Overhead Crane Conversion from CMAA to a Winch 1

[ECD40]

We have a CMAA standard 20 tonnes capacity overhead crane with a 4 x 2 rope reeved drum, which is 6 meters long x 487 mm in diameter x 18 mm diameter rope. Unfortunately, the fleet angle of the crane hook ropes is such that the crane hook cannot be lowered through a lift well located at about half of the total required crane hook vertical travel distance. As this crane is in an existing operational building, it is not practical to replace the entire crane trolley with a larger diameter drum to reduce the rope fleet angle.
What I wish to do is to convert the existing single layer of rope CMAA standard drum into a 3 rope layer winch drum, with less horizontal rope travel and therefore a smaller rope fleet angle that will allow the crane hook to pass through the existing lifting well in the building.
I've been following the thread of the forum 'How to calculate load imposed on tube by winding string tightly', and my problem seems to be very similar. It also seems to be difficult to calculate. So what I need is to determine if the existing drum that is designed for a single layer of rope can carry the same load when the rope is triple layered, but with a smaller fleet angle by adding 2 false flanges to control the lateral rope travel on the drum.

Any ideas/solutions/reliable formula will be gratefully received.

 

[dhengr]

ECD40:
You can see it, but we can’t see it from here. So, some good sketches with sufficient details, dimensions, good proportions, controlling conditions etc. would really be helpful in your attempt for some help or ideas. I’m not sure I understand the meaning and arrangement of the “4 x 2 rope reeved drum,” and “the crane hook cannot be lowered through a lift well located at about half of the total required crane hook vertical travel distance.” That’s a pretty long drum and you have to know much more about it. Adding two “false flanges” may not improve the strength of the drum, from the welding and detailing standpoint. And, remember when you look at that drum you must add the bending stresses in the drum to the crushing stresses caused by the multiple layers of wire rope. From the earlier thread you know that multiple layers of wire rope increase the compressive stresses in the drum, at least up to about three or four layers deep. Also, when you bunch the wire rope up at a couple locations on that 20' long drum, you might likely increase the bending stresses in the drum; akin to applying most of the as a concentrated load near the center of the drum span. Is there someplace, or some way, in the system where you could add a ‘deflector block, or pivot block, or fleeting sheave’ to reduce the fleeting angle near the hook? But then, remember that you must keep the fleet angle at the drum within certain limits also.

 

[ECD40]

The attached 3D view shows the 4 x 2 rope reeving and the 18 feet rope travel across the drum as a single layer of rope in accordance with the CMAA specification. To limit the travel to 6 feet and 3 layers of rope would mean that the crane would no longer meet the CMAA specification.
Firstly, what can we do technically, to reduce the fleet angle and solve the problem of the getting the hook and ropes through the lift well and secondly, what is the relevance of having a CMAA specification crane at this stage (post-purchase).
To increase the 'stiffness' of the existing drum to resist the crushing stress from the extra layers of rope, maybe we could add a Lebus shell to the existing drum? Does anyone have experience in this type of retrofit?
This problem would not have occurred if we had bought a crane with a larger diameter (about 5 feet) drum with a single layer of rope when we purchased this crane. Such is hindsight!
I'll appreciate any thoughts and ideas.

 

[BrianE22]

I can't really address your problem but here are some other considerations. When you're off the first layer the cable will not wrap so nicely for subsequent wraps. It's tough on the cable. You'll probably have to inspect your cable more often. Also, with the not-so-nice spooling, one side will probably be at a different drum wrap radius for a short period of time.

 

[ECD40]

BRIANE22
Isn't what you are saying, exactly what happens with a winch with multi layers of rope as it spools back and forth the drum width? Is this a problem for the winch or rope life? We'll be hoisting the full 300 feet lift about twice a year for maintenance parts.

 

[MikeHalloran]

I'm assuming that the existing Lebus shell, or grooves in the drum, are arranged left/right, so that the hook trajectory is straight down as the hoist unreels with the crab stationary, but the fleet angle changes.

If, instead, the shell or grooves were arranged right/right or left/left, then the fleet angle of the wire would be constant as the hoist unreeled, but you would have to move the crab a little to make the hook trajectory perfectly vertical. I suppose you could program a PLC to coordinate the motion if you didn't want to rely on operator skill.

That may or may not be a major modification to the hoist.

Have you spoken to the hoist manufacturer about other possibilities?


