Abstract:
A safety brake device for a theatre hoist to prevent the uncontrolled release of a load that is suspended above or below people includes an overrunning clutch and a torque disc. The torque disc only rotates with the overrunning clutch when the load is lowered, but must overcome friction forces applied to the surface of the torque disc to do so. The friction forces are constantly applied to the torque disc by maintaining friction material in contact with the torque disc. The friction material is a non-asbestos, non-metallic composite saturated with a lubricant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. application Ser. No. 12/567,338, filed Sep. 25, 2009, now U.S. Pat. No. 8,448,922 B2, issued on May 28, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to safety brakes and, more specifically, to a safety brake device applied to theatre hoists that lift and maintain heavy loads suspended. 
     2. Description of Prior Art 
     Hoists that lift loads in a vertical direction are used in many industries for a variety of applications. For theatrical settings, athletic and entertainment arenas, overhead lifting with higher safety standards are routinely required because hoists are lifting loads directly over human beings. It is also common for portions of the staging in these theatrical settings to be lifted. Similar safety standards are required in these instances because people may be standing on the portion of the stage being lifted. 
     Live performances in a theater typically employ a number of curtains and backdrops to convey to the audience different settings, environments, moods, and the like. These curtains and backdrops must be changed throughout the course of a performance within a fairly short time frame without interrupting the performance. Typically this is done by raising a particular backdrop above the stage and out of sight of the audience when it is not being used. When a particular backdrop is needed, it is lowered into place on the stage. 
     Theatrical backdrops and curtains are typically suspended from battens, which are pipes or trusses that span the width of the stage. Battens can be 20 feet or more in length, depending on the size of the stage. As should be apparent, the weight of the battens and the items suspended from them can have substantial weight. As the weight of the load increases so does the power required to raise the load. Counterweights are employed to balance the load of the batten and its associated load. However, if the load is not closely balanced or if there is a failure in the motorized drive lifting the hoist, the system may get out of control, dropping the load or the counter-weight, causing injury or death to people nearby and/or collateral damage. 
     Therefore, because of the risk of hoist failure, there is a need for a safety device to prevent the uncontrolled release of heavy loads and staging that are either supported above or below human beings. 
     SUMMARY 
     The present invention comprises a combination overrunning clutch, torque disc, and friction material for preventing the uncontrolled lowering of a load. An axle connected to a motorized drive engages the overrunning running clutch. A torque disc fixedly attached to the overrunning clutch will rotate with the overrunning clutch when a load is lowered; however, resistance against rotation is generated by a set of fixed friction discs applying pressure to the sides of the torque disc. In order to lower a load, the motorized drive must overcome the friction forces applied to the side surfaces of the torque disc, thus enabling the axle to rotate. 
     None of the prior art devices are seen to offer the advantages of the present invention that will become apparent from the detailed description of the invention provided below. 
     It is an advantage of the present invention to provide a safety brake device that prevents the uncontrolled release of a suspended load. 
     It is a further advantage of the present invention to provide a safety brake device that provides smooth consistent resistance without producing excessive noise during operation. 
     It is a further advantage of the present invention to provide a safety brake device that uses a friction material that operates consistently at different temperatures and irregular use. 
     It is a further advantage of the present invention to provide a safety brake device that uses a friction material that is long wearing, thereby reducing the need for adjustment. 
     It is an advantage of the present invention to provide device of simple design and manufacture that can be fitted to current hoisting systems. 
     Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view of a first embodiment of the present invention. 
         FIG. 2  is left-side elevational view of the first embodiment of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the first embodiment along line “A-A” of  FIG. 2 . 
         FIG. 4  is a right-side elevational view of a hub for the first embodiment of the present invention. 
         FIG. 5   a  is an elevational view of a first embodiment of the present invention mounted on an axle and suspended from a frame. 
         FIG. 5   b  is an elevational view of an installed first embodiment of the present invention above a stage. 
         FIG. 6  is a front elevational view of a second embodiment of the present invention. 
         FIG. 7  is a left-side elevational view of the second embodiment of  FIG. 6 . 
         FIG. 8  is a rear elevational view of a friction disc of the second embodiment of the present invention. 
         FIG. 9  is a left-side elevational view of the friction disc of  FIG. 8 . 
         FIG. 10  is an elevational view of an installed second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment of the present invention is shown in  FIGS. 1 through 4 . The first embodiment of the safety brake device comprises an overrunning clutch  12  and a disc assembly  10 . The overrunning clutch  12  may be of any suitable design known in the art, such as a ramp and roller or sprag, and includes a keyed bore enabling the inner race of the overrunning clutch to rotate with an axle installed through the keyed bore. The clutch is designed and installed such that the outer race will rotate with the inner race and axle only when a load is lowered. 
     The disc assembly  10  comprises a hub  22  which is installed adjacent to the overrunning clutch  12  along the axle. The hub  22  also has a bore enabling it to be installed onto the axle; however, the diameter of the bore is not keyed and is larger than the diameter of the axle, so that the hub  22 , if fixed, will not rotate with the axle. Mounted onto the hub is a torque disc  14  sandwiched between a set of friction discs  18  and backing plates  20 . A securing means is required to constantly maintain a force that presses the friction discs  18  against the sides of the torque disc  14 . In the first embodiment, the securing means comprises a nut  26  and Belleville washer  24  that is screwed onto a threaded end of the hub  22 , such that the Belleville washer is pressed against one face of a backing plate  20 . The disc assembly  10  further comprises an adaptor  16  fixed to the torque disc  14 . A first set of screws  28  are used to attach the torque disc  14  to the adaptor  16 . 
