Tension Mast Spring - Knitting Machine. Spring Only! Used to keep tension on your knitting yarn so that it keeps a steady tension on your knitting plus it does not become tangled in the mechanisms of your knitting machine. These do not normally need replacement except when lost, Broken or mistreated. Two basic styles are used however, they differed by manufacturer. The Brother/KnitKing style snaps into place (Brother), while the Studio/Singer utilizes a wire (spring wire) held in place by a screw(s) and has been bent in the middle making two tension mast springs with one continuous strand of wire - that is considered as one piece.