Abstract:
A lifting apparatus for a vertical vacuum furnace is disclosed. The apparatus includes first and second support modules arranged in spaced parallel alignment and first and second reversible lifting mechanisms mounted on respective ones of the first and second support modules. The apparatus also includes first and second motive means coupled to the first and second reversible lifting mechanisms for driving the reversible lifting mechanisms. First and second trolleys are operatively connected to the first and second reversible lifting mechanisms and adapted for engaging with a payload. The apparatus further includes a control system connected to the first and second motive means for controlling the operation of the first and second reversible lifting mechanisms whereby the first and second trolleys can be raised or lowered. A vertical vacuum furnace assembly including the lifting apparatus is also disclosed as well as a support module and a bottom head assembly for the lifting apparatus.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/581,298, filed Dec. 29, 2011, the entirety of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to vacuum heat treating furnaces and in particular to a vertically-oriented vacuum furnace and an apparatus for lifting a work load into the vacuum furnace and lowering therefrom. 
         [0004]    2. Description of the Related Art 
         [0005]    Industrial vacuum heat treating furnaces having either a horizontal configuration or a vertical configuration are known. In a vacuum furnace having a horizontal configuration, a work load of parts to be heat treated is transported into the furnace chamber with an apparatus that provides horizontal translation of the work load. In a vacuum furnace having a vertical configuration, a lifting apparatus is used to raise the work load from the factory floor up to the furnace chamber which is elevated. 
         [0006]    A known arrangement for a lifting system for a vertical vacuum furnace utilizes a four-point lifting apparatus. The apparatus typically includes four ball screws that operated synchronously so that the work load is lifted evenly. In order to keep the ball screws in synchronism with each other, multiple gear boxes and connecting shafts with couplings are utilized. The known designs for such lifts were composed of many parts that had to be assembled and aligned at the manufacturing site, and then disassembled for shipment. When the furnace arrives at the customer site, the lift apparatus must be re-assembled and aligned again. That is a time consuming process that usually adds several days to the delivery time schedule. 
         [0007]    The gear boxes, drive shafts, and couplings used in the known lift mechanisms generate a considerable amount of noise when operating to lift or lower a work load. The couplings that connect the drive shafts, motor shafts, gear box shafts, and the ball screw shafts become loosened over time. When that occurs, it causes one or more of the ball screws to become un-synchronized with the other ball screw(s). Such out-of-synch operation can cause catastrophic damage to the lifting mechanism. If the ball screws get too far out-of-synch, the work load itself and even the hot zone inside the furnace can be damaged. 
         [0008]    The known lift mechanisms for vertical vacuum furnaces have lifting points that contact the bottom lifting structure of the furnace through coil springs. The lift mechanism is operated to lift the bottom door toward the furnace until a mechanical limit switch is tripped, thereby providing an indication that the lifting structure was in its final, fully-lifted position. In the final lifted position, the springs are compressed a small amount as the bottom lifting structure contacts the upper part of the furnace vessel. If the mechanical switch is not adjusted properly or becomes out-of-adjustment, the springs over-compress and the lifting structure and door can be subject to bending damage. 
         [0009]    In view of the foregoing problems with the known lifting systems for vertical vacuum furnaces it would be desirable to have a lifting apparatus for a vertical vacuum furnace that overcomes the problems associated with the known lifting systems. 
       SUMMARY OF THE INVENTION 
       [0010]    The lifting system for a work load into a vertical vacuum furnace in accordance with the present invention includes two ball screws each driven by a servo-type motor and synchronized with each other through an electrical servo drive system using encoders and/or resolvers to provide position feedback. Each ball screw is constructed and arranged to lift or lower an elevator that is guided in tracks. The elevator system with ball screw and motor are assembled into the leg structure of the vertical furnace. This leg/elevator/ball screw/motor combination is a modular assembly that remains intact for shipment and installation at the end user&#39;s site. 
