Abstract:
The invention is a magnetic lifting device having a lever-operated cam and roller combination, which facilitates removal of the magnetic lifting device from a workpiece, wherein the cam-mounted roller is free to rotate, eliminating the potential for marring the service of the workpiece when the cam is operated.

Description:
FIELD OF THE INVENTION 
     The invention pertains to permanent magnetic work holders, and, more specifically, to magnetic work holders having release mechanisms to facilitate separation of the work holder from the workpiece. 
     BACKGROUND OF THE INVENTION 
     Both permanent magnet and electromagnetic devices are commonly used in industry for handling ferromagnetic materials. In a typical installation, magnetic lifts are used to lift, or otherwise manipulate, workpieces of ferromagnetic material, such as steel. By way of example, a magnetic lift, secured to a hoist or robotic arm, is brought into contact with a workpiece, such as a steel sheet. So long as the magnetic attraction or holding power of the magnet exceeds the weight of the workpiece, the workpiece may be lifted, moved or otherwise manipulated so long as the magnet remains in contact with the surface of the workpiece. In this fashion, ferromagnetic metallic components may be conveniently transported between locations in the industrial environment, for example, without the need for otherwise gripping the workpiece or providing attach points on the workpiece. 
     This type of magnetic lift is very well known, and has been in industrial use for a long period of time. Typically, such lifts utilizing permanent magnets are comprised of a housing or body containing one or more permanent magnets. The body is provided with a lift point to which may be attached a hoist or similar device. One surface of the complete magnetic lifting assembly so described is designed to be complimentary in shape to the workpiece being lifted. In a wide variety of applications, the workpiece is relatively flat, meaning that the corresponding contacting face of the magnetic lift is also flat. 
     While this basic configuration is both convenient and effective, it presents certain significant problems when permanent magnets are the magnets of choice for the lift. Permanent magnets are capable of exerting substantial magnetic forces. In particular, modern rare earth-type magnets, such as neodymium, exhibit enormously powerful magnetic attraction when properly selected for size, number and alignment. Since the forces exerted by permanent magnet assemblies are constant, separating the magnetic lift from the workpiece after the workpiece has been manipulated or moved presents serious challenges. 
     Basically, in order to separate the magnetic lift from the workpiece, the workpiece must be held, in some fashion, while sufficient force is applied to the magnetic lift to cause it to part from the workpiece. In most applications, this separation is most easily accomplished by lifting one end or one side of the permanent magnet lift from the workpiece, thereby creating an air cap. Once a substantial portion of the contacting surface of the magnet has been separated from the workpiece, the magnetic attraction between the lift and the workpiece is sufficiently reduced to permit the magnetic lift to be completely withdrawn. 
     A number of methods have been devised for separating the magnetic lift from the workpiece. Smith, in U.S. Pat. No. 3,014,751, teaches the placement of a camming release handle in the center of the magnetic assembly. The camming element, pivotably connected to the handle, is brought into contact with the surface by rotation of the handle. Ross, in U.S. Pat. No. 3,319,989, teaches a similar technique, whereby the camming element and handle combination are mounted near one end of the magnetic lift body. Both of these techniques, however, result in substantial marring of the surface of the workpiece. The degree of marring is directly related to the hardness of the workpiece, camming element, and the force required to separate the lift from the workpiece. In addition, earlier designs exhibit substantial friction between the camming mechanism and the workpiece. 
     There is a need, accordingly, for an improved magnetic lift and associated release mechanism, which I describe herein. 
     SUMMARY OF THE INVENTION 
     The invention is a self-contained, portable, permanent magnet-type lift assembly which is provided with a non-marring, low friction release mechanism. The release mechanism is secured to one end of the magnetic lift assembly, and is in the general form of a rotatable cam/handle combination. The surface of the cam which contacts the workpiece is a pivoting roller provided with a non-marring surface. Disengagement of the lift from the workpiece is accomplished by bringing the roller cam into contact with the surface of the workpiece, and using the mechanical advantage provided by the cam handle to separate the lift from the workpiece. The cam is designed as a pivoting roller mounted on a bearing, permitting the cam roller surface to roll across the surface of the workpiece as pressure is applied to the operating handle, thereby reducing both friction and marring of the workpiece surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a collection of side views of the invention shown in relationship to a workpiece, with the release mechanism in the retracted intermediate and extended positions. 
     FIG. 2 is a front view of the invention showing the release mechanism. 
     FIG. 3 is a side view of the release mechanism. 
     FIG. 4 is a top view of the release mechanism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, the present invention, comprising a magnetic lift assembly  10 , is further comprised of a magnet housing  20  having a top  22  and sides  24 A,  24 B,  24 C and  24 D. Preferably, the housing is in the form of a rectangular box, and is provided with a lift point  40  attached to the housing  20  through lift point attachment  42 . Typically, lift point attachment  42  may be a threaded fastener, such as a nut, or a reinforced threaded recess, either of which may be attached to the top  22  of housing  20 . The lift point  40  is then threadably attached to attachment  42 . Other methods of attachment, such as welding, however, are equally useful. 
     With reference now to FIGS. 1 a - 1   d , further detail of the invention is displayed showing a plurality of magnetic elements  21 . Preferably, the magnetic elements are selected from the various classes or types of magnets material, such as ceramic or rare earth magnets. Typically, an array of magnets  21  and intervening pole pieces is utilized to concentrate the magnetic flux of the magnets  21 , thereby in creasing the lifting power of the assembly. The magnets  21  and pole pieces are secured within the housing utilizing any of a variety of well known means. 
