a yarn tensioning unit for use in a textile machine that includes a shaft having a pulley mounted for rotation thereon over which yarn is conducted . a pneumatic cylinder is slidably mounted on the shaft which contains a piston that is affixed to the shaft to form an expandable chamber with one end wall of the cylinder . the opposite end wall of the housing is magnetically coupled to the pulley to establish a magnetic brake for the pulley . the spacing between the cylinder end wall and the pulley is adjusted by controlling the flow of fluid into and out of the expandable chamber to vary the magnetic braking force exerted upon the pulley .