Abstract:
A release device for a draft gear assembly comprises a draft gear housing for slidably supporting a yoke shaft and capturing a cushion unit associated with the yoke shaft between buff and draft stops. The buff stop comprises a release ring within the interior of the draft gear housing being radially divided into two or more sections. A plurality of release bolts designed to shear under emergency buffing forces extend through the draft gear housing and into the sections of the release ring to draw the sections radially outward to the interior surface of the draft gear housing eliminating gaps and minimizing flexing of the release bolts.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the mechanical connection between rail vehicles and, in particular, between vehicles for carrying passengers in mass transit applications. This invention more specifically relates to the emergency release of the draft gear or cushioning assembly permitting the vehicles to come together for more controlled absorption of energy and to prevent climbing after collision. 
   2. Description of Related Art 
   In railway and transit vehicles, buffing and draft forces between connected vehicles are transmitted to the under frames of the vehicles through drawbars, draft gears and coupler heads (coupler assemblies). The draft gears have cushioning devices which accommodate normally expected forces. Adjacent vehicles are held coupled spaced apart. In the case of abnormal buffing forces which might be encountered on collision, it is desirable to enable the vehicles to come together so that anti-climbers on the ends of the vehicles prevent the end of the trailing vehicle from overriding the lead vehicle. Typically, this function is provided by collapsible draft gears which having release mechanisms based on shear bolts. 
   As is generally recognized in the railway coupling art, rail transit vehicle coupler assemblies make use of emergency release bolts that break at a designed buff force allowing the draft gear device to telescope into the draft gear housing. The emergency release bolts extend radially through the draft gear housing and into an emergency release ring. The draft gear housing contains an energy-absorbing device that bears against the emergency release ring and emergency release bolts. 
   The draft gear housing is an integral part of a coupler assembly which is mechanically secured to the underside of its associated vehicle. Coupling and inter-car forces are transmitted from the draft gear assembly to the emergency release ring and to the release bolts by the release ring. Existing coupler assemblies normally employ a rigid emergency release ring suspended in a rigid draft gear housing by radially extending emergency release bolts. A clearance must exist between the rigid emergency release ring and the rigid draft gear housing to permit assembly. The emergency release bolts are designed to shear and break in two pieces when the coupling forces between two vehicles exceed a designed limit as determined by the strength of the emergency release bolts. 
   Normally, the coupling forces that occur when a consist of rail vehicles is being assembled and connected together for travel with a locomotive or lead vehicle exert less load on the emergency release bolts than the designed limit. The assembly stays intact. 
   On hard coupling or collision events, forces in excess of the designed limit will be exerted. At this time, the emergency release bolts will break or shear. This allows portions of the draft gear assembly to slide within the draft gear housing and engage a secondary energy dissipation device. 
   In existing coupler assemblies, the emergency release ring is pulled against the inside wall of the draft gear housing to form a contact at a single location which corresponds to the first emergency release bolt tightened. A gap is formed between the housing and the release ring near the remaining release bolts. This gap allows for forces to repeatedly flex the emergency release bolts in a bending mode. This bending results in reduction in the fatigue strength of the bolts. 
   The overall structure of the drawbar, draft gear (cushioning unit) and coupler head of one type of mechanical connection for rail vehicles is disclosed in Grau et al. U.S. Pat. No. 6,499,613 entitled “Coupler with Extended Emergency Release and Towing Feature.” This patent discloses primary and secondary release devices, the primary release device being most relevant to this application. The structure of a shear bolt and the need of well-defined shear planes are disclosed Grau et al. U.S. Pat. No. 6,981,599 entitled “High Capacity Shear Mechanism.” 
   SUMMARY OF THE INVENTION 
   It is an object of this invention to provide a fatigue-resistant emergency release device for rail transit vehicle coupler assemblies. Specifically, the release device is provided with enhanced fatigue life performance for emergency release bolts by eliminating bending forces exerted on the emergency release bolts. 
