Abstract:
A crosshead for a piston rod comprises a main body having a first seat and a second seat being configured to hold a connecting rod, the second seat being configured to hold a piston rod; the main body comprising a linking portion defining at least partially the first seat and the second seat for holding respectively the connecting rod and the piston rod; the main body comprising a closing portion defining at least partially the second seat and being configured to secure the piston rod inside the second seat.

Description:
BACKGROUND 
       [0001]    Embodiments of the invention relate to a crosshead for a piston rod, 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]    Such crosshead is known in the state of the art. The crosshead comprises a main body having a first end facing the crankshaft and a second end facing the piston. A connecting rod is hinged on the first end, thus connecting it to the crankshaft. A piston rod is attached to the second end. Specifically, the piston rod is engaged into a receptacle of a flange, and it is retained therein primarily by mechanical interference. The flange itself is bolted onto the second end of the main body. Additionally, crosshead is provided with sliding shoes attached to the main body and slidably engaged on an internal wall of the cylinder. The sliding shoes themselves absorb the above mentioned sideway forces, allowing the crosshead to maintain the alignment with respect to the longitudinal axis of the piston. 
         [0006]    Several disadvantages are apparent in the above described prior art crosshead. The crosshead itself is a particularly complex device, which necessitates a lot of components that need to be machined within strict tolerances. The assembly procedure is also long and complicated, as it is necessary to bolt the piston rod to the flange before the flange itself can be connected to the main body of the crosshead. Specifically, a counterbolt is screwed onto the threaded end of the piston rod, so as to secure firmly the piston rod onto the flange. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    A first embodiment of the invention relates to a crosshead for a piston rod comprising a main body. The main body has a first and a second seat in order to hold a connecting rod and a piston rod respectively. The main body comprises a linking portion, which defines at least partially the first and the second seat. The main body also comprises a closing portion, which defines at least partially the second seat and is configured to secure the piston rod inside the second seat. 
         [0008]    This embodiment achieves several advantages over the prior art. Indeed, the two portions can retain the piston rod purely by mechanical interference, removing the need for a screwed counterbolt. 
         [0009]    Also, the number of components is reduced, along with their complexity. This simplifies the assembly of the crosshead. 
         [0010]    An embodiments of the present invention is also embodied in a method for assembling a crosshead according to the first embodiment. The method comprises the steps of joining the linking and the closing portions while placing a piston rod inside the second seat. Securing the two portions together, more particularly with a fastening element. 
         [0011]    This embodiment is advantageous over the prior art, because there is no need to force the piston rod inside a precisely sized receptacle. Also, screwing the counterbolt directly over the piston rod is no longer necessary, as the load applied by the fastening element is sufficient to hold it in place. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further details and specific embodiments will refer to the attached drawings, in which: 
           [0013]      FIG. 1  is a perspective view of a crosshead for a piston rod according to an embodiment of the present invention; 
           [0014]      FIG. 2  is an exploded perspective view of the crosshead of  FIG. 1 ; 
           [0015]      FIG. 3  is a sectional view of the crosshead of  FIGS. 1 and 2 ; 
           [0016]      FIGS. 4A and 4B  are plan views of the crosshead according to respective embodiments of the present invention; 
           [0017]      FIGS. 5A, 5B, 5C, and 5D  are longitudinal sectional views of a detail of the crosshead of  FIG. 1  according to different embodiments of the present invention. 
       
    
    
     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 for a piston rod according to an embodiment of the present invention. The crosshead  1  has the purpose of connecting comprises a main body  3 . The main body  3  is used to link a connecting rod  2  with a piston rod  4 , as shown for example in  FIG. 1 . 
         [0021]    Indeed, the main body  3  has a first  5  and a second seat  6 . The first seat  5  is configured to hold the connecting rod  2 . The second seat  6  is configured to hold the piston rod  4 . In an embodiment, the first  5  and the second seat  6  are positioned on opposite ends of the main body  3 . 
         [0022]    The main body  3  has a longitudinal axis “A”, along which the first  5  and the second seat  6  are substantially aligned. Indeed, the longitudinal axis “A” is the longitudinal axis of the piston rod  4 . In other words, the longitudinal axis “A” identifies the direction along which the crosshead  1  moves during normal functioning. The main body  3  also has a hinge axis “B” in the first seat  5 , around which the connecting rod  2  can rotate. The hinge axis “B” is in an embodiment perpendicular to the longitudinal axis “A”. 