         [0011]    The movement of the lifting elevator is very quiet because there are no gear boxes, shafts, and couplings. Each servo motor is directly coupled to a respective ball screw. The servo drive system can be programmed for acceleration and deceleration of the elevator movement near the end of its travel. This allows for the elimination of the springs between the lifting structure and the pick-up points of the elevator. Encoder feedback precisely locates the elevators in either the full up or full down positions. 
         [0012]    Another beneficial feature of the lift system according to this invention is that the ball screw attachment point on the elevator mechanism has a jointed linkage that allows for misalignment with little or no stress to the ball screw. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The foregoing summary as well as the following detailed description will be better understood when read with reference to the drawing views, wherein: 
           [0014]      FIG. 1  is a perspective view of a vertical vacuum furnace assembly in accordance with the present invention; 
           [0015]      FIG. 2  is a front elevation view of the vertical vacuum furnace assembly of  FIG. 1 ; 
           [0016]      FIG. 3  is a perspective view of a leg assembly used in the vertical vacuum furnace assembly of  FIG. 1 ; 
           [0017]      FIG. 4  is a rear elevation view of the leg assembly of  FIG. 3 ; 
           [0018]      FIG. 5  is a perspective view of a bottom head assembly used in the vertical vacuum furnace of  FIG. 1 ; 
           [0019]      FIG. 6  is a top plan view of the bottom head assembly of  FIG. 5 ; 
           [0020]      FIG. 7  is a front elevation view of the bottom head assembly of  FIG. 5 ; 
           [0021]      FIG. 8  is a front perspective view of an elevator trolley used in the leg assembly of  FIG. 3 ; 
           [0022]      FIG. 9  is a rear perspective view of the elevator trolley of  FIG. 8 ; 
           [0023]      FIG. 10  is perspective view of ball screw jack used in the leg assembly of  FIG. 3 ; 
           [0024]      FIG. 11  is an elevation view of the ball screw jack of  FIG. 10 ; and 
           [0025]      FIG. 12  is a block diagram of a servo-motor control system used in the vertical vacuum furnace of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring now to the drawings and in particular to  FIGS. 1 and 2 , there is shown a vacuum furnace  10  in accordance with the present invention. The vacuum furnace  10  has a vertical orientation and is elevated relative to the floor of the facility in which the vacuum furnace is located. The vacuum furnace  10  includes a pressure/vacuum vessel  12  and a vacuum pump  14 . The pressure/vacuum vessel  12  is supported by a pair of leg assemblies including, a first leg assembly  16  and a second leg assembly  18 . A service platform  20  is mounted at the upper ends of leg assemblies  16  and  18 . A control cabinet  22  and a control console  24  are provided, preferably adjacent to the vacuum furnace  10 . 
         [0027]    The pressure/vacuum vessel  12  includes a body  26  having an opening  28  at the lower end of the body. The pressure/vacuum vessel  12  also has a bottom head assembly  30  that is movable for closing the opening  28  when the vacuum furnace is to be operated for heat treating a work load of metal parts. The body  26  of pressure/vacuum vessel  12  is mounted on the leg assemblies  16  and  18  with a plurality of support arms. Support arms  40   a ,  40   b ,  40   c , and  40   d  are provided to attach the front portion of pressure vessel body  26  to the front portions of leg assemblies  16  and  18 . A similar group of support arms (not shown) are provided to attach the rear portion of pressure vessel body  26  to the rear portions of the leg assemblies  16  and  18 . 
         [0028]    Referring now to  FIGS. 3 and 4 , there is shown in greater detail the construction of the leg assembly  16 . Leg assembly  18  is constructed and arranged essentially the same as leg assembly  16 . Therefore, only leg assembly  16  will be described. 
