WEB INFEED TENSION CONTROL SYSTEM WITH DANCER ROLL

An improved web infeed system feeds a web of material to printing plate roll having a tension sensor, a compact differential drive unit and a dancer roll mechanism. A main rotation input is imparted from a drive motor to the flexible spline while a portion of main rotation input is imparted via a clutch to the wave generator in the direction opposite to the rotation of the flexible spline, and the result of computing said two rotation inputs is obtained from the circular spline to produce an output, then the state of engagement between the clutch and output is changed by a controller which computes the signal of a tension sensor in a place advance circuit, thereby controlling the rotation speed of said output to maintain constant the tension of the web. Dancer roll mechanism is interposed between the infeed roll and a pinch roll mounting the tension sensor thereon and is so adjusted as to be in an equilibrium of force to the tension of the web of material by air cylinder or air spring via a lever arm for eliminating any minute tension variation. A control valve applies a control pressure, in response to a change in the preset tension, to the air cylinder or air spring for enabling the dancer roll to continue the action at a fixed position. In the case of needing a sudden decrease of the web tension, a solenoid valve interposed between the air cylinder or air spring and the control valve is energized to evacuate the air cylinder or air spring to break said equilibrium of force.