The typical questions asked in such situations are:

1)    Was the belt tracking true - Were the top and bottom drums crowned?  Were the top and bottom drum centres in-line to within 5 mm?  Were the drums in good condition and provided good service?  Were the top and bottom drums clean where the belt ran with no build-up of any sort under the belt (the belt must run on a clean surface)?  Could the drum shafts be moved far enough to allow tracking? Did the maintenance guys have a well tested and proven written procedure for tracking the belt (you should be running for months without constantly adjusting the tracking)?

2)    Was the belt damaged or fatigued – Was it a true bucket elevator belt or just a conveyor belt (true elevator belts have more fibres in the belt to take the stress)?  Was the lap joint(s) where the belt joins straight and letting the belt run true or was the join deforming the belt?

3)    Was the product causing the buckets to grab and drag?  Was the elevator boot clear of product inside the elevator that could have caught buckets?

To help clear product away and leave room for the buckets to move through you install stainless steel metal buckets, every fifth bucket, that are a couple of millimetre wider and deeper than the plastic buckets.  The metal buckets act to cut through the product and clean it away.  You may need to get them specially fabricated if the bucket supplier cannot provide them. (Be careful that installing metal buckets does not lead to added safety risks.)

Even after checking all the usual issues they continued to have more bucket loss events, including 3 hits in one day on one shift.

To assist in the investigation the maintenance records for the elevator were sent to me for review.  There were many instances of buckets being damaged or ripped and extensive downtime had occurred many times.  Throughout the maintenance work records were comments that the elevator boot was full of sand and that the buckets were dropping sand on the return leg.

Ripping buckets off the belt sounded like a belt tracking problem.  Was the belt wandering to the side of the elevator and getting caught on bolts or flanges and these were damaging the buckets?

But the work record notes indicated there was a lot of sand returning instead of being discharged.  The resultant build-up bogged the belt because the buckets could not haul their way through the sand.  I wondered if the belt was travelling fast enough to sling the sand out of the top discharge chute.

It was necessary to do some engineering calculations to check if the elevator was running fast enough.  To do the calculation required the distance between top and bottom drum centres, the current belt speed, the bucket spacing and the bucket size.  Was the elevator vertical or inclined?  What was the throughput required from the elevator in tons per hour?  What did it discharge into at the top - a chute, a hopper, etc?

You need to know about the damp sand’s materials handling characteristics.  What was the particle size range and D50 size (mid-range size)? What made the sand damp - just water or was a liquid chemical mixed in with it?  How much dampness - how much liquid for each measure of sand (say in a gallon of sand how much liquid as a percentage of weight)?

Had the sand changed from what it used to be? Was it a new sand supplier? Had the process changed from what it used to be?  How was the sand fed into the elevator - screw conveyor, hopper and chute, etc?  What feed rate was the sand being fed to the elevator boot?

By going through the design calculations for belt speed and throughput you can check if the belt speed was travelling fast enough to flick the sand out at the top.  But damp sand will behave differently to loose sand.

On replying to the people at site I suggested that it would first be worthwhile slowing the feed into the elevator to test if it was being over-fed.  If it could not take the sand away fast enough the excess sand feed would bog the buckets.  At least that would give the elevator a chance to clear-out the bottom boot and not catch the buckets.  The second thing to do was to speed the belt fast enough to discharge the buckets at the top drum with centrifugal force.

Shortly after the above information was sent good news arrived from site.  They had got control of the sand elevator.  On site they had made several modifications such as increasing the speed and had moved the feed chute up.  Instead of feeding into the bottom of the elevator close to the tail pulley they fed-in higher up.

The faster belt speed was the reason the bucket elevator behaviour improved.  The speed increase did two things.  It moved the buckets fast enough to fling the sand-out of the buckets at the top pulley.  Secondly it let the buckets remove the sand at a faster rate than the feed into the boot.

Finally, after years of problems,  the bucket elevator was working as it should and no longer was it a production bottleneck (and a pain in the neck)!

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