 . 
         [0036]    Indeed, the connection element  6  extends through the bore  5  across the flange  14 . In other words, the connection element  6  crosses the flange  14 . The above described abutment surface  16  is therefore placed in contact with the flange  14 . The screwing of the nut  12  onto the second end  6   b  provides the connection element  6  with an axial load, thereby pressing the abutment surface  16  against the flange  14 . 
         [0037]    A plurality of holes  17  are located on the flange  14 . Specifically, the holes  17  ere located on two sides  14   a  of the flange  14 . With more detail, the flange  14  has two holes  17  on each side  14   a.  A bolt  18  is provided for each hole  17 , in order to attach the flange  14  to the main body. 
         [0038]    The assembling operation of the piston rod  4  with the crosshead  3  is as follows. The connection element  6  is attached to the piston rod  4 . Specifically, the first end  6   a  of the connection element  6  is screwed into the seat  4   a  of the piston rod  4 . The connection element  6  is therefore inserted into the bore  5  of the flange  14 . The nut  12  is screwed onto the second end  6   b  of the connection element  6 , thereby securing the flange  14  against the piston rod  4 , in particular on the abutment surface  16 . The flange  14  is therefore attached to the main body  13 . Specifically, the bolts  16  are installed each in its respective hole  17 . 
         [0039]    This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.