Dear Sunil..

The forward tilt on the wing rolls helps the belt to centralize in the curve, and not run off to the inside of the horizontal curve.

If the belt wanders off centre, the additional belt weight on the wing roll will drive the belt back again.

If the belt is pulled in backwards, the opposite will happen.

The belt will wander off centre, and the wing rolls will de-train the belt further until the belt is on the road next to the conveyor.

If the idler frames are super-elevated however (which they should be), this effect will be significantly reduced.

The lower the speed, the longer it takes to end up in the road.

You have to pull the belt in along the direction of travel for both carry and return strands.

Good luck

LSL Tekpro ■

Hi Sunil Prabhakar

The designer may be requesting this so the belting is pulled from the correct direction of travel as it has been manufactured.

Steel cord must be pulled on with the manufacture Stamp / Brand on the same side of the system or there will most likely be tracking problems. ■

Not so Bruce..

As long as the top cover is towards the burden the belt carries, then the belt can be installed either way round, and there will not be any untoward tracking problems.

There is nothing special about the construction of a belt to be installed on a horizontally curved conveyor. You just have to pull it in off the roll, if at all possible in the direction of belt travel so it sits down nicely, with top side up.

LSL Tekpro ■

Some Points:

1. The belt should be pulled in the direction of intended operation due to the banking angle being adjusted for tension changes which favor the particular direction of travel (ie carry & return)

2. You can pull in the opposite direction if the banking angle does not vary within the particular curve of interest, however the belt will reseat itself once it goes into operation. Assume there are two differing banking angles at opposite ends of the curve arc. What do you think will happen when the belt is strung in reverse?It will not behave as the designer intended until the tensions are adjusted per operation. Given significant banking angle changes, you may wish to reconsider.

3. I disagree with Graham on the issue of belt installation direct and fabrication direction. CDI has witnessed a number of belts with a bias to track left or right depending on fabrication and installation direction. It must be consistent between both. By example, we obsereved a 150mm tracking error, due to the above error, during the initial Channar 20km installation. The rolls were marked from the factory with the fabrication direction. Still the installer placed 3 600m rolls in the reverse direction at differing locations. After measureing the tracking with instruments, it was clear to be a finstallation direction problem. Also, the mfr. had changed his practice and made two widths in the same pressing contrary to the contract for 69 of the 77 rolls. We had to cut the three rol sets out and reverse them to control the hugh error in tracking. Once changed, the error was within our specifications. It was also noted that a single press construction was superior to th two press construction. by about +/- 30mm. WIth the single wide belt construction, by the eight roll group the fab tracking error was down to +/- 8mm. With the two wide set the error was about +/-40mm. These errors are exclusive of splice errors and wire cord anamolies.

4. Is this the Lafarge 17 km overland? ■

Hi Graham

That my be so for Fabric (debatable) but for Steelcord it is critical that all are installed with the brand on the same side.

They can be pulled in the opposite direction if the rolls have been wound accordingly.

I guessing the system mentioned is the

Details from http://www.conveyor-belt-guide.com/Extremes.htm ■

Bruce & Graham,

Please don't confuse the troops.

I believe if you take the referenced downhill Lafarge conveyor and its curve assemblies and pay attention to the buildup of belt line friction and gravity forces, you will realize that you should not pull the belt helter-skelter without regard to its travel direction. It must be pulled per the direction of travel.

The carry side moves downhill, with material flow, the belt tensions have additive friction and negative gravity components, and have very different banking angles and lateral behavior than if the same belt had to move uphill adding friction and gravity. Same is true of return side.

We did a design for this system to Lafarge about 2 years ago. ■

Hi Lawrence Nordell

Who’s confusing who ?

Who mentioned the Lafarge system ?

Who mentioned “belt helter-skelter without regard to its travel direction” ?

I stated that

And

Due to the size of some systems there may be multiple crews installing the belting, these may be at the head , centre and tail areas.

Or they may have 2 crews working from the centre towards the head and tail sections.

Pulling the belting on the return side as well as the carry side simultaneously, one will be the direction of travel and the other will be opposite.

As long as the Stamp / Brand are on the same side and the sequence of installation is correct there should not be a problem.

If you are referencing a certain system as you seem to be this might be the case, but please do not assume that all overland systems are the same. ■

Dear Bruce,

I believe you are a little confused. First, I mentioned Lafarge, and I believe the thread starter is talking about the Lafarge 17 km horizointally curved installation as you referenced in:

http://www.conveyor-belt-guide.com/Extremes.htm

Second, we make statements in this forum that can be misunderstood. or too generalized. Is the thread starter requesting knowledge about how to handle the 17 km Lafarge conveyor with a horizontally curvd belt installation?

Third, I disagree with your comment on having the brand name up, or at least consistent, as sufficient for threading the Lafarge system.

