Analysis of Tiplock Phenomenon in Extension Ladders

Extension ladder accidents injure thousands of people each year in the U.S. One cause of these accidents is false lock of the flylocks. Flylocks are the devices that support the upper or fly section of an extended ladder. False locked flylocks are not fully locked, but nevertheless support a load. False lock is not stable and may release under disturbance of the ladder, e.g. from climbing. False lock modes include tiplock, flipperlock, and camlock. If a false lock releases the ladder will telescope, and may cause serious injury. One false lock mode is tiplock, where the tip of the flylock is resting on the rung of the ladder in a metastable position. The tiplock may be held in place by mechanical locking, or by friction between the tip and the rung. Tiplock may occur when the ladder is being extended and the fly section is not raised enough to fully engage the flylocks. Ladder labels usually instruct “Securely engage ladder locks before climbing,” but typically do not instuct a user how to do this. Ladder users employ various methods to securely engage flylocks. These methods do not always prevent false lock. No gravity or spring actuated flylock design is known which will completely eliminate tiplock, but various designs have different ranges of fly section travel over which tiplock can occur (tiplock “window”). A mathematical expression defines the conditions under which friction tiplock can occur. Two methods of evaluating the tiplock window are discussed: geometric analysis and physical measurement. Tiplock propensity during random ladder extension is calculated. A geometric analysis is made of two flylocks with different tip radii. The flylock with the smaller tip radius is observed to have a smaller tiplock window.Copyright © 2004 by ASME