Hydraulic Cylinder - Buckling

Sorry can't help, but WOW that's impressive!

But I don't suppose you were too impressed.

Hi Keverne,

Unable to see details on drawing but from photo's, the piston rods look far too small for the size of the Jib and there would need to be stop tubes in the cylinders to increase the bearing length when fully extended. More details needed. Weight and length of jib cylinder dimensions, hydraulic pressure etc.

Best regards,

John

I'm not sure about the presence of stop tubes. Apart from the listed info, what else is needed? I try to get the info.

This is speculation, but I wonder if the outer ends of the cylinders touched the lower frame of the jib. In the drawing, they look very close. If so, this might have placed a slight bend in the rods, in the direction of the subsequent failure (which appears to be from bending as the jib was lowered.

I agree that the rods appear to be much too slender. If the drawing is to scale, the rods in the drawing look better.

No, the end of the cylinders did not touch the jib. Yes, I do agree the diameter of the piston rod seem inadequate to support the load as well. However I have a concern as to whether why the piston rods could not be further extended at 70 degrees, could it be due to side loading or simply it just can support the load?

Hi keverne,

It most probably failed at 70º as just below that was the maximum total length of the cylinder from pivot to pivot that could support the weight, hence the catastrophic failure.

Best regards,

John

Hmmm, then does this mean it can be conclusive that the hydraulic cylinder had the wrong specs?

1. Do you know the magnitude of the load the cylinders are lifting?
   
2. Do you know the internal design of the cylinder? Is it hydrostatic?
3. The weight of the cylinder itself will result in a side load while the cylinder is horizontal. The effect of the weight is reduced as the cylinder approaches vertical, however as the cylinder extends the internal bearing features (hydrostatic bearings, piston rings or whatever) get closer together and are thus less able to resist a side load.
   

1. The bore of the hydraulic cylinder 125mm, rod 65mm, and stroke 3000mm. If not mistaken the lifting load is approximately 7 tonnes based on the information provided.

2. Not sure about the internal design, perhaps its incompressible oil.

3. Could the internal bearing features become damage following the buckling? If not, maybe dismantling it could give a clearer indication?

A few more facts might help.

What is the Hydraulic Pressure?

What is the cylinder size?

What is the rod size?

What is the load?

Stroke?

Do you know where the center of gravity is for the frame?

hydraulic pressure ~2000psi

hydraulic cylinder: bore 125mm, rod 65mm, stroke 3000mm

load: ~7 tons

i don't know the centre gravity for the frame

There are lots of questions here, but at 1st thought, your geometry is off on your extension system. This also leads to the smaller rod diameter bending much easier. As other have said without a lot more information it is very hard to work up a solution to your issue here. Seek a good engineering firm to help assess the problem and give you needed answers.

Thanks for the input. May I ask how to determine the geometry is off?

I have designed large aircraft hanger doors that work off hydraulic cylinders in a tight configuration before. It takes quite a bit of mathematical equations to come up with the proper geometry along with the proper cylinder rod, cylinders and mount designs capable of the loads required. If you have to ask where to begin, you need to seek an on site engineering firm to help you in your design. That is the best I can offer you. Trying to engineer something over the internet is not something I would want to have any liability with.

The cylinder rods may be adequate as it did get it to 70 degree angle at that point most of the load would be on the pivot. What about the external forces that may have been applied like the wind? There is also the weight that is above the tie point of the rod of cylinder. Was it taken in to consideration as the mast reaches the point where most of the load is down on the pivot the upper part of the mast is going to want to keep on traveling due to momentum. Which could cause the upper part of the mast to sway that could create forces beyon the limits of the cylinder.

It is not reported that there was strong winds. However, at 70 degree, the piston rods could not be further extended and it was reported there was slight jerking (up & down motion) before the piston rods started to bend. Buckling ensued when the assembly was brought down and resulted in incomplete retraction before the barrel of the cylinder slowly drop down to the ground with the 7 ton load of the assembly.

Angles are off, cylinder not specd' properly.

Need to completely re-evaluate the design, and have a competent mechanical engineer look over the work.

In short, It was a combination of poor initial design, when choosing the angles, and very poor selection of the cylinders using an appropriate safety margin. Your cylinders should be able to push up two of those bad-boys before they buckle if they were chosen correctly.