   Briefly, according to one embodiment of this invention, there is provided a release device for a draft gear assembly. The draft gear assembly comprises a draft gear housing for slidably supporting a yoke and integral yoke shaft and capturing a cushion unit associated with the yoke shaft between buff and draft stops. The buff stop is designed to release from the housing under emergency buffing forces. The buff stop comprises a release ring having a cylindrical axis and an outer cylindrical surface with a diameter permitting sliding engagement within a cylindrical interior surface of the housing. The release ring is radially divided into two or more sections. There are flat chord surfaces parallel to the cylindrical axis on the outer surface of each section. A threaded bore extends into each section through and perpendicular to the flat chord surface in each section. 
   A retention system holding the sections of the release ring together includes at least one circumferential groove provided in the outer cylindrical surface of the release ring and an expandable split ring positioned in the groove. The retention system allows the independent release ring segments to accommodate radial and longitudinal misalignment of the sections relative to each other. 
   A plurality of emergency release bolts designed to shear under emergency buffing forces has threaded ends. There is a flat radial bearing surface between the ends of the emergency release bolts. When the release ring is positioned in the draft gear housing, release bolts may extend through the draft gear housing and into the sections of the release ring so that a flat surface extending radially from the release bolt will abut a flat chord surface of a release ring section establishing a well-defined shear plane. Nuts draw the sections radially outward minimizing flexing of the release bolts. The plurality of emergency release bolts directed radially through openings in the draft gear housing and corresponding to threaded holes in the sections of the emergency release ring enable each section to be drawn tightly against the inside surface of the draft gear housing facilitating pure longitudinal shearing load. 
   Briefly, according to this invention, there is also provided a draft gear assembly comprising a draft gear housing, a yoke and integral yoke shaft slidably secured in a housing, a cushion unit associated with the yoke shaft and captured between buff and draft stops secured in the housing. The buff stop comprises a release ring having a cylindrical axis and an outer cylindrical surface having a diameter permitting sliding engagement within a cylindrical interior surface of the housing. The release ring is radially divided into two or more sections. A threaded bore extends into each section. At least one circumferential groove is provided in the outer cylindrical surface of the release ring. An expandable split ring is positioned in the groove holding the sections of the release ring together. 
   A plurality of release bolts designed to shear under emergency buffing forces has threaded ends. When the release ring is positioned in the draft gear housing, release bolts may extend through the draft gear housing and into the threaded bore in the sections of the release ring. A nut turned on the other threaded end of the release bolts draws the section radially outward to the interior wall of the draft gear housing eliminating any gap and thus minimizing flexing of the release bolts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and other objects and advantages will become clear from the following detailed description made with reference to the drawings in which: 
       FIG. 1  is a perspective view of drawbar, draft gear and coupler assembly for a modern transit vehicle; 
       FIG. 2  is an exploded perspective view of a draft gear according to this invention attached to a drawbar; 
       FIG. 3  is a section view of a draft gear according to this invention; and 
       FIG. 4  is an exploded perspective view of a release ring according to this invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 1 , a mechanical connection between transit vehicles comprises a drawbar  10  that is secured to the underside of the vehicle (not shown) by an anchor ring  12  enabling rotation about a generally vertical axis. The draft gear housing  14  is bolted to the drawbar by secondary release bolts  16 . On the upper side of the draft gear housing  14 , a follower slot  18  is supported to receive a curved guide rail  20  secured to the underside of the vehicle which also accommodates the rotation of the drawbar  10  and draft gear housing  14  about the vertical axis through the anchor ring  12 . A coupler head  22  for capturing the coupler head of an adjacent vehicle is mounted to a yoke  24  for rotation about a generally horizontal axis perpendicular to the axis of the drawbar  10 . The yoke  24  is integral with a yoke shaft  32  (not visible in  FIG. 1 ) which is journaled in the draft gear housing  14  for some rotation about a horizontal axis parallel to the axis of the drawbar  10 . The yoke shaft  32  also moves slidably within the draft gear housing  14 . Also shown in  FIG. 1  are the electrical coupler  26  and the pneumatic conduits for a brake pipe and reservoir supply pipe. The purpose of  FIG. 1  is to illustrate one setting in which the present invention may be found. The multi-segment release ring arrangement which is the subject of the present invention can be used in many drawbars or couplers having draft gear configurations other than those shown in the figures. For example, it has application in light rail type couplers with a tail eye type anchor and integral coupler head. 
