Patent Abstract:
A duct access door for use with ducts of circular cross-sections that carry cooking vapors and greases is a “sandwich” type door that has inner and outer arcuate plates, the inner plate being positioned inside the circular duct and the outer plate outside the duct with a heat resistant gasket attached to the outer plate. Strategically located bolts draw the plates together to compress the gasket and prevent leaking of vapors and greases. Sliding captive nut assemblies accommodate the arcuate shape of the inner and outer plates so that the bolts may be tightened.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/437,283, filed on Jan. 28, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not Applicable 
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
       [0004]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0005]    1. Field of the Invention 
         [0006]    This invention relates generally to the field of access doors providing access to the interior of a duct, and specifically to an access door designed to provide easy access to the interior of grease ducts and other ducts having a circular cross-section serving, for example, commercial and institutional kitchens, allowing the cleaning, servicing, and inspection of the ducts, or any wires, equipment, or other components located within the ductwork. 
         [0007]    2. Description of Related Art 
         [0008]    Access doors for ductwork are well known in the prior art. Historically, access doors formed from two plates with a gasket between the plates have been known. The access doors are bolted together so that the two plates may be separated in distance and then the inner plate is inserted into the duct through the duct opening. The outer plate and the gasket remain outside the duct and, when the bolts are tightened, the two plates are drawn together to seal the opening in the duct. This generic type of duct access door has become known as a “sandwich” door for obvious reasons. 
         [0009]    To enter the sheet metal ductwork, access openings are usually cut in a rectangular shape which permits the inner plate of a “sandwich” access door to be maneuvered through the diagonal of the rectangular opening and to be positioned inside the duct. 
         [0010]    As ducts have come to be used in kitchen service to remove vapors and grease from cooking areas, duct access doors have become more sophisticated in order to obtain approval by Underwriters Laboratories for use in an environment where grease and cooking vapors are present since a fire may occur inside the duct under such conditions. Access doors approved by Underwriters Laboratories must completely seal the duct in order to obtain such approval. 
         [0011]    One type of access door which has obtained Underwriters&#39; approval is described in U.S. Pat. No. 5,165,189 issued Nov. 24, 1992 to Bernard P. Besal. This access door requires a frame that must be retrofitted into the duct opening in order to receive a cover with a gasket. This retrofitting is time consuming and expensive and requires a multiplicity of fasteners in order to obtain the necessary sealing. 
         [0012]    Until the invention described and claimed in U.S. Pat. No. 6,505,441 B1 issued in Jan. 14, 2003 and assigned to the assignee of the present invention, sandwich-type access doors were not able to meet the sealing requirements of the Underwriters Laboratories for use with grease ducts serving commercial and industrial kitchens. The invention of U.S. Pat. No. 6,505,441 B1 has proven to be very successful commercially for ducts having a rectangular cross-section. 
         [0013]    In recent years, ducts having a circular cross-section have become more popular and it has become necessary to provide sealable access doors for those ducts. An attempt was made to form the parts of the invention of U.S. Pat. No. 6,505,441 B1 into an arc in order to be used on a round duct. It was determined that the curvature of the arc caused misalignment problems for the corner bolts of that invention. The present invention was developed to solve the bolt misalignment problems induced by the arcuate shape of the door as well as to permit the doors to be bent into arcuate shapes of varying diameters to accommodate various sizes of circular cross-section ducts. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    An access door to allow access to the interior of a circular cross-section duct having a rectangular opening formed in the duct is provided. The access door includes first and second arcuately formed rectangular plates which are larger in both dimensions than the rectangular duct opening. The first rectangular plate has at least two bolts fixed to it that project outwardly from the first rectangular plate and the first rectangular plate also has movable nuts slidably secured to at least four corners of the plate to receive bolts. The second rectangular plate which is generally of the same size as the first plate has holes registering with the bolts fixed to the first plate and with certain positions of the nuts slidably secured to the corners of the first plate. A non-flammable gasket which can withstand temperatures resulting from burning debris located within a duct is fixed to the second plate and positioned between the first and second plates. Nuts are threaded onto the two bolts fixed to the first plate. Bolts extending through the holes in the second plate and threaded into the slidably secured nuts are provided. The nuts and bolts are unthreaded sufficiently to permit the first plate to be inserted into the duct and thereafter the nuts and bolts are tightened to draw the first and second plates together and to exert a compressive force upon the gasket. 
         [0015]    The access door of the present invention can be inserted into any circular cross-section duct rectangular opening without modification of the rectangular opening in any way whatsoever. The usual procedure is to provide an access opening template with the access door of the present invention so that the template fixes the size of the rectangular duct opening and provides a rectangular opening which when the access door is positioned within the opening, there is a one inch space where the access door overlaps every side of the rectangular opening. 
         [0016]    By strategically positioning additional bolts at the four corners of the access door, and in some cases along the edges of the access door, the access door has been made air tight, vapor tight, and grease tight so that Underwriters Laboratories&#39; approval may be obtained for the access door of the present invention. 
         [0017]    Accordingly, it is an object of the present invention to provide an access door for circular cross-section ductwork which remains structurally intact and leak-proof under duct fire conditions. 
         [0018]    It is another object of the present invention to provide an access door which provides easy access to the interior of circular cross-section ductwork. 
