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Thread: Microslip

  1. #1
    Join Date
    May 02
    Posts
    23

    Microslip

    Breakage of 4 to 10 cords around the centre of a steel cord belt, some within a metre of each other has recently been experienced. The belt is ST 1250, 1800 wide, narrow trough 45 deg. THe belt was replaced with more breaks appearing soon after. Head pulley and non drive 270 wrap snubber were replaced and seems to have fixed the problem. Microslip is suspected, are there any other case studies to substantiate this theory ? Much cost was incurred. Anyone have any other theories?

  2. 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

  3. #3
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc. [eDir]

    Conveyor Dynamics Inc.

    Professional Experience 58 Years / 8 Month Lawrence K. Nordell has 58 Years and 8 Month professional experience

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    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

  4. #4
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc. [eDir]

    Conveyor Dynamics Inc.

    Professional Experience 58 Years / 8 Month Lawrence K. Nordell has 58 Years and 8 Month professional experience

    Discussions 2608 Lawrence K. Nordell acceded to 2608 discussions, Articles 0 Lawrence K. Nordell wrote 0 articles, Publications 0 Lawrence K. Nordell Nordell released 0 publications

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    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.

  5. 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
    Graham Spriggs

  6. #6
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc. [eDir]

    Conveyor Dynamics Inc.

    Professional Experience 58 Years / 8 Month Lawrence K. Nordell has 58 Years and 8 Month professional experience

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    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

  7. #7
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc. [eDir]

    Conveyor Dynamics Inc.

    Professional Experience 58 Years / 8 Month Lawrence K. Nordell has 58 Years and 8 Month professional experience

    Discussions 2608 Lawrence K. Nordell acceded to 2608 discussions, Articles 0 Lawrence K. Nordell wrote 0 articles, Publications 0 Lawrence K. Nordell Nordell released 0 publications

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    Know-How Design (1521) Lawrence K. Nordell used this tag 1521 times, Pipe Conveyor (238) Lawrence K. Nordell used this tag 238 times, Chutes (119) Lawrence K. Nordell used this tag 119 times

    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

  8. #8
    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

  9. 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
    Graham Spriggs

  10. #10
    Lawrence K. Nordell

    Lawrence K. Nordell

    President and CEO

    Conveyor Dynamics Inc. [eDir]

    Conveyor Dynamics Inc.

    Professional Experience 58 Years / 8 Month Lawrence K. Nordell has 58 Years and 8 Month professional experience

    Discussions 2608 Lawrence K. Nordell acceded to 2608 discussions, Articles 0 Lawrence K. Nordell wrote 0 articles, Publications 0 Lawrence K. Nordell Nordell released 0 publications

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    Know-How Design (1521) Lawrence K. Nordell used this tag 1521 times, Pipe Conveyor (238) Lawrence K. Nordell used this tag 238 times, Chutes (119) Lawrence K. Nordell used this tag 119 times

    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

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