Abstract:
A device and method for machining low case hardened steel connecting rods, whereby a big end of a connecting rod is subjected to traction to break a cap off the big end; traction is exerted by two traction members, which are inserted inside an eye of the connecting rod and are parted in a direction parallel to a rod of the connecting rod; and the traction members have thrust surfaces with different radii of curvature.

Description:
[0001]    The present invention relates to a device for machining connecting rods.  
           [0002]    The present invention may be used to advantage for machining low casehardened steel connecting rods, to which the following description refers purely by way of example.  
         BACKGROUND OF THE INVENTION  
         [0003]    Here and hereinafter, low case hardened steel connecting rods are intended to mean connecting rods made of case hardened steel with a case depth of less than eight millimetres.  
           [0004]    Connecting rods made of low case hardened steel have been proposed to enable them to better withstand the stress imposed by modern combustion engines.  
           [0005]    Machining low case hardened steel connecting rods, however, poses various problems, owing, in particular, to the relatively high residual “ductility” of this type of material.  
           [0006]    A connecting rod normally comprises a rod, and a big end in turn comprising an eye bounded by an inner annular surface of the big end. To fit the connecting rod to the engine and so insert the crankshaft through the eye, the big end is normally divided into two portions, a first of which remains attached to the rod, and the second of which is separated to form the so-called cap.  
           [0007]    The big end is normally cut or, preferably, broken by traction into the two portions.  
           [0008]    Since the cap must eventually be reconnected to the first portion at assembly, the contact surfaces between the cap and the first portion must mate as accurately as possible to reduce the formation, during operation of the engine, of stress and/or deformation which may damage the connecting rod or other parts of the engine.  
           [0009]    For this reason, the break must be as “fragile”, i.e. result in as little inelastic deformation, as possible. In which connection, it is important to note that any deformation, even not in the area of the break, may still promote failure and stress during operation of the engine.  
           [0010]    The break is normally made using a device comprising two traction members, which, in use, are inserted inside the eye and pulled apart to detach the cap from the first portion of the big end. The traction members have respective thrust surfaces in the form of a cylindrical sector, and which mate with said annular surface and have substantially identical angles of curvature.  
           [0011]    Though long used for machining connecting rods of relatively high-carbon steel, attempts so far to apply the above device to low case hardened steel connecting rods have resulted in breaks of relatively poor “fragility” and therefore in parts substantially unsuitable for use in modern combustion engines.  
         SUMMARY OF THE INVENTION  
         [0012]    It is an object of the present invention to provide a device for machining steel connecting rods, designed to eliminate the aforementioned drawbacks, and which at the same time is cheap and easy to produce.  
           [0013]    According to the present invention, there is provided a device for machining steel connecting rods, as claimed in Claim  1 .  
           [0014]    The present invention also relates to a method of machining steel connecting rods.  
           [0015]    According to the present invention, there is provided a method of machining steel connecting rods, as claimed in Claim  8 .  
           [0016]    The present invention also relates to a steel connecting rod.  
           [0017]    According to the present invention, there is provided a low case hardened steel connecting rod, as claimed in Claim  15 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    A non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which:  
         [0019]    [0019]FIG. 1 shows a plan view of a connecting rod in accordance with the present invention;  
         [0020]    [0020]FIG. 2 shows a longitudinal section of a detail of the FIG. 1 connecting rod;  
         [0021]    [0021]FIG. 3 shows a plan view of a connecting rod from which to form the FIG. 1 connecting rod;  
         [0022]    [0022]FIG. 4 shows a view in perspective, with parts removed for clarity, of one embodiment of the device according to the present invention;  
         [0023]    [0023]FIG. 5 shows a lateral section of the FIG. 4 device;  
         [0024]    [0024]FIG. 6 shows a plan view of parts of the FIG. 4 device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    With reference to FIGS. 1 and 2, number  1  indicates as a whole a low case hardened steel connecting rod comprising a rod  2 ; and a big end  3  in turn having a cylindrical opening or eye  4 , which has a longitudinal axis  4   a  substantially crosswise to rod  2 , is defined laterally by an annular surface  5 , and houses a portion of a crankshaft.  
         [0026]    Big end  3  is divided into two portions: a first portion  6  integral with rod  2 ; and a removable cap  7 . First portion  6  is saddle-shaped and forks into two legs  8 , each having a respective end  9 ; and cap  7  is also saddle-shaped, and forks into two legs  10 , each having a respective end  11 .  
         [0027]    Each end  9  is gripped to a respective end  11  by a relative bolt  11   a , so that portion  6  and cap  7  define eye  4 . Big end  3  also comprises two notches  12  formed at the outer edges of ends  9  and  11  and extending parallel to axis  4   a.    