         [0029]    Leg assembly  16  includes a pair of support columns  32   a  and  32   b . The support columns  32   a  and  32   b  are connected together with a cross beam  34  and a floor plate  35 . The floor plate  35  is attached to the bottoms of support columns  32   a  and  32   b , preferably by being welded thereto. The cross beam  34  is attached between the columns  32   a  and  32   b  at a location that is intermediate to the bottom ends and the top ends of support columns  32   a  and  32   b    
         [0030]    Leg assembly  16  also includes an elevator mechanism  46  that is configured for lifting or lowering the bottom head assembly  30  relative to the pressure vessel body. The elevator mechanism  46  includes a mechanical lifting device, preferably a ball-screw jack  50 , and a lifting trolley  52  that is operative coupled to the lifting device. Guide channels  42   a  and  42   b  are formed or mounted on facing surfaces of columns  32   a  and  32   b , respectively. The guide channels  42   a  and  42   b  provide tracks for the lifting trolley to move in, along the leg assembly  16 . The ball-screw jack  50 , shown in greater detail in  FIGS. 10 and 11 , includes a threaded shaft  58  and a ball nut  59  in accordance with the known construction. A drive gear box  60  is coupled to the threaded shaft  58  at one end thereof. The drive gear box  60  includes a motor mount  61  for attaching an electric drive motor  51  thereto. It will be appreciated by those skilled in the art that a machine-screw jack can be used instead of the ball screw jack shown in  FIGS. 10 and 11 . 
         [0031]    Referring now to  FIGS. 8 and 9  of the drawing, the lifting trolley  52  includes a pair of L-shaped side plates  70   a  and  70   b . The side plates are connected together with an upper cross beam  72  and a lower cross bar  74 . In the embodiment shown, the lower cross bar  74  is formed of a pair of bars  74 ′,  74 ″ in spaced parallel relation to each other. The upper cross beam  72  has an opening  88  formed therethrough at a location that is preferably midway between the side plates  70   a ,  70   b . The opening  88  is dimensioned and position so that the threaded shaft  58  of the ball-screw jack  50  can pass therethrough. The lifting trolley  52  includes a ball-screw attachment assembly  76  for coupling the ball-screw jack  50  to the trolley  52 . Lift bars  80   a  and  80   b  are mounted between the bars  74 ′,  74 ″ of lower cross bar  74 . A bracket  78  is provided for connecting the ball-screw jack  50  to the lifting trolley  52 . As shown in the embodiment of  FIGS. 8 and 9 , the bracket  78  includes a mounting plate  79  that has a central opening  89  and plurality of bolt-holes. The central opening is dimensioned to permit the threaded shaft  58  of the ball-screw jack  50  to pass through the lifting trolley  52 . The bolt-holes are provided for attaching the ball nut  59  of the ball-screw jack  50  to the mounting bracket  78 . 
         [0032]    The mounting bracket  78  is coupled to the lifting bars  80   a  and  80   b  by means of link bars  82   a ,  82   b ,  82   c , and  82   d . The link bars  82   a  and  82   b  are pivotally connected between lift bar  80   a  and the mounting bracket  76  with pivot pins  84   a  and  84   b . Link bars  82   c  and  82   d  are pivotally connected between lift bar  80   b  and mounting bracket  76  with pivot pins  84   c  and  84   d . The jointed linkage provided by the link bars between the lift bars and the mounting bracket prevents significant lateral stress to the ball-screw jack when minor misalignment of the elevator trolley and the ball-screw shaft occurs. 
         [0033]    Guide wheels or bearings  180   a  and  180   b  are provided on the outward facing surface of end plate  70   a . In like manner, guide wheels  180   c  and  180   d  are provided on the outward facing surface of end plate  70   b . The guide wheels  180   a  and  180   b  are affixed to end plate  70   a  on mounting pads  182   a  and  182   b , respectively. Similarly, guide wheels  180   c  and  180   d  are affixed to end plate  70   b  on mounting pads  182   c  and  182   d , respectively. The guide wheels  180   a - 180   d  are dimensioned and arrange to fit and travel in the guide channels  42   a  and  42   b  of leg assembly  16 . 
         [0034]    End plates  70   a ,  70   b  are L-shaped and include feet  86   a  and  86   b  which extend laterally. The feet  86   a  and  86   b  are constructed and arranged to engage with the support structure for the bottom head assembly of the pressure/vacuum vessel as described in greater detail below. 
