Hi, I'm doing a project on ways to improve load control on conventional tower cranes. I'm going to start off by explaining the areas where there is room for improvement, the dangers of load control, for example load swing. There is obviously the problem of the loads swinging from the lifting cable, or rotating. What other problems or dangers are there? Ultimately it will lead on to the idea of using a cable based stewart platform manipulator to automate the lifting process to improve load positioning. So I'm looking for ideas on why the lifting operation is not ideal. What is unsafe about it? Why is it inaccurate? What would automation improve? Any help would be greatly appreciated Thanks
School project?
Yes
One of the areas you want to look into deeper as you go through this exercise is the need to keep what is referred to as 'above the hook' weight to a minimum. That assumes your stewart platform manipulator would be below the boom and above the hook. The ability of a crane to lift a load is predicated on it's ability to withstand the 'pound feet' the load puts on the apparatus. As the radius from the slewing center axis is increased the amount of weight a crane can handle decreases. For instance, a 1,000 pound load 10 feet from the slewing center axis puts a 10,000 foot pound load on the slewing ring. Move that same 1,000 pound load out to 100 feet and the slewing ring will see a 100,000 foot pound load. This is offset by the ballast on the opposite end of the boom, the counter weight. One of the goals in lifting is to keep all unnecessary rigging weight to a minimum. If you think adding a stewart platform manipulator is the answer you'll need to consider the disadvantages of extra suspended weight that will subtract from the 'money weight', e.g. the load carrying capacity of the crane. If your stewart platform manipulator is below the hook, the same thing applies. But as you described in your original post my guess is you're looking at an above the hook solution.
Thanks for the reply Shenandoah, very helpful. I will mention the consideration needed to minimise above the hook weight. Would Here is a diagram of the configuration. Would you think this configuration is better in terms of above the hook weight - it seems as if it would be better having the load constrained along the 6 wires than the load only being constrained along 1 lifting cable This configuration as far as I can see would increase accuracy, there would be no need for those on the ground to reposition the load, it would reduce load swing Are there any more considerations anyone can think of?
Also does anyone know the measures currently in place to reduce sway, the load on a tower crane is only stable vertically, so does anyone know of methods they use to counteract the swaying of the load?
please explain a stewart platform manipulator. all cranes require the operator to control the load movement. all crane loads are generally free to swivel from the hook unless restrained by tag lines-this is not strickly a tower crane issue. crane work is not a robotics issue. operators are required to continually adjust for variables that occur throughout the workday.
it would basically be a platform attached to a carriage, and then 6 wires would attach to another triangular platform beneath it, in 3 pairs. The general function of the stewart platform is that when the length of the legs is altered then the position of the moveable platform can move in 6 degrees of freedom. But in this case its using wires, and winches allow the wires to change length. The length is controlled through the winches, via computer.
thank you for the drawing. when you consider the typical line speeds involved with production tower crane operations, and the complexity, and maintenance factors involved with the additional reeving this would be a hard sell.
Can you elaborate on that? I'm not familiar with cranes so your help is appreciated.
Couple of thoughts...more cables equals more weight. Not only in a static scenario, but also in a dynamic scenario. And the dynamic scenarios is where your device will both help and hinder load control. When the load is swinging due to wind, boom movement, et al, the load will put stress of varying quantities on different lines of your stewart platform manipulator. In essence, there is no guarantee all the line will share the same load factors as the item being lifted moves side to side, or fore and aft. Additionally, the added weight of the corrective device will introduce unwanted variables into the mix such as added swinging momentum that will have its' own unique harmonic. This will need to be overcome in order to place the lift in the desired location. This could be minimal, or it could be a nuisance. A lot would depend on the harmonics induced. You could test this to a small degree by creating a model of the device and initiating a torque swing in the lifted item whereby you twist the item so it rotates in its' vertical axis, as an example, so you see how it's constrained and what harmonics it induces into the equation. You want your lift to put as few shock loads on the crane as possible, and as the lines of your SPM tighten and loosen due to the shifting load of the lift you want to understand where those force ultimately get dampened. And how fast, and at what frequency keeping in mind the frequency will change over time. Also, the aspect ratio of your device, fore and aft vs side to side is considerable. If the length of the lift is long, you may reach a point of diminishing returns due to the small included angle of the lines of the device. A fore and aft dimension of 20 feet for the SPM on a 200 foot lift will yield such a small angle. And given the necessary narrowness of the device the angle will be even smaller in the side to side plane. This will tend to negate a bit of the corrective powers of your SPM. Ironically, the biggest hurtle you will face will be momentum. Getting folks to try new stuff is an uphill battle regardless of how innovative or worthy a device is. So don't give up on that front. You can always do what most successful innovators do and that's to push enough money through the political process that your device becomes mandatory.
Great thoughts but I must stress this is not my device, this is an invention by Stewart Albus, and I'm writing a paper on the NIST RoboCrane and how it could be used on a tower crane.
its still going to swing