         [0019]    Another object of the present invention is to provide a sandwich-type duct access door for which approval by Underwriters Laboratories may be obtained for use with various circular cross-section ducting systems. 
         [0020]    These and other objects of the present invention all become readily apparent as this description proceeds in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is an exploded perspective view of the access door of the present invention. 
           [0022]      FIG. 2  is a top plan view of the inner arcuate rectangular plate of the present duct access door. 
           [0023]      FIG. 3  is an end elevation of the inner rectangular plate of  FIG. 2 . 
           [0024]      FIG. 4  is a top plan view of the outer arcuate rectangular plate of the present duct access door. 
           [0025]      FIG. 5  is an end elevation of the outer arcuate rectangular plate of  FIG. 4  and the gasket affixed thereto. 
           [0026]      FIG. 6  is a perspective view of one of the captive nuts viewed from outside and slidably connected to the inner arcuate rectangular plate. 
           [0027]      FIG. 7  is a perspective view of one of the captive nuts viewed from inside and slidably connected to the inner arcuate rectangular plate. 
           [0028]      FIG. 8  is a perspective view of a frame for an access door 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    Referring to the drawings, there is shown a duct access door indicated generally at  10  having a first inner arcuate rectangular plate  12  and a second outer arcuate rectangular plate  14 . Gasket  16  which is coextensive with the rectangular plates  12  and  14  is fixed to the outer rectangular plate  14 . The rectangular plates  12  and  14  must be rigid and are preferably formed of steel having a thickness of at least 1/32″ to prevent warping of the plates by heat and most preferably 1/16″. 
         [0030]    Bolts  18  are fixed to a first plate  12  and project outwardly from the first plate  12 . Nuts  20  are provided to be threadingly applied to bolts  18 . Captive nuts assemblies  22  are provided at the four corners of first plate  12  and may also be provided along the edges of the plate  12  depending upon the size of the access door  10 . These captive nuts assemblies  22  are best shown in  FIGS. 6 and 7  and are received in rectangular holes  34  formed in plate  12 . Rectangular holes  12  are oriented to be parallel to the ends of arcuate plate  12 . It should be noted that in an alternative embodiment of the captive nut assemblies  22  could alternatively be placed in the outer rectangular plate  14 . 
         [0031]    Formed in the second outer rectangular plate  14  are holes  24  that register with bolts  18  fixed to plate  12 . Also formed in outer arcuate plate  14  are holes  26  at the corners of the plate that register with captive nut assemblies  22  on plate  12 . Bolts  28  are provided to pass through holes  26  and thread into the captive nut assemblies  22  on arcuate plate  12 . 
         [0032]    As best seen in  FIGS. 6 and 7 , the captive nut assembly  22  has supporting wings  22   a  and a nut  22   b  secured between the wings  22   a . The nut assembly  22  is positioned within a rectangular slot  34  formed in plate  12 . The slot  24  is oriented to be parallel with the end of plate  12  and permits the captive nut assembly to slide along slot  34  to positions that readily receive the bolt  28  associated with each nut assembly  22 . A sealing plate  36 , the position of which is indicated by hidden lines in  FIG. 6 , is positioned around the nut assembly  22  and next to the concave surface of arcuate wall  12 , as shown in  FIG. 3 . The sealing plate  36  slides with the nut assembly  22  and covers the portions of slot  34  not occupied by nut assembly  22 . It should be noted that in an alternative embodiment that the sealing plate  36  can be used in connection with a bolt assembly in which the position of the bolt is adjustable within the slot  34  and a nut would be used in connection with the hole. Typically in this alternative embodiment the holes would be in the first inner rectangular plate  12  and the slots would be in the second outer rectangular plate  14 . The adjustable bolt assembly could be the same as the adjustable nut assembly wherein a bolt would extend through the adjustable nut assembly on the second outer rectangular plate  14  and then extending through the hole in the first inner rectangular plate  12  where there would be a corresponding nut which would allow the plates to be compressed together. This embodiment would allow the bolt to slide in the slot in the second outer rectangular plate  14 . Additionally, there could be a corresponding sliding nut assembly in both the first rectangular plate  12  and the second outer rectangular plate  14 . 
         [0033]    During the manufacturing process, the plates  12  and  14  are first formed as flat plates. The plates  12  and  14  are thereafter further formed into their final arcuate shapes. Because of the sliding movement of the nut assemblies  22 , the initially flat formed plates  12  and  14  may be formed into shapes with different arcuate radius measurements which permits use with several different sizes of circular cross-section pipe. In addition to being able to form the various radius measurements of the arcuate plates during manufacture, the sliding nut assemblies  22  permit the bolts  28  to align themselves with the nuts  22   a  which would not be possible with fixed nuts in plate  12 , because of the nested nature of the arcuate plates  12  and  14 . 
         [0034]    As seen in  FIG. 1 , an opening  30  in a duct is illustrated in phantom lines. The gasket  16  is attached throughout at least the perimeter of the outer plate  14  and is formed from ceramic fiber board material which is sold commercially by the Unifrax Corporation of Niagara Falls, N.Y. under the trademarks FIBERFRAX® and DURABOARD®. This material is resistant to flame, heat, and grease. 
         [0035]    To install access door  10  in a duct, the nuts  20  are loosened on bolts  18  and the bolts  28  are loosened or removed from the corners of the access door. With the outer plate  14  and gasket  16  separated from the plate  12  but still loosely bolted to inner plate  12 , the inner plate  12  is maneuvered through the diagonal of the rectangular opening  30  to position inner plate  12  within the duct. The bolts  28  are then inserted through the corners of outer rectangular plate  14  and bolted to inner rectangular plate  12  through nut assemblies  22 . The nuts  20  and bolts  28  are then tightened to create a compressive force on gasket  16  and to provide a seal to the access door  10 . 
       DEFINITIONS 
       [0036]    “arced rectangular duct opening member” can be a plate or a frame. As referred to in the above the preferred embodiment it is referred to as a first inner arcuate rectangular plate  12 . It can have any shape including rectangular, oval, circular, racetrack (rectangular with curved ends), etc.
 