     As shown in FIGS. 2-4, one end of the housing is provided with one or more cam attachment brackets  34 . Cam assembly  12  is pivotably connected to the cam attachment bracket  34  in such a way as to permit the roller element  38  of cam assembly  12  to rest on or slightly above the surface of the workpiece  60  when the magnetic lift assembly  10  is in the lifting configuration depicted in FIG. 1 a . Cam assembly  12  further comprises handle  30  and grip  32  which are unitary with roller arms  33   a ,  33   b . Roller arms  33   a  and  33   b  are provided with bearings  37  and with axle element  31 . Axle element  31  provides the pivot for roller  38 . Preferably, sealed ball or roller bearing assemblies are utilized for the bearings  37 . 
     Cam attachment bracket  34  is likewise provided with cam bolts  39 , and cam assembly  12  is attached to cam attachment flange  34  at a position offset from the centerline of the roller axle utilizing cam bolts  39 . This pivotal connection between the cam assembly  12  and cam attachment flange  34  permits the handle of assembly  12  to be rotated toward the workpiece, thereby bringing the roller surface  50  into contact with the workpiece  60  as shown in FIGS. 1 a - 1   d . In the preferred embodiment, cam attachment flange  34  is provided with a stop  52  which limits the rotational movement of handle  30 , to prevent trapping or pinching of the operator&#39;s hands between the handle and the workpiece. Further, handle  30  is provided with return springs  54  interconnecting the handle and the magnetic housing, reducing the amount of force required to return the handle to its upward and disengaged position. 
     The roller surface  50  is a durable machined surface preferably of brass or steel with rounded edge for long life, thereby reducing the amount of damage done to the surface of the workpiece as the handle  30  is rotated from its vertical to its horizontal position. Further, the surface  50  of the roller  38  is free to travel or roll across the surface of the workpiece, thereby preventing the roller surface from scuffing or otherwise marring the surface of the workpiece. 
     Operation of the device will be best understood by examining FIGS. 1 a - 1   d . In FIG. 1 a , the cam assembly is in the retracted position, with the surface of roller  38  either slightly above or just barely in contact with the upper surface of workpiece  60 . In this position, the magnetic elements  21  are in intimate contact with the surface of the workpiece, creating strong magnetic holding. When it is desired to release the magnetic lift assembly from the workpiece, the handle  30 , provided with a grip  32 , is manually rotated downward toward the work surface as shown in FIG.  3 . Because the pivot point at which the magnetic assembly is attached to the cam attachment flange  34  is selected to insure a mechanical advantage, forcing the roller  38  downward against the upper surface of the workpiece  60 , prying the lift assembly away from the work surface and creating an air gap  62  which grows progressively larger as the handle is rotated from the vertical to the horizontal position. The roller is free to rotate on its axle/bearing combination, allowing the roller surface  50  to roll smoothly along the upper surface of the workpiece  60 . Stop  52  prevents the handle  30  from contacting the workpiece, and insures that a gap between the workpiece and the handle remains even at full downward travel of the handle, thereby protecting the hands of the operator. 
     Occasionally, the magnetic attraction between the workpiece and the magnetic lift is sufficiently large as to make it difficult for the magnets and workpiece to be separated, even utilizing the mechanical advantage associated with the operation of the handle  30  in relationship to the workpiece  60  and the magnetic housing  20 . In that circumstance, it may be necessary to apply substantially greater leverage to the lift and the workpiece to effect separation. To this end, the housing  20  is provided with a threaded release  12 , as shown in FIGS. 1 a - 1   d . The threaded release is threadably coupled to the housing in such a fashion as to permit the threaded release to be rotated within a corresponding threaded cavity in the housing  20 . Rotating the threaded release causes the end of the threaded release opposite the head thereof to be brought in contact with the workpiece. Further turning or tightening of the threaded release  12  drives the end of the threaded release against the workpiece, affecting separation of the workpiece and the magnetic lift at the end of the housing  20  opposite the release mechanism  12 . Once an air gap has been created between the end of the housing  20  and the workpiece  60  in the proximity of housing side  24   b , further separation of the lift and the workpiece can be easily effected by operation of the handle  30 , if necessary. 
     To better understand the functionality of the stop  52 , it will be appreciated that the axis about which the handle assembly  12  rotates is defined by the axial center of cam bolts  39 . The position of cam bolts  39  in relation to roller arms  33   a  and  33   b  is selected so as to offset the axis of axle element  31  from the axis of cam bolts  39 . In this fashion, rotation of the handle assembly  12  from the retracted or upright position shown in FIG. 1 a  to the extended position shown in FIG. 1 d  serves to move roller surface  50  from a point distant from housing side  22   c  to a position wherein the roller surface  50  is brought into contact with the lower portion of side  24   c , designated as stop  52 . As will be apparent to those skilled in the art, proper selection of the distance of the axis of cam bolts  39  from the side  24   c , with due consideration for the diameter of roller  38  insures that roller surface  50  will be brought into contact with stop  52 , after lifting housing  20  from the workpiece, but prior to the time handle  32  contacts the plane of the workpiece. 
     Once the magnetic lift assembly has been separated from the workpiece as above-described, the handle may be rotated again to its vertical position, thereby retracting the roller cam to its initial position in preparation for subsequent use.