Originally Posted by sganesh

Thanks a lot Mr.Lyle Brown.

Reverse running of conveyors have occurred many times and seen by me personally. But explosion of the fluid couplings has happened for the first time. There could be more chance that forward direction by motor ( as command could have been given from remote unknowingly ) and reverse running of conveyor happened at same time.

This is also evident from the case 1 in your attached link. But not in others.

Also few more questions :

1. How ratio of gearboxes can change if they are of same model and from same company?

2. If gearboxes are installed in different pulleys, how can we be sure that all the hold backs will have same load? It is probably that the first gearbox at the highest elevation and nearest to the discharge point will be subjected to more load.

3. Why hold backs are not recommended with in gearbox?

There could be more questions, which need to be analysed. Doubt on "overfilling of the fluid coupling" was eliminated by checking the stand by drive. It was found OK. In the attached link also nothing is mentioned about over filling.

Thanks a lot to Mr.Roland also for participating.

Regards.

Dear Ganesh,

This is an interesting incident.I have been ussing fluid coupling in steep conveyors for many years,never had any blasting other than fusible plug failure.

In all our conveyors the hold back is in built with the gear box.In slopy conveyor normally brake is not fitted.

Only reason I can conceive is starting the conveyor when it is moving backwards with a high moment of inertia due to load on the belt and failure of hold back.

There is no other reason for motor failure along with the fluid coupling blast.

Narayanan Nalinakshan ■

I find it disturbing, and in consequence not surprising that there has been no input to this thread from any fluid coupling supplier.

If the fusible plug did not alleviate the system then there is a fault either with the plug, its selection or specification. The failed component needs to be salvaged and presented to the Vendor for his comment.

From the failure described it is probable the motor shafts are jammed rather than seized. Be very careful when releasing them.

In 1972 I had multiple failures of clutch pressure plates. These were suffering overspeed flywheel explosion when installed on rotary engineed cars. The Japanese principal told me not to worry: they were having identical problems.

Recently a pump Vendor assured me about his casing pressure ratings. Some months later and on a different continent I discussed radiographic examination for pump castings with the Vendors home works. There is no NDT at all. A blow hole can sit undetected waiting for pressure cycling and celerity to do the business. Think Mazda and Warman.

With capital equipment we are at the mercy of capital. The level of risk is comparable to the old cliche Aeronautical, Mechanical, Civil and what measures they employ to place a bolted fastener. Unfortunately nowadays the Civil method makes far too many inroads into Mechanical expectations. ■

Who then, in your opinion, is the responding fluid coupling supplier?

Yet further: regardless of rapid heating, you say you have seen the conveyors running backwards on previous occasions. Why didn't you do something about it then? ■

On occasion the fluid coupling bearings fail causing the rotor to engage the impeller vanes which results in internal disintegration.

If the holdback cannot hold the reverse belt's load and starts the coupling's runner (reducer side fed from the reverse belt motion) to run at a higher relative speed to the impeller, the fluid coupling will generate a higher relative torque transmission against a motor still engaged. The motor will go into maximum pullout torque. This, in turn, will cause the coupling to go into torque overload and may happen before the fusible plug can melt. ■

Originally Posted by sganesh

Thanks a lot Mr.Lyle Brown.

Reverse running of conveyors have occurred many times and seen by me personally. But explosion of the fluid couplings has happened for the first time. There could be more chance that forward direction by motor ( as command could have been given from remote unknowingly ) and reverse running of conveyor happened at same time.

This is also evident from the case 1 in your attached link. But not in others.

Also few more questions :

1. How ratio of gearboxes can change if they are of same model and from same company?

2. If gearboxes are installed in different pulleys, how can we be sure that all the hold backs will have same load? It is probably that the first gearbox at the highest elevation and nearest to the discharge point will be subjected to more load.

3. Why hold backs are not recommended with in gearbox?

There could be more questions, which need to be analysed. Doubt on "overfilling of the fluid coupling" was eliminated by checking the stand by drive. It was found OK. In the attached link also nothing is mentioned about over filling.