         [0035]    As will be apparent to those skilled in the art, the construction features of leg assemblies  16  and  18  provide the advantage that they can be preassembled as modules prior to shipment with the vacuum furnace. The ability to ship the leg assemblies as preassembled modules significantly reduces the time needed to ship the vacuum furnace and assemble the vacuum furnace at the user&#39;s facility. Moreover, it is also apparent from the foregoing description, that there is no mechanical linkage required between the lifting mechanisms on each leg assembly. Thus, the modular construction of the leg assemblies  16  and  18  avoids the need for the installation of such linkage and the need for proper alignment and realignment of the lifting mechanisms and the linkage that is necessary in the known lift mechanisms. The omission of the multiple gear boxes, drive shafts, and couplings that are usually part of the mechanical linkage between the lifting mechanisms, also results in significantly quieter operation. 
         [0036]    Referring now to  FIGS. 5 ,  6 , and  7 , there is shown a preferred embodiment of a bottom head for the pressure/vacuum vessel  12 . The bottom head assembly  102  has a generally flat profile for compactness. The bottom head  102  includes a generally round plate  104  that is dimensioned to cover the opening in the bottom of the pressure vessel body. The plate  104  is preferably formed with a peripheral groove that receives a sealing ring  107 . A flange  106  is formed around the circumference of the plate  104  and is dimensioned and arranged to engage with a corresponding, mating flange about the opening  28  in the pressure/vacuum vessel body  26 . The flat bottom head arrangement is suitable when high gas quenching pressures, i.e., gas pressures greater than about 2 bar, are not used in the vacuum furnace. Therefore, when gas quenching pressures greater than about 2 bar are used, a conventional dished or domed bottom head must be used to meet the requirements of the pressure vessel code. 
         [0037]    A pair of support beams,  108   a  and  108   b , are attached to the exterior of the plate  104  and extend transversely across the plate in spaced parallel relation. The support beams have portions that extend beyond the plate  104 . In particular, support beam  108   a  has extension portions  110   a  and  110   b  and support beam  108   b  has extension portions  112   a  and  112   b . The extension portions,  110   a  and  110   b  are constructed for engaging with the feet,  86   a  and  86   b , of the lifting trolley  52 . The extension portions,  112   a  and  112   b , are similarly constructed to engage with the corresponding feet of the lifting trolley on leg assembly  18 . As seen in  FIG. 7 , support legs,  114   a  and  114   b , extend vertically from the exterior side of plate  104 . A second pair of support legs is provided behind and spaced from support legs  114   a  and  114   b , but are not shown in  FIG. 7 . The support legs are constructed and arranged to add stiffness to the plate and to support the bottom head assembly  102  when it is resting on the floor. The support legs are dimensioned to provide sufficient height above the floor so that the lifting trolleys can readily engage with the extension portions  110   a ,  110   b ,  112   a , and  112   b  of the support beams  108   a  and  108   b.    
         [0038]    A plurality of sockets or receptacles  118  are arranged and affixed in the central area of the interior side of plate  104 . The receptacles  118  extend vertically and are dimensioned to receive the posts that support the furnace hearth rails (see,  FIG. 1  for example). A cover plate  122  is mounted on the interior side of the plate  104  to cover the central area plate. The cover plate  122  sits on and is attached to a spacer ring  124  that is affixed to the inside surface of plate  104 . The cover plate  122  has openings formed therein to permit the receptacles  118  to extend therethrough. Preferably, the bottom head assembly  102  includes means for cooling the head assembly from the intense heat produced in the furnace during a heat treating cycle. The cooling means is preferably realized by the combination of the spacer ring  124  and cover plate  122  which defines an enclosed space that functions as a coolant jacket. The coolant jacket preferably includes channels (not show) for directing the flow of a coolant, such as water, across the interior surface of the plate  104 . The channels are preferably arranged so that substantially the entire surface of the central area of the plate  104  can be contacted with the coolant. The channel arrangement can be readily designed by those skilled in the art to ensure that the plate  104  is adequately cooled and so that there are no dead-flow spots or eddy currents that would adversely affect the cooling of the bottom head assembly  102 . The cover plate separates the cooling channels from the interior of the vacuum furnace when the bottom head assembly  102  is in the closed position relative to the pressure/vacuum vessel body  26 . 