         [0028]    In variations not shown, notches  12  may be formed at the bottom and/or top inner edges of ends  9  and  11 .  
         [0029]    Ends  9  and  11  include surfaces lying in two break planes  13  and  14  and formed by “fragile” breakage of a connecting rod  15  (FIG. 3) on which notches  12  are  15  formed beforehand. Break planes  13  and  14  are coplanar, and lie substantially in a diametrical plane (not shown) of eye  4  substantially crosswise to rod  2  and through notches  12 .  
         [0030]    In further embodiments not shown, break planes  13  and  14  may be at an angle to each other and/or need not lie in said diametrical plane.  
         [0031]    Connecting rod  15  (FIG. 3) is substantially identical with connecting rod  1 , the only difference being that big end  3  has no notches  12  and is formed in one piece, i.e. with portion  6  integral with cap  7 .  
         [0032]    With reference to FIGS. 4 and 5, number  16  indicates as a whole a device for producing connecting rod  1  from connecting rod  15 , and which comprises a rectangular parallelepiped-shaped bed  17  supporting two uprights  18  and  19  at opposite ends. Uprights  18  and  19  are substantially parallelepiped-shaped and connected by a horizontal guide  20  comprising two slide cylinders  21  (only one shown) extending longitudinally with respect to bed  17 .  
         [0033]    Upright  18  is defined at the top by a supporting surface  22  for supporting rod  2  and first portion  6 , and which has a fixed traction pin  23  in the form of a vertical half-cylinder. Traction pin  23  is inserted inside eye  4  and is defined laterally by a thrust surface  24  in the form of a cylindrical sector and which mates with annular surface  5  of first portion  6 .  
         [0034]    Device  16  also comprises a slide  25  movable along horizontal guide  20  and having a seat  26  for housing cap  7 . Seat  26  has a supporting surface  27  substantially coplanar with surface  22  and for supporting cap  7 , and is fitted with a traction pin  28  in the form of a vertical half-cylinder.  
         [0035]    Traction pin  28  is fitted integrally to slide  25 , is inserted inside eye  4 , and is defined laterally by a thrust surface  29  in the form of a cylindrical sector and which mates with annular surface  5  of cap  7 .  
         [0036]    As shown clearly in FIG. 6, thrust surface  24  has a smaller radius of curvature than thrust surface  29 . In preferred embodiments, the radius of curvature of annular surface  5  is greater than or equal to that of thrust surface  29 . More specifically, the radius of curvature of annular surface  5  preferably ranges between 100% and 110% of the radius of curvature of thrust surface  29 .  
         [0037]    Preferably, the radius of curvature of thrust surface  24  ranges between  80 % and  99 %, in particular between 92% and 98%, of the radius of curvature of thrust surface  29 . Particularly preferred embodiments are those in which the radius of curvature of thrust surface  24  is 97% of the radius of curvature of thrust surface  29 .  
         [0038]    Device  16  also comprises an actuating unit  30  for parting traction pins  23  and  28 , and which comprises two horizontal hydraulic cylinders  31  connected at the front to slide  25  by means of a flange  32 , and supported at the rear by a bracket  33  fitted to upright  19 .  
         [0039]    Device  16  also comprises a known laser source  34  (shown schematically) for making notches  12 .  
         [0040]    In actual use, connecting rod  15  is positioned so that traction pins  23  and  28  are inserted inside eye  4 , cap  7  is housed inside seat  26 , and rod  2  is supported on surface  22  and oriented substantially parallel to horizontal guide  20 .  
         [0041]    To increase the fragility of the low case hardened steel of connecting rod  15 , connecting rod  15  is preferably cooled using known methods, e.g. by immersing it in liquid nitrogen before it is set up on the device.  
         [0042]    Once connecting rod  15  is set up, laser source  34  is activated by a control unit  35  (shown schematically in FIG. 4) to form notches  12 .  
         [0043]    At this point, actuating unit  30  is operated (also by control unit  35 ) to move slide  25  towards upright  19 . At the same time, traction pins  23  and  28  are parted in a direction substantially parallel to rod  2 , and thrust surfaces  24  and  29  mate with annular surface  5  until “fragile” breakage of big end  3  along break planes  13  and  14  is achieved.  
         [0044]    In this connection, it should be pointed out that, during traction by traction pins  23  and  28 , the particular geometric configuration of traction pins  23  and  28  provides for a relatively high concentration of mechanical stress in break planes  13  and  14 . In particular, being connected to rod  2 , first portion  6  is more rigid than cap  7 , thus reducing inelastic deformation of big end  3  and, in particular, end surfaces  9  and  11 .  
         [0045]    Once cap  7  is detached from big end  3 , connecting rod  1  can be machined, e.g. ground and lapped, in known manner.