Thanks a lot to Mr.Roland also for participating.

Regards.

Dear Ganesh,

I have been using gear boxes with built in hold back for steep conveyors since 1980,prior to this the hold back was fitted on the extended intermediate shaft of the gear box.Few gear boxes of 900 KW,manufactured as late as2008,also is running with inbuilt hold backs.I have not come across such a problem.In my opinion the topmost hold back shold be designed to hold the belt from reverse movement.A correct analysis can be made only after seeing the layout of this conveyor,with profile.

Narayanan Nalinakshan. ■

Hi

Sorry I'm no Fluid Coupling expert, though I have like others come across dramatic failures like yours. When you get to looking to replace the drives have a thought for an alternative, look at www.magnadrive.com this company product replaces many fluid couplings.

5 years ago we replaced a fluid coupling in the same circumstance.

Regards

Frank ■

Conveyor Dynamics, Inc. (CDI) have made extensive dynamic analysis soft-start studies of the Magna-Drive systems on behalf of a major equipment supplier in Brazil. A recent CDI presentation to many major mining companies, which either were considering or were specifing use of the Magna-Drive, have altered their collective company views. This has led to canceling ongoing orders until a thorough independent study is concluded.

CDI claims, some Magna-Drive systems may be configured to improve "soft-start" mechanics versus across-the-line starting (DOL). It is not clear that Magna-Drive claims are "all valid" and may mislead users into undesirable choices.

CDI has invited Magna-Drive to our office to demonstrate our evidence and collaborate with their engineers. To date, weeks have passed and they have not made the 100 mile trip. CDI did evaluate their early claims from more than 5 years ago. Since CDI wishes to support the end users, we have everything to gain and nothing to lose in endorsing any good or improved soft-start product.

Our door is open. We are willing to discuss our findings with all interested parties.

The holdback issue seems obvious and fixable. Some of the facts may also be a little vague and need further clarification. ■

As already mentioned, a dodgy application of any equally qualified bit of kit will give the same, or similar bother. This thread is surely now exhausted when the conversation refers to rare earth metal prices in relation to somebody not replacing a holdback after they'd seen it fail. Give us a break! ■

Dear Sganesh,

You historical record leaves many facts unclear. If the drive was in a shutdown mode, the motor would have been turned off. If not turned off, then I would assume the power would be sufficient to keep the conveyor running until the operator shut the drive off. If the drive was still engaged until the overload protection would have cut power, then under all three scenarios the motor would be off.

Given the motor is turned off, in order for the holdback to engage, the system was coasting to a stop without power. Then we reach zero speed at the drive and the gravity force of the incline load would accert itself and begin a reverse drive action. If no holdback engagement or failed engagement, then we see an acceleration of the conveyor driving the fluid coupling in reverse with a retarding load of the motor being accelerated.

The coupling driving mechanism is motor inertia against an accelerating conveyor load in reverse. Big question, if the load was forcing the fluid coupling into reverse, how long did it take to reach critical overspeed? A second question is where did the reverse loaded belt ore body go? Belts can double their speed in less than 10 seconds i.e. can run in reverse to full speed in less than 10 seconds and 2 x speed in <20 seconds, or 3 x time speed in <30 seconds. In all cases, the material on the belt is also flowing in the reverse direction for 10 seconds onward. What happened to the ore? What happened to the manufacturers specification of super critical speed. The fluid dynamics by itself would not be likely to make the blast effect. More likely it would be a critical overspeed "REED FREQUENCY" problem, enhanced by the imbalance between runner and impeller with fluid contribution.

Anyway these are all conjecture. We have insufficient information to conclude cause and effect. Let this be enough. ■

Your question seems to have left a lively debate. I'm sorry I did not reply to your question some days ago, I must have forgot to tick to get email updates. We are based in UK so our experience with MagnaDrive is not worldwide. We are rotating engineers and UK distributors for MagnaDrive. The best place to get case studies is from MagnaDrive direct www.magnadrive.com . They will also put you in contact for the closest agent to you.

