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BACKGROUND 
     This disclosure relates to a continuous mining machine, and more particularly, to a continuous mining machine having a cutter drum member with extendable end portions. The present disclosure is directed to a means for extending, retracting and locking the cutter drum extensions in their extended position. 
     In a continuous-mining apparatus of the type employed in this invention, a cutting head mechanism is used to dislodge material from a mine vein and is operable to provide a mine passageway or room into which the apparatus advances and mining progresses. The cutting head mechanism is pivotably mounted on a mobile base to swing in a vertical plane between the mine roof and floor and includes a rotary drum cutting head assembly arranged on a horizontal transverse axis and having teeth or bits which tear away and dislodge the mineral. The apparatus also includes a conventional loading head for gathering the loose mineral on the mine floor and moving it rearwardly and inwardly towards the forward receiving portions of the conveying means of the apparatus. The rotary drum cutting assembly has end portions thereof which can be selectively extended or retracted, to respectively, dislodge mineral from the ribs and corners of the mine passageway and reduce the effective length of the cutting head assembly to provide clearance at the sides of the assembly. 
     As shown in  FIG. 1 , a continuous mining machine generally designated as  10 , may assume various forms but, for illustrative purposes, herein comprises a crawler base  14  carrying a frame  16  on which a forwardly extending mining boom or support member  18  is pivotally mounted at the forward end of the frame  16  to swing up and down between a mine roof and a mine floor. An elongated mining head member  24  (also called a cutter head) extends transversely to the support member  18  and is rotatably secured thereto at the forward end thereof. The mining head member  24  is mounted for powered rotation about a longitudinal axis X-X of the head member. Pivotally mounted at the forward end of the frame  16  and extending forwardly there from beneath the support member  18  is a conventional loading head  26  having oscillatory gathering arms  28  for engaging mine mineral and moving such mineral rearwardly and inwardly towards well known conveying means  30  of the mining machine  10 . Conventional hydraulic jacks (not shown) serve to swing the support member  18  in a vertical plane about the pivot axis thereof and to tilt the loading head  26  about the horizontal axis. 
     The elongated mining head member  24  is driven from a pair of motors  34 . As shown motors  34  are in spaced axial alignment and extend generally parallel to the head member  24 . A support member  18  at opposite side portions thereof suitably carries motors  34 . 
     The drive from motors  34  rotatably drives a cutting head assembly head shaft  36 . Head shaft  36  extends parallel to the longitudinal axis of the head member  24  and is captively and rotatably supported by tubular gear casing extensions  38  that also extend parallel to the longitudinal axis of the head member. The gear drive system for the mining head member may be that shown in either U.S. Pat. No. 3,617,093 or U.S. Pat. No. 3,695,725, or of other conventional construction. 
     As hereinafter described, head shaft  36  rotatably drives: an endless circulating belt type continuous hinge cutter chain  48 , hollow cylindrical rotary drum cutting heads  50  and hollow cylindrical rotary drum cutting head extension portions  52  which include cylindrical portion  66  which are partially slideably, received within the ends of respective cutting heads  50 . The portions  52  are selectively hydraulically extendable and retractable there from. An operator can selectively extend or retract the extension portions via a hydraulic control system. 
     Examples of conventional mining machine drum cutter extension means are shown in U.S. Pat. No. 4,489,985, and U.S. Pat. No. 3,516,712. 
     In U.S. Pat. No. 4,489,985, as can be seen in prior art  FIGS. 2 and 3 , the head shaft  36  drives the outer peripheral member  60  of the cutting head  24  via drive ring  72  which engage a plurality of circumferentially spaced keys  74 . The outer peripheral member  60  is supported on casing extension  38  by bearing  76  and other internal bearings (not shown). 
     The hollow cylindrical rotary drum extendable end portions  52  have an inner diameter thereof larger than the outer diameter of cutting head  60  and are rotatably driven by the cutting head  60 . As shown in  FIGS. 2 and 3 , a plurality of circumferentially spaced keys  82  has the inner ends thereof fixedly secured to the outer peripheral portion of the cutter head  60 . Consequently, the head  60  drives the end portions  52  by means of keys  82  drivingly engaging respective ones of a plurality of cooperating key ways in the end portions  52 . The keys  82  and the key ways  84  are circumferentially spaced about the inner periphery of the end portions  52 . The above described key and key way driving arrangement additionally allows for the reciprocal axial movement of cutting head end portions  52  with respect to cutting heads  60  by the axially sliding relationship which exists between keys and key ways  82  and  84 , respectively. 
