Originally Posted by NarayananNalinakshan

.......and saw the mast and counter weight booms buckling.

With some risk,we have immediately gave supports to the bucket wheel boom with wooden sleepers which were readily available.

The mast and counter weight boom totally collapsed. There was no damage to the slew deck.

With local contractors we had reconstructed the mast and counter weight boom and put back the original counter weight.

The Reclaimer was recommissioned with the original Oscillations.The machine is still working satisfactorily.(The Oscillation still persists).

Rgds,

Narayanan Nalinakshan.

Now there's an understatement if ever I heard one. I hope you regularly polish the medals they gave you!

This is a thread which should thoroughly engross many forum members. My own bob's worth follows:

Perhaps there was a mismatch in the resonance. I thoroughly examined a German design for a BWR around 1999 and was highly impressed to find that the boom had been stressed in both directions because the imposed self-weights and material loads act in opposite directions. Of course, being Central Europeans, they were absolutely correct. Your vibrations only worked in the downward direction which supports this long shot, I hope. While that might explain the source of the oscillations it does not explain the counterweight paraphernalia buckling. Ten percent reduction in counterweight would alter the hydraulic pressures considerably but I would not expect relieved counterweight supports to fail under less load.

Do you know if the 'exigencies' during commissioning and handover recognised the oscillations? Similarly, was the counterweight within the balancing pressure limit setting? If the counterweight was excessive the 'annuli on descent' could have raised the pressure to trip the relief valve; the counterweight pulled the boom up until the pressure drop allowed the cylinders to progress to the next blow-off. This latter is a more likely reason for the oscillations. Now, when the remedial contractor reduced the counterweight he increased the boom moment which stopped the annuli pulsations and on the flip side sent the crown pressure soaring. Each time up-luffing finished there would have been a greater dynamic unloading within the counterweight supports which might have accelerated buckling although we'd need to see the actual drawing because this failure mode is another long shot. ■

Dear Narayanan Nalinakshan,

The reason that the bucket wheel boom oscillated while lowering, is that the over centre valve was not properly adjusted or too small.

The boom was lowering faster than the hydraulic oil supply could deliver.

Increasing the counter pressure of the over centre valve while lowering should solve the problem.

Also, if the cause is resonance oscillation, the remedy is to consider the boom as a mass-spring system.

The spring part is the stiffness of boom and the hydraulic system.

The resonance behavior can be eliminated by proper dampening.

A matter of a 5 mm throttle piece in the over centre valve or the hydraulic proportional control valve.

Keep in mind that oscillations also cause significant additional stresses to a structure, causing fatigue failures.

Changing the counterweight also changes structural stresses and certainly increases the hydraulic pressures while luffing.

Increased hydraulic pressures increase the material stresses, especially at the connecting point.

Not an exceptional case.

Take care ■

It is not often we get a thread of this quality and I certainly don't mind.

I'm no spring chicken myself and really appreciate the mental activity coming through the better threads.

As you say, it is all about active discussion and understanding. Sometimes the thread starter has, with a bit of over anxiety, answered his own question which is understandable since youth is often unable to push a problem towards those who created it in the first place. We all, young and older, need more of these threads. ■

I don't believe that I can add to the expertise shared by Mr. Gately and Mr. Teus but I can describe my similar experience though not to the extent of the structural failure that you experienced.

As a young engineer (within my first three years) my first field assignment was the commissioning of a bucket wheel stacker/reclaimer. Hydraulically luffed by twin cylinder the stacker reclaimer was counter-weighted for all of the dead load and 40% of the live load as this was typical. I too experienced a bouncing boom when luffing up. This was in the winter in Iowa, USA. I was instructed through various possible remedies including bleeding the hydraulics in hopes of eliminating any air in the system. Ultimately it was determined that a velocity fuse would lock up and release, causing the bouncing. The cure included a complete redo of the hydraulics plumbing including larger piping and hoses that allowed higher flow rates, thus more free flow.