Your comment/method may be possible, but has consequences which I have already commented on.. As the designers told him, "pull the belt on in the direction of travel". This has to do with the nature of the horizontal curve physics during threading. If no horizontal curves, then your comment is OK.

Fourth, on helter-skelter, I believe your third paragraph makes my point wrt Lafarge.

Fifth, by my initial comments, I just wanted the interested parties to know -- the installer can place the brand up but only on the left or right side as you stated. Not placing the belt, with the brand up and on the same side, may induce mal-tracking.

On your last paragraph, please go to the thread starters question. I do appologize for not clarifying what I believe the thread starter is refering to - Lafarge. He may not be, and, I may be too presumptous or prejudiced with zeal. For this I may owe you an appology. There are very few belts of this size in the wolrd, much less in the near east.

I have not made a general statement on threading methods. Had I done so I, again, would owe you an appology.

Mr. Sunil Prabhakar, is this the Lafarge installation? ■

Hi Sunil Prabhakar

We have given our reasoning for this not knowing the system first hand.

I would be interested in knowing why the designer Aumund insist’s on it.

Look forward to your reply. ■

Sunil,

You can see that opinions differ. Why do they differ?

I offered points and have taken detailed measurements to back up the notations. I will say again:

When the belt is horizontally curved, and you wish to control its behavior during threading, the tension pattern and belt response to the threading tension are more likely to behave as the operating belt will behave. Thus, if the horizontal curve has differing banking angles at its beginning and end, the belt, pulled in the travel direction, will have transverse displacements similar to the bankinga angle adjustments. To do the contrary will invite adverse displacement behavior.

If the banking angles are the same throughout the curve there is no need for such a procedure.

Do you have your answer? ■

Larry..

What about the forward tilt of the idlers?

If you pull the belt in the wrong way the idlers have a backward tilt, and the belt is likely to climb out.

This is what happened at Namakwa sands long curved conveyor, and all idlers were banked at 4 degrees, but the belt would not stay in the trough and climbed out. (3km horizontal radius).

I doubt if it would have climbed out if the banking were steeper, but I don't know the details of Sunil's banking angle. We do know however that the forward tilt is adjustable, which is rather strange, and implies that the designer is possibly not really sure of himself.

Cheers Larry

LSL Tekpro ■

Dear Graham

We never apply idler forward tilt due to the uncertainty of moisture. However, I agree with you, if forward tilt is used.

I am working on an interesting problem, on 60 and 72 inch big and long belts where a firm applied forward tilt, as is their practice. The belt cover was abraded by the forward tilt. Millimeters were lost in less than two years. My estimate is the belt life would have been reduced by a factor of 5X, had the condition been allowed to run with the 2 degree forward tilt.

The forward tilt also was instrumental in pulley lagging damage.

We installed trays to catch the residue and estimate the damage. The particular installation has other wear factors, which I will not comment on here. We requested they not use forward tilt. Now I believe they believe no more forward tilt.

Cheers to you too.

Lawrence Nordelll

www. conveyor-dynamics.com ■

Graham / Mr Nordell

Hypothetically, if the rolls were in the correct sequence and it was necessary to install a section or sections of this belt in the opposite direction of travel, why could you not use temporary tracking assistance via guide rollers or tracking frames to assist with positioning of the belt on the system.

It is after all, not under running conditions by way of speed or tension and a relatively small belt in regard to width and rating.

Is this not possible. ? ■

Larry,

How does the forward tilt contribute to pulley wear?

Shane ■

Dear Bruce,

I did mention, in my initial comment point no. 2 to this thread, that it can be done, and it is often practiced on straight conveyors and with horizontal curves if the banking angle is not significantly varied.

It can be and is done with side guide rolls, as you suggest, give the roll pressure is not beyond our criteria. We often specify side guide rolls in critical zones to facilitate installation, commissioning and early operation. THis insures fast commissioning and early production to the belts intended specifications.

I was defending the criteria issued by the 17 km Lafarge designers. I would have issued the same instructions to the belt installer. ■

Dear Shane,

First The Idlers:

THe forward tilt is a abrasive procedure due to the lead angle trying to push the belt sideways as do steering idlers. With both wing rolls trying to push the belt sideways, there is a grinding effect when they are restrainted by the counter forces between opposing wing rolls with forward tilt.

Wear rate has an exponential component to the force applied. In the case I referenced, without naming, the level of force was significant on 1500 mm and 1800 mm wide belts with significant tonnage.

Second The Pulley Lagging:

There are many forces the exaccerbate rubber wear. In the particular instance, the rubber's threshold capacity to wear was increased in the belt wear region noted above. THis occurs when the spin radius of the pulley lagging & belt cover differ along their interface. Shear stress builds and creates heat with consequential wear as the surface speed differential increases between regions of the interface with the differing spin radii (worn and unworn belt).

