A crane overrun is the distance between where a crane driver stops the motion of the crane and when the motion actually stops.
In other words, when the operator lets go of the lever after doing an action, the instigated action will continue until it slows to a stop.
Another term for overrun is run on.
Why do overruns happen?
Overruns prevent damaging the crane. If the motion stopped dead, the moment of the movement could shock load the crane. Especially if the crane is carrying a heavy load.
It’s similar to a falling object hitting the ground. The resulting impact from the gravitational force can be catastrophic. When the object hits the ground, its kinetic energy transfers into the ground, so the force continues to operate. F=Mg.
You could also relate it to the braking distance of a car. If a car immediately stops dead when the driver slams the brakes on, the force would be equal to crashing the car, potentially forcing the driver through the windscreen and onto the road.
When a load being lifted by a crane creates a shock load while being hoisted down, its weight can increase by up to four times.
What types of overruns are there?
The three main types of overrun on cranes are hoist, slew, and luffing jib overruns. The trolley on saddle a jib doesn’t get much of an overrun. Read here to find out the difference between a luffing jib and saddle jib crane.
The amount of overrun can depend on crane size, crane configuration, the speed the crane is moving at, and the weight of the load. Heavier loads can make cranes hoist slower, which means less overrun.
How to avoid overruns.
The best way to avoid crane overruns is to slow down before reaching the destination.
With any of the three actions, a slinger should notify the crane operator when they’re approaching the destination, so that they can make the appropriate adjustments.
Slew
With a slew action, the crane driver should slow down when approaching the target, then catch the swing once there.
Hoist
When the crane hoists down, the slinger should give the driver as much information as possible. Some slingers will tell the driver what gear to hoist down in, reducing the gear speeds as the load or hook block gets closer to the ground.
This can get confusing, because every crane is different. They have different amounts of gears. Plus, each of the gears on one crane might be different to the next. So, the slinger will often instruct to use the wrong gears.
Other slingers like to say how many metres or feet are left. This is the preferred option for most crane drivers, because they can gauge for themselves how fast they need to be going.
Other information helps. Such as telling the driver what floor of a building they’re level with, then when the tag line is in the slinger’s hand, then when the load is in hand. All this information paints a picture for the crane driver.
Jib
The same principle applies when avoiding jib overruns. Especially when the load is low to the ground or anything/anyone it could strike.
When the load is low, the slinger might ask the driver to float it in or out, whatever the case may be. To explain this, when jibbing down, the natural arcing motion lowers the load/hook block. So floating means to hoist up while jibbing down to keep it level, and vice versa when jibbing up. This is where things can get more technical.
On another note, if the crane is jibbing down with the load/hook block close enough to strike something, then the slinger suddenly instructs the driver to stop jibbing as mentioned, the jib may continue to move down, depending on the speed. A way for the crane driver to counteract this is to slowly hoist up a bit while the jib overruns. This, in itself, is like floating the load. It prevents the load from scraping the ground, crushing limbs, colliding with obstacles, or such like.
The same action can be done when jibbing up. But instead, the driver would hoist down a bit during the overrun.
Check out this detailed explanation and visual example of a crane overrun.
What to take away.
Problems can occur when crane drivers or slingers are either unaware of overruns or are unsure of how much overrun the particular crane has. I’ve been caught out with this myself a few times.
The first was when I was an inexperienced crane operator. I didn’t anticipate the overrun when lowering the hook block for the slinger to put the chains on. It subsequently made the hook block hit the ground.
The second was when a slinger instructed me to fire the hoist down when I had a load on. The load wasn’t too far from the ground. By the time the slinger told me to stop, the run-on made the load squash a barrier.
The third time was a lack of concentration on my part. Depth perception can be difficult to gauge when up a tower crane. In this instance, I didn’t realise how close to the ground the load was. When the slinger noticed, there wasn’t enough time to stop, and the load clipped a scaffold railing. Luckily, in each instance, nobody got injured. But they could have been. Which is why full concentration and communication are important for all involved with lifting.
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