Abstract:
A easily rebuildable pneumatic cylinder for a railroad track switch operator utilizes a fiber-reinforced resin composite cylinder tube and a piston wear band of a composite material. The cylinder is easily disassembled for replacement of all of the seals and the rod bearing, along with the cylinder tube and wear band. All wear and replacement parts are identically used in both common types of pneumatic cylinders in present use.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention pertains to a pneumatic operator for a railroad track switch and, more particularly, to an improved pneumatic cylinder for such an operator providing universal replacement for existing cylinders in an assembly which is quickly and economically rebuildable on site.  
           [0002]    Railroad track switch operators or switch machines of the prior art are powered by a number of different types of actuators. Pneumatic cylinders and hydraulic cylinders are commonly used, with the piston rod ends of such cylinders operatively connected to the switch points to selectively move the points between the two positions. Electric motor and fluid motor-driven actuators are also used. In addition, many switch operators also provide the capability for alternate manual operation.  
           [0003]    In the United States, pneumatic operators are commonly used and comprise two basic types. The differences between these two types of pneumatic operators include the manner in which the pneumatic cylinder is mounted with respect to the other components of the switch operator and the basic construction of the cylinder itself. Both types of pneumatic switch machines uses a cast iron cylinder housing in which the nose end of the cylinder is an integral part of the casting. In order to rebuild this cylinder, the nose through which the piston rod extends must be first rebored for the replacement rod bearing bushing and, because concentricity is difficult to maintain, reassembly after rebuilding typically results in considerable leakage and loss of time. In addition, these cylinders must be lubricated regularly and are subject to premature failure if not properly maintained. These cylinders also utilize a piston that is of a multi-piece construction. The piston seals or packings are difficult to orient properly during assembly and often are found not to properly seat and to leak. The rod seals at the nose of cylinder are also difficult to install and remove and are also a source of high shaft friction. The piston rod wiper seal is fastened with a snap ring that provides less than adequate holding power and thus does not adequately prevent the ingress of dirt into the cylinder. Finally, the rubber seals are not chemically compatible with the cylinder environment, for example, the prior art seals are attacked by alcohol which is used to eliminate moisture from the compressed air.  
           [0004]    Attempts have been made to substitute more conventional pneumatic cylinder constructions for cast iron cylinders. However, these off-the-shelf cylinders cannot be easily rebuilt and, as a result, they are simply replaced and discarded.  
           [0005]    There is a need, therefore, for a reliable pneumatic cylinder for the harsh environment of a railroad switch operator that can be easily, effectively and quickly rebuilt in the field. To provide such a cylinder that is also virtually maintenance free would also be very desirable.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, a pneumatic cylinder is provided for a railroad track switch operator that is capable of being easily and quickly rebuilt in the field and, with only two part changes, can be used in either of the two most common types of switch machines using pneumatic cylinder actuators. Further, this cylinder is adapted for direct field replacement of prior art cast iron cylinders and both types utilize the same direct mount cylinder head and control valve for operating air distribution to the cylinder.  
           [0007]    Thus, the universal and easily rebuildable pneumatic cylinder of the present invention includes a field-replaceable cylinder tube of a plastic composite material, a back end cap and a nose end cap closing the opposite ends of the cylinder tube, said caps having opposed annular bosses sized to be concentrically received in one end of the tube; a plurality of tie rods interconnecting the end caps; a one-piece piston mounted to reciprocate within the cylinder tube and including a piston rod extending axially through a bore in the nose end cap; a replaceable annular wear band mounted in a recess in the periphery of the piston, said wear band also made of a composite plastic material; and a plurality of replaceable sealing rings for each of the respective interfaces between (a) the cylinder tube and the end cap bosses, (b) the OD of the piston and the ID of the tube, and (c) the nose end cap bore and the piston rod. The assembly also includes a replaceable composite plastic rod bearing mounted in a bore in the nose end cap.  
           [0008]    The plurality of replaceable sealing rings preferably comprises (a) a pair of D-ring seals for each of the nose and back end cap bosses, (b) a pair of packing ring seals, disposed in an annular groove in the piston on each side of the wear band, (c) a rod seal mounted in a counterbore in the nose end cap bore, and (d) a piston rod wiper seal recessed in the axially outer face of the nose end cap bore.  
           [0009]    The assembly also includes a connector on the outer end of the piston rod for operative connection to a switch mechanism to move a track switch point between two positions corresponding to extended and retracted piston positions. In one embodiment, a signal generator is attached to the piston and a piston sensor is mounted on the outside of the cylinder to sense the position and to generate a signal representative of the piston positions. The signal generator preferably comprises a magnetic material attached to the piston head and the sensor comprises a proximity switch.  
           [0010]    The back end cap for the cylinder may be selected from a pair of back end caps that have different lengths in the direction of the cylinder axis to accommodate either of the two common types of pneumatically operated switch machines. A common cylinder head is adapted to mount directly on either of said back end caps to direct compressed air from a supply to the cylinder. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIGS. 1 and 2 are isometric views of the two common types of pneumatic cylinders utilizing the rebuilding assembly and method of the present invention.  
         [0012]    [0012]FIG. 3 is an exploded view of the cylinders shown in FIGS. 1 and 2 showing both the common parts and the differing parts.  
         [0013]    [0013]FIG. 4 is a vertical section through the cylinder showing the arrangement of the common parts.  
