Method and apparatus for applying a constant tension to a film

A film wrapping machine for wrapping a film about a load, comprising:

• a stretch head having a film supply roll operatively connected thereto;
• means for moving said stretch head with respect to a load whereby said stretch head applies film from said film supply roll to said load as said stretch head is moved with respect to said load;
• a film roller operatively connected to said stretch head and rotated by said film as said film is drawn over said film roller wherein said film rotates said film roller at a variable rate of speed dependent upon the rate of speed at which said film is drawn over said film roller;
• a hydraulic pump operatively connected to said film roller such that said rotating film roller drives said hydraulic pump at a variable rate of speed dependent upon the rotation rate of said film roller;
• an oil reservoir fluidically connected to said hydraulic pump so as to form therewith a hydraulic circuit wherein said hydraulic pump pumps oil through said hydraulic circuit as said film roller drives said hydraulic pump;
• and
• constant pressure valve means incorporated within said hydraulic circuit for maintaining a constant oil pressure within said hydraulic circuit whereby said hydraulic pump maintains a constant drag upon said film roller independent of the rate of speed at which said film roller drives said hydraulic pump, and wherein further said constant drag impressed upon said film roller applies a constant tension to said film independent of the rate of speed at which said film is drawn over said film roller.

A method for applying a constant tension to a film in a film wrapping machine, comprising the steps of:

• drawing a film over a film roller so as to rotate said film roller at a variable rate of speed dependent upon the rate of speed at which said film is drawn over said film roller;
• operatively connecting a hydraulic pump, incorporated within a hydraulic circuit, with said rotating film roller such that said hydraulic pump is driven by said film roller at a rate of speed which is dependent upon the rotational rate of speed of said film roller whereby said hydraulic pump pumps oil through said hydraulic circuit;
• and
• maintaining a constant oil pressure within said hydraulic circuit independent of the rate of speed at which said film roller drives said hydraulic pump so as to maintain a constant drag upon said film roller and thereby apply a constant tension to said film independent of the rate of speed at which said film is drawn over said film roller.