Mike Halloran
Pembroke Pines, FL, USA

 

[dhengr]

ECD40:
I certainly wouldn’t be adverse to going directly to the real experts on this topic, don’t be so bashful, there are plenty of people who know more about the subject than you and I do. Go back to the original crane manufacturer, or to Lebus, those people are generally very willing to help and they have a lot more experience and many more good ideas than I do. Go to their engineering dept., not to some salesman who only wants to sell product. Call them, don’t do this through 200 e-mail exchanges, where they can’t ask questions to draw you out so easily. Go to them with complete details on the situation, so they have all the info. they need to make a judgement, and with knowledgeable (intelligent) questions and you have nothing to be embarrassed about in asking your questions. Also, be knowledgeable enough to know when someone is blowing smoke up your leg, or you don’t understand, and then ask a follow up question.

Like I said, “That’s a pretty long drum and you have to know much more about it.” The wire ropes are attached (dead ends) out at the ends of the drum, right? What kind of crane is this hook/drum on? How often do these lifts occur, how automaticly does this have to occur? Right after the hook block drops through the top of the lifting well, could you apply some temporary deflector sheaves, at essentially the width dimensions of the rope coming off the top of the hook block? This would be two sheaves (one for each wire rope from the drum) in one, essentially self equilibrating, block with two side plates. This might actually engage the lifting well edge to limit lateral movement. In fact, it might be permanently fixed to the edge of the lifting well if you use it twice a year.

What are the dimensions of the hook block and of the lifting well, and at what elev. is the well at in the total 300' lift? You said the lifting well starts at about mid-height, at about 150'; (8-9')/(2) rope travel on each side; tan^-1(4.5'/150') = 1.8̊ fleet angle. And, at 300'; 8-9' rope travel on each side; tan^-1(9'/300') = 1.8̊ fleet angle. That really seem to be threading the needle at the lifting well. Do you really mean “4 ropes x 5 t each” or do you mean 8 parts at 2.5 tons each at the hook block?

I can also imagine two new split drums which would attach to your existing drum, at about the 3rd points (20'/3) from each end of the drum. These new drums would take the drum compression from the wire rope wraps, and impart are fairly reasonable bending moment and torque to the existing drum. This would be something to speak with people like Lebus about. But, this might be fairly expensive and complicated, when you only do this for a short time, a couple times a year, and the crane works well otherwise.

 

[ECD40]

MikeHalloran:
There is no Lebus shell on the drum - this was just a thought of an addition to add stiffness. The existing drum grooves are cut both hands to keep the crane hook centered for the 300 feet lift. Even if the grooves were cut r-r or l-l, the distance between ropes would be about 9 feet, which is still wider than the existing lift well floor opening, which is only 6'-8" wide.
No - the crane manufacturer has not yet been involved in this alternative technical solution, as we have a contractual issue still to resolve.

 

[MikeHalloran]

All I can think of is a pair of big roller fairleads at, or just under, the lift well floor opening.


Mike Halloran
Pembroke Pines, FL, USA

 

[BrianE22]

BRIANE22
Isn't what you are saying, exactly what happens with a winch with multi layers of rope as it spools back and forth the drum width? Is this a problem for the winch or rope life? We'll be hoisting the full 300 feet lift about twice a year for maintenance parts.

Yes, cable life is shorter when compared to a hoist with a single layer.

Have you thought about building a drum on your drum - a split drum (bolt the two halves together) with a large enough diameter so that you have only one layer.

 

[ECD40]

All - The comments to date from all of you are very much appreciated.

Whilst technically, it looks like adding a Lebus shell to the existing drum could modify the fleet angle to get the ropes through the lifting well, there is one other item that is of concern. The current crane meets the CMAA specification with a single layer of rope on the drum (albeit the crane is not 'fit for purpose'). If this is changed to a multi-layer of ropes on a new drum (i.e., Lebus shell), we will no longer have a CMAA specification crane.
Does this matter, now that we have the crane installed?
Are there any codes (Federal or State) that you are aware of, that require that we have a CMAA specification crane? If we have the modified crane certified by a P.E., are we OK to operate it as a winch? This may or may not be a 'show-stopper'.

Eventually, the crane supplier will be required to participate in this resolution/modification, but not for the next month or so, when the dispute will reach a certain stage.

Thanks for spending your time helping me resolve this dilema.

 

[BrianE22]

I'd check with your local OSHA inspector.

 

[ECD40]

BrianE22

Thank you - I'll do that.

In a couple of months time, I'll post how this was finally resolved. In the meantime - thanks to you all for your contributions.