Taking up the slack

I had a problem with my HDPE filament getting unwound from its reel. Because my extruder is attached to the z-axis, the filament gets pulled off the reel as the z-axis descends, but when it rises back to the home position there was nothing to take up the slack. Also the springiness of the HDPE makes it want to unwind. It needs a constant back tension to take up the slack and keep the filament on the reel.

My first idea was to attach a small DC motor to the roller to provide a backwards pull. As the motor would be permanently stalled I would have had to limit the current to something reasonable. After some thought I came up with a much simpler solution. I wound some picture cord around the roller and hung a weight from it. As the filament unwinds it lifts the weight. The weight is also tethered to the top of the machine, so once it gets to the maximum height it stops. The reel is only a friction fit on the roller so it starts to slip at that point. When the axis ascends again the weight falls and winds the reel backwards, taking up the slack. There is enough travel on the weight to cover the full z-axis travel, even when the filament has been used down to the inner diameter of the reel.