Have you ever come across the term ‘jib deflection’ and wondered what it is? Let’s answer that for you in short right now.
Jib deflection often happens when a heavy load gets lifted by a crane. The weight of the load bends the crane’s jib downwards, and on tower cranes, also bends the mast forwards towards the load.
Now that you’ve got a rough idea, we’ll get into more detail. This article will talk you through when and why jib deflection happens. We’ll also discuss what a crane driver can do to overcome it.
What is jib deflection?
Imagine holding your arm out straight with your palm facing upwards, then someone puts two bags of sugar on your palm. Unless you’re super muscly and have very good core strength, the chances are your arm is going to lower. If your arm is really rigid, your body might even lean over a bit.
This is what can happen when cranes lift loads that are remotely heavy. Nearly all cranes experience this. Some cranes flex less than others, depending on the size and configuration of the crane.
When it happens to tower cranes, not only does the jib bend away from the crane, the mast also leans forwards. Although, it leans less if tied into a building, as shown below.
The ballast weights of a tower crane rest at the back if the crane. This means that the resting position of the crane, without a load being lifted, is tilting backwards. When a heavy load gets lifted, the weight of it pulls the crane to a more upright position.
But what effect does this have on the lift? Is it dangerous?
The answer is, yes, it can be dangerous if not compensated for.
Jib deflection causes the radius of the hook to increase. The radius is measured from the centre point of the crane to the hook block.
Check out the diagrams below. Diagram one shows the hook block when the crane is empty (has no load on). Diagram two shows the crane lifting a heavy load.
When the operator is about to lift the load, as soon as the slack in the chains has gone and tension is on, the jib will bend until full weight is taken. Which means the radius will increase until the load is in the air.
Therefore, as the load leaves the ground, the optimal pressure of lifting isn’t directly above the centre of the load. On a luffing crane, this would be the end of the jib, with the hook block in a plum line below it. On a saddle jib, it would be the trolley, with the hook block in a plum line below that.
As this point will be further out than the centre of the load (sometimes as much as a metre) the load will swing outwards as soon as it leaves the ground.
This can be dangerous because the load could either hit people or objects. If the load is extremely heavy, this could cause a lot of damage or injury.
As a crane driver myself, this has happened to me a couple of times. The first time, I was inexperienced. I was taking the head of a pump off the back of a wagon. It weighed around 6.5 tonnes. I didn’t allow for the jib deflection, so the load swung into the bulkhead of the lorry. Luckily, no damage was done.
The second time was with a concrete skip. The concrete lorry had poured into it. As I lifted the skip to take up to the top floor, it swung out and hit the lorry. This time, it bent something on the lorry.
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How does a crane driver deal with jib deflection?
There are a couple of ways to compensate for jib deflection. But before doing these, there are things to consider.
Before lifting
If the crane driver knows the load is going to be heavy, they should make sure there are no people or objects nearby. A delivery vehicle or other plant might park right next to the load or pass by while lifting. It’s advisable to make sure there is a clear space around the lift, just in case the load does swing when lifting.
The crane driver should also be aware of how much the crane they’re driving deflects. As stated before, some flex more than others. If they’re new to the crane, they can do this by asking other drivers who have already been driving the crane. They can also be conscious of the movement with each lighter lift as they get used to driving the crane.
When lifting.
1.
Start the lift with the hook block at a shorter radius than what the load is. Then, when the weight is being taken, the deflection causes the optimal point of pressure to be closer to being directly above the load.
This isn’t the best option, because it’s kind of like guess work. The driver has to hope that the hook is just the right amount of distance to the load in order to make it centre when full weight is taken.
2.
For this exercise, bear in mind the principal that jibbing up on luffing cranes and trollying in on saddle jibs decreases the radius.
Start the lift with the hook block centre of the load. So, directly above it.
If the crane has a luffing jib, as soon as the tension is taken, the driver should slowly jib up instead of hoisting up.
If the crane has a saddle jib, the principal is the same, but the driver should slowly trolley back.
In both types of cranes, the driver should keep an eye on their Rated Capacity Indicator (RCI) readings. They take a note of the radius at the start of the lift, then maintain that radius when they’re working the jib/trolley until full weight is on.
The figure will fluctuate slightly. It might flicker up and down. This is normal. It happens with the bounce, sway, and general movement of the crane.
This is my preferred option, as the radius is fully under the driver’s control.
If the crane driver believes the load is going to be heavy, they should always ask the slinger/signaller, crane supervisor, or Appointed Person (AP) what the weight is.
This is so that they can see on their RCI screen when full weight is approaching, in order to stop jibbing or trollying. If they do continue to jib or trolley once the load is lifted, it might create a swing or unintentionally reduce the radius.
When landing.
The opposite action to when taking the full weight should be done when losing all the weight after landing the load. Although, maintaining the exact radius isn’t entirely necessary. The idea is to minimise the swing of the hook block once all weight has left the crane.
This is because, as the weight decreases, the jib goes back to its natural state, which decreases the radius. Therefore, the hook block swings inwards.
When this happens, the block can smash into objects. Also, slingers might be holding onto the slings or chains, so when the block swings inwards, it might create tension on those accessories. This tension could trap fingers or hands if they’re in gaps of the accessories.
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Conclusion
Some things aren’t taught in crane driver training. For me, jib deflection was explained, but we didn’t do any exercises on how to compensate for it. So, when I got onto site, I had to learn the hard way. Sometimes this can be good. But it can also be dangerous. Especially with safety critical work.
Hopefully, you now have a clear idea of what jib deflection is. This is useful for both crane drivers and slinger/signallers. If you’re new to both professions, you should now be more confident when experiencing jib deflection.
If you have any questions, please feel free to comment below. Also, I’d love to know of your own experiences with jib deflection.