NordicTrack Ski / Skier Machines Arm Exerciser AE Tension Adjustment Knob / DialThe Tension Adjustment knob screws down on top of the large bolt spindle which comes through the center of the arm cord DRUM. Below this KNOB is a SPRING followed by a Super thin BEARING WASHER < BEARING > Sandwiched between another Super thin BEARING WASHER which sits on top of the arm exerciser cord drum. (Special steel bearing washers are super smooth and wafer thin). ALL of our original OEM knobs come triple reinforced which we do here. Plastic