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TECHNICAL FIELD 
   Within the field of window hardware, the invention relates to linkage assemblies for pivotally mounting sashes within frames and, more particularly, to adapting 4-bar linkage assemblies to the constraints of conventional channel mounts for anchoring hardware components. 
   BACKGROUND OF INVENTION 
   A standardized mounting system for attaching window hardware and accessory options to sashes and frames of window assemblies features elongated channels referred to as “Euro-Grooves”. The channels have a T-shaped cross section, comprising a channel bottom and two sidewalls with overhanging rails. Hardware can be slid along the channels to desired positions and anchored to the overhanging rails of the sidewalls. 
   Although many benefits accrue from such standardized mounting systems, the channels can impose clearance limitations, particularly for linkages that support pivotal and translational motions of casement windows. For example, such channels can provide strong anchors for securing conventional 4-bar linkages to mating sashes and frames of casement windows, but the opposing channels between the sashes and frames can limit the space available for accommodating a stack height of the linkages. 
   The clearance problem can sometimes be resolved by reducing the thickness of individual bars of the linkages so that the collective stack height of the bars matches the available space. However, this solution is not workable for some heavy-duty applications, where the demands on the linkages are severe. In such instances, the removal of one or more of the channels may be needed to establish the required space. 
   SUMMARY OF INVENTION 
   The invention features a linkage assembly, which among its various embodiments incorporates adaptations for exploiting advantages of the “Euro-Groove” mounting system while overcoming attendant clearance limitations. The embodiments include 4-bar linkage assemblies in which modified vent bars engage conventional mounting channels. A base portion of one such vent bar slides within the conventional channel, an offset portion of the same bar abuts one end of the channel sidewalls, and a fixed connection between the base and offset portions projects through overhanging side rails of the channel. A spacing feature provides rotational clearance between the intermediate linkage and the sidewalls, supporting rotation of the intermediate linkage with respect to the vent bar mounted within the channel. Thus, the modified linkages can be both anchored to and located along conventional mounting channels, while preserving the intended functions and desired thicknesses of their linkage bars. 
   One version of the invention as a linkage assembly for pivotally mounting a sash to a frame includes a vent bar that is slidable within a channel of the sash into a fixed mounting position. The vent bar has a length that can be oriented along the sash channel and a width that can be overhung by sidewalls the sash channel. One end of an intermediate linkage pivotally connects to the vent bar. A track is attachable to a channel of the frame, and a shoe slides along the track. Another end of the intermediate linkage pivotally connects to both the track and the shoe. A spacing feature provides rotational clearance between the intermediate linkage and the sidewalls of the sash channel in support of a rotation of the intermediate linkage with respect to the vent bar mounted within the sash channel. 
   The intermediate linkage pivotally connects to the vent bar about first and second pivot axes, and the spacing feature is preferably a first of two spacing features. The first of the two spacing features provides rotational clearance along the first pivot axis between the intermediate linkage and the sidewalls of the sash channel, and a second of the two spacing features provides rotational clearance along the second pivot axis between the intermediate linkage and the sidewalls of the sash channel. The first spacing feature, which preferably takes the form of a bushing, fits between the sidewalls of the sash channel. The first and second spacing features locate the intermediate linkage with respect to the vent bar in a position for overlying the sidewalls of the sash channel. 
   An offset portion of the vent bar preferably forms the second spacing feature. A connection between the offset portion of the vent bar and a remaining base portion of the vent bar can be narrowed to provide clearance for projecting between the overhanging portions of the sidewalls. The offset portion of the vent bar can be formed as a tip at one end of the vent bar for engaging an end cap mounted on the track. The tip also forms a stop for locating the vent bar at the fixed mounting position along the sash channel. The connection can be formed by a bend in the vent bar that is relieved (e.g., notched) for projecting between the overhanging portions of the sidewalls. The stop can be formed by a land on the offset portion adjacent to the relieved bend in the vent bar. The stop engages an end of the sash channel. 
   Another version of the invention as a window mounting system includes a track attachable to a channel formed in a frame and a vent bar slidable along a channel formed in a sash. A shoe slides along the track. An intermediate linkage connects the vent bar to both the track and the shoe. The vent bar has a base portion that is overhung by sidewalls of the sash channel and an offset portion that supports a pivotable connection to the intermediate linkage. A fixed connection between the offset portion of the vent bar and the base portion of the vent bar is narrowed with respect to the base portion of the vent bar for projecting between the overhanging portions of the sash channel sidewalls. 
   The offset portion of the vent bar preferably includes a stop positioned for engaging at least one of the sidewalls of the sash channel. The fixed connection is also preferably narrowed with respect to the offset portion of the vent bar, and the stop is preferably formed in the offset portion at an intersection with the fixed connection. The stop can be positioned for engaging the sidewall at one end of the sash channel. 