THe lagging wear may not have been as siginificant if it were not for other factors that added to the interface forces. The pulley lagging was destroyed in local areas of the pulley that aligned with the belt wear. THis occured in a matter of months. ■

Gentlemen...

We had a similar problem with lagging failure, which turned out to be a bit of a pandemic. Many reports of premature lagging failure all over the place.

On one of our conveyors the rubber pulley lagging failed in about 2 months. It was on a conveyor in parallel with, and almost identical one next to it where the lagging lasted for a couple of years.

At first we blamed the lagging, but then we realised that the only difference between the conveyors was the type of belt.

The one that lost the lagging had one of these new belts with the fancy new abrasion resistant covers.

It stuffed the lagging out of sight. Don't ask me how or why, but it did.

(This is another reason I go for ordinary belts with ordinary covers and smooth ceramic stick on lovely long lasting lagging..Larry)

Regarding the forward tilt in the horizontal curves, I wish you could see the belts on my conveyors with such configuration.

The belts look like they have been polished to a glass like finish, with negligible wear to belt.

When we extended one of them from 1.5km to 4km long after 3 years and spliced in the new belting, we actually found little difference between the old and new belts, which surprised me no end.

LSL Tekpro ■

Gentle Graham,

Wonders never cease. I get the sense that there is a love fest between man and belt.

Excuse my observation, it must be that Peter Barlow talking. ■

Gentlemen,

This topic has roamed far and wide but as far as pulling on an overland belt with a horizontal curve, Larry is right. If the designer is using forward tilt on the idlers as well to assist tracking he has got it wrong, also something Larry has raised. It is an extremely easy exercise to work out the differential speed between the belt and the idler across the idler face if you forward tilt the idlers. We have a number of examples where, as Larry has said, this practice has led to a premature loss of the belt due to bottom cover wear. I would also agree with Larry that belt life could be shortened by up to 5X. In simple terms, with todays high modulus belts and steel cord belts forward tilting of idlers to assist tracking should be absolutely avoided. There are better ways to achieve this outcome.

Col Benjamin ■

Colin,

You mentioned it is an easy exercise to measure the differential speeds when you forward tilt idlers

How is this achieved?

Shane ■

Shane,

Unless the belt is square to the contact points along the idler face there will be a differential between the idler speed and the belt speed at points along the idler face. The idler speed is determined by the belt speed either at the point where the belt first contacts the idler or if the contact pressure is sufficient at the point of highest belt to idler contact pressure ( usually identified as the shiniest part of the idler face). If you lay out the geometry of the idler configuration relative to the belt, set the idler speed based on observations as above, then it is an easy exercise to calculate the relative speed differentials. It is this that induces wear on the idler shell and the bottom cover of the belt in the trough section. We have done a great deal of work on this and other belt wear/idler shell wear issues and what we find is that idler alignment is a critical maintenance issue particularly where there are high loads or heavy belts with troughing angles of 35 degrees or greater. As a consequence we do not advocate lagging or leading idlers nor tilting idlers to facilitate tracking in any but extreme circumstances nor for similar reasons to we advocate offset idlers.

Colin Benjamin ■

Hello Colin..

I read your contribution with interest.

What I don't understand however is, that with all the horizontally curved conveyors which we have either designed and installed or just been involved with over the last 15 years ( which is a considerable number of conveyors), why have we never experienced any of the wear problems you mention.

We always use off-set, forward tilting, super-elevated idler frames and plain steel rolls, as do the other main contenders here.

In practice they work perfectly over a wide spectrum of duties.

We have even installed two horizontally curved conveyors of 3000m radius working with the return belt dirty side down . On one of them, this causes a slight reduction in shell life as the curve starts immediately after the material is discharged, but in practice does nothing whatsoever to the belt cover life, and the roll life is acceptable.

On the other, with dirty side down on the return curve, the belt has a few hundred metres to clean itself, and the roll and belt life is really excellent, despite being horizontally curved over most of its 4km length.

Regards

LSL Tekpro ■

Hi Graham,

I have also seen and looked at many conveyor systems with idlers that have been led, tilted plus offset idler systems where the maintenance impact has been such that no one is concerned nor has the design been seen as an issue. In fact during a visit to South Africa I was assured this was not an issue in SA until I went underground at a coal mine to find bottom cover wear on offset idlers being a major reason for a belt being changed out. 3 roll offset idlers generally do not cause much of a problem if set up correctly, 5 roll are much more of an issue especially if designed with pipe structure as it tends to increase the offset distance. I guess our interest stemmed from when.some years ago we were involved in a major and very serious warranty claim. It tends to heighten your focus and analysis skills as to why something happens.

