Work around cranes enough and you’ll see moments where a heavy load slowly bounces up and down on the hook. This bouncing is unstable, and the more it happens, the better the operator could have been.
OSHA even steps in here saying “The hook shall be brought over the load in such a manner as to prevent swinging” and “During hoisting care shall be taken that there is no sudden acceleration or deceleration of the moving load.” These standards can be found in the 1910.179 (n)(3).
How Dynamic Forces Make Loads Unstable
In short, a static load is stationary, while a dynamic load is moving. The reason why dynamic loads can become unstable is because bouncing around can add weight beyond what the crane was designed for. The more a load jerks around, the more additional forces are applied to the load, forces that are typically not planned or calculated into lifts and can cause cranes to fail.
All true weight measurements for lifts are based on static, perfectly still loads. This applies to all crane counterweights, load charts, load moment indicators, and any other performance data. This is the same reason nurses want you to stand perfectly still on the scale. They are looking for your static weight. If you move at all, the scale indication changes, resulting in an inaccurate reading.
Once an object of any kind starts moving (in any direction) the physical forces involved to start; and especially to stop said motion safely should be thoroughly understood by everyone associated with this activity. It took some form of energy to move the load; some form of energy will be required to stop it. Careful and precise acceleration and deceleration is the key to smooth crane operations. That applies to any direction – hoist, swing, bridge, trolley, etc.
Any form of crane motion – hoisting, swinging; bridge or trolley movement – needs to be carefully controlled with regards to the actual forces involved. Slow starts and stops, no sudden moves; these are the key considerations when operating almost any kind of machinery.
How “Zero G” Helps Us Understand Lifts
Ideally these forces are considered and accounted for in the design and manufacturing process, enabling the machine to accomplish specified movements without overloading or destabilizing. Essentially: the structural design should be able to handle any applied dynamic force under NORMAL operating conditions.
ARE YOU SITTING DOWN?
If not, you may wish to. Most people have seen the famous ‘dogfight’ movies where the fighter pilots are getting crushed into their seat by “G forces” as they try to outmaneuver their opponent. Simple question: How many vertical G forces are you experiencing right now, sitting there in your chair? Unless you are occupying an elevator, car, airplane, train or other moving vehicle; the correct answer is most likely…one. I am increasingly amazed by how many educated and experienced people want to say ‘zero’. ‘Zero G’ is effectively weightless. If you are a scuba-diver, you will understand this. Skydivers know all about it - exactly what they live for; even though it only lasts for a very short time.
How many Gs do we want to apply to our cranes; our rigging; along with our load? The only proper answer is...one. The problem is it is impossible to lift a load at our desirable one G. You need more lift than you have weight.
Zero G, or anywhere even close to it, is a very bad idea when we’re talking cranes. The closer you get your load to weightless, the further it will come crashing back down to earth on the way back. Our ultimate desire when lifting is to keep the dynamic load as close to ONE G as possible, and still conduct the lift.
Applying Lifting Physics as an Operator
What would you suppose the actual dynamic forces applied to lifting a load clear of the surface would be? The first thing any lifting machine must accomplish to pick a load is: develop more lift than the weight of the load. Exactly HOW much ‘excess lift’ is applied to the crane and rigging, as well the load itself, is determined by the actual movement of the hoisting mechanisms, as directed by the operator.
A smooth operator will lift the load as gently as possible, make the horizontal move with a minimal load swing, and set it down nice and easy. The engineering calculations involving complex strain vectors associated with any crane are based (once again) on NORMAL operating techniques.
What happens to our collection of lifting gear when any part of it is subjected to sudden acceleration or deceleration? That event is accurately described as a ‘shockload’. Remember that shockloads can occur in ANY direction, not just the vertical. Any change in direction or speed of our load will affect the dynamics of the entire assembly.
Per OSHA’s Slings and Rigging Standard, 1910.184(c)(11): Shock loading is prohibited.
What Do Cranes and Chicken Eggs Have in Common?
A chicken egg has that certain oval shape to it. Why is that, and what possible connection does the egg have with our crane? If we want scrambled eggs, we usually break the shell on the side. Never the butt, or the point. We don’t want shattered eggshell in our food. The egg is intended to withstand that first VERTICAL force imposed upon landing in the nest. If the egg lands on the side, then we already have our scrambled eggs.
What happens to our lifting gear when we side-load any part of it? Cranes and other lifting systems are normally designed to accommodate a vertical strain. It is not acceptable practice to utilize the overhead or mobile crane for dragging loads off shelves or across the floor. If we really need to dislodge our Jeep from the riverbed, we need a wrecker, not a crane or boom truck; for what should be obvious reasons.
Every machine I have ever encountered could be compared to the eggshell: strong points, and weak spots. I can indeed (at 200 pounds even) stand on an empty soda can with one foot. I’m NOT suggesting that anyone try this. It is, however, possible to do, but ONLY if the weight is applied slowly and evenly. If any part of the can receives a shock or point load, the result is a crushed soda can. This analogy also works if you substitute “soda” with “beer.”
This same principle applies – in Spades – to our crane and lifting activities. Cranes are like eggshells. So are automobiles; ships; rail cars and locomotives; airliners; and even your favorite monster pick-up truck. They are all designed to take the major portion of the strain vertically, not sideways. If you don’t believe that, loan me a dozer, or even a small forklift for a few minutes and I will be happy to demonstrate on the monster truck.