Joe Dos Santos ■

Dear Joe,

Boom oscillations induce oscillating stresses, that can cause fatigue failures in the structure.

In case of an oscillating movement of a heavy boom, one should never wait until the imminent failure occurs as Mr. Narayanan Nalinakshan obviously did and still does after recommission.

Knowing the problem and the risks, the responsibility and accountability needs to be clarified in advance.

Have a nice day. ■

Hello,

This does not address the issue being discussed directly, but could be of interest as a collateral information in context of luffing system.

Referring to the article by Werner Fletchner - PHB Germany in TransTech publication, the gist of pertaining information is as below:

1) Rope winches are preferred mechanism in luffing system for bucket wheel excavators and also for very large boom stacker and reclaimer machines. The winch system has two independent ropes. These have adequate safety factor so that even during failure of one, the other will take care of load at reduced safety factor, during shutting down and repair.

2) The present day practice is for hydraulic luffing system for stacker and reclaimer (generally for most of the machines). The chances for boom luffing vibrations are mentioned as below:

- Less chance during boom raising.

- More chance during boom lowering.

- Still more chances during boom direction reversal.

As from my side:

In case of rope system; the force being acting is quite away from boom hinge and hence it is less prone to vibration.

In case of hydraulic system; the force being acting is quite near to hinge point and any oscillation in hydraulic system will get magnified many times at boom end.

Regarding general rules about vibration:

The large amplitude of vibration implies that the system is springy (less stiffness). In this case vibration frequency will be less but amplitude will be more and it can become visible.

The system having adequate / more stiffness will have vibration frequency more but amplitude will be small, and may not be perceptible. The stiffness is an engineering quantity force per unit displacement.

Ishwar G. Mulani

Author of Book: Engineering Science And Application Design For Belt Conveyors (new print November, 2012)

Author of Book: Belt Feeder Design And Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■

The problem described was already clearly identified as a(n) hydraulic phenomenon. Rope winches are the norm in crane applications and can often be used when a suitable mast is built into the design. When the machine is not primarily hoisting then most companies, especially PHB or whatever it is called these days, opt for the lighter hydraulic luffing rams. Mast construction calls for a lot more steel and shed clearance. Moreover the active length of rope in a luffing hoist is very short which leads to greater hardening of the wires which in turn shortens the life of the ropes which is why 2 ropes are necessary. Did PHB mention that? Ropes are extremely dangerous. My dad nearly lost his sight through whiplash from an underground rope so maybe I'm biased there: but ropes are still very dangerous!

I am now wondering if any fledgling manufacturer has explored using 4 rams, 2 opposing arrangements on each side. That way there would be equal areas on either side during up and down travel to eliminate the crown and annulus differential. What does the team think? ■

Hello,

Referring to my earlier reply, the excavators mentioned therein (as a part of article) are in context of the subject that is bucket wheel on boom large size excavators. It does not pertain to other different type of excavators.

It is clarified that there are also bucket wheel on boom excavators having hydraulic luffing for certain types and sizes.

The interested readers can refer to the concerned manufacturers' information on their website, leaflet, etc.

Regards,

Ishwar G. Mulani

Author of Book: Engineering Science And Application Design For Belt Conveyors (new print November, 2012)

Author of Book: Belt Feeder Design And Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■

In the TransTech book on Bucket wheel reclaimers there was often mention of German constructors charging the pipe structures with nitrogen and connecting to pressure gauges (the days before transducers). Operators or maintenance staff could detect a weld, etc. failure by the pressure drop. I never came across this system but I admired, as usual, the outright Teutonic professionalism. Is the method still used and has it been adopted to give alarm signals from transducers? Because of the large volumes in the structural piping the weld would probably need coding but considering the price of such machines that would be a small price to pay for reducing the fatigue failures mentioned. But it is probable too rich for the FEM committee's blood. ■