Abstract:
A displaceable drive ball safety element type torque limiter is installed in a variable speed AC motor drive crusher mill drive which can include a selective operable locking brake to hold the crusher components stationary during maintenance operations.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 61/209,086 filed on Mar. 3, 2009. 
     BACKGROUND OF THE INVENTION 
     The present invention concerns variable speed AC driven mining machinery such as crusher mills. Such mills involve massive components and very large horsepower motors in excess of 1000 hp. This type of AC motor sometimes malfunctions in such a way as to create a very large momentary spike in torque sufficient to destroy the pinion ring drives for the mills. This is a catastrophic failure and most require months to bring the equipment back on line. 
     The industry has previously exclusively used “SAFESET” (trademark of Voith Company of Germany) torque limiters, which are costly and require a long lead time to obtain. They also require part replacements when an overload occurs and involve friction such as to be subject to wear. Ordinary friction torque limiters have been thought to be inadequate for such an application, and shear pins are not consistent in releasing drive and must he replaced. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes torque limiters of a type sold by Brunel Corporation and described in copending U.S. application Ser. No. 12/704,581 filed on Feb. 12, 2010, incorporated by reference herein. The safety elements are mounted radially (or axially) on a pitch circle diameter. These torque limiter safety elements include balls held down in detents (drill point type holes) by spring forces acting on locking balls. Shearing (torque) forces are applied to the complement of balls in their detent pockets. This produces driving torque, which is transmitted from input to output through the drive balls. When the driving torque exceeds the allowable torque set by spring forces holding locking balls which hold the plungers keeping the drive balls in the detent pockets, the locking balls are displaced and the drive balls ride out of the detent pockets. The locking balls ride up ramps to an outside diameter which produces a complete disconnect of the torque limiter allowing relative rotation to continue indefinitely without any frictional drag. 
     Reengagement is accomplished by rotating the now free drive limiter half to align match marks on the driven hub. Plungers which reset the spring loaded balls into their pockets are tapped with a. dead bow hammer reseating the locking balls to reapply the spring load and reset the torque limiter to the preset release torque setting. 
     An important additional feature is the provision of a large brake disc and brake caliper integrated in the torque limiter. 
     When the torque limiter is disengaged, the brake caliper is activated to hold the mill drive stationary and prevent the mill from moving, which would present a safety hazard to a maintenance worker entering into these very large machines to do repairs. Movement of the components may otherwise occur. 
     The torque limiter design:
         1. Can include a brake disc for use in a holding brake system used on the ball and sag mill application.   2. Is less expensive than other types of styles of products.   3. Takes less time to reset after an overload event that disengages the ball detent safety elements.   4. Has a resetting reliability which doesn&#39;t rely on highly skilled maintenance personnel.   5. No spare or replacement parts are required after one or multiple overload events that disengage the ball detent safety elements.   6. Can be used in low and high speed applications.   7. Can accommodate a variety of mounting configuration based on shaft arrangements.       

    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an apparatus according to the invention including a ball or sag crusher mill in combination with a low speed drive incorporating a torque limiter of a type depicted below in the other Figures. 
         FIG. 2  is a diagram of another apparatus according to the invention including a ball or sag mill in combination with a high speed drive incorporating a torque limiter as in  FIG. 4 . 
         FIG. 3  is a diagram of another apparatus according to the invention including a gyratory cushion mill and drive incorporating a torque limiter as in  FIGS. 1 and 2 . 
         FIG. 4  is a partially sectional view of a torque limiter of a type utilized with the invention. 
         FIG. 4A  is a pictorial view of the torque limiter shown in  FIG. 4 . 
         FIG. 4B  is an enlarged view of a portion of  FIG. 4 . 
         FIG. 5  is a partially sectional view of a torque limiter of a smaller capacity and without a locking brake. 
         FIG. 5A  is a pictorial view of the torque limiter shown in  FIG. 5 . 
         FIG. 5B  is an enlarged view of a portion of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain specific terminology will he 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. 
       FIG. 1  depicts a low speed crusher mill  10  having a ring gear  12  affixed to its perimeter at one end driven by two pinions  14  on a shaft  16  supporting a pillow block hearings  18 . A pair of variable speed AC motors  20  each have an output shaft  22  which each in turn drive an input of a torque limier  24  of a type described herein and in the copending patent application referenced above. A flex coupling  22  can be included for easier alignment. 
     A disc locking brake  26  can be included as a safety feature. 
       FIG. 2  depicts a high speed drive for a crusher mill  10  which includes reduction gearing  28  interposed between the torque limiter  24  and the pinion drive shaft  16 . 
       FIG. 3  depicts a gyratory crusher mill  32  with a drive gear  34  driven by a pinion  36  on a shaft  38 . 
     A flex coupling  40  and spacer shaft connect the torque limiter  24  connected to a motor shaft  42 . 
       FIG. 4  is an enlarged sectional view of a torque limiter  24  of the type combined with the crusher mill drive, and  FIG. 4A  is a pictorial view thereof. 
       FIGS. 5 and 5A  show a lower capacity torque limiter  24 A with only two safety elements  74 . 
     Axially extending safety elements can also be used as described in the copending patent application referenced above. 
     The torque limiter  24 ,  24 A are of a type previously commercialized but not heretofore combined with crusher mills, and includes a series of “safety elements”  46  which are each comprised of an outer module carrier  48  carrying a plurality of radially oriented module assemblies  50 . The module assemblies  50  each include a plunger  52  engaging a drive ball  54  resting in a seat  56  carried in a detent pocket carrier  58 . The carriers  48  and  58  are respectively attached to respective drive and driven members  60 ,  62  in the mill drive so as to establish a rotary drive from the motors  20  to the crusher mill  10  through the drive balls  54 . 
     At a preset torque level, which can be as high as 700,000 lb-ft, the reaction of each drive ball  54  overcomes the force exerted by a series of Belleville springs  64  acting on a ramp ring  69  restraining radial movement of locking balls  66  allowing all of the plungers  52  to move radially out. Locking balls  66  are displaced to ride out over ramps  67  and onto an outer race  68 . In this position, the locking balls  66  do not constrain the drive balls  54  to stay in the pockets  56 . This action is described in detail in the above referenced copending application and is known in the torque limiter art. 
     A brake disc rotor  90  ( FIG. 4 ) is affixed to detent pocket carrier  58  filed to the driven member  62 , engaged by a caliper  72  when the torque limiter is disengaged, which can apply an engagement force to lock the rotor  70  and thus the driven member  62  and thereby prevent any motion of the mill components when the torque limiter is disengaged.