Patent Abstract:
A twist lock attachment system for attaching a fixture having a sheet metal mounting plate that is part of a sheet metal housing to a structural support. The exemplary fixture disclosed is a lighting fixture attached to a wiring conduit. The mounting plate has a generally circular mounting aperture. A twist lock bushing includes a tubular body correspondingly generally circular in cross section. The mounting plate and the twist lock bushing releasably engage each other by relative movement to insert the tubular body into the mounting aperture with the mounting plate and the bushing angularly oriented in an insertion and removal position with reference to each other, followed by rotation of the mounting plate and the twist lock bushing relative to each other to an installed position.

Full Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a twist lock attachment system for attaching a fixture, such as a lighting fixture, to a structural support such as a wiring conduit. The invention particularly applies to lighting fixtures having sheet metal housings. 
     Fixtures, for example commercial and industrial lighting fixtures, are often installed by hanging from a structural support such as a threaded conduit. A commercial or industrial lighting fixture that includes a ballast transformer can have a substantial weight, making the installation task difficult. 
     Thus, hanging a heavy, sheet metal housed lighting fixture from a conduit typically requires either extensive bracketry, or expensive permanently-attached threaded bushings. As an example of bracketry, some lighting fixture designs include a relatively small cast plate-like mounting bracket that threads onto the conduit. The heavier, main part of the lighting fixture then slides onto the mounting bracket plate. With permanently-attached bushings, an installer must twist a heavy lighting fixture onto a threaded conduit without damaging the threads or dropping the fixture. 
     In the case of lighting fixtures that have a die cast housing, in contrast to a sheet metal housing, a current practice is to include a twist lock bushing as part of the housing. Thus, integral hook-like tabs are cast into the housing, and cooperate with a separate locking ring that threads on to the conduit. Twist lock bushings are particularly convenient to use. Mating elements of a twist lock bushing assembly are engaged and then rotated relative to each other a relatively short angular distance, such as 90°, which is far simpler for an installer compared to the multiple rotations required to attach a conventional threaded bushing to a conduit. However, a die cast housing with an integral twist lock bushing element is more expensive than a sheet metal housing. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore seen to be desirable to provide a twist lock attachment system suitable for use with a fixture, such as a lighting fixture, having a housing made of sheet metal. 
     In an exemplary embodiment, a twist lock attachment system includes, in general, a mounting plate made of sheet metal included as part of a fixture, as well as a twist lock bushing attachable to a structural support, such as a threaded conduit. The mounting plate has an exterior surface and an opposed interior surface separated by a mounting plate thickness, as well as a generally circular mounting aperture. The twist lock bushing includes a tubular body correspondingly generally circular in cross section, and having an insertion end and an opposite end. The mounting plate and the twist lock bushing releasably engage each other by relative movement to insert the tubular body into the mounting aperture with the mounting plate and the bushing angularly oriented in an insertion and removal position with reference to each other, and subsequent rotation of the mounting plate and the twist lock bushing relative to each other in a first rotational direction to an installed position. 
     The tubular body more particularly includes a radially-extending top flange located intermediate the opposite end and the insertion end. The top flange has a flange bearing surface axially facing the insertion end for limiting relative axial movement in an insertion direction by bearing against the exterior surface of the mounting plate. The flange bearing surface accordingly defines a flange bearing surface plane. The tubular body additionally includes at least two radially-extending locking tabs intermediate the top flange and the insertion end generally adjacent the insertion end. The locking tabs have locking tab bearing surfaces axially facing the opposite end for retaining the tubular body within the mounting aperture by bearing against the interior surface of the mounting plate in the installed position. Thus, the tab bearing surfaces are separated from the flange bearing surface plane a distance corresponding to the thickness of the mounting plate. For clearing the locking tabs as the tubular body is inserted into or removed from the mounted aperture in the insertion and removal position, the mounting plate has at least two locking tab clearance slots extending radially from the mounting aperture. Releasable elements are provided for preventing relative rotational movement of the mounting plate and the bushing in the installed position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevational view, partially broken away, of a fixture embodying the invention; 