         [0023]    The crosshead  1  also comprises sliding shoes (not shown) attached to the main body  3  and designed to allow a reciprocating movement along the longitudinal axis “A”. The sliding shoes will not be further described, as they are themselves known to the person skilled in the art. 
         [0024]    In detail, the main body  3  comprises a linking portion  3   a , which defines at least partially the first  5  and the second seat  6 . The main body  3  comprises a closing portion  3   b , which defines at least partially the second seat  6 . The closing portion  3   b  is configured so as to secure the piston rod  4  inside the second seat  6 . 
         [0025]    According to the embodiments of the present invention, the shape of the portions  3   a ,  3   b  are designed so that, when linked together, they define the main body  3  according to the desired dimensions and specifications. 
         [0026]    With regards to the shape of the portions  3   a ,  3   b , there are two different embodiments of the present invention: one in which they are symmetrical and the other one in which they are not. 
         [0027]    In the embodiment shown in  FIG. 4 a   , the linking portion  3   a  and the closing portion  3   b  are substantially symmetrical. The plane of symmetry is perpendicular to the hinge axis B. In other words, the closing portion  3   b  of the main body  3  also defines at least partially the first seat  5 . Indeed, as shown in  FIG. 4 a   , the linking portion  3   a  and the closing portion  3   b  are two half-parts of the main body  3 . More particularly, in this embodiment the connecting rod  2  and the pin  12  can be integrally formed. 
         [0028]    In the embodiment of  FIG. 4 b   , the first seat  5  is located exclusively on the linking portion  3   a.    
         [0029]    Further details of the linkage between the portions  3   a ,  3   b , will be given in a following part of the present disclosure. 
         [0030]    The above mentioned first seat  5  is designed to provide a way to link the crosshead  1  to the connecting rod. In order to achieve this purpose, the main body  3  is provided with two holes  7 , more particularly coaxial, which can hold a pin  12 . Indeed, the above mentioned hinge axis “B” is defined by the axis of the holes  7 . As discussed above, according to different embodiments of the present invention, the pin  12  can be either inserted in a further hole  8  of the connecting rod  2 , or can be integrally formed along with the connecting rod  2 . 
         [0031]    The second seat  6  is formed as a bore  9  in the main body  3 . Specifically, the bore  9  develops along the longitudinal axis “A” of the main body  3 . With greater detail, the second seat  6  has a lateral surface  6   a  configured to engage the piston rod  4 . Indeed, the lateral surface  6   a  of the second seat  6  extends at least along the aforementioned longitudinal axis “A”. In other words, the lateral surface  6   a  develops around the longitudinal axis “A”. 
         [0032]    More particularly, the lateral surface  6   a  is defined by both the linking portion  3   a  and the closing portion  3   b . Specifically, the linking portion  3   a  and the closing portion  3   b  have each an angular extension of 180° with respect to the longitudinal axis “A”. In other words, half of the lateral surface  6   a  is on the linking portion  3   a , while the other half is on the closing portion  3   b.    
         [0033]    Also, the piston rod  4  has an engagement surface  4   a  configured to contact at least partially the lateral surface  6   a  of the second seat  6 . Therefore, the shape of the engagement surface  4   a  will be correlated to the shape of the lateral surface  6   a  of the second seat  6 . In other words, the lateral surface  6   a  is at least partially complementary to the engagement surface  4   a  of the piston rod  4 . 
         [0034]    Additionally, in the embodiment shown in  FIG. 5 b    is shown that the piston rod  4  can also be provided with at least one shoulder  13 , configured to contact the main body  3 . In an embodiment, the piston rod  4  can be provided with two shoulders  13 . In this case, the engagement surface  4   a  of the piston rod  4  is defined between the two shoulders  13 . If the distance between the two shoulders  13  is wider than the lateral surface  6   a  one or two gaps  14  will be defined between the main body  3  and the shoulders  13 . 
         [0035]    These gaps  14  can either stay empty if the load transmission is left to the friction between the piston rod  4  and the second seat  6 . Otherwise, a filler element (not shown) can be inserted in one or both of the gaps  14 . More particularly, this can be used to fine-tune the piston-crosshead-cylinder assembly inside a reciprocating compressor. 
         [0036]    In an embodiment of the invention, shown in  FIG. 1 , the lateral surface  6   a  is a semicylindrical surface. Within the present disclosure, the term “semicylindrical surface” is meant as half the lateral surface of a right circular cylinder cut along an axial plane of symmetry. In this embodiment, the engagement surface  4   a  of the piston rod  4  will also be semicylindrical. 