“sliding fastener” can be a captive nut assembly or a bolt assembly in which the nut or bolt is able to slide or adjust its position. It can be within the slots.
 
“a rectangular plate” can be demonstrated as the outer rectangular plate  14 . The rectangular plate can have any shape including rectangular, oval, circular, racetrack (rectangular with curved ends), etc.
 
“rectangular slot” is a slot that has a rectangular shape. The rectangular slot can have square or curved corners.
 
“a rectangular arced duct opening member complementary sliding captive fastener or hole” can be a hole, a sliding nut assembly, sliding bolt assembly, or an attachment member within the rectangular arced duct opening member that corresponds to a hole, a sliding nut assembly, a sliding bolt assembly, or an attachment member on a rectangular plate.
 
“a rectangular plate complementary sliding fastener or hole” can be a hole, a sliding nut assembly, sliding bolt assembly, or an attachment member within the rectangular plate that corresponds to a hole, a sliding nut assembly, a sliding bolt assembly, or an attachment member on a rectangular arced duct opening member.
 
“co-fastener” can be the second part of a fastener. For example a nut could be the co-fastener to a bolt. Alternatively a bolt could be a co-fastener to a nut.
 
“threadably connects” is connecting using threads
 
“creating a sealed connection with the circular duct” can be a when the access door is installed in the circular duct a seal is created. The seal may pass the UL tests and prevents smoke and other leakage.
 
“a second arcuate rectangular plate” can have any shape including rectangular, oval, circular, racetrack (rectangular with curved ends), etc.
 
“a first arcuate rectangular plate” can have any shape including rectangular, oval, circular, racetrack (rectangular with curved ends), etc.
 
“a rectangular plate” can have any shape including rectangular, oval, circular, racetrack (rectangular with curved ends), etc.
 
“corners” toward the edges of the object if the object is round. In a square or rectangular shape it is the standard definition.
 
“Nut”—the nut can be a square nut or hex nut.
 
         [0037]    In accordance with the provisions of the patent statutes, I have described the principle, mode of operation and the preferred embodiment of my invention. It should be understood that the invention may be practiced otherwise than as precisely illustrated and described herein in accordance with the claims affixed hereto.

Technology Classification (CPC): 5