Abstract:
A device for removing static charges from a moving paper web includes two grounded and shielded permanent magnets with means to support each magnet on opposite sides of the moving web with the polarities of each magnet so oriented that the charge-neutral center plane of the magnet is perpendicular to the path of the web. The device removes static charges by exposing the paper web to a grounded magnetic field first of one polarity and then to a grounded magnetic field of the opposite polarity.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for removing static electricity by use of a grounded magnetic field. 
     2. Description of the Related Art 
     Static electricity as is generated by the passage of webs of electrically non-conductive material over industrial equipment, creates substantial problems in many industries, for example the printing industry. Thus, charges caused by friction of paper passing against the dissimilar material of a printing press, conveyer or other machine may interfere with downline processes such as folding or stacking. 
     Methods heretofore used in dealing with static electricity include the use of powder sprays, silicon, etc. Such methods in reducing friction may increase maintenance problems, as those due to spray build-up, and may be unsuitable for use with many types of materials. 
     &#34;Static bars&#34; have been designed to discharge a constant current from a high voltage source to the paper to neutralize static build-up. These have been generally ineffective because of the continual variations in charge and in the location of the static charge build-up. 
     SUMMARY OF THE INVENTION 
     In the preferred embodiment of the present invention a strong grounded magnetic field is used to remove static charge build-up. As utilized with paper webs, for example, a statically charged web is passed between two elongated permanent magnet assemblies closely spaced in parallel alignment, and positioned traverse to the movement of the web. The magnetic assemblies are shielded along three elongated sides of each magnet to increase the density of the field emitted by the exposed side. Each magnet has its polarities oriented so that the charge-neutral center plane is parallel to the two opposite shielded sides. The result is that the opposite sides of the passing web are first simultaneously exposed to one polarity, and then to the opposite polarity. 
     While we are aware of no accepted theory regarding how magnetic flux interacts with static charges, it is believed that flux of one polarity attracts the static charges of opposite polarity and leads them to ground. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a magnetic device shielded on three sides embodying the present invention. In this illustration, the side shielding is broken away to show its assembly of several magnetic blocks of equal size. The base extends beyond the magnet assembly to allow clamping in position and for convenient grounding. The polarities of the magnet assembly are marked by plus and minus signs. 
     FIG. 2 is an end view of the FIG. 1 device. The dotted lines illustrate the idealized shape of the magnetic field as shielded and ideally emitted by the exposed surface of the device only. The charge-neutral center plane where the oppositely charged flux fields meet is also shown. 
     FIG. 3 is an isometric view of an alternative magnetic device in which a single magnet of the desired length is used. 
     FIG. 4 is an isometric view illustrating movement of a web between the grounded magnetic devices of either the FIG. 1 or FIG. 3 type and supporting end brackets which maintain the parallel alignment and spacing of the device and to which grounds are connected. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The magnetic device 10 shown in FIG. 1 utilizes a permanent magnet 16 constructed of several permanently magnetic blocks 17 of equal size. Each block has its polarities oriented along its elongated sides, their polarities being marked in FIG. 1. The blocks are assembled end-to-end. This construction allows use of a plurality of standard sized magnets 17 to construct a magnetic device 10 of custom length. This type of assembly is especially advantageous when the permanent magnet material is soft or brittle making manufacture of a single magnet of sufficient length impractical. 
     The permanent magnet 16 preferably made of the metal hereafter described isshielded on three sides by a base plate 11 and two side plates 12, 13 held to it along the sides of opposite polarity of magnet 16 by welds 14, 15. The shielding serves in effect to shape and partially concentrate the magnetic field emitted; such a shielded field is ideally illustrated in FIG. 2, with the field of one polarity a and the field of opposite polarity b. The base shielding plate 11 is preferably longer than the magnet 16, its projecting ends haying bores 24, 25 to allow bolting of theapparatus in place, and preferably both ends of the base plate are connected to a relatively unimpeded electrical ground 22, 23. Side shielding plates 12, 13 are of a height slightly greater than the magnet 16, to protect the exposed surface 25 of the magnet and to improve the &#34;focus&#34; of its magnetic field a, b. 