     FIG. 2 is a top plan view of a mounting plate included as part of the fixture of FIG. 1; 
     FIG. 3 is a bottom plan view of the mounting plate; 
     FIG. 4 is a side elevational view of a twist lock bushing in isolation; 
     FIG. 5 is a top plan view of the bushing, taken on line  5 — 5  of FIG. 4; 
     FIG. 6 is a bottom plan view of the bushing, taken on line  6 — 6  of FIG. 4; 
     FIG. 7 is a side elevational view of the twist lock bushing in a different orientation compared to FIG. 4, taken on line  7 — 7  of FIG. 5; 
     FIG. 8 is an enlarged top plan view, showing the twist lock bushing and a portion of the mounting plate in the insertion and removal position; 
     FIG. 9 is an enlarged bottom plan view corresponding to FIG. 8, showing the twist lock bushing and a portion of the mounting plate in the insertion and removal position; 
     FIG. 10 is an enlarged top plan view, taken on line  10 — 10  of FIG. I, showing the twist lock bushing and a portion of the mounting plate in the installed position; 
     FIG. 11 is an enlarged bottom plan view, taken on line  11 — 11  of FIG.  1  and corresponding to FIG. 10, showing the twist lock bushing and a portion of the mounting plate in the installed position; 
     FIG. 12 is an installed-position cross-sectional view taken on line  12 — 12  of FIG. 10; 
     FIG. 13 is an installed-position cross-sectional view taken on line  13 — 13  of FIG. 10, with a set screw additionally included; 
     FIG. 14 is a view taken on line  14 — 14  of FIG. 10; 
     FIG. 15 is a view, in the same orientation of FIG. 1, taken on line  15 — 15  of FIG. 10; 
     FIG. 16 is a view taken on line  16 — 16  of FIG. 10; and 
     FIG. 17 is a view taken on line  17 — 17  of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1, a representative lighting fixture  20  includes an upper housing  22  made generally of sheet metal, a lower housing  24  containing a relatively heavy ballast transformer, and a socket  26  for a high intensity discharge lamp (not shown). The fixture  20  typically has additional elements, not shown in FIG. 1, such as a reflector and a lens. 
     The upper housing  22  includes an upper mounting plate  28  made of sheet metal. The mounting plate  28  has an exterior surface  30  and an opposed interior surface  32 , separated by a mounting plate thickness. A suitable mounting plate thickness for a fixture weighing 23 pounds (10 kg) is 0.05 inch (1.27 mm). The upper housing  22  also has an end cover  34 , which is removable to provide wiring access, as well as an opposite end cover. 
     Also shown in FIG. 1 is a twist lock bushing  40  which is attachable to a structural support. In the example of FIG. 1, the structural support is a threaded electrical conduit  42 , shown in phantom. The threaded electrical conduit  42  may comprise the lower end of a hanger hook (not shown), which includes an aperture for electrical conductors. Twist lock bushings embodying the invention may be provided which are attachable to other forms of structural support, such as directly to beams, or to other ceiling structures, including electrical junction boxes. 
     The twist lock bushing  40  and the mounting plate  28  together comprise a twist lock attachment system  44 . 
     FIG. 2 is a top plan view of a portion of the mounting plate  28  (generally on line  10 — 10  of FIG. 1, but with the twist lock bushing  40  not installed). The exterior surface  30  is visible in FIG.  2 . The mounting plate  28  has a generally circular mounting aperture  46 , with at least two locking tab clearance slots  48  and  50  extending radially from the mounting aperture  46 . In the illustrated embodiment, there are a pair of diametrically-opposed locking tab clearance slots  48  and  50 . 
     FIG. 3 is a corresponding bottom plan view of the mounting plate  28 , with the opposed interior surface  32  visible, as well as the mounting aperture  46 , and the locking tab clearance slots  48  and  50 . 
     FIGS. 4,  5 ,  6  and  7  show the twist lock bushing  40  in isolation. In particular, FIG. 4 is a side elevational view in the same orientation as FIG. 1; FIG. 5 is a top plan view taken on line  5 — 5  of FIG. 4; FIG. 6 is a bottom plan view taken on line  6 — 6  of FIG. 4; and FIG. 7 is another side elevational view, taken on line  7 — 7  of FIG.  5 . The twist lock bushing  40  has a tubular body  52  which is generally circular in cross-section. The tubular body  52  has an insertion end  54  and an opposite end  56 . 
     Referring to the cross-sectional views of FIGS. 12 and 13, in addition to FIGS. 4-7, the tubular body  52  has a tapered bore  58 , the interior surface of which is threaded for attachment to a conduit such as the FIG. 1 conduit  42 . The interior bore  58  terminates at its lower end in an aperture  60  through which electrical conductors (not shown) pass. 