     A hydraulic cylinder  90 , in conjunction with its piston rod  92 , accomplishes the extension of the end portions  52 . The rod  92  is fixedly connected to plate  94  that is slidably mounted within the inner diameter of cylinder  66  of end portion  52 . An annular drive ring  96  is fixedly attached to the inner diameter of cylinder  66  for movement therewith. Drive ring  96  has a plurality of holes  99  within its outer annular surface to accommodate a plurality of bolts  98  which have one end threadably engaged with the piston  94  for movement therewith and the other end fixedly attached by threads to latch release member  100 . The holes  99  located on drive ring  96  have a diameter slightly larger than the diameter of bolt  98  to permit the bolts  98  to slide there through. Consequently the piston  94  and the latch release member  100  move as a unit in the direction of longitudinal axis X-X. 
     As can be best seen in  FIG. 2 , at least one latch member  102  is pivotally mounted on a mounting ring  104  that is fixedly attached to head shaft  36  for rotation therewith. 
     U.S. Pat. No. 3,516,712 discloses a mining machine with an extendable head having a spring return mechanism. The means for extending the extensible head is a hydraulic cylinder. More particularly, as shown in  FIG. 4 , there is a shaft  87  extending axially through the mining head  13 . The end mining head section  70  has its telescoping cylinder part  71  slidably mounted on the shaft  87  with a key  88  in a keyway  89 . A slide collar  90  is secured to the outer end of the shaft  87 , and a mating inner cylinder  91  is slidably engaged with the slide collar  90  in sealed engagement. A bore  92  extends through the shaft  87  to the inner cylinder  91 . Hydraulic fluid is delivered through the bore  92  into the inner cylinder  91  to move the telescoping cylinder part  71  and the end mining head section  70  outwardly relatively to the shaft  87 . 
     A coil spring  93  is disposed between the outer mining head section  65  and the telescoping cylinder part  71 . A ring  94  is secured to the outer mining head section  65  abutting one end of spring  93 , and a ring  95  is secured to the inner end of the telescoping cylinder part  71  abutting the other end of spring  93 . In the extended position of the end mining head section  70 , as seen in  FIG. 5 , the spring  93  is compressed between the rings  94 ,  95 , and the end mining head section  70  is held in its extended position by hydraulic fluid trapped within the inner cylinder  91 . The end mining head section  70  is retracted by release of the hydraulic fluid from the inner cylinder  91 , and the force of the spring  93  between rings  94 ,  95  withdraws the telescoping cylinder part along the shaft  87  and within the outer mining head section  65 . 
     SUMMARY 
     It is an object of this disclosure to provide a better drum extension portion than that provided in the above cutting heads. In conventional cutting heads, the forces acting to extend and lock the drum extension portions is sometimes inadequate, resulting in machine operators sometimes welding the drum extension portions in their extended position. It is an object of this disclosure to provide a drum extension portion with more extension force and locking force than in conventional cutting heads. More particularly, the disclosed drum extension force has extension and locking forces more than four times stronger than in conventional cutting heads. 
     In this disclosure, there is provided an improved continuous mining machine of the type that includes an elongated body portion mounted on devices for propelling the body portion. A support member is pivotally secured to the body portion and extends forward there from. An elongated mining head member is mounted on the forward end of the support member for powered rotation about a longitudinal axis of the head member. The head member has axially extensible and retractable drum extension portions. 
     The drum extension portion includes an inner end structure attached to the main cutting head portion, and an outer cylinder attached to the inner end structure. The drum extension portion also includes an inner cylinder attached to the inner end structure and having an outer surface, and the inner cylinder is coaxial with and received within but spaced apart from the outer cylinder. The drum extension portion also includes an extendable cylinder having an inner annular enlarged end, with a portion of the extendable cylinder being received between the inner cylinder and the outer cylinder. A first space is defined between the inner end structure and the enlarged end of the extendable cylinder. The drum extension portion also includes an end cap having a flange that receives coaxially within it the outer end of the inner cylinder, and the extendable cylinder enlarged end is received between the end cap flange and the outer cylinder. A second space is defined between the inner cylinder and the extendable cylinder and between the extendable cylinder enlarged end and the end cap flange, so that when fluid enters the first space and leaves the second space, the extendable cylinder moves to the extended position, and so that when hydraulic fluid enters the second space and leaves the first space, the extendable cylinder moves to the retracted position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial plan view of a mining machine; 
         FIG. 2  is an enlarged view showing a prior main gear casing in a portion of a cutter head assembly, with a latch mechanism in its locked position; and 
         FIG. 3  is an enlarged view showing the half of the cutter head assembly as is shown in  FIG. 2  with the latch mechanism shown in the unlatched position with the extension retracted. 
