Improved gripper return means for a machine for simultaneously biaxially stretching thermoplastic film

In a machine for simultaneously biaxially stretching a web of thermoplastic sheet material under thermally varying conditions in a continuous process, said machine including:

• a web preliminary treatment zone, a web biaxial stretching zone, and a web after-treatment zone;
• web grippers movable along opposite sides of said machine and arranged to engage opposite edges of web material advanced through said zones;
• continuous gripper guide rails on each side of the machine for slidably engaging and guiding said grippers along predetermined closed-loop paths on both sides of said machine;
• means on opposite sides of said machine for engaging and moving said grippers along said guide rails, including gripper advancing drive means in said web stretching zone, a gripper return means following said web after-treatment zone, a gripper input means in advance of said web preliminary treatment zone, and a gripper return feeding means in advance of said gripper return means;
• means for actuating said gripper advancing drive means, gripper return means, gripper return feeding means, and gripper input means;
• said gripper advancing drive means being arranged to cause a progressively increasing rate of advancement of each of said grippers through said web stretching zone on opposite sides of the machine to cause stretching of said web engaged by the grippers along the direction of web and gripper advancement in said stretching zone;
• said gripper guide rails on opposite sides of the machine diverging from each other in the direction the grippers are advanced to cause stretching of said web transversely of said web advancement direction in said web stretching zone when said grippers are advanced therethrough in engagement with opposite edges of the web material;
• said gripper return feeding means advancing said grippers to said gripper return means at a rate of motion corresponding to the velocity of said grippers at the end of the web stretching zone, said rate of motion being greater than the velocity at which said grippers are advanced through said gripper input means;
• the improvement comprising;
• each of said gripper return means comprising a variable length return screw assembly for advancing said grippers at a progressively slower rate of movement, and a gripper storage area extending along said guide rail between said screw assembly and said gripper input means for receiving grippers advanced by said gripper return means, said grippers being moved along said rail in said storage area and towards said gripper input means by reason of the continuous advancement of grippers into the said storage area by said gripper return means, said grippers abutting each other and being freely slidable along said guide rail in said storage area;
• said variable length return screw assembly comprising a core shaft;
• means for rotatably driving said core shaft synchronously with said gripper return feeding means;
• a first cylindrical sleeve element having helical grooves of uniform pitch thereon concentrically fixed to said core shaft at the gripper receiving end thereof, said first sleeve being disposed adjacent said gripper return feeding means and extending along a portion of said guide rail;
• a second cylindrical sleeve element having helical grooves of diminishing pitch thereon mounted concentrically on said core shaft at the gripper discharge end thereof adjacent said storage area, said second sleeve element being mounted for slidable movement along said core shaft while being fixed thereto in a rotational sense;
• and an axially resilient helical guide means in the form of a coil spring concentrically mounted on said core shaft between said sleeve elements and secured at its opposite ends to said sleeve elements in such a manner that the opposite ends of the helical guide means intersect and interfit with the helical grooves of said sleeves in a continuous manner;
• said grippers each having a projection thereon for engaging said helical grooves in said sleeve elements and said helical guide means;
• said return screw, when rotated, advancing said grippers along said guide rail at a progressively decreasing velocity between said gripper return feeding means and said storage area by engaging each gripper through its respective projection at said first sleeve element as each gripper is discharged from said gripper return feeding means, and thereafter advancing each gripper along said first sleeve element, helical guide means, and second sleeve element by means of said projection;
• said second sleeve means slowing the velocity of each gripper before its advancement to the said storage area by reason of the said grooves of diminishing pitch thereon;
• said second sleeve element of each return screw being free to react to variable rate of gripper movement into and through said storage area resulting from temperature induced dimensional variations within the said machine and for ensuring reliable advancement of grippers to said gripper input means by sliding axially along said core shaft to vary the effective length of said return screw assembly along said guide rail, said helical guide means normally being precompressed a selected amount in its assembled working position when a predetermined number of grippers are in said storage area in abutting relationship along said guide rail, said second sleeve element normally slidably axially engaging the projection of the last gripper discharged into said storage area by said gripper return screw assembly and being resiliently urged thereagainst by said precompressed helical guide means.