     To facilitate threading on to the conduit  42 , the bushing  40  tubular body  52  additionally includes an integral hex nut structure  62  having surfaces  64 ,  66 ,  68 ,  70 ,  72  and  74  engageable by a wrench (not shown), in a conventional manner. 
     Intermediate the opposite end  56  and the insertion end  54  is a radially-extending top flange  78 . In the illustrated embodiment, the top flange  78  comprises a plurality of top flange segments, in particular a pair of top flange segments  80  and  82  separated by annular gaps  84  and  86 . The top flange  78  has a lower flange bearing surface  88  axially facing the insertion end  54 , and defining a flange bearing surface plane. The flange bearing surface  88  limits relative axial movement in an insertion direction by bearing against the exterior surface  30  of the mounting plate  28 . Opposite the flange bearing surface  88  is a flange upper surface  90 . 
     In addition, intermediate the top flange  78  and the insertion end  54  generally adjacent the insertion end  54  are at least two radially-extending locking tabs  92  and  94 . The locking tabs  92  and  94  have respective locking tab bearing surfaces  96  and  98  axially facing the opposite end  56  of the tubular body  52 . The locking tab bearing surfaces  96  and  98  retain the tubular body  52  within the mounting aperture  46  in the installed position by bearing against the interior surface  32  of the mounting plate  28 . The locking tab bearing surfaces  96  and  98  accordingly are spaced from the flange bearing surface plane defined by the lower flange bearing surface  88  a distance corresponding to the thickness of the sheet metal mounting plate  28 , including an allowance for parts tolerances. For use in combination with a mounting plate having a maximum thickness of 0.05 inch (1.27 mm), the locking tab bearing surfaces  96  and  98  may be spaced from the plane of the flange bearing surface  88  a minimum distance of 0.05 inch (1.27 mm). 
     Referring also to FIGS. 8-15, during operation, and referring initially to the top plan view of FIG.  8  and the corresponding bottom plan view of FIG. 9 which show the mounting plate  28  and the bushing  40  in an insertion and removal position, the locking tabs  92  and  94  are cleared by the respective locking tab clearance slots  48  and  50 , allowing the twist lock bushing  40  to be inserted into the mounting aperture  46  until the lower flange bearing surface  88  contacts the upper, exterior surface  38  of the mounting plate  28 . 
     Referring next to the top plan view of FIG.  10  and the corresponding bottom plan view of FIG. 11 which show the mounting plate  28  and the bushing  40  in an installed position, the bushing  40  is rotated in a first rotational direction relative to the sheet metal mounting plate  28  to the installed position of FIGS. 1,  10  and  11 . Referring particularly to the cross-sectional view of FIG. 12, in the installed position, the locking tab bearing surfaces  96  and  98  bear against the lower, interior surface  32  of the mounting plate  28 , thereby supporting the weight of the fixture  20 . 
     Contrasting FIG. 8 (insertion and removal position) and FIG. 10 (installed position), when viewed from the top, as the first rotational direction the bushing  40  is rotated clockwise relative to the mounting plate  28 . In practice, however, the bushing  40  remains stationary, and the mounting plate  28  is rotated counterclockwise to the installed position, when viewed from the top. From the point of view of an installer, when viewed from the bottom as in FIG. 9 (insertion and removal position) and FIG. 11 (installed position), the fixture  20 , including the mounting plate  28 , is rotated clockwise to the installed position. 
     To define the installed position by limiting relative rotation of the mounting plate  28  and the bushing  40  in the first rotational direction past the installed position of FIGS. 1,  10  and  11 , the mounting plate  28  includes at least one installed position rotational stop  100 . The installed position rotational stop  100  extends from either one of the exterior  30  or interior surfaces  32 , and is positioned and angularly located so as to contact a portion of the twist lock bushing  40  when the mounting plate  28  and the twist lock bushing  40  are rotated relative to each other in the first rotational direction to the installed position. 