Anyway the above experience led us to do a lot of research in this area and while we have not published much of it we have applied what we have learnt to developing management systems and software for idler maintenance plus applied it to any designs we consult on. It was therefore very easy for me to agree with Larry. As far as your experience , good luck but if you get over here to Australia I am more than happy to take you to some installations where their luck ran out.

All the best

Col Benjamin ■

I noted in an earlier response that a key factor is force or pressure between roll face and belt surface. Rubber fails exponentially with increasing force, once that force/pressure exceeds the specific compound's shear work limit, exagerated wear will occur.

RSA does not have a significant number of conveyors, that exceed 1400mm in width, which carry dense product, medium to high tonnage, with large apparent idler spacing, at medium to high speed, and have forward tilted rolls.

CDI has never experienced my earlier noted story until these factors came together.

Offset rolls have, by themselves, caused no grieve for us.

Installation and manufacturing tolerances must allso be considered. Engineers and standards do vary in what is acceptable. THe larger, state-of-the-art conveyors do need special attention to these details.

My point is rubber has a limit. Once exceeded, it will fail at a disproportional rate with respect to increasing stress. Thus, the limit must be know to stay clear of trouble.

The manufacturers do not have the lab data to provide the criteria. The engineer does not have the tools to predict the stresses. The criteria also changes with compounds and many other factors. As they say experience is the belt teacher. We have some dots on the curve. We consider that much is yet to be learned, and we learn from others as is the strength of this forum . ■

Just a short addition to Larry's comments, the wear/failure mode we have encountered with offset idlers is different to wear induced by idlers that have been lagged or led or forward tilted. The high face pressure between the idlers and the belt plus a speed differential is definitely the factors in the latter case. In the offset case it is the lack of contact pressure that creates a speed differential and this together with any build up or corrosion of the idler face that means the idler has a rough surface. In many instances the second pre-requisite does not occur hence there is no problems with the offset idlers. When it does bottom cover wear can be quite rapid.

Col Benjamin ■

Colin,

I am trying to understand your comment on "offset idler wear is induced by lack of contact pressure". Could you give a little more detail on how this is manifested. Where does the speed differential come from? ■

Larry,

The more you say the more you get asked to explain. In these pages it is very difficult nor do I have a paper published on our findings but it relates to the geometry of the system relative to the high modulus belts we now use. In essence on a flat conveyor utilising 5 roll offsets the belt will contact the leading offset rollers first and unless the geometry of the design allows for this the belt will tend not to contact the lagging offsets with the sufficient contact pressure to ensure they rotate at the speed of the belt at all times. If these lagging idlers have in any way a roughened surface due to corrosion etc (usually caused in itself because the belt does not keep the idler face polished) then these idlers can act almost like grinding wheels. We have some wonderul photos and evidence of this happening. The problem can be duplicated with 3 roll offsets and is made worse the wider the centre line offset of the idlers is hence my comment about structures designed with a pipe like structure from which the idlers are suspended as this design tends to exaggerate the offset.



Col Benjamin ■

Dear Lawrence,

Sorry for the late reply on this, I should have posted it last year.

Confused or not, it can be done, it has been done, the impossible just takes a little longer. ■

Dear Shri Sunil Prabhakar,

It is advisable / safe to pull the belt in direction of belt travel as instructed by designer / manufacturer of conveyor and Graham and others.

Belt is subjected to centripetal (centre-in) force due to belt tension at curvature. This is countered by centrifugal (centre-out) forces applied by idlers on belt, so that maximum portion of frictional grip between belt and idlers, is available to keep the belt stable. The centrifugal forces by idlers can arise due to radial offset of idlers in horizontal plane and radial offset in vertical plane. The centrifugal forces due to vertical offset will remain same whether belt is pulled forward or backward. But the centrifugal force due to offset in horizontal plane, will become negative when the belt is pulled backward i.e. it will also tend to shift the belt inward. Thus, net effect for force balancing is reduced.

The centrifugal forces applied by idlers, are designed for belt tension during steady running and starting. This tension is quite high compared to the belt pulling tension during installation. So, the adverse effect on belt alignment by pulling the belt in reverse direction will not be so much pronounced, or even may remain within allowable margin, as a total effect. The degree of this depends upon specific design of the horizontal curved portion. However, belt pulling in direction of belt operational travel is safer.

The belt manufacturer might have given the erection instructions covering various issues, but it seems that the belt placement should be made realigned after say every 100 m of installation, etc., in tune with the progress of the belt installation. Once the belt is placed at proper position, the frictional grip between belt and idlers will tend to keep it at proper place.

Conveyor designer can analyse belt shifting tendency / forces during erection stage when the belt is empty and the magnitude of pull considering the specific length of belt which is to be pulled. The subject / decision is combination of design analysis and field experience.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■