MagnaDrive will not solve your problem, I agree with the postings you need to address and guard against your reversing problem.

I made my posting because once you have resolved your reversing issue and if you are not confident with fluid couplings there is an alternative. In our experience MagnaDrive is a viable product. We have replaced many fluid couplings. These units still in operation after 4, 6, 8 years with no issues and no maintenance requirements and no inventory. There are many thousands MagnaDrive units in operation not every one have issues with them. We look at every application on its own merits and do decline MagnaDrive as the best solution. You need to make the choice whether it is right for you. Sorry Sganesh I've taken your posting to respond to other MagnaDrive comments.

Regards

Frank ■

Thanks for the paper adress.

I was always had restrictions against the use of fluid couplings at descedent conveyors, but now I am feared in to use it also in other cases.

Thank you.

Alexandre ■

Originally Posted by sganesh

Dear all,

Hopefully, we have solved this problem. That means this problem should not occur in other conveyors by providing two simple mechanisms.

1. We have provided long belt pieces nearer to suitable pulley where we like to face such problems. During normal forward running, this long belt piece will be riding on the conveyor. If hold back fails, it will go reverse and wedge between the pulley and the conveyor belt. It will act as a brake and avoid the conveyor to go in reverse.

Careful analysis is required for effective working of this arrangement. THIS CAN BE ONLY A STOP-GAP ARRANGEMENT IN THE ABSENCE OF ORIGINAL HOLD BACKS. PLEASE KEEP ALWAYS THE ORIGINAL HOLDBACKS AS SPARES.

2. In addition to normal zero speed switch, we have provided addtional proxy switch in another pulley of the same conveyor.

If the motor is stopped, the holdback should hold the conveyor. If holdback is failed, it will allow the conveyor to go in reverse. Then this switch will sense the reverse running of the conveyor. It will send the signal to PLC and will not allow the motor to be started again. So no forward & reverse running of the vanes in the fluid couplings at the same time. So possible blasting would be avoided.

Hope the above suggestions may work for you also.

Regards to all,

Hi All, at the risk of kicking this off again I have just read the posts with interest. The final solution and some of the discussion still seems to have missed the point. The detail is there but not the conclusion. I have no connection with FC suppliers, but they are a good part of a drive solution if (1) integrated and (2) operated and (3) maintained correctly. I have seen some interesting FC issues over the years and they always result from a lack of at least one of these three issues. I haven't actually seen an FC fail per se but accept that this is possible. Nothing lasts forever, and as I was quoted many years ago by a supplier "this has a lifetime guarantee - when it fails it has reached the end of it's life"

That this conveyor was deliberately operated with a failed holdback; and apparently no thought given to consequence is interesting.

That all three drives failed in the same way at the same through hand grenading time surely is an indication of overspeed, and it doesn't matter whether the motor was trying to pull away or not, and I can't see much relevance over hot oil and whether the fusible link operated or not. As stated the failure would have happened with no oil. There is not even any conclusive evidence that it was the oversped coupling that was the cause, as the conveyor reverse operation would also overspeed the gearbox and the motor I believe. The description of the damage seems to confirm this.

That some drives fail on running back and some don't is probably due to gravity. If the load on the belt and the belt length and inclination are sufficient then gravity will accelerate the belt backwards for sufficient time to get an overspeed. A shorter belt with a low angle may not develop sufficient accelerating force for the problem to occur.

The makeshift holdbacks described above may actually work, but I would be extremely wary of initiating a fire through friction between the normally running system. That is if I understand the description correctly.

Having an interlock to stop the motor starting when the conveyor is running back may have a use, but will not prevent a repeat of overspeed, and different couplings may change the speed at which they fail but none will be unable to resist at the particular critical speed. The gearbox and motor will still have the same critical speed anyway.

I would be looking at installing duplicate holdbacks or brakes and making sure that they are kept in good condition. I have seen a couple of failed holdbacks but these had never been checked or maintained.