         [0039]    Referring now to  FIG. 12 , there is shown a preferred arrangement for controlling the operation of the lift apparatus according to the present invention. The control system  90  is configured to provide failsafe operation of the lift apparatus by providing a means to interlock movement of the ball screw drives and to synchronize the operation of the lift motors so that the bottom head assembly can be lifted or lowered in a level condition to avoid damage to the bottom head and/or to the lifting mechanism. The control system  90  includes a programmable logic controller (PLC)  100 , a master servo drive circuit  92 , and a follower servo drive circuit  94 . Lift motor  50  is connected to master servo drive circuit  92 . An encoder  96  is mechanically coupled to the drive shaft of motor  50 . In like manner, lift motor  55  is connected to follower servo drive circuit  94  and a resolver  98  is mechanically coupled to the drive shaft of motor  55 . 
         [0040]    The PLC  100  includes a processor that is programmed to provide electrical command signals for operating lift motors  50  and  55  to raise or lower the bottom head assembly in response to commands input by a furnace operator. The operator commands may be input to the PLC by any convenient means such as by push buttons or by a keyboard. PLC  100  is programmed to receive status information from the master and follower servo drive circuits indicating whether the bottom head assembly  102  is in its raised or lowered position. When the PLC determines the location of the bottom head assembly, it sends an interlock signal to the servo drive circuits which indicates that movement can be executed. The master servo drive circuit  92  is connected to the PLC  100  for receiving the command signals and to provide first feedback signals to the PLC. The follower servo drive circuit  94  is connected to the master servo drive circuit  92  for receiving the command signals and to provide feedback signals to the PLC through the master servo drive circuit. The PLC is also programmed to monitor the feedback signals from the master and follower servo drive circuits and to provide updated command signals to maintain synchronism between the lift motors  50  and  55 . The feedback signals may include indicia of position and/or speed. The encoder  96  is adapted to generate a first feedback signal based on rotation of the drive shaft of lift motor  50 . Encoder  96  is connected to the master servo drive circuit  92  for communicating the first feedback signal thereto. In like manner, resolver  98  is adapted to generate a second feedback signal based on rotation of the drive shaft of lift motor  55 . Resolver  98  is connected to follower servo drive circuit  94  for communicating the second feedback signal thereto. Preferably, the system includes a homing limit switch (not shown) which is connected to the lift controller. The homing limit switch is positioned to detect when the lifting mechanism is in it fully lowered position and operates to send a signal to the lift controller so that the system zeros itself relative to the position indication. 
         [0041]    The servo-drive control system of the present invention provides synchronized movement of the lifting mechanisms on each leg assembly without the need for mechanical linkages including multiple gear boxes, shafts, and couplings, between the lifting mechanisms. The omission of such mechanical linkage results in a significant reduction in the time needed to assemble the vacuum furnace at a customer&#39;s facility. The lifting mechanism is much quieter in operation than the known lifting mechanisms for vertical vacuum furnaces. Moreover, the omission of mechanical linkage avoids misalignment problems resulting from long term use. Also, the control system according to this invention is programmable to provide precise lifting/lowering cycles and to be self-limiting with regard to the torque or lifting force the drive mechanisms produce so that accidental damage to any of the elevator components can be avoided. The drives for the lifting mechanisms are self-limiting with regard to the torque or lifting force they produce in order to substantially avoid accidental damage to the elevator components. 
         [0042]    The terms and expressions which have been employed are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features or steps shown and described or portions thereof. It is recognized, therefore, that various modifications are possible within the scope and spirit of the invention. Accordingly, the invention incorporates variations that fall within the scope of the invention as described.