A monodirectional braking mechanism is disclosed, for use with a device for extending and retracting a sheet of material on a roller, the braking mechanism comprising: one or more than one housing containing viscous fluid, one or more that one rotor within the housing; a brake connected to the rotor and placed within the housing, the brake comprising a projecting pin; a braking spring wound around the pin of the brake and placed outside the housing; where the braking mechanism reduces the force or rate, or both the force and rate, of rotation of the roller as the roller is rotated in a second rotation direction but has substantially no effect on the force or rate of rotation of the roller when the roller is rotated in a first rotation direction; and where the first rotation direction is the reverse of the second rotation direction.
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