   Referring to  FIG. 2 , the draft gear housing  14  is shown secured to the drawbar  10 . The housing has a hollow generally cylindrical interior. A release ring  30  is shown exploded out of the housing interior as are the yoke  24 , yoke shaft  32  and cushioning assembly  34 . As shown in  FIG. 2 , this release ring  30  is a separate and independent member. When these components are slid into the housing, the release ring  30  is secured to the interior wall of the housing by release bolts  36  and nut  38 . 
   Referring to  FIG. 3 , the yoke  24  is threadably connected to the yoke shaft  32 . At the far end, the yoke shaft  32  is threaded to a tail stud nut  42 . The release ring  30  is located radially outward of the tail stud nut  42 . The release ring  30  comprises the buff stop. A sleeve bearing  44  is positioned between the release ring  30  and the tail stud nut  42 . At the yoke end of the yoke shaft  32 , plug nut  46  is threadably secured to the interior of the housing and comprises the draft stop. Sleeve bearing  48  is positioned between the plug nut  46  and the yoke shaft  32 . Adjacent the buff and draft stops are followers  50  and  52 . Captured between the followers is the cushioning assembly  34 . Thus, in normal operation, the yoke shaft  32  can rotate about its axis and can shift in and out of the draft gear housing  14  restricted by the compression of the cushioning assembly  34  against either the buff stop or draft stop. 
   Referring to  FIG. 4 , the release ring  30  has a cylindrical axis, an inner circular cylindrical surface and an outer mostly circular cylindrical surface. The ring is divided into four sections  30   a ,  30   b ,  30   c  and  30   d  that are separated by radial planes intersecting the axis of the ring. Each section has on its outer surface a flat chord surface  30   e  and threaded bore  30   f  with an axis perpendicular to the flat chord surface  30   e  for receipt of release bolts  36  (see  FIG. 3 ). The axis of the threaded bore  30   f  intersects the cylindrical axis of the release ring  30 . 
   The release bolts  36  are designed to shear under emergency buffing forces. The release bolts  36  have two threaded ends of different diameters. One threaded end is turned into a threaded bore  30   f  in a release ring section. The other threaded end extends through the draft gear housing  14  and has a torque nut  38  turned thereon. Preferably, a cylindrical shank extends between the threaded ends of the release bolts  36 . A flat radial surface is provided where the shank meets the threaded end for turning into a section of the release ring. A preferred release bolt is described in U.S. Pat. No. 6,981,599 noted above. 
   At each axial end of the release ring circumferential grooves  60 ,  62  are provided to receive expandable split rings  64 ,  66 . The rings  64 ,  66  may make several loops and are preferably configured so that each loop of the ring lies on the same cylindrical plane. The split rings  64 ,  66  when emplaced hold the sections of the release ring  30  together. The expandable split rings  64 ,  66  provide the sections of the release ring with the ability to expand outward and to move relative to each other in the radial direction. 
   When the release ring  30  is positioned in the draft gear housing  14 , threads on one end of release bolts  36  extend through the draft gear housing  14  and into the sections of the release ring  30  until the flat radial surfaces of the bolts abut the flat chord surfaces  30   e  of the release rings  30 . Thereafter, the torque nuts  38  are tightened to draw the sections of the release ring radially outward so that the outer surface of the release sections abut the inner surface of the draft gear housing eliminating any gap and minimizing flexing of the release bolts. The retention system for holding the release ring segments together can take other forms than split rings. Expandable retainers of various types may be applied to the outer diameter, inner diameter or the axial ends of the release ring sections. 
   It is an advantage of this invention that the multi-segmented release ring can be expanded radially outward to effectively contact the inside of the wall of the draft gear housing adjacent each emergency release bolt, thus eliminating any gap between the inside wall of the draft gear housing and the emergency release ring. This has the effect of equalizing the loads on the emergency release bolts. It also permits the equal preloading of all emergency release bolts at the time of assembly. Most important, this provides a close contact fit between the release ring and the housing at the shear plane eliminating bending forces that can fatigue the release bolts. Finally, the release ring can have an initial (unexpanded) diameter that facilitates in the assembly of the release ring in the draft gear housing. 
   Having thus described my invention in the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent are set forth in the following claims.