     A second set of screws  30  are used to attach the adaptor  16  to the outer race of the overrunning clutch  12 . The adaptor  16  and torque disc  14  are fixedly attached to the outer race of the overrunning clutch  12 , so that the three elements will rotate together when lowering a load; however, the friction discs  18  and backing plates  20  remain fixed on the hub  22  and will not rotate with the torque disc  14 , thus generating a friction forces between the torque disc  14  and friction discs  18  when a load is lowered. The disc assembly optionally includes a bearing  32  that keeps the torque disc  14  aligned with the friction discs  18  and prevent uneven wear of the friction discs. 
     Referring now to  FIGS. 5   a  and  5   b , a typical environment in which the present invention may be installed is shown. The safety brake device is mounted on the opposite end of an axle  34  from a motorized drive  38  and suspended from a frame  39  above a stage. The dashed outline in  FIG. 5   b  provides a cut-away view of the area above the stage where the first embodiment of the invention is typically installed. The exposed face of the hub  22  is attached to a bracket  40  which keeps the hub  22  fixed as the axle  34  rotates. A set of winch drums  36  fixed onto the axle  34  may also be present. Cables  33  wound around the winch drums  36  are attached to a batten  35  from which a load  37 , such as a curtain  37   a  or theatrical scenery  37   b , is suspended. The cables  33  can also be directly attached to a load, such as a platform  37   c . When the motorized drive  38  rotates the axle  34  to lift a load, the inner race of the overrunning clutch  12  rotates with the axle  34 , but the remaining parts of the safety brake device remain fixed. 
     Once a load is suspended, the motorized drive  38  stops. The weight of the load will force the axle  34  to rotate in the opposite direction to lower the load; however, at this instance, the overrunning clutch  12  will lock, so that the outer race, adaptor  16 , and torque disc  14  will attempt to rotate, but will be held in place because of the friction forces between the torque disc  14  and the friction discs  18  which remain stationary with the hub  22 . If a friction disc is selected such that the friction forces are equal to the gravitational forces of the load, the motorized drive is not taxed and only a slight application of rotational force to the axle is necessary to set the load in motion. 
     An appropriate friction material must be selected for the friction discs  18  which has a low differential between static and dynamic coefficients of friction, such that a motorized drive is not heavily taxed when started and loads may be raised and lowered at a slow speed. It is preferred that the ratio between the static coefficient of friction and the dynamic coefficient of friction for the friction material be equal to or greater than 1.05 and less than or equal to 1.15. The friction material needs to provide smooth consistent resistance without producing any squeal, as excessive noise would be unwanted during a performance. Eliminating squeal can be achieved by saturating the friction material with a lubricant. Given the often unpredictable system usage, the friction material needs to be consistent at different temperatures and irregular use. Finally, the material needs to be long wearing reducing the need for adjustment and replacement. Any frictional material known in the art to include these characteristics, for example the frictional materials disclosed in U.S. Pat. No. 6,630,416, the disclosure of which is incorporated herein by reference, is acceptable. 
     Referring now to  FIGS. 6  though  9 , a second embodiment of the invention is disclosed wherein the disc assembly has been replaced with a caliper and pad assembly. The second embodiment of the invention does not require the use of an adaptor as the torque disc  14  is secured directly to the overrunning clutch  12 . The friction material is now in the form of a pair of friction pads that sandwich the torque disc  14 . The friction pads are comprised of a shoe  48  to which the friction material  50  is bonded. An intermediate backing layer may be employed between the shoe  48  and the friction material  50 . The friction pad shoes  48  are attached to a caliper  44  which applies the necessary force to the sides of the torque disc  14 . Turning the knob  42  of the caliper  44  increases the distance between the ends of the caliper arms  46 . Because the caliper arms  46  are pivotally connected, the distance is decreased between the opposite ends of the caliper arms  46  to which the friction pads are attached. The knob  42  is turned and left in position to constantly maintain a force on the sides of the torque disc  14 .  FIG. 10  demonstrates second embodiment of the invention installed in the same typical environment shown in  FIG. 5 . The caliper  44  is braced to the frame to which the motorized drive and axle are suspended. The torque disc  14  has a bore enabling it to be installed onto the axle; however, the diameter of the bore is not keyed and is larger than the diameter of the axle, so that the torque disc  14  will only rotate with the outer race of the overrunning clutch  12  when a load is lowered. The friction forces applied by the friction pads on the caliper  44  should be equal to the gravitational forces of the load, such that the motorized drive is not taxed and only a slight application of rotational force to the axle is necessary to set the load in motion. 
     Thus, there has been described and illustrated herein a safety brake device that prevents the uncontrolled release of a suspended load. However, those skilled in the art will recognize that many modifications and variations besides those mentioned specifically may be made in the technique described herein without departing substantially from the spirit and scope of the present invention. For example, the safety brake device may be designed as a drum brake wherein the friction material is in the shape of a collar that applies frictional forces to the circumference of the torque disc. Accordingly, it should be clearly understood that the forms of the invention described herein are exemplary only, and are not intended as a limitation on the scope of the present invention.