The Dangers of Shock Forces

Definition
To understand shock forces you must know that there exists static (not in motion) forces and dynamic (in motion) forces. In the real world we rarely have just simply static forces occurring during lifts. Even small amounts of speeding-up or slowing-down of the load result in dynamic loads. The very act of even slow lifting often results in some forces caused by movement such as swinging and drifting.

However, the force we are talking about it the one that occurs rapidly as opposed to slow dynamic forces. Shock force is more commonly referred to as shock load. This derives from the fact that engineers routinely refer to forces occurring in and on structural members as loads. In rigging a load is an object to be lifted or flown from one point to another point, hence the use of the phrase shock force in this article.

How Do Shock Forces Occur?
Shock forces can occur for any number of reasons, most notably, an operator taking-up sling slack with a sudden jerk; rapid acceleration (or deceleration) of the load; failure of fair leads or sheave guides to prevent the rolling out of a slack line.

What Is The Significance of Shock Forces?
The magnitude of a shock force can be many times that of the weight of the load be lifted. This is why the safe working load of rigging equipment is substantially lower than the minimum breaking strength. Minimum breaking strengths are stated for static, straight pulls. A factor of safety must therefore exist.

How Are Shock Forces Calculated?
The amount of force created in a shock situation is dependent on, among other things, the weight of the load and the distance of travel. The exact determination can be quite complicated because the value of the load's stopping distance is based on the amount of elastic stretch.

In order to precisely calculate the stopping distance we would need to know the exact composition of the wire rope, the equivalent cross sectional area, and the apparent modulus of elasticity of the wire rope composite, and then use a complicated formula to calculate the exact amount of elastic stretch*.

023bWeightMany manufacturer's websites state that there exists no practical method to estimate shock force. Gelrum** provides an example of a 75 foot long (L) 1⁄4 inch diameter galvanized cable sling subjected to a shock force by the sudden dropping of a 500 pound load 6 inches. The resulting shock force is 2,296 pounds, a value over 4 times the weight of the load!

*A free applet, or automated calculator, for wire rope elastic stretch is located online at:
http://www.macwhyte.com/Technical/Metallic-Area-Elastic-Stretch-Calculator

**Gelrum, Jay O., 2007 Third Edition of Stage Rigging Handbook, ISBN-13:978-0-8093-2741. Similar information is available from the publication Entertainment Rigging: A Practical Guide for Riggers, Designers, and Managers, by author Harry Donovan.