   The offset portion of the vent bar also preferably includes a tip formed at one end of the vent bar for engaging a camming surface at one end of the track. The offset portion can be spaced from the base portion of the vent bar in a position that provides rotational clearance between the intermediate linkage and the sidewalls of the sash channel. The base portion of the vent bar also supports a pivotable connection to the intermediate linkage. A spacing feature between the base portion of the vent bar and the intermediate linkage can be sized for projecting between the overhanging portions of the sidewalls of the sash channel. The spacing feature provides rotational clearance between the intermediate linkage and the sidewalls of the sash channel in support of a rotation of the intermediate linkage with respect to the vent bar mounted within the sash channel. 
   The intermediate linkage preferably includes a first link connecting the offset portion of the vent bar to the shoe, a second link connecting the base portion of the vent bar to the track, and a third link connecting the first and second links. The spacing feature can be formed as a spacer between the base portion of the vent bar and the second link. Preferably, the spacer has a thickness not less than a thickness of the overhanging portions of the channel sidewalls and is sized in width to fit between the overhanging portions of the channel sidewalls. 
   Yet another version of the invention as a window assembly includes channels in both a frame and a sash for mounting hardware. The sash channel has a channel bottom and sidewalls that include overhanging portions that overhang the channel bottom. The frame channel and the sash channel define a limited clearance between positions of overlap. A track is anchored to the frame channel, and a shoe slides along the track. A vent bar located along the sash channel has a base portion overhung by the overhanging portions of sidewalls. An intermediate linkage connects the vent bar to both the track and the shoe. A spacing feature provides rotational clearance between the intermediate linkage and the sidewalls of the sash channel in support of a rotation of the intermediate linkage with respect to the vent bar. 
   The spacing feature preferably is preferably spacer that is located along a pivot axis between the base portion of the vent bar and the intermediate linkage and has a thickness not less than a thickness of the overhanging portions of the sidewalls. The vent bar preferably includes an offset portion supporting a pivotable connection with the intermediate linkage. Preferably, a fixed connection between the base portion of the vent bar and the offset portion of the vent bar is narrowed with respect to the base portion of the vent bar and projects between the overhanging portions of the sidewalls. The preferred offset portion includes a stop that engages one end of the sash channel for positioning the vent bar along the sash channel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a 4-bar linkage assembly adapted in accordance with the invention and shown in a partly open position. 
       FIG. 2  is a side view of the same 4-bar linkage assembly shown in a closed position showing overlapping bars of the assembly. 
       FIG. 3  is an end view of the 4-bar linkage mounted between channels incorporated into a sash and frame of a casement window. 
       FIG. 4  is a cut-away side view showing one end of the linkage assembly engaged with the sash channel. 
       FIG. 5  is a plan view showing an engagement of one end of a vent bar of the linkage assembly with the sash channel. 
       FIG. 6  is a perspective view of the one end of the linkage assembly mounted to the sash. 
   

   DETAILED DESCRIPTION 
   Referring to the drawing figures and in particular to  FIG. 1 , a 4-bar linkage assembly  10  of the type contemplated by this invention, includes a track  12  having a channel  14  along which a shoe  16  is guided for translation (i.e., slides) along a limited length of the channel  14 . Intermediate linkage bars  20 ,  22 , and  24  connect the track  12  to a vent bar  26 . The shoe  16  is preferably made out of brass together with a resin friction-adjusting pad, but all of the linkages including the track  12 , the vent bar  26 , and the intermediate linkage bars  20 ,  22 , and  24  are preferably made of stainless steel. The choice of materials can be made in accordance with requirements of the design and is not a limitation of the invention. 
   Intermediate linkage bar  20  has a first end  31  that is attached by way of a pivot pin  32  to the track  12  and a second end  33  that is attached by way of a pivot pin  34  to the vent bar  26 . Similarly, the intermediate linkage bar  22  has a first end  35  that is attached by way of a pivot pin  36  to the shoe  16  and a second end  37  that is attached by way of a pivot pin  38  to the vent bar  26 . The intermediate linkage bar  24  interconnects the other two intermediate linkage bars  20  and  22 , sharing the pivot pin  36  with the linkage bar  22  at a first end  39  but attaching to an intermediate length of the linkage bar  20  by way of another pivot pin  40  at a second end  41 . 
   As shown in  FIG. 2 , the first ends  31  and  35  of the linkage bars  20  and  22  are elevated at a first level above the track  12 . A spacer  42  surrounding the pivot pin  32  elevates the first end  31  of the linkage bar  20  above the channel bottom  14  of the track  12 . The first end  35  of the linkage bar  22  rides directly on the shoe  16  or through the intermediacy of a thin friction-reducing washer (not shown). The second end  33  of the linkage bar  20  and the first end  39  of the linkage bar  24  occupy a second level. A thin friction-reducing washer  46  around the pivot pin  36  separates the first ends  35  and  39  of the linkage bars  22  and  24 . A thin friction-reducing washer  50  around the pivot pin  40  separates the second end  41  of the linkage bar  24  from an intermediate portion of the linkage bar  20 . The vent bar  26  occupies the remaining third and fourth levels. Spacers  44  and  48  elevate the vent bar and are described in more detail below. 
