Abstract:
A resettable torque limiter in which major components are cast from stainless steel in order to minimize material requirements and which are thereafter annealed to provide corrosion resistance in service.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. provisional application No. 61/279,134 filed Oct. 16, 2009, incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention concerns safely torque limiters used in waste water treatment facilities. 
         [0003]    Such facilities typically include rectangular settling tanks which have scrapers or flights connected by a drag chains which are driven by electric motors and reducers. Safety overload torque limiters are incorporated in the drive to protect the drive equipment in case a jam condition develops. 
         [0004]    Simple shear pin designs are in wide use but are less reliable and more labor intensive to maintain and create longer down times since they must be replaced when sheared. Failures due to the corrosive environment have often occurred with other designs. 
         [0005]    A superior resettable torque limiter design has been developed for this application, using a drive ball which is spring urged into a position by a plunger where it creates a driving connection between two rotary members in the torque limiter. The driveball is driven out of a receiving recess in one member to overrun when the torque exceeds a preset maximum. The drive ball will not resume its driving position when the torque level declines due to an arrangement which prevents the spring force from being active once an overrunning event has occurred. The torque limiter is reset by repositioning the drive ball into its driving position as by tapping the plunger with a hammer or mallet. 
         [0006]    Such torque limiters have operated very reliably over many years in service which is important as the waste treatment equipment is costly and must be kept operational at all times. 
         [0007]    The corrosive environment in which these torque limiters operate has dictated that stainless steel be used in the manufacture of its components and this causes that limiter to be relatively costly to manufacture due to the considerable amount of expensive stainless steel which is required to manufacture these torque limiters. 
         [0008]    Such torque limiters have conventionally been constructed from stainless steel bar stock which is rolled when produced, which rolling develops a metallurgy in the material which is highly corrosion resistant. Thus, use of bar stock for the torque limiter parts results in highly corrosion resistant parts and provides a very good long term service performance. 
         [0009]    Using stainless steel castings has been regarded as unsatisfactory for applications subjecting parts to corrosion as being much more subject to corrosion due to the different metallurgy produced by the casting process. Such parts are likely to fail in a relatively short time when used in water treatment facilities. 
         [0010]    There is considerable stainless steel material lost in the machining of these large parts from bar stock, since it requires extensive machining away of the material, causing substantial waste of stainless steel in their manufacture. The need for such extensive machining of those parts also adds significantly to the cost of such torque limiters. 
         [0011]    It is an object of the present invention to provide a corrosion resistant resettable safety torque limiter for waste water treatment facilities of the type described which is highly corrosion resistant to be well suited to waste water treatment applications but which can be manufactured at significantly lower cost. 
       SUMMARY OF THE INVENTION 
       [0012]    The above recited object and other objects which will be understood upon a reading of the following specification and claims are achieved by precision casting the major torque limiter rotary components from stainless steel, which allows the components to be cast close to their final dimensions, eliminating the extensive machining operations which were previously required to produce such components and the waste of stainless steel metal which is machined away. 
         [0013]    In addition, recess features are preferably molded into these components at locations which will not affect their proper performance during the service life of the torque limiter to achieve additional material savings. 
         [0014]    The stainless steel alloy used is a low carbon alloy, i.e., 0.03% carbon 316L (CF3M) stainless steel. 
         [0015]    In order to achieve the necessary corrosion resistance, the cast components are annealed after casting by being heated to a temperature in the range of 1850° F. to 2050° F. in an oxygen free atmosphere provided by a vacuum furnace, with subsequent rapid quenching as in a sand bed fluidized with nitrogen gas, preferably in a multiple chamber furnace to achieve sufficiently rapid quenching. 
         [0016]    This annealing process largely eliminates precipitate carbon which is believed to make the cast material subject to corrosion, and have produced corrosion resistant parts. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a sectional view through a safety element torque limiter of type which the present invention is concerned. 
           [0018]      FIG. 1A  is a fragmentary enlarged view of a portion of the torque limiter shown in  FIG. 1 . 
           [0019]      FIG. 2  is an exploded pictorial view of the torque limiter shown in  FIG. 1 . 
           [0020]      FIG. 3  is an enlarged pictorial view of a module carrier hub component included in the torque limiter shown in  FIG. 1 . 
           [0021]      FIG. 4  is a pictorial view of a detent pocket plate included in the torque limiter shown in  FIG. 1 . 
           [0022]      FIG. 5  is a pictorial view of a retaining plate included in the torque limiter shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
         [0024]    Referring to the drawings, and in particular  FIGS. 1 and 2 , a resettable torque limiter  10  is shown of the type described in U.S. 2010/0224713A1, incorporated herein by reference. 
         [0025]    A carrier hub  12  has a relatively rotatable pocket plate  14  supported thereon. 
         [0026]    A retainer plate  16  is affixed to one end of the carrier hub  12  and together with a flange on the other end of the carrier hub  12  axially captures the pocket plate  14  on the carrier hub  12 . 
         [0027]    A drive shaft (not shown) is received in a common bore  18  machined to the size of a shaft to be received therein (not shown) in the retaining plate  16  and carrier hub  12 , with a key groove  20  and set screw hole  22  also provided in the well known manner. 
         [0028]    An output member  24  is connected to be driven by the pocket plate  14 . 
         [0029]    A disconnectable drive unit  26  is mounted on a hub of the carrier hub  12  and includes a primary drive ball  28  urged against a seat  30  carried by the pocket plate  14  by a spring loaded plunger  32  so that the drive ball  28  is positioned partially within aligned respective recesses in the carrier hub  12  and the pocket plate  14  to establish a rotary drive connection between the carrier hub  12  and pocket plate  14 . 
         [0030]    The torque transmitted from the carrier hub  12  through the drive ball  28  to the pocket plate  14  tends to cam the drive ball  28  out of the recess in the pocket plate  14  and back into the recess in the carrier hub  12  since the center of the ball  28  lies within the recess in the carrier hub  12  when on the seat  30 . This movement is resisted by the Belleville springs  40  urging the plunger  32  and engaged drive ball  28  to the left. 
         [0031]    There is a counterforce acting on the plunger  32  tending to move the plunger  32  back to the right generated by the torque transmitted by the drive ball  28 . When a predetermined maximum load is exceeded, a set of secondary balls  34  are moved by the force acting on the plunger  32  to ride out on ramp ring  36  to an overrunning position on an outside diameter  38  of the plunger  32 , releasing the spring force normally exerted on the ball  28  by the Belleville springs  40  ( FIG. 1A ). 
         [0032]    This allows the drive ball  28  to be moved completely to the right out of the recess in the pocket plate  14  by the reaction to the torque acting between these components to disconnect the drive between the carrier hub  12  and pocket plate  14 . This continues until the drive unit  26  is reset as by driving the plunger  32  to the left to cause the balls  34  to re-enter the space between the ramp  36  and facing ring  37 . 
         [0033]    The major components of the torque limiter  10  are the carrier hub  12 , the pocket plate  14  and the retainer plate  16 , and each have portions thereof that are only light stressed in service. 
         [0034]    According to the invention, these parts are cast from an austenitic stainless steel, using a precision casting process such as investment casting to minimize the material to be machined away, saving on material and machining time. The cast material is rendered corrosion resistant by a subsequent annealing process, known per se but not heretofore applied to this application. A preferable material austenitic stainless steel is low carbon 316L stainless steel (CF3M), i.e. containing approximately 0.03% carbon. The composition is as follows: 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 ASTM 351 
               