     The preferred material presently available for the permanent magnet 16 is believed to be strontium ferrite oxide (SrO·6Fe 2  O 3 ), aceramic. 
     The shielding on the sides 12, 13 and base 11 of the magnet should, at current state of the art, be at least 1/4 of an inch thick, preferably 3/8ths of an inch thick. The side shields 12, 13 preferably extend 1/16&#34; beyond the exposed surface face 25 of the magnet. The presently preferred shield material is a steel alloy containing 30% to 50% nickel, commercially available from Ford Steel Co., St. Louis. 
     The magnetic device 10 shown in FIG. 3 uses a single permanent magnet 30 asan alternative to the FIG. 1 assembly 16. 
     For removing static electricity from both sides of a moving web c, use of two magnetic devices 10 of the FIG. 1 or FIG. 3 type assembled together opposite each other as shown in FIG. 4 is preferred. Each of the two magnetic devices 10 is constructed preferably slightly longer than a widthof a web c from which static charges are to be removed. The web c passes between the two narrowly spaced magnetic devices 10, they being supported and secured by brackets 31, 32 in parallel alignment, with their flux-emitting surfaces 25 facing each other, their like polarities being opposite to each other, and their common charge-neutral planes d perpendicular to the path of the moving web c. Each bracket 31, 32 preferably has a slotted mounting hole 33 and an arcuate slot 34 to facilitate relative spacing and alignment. Preferably each end of both themagnetic devices has electrical grounds 22, 23. The brackets 31, 32 are mounted so as to avoid metal-to-metal conductive contact between the magnetic apparatus 10 and the printing press or other equipment which is generating the static electricity on the moving web. 
     As used with a printing press, the two magnetic devices 10 are so positioned by brackets 31, 32 that the paper passes between them after exiting the printing press. The magnetic devices 10 are preferably spaced as close as is practical, say within one inch of each other, so that each side of the paper must pass within say 1/2 inch of an exposed surface 25 of the magnet. 
     The length of each magnetic device 10 is preferably slightly greater than the width of the web c, say a minimum one inch overlap on each end. The physical proportions of each device 10 must be determined by the magnitudeof the static electricity present on the web c. We know of no mathematical equation to calculate the field strength required. 
     Readings of static electricity may be taken by a standard static charge meter such as Model ACL 400 sold under the trade name &#34;Static Locator&#34;, commercially available from ACL, Inc. Elk Grove, Ill., at several locations on the web c as it exits the printing press. Variables will affect the amount of static electricity; common variables include the speed of the press, the humidity in the air, and the type of web material moving through the press. 
     A magnetic device 10 of strontium ferrite oxide made of available standard sized 1×2×6 inch blocks 17 having nominal properties of residual induction 390 m Tesla, commercially available from Crucible Magnetics Corp., Elizabethtown, Kty., as Ferrimag®8A, when shielded asdescribed herein will eliminate static charges up to (+) or (-) 30,000 Volts. An apparatus 10 constructed of blocks 17 of magnets of greater mass, for example 2×2×6 inch, may be necessary to eliminate greater quantities of static charge build-up. 
     Preferably each end of each magnetic device is connected to an isolated ground 22, 23. 
     Paper webs passing through this magnetic apparatus have been found to be substantially static free, eliminating a variety of common problems encountered in downline processes such as stacking and folding. 
     While grounding of the magnetic device at each end is generally preferred, in certain instances, such as when an extremely long magnetic device is constructed, additional intermediate grounds may be desirable. 
     While this apparatus has proven effective for use with moving webs, it may also be used with any material, regardless of shape, which is relatively non-conductive and therefor likely to accumulate static charges during manufacture or subsequent processing or use. Likewise, this apparatus may also be used in cases where the magnet moves relative to the materials, such as reciprocating movement by the magnet. 
     Since various modifications may be made in the apparatus and use herein described without departing from the scope of the invention, all matter contained in the foregoing description shall be taken as illustrative rather than limiting.