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Re: Opinion on a structural dynamics book

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On Feb 15, 2006, at 6:39 AM, Gary Hodgson & Associates wrote:

Just what I need--I got a call from a crane customer on
Monday whose customer is complaining about the crane
yo-yoing.  Does Timoshenko have anything on yo-yoing?
My considerable experience with yo-yo's is that they have almost nothing in common with cranes. Nor does Irvine mention it. So does the hook load go to end-of-travel and then bounce back upward all on its own? Or does the load just descend and stay there spinning until the operator give the hoist a little tug? I bet Around the World or Walking the dog is really quite something to see. (Sorry, couldn't resist. Won't happen again);->

I'm assuming this is a bridge crane--is it? Sometime very early, you should do a very careful inspection of the structure to make sure nothing's broken. Structures tend to get bouncy when they're coming apart or fracturing. If the motion has only recently become a problem, the structure has changed, and loose bolts or broken welds could be responsible. If the crane has been structurally soft since installation, check it anyway because the continuous cycling tends to loosen things. You might also check the rails to make sure they're straight.

When you know that the structure isn't damaged, you want to try to see if it's a control problem or a structural problem first. If it's a bridge crane get an idea of the frequency and direction of motion and any variation with load and trolley position. The softest element of the load path is generally the bridge girders, so make yourself a quick estimate of the corresponding frequencies, both vertical and horizontal to see if there's a match. The frequency of motion will vary with trolley location if the bridge girders are chiefly involved. Horizontal (bridge travel direction) motion may involve either support deformation or girder stiffness.

If you get lucky, you may find that the operation is at fault, either because of a control malfunction that makes a drive stop suddenly or operator technique. A structural problem may end up being a serious nightmare, because it's not all that easy adding enough stiffness in the right place to make a big difference. It also means bringing the machine down and greatly affecting operation, which your customer won't like.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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