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Mechanical advantage means that we get more effort out of our lever than we put into it. With our lever, the mechanical advantage is calculated by taking the length of the pry bar on the working side of the cribbing fulcrum (the effort arm) and dividing it by the length of the pry bar on the load side of the cribbing fulcrum point. This typically shorter length is called the resistance arm. For example, if the long portion of a pry bar on the effort side is five feet (60 inches) and the shorter portion of pry bar on the resistance or load side of the fulcrum point is one foot (12 inches) long, then the mechanical advantage of our simple lever is described as a ratio of 5:1. Using this same mechanical advantage, we can theoretically use 200 pounds of downward force on our pry bar lever to lift approximately 1,000 pounds of weight. Where a simple lever demonstrates its mechanical advantage is when without it, the person would be unable to move or lift the weight.
Once lifted off the ground, our scenario calls for the rescuers to move the concrete block horizontally. To accomplish this, one technique is to use our pry bars or long length timbers as a Second-Class or second-order lever. Personnel place the pry bars or long timbers low at one end of the block and push upward. This time, the fulcrum is the ground. The load is on the pry bar or timber as we lift upward at the upper end of the bar or timber. Once again, the further away the effort is from the fulcrum and the load, the greater the mechanical advantage of the lever. This Second-Class lever prying action against the concrete block moves the object forward horizontally. If the steel pipes in the tool inventory for this exercise are placed beneath the heavy load, then the block can be moved relatively easily. One trick when rolling your lifted load is to learn to "steer" the load as it moves forward. By sticking a pry bar inside the steel roller pipes, a rescuer can twist the long bar, thus moving the roller pipe as it changes the angle of the pipe beneath the load. This effectively turns the load as it moves.
A trained rescue team, given this assignment bur limited to only the listed tools and equipment, will find that this is a challenging task. It takes coordination, teamwork, knowledge of levers, fulcrum, mechanical advantage and a desire try something you probably haven't done before. Try it. You'll like it, I'm sure!
TASK: Given a heavy object, the rescue team shall lift, move and stabilize the object utilizing only cribbing blocks, wood timbers and 60-inch pry bars.
RON MOORE, a FirehouseÂ® contributing editor, is a battalion chief and the training officer for the McKinney, TX, Fire Department. He also authors a monthly online article in the Firehouse.com "MembersZone" and serves as the Forum Moderator for the extrication section of the Firehouse.com website. Moore can be contacted directly at Rmoore@firehouse.com.