Manufacturing Engineer and Factory Safety Engineer Discusses Bungee Cords

Manufacturing engineer discusses bungee cord safety from a safety engineering view point. Stretching a bungee cord imparts a significant amount of potential energy into the system that can be released and make the end hook a dangerous projectile. There are safety warnings on most cords; however, these are often ignored or torn off. It has been suggested to limit the extension (to 50%) to limit the danger. Streth tests were performed (upper left photo). to examine the difference between a limited bungee cord and one without limits. The bungees were stretched and the force-distance data ploted (upper right figure) The area under each curve is the stored potential energy. This is converted to kinetic energy (KE). From the KE the hook velocity can be calculated. The unlimited bungee generated a maximum hook velocity of 100mph. The maximum hook velocity for the limited bungee is 70mph. Both bungees can still be dangerous.

It appears that the limited bungee was made more robust to increase the holding force for 50% elongation. This has a negative effect on this cord's safe use.

Machine Controls: Factory Safety or Manufacturing Safety Issue

Factory safety expert witness was engaged to determine the cause of a worker injury to an experienced operator/supervisor. The superviser was threading the machine with plastic sheet material when her hand was crushed. The accepted practice for threading was to feed the plastic sheet from the top and reach underneath to pull it through while another operator (i.e helper) pressed the "advance" (Arrow C) function on the control panel. In this instance the helper accidentally pushed the start button ( Arrow A). Both buttons are the same color and are very near each other. When the supervisor realized that her hand was being squeezed by the machines heated platens, she ordered the operator to "stop" the machine. The operator pressed the red stop button (Arrow B) instead of the E-stop. The stop button stops the machine after it has completed a full cycle. The "E-Stop" immediately stops all functions. Since the machine completed its cycle, the supervisor's hand was severly injured. This is a case of poor machine design with respect to machine safety and operator safety. Because both the advance and start buttons are adjacent and the same color, it is forseable that one could push the wrong button. In addition to the "helper"the emergency stop and the "stop" buttons are the same color and perform the same apparent function (i.e. it is forseeable that an operator would assume that both perform the same function). Machine guards are not appropriate because the operator must reach in to the thread the machine; in addition, the machine must be on for the threading operation to be performed.

Solution: Provide an on/off foot switch to control the advance function. This assures operator safety by allowing the person threading the machine to have absolute control over starting and stopping the machine as it is being threaded.

Safety Expert Examines a Personal Injury from Bungee Cord

Safety Expert analyzes the cause of injury to a wake board user from a bungee cord on the wakeboard rack. The rack is a piece of sports equipment, and it is designed to hold two boards in place with two 19" long bungee cords; each has a 1" plastic ball on one end. The cord is stretched to a length of 40" in order to get the ball over the racks,and the cord is fit into a slot. The ball holds the board in place. During the mounting of the wakeboards, the user is looking under the rack to perform the slotting operation; this puts his face in the 'line of fire" if the user accidentally lost hold of the ball as he is stretching the cord. This is exactly what happened, and the user lost an eye. According to the safety expert, this design is even more dangerous because boaters hands may be wet or coated with sun tan oils etc. Tests on the cord indicated that the ball velocity is greater than 100 mph. The photo on the upper left is the users view. The two balls are in their appropriate slots. Upper right is looking at the rack with two mounted wakeboards. This rack model has been discontinued.