Dear Mr. Glenn,

The belt failure which you have mentioned can be by many reasons such as excessive impact at loading point, weak belt compared to actual occurring tensions (momentarily and continuously), pulleys of inadequate size, inadequate transition length, excessive crowning (if it is) of pulleys, improper starting factor, improper braking factor / time, faulty manufacture of belt, idler spacing etc.

I suggest you to mention sufficient details of this conveyor such as speed, conveyor length, lift, some indication about profile of conveyor, belt particulars, lump size, lump falling height at feed point, material name, pulley diameters, transition length, etc. This can help the readers to give you some suggestion / opinion.

In reality your problem pertains to design analysis of conveyor, by competent designer of your choice. However, still you may get some useful / interesting suggestion in this forum.

Regards,

Ishwar G Mulani.

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

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 5882916 ■

G'day Glenn,

As previously suggested there can be a number of reasons for the center cable damage.

Most likely conditions, with the given information, would be: a) drive and /or snub pulley diameter irregularities with either crowned geometry, muck buildup on snub pulley from dirty side contact or irregular lagging proud in the center on either pulley or, b) impact damage with big lumps and not so small cable diameter (5.8mm). I would put money on the pulley geometry and a to small a distance between the drive and snub. A short distance will exacerbate local cord stresses with poor pulley geometry.

Although the impact can produce such damage in the belt center region, it would be unlikely at ST-1850N/mm. It would be more likely at small cable diameter (

Head transition trough region is not likely. This area would promote edge cord failure not center cord failure. Tail transition trough might, but it would be extreme and noticable from heavy belt center buckling. The failure mechanism would be fretting stress.

Acute convex curve radii could, under extreme conditions, cause center cord compression or fretting failure as have concave radii with edge cord buckling. You would observe belt buckling in the center region.

Neither of the two vertical radii conditions noted above are likely. The cords would be failing continuously all along their axes.

I highly doubt dynamic starting and/or stopping conditions would cause the condition. The normal high cord stress is near or at the belt edge.

We could talk about belt construction, alignment, idler wing to center roll junction stress ( more likely fail rubber than cable), turnover stresses, mal-tracking, rock entrapment under load point skirts, and on and on. An expert would need to see the installation to render an accurate cause and effect relationship. An expert, in such detail, would likely spot the problem in short order.

I will stop blathering and let you elucidate more.

Good Luck,

Lawrence Nordell

Conveyor Dynamics, Inc.

web: www.conveyor-dynamics.com

email: nordell@conveyor-dynamics.com■

MOre..........

Micro-slip occurs on all drive and most non-driven pulleys with varying degrees. It is not the problem. There are papers on the mechanics of belt action around pulleys - two German doctorals - Zeddes & Hohman(sic?). The best pubished works can be found in the studies of metal levelers that stretch sheet metal ( alias belt) beyond yield. A series of small rollers have the thin metal sheet bent at about 90 degree bends back and forth to force elongation in small steps to iron out local strain discontinuities. This metal creep analysis is similar to belt creep (alias micro-slip) over pulleys. This use to be my field (rolling mills and related equipment) before belt conveyors.

The process can be modelled using DEM (discrete element method). I won't go into the details here.

Drives take out belt stretch that has built up along the conveyor path from gravity and friction rolling resistance. Belt tension drop (T1 to T2) changes belt length as happens over drive pulleys. The stretch occurs in small increments (micro-slip??) that can be heard and identified by their frequencies.

I am curious about your 270 degree of wrap. This would be a whopper and a record breaker. May your frame of reference is different from convention and needs description.

Lawrence Nordell

Conveyor Dynamics, Inc. ■

Hello Glenn..

I have seen your problem many times, funny enough more often than not on St 1250. It has often been due to the odd piece of rock getting into the return system. Being steelcord, the return idlers are Vee and concentrates any foreign material to the center of the belt and carries it to the next pulley. If high tension, then the rock goes rond the pulley and breaks the cords. often the rock cannot get out and breaks the cords every Pi x D.

Pulley build up has the same effect, especially on the high tension pulleys, and messes up the splices and breaks cords. Here you can use a pulley scraper, but keep it away from the belt line as I have known such scrapers to get caught on the odd cable poking out of the belt and causing a rip.

The last case is pure impact damage, where large angular dense lumps hit straight onto the belt and broke the cords. We were consulted to re-design the transfer chutes for this poor client. We found the impact energy per lump was over 3000 Joules. We managed to get this down to 1000 Joules and all is now well.

Assuming you have a well behaved counterweight, If you have no Impact to speak of, no return spillage and flat pulleys, all will be well and you wont break cords.

