The Lowdown on Load sway

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Anti-sway technology was revolutionary when it was first introduced – but back then it did not actually work very well. Today’s systems, in contrast, offer safety and economy, and allow even a non-specialist to operate a crane. Julian Champkin reports.

Hoist Magazine began writing about anti-sway systems almost 20 years ago, but the origins of anti-sway go back much further. Some 420 years back further, in fact, to around 1602, to a day when Galileo found himself in Pisa cathedral. Bored, we presume, by the sermon he found himself watching the heavy chandelier swinging on its chain from the roof.

A small swing, he realised, takes just as much time as a big swing. The period of the swing depends only on the length of the supporting rope, not on the amplitude. The equation that governs pendulums is taught in most introductory physics courses:

And the euation shows that the time of each oscillation, T, depends only on the length of the rope and the acceleration of gravity. The weight that you hang on the end does not affect the timing at all.

Hence the invention of the pendulum clock, and, more relevantly for our purposes, the somewhat later invention of anti-sway. A load swinging on a wire is nothing more than a pendulum. So it obeys that same equation. To find the period of each swing you don’t need to know what load you have picked up – you only need to know how much wire you have paid out, and that is reasonably easy to measure.

And to cancel out the swing – to make the rope hang practically vertically as you move the load along – all you have to do is move the support – the trolley – at the same speed as the load at the end of the rope. A simple mathematical algorithm could do the sum and adjust the trolley speed accordingly.

In a simple, ideal world, that would be all. The world, however, is rarely simple and is ideal only sometimes. If your hoist is in the open air there is wind that can add an extra, unpredictable sway to your load. Even without wind your load is not actually hanging from the rope: it is the hook that is hanging from the rope, and the load is hanging from the hook, perhaps via a length of rigging – so the load can swing from the hook while the hook is swinging from the rope. In which case you don’t have a single pendulum, you have a double one – a pendulum hanging from a pendulum, and each pendulum swinging at its own speed.

As we have said above, the basic piece of information that governs sway is the length of your paid-out rope. Without that, how on earth can his system work?

Sorensen explains: “Suppose you had a crane and you knew everything about it. You knew that you had 5m of hoist length and you had a 1.0t load that was a certain shape and you knew how fast the frame was able to travel; if you knew all those parameters it is pretty straightforward to do the math and figure out how you can move that crane from point A to point B without very much swaying. It’s kind of a second-year engineering problem. And there’s not just one solution you could come up with. There’s actually an infinite set of solutions that would work to move the crane from point A to point B without swinging. You could move it very, very slowly all the way, or you could move it slow then faster then slower, and so on. Call that group of solutions Group A. It is all the solutions for doing Configuration A.

“Now suppose that you have a different configuration of a crane. Maybe instead of 5m of rope length it is 10m, and you have some additional rigging hanging off the bottom block, and you’re picking up a large horizontal I-beam that will itself add sway. Once again, as long as you know everything about it, all those details, you can do the math and come up with a family of solutions that works on that particular rigging configuration. Call that Group B.

“Then you do a Group C, for another configuration, then groups D, E, F, all the way through the alphabet for all the configurations you can think of. Now here comes the nice bit: there will be some overlap between the groups. There will be solutions in Group A that are also in Group B, and some of those will even be in Group C, and so on. It is like a Venn diagram: there will be a subset of solutions that work for every configuration of load and rigging that that crane can have. And what ExpertOperator does is just choose the fastest one.” So, bingo: you have an anti-sway procedure that works for all configurations of your crane without needing to know even how much rope you have paid out.

Note that it only works for that first classification, of operator-induced sway. If your crane is outside, subject to wind and weather, you would need to add a sensorbased system. “You have to add a camera,” says Sorensen. “Or if you’re wanting to detect when the operator has a side load or snags the hook on something, then you need a camera for that.” That scenario, for example, might deploy CraneVision.”

COST AND CONSISTENCY

A sensor-based system, as we have said, gives real information about the real position of the crane. Why then would anyone want a sensor-less system?

One reason, Sorensen says, is cost: without sensors to pay for and to calibrate on-site they are quick and simple to install in a new crane, or to retrofit to an existing one. Another, less obvious, reason is consistency.

“Crane operators are a picky bunch,” says Sorensen. “They get used to a crane the way it is, and one thing they really like is for the crane to behave the same way every single time they use it. If it takes five seconds to decelerate from top speed, they want it always to take five seconds to decelerate from top speed no matter what the load is or the lift height may be. They don’t want it to take three seconds with the load held high and six seconds when it is low. But that is what anti-sway with sensors delivers: it actually changes the way that it decelerates and accelerates depending on what that sensor is telling it. And that is one of the reasons why crane operators would turn off the anti-sways that were based on sensors – it is because it caused inconsistency.

“A sensor-less system, on the other hand, doesn’t know whether the lift is high or low, and it doesn’t care: it delivers the same pattern of movement for both. That makes the operator feel more comfortable with his crane and more at home with it.”

The advantage of anti-sway – whether open-loop or closed – are many. It takes less skill on the part of the operator – and skilled crane operators are hard to find.

“We have done the test many hundreds of times,” says Sorensen, “where we have taken an experienced operator and have them do certain manipulation tasks, and then a brand new operator and have them do the same tasks but with the anti-sway technology. And it is tremendous how quickly the new person can exceed the performance of the experienced operator.”

Two decades ago, anti-sway may have been clunky and not very efficient. Today, it actually works. Given all the advantages, of safety, speed and productivity, and the small cost of an anti-sway system in comparison to the cost of a crane, it is perhaps surprising that there are still overhead cranes without them. But the retrofitting of older cranes is still a flourishing business. If your own system is still swaying, perhaps it is time to put a stop to it.

The Lowdown on Load sway