   The pivot pins  32 ,  34 ,  36 ,  38 , and  40  are preferably formed as rivets with heads recessed within the linkage bars  20 ,  22 , and  24  and the vent bar  26  to avoid interference between the linkage bars or other neighboring structures. The spacers  42 ,  44 , and  48  can be formed similar to the friction-reducing washers  46  and  50 , differing only in thickness. The designation of spacer is applied where more than a minimum thickness is required to provide rotational clearance. 
   The 4-bar linkage assembly  10  is adapted as shown in  FIG. 3  to interconnect a sash  62  with a frame  64  of a casement window  60  in support of opening and closing the sash  62  within the frame  64 . Both the sash  62  and the frame  64  can be constructed from aluminum extrusions or made from other common materials within the window art. A “Euro-Groove” mounting channel  66  is formed in the sash  62 , and another “Euro-Groove” mounting channel  68  is formed in the frame  64  to provide conventional window hardware mounts. Both “Euro-Groove” mounting channels  66  and  68  include elongated channel bottoms  70  and  72  and two sidewalls  74  and  76  with overhanging rails  78  and  80 . In cross section, the mounting channels  66  and  68  appear as T-shaped slots with the overhanging rails  78  and  80  overhanging portions of the channel bottoms  70  and  72  for confining mating features of the window hardware within the channels. 
   The track  12  attaches to the mounting channel  68  of the frame  64  in a conventional manner through a bracket  52  that extends along the mounting channel  68 . Screws  54 , which extend through openings in the track  12  into threaded engagements with the bracket  52 , draw the bracket  52  and the track  12  together against the overhanging rails  80  of the mounting channel  68 . 
   In contrast, the vent bar  26 , as shown more clearly in  FIGS. 4-6 , includes a base portion  84  that makes a sliding fit within the mounting channel  66  of the sash  62  and offset portion  86  at one end of the vent bar  26  that extends beyond one end  58  of the channel  66 . A fixed connection  88  is made between the base portion  84  and the offset portion  86  by a bend in the vent bar  26 . The overhanging rails  78  of the mounting channel  66  overhang the base portion  84  as a constraint. However, the fixed connection  88  is notched, relieved, or otherwise narrowed to project between the overhanging rails  78  for elevating the offset portion  86  of the vent bar  26  above the sidewalls  74  of the mounting channel  66 . A stop  90  is formed by the offset portion  86  at an intersection with the fixed connection  88  for engaging the one end  58  of the mounting channel  66 . The stop  90  locates the vent bar  26  in a desired position within the sash  62 . 
   The spacer  44  is sized in thickness to separate the second end  33  of the intermediate linkage bar  20  from the vent bar  26  by an amount that provides rotational clearance between the linkage bar  20  and the sidewalls  74  of the mounting channel  66 . The spacer  48  fills a space between the offset portion  86  of the vent bar  26  and the second end  37  of the intermediate linkage bar  22 . A further offset of the offset portion  86  could make the spacer  48  unnecessary, but the offset of the offset portion  86  is preferably limited for engaging the end  58  of the mounting channel  66  as the stop  90 . 
   The linkage assembly  10 , as adapted in accordance with the invention, occupies space (see  FIG. 3 ) within the mounting channel  66  beyond the usual space afforded by the hinge gap HG between the mounting channels  66  and  68 . The additional space allows the various components and linkage bars of the linkage assembly  10  to be formed at desired thicknesses in support a range of window applications, including heavy-duty applications such as for commercial purposes. 
   The offset portion  86  of the vent bar  26  also includes a tip  92  at its open end shaped for engaging an end cap  94  at one end of the track  12 . The end cap  94  includes a camming surface that functions as a dual-sided containment structure to assure that the sash  62  fits tightly against the frame  64  in a closed position and opens without binding. The base portion  84  of the vent bar  26  adjacent to the fixed connection  88  can be swaged or otherwise altered to make a more secure frictional connection to the mounting channel  66 . Screws (not shown) through openings in the vent bar  26  can be used to secure the vent bar  26  to the mounting channel  66 . Additional details of a 4-bar linkage assembly capable of adaptation for the purposes of the present invention are disclosed in co-assigned U.S. Pat. No. 5,898,977 to the present inventor, which is hereby incorporated by reference. 
   Although described above in its preferred form, the invention is capable of various modifications in accordance with the overall teaching of the invention including making similar adaptations to other linkage structures for better exploiting hardware mounting channels for both in-swing and out-swing window assemblies.

Summary:
A 4-bar linkage assembly for mounting a sash to a frame member of a window assembly. The linkage assembly including a vent bar having an offset portion and a base portion that fits within a sash channel of the sash. The vent bar includes a fixed connection extending between the offset portion and the base portion for supporting the offset portion and projecting between overhanging rails of the sash channel. The offset portion forms a stop which engages an end of the sash channel for locating the vent bar within the sash channel.