Crane Runway Straightness and Specs 2

[dmalicky]

I am designing an overhead under-running bridge crane for a manufacturing company: 1 ton capacity, 46 foot span, 75 foot runway length (supported at 25 foot intervals), Class C service. I have ordered the CMAA 74 spec and plan to follow it as much as possible. From preliminary data and calcs (bridge weight of 4500lb, 2000 lb load + 500 lb trolley all on one side w/ 20% impact, stress 20% of UTS) the runways will probably be S12x40.8, A572-50.

My main question is on the runways: I understand from this article:

http://www.dearborncrane.com/Downloads/Avoiding Pitfalls of New Construction.pdf

that CMAA specifies a very tight tolerance on runway beam straightness.

On buying straight beams, Our local Ryerson and Tull rep said he had never had anyone ask for special runway steel for cranes. And, another plant in our company built a similar 30' span bridge crane, did few if any calculations, didn't follow any specs, didn't order special runway steel, didn't hang the runway especially straight... and they say it works just fine. Of course, this results in CMAA seeming irrelevant to many in the company.

So, a first question is, do we special order "runway steel" or use regular I beams? If the former, who sells it?

Here are some factors as I see it:
1) Following applicable codes is good practice and if it is safety related and up to me alone, that is what I prefer to do.
2) Specs/codes/standards are often conservative in order to meet special/extreme cases, and if one's situation is not special/extreme, a relaxed standard is often fine. Our crane is class C and low load (compared to other cranes) so might not need such precise runways.
3) My company wants this done as soon and simple as possible, and will require justification for any delays, extra costs, or special orders. And, I will need more justification than "this is the CMAA standard" for both mgmt and labor to buy and build our runways to that standard.

In short, I don't want to push a standard that is more strict than needed, but I also don't want to water down the standard and have a problem later.

The above article says if the runways aren't straight, wheels/motors/gearboxes can fail, or runways can need premature replacement. These doesn't sound particularly safety related, as in, "the crane will fall down". And, I would think these problems would be much more likely on the high capacity and heavy service cranes than for a 1 ton Class C.

If anyone has some insights on buying and building sufficiently accurate runways, or consequences of inaccurate runways over time--and the circumstances in which they occurred--I'd appreciate it.

Thanks,
David Malicky, Ph.D.

 

[jec67]

I advise that you adhere to the CMAA specifications. There are two considerations: safety and serviceability. While the problems you mention may not cause a life-safety issue, there is the potential for lost revenues due to downtime of the system. There is also the possibility of higher maintenance costs for this system as opposed to one designed in accordance with the CMAA specifications.

I prefer sections that are produced for use as runways. For an underhung system, these sections are manufactured to tighter tolerances than is rolled steel. The sections also (typically) incorporate a hardened steel lower flange, which provides for greater resistance to wear and smoother operation of the system.

Jeff Crandlemire, P.E.
Archer Engineering
203-775-5673

 

[arto]

Look at the ASME B30 series too:

B30.2 "Overhead and Gantry Cranes (Top Running Bridge, Single or Multiple Girder, Top Running Trolley Hoist)"

B30.17 "Overhead and Gantry Cranes (Top Running Bridge, Single Girder, Underhung Hoist)"

etc., etc.

 

[ehcet]

During my over 3 decade of crane and runway design, special beam for underrunning runway was used only if the trolley i.e. endtruck construction and wheel type required it.
For normal contoured wheel you can use std. beam there are built in clearance for the beam flange width fluctuation.
Never ever faced underrunning beam problem regarding premature failure if the stress design is correct , the crane bridge is aligned properly.
The special beam is just an excuse.

 

[RockEngineer]

With 1 ton capacity and 46 ft span, there should not be any need for ordering special steel for your runway girders. Large capacity, long span high speed cab operated cranes can be critical in having straight parallel runways but the small pendant operated cranes are not as critical.

 

[dmalicky]

Thanks to all for your helpful posts. I received CMAA 74 and have a few follow-up questions--

In determining "P" in section 3.3.2.6, do we generally assume the endtruck provides less than 4 wheel contact to the runway? I.e., local irregularities in the runway (even within CMAA specs), plus endtruck build tolerances, will probably result in non-4 wheel contact, giving twice the P as would ideally be expected. The "max wheel load" published by various vendors (Ace, Munck, Demag) is more consistent w/ non-4 wheel contact. But, Figs 3.3.2.6 in CMAA 74 imply 4 wheel contact. Or, are dual C-channel endtrucks torsionally flexible enough to provide a crude suspension? I see section 3.6.5 discusses equalizer trucks, though I don't know when these are normally used.

For "Case 2 Load Combination" (Section 3.3.2.5.2), the formula doesn't make sense to me--it adds vertical and lateral forces directly, w/o vectors.

Thanks again,
David

 

[ehcet]

You are overreactin for a little 1 ton crane runway.
Call me at 905-712-2373 if you want some solid info.
Ernie

P.S. I did those things for 32 years at DEMAG. Trust me,

 

[sacem1]

I agree with ehcet yours is really a small crane and standard beams will be enough, trust ehcet and myself I'm right now commisioning 7 diferent size overhead cranes.

Cheers

SACEM1