     In the illustrated embodiment, the installed position rotational stop  100  takes the form of at least one interior rotational stop  100  projecting downwardly from the interior surface  32 . In FIG. 15, which is a view taken on line  15 — 15  of FIG. 10, and in FIG. 16, which is a view taken on line  16 — 16  of FIG. 10 showing the interior rotational stop  100  in cross section, the installed position or interior rotational stop  100  takes the representative form of a “dimple” formed in the sheet metal mounting plate  28 , concave when viewed from the top as in FIGS. 2 and 10, and convex when viewed from the bottom as in FIGS. 3 and 11. The installed position or interior rotational stop  100  is positioned and angularly located so as to contact one of the locking tabs  92  and  94  when the mounting plate  28  and the twist lock bushing  40  are rotated relative to each other in the first rotational direction to the installed position. In the illustrated embodiment, the installed position or interior rotational stop in the form of a dimple  100  contacts the locking tab  92  to prevent further relative rotation. 
     Similarly, to facilitate alignment of the locking tabs  92  and  94  with the locking tab clearance slots  48  and  50  for disengaging the mounting plate  28  and the twist lock bushing  40  in the event the fixture  20  is to be removed, there are provided elements for limiting relative rotation of the mounting plate  28  and the bushing  40  in a second rotational direction opposite the first rotational direction beyond the insertion and removal position of FIGS. 8 and 9. In the illustrated embodiment, the bushing  40  includes at least one rotational stop tab extending further radially from the radially-extending top flange  78 . A pair of rotational stop tabs  102  and  104  extend further radially from the top flange  78 , the rotational stop tab  102  extending from the top flange segment  80 , and the rotational stop tab  104  extending from the top flange segment  82 . 
     Correspondingly, the mounting plate  28  includes at least one exterior rotational stop  106  projecting from the exterior surface  30 . The exterior rotational stop  106  is positioned and angularly located so as to contact one of the rotational stop tabs  102  and  104  when the mounting plate  28  and the twist lock bushing  40  are rotated relative to each other in the second rotational direction to the insertion and removal position of FIGS. 8 and 9. From the point of view of an installer, when viewed from the bottom as in FIG. 9, the fixture  20 , including the mounting plate  28 , is rotated counterclockwise to the insertion and removal position. 
     In FIG. 17, which is a view taken on line  17 — 17  of FIG. 8, the exterior rotational stop  106  is shown in cross section, and also may be seen to take the representative form of a dimple formed in the sheet metal of the mounting plate  28 , in this case projecting upwardly. In the top plan view of FIG. 2, the exterior rotational stop  106  is convex, while in the bottom plan view of FIG. 3, the exterior rotational stop  106  is concave. In the illustrated embodiment, the exterior rotational stop  106  contacts the rotational stop tab  104 . The exterior rotational stop  106  is radially spaced from the mounting aperture  46  a sufficient distance to avoid interference with the top flange  78 , except for intentional engagement with the rotational stop tab  102  or  104 . 
     Finally, in order to secure the fixture  20  in the installed position, the attachment system additionally includes releasable elements for preventing relative rotational movement of the mounting plate  28  and the bushing  40  in the installed position. A variety of structures may be employed to accomplish this, such as a set screw, latch, spring, bend tab, snap pin, flexible body detent or friction fit. 
     In the illustrated embodiment, to prevent relative rotational movement of the mounting plate  28  and the bushing  40  in the installed position, the top flange has at least one flange set screw aperture. More particularly, a flange set screw aperture  110  formed perpendicularly through the flange  78  segment  82  angularly co-located with the rotational stop tab  104 , as well as another flange set screw aperture  112  angularly co-located with the rotational stop tab  102 . For convenience of use, the flange set screw aperture  112  is angled at an angle intermediate 0° and 90° with reference to the flange bearing surface  88  plane. At least a portion of which aperture  112  extends through the exterior rotational stop tab  104 . 
     Correspondingly, the mounting plate  28  has at least one mounting plate set screw aperture, in the illustrated embodiment mounting plate set screw apertures  114  and  116 , angularly located with reference to the flange set screw apertures  110  and  112  so as to be in alignment in the installed position so that a set screw  118  (FIG. 13) within the set screw apertures  110  and  114  or  112  and  116  prevents relative rotational movement of the mounting plate  28  and the bushing  40 . In the illustrated embodiment, the set screw  118  has self-tapping threads that cut into the sides of angled flange set screw aperture  112 . The end of the set screw  118  extends through the mounting plate set screw aperture  116  to provide rotational lock. Alternatively, a set screw could be threaded into the straight set screw aperture  110 , extending through the mounting plate set screw aperture  114 . 
     While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.

Technology Classification (CPC): 5