People often give me a hard time when I over-design things, they worry about the added cost... this is a perfect example of where that little added cost on the front end will pay for itself when catastrophic failures don't end up happening.

NEVER design ANYTHING that is not safe enough to bet your own life on. Glad no one was hurt...

Thank you for the input. I am a layman here and how to know if the angles are off?

"I am a layman here and how to know if the angles are off?"

Many Mechanical Engineers have gone through many years of schooling to know this information.

Is there a reason why the cylinders are not coupled together at the piston rod end?

Seems to me that if you are lifting 7 tons, you would want to make sure that there will be no latteral movement on the piston rod end and that they act together. I can vision 1 cylinder extending and wanting to move outward, as that happens the effects on the other will pull it and then you have both cylinder rods shaking opposite creating a harmonic. Once this begins, you can not stop it. You did mention that some said that the cylinders "chattered" as what I call it. It is a very poor design.

Awesome pictures though. I have never seen a rod bend like that. Amazing.

Best to get the engineer and all who specified the equipment on the scene to hash it out and make it right.

Thanks for the input.

Hi keverne,

From application details given, I checked a cylinder manufacturers catalogue I have, on a range of cylinders, similar in construction to those in your application, with a view to doing a quick check on the "Rod Strut Effect". The correct rod strut length figure to use, is dependant on the mounting style. For a cylinder with a rear pivot mount and a piston rod clevis the strut length "L" is equal to "2S" where "S" is the length from pivot to pivot, with the cylinder fully extended. With your cylinder stroke of 3000mm "L" will be 6000mm, + the zero stroke basic dimensions of the cylinder (likely to be about 500 to 700mm). This will give a "2S" of approximately 13000mm!!!!!

At 2000psi, the cylinder will develop a maximum thrust of approx. 38000 lbs., (it will, of course, only develop that pressure required to raise the jib from the worst condition, which is from the horizontal).

I can only give figures from the chart in the catalogue and without knowing the mechanics of the jib, I have assumed the worst case senario of full thrust being required (unlikely).

The Rod Strut Effect chart, maximum corrected strut lenght "L" is 4000mm (we have about 13000mm) and goes off the chart on rod diameter required (being greater than 110mm). As another option, I checked the maximum stroke and thrust, a 110mm rod would allow and it was 4000mm stroke at 20,000 lbs thrust.

The details above don't realy solve your problem but I hope it gives you some idea of how far you might have to go to put things right.

By the way, I don't think the mechanics of the jib are necessesarily wrong. As long as the pivots are free to move, throughout the cylinders stroke, there should not be a problem, BUT the mechanics might mean the cylinders have to be far stronger to cope with the forces involved.

All the best in your endevours to solve your problem,

John

Thank you alot for the extensive input. As I am a layman in this field, and if I get your drift; are you suggesting the geometry of the design might not be necessarily flawed but somehow it was equipped with cylinder jacks having the wrong diameter hence could not support the lifting load (exceeding critical load)? Kindly correct me if I have misinterpreted the information provided.

Really appreciate your kind patience and interest in providing the necessary details to the posted enquiry.

Hi keverne,

Basically yes.

The important thing with cylinders, hydraulic or pneumatic, is to isolate them from side load by ensuring the mountings are correctly selected. Also that the cylinder is sized correctly in bore and particularly with hydraulic cylinders, piston rod diameter and the "Rod Strut Effect" is checked.

From the photos the hydraulics look "small" when compared to the Jib. An old saying "if it looks right it probably is right" is still valid. But still do the calculations.

The jib weight of 7 tons (15680 lbs.) means little, if the mechanics of the application give a mechanical disadvantage that could multiply it by several times.

The cylinders will most probably also need stop tubes fitted to increase the bearing length when fully outstroked.

Best regards,

John

Thank you very much John. In fact I am also concern as to whether the failure could be due to side loading resulting from misalignment of the piston rods. But based on most views I received from the forum thread; all condemned the diameter of the hydraulic cylinders being not suitable for the application. So indirectly, can I safely rule out side loading perhaps since the diameter of the piston rods was not suitable in the first place?