               
                 316L (CF3M) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 C 
                 &lt;0.03% 
               
               
                   
                 Mn 
                 &lt;1.50% 
               
               
                   
                 Si 
                  &lt;1.5% 
               
               
                   
                 P 
                 &lt;0.04% 
               
               
                   
                 S 
                 &lt;0.04% 
               
               
                   
                 Cr 
                 18.00%-21.00% 
               
               
                   
                 Ni 
                  9.00%-12.00% 
               
               
                   
                 Mo 
                 2.00%-3.00% 
               
               
                   
                   
               
             
          
         
       
     
         [0035]    In the as cast state, carbon tends to be precipitated and forms localized areas subject to initiation of corrosion. 
         [0036]    By heating the casting to approximately 1900° F. (1850-2050° F.) the carbon precipitate is largely eliminated by causing the carbon to re-enter solution. 
         [0037]    The parts are then quenched to prevent the carbon from again precipitating. 
         [0038]    The quenching is preferably carried in a sand bed fluidized by nitrogen gas, in a two or three chamber vacuum nitrogen back fill. 
         [0039]    In  FIG. 3 , the carrier hub  12  is shown having two voids  42  cast therein intermediate the two holes  44  provided to accept the overload release units  26  (only one can be provided and one hole  44  covered by a cover plate  46 ). Full thickness material is maintained in the area surrounding these holes  44 . 
         [0040]    In  FIG. 4 , the pocket plate  14  has four cast in voids  48  located between bolt holes  50  except where the pockets  56  ( FIG. 1 ) are formed (on the opposite side) to receive the drive ball  28  and seat  30 . 
         [0041]      FIG. 5  shows voids  58  cast in between the bolt holes  60  except in the location aligned with the key way  20  and set screw hole  22 . 
         [0042]    Accordingly, a torque limiter suitable for waste water treatment drive equipment can be provided at a substantially lower cost than previous designs of such devices while insuring an extended service life.