Good luck

LSL Consulting / Tekpro Projects ■

Hello Gram,

I agree with the idea of center cord breakage from tramp rock recycling around the pulley and may be the ticket in this case. Don't you think the scenario you describe would more likely be a condition at the conveyor's tail pulley, not his evidence with the noted drive and snub cause-and-effect?

I got the ST value bogussed transposing belt width and strength. In looking at the break frequency, the pulley diameter would have to be around 300mm. This is in the possible realm, although the tail pulley is more likely to be on the order of 600-750 mm diameter. The breaks would then be about 2m apart unless there were multiple rocks. However, somebody might try such a small diameter on a snub.

Maybe when the pulleys were changed the belt cleaners were adjusted to control buildup and the problem ceased.

LKN ■

Another after thought,

Cord breakage, from dirty side build-up on the drive snub, can be significantly reduced if the snub is positioned further away from the drive. You do loose on drive wrap and this may need to be compensated with added takeup force. The short distance between drive and snub makes for high local cable elongation when build-up occurs.

Rock or muck build-up induced cord breakage would produce evidence of cord displacement away from the localized pulley build-up deformation. When you look at the cable breakage, observe whether the breakage protrudes toward the belt return cover or top cover. If it pushes toward the top cover, then this likely indicates breakage starting on the underside and the probables of pulley buildup or rock entrapment and not load station impact.

LKN ■

We too had similar problems on an ST1000 steel cord belt. The problem was created by a worn V blade scraper a the tail pulley. We actually had experienced punctures through the belt. I would concentrate my efforts on that aspect. Fines can accumulate at the tail and also cause glazing on the tail pulley, especially under high tension applications.

Good luck

Gary Blenkhorn ■

Hi there Larry..

I don't know the pulley configuration on this conveyor, so I can't really comment on the most likely pulley to point the finger at.

Suffice to say though, the more pulleys there are the more vulnerable is the conveyor to cord damage.

As for keeping the pulleys away from each other, you have hit the nail on the head as far as I am concerned. They should be as far away from each other as possible to recuce the possibility of loacalised strain in the belt due to irregularities in the pulley surfaces.

I cannot remember how many times I have had to try and solve such problems by putting in stools and additional steelwork to move pulleys further apart.

On a number of occasions I have managed to remove pulleys altogether, as they weren't really doing anything usefull...why do they put them there?

(If a pulley isn't there then not only can it do no damage to the belt, but you don't have to pay for it or maintain it.)

A cord in an St 1250 is only about 4mm in diameter and is very vulnerable, as an 1800mm wide belt of that class takes a lot of pull.

And Gary..

Our clients are also reporting more and more about the dangers of the Vee Plough as you describe.

As a result, we only use straight ploughs, diagonally across the belt.

Better...more economical...simpler...

Cheers

LSL Consulting / Tekpro Projects ■

Glenn,

Could you give feedback on the belt top(carry) or bottom(pulley) cover surface that is or maybe bruised above the broken cables?

Larry ■

Glenn,

I have just read your private email. I believe, from your description, the problem might be the belt-to-pulley lagging motion. This is induced from the dirty size belt and ore grit rubbing on the snub as they interact with the transition idler differential cord stresses across the pulley face.

Your term "microslip" I call shear work. The work of Zeddes documents ( lab measures) the shear work as the belt approaches and discharges from a non-driven pulley. By example, on a 180 degree wrap, the pulley lagging and belt cover differential motion or zero shear and zero slip occurs 90 degrees from the entry and exit belt contact points. Just before contact, between belt cover and pulley lagging, the belt dirty top cover and pulley lagging are not deformed. Just after contact, near zero and 180 degree wrap, the pulley steel shell pulls to deform both lagging and belt cover against the steel cord's tension. Here you will get your microslip.

Depending on the proximity of the drive drum, you can also get some of the belt stretch recovery from the relaxation of T1 to T2 tension. This is exacerbated by the idler transition elongation recovery. The grit from the dirty side contact acts as a grinding paste as these relaxations occur at the belt cover and lagging slip interface.

The fixes are: 1) move the snub about 0.5 to 1.0 seconds belt travel time from the drive exit, 2) get better belt cleaners or adjust them more often, 3) don't damage the carry side cover from high rock impact and cover gouging at the load point making cleaning ineffecive, 4) change the drive and/or snub lagging before it becomes highly worn.

The above comment is a generalization of your particular snub pulley with a low wrap angle. A good FEA or DEM model, with non-linerar elasticity and contract slip interaction, could show the mechanisms at work.

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■

Just an after thought:

The drive pulley is straight faced or crowned? I would think you would see a crowned pulley is this design. However, you never know. If it is crowned - bing-o!!! This would cause all you grief.

LKN ■

Golly,

I dont read what I type. The above should have read -- " I would not think you would see a crowned pulley used in this design."

LKN ■