MATHEMATICAL ANALYSIS OF EXTENSION LADDER FLIPPERLOCK

Most extension ladders employ pivoting flylocks or rung locks to support the retractable fly section against the force of gravity. Such flylocks can be prone to a condition known as flipperlock, where the flipper or tongue of the flylock is held by friction and the fly section is prevented from retracting. A user may not be aware of this condition, which is metastable. A retraction of the ladder may occur in use, with accompanying injuries. The ability of a flylock to flipperlock can be analyzed mathematically. One key variable is the angle between the tangent of the flylock tongue and the perpendicular to a line connecting the flylock pivot point with the rung/flipper contact point. The other key variable is the coefficient of friction between the flipper and the rung. An equation is developed for the minimum force developed by the flipperlock condition. The design features necessary to reduce the incidence of false lock are discussed. Other methods for reducing false lock accidents are also discussed, including safety devices and specific training and instruction topics and techniques.Copyright © 2008 by ASME