vertical support column 18 when crank 62 is pivotally displaced outwardly about pivot pin 79 as shown in fig6 b the distal end of lock pin 78 is inserted between adjacent teeth within sprocket 68 to prevent rotation of the combination of crank 62 shaft 70 and spur gear 72a to prevent displacement of the film wrap carriage 60 on the vertical support column 18 torsion spring 64 disposed about pivot pin 79 and engaging crank 62 urges the crank outwardly from the film wrap carriage 60 as shown in fig6 b to maintain lock pin 78 in engagement with sprocket 68 in this manner the film wrap carriage 60 is maintained in the locked position to prevent movement of the film wrap carriage on the vertical support column 18 until crank 62 is urged inwardly to the position shown in fig6 a by overcoming the force of torsion spring 64 when crank 64 is displaced inwardly as shown in fig6 a crank 62 may be rotationally displaced for moving the film wrap carriage 60 either up or down along the vertical support column 18 polyethylene contact buttons 82a and 82b comprised of an ultra high molecular weight material are inserted through the film wrap carriage 60 and engage.