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A long time ago Dave Spencer and Ken Casey/Spidey, did a very accurate calculation of impact loads on rigging points. The accuracy of the math was confirmed using their dynamometers.

Don's published rule of thumb is plenty accurate to figure out impact loads and size the rigging gear, with proper safety factors.

Weight of piece/500#

Drop of 4'

[500x4]+500=2,500 impact on the rigging point. The actual in the field numbers were very close to that point.

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This "rule of thumb" business seems to come back to life every so often no matter how many times I try to shoot it dead.

What is the rule of thumb for how fast a motor vehicle can accelerate? How far it can go on a tank of gas? How much runway an airplane needs to get airborne? Everyone can see these are ridiculous questions because there are lots of variables that must be specified before any of the questions could possibly be answered.

For the rigging question, for a given distance of drop, by far the most important variables are the stretchiness of the rope and the total amount of rope available to absorb the energy of the fall and stop the load, yet no one has mentioned either of these!

Let's reverse analyze the example given by assuming the load starts falling right at the anchor point and falls 4 feet. At that point the rope goes taut and begins to stretch. The rope keeps stretching and the load keeps slowing down until finally it stops. At that point we measure 2500 lbs. force on our dynamometer. Since we know nothing about the rope we have to make some reasonable assumptions. Most ropes behave more or less like springs, that is, the amount of stretch is directly related to the force applied (look on the Yale Web site for some excellent graphs). What this means is the rope's stretch at 2500 lbs. is twice what it is at 1250 lbs. We can now say the average force the rope felt over the full distance, x, that it stretched, is 1250 lbs. Applying a little algebra we can solve for x and find that it is 2.67 feet.

The "rule of thumb" is looking really bad right about now, because none of us owns a rope that can stretch 66% of its length without breaking. That would take a bungee cord. This result also implies that the actual force on a real rigging rope that could withstand the 4-foot drop would be much much greater than 2500 lbs.

There is no rule of thumb. You simply must have at least a ball-park idea of the stretchiness of the rope, usually clearly indicated in the online catalogs, the expected distance of fall, the weight of the load, and the total length of rope available to stretch. There are just too many factors here to lead to a useful rule of thumb.