Cranes

breadmonkey

Maybe this is a bit sily but I've been thinking about it a bit recently.

How is it that a tower crane (big ones used on site) can be balanced when it is unloaded as well as when it is loaded to capacity. Why aren't there problems in one case or the other?

godtabh

I never thought of it before either but I reckon its the way the forces are transfered.

In most cases (as far as I can remember anyways) there is a strut attached to the weights. When the crane is unloaded the struct transfers the load down to the base. When the crane is loaded to goes in to equilbrium (with any left over weight transmitted via the strut to the groun).

I'm sure if you thought about it you can work out the forces.

cargrouch

The counter weight can be moved in and out along the beam. Close to the cabin when unloaded, furthest away from the cabin when lifting max weight. (Am I right? Do I get a biscuit?)

crosstownk

Some basic info here > http://science.howstuffworks.com/tower-crane.htm

Borzoi

breadmonkey wrote:

How is it that a tower crane (big ones used on site) can be balanced when it is unloaded as well as when it is loaded to capacity. Why aren't there problems in one case or the other?

The conter weights are in a fixed postion, but AFAIK the system is only in equilibrium when the crane is half loaded. That minimises the bending forces on the tower. The reason the whole thing doesn't topple over is due to the enormous weights on the base of the tower - usually not visable to Joe Public, because it's hidden behind the building site hoardings.

The other thing to bear in mind is that the Max Lift of the crane decreases along the jib. The max loads are marked on the jib arm so that the operator doesn't inadvertantly lift too heavy an object, too far out, and cause the TC to collapse.

The Liebherr company are a huge crane manufacturing firm, with a facility in Killarney. Their website might have more info

Litcagral

cargrouch wrote:

The counter weight can be moved in and out along the beam. Close to the cabin when unloaded, furthest away from the cabin when lifting max weight. (Am I right? Do I get a biscuit?)

Sorry - no buiscuit. The counter weights do not move along their jib.

breadmonkey

crosstownk wrote:

Some basic info here > http://science.howstuffworks.com/tower-crane.htm

That was very informative but it doesn't address my original question.

Litcagral

breadmonkey wrote:

That was very informative but it doesn't address my original question.

Tower cranes are anchored by massive concrete bases for stability (as borzoi said the anchor base is usually hidden from view by hoardings). If you were to stand in concrete until it hardened you would be more stable. Tower cranes are generally utilised for relatively light loads seldom more that several tonnes and therefore it is relatively easy to balance them. The immovable counterweights give it extra balance. At the extreme end of the jib, the crane can lift the least weight and this increases as the trolley moves towards the centre.

Mobile cranes (those on trucks) are capable of lifting much greater weights, sometimes up to 1,000 tonnes, but they are much more expensive to hire.

ciaranr

The unbalanced moment is probably taken up by the vertical pylon as bending moment. If you consider that the forces on the other plane, perpendicular to the main axis (wind, load sway), are taken by the main pylon, i suspect it can deal with the unbalance produced by different load states without any external help.

Voodu Child

In laymans terms-

Stand with your legs apart and a weight in one hand, down by your side. Lift your arm up so its pointing straight out. You dont fall over (unless youre drunk) and you dont need to counterbalance the weight with something else in your other hand. You will feel a stress in your torso though.

You dont fall over because youve got a good solid base. Simple

Also, if you have a lighter weight held out in the other hand, you will feel less stress in your torso when you lift out the heavy weight with the other hand, but conversely, you will feel a slight stress in your torso when your other hand is empty. So you are kind of balancing the max and min moments with the counterweight, and taking the difference up with your torso. A crane is the same. Hmmmm i could probably write this better but i couldnt be bothered...

Also note that some cranes DO have moveable counterweights. (not any ive come across though)

Plug

If you look up at the crane you can see little registration plate type thing's that show the weight it can lift. Most tower crane's can left only about a ton at the front and anything up to 8-10 ton's just in front of the cab.
The biggest tower crane in the world has sliding counter weight's.

Barney04

Yea, it's about balancing the forces. The counterweights are mounted toward the rear of the mast, so they put so much force when crane is loaded. I allways loved the look of tower cranes, and i enjoyed watchin them working. Now they are mainly produced in China, but i love classic manufactor like Bigge Equipment

Surveyor11

The counterweight is located close to the cabin and exerts a force (moment). The crane operates on the principle of moments. So a 1 ton counterweight located at, say 2m behind the cabin will mean the crane can lift an opposing force (one tonne) at 2m along the boom on the opposing side. The load the crane can lift will decrease as you move away from the cabin to the end of the boom, so it a 10m boom may only be able to lift 0.5 tonnes at the end.

Hopefully, the attached illustrates what I'm trying to explain!

http://www.bigge.com/crane-charts/tower-crane-charts/SK575.pdf

Mr. Tezza

Its been pretty much covered by everyone else that has replied but I can ask my mate who works for Irish Crane & Lifting tomorrow if you want?

Mr. Tezza

Was talking to my mate there, its basically all about forces and moments... A crane is ALWAYS overloaded by its ballast (weights behind the cab) but never to such an extent as to make the crane unstable, the shorter the jib, the less ballast you need, simples right?

When a crane is not carrying a load the weight of the ballast actually causes in to lean back but never so much as to danger the structural integrity of the crane, so when a weight is applied to the front of the crane it begins to lean forward again. The idea is to have the ballasts take up the weight so as to put the least amount of stress on the crane body as possible. So as I understand it cranes are pre-stressed to be able to take the weight of their ballast when not loaded...

Tower cranes do not have moving ballasts but there are types of cranes out there that use hydraulics to move ballast back and forth as necessary.

His website is www.irishcranes.com, he deals in tower cranes and knows everything about them, if he doesn't know it, its not worth knowing!

I hope this helps answer/understand what you were asking.