Finally, I have looked at some other overspeed issues over the years all gravity driven. The effects are always impressive - gravity is an incredible force and we rarely design for the effect of overspeed. The history of the lead up to the effects is always impossible to pin down, especially if there is a chance that someone will get blamed. ■

Hello,

I spotted your post whilst searching the net for something else. Your problem is not unique and we have seen the same issues at other plants.

We solve this issue at what was a Corus Steel Works by replacing the Fluid coupling with an alternative soft start coupling that was five years ago.

The replacement soft starting coupling contains no oil, needs no maintenance, has no touching parts, can accept high levels of unavoidable misalignment, has an extremely long service life beyond that of the conveyor and can withstand the high rotational speeds associated with this type of run back.

Whilst I accept there are other was of mechanically or electrically protecting the system they do not address the fundamental issue that fluid couplings turning at high speed and beyond there capability can be dangerous.

Is see that yet another fluid coupling exploded recently at a UK quarry causing severe damage to surrounding equipment but without any personal injury.

In my opinion not is just a matter of time before we see someone injured or worse.

Please let me know if you would like more information on the fluid coupling alternative?

Regards, Paul. ■

I come to this post obviously very late. I wish that I had seen it much earlier as I have experience with such an occurrence. In 2005 the 5000 HP slope conveyor at Oak Grove coal mine, in Alabama, suffered a failure of the two high speed backstops. The loaded belt ran away backwards until it finally stopped due to jamming of the accumulated material at the bottom of the slope. Much damage was done including explosion of the fluid couplings due to centripetal acceleration. The shrapnel perforated the walls of the head house. Thankfully there was no one there so there were no injuries. The backstops were of the centripetal type and disengaged at any speed above 90 RPM (where the centripetal acceleration exceeded gravity). As the backstops were not damaged we concluded that they just failed to engaged. With the motor off it offered no regenerative resistance. We simulated the back-running for the known travel distance before the conveyor came to a stop due to jamming. The backward speed attained corresponded to centripetal forces at the couplings that exceeded the ultimate strength of the coupling material, thus the centripetal explosion at both couplings. Indeed they served as protection for the motors which disengaged when the couplings exploded, otherwise they too would have exploded as the backward speed increased further. The fusible plugs could not save the couplings as they melt away due to heat which is caused by slip, There was no reason to expect the slip that would cause the heat to trigger the fusible plugs.

Because the backstops were not damaged we knew that they merely failed to engage. Though this could be due to local hang-up say due to dirt, this was highly unlikely because there was no evidence of dirt ingress and because they both failed to engage (what is the probability of that). Thus we concluded that a shock wave through the belt caused a speed reversal from 90 RPM to -90 RPM within a time lapse so short that the pins did not have the time to drop. Our working model simulations revealed a reversal time required that could not be explained by the calculated deceleration of the loaded conveyor, hence the shock wave theory by default.

I welcome comments of the forum participants.

Joe Dos Santos ■

Joe,

Thanks for describing the mystery. Perhaps there was a localised carcass failure which might produce a shock wave. I am surmising there.

A few months back there was a thread covering arrestors for snapped belts. At the time the device seemed rather impractical but in the light of this thread it assumes a lot more credence. As I recall, belt wide nipping jaws slid over the return strand in normal running and if the belt broke the jaws bit the backrunning belt into standstill. If the salesman changed his pitch he might be onto a winner. ■

Luis,

The belting was high strength steel cord. Even after the run-back and jamming the belting was not badly damaged. The idea of an elastic shock wave was by deduction because of the forensics. The backstops were not damaged they just never engaged. To put it into perspective the backstops were of the high speed type turning at 1190 RPM while the conveyor ran. The pins that dropped into the slots, to engage, separated centripetally at the relatively low speed of 90 RPM, only 7.6% of running speed. Thus the window between being centripetal while slowing down (until 7.6% of running speed) and again becoming centripetal, when running backwards, is relatively small.

I would love to hear from some of the other experts on this. Has anyone else experienced such a run-back failure when the backstops were not damaged but just failed to engage, i.e. the pins did not drop into the slots.

Joe Dos Santos ■