Shock load is the term used to describe the sudden force exerted when an object suddenly accelerates or decelerates, such as when a falling object hits the ground, a fastball strikes a catcher's glove or a diver begins to leap off a diving board. This force is exerted on both the moving object and the object being acted on. Determining shock load can be very important in a variety of safety-related situations, for example, determining the effectiveness of a safety harness or the wire lanyard attached to it. Most harness lanyards are made to withstand a certain amount of force, and you can calculate the shock load for a falling object attached to a somewhat elastic wire rope.
Determining Shock Load
- Calculating shock load for an elastic wire requires knowing several factors:
- Weight of the object in pounds (load)
- Falling distance in inches (FD)
- Length of the cord in feet (L)
- Modulus of elasticity (E) = 11,500,00 pounds per square inch (new rope) or 15,000,000 pounds per square inch (stretched rope)
- Area factor of rope (area factor)—each particular kind of rope has an associated area factor
- Metallic area of the rope (A) = diameter of the rope in inches x diameter of the rope in inches x area factor
Area factors for wire ropes typically range from around 0.35 to over 0.55.
Safety harnesses should be checked regularly for damage and replaced as necessary.
Write down the equation to determine shock load in pounds: shock load = load x [1 + (1 + (2 x FD x A x E)/(load x L))^1/2].
Plug in the values in the following example: load = 200 pounds, falling distance = 12 inches, area factor = 0.472, diameter of rope = 0.25 inches, metallic area = 0.0295 inches^2, modulus of elasticity = 15,000,000 pounds per square inch, and length of cord = 10 feet (120 inches). Therefore, in this example, shock load = 200 x [1 + (1 + (2 x 12 x 0.0295 x 15,000,000)/(200 x 120))^1/2].
Calculate the numerator then the denominator separately, as per the order of operations. So in this example, the equation simplifies to shock load = 200 x [1 + (1 + (10,620,000)/(24,000))^1/2].
Divide the numerator by the denominator, as per the order of operations. So now you have shock load = 200 x [1 + (1 + 442.5)^1/2]. Add 442.5 to 1 within the parentheses to get shock load = 200 x [1 + (443.5)^1/2].
Take the square root of 443.5 and then add 1 to perform the calculations within the brackets and get shock load = 200 x 22.059.
Multiply for the final result: shock load = 4,411.88 pounds.
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