         [0037]    In an alternative embodiment, the lateral surface  6   a  is at least partially a semiconical surface. Within the present disclosure, the term “semiconical surface” is meant as half the lateral surface of a right circular cone cut along an axial plane of symmetry. Specifically, in the embodiment of  FIG. 5 a    the lateral surface  6   a  is doubly semiconical, meaning that it comprises two semiconical surfaces joined together. In this case the engagement surface  4   a  of the piston rod  4  will be semiconical. More particularly, the partially semiconical embodiment of the lateral surface  6   a  can provide an improved hold on the piston rod  4 . 
         [0038]    In the alternative embodiment of  FIG. 5 c   , the second seat  6  is provided with a plurality of ridges  16  on the lateral surface  6   a . These ridges  16  can develop circumferentially around the longitudinal axis “A”. Alternatively, the ridges  16  can be formed out of a single helical structure, in a manner substantially similar to a screw thread. The piston rod  4  is correspondingly provided with teeth  15  on its engagement surface  4   a . These teeth  15  also develop around a central axis of the piston rod  4  (corresponding to the longitudinal axis “A” in the drawings) so that they can engage the ridges  16  on the lateral surface  6   a  of the second seat  6 . 
         [0039]    In the embodiment shown in  FIG. 4 c    the teeth  15  have progressive growing length from the direction of the piston head to the crosshead  1 . More particularly, this solution allows for better handling of compression loads. In a corresponding embodiment (not shown in the figures) the teeth  15  have progressively decreasing length along the same direction. This enables a better handling of traction loads. In further embodiment these profiles of the teeth  15  can be combined together, in order to optimize the load bearing capability for a specific application. Indeed, the profiles of the teeth  16  can either first decrease and then increase, so as to have the maximum length in the middle. Alternatively, the teeth  16  can first decrease and then increase their length, in order to have a tooth  16  of minimum length in the middle. In other words it is possible to vary the shape of the teeth  15  on order to distribute uniformly the load on all teeth  15 . 
         [0040]    In a further embodiment, shown in  FIG. 5 d   , the lateral surface  6   a  has a sinusoidal profile in the plane of the longitudinal axis “A”. More generally, any periodic and/or repeating profile that can provide an improved grip on the piston rod  4  can also be used. The choice of the specific profile will be made depending on the requirements of the specific application. In this embodiment the engagement surface of the piston rod  4   a  has a matching profile to the lateral surface  6   a.    
         [0041]    In all the described embodiments, the piston rod  4  is held in the second seat  6  primarily by mechanical interference. Indeed, the second seat  6  has an internal diameter which is less than an external diameter of the piston rod  4 . More particularly, the difference between the internal diameter of second seat  6  and the external diameter of the piston rod  4  is comprised between 0.05 and 0.3 percent of the external diameter of the piston rod  4 , so that the second seat  6  can grip onto the piston rod  4  by mechanical interference. 
         [0042]    To join the linking  3   a  and the closing portions  3   b , the crosshead  1  comprises a fastening element  10 . Additionally, the fastening element  10  also locks the piston rod  4  in the second seat  6 . In greater detail, the fastening element  10  is configured to apply a load to the linking  3   a  and to the closing portion  3   b , in order to push them together in a direction substantially perpendicular to he longitudinal axis “A”. 
         [0043]    In some embodiments of the present invention the fastening element can comprise one or more bolts  11 . Each bolt  11  develops along a transversal direction “C”, perpendicular to the longitudinal axis “A”. As can be seen from  FIG. 1 , the fastening element can comprise six bolts  11 , symmetrically placed with respect to the bore  9 . The number of the bolts  11  can vary as required by the specifications in a particular application. 
         [0044]    To assemble the above described crosshead, the operator has to force the piston rod  4  inside the second seat  6 . This can be accomplished either by a purely mechanical forcing or by pre-heating the linking portion  3   a . If a connecting rod  2  with an integral pin  12  is used, they are also placed in the first seat  5  at this time. The closing portion  3   b  is then joined to the linking portion. If it is deemed appropriate, the closing portion  3   b  can also be pre-heated. Alternatively, the piston rod  4  can be cooled. 
         [0045]    The portions  3   a ,  3   b  are then secured together, in an embodiment with the fastening element  10 . Specifically, a load is also applied to the fastening element  10 , in order to block the piston rod  4  between the portions  3   a ,  3   b  by mechanical interference. This is done more particularly by screwing the nuts on the bolts  11 . 
         [0046]    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.