         [0014]    [0014]FIG. 5 is an exploded view of the cylinder shown in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Referring first to FIGS. 1, 3 and  4 , an easily rebuildable pneumatic cylinder  10  for one type of track switch operator utilizes a cylinder tube  11  made of a fiber-reinforced plastic resin composite material. The cylinder tube  11  is mounted between an enclosing back end cap  12  and a nose end cap  13 . The assembly is held together with four axially extending tie rods  14 , each secured with a nut and lock washer pair  15 . Each of the back and nose end caps  12  and  13  includes an annular boss  16  and  17 , respectively, which is received in one end of the cylinder tube  11  during assembly. Each of the bosses  16  and  17  includes a pair of D-ring seals  18  set in axially spaced annular grooves in the surface of the boss.  
         [0016]    Referring also to FIG. 5, a piston  20  is mounted for reciprocal movement in the cylinder tube  11  and has a piston rod  21  demountably attached centrally to it, as with a cross pin  19 . The piston rod  21  extends axially through a center bore  22  in the nose end cap  13 . The piston  20  is of a unitary construction, preferably either steel or cast iron, and is provided with a wide annular recess  23  in which is seated a replaceable annular wear band  24 . The wear band is made of a plastic material, preferably a fiber-reinforced plastic resin composite. The surface-to-surface contact between the ID of the cylinder tube  11  and the OD of the wear band  24  has been found to provide an excellent low friction, high wear life interface for low maintenance operation. The main piston seal is provided by a pair of oppositely disposed packing ring seals  25  disposed in annular grooves  26  on either side of the wear band  24 . The bore  22  in the nose end cap  13 , which is preferably made of steel, is provided with a replaceable reinforced plastic composite rod bearing  27 , such as a glass-filled nylon. A piston rod seal  28  is positioned adjacent the rod bearing on the axial inner end thereof and a rod wiper seal  30  is positioned on the axial outer face of the nose end cap  13 .  
         [0017]    In rebuilding the cylinder  10 , no machining is required and the cylinder may be disassembled and rebuilt on site. In the rebuilding process, the composite tube  11  and wear band  24  are replaced, as are the D-ring seals  18 , the packing ring seals  25 , the rod seal  28  and the wiper seal  30 . The reinforced plastic rod bearing  27  is also replaced.  
         [0018]    Referring particularly to FIGS. 4 and 5, the rod bearing and seal assembly is shown in detail. A deep first counterbore  32  extends from the outer face of the nose end cap  13  axially into the annular boss  17 . The piston rod seal  28  is located in the bottom of the first counterbore  32 . The rod bearing  27  is held in the ID of a flanged insert  33  where one end of the bearing is captured by a reduced diameter end flange  34  and the opposite end is held by a snap ring  35 . The subassembly of the flanged insert  33 , bearing  27  and snap ring  35  is inserted into the first counterbore  32  after the rod seal  28 , and the main end flange  37  of the insert  33  is received in a shallow second counterbore  36  in the face of the nose cap  13 . The rod wiper seal  30  is then placed over the outer end of the piston rod  21  and held against the outside face of the flanged insert  33  with a recessed keeper plate  38 . The keeper plate is also received in the second counterbore  36  and is secured to the main end flange  37  of the insert  33  with a number of machine screws  40 .  
         [0019]    All of the seals  18 ,  25 ,  28  and  30  are made of a synthetic material, such as a urethane, which is inert to the contaminants and solvents normally encountered in use. These seals need only be greased lightly upon assembly and no relubrication whatever is required during their full service life. Preliminary tests have indicated a normal wear life far in excess of the prior art cast iron cylinders using rubber sealing rings.  
         [0020]    The cylinder  10  includes a mounting plate  41  that is bolted to respective flats  42  and  43  machined on the surfaces of the nose end cap  13  and back end cap  12 . Operating air pressure is supplied to the rod end of the cylinder  10  by an air line  44  extending between the back end cap  12  and the nose end cap  13 . Operating air is supplied to the cylinder via a cylinder head  45  bolted directly to the end face of the back end cap  12 . The cylinder head  45  is preferably of the type disclosed in my copending U.S. application Ser. No. 09/643,711, filed on Sep. 1, 2000, and entitled “Railroad Track Switch Operator”, the disclosure of which is incorporated herein by reference.  
         [0021]    Another type of cylinder  46 , also commonly used in pneumatic operated switch machines, is shown in FIG. 2. This cylinder  46  is identical in all respects to the cylinder  10  described above, except that it does not utilize a mounting plate  41  and it utilizes a back end cap  47  and a piston rod  49  of a slightly different construction. However, the back end cap  47  has an integral annular boss  48  adapted to utilize the same D-ring seals  18  as in the previously described cylinder. Thus, to rebuild either of cylinders  10  or  46 , the wear and other replaceable parts are identical, namely, the cylinder tube  11  and wear band  24 , the D-ring seals  18 , the piston packing ring seals  25 , the rod seal  28 , the wiper seal  30  and the rod bearing  27 . As indicated above, no machining whatever is necessary in the rebuilding process.  
         [0022]    Furthermore, the back end cap  47  of the cylinder  46  is adapted to utilize the same cylinder head  45  described above.  
         [0023]    The switch machine which utilizes this type of cylinder  46  includes a cylinder mounting pin  50  that extends through a through bore  51  in the back end cap  47 . At the opposite end of the cylinder  46 , the piston rod  49  is provided with a rod end bearing  52  for attachment to a clevis pin (not shown) in the switch machine.