Foredom Duplex Spring for 10016 Handpiece Style HP76. This spring connects the two separated ends of a handpiece and is all that distinguishes these handpieces from standard models. The duplex spring component requires periodic lubrication. Duplex Springs provide extra flexibility to a handpiece when more precise control for delicate work is required. The spring drive is made of special steel double wound tubular construction with a protective flexible spring cover. Replacement springs can be installed by the user. Duplex springs are subject to breakage and are NOT covered by warranty. We recommend that only experienced users of flex shaft tools and/or those whose work demands added flexibility consider this style handpiece.