         FIG. 4  is a section view of one end of a prior art cutter head assembly. 
         FIG. 5  is a section view of one end of a cutter head assembly according to this disclosure with the drum extension portion in its retracted position. 
         FIG. 6  is a section view of one end of a cutter head assembly according to this disclosure with the drum extension portion in its extended position. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrated in  FIGS. 5 and 6  is a mining machine cutting structure  200  according to this disclosure. Where like elements are found in  FIGS. 1 and 2  of the drawings, reference will be made to  FIGS. 1 and 2 . 
     The mining machine cutting structure  200  includes a support member  18 , and an elongated cutting head assembly  204  mounted at the forward end of the support member for powered rotation about a longitudinal axis. The cutting head assembly  204  comprises a centrally located main cutting head  50 , and at least one drum extension portion  208  located at an end of the cutting head assembly  204  axially outward of the main cutting head  50 . The drum extension portion  208  is capable of moving in an axial direction along the longitudinal axis from a retracted position to an extended position with respect to the main cutting head  50 . More particularly, the drum extension portion  208  includes a toroidal hydraulic cylinder for selectively extending and retracting the drum extension portion  208 . 
     The drum extension portion  208  has an axial inner end adjacent the main cutting head  50  and an axial outer end, and the drum extension portion  208  includes an inner end structure  212  attached to the main cutting head  50 . The drum extension portion  208  also includes an outer cylinder  216  attached to the inner end structure  212 , and an inner cylinder  220  attached to the inner end structure  212 . The inner cylinder  220  also has an outer surface  224 , and the inner cylinder  220  is coaxial with and received within but spaced apart from the outer cylinder  216 . The inner cylinder  220  is also adapted to be connected (not shown), as is cylinder  66  in  FIGS. 2 and 3 , to an outer cutter head  60 . 
     The drum extension portion  208  also includes an extendable cylinder  228  and an end cap  230 . The extendable cylinder  228  has an inner annular enlarged end  232 , and a portion of the extendable cylinder  228  is received between the inner cylinder  220  and the outer cylinder  216 . The enlarged end  232  has seals and bearings to support sliding movement of the extendable cylinder  228  between the inner and outer cylinders. A first space  218  is defined between the inner end structure  212  and the enlarged end  232  of the extendable cylinder  228 . 
     The end cap  230  has an axially extending flange  236  that receives coaxially within it the outer end of the inner cylinder  220 , the extendable cylinder enlarged end  232  being received between the end cap flange  236  and the outer cylinder  216 . The flange  236  has seals and bearings to support sliding movement of the extendable cylinder  228  between the flange  236  and the outer cylinder  216 . A second space  240  is defined between the inner cylinder  220  and the extendable cylinder  228 , and between the extendable cylinder enlarged end  232  and the end cap flange  236 , so that when fluid enters the first space  218  and leaves the second space  240 , the extendable cylinder  228  moves to the extended position, and so that when hydraulic fluid enters the second space  240  and leaves the first space  218 , the extendable cylinder  228  moves to the retracted position. 
     The inner cylinder  220  has an extend port  244  through the inner cylinder  220  and communicating with the first space  218 , and a retract port  248  through the inner cylinder  220  communicating with the second space  240 . Conventional porting (not shown) through the elongated cutting head assembly  204  causes fluid under pressure to selectively enter the ports  244  or  248  to extend or retract the drum extension portion  208 . 
     Various other features of this disclosure are set forth in the following claims.

Summary:
A continuous mining machine cutting head structure is set forth. The head member is of the type having axially extensible and retractable end portions. Included within the head member is a toroidal hydraulic cylinder for selectively extending and retracting these end portions.