Shutter construction

Claim 1

IN A SHUTTER APPARATUS, A SUPPORTING FRAME, A PLURALITY OF PARALLEL SHUTTER VANES PIVOTALLY MOUNTED ON SAID FRAME SELECTIVELY PIVOTABLE BETWEEN OPEN AND CLOSED POSITIONS, AN ACTUATING ARM DEFINED ON EACH OF SAID VANES HAVING A CRANK PIN IN SPACED RELATION TO THE PIVOT AXIS OF THE VANE, A CONTROL ROD INTERCONNECTING THE CRANK PINS OF SAID ARMS PROVIDING LIKE AND SIMULTANEOUS OPERATION OF SAID VANES, MEANS SELECTIVELY POSITIONING SAID CONTROL ROD, A TENSION SPRING BIASINGLY ASSOCIATED WITH SAID CONTROL ROD TENDING TO BIAS SAID ROD IN A GIVEN DIRECTION, A LEVER PIVOTALLY MOUNTED ON SAID FRAME ABOUT A SUPPORT PIVOT AXIS AND PIVOTALLY ATTACHED TO SAID CONTROL ROD, AN ARCUATE SPRING EXTENSION ENGAGING SURFACE DEFINED ON SAID LEVER ECCENTRICALLY RELATED TO THE PIVOT AXIS THEREOF, SAID ARCUATE SURFACE HAVING A MINUMUM RADIAL DIMENSION FROM SAID LEVER PIVOT AXIS AT ONE END OF SAID SURFACE PROGRESSING TO MINIMUM RADIAL DIMENSION AT THE OTHER END OF THE SURFACE AND A FLEXIBLE SPRING EXTENSION ATTACHED TO SAID SPRING AND FIXED TO SAID LEVER AS TO FRICTIONALLY ENGAGE SAID ARCUATE SURFACE THEREOF DURING PIVOTING OF SAID LEVER, SAID SPRING EXTENSION ENGAGING SURFACE BEING OF SUCH CONFIGURATION AND BEING SO RELATED TO SAID LEVER PIVOT AXIS THAT SAID SPRING IMPOSES A SUBSTANTIALLY EQUAL BIASING FORCE ON SAID CONTROL ROD THROUGHOUT THE OPERATIVE PIVOTING RANGE OF SAID LEVER, SAID EXTENSION ENGAGING THE MINIMUM RADIAL DIMENSIONS OF SAID SURFACE AT MAXIMUM EXTENSION OF SAID SPRING.