Hi keverne,

"So indirectly, can I safely rule out side loading"

I think that the catastrophic failure of the rods could be looked upon to be a side loading failure because, when the extended length of the cylinders could not support the weight of the jib they buckled (bent) at the junction of the piston rod and the piston rod gland on the front cover of the cylinder but the failure can be attributed to the too light construction of the cylinders and not to the mechanics of the jib. I would however, suggest that the new bigger cylinders that will be needed, require the mountings of the cylinders to be much heavier and stronger than the existing units which will be a good thing.

Have a good weekend

John

Thanks a lot for the opinion and technical information. Really appreciate your help. :)

The current geometry will require a massive bore rod. Slight changes in your geometry can yield much lower forces, and therefore smaller dia rods. This is the job of a Mechanical Engineer.

Thank you very much for the input.

I think I could have an explanation but for a succinct analysis of it I would need the dimensions you indicated in the picture but who are impossible to read. Could you please try to write them with bigger characters on the drawing ? I would also like to know the cylinder length in the full collapsed position: i.e. with piston rod totally inside. Which is the full stroke from afore mentioned position to the one with the piston rod totally outside (from end to end of stroke)?

Unfortunately I do not have the permission to reveal the dimensions from the client's design. I also do not know the cylinder length in the full collapsed position.

GA

I do have difficulty to 'digest' what you have posted. But from my understanding, you have recommended a piston rod with diameter 80 or bigger for the design; and unfortunately for my case a piston rod with diameter 65 was used. Obviously, it was a wrong choice of diameter to be used.

So when you opined that it is a simple design problem, are you suggesting the construction of the assembly as well or just the choice of the size of hydraulic ram?

Btw, thanks a lot for the extensive amount of information. Really appreciate your interest and time spent in the discussion.

It seems to me that you would be better off with a multi-stage cylinder, such as the type used in dump trucks. One advantage of this style is that the larger piston is at the bottom of the arrangement, providing the most capacity where it is needed, along with a slower hoist speed. As the smaller stages engage, the load is decreasing and the speed increases. You could get by with one such assembly, mounted in appropriately-designed trunnions at each end.

It also seems that the cylinder alignment in your diagram is too near parallel with the truss frame being raised when it is fully erect, which could result in stresses that are not readily evident, particularly if there are any misalignment issues between the two cylinders.

Thank you very much for the insight.

Were the cylinders properly purged so that there was no air in the cylinders?

Especially after on the full extension, then retracts, it looks like the pressure is on the rod side to start to bring it down, After the crane moment of enertia is transfered over to the cylinders then the cylinder is bled on the cylinder side.

May need to do a force analysis on the motion

I am not sure about this and no one can verify this. As a matter of fact, it was a new installation failure on testing. Does new installation require purging on the cylinders?

I'm a little gob smacked that the cylinder supplier did not say "these cylinders will not support their own weight when extended"

Gravity and way too smaller rod diameter caused this - along with that Euler fellow as Nick pointed out {though I'd say 88 mm is still way under}

You need a cylinder with a tubular rod [less mass] near the bore diameter, having just enough 'pull' to start the raised truss 'falling' - assuming wind, or some such, is against a straight fall.

You also need extension limiters so lateral thrust is within the capacity of load rings.

My advice is to find a supplier of cylinders who has some idea of what they are doing.

In this geometry, as the cylinder approaches its full extension, the jib will swing quite a bit with only small cylinder movement. I would worry about inertia, wind loads, and play in the cylinder attachments. And I still would suspect that the cylinder could have contacted the jib, without being noticed if no one was looking at the the zone of close approach when it happened. The dimensions in the drawing are hard to read, so these comments remain uncertain.

I do agree there maybe a possibility of what you have describe but unfortunately that cannot be verified because the assembly has been fixed.

Fixed how? And how will the the second attempt fare?

This whole thread suffers from a dearth of basic information such as weights being lifted and their locations, structural steel dimensions, etc. Everybody is guessing in the dark, though nick name's guesses are among the best.

When the jib is horizontal, the cylinder looks like about 25°; when the jib is vertical, the cylinder looks like about 93°. If so, this would be a swing of 68°; 70° has been mentioned; is that what the 70° is meant to describe? Etc., etc.

I'm sorry this annoys you but that's all I am provided with also. And thanks for the views but I get that most have opined that wrong diameter or perhaps wrong choice of hydraulic system was used so that's more than enough for me given the limited data at hand.