Tensioning of flexible sheets across frames

Claim 1

A device for supporting a flexible sheet in tension comprising: a planar peripheral frame bounding a space, said frame comprising a series of elongate side members; a plurality of tensioning devices carried by the frame, each tensioning device having at least one passage therethrough for receiving at least one of said side members, said tensioning device including a hollow casing having a wall extending parallel with and adjacent to the plane of said peripheral frame, an elongate slot in said wall extending from near said passage receiving said side member towards the middle of the frame and away from said passage; a movable sheet engaging element slidable within said slot, said sheet engaging element having an outer part projectng from said wall of said casing for engagement with a sheet to be supported, said outer part having a surface engaging the outer surface of said wall, said sheet engaging element further having a stem extending freely through said slot and an enlarged root portion within said casing which is wider than said slot and cooperates with the inner surface of said wall to hold said sheet engaging element captive in said slot while allowing free movement of the sheet engaging element along said slot towards and away from the middle of said space; a spring fixed within said casing, said spring including a coil portion and an end portion projecting radially from said coil portion, said tensioning device including a boss fixed with respect to said casing and receiving said coil portion to form a pivotal axis for quasi-pivotal movement of said end portion for tensioning and untensioning said spring, said quasi-pivotal movement being in a plane parallel with said wall, said enlarged root portion of said sheet-engaging element having a groove therearound in which said end portion locates, said end portion, in a relatively unstressed state of said spring engaging a side of said root portion when said sheet engaging element is positioned adjacent said frame within said slot, said leg portion, in a relatively stessed state of said spring, engaging a cam surface on the bottom of said groove in said root portion when said sheet engaging element is positioned towards the middle of said space within said slot whereby force from said spring substantially laterally of the slot so that, in the absence of externally applied forces, said sheet engaging element is retained against movement along said slot; said sheet engaging elements of said tensioning devices movable within said slot against said spring from a position nearer the middle of the space, where a sheet will not be in tension, to a position away from the middle of the space, where a sheet is held in tension by the aggregate effect of the spring bias of said springs on said sheet.

Claim 2

A device for supporting a flexible sheet in tension comprising: a planar peripheral frame bounding a space, said frame comprising a series of elongate side members; a plurality of tensioning devices carried by the frame, each tensioning device having at least one passage therethrough for receiving at least one of said side members, said tensioning device including a hollow casing having a wall extending parallel with and adjacent to the plane of said peripheral frame, an elongate slot in said wall extending from near said passage receiving said side member towards the middle of the frame and away from said passage; a movable sheet engaging element slidable within said slot, said sheet engaging element having an outer part projectng from said wall of said casing for engagement with a sheet to be supported, said outer part having a surface engaging the outer surface of said wall, said sheet engaging element further having a stem extending freely through said slot and an enlarged root portion within said casing which is wider than said slot and cooperates with the inner surface of said wall to hold said sheet engaging element captive in said slot while allowing free movement of the sheet engaging element along said slot towards and away from the middle of said space; a spring fixed within said casing, said spring including a coil portion and an end portion projecting radially from said coil portion, said tensioning device including a boss fixed with respect to said casing and receiving said coil portion to form a pivotal axis for quasi-pivotal movement of said end portion for tensioning and untensioning said spring, said quasi-pivotal movement being in a plane parallel with said wall, said enlarged root portion of said sheet-engaging element having a groove therearound in which said end portion locates, said end portion, in a relatively unstressed state of said spring engaging a side of said root portion when said sheet engaging element is positioned adjacent said frame within said slot, said leg portion, in a relatively stessed state of said spring, engaging a cam surface on the bottom of said groove in said root portion when said sheet engaging element is positioned towards the middle of said space within said slot whereby force from said spring substantially laterally of the slot so that, in the absence of externally applied forces, said sheet engaging element is retained against movement along said slot; said sheet engaging elements of said tensioning devices movable within said slot against said spring from a position nearer the middle of the space, where a sheet will not be in tension, to a position away from the middle of the space, where a sheet is held in tension by the aggregate effect of the spring bias of said springs on said sheet.