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TOPIC: Levers, Fulcrum and Mechanical Advantage
OBJECTIVE: Understand Basic Lifting Principles using Levers, Fulcrum, and Mechanical Advantage
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.
This University of Extrication column is Part 3 in a series on lifting and stabilizing heavy loads. In this column, we provide a challenging yet fundamental training exercise for rescuers to fulfill that requires an understanding of simple levers, effective use of limited basic tools and application of mechanical advantage.
The training task is for a team of rescue personnel to lift, stabilize and move a large, heavy object from one point to a different location using only specific and limited hand tools. What makes this challenging is that no hydraulic, air, electric, or gas-powered tools can be used. The team must move the object using only simple pry bars, cribbing, steel pipes and some long pieces of wood timber.
The training scenario is intentionally limited to an inventory of basic, manual tools. Our inventory consists of the following tools and equipment:
- Two 60-inch pry bars with pinch point ends
- Four two-inch-diameter-by-24-inch steel pipes
- Four two-inch-diameter by 72-inch steel pipes
- 40 eight-inch-by-eight-inch-by-24-inch cribbing blocks
- 10 eight-inch-by-eight-foot wedge blocks
- 40 four-inch by four-inch by 18-inch cribbing
- 10 two-inch-by-four-inch-by-18-inch cribbing blocks
- 10 four-foot-by-four-inch wedge blocks
- Two eight-inch-by-eight-inch-by-eight-foot wood beams
To accomplish this training challenge, a heavy object is needed. One such ideal training object is a two-foot-by-two-foot-by-four-foot concrete block. The solid-concrete block selected as our example is typically used by sand and gravel companies as divider walls between their various products. Our concrete block weighs just below 4,000 pounds.
The site location selected for this training had an acquired structure with a small concrete porch. The block resting on the ground will have to be repositioned to this porch using only the simple tools, simple levers, mechanical advantage and lots of brute force. For your training, you must obtain similar heavy objects to be lifted and moved. If you get two of these gravel company blocks, design the task to require setting one on top of the other. This sounds simple but it is a monstrous task. It will take a rescue team in the neighborhood of an hour to accomplish this end result.
The assignment is to lift the object and then move it horizontally. In this case, our two-ton block will be moved from its starting point at ground level to the porch slab six-plus feet away and two feet above the ground. To accomplish the challenge set forth in this column, the rescuer must have an understanding of simple levers, know how to use a fulcrum and must be able to use the principle of mechanical advantage to assist with lifting the heavy load.
There are three classes of simple levers: First Class, Second Class and Third Class. Using the pry bars or the long wood timbers from our limited equipment inventory to pry on the cribbing and lift the concrete block is an example of a simple First-Class lever. The First-Class lever we create when we lift with the pry bar or the long timbers uses a contact point on the cribbing, called the fulcrum, to create a force multiplier known as mechanical advantage.
To lift with the pry bars requires that a piece of cribbing be positioned near the end of the block to be lifted. The pry bar is positioned over top of the cribbing as the working end of the bar is strategically positioned under the block. The effort it takes to lift the end of the object depends upon how effectively the pry bar and cribbing are positioned. With practice, the rescuers will soon learn how just a few inches difference in the position of the pry bar and the cribbing block make a big difference. As the weight is lifted, cribbing is positioned beneath the lifted load. Wedges help take up the space as the load is pried up. Box cribbing must be designed so that the load is adequately supported and balanced.
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.