First, sorry for the late reply. This is an academic point of interest.

I am not sure I get your definition of "eccentrically loaded". I have witnessed Rio loading stations with high tonnage that cause an initial laterally reduced ore center and high skirt load configuration, like an inverted ore surcharge. Kind of like a large water jet stream directed mainly into the belt center line. This causes a flushing action that pushes the ore up against the side skirts. In time and motion, the load reestablishes itself into a normal surcharge shape. I have witnessed this at Intercourse Island 9,000 t/h system.

Also at Intercourse Island, the structure had a high structural compliance (deflection). The structure high compliance coupled with the belt natural frequency and with roller spin rpm caused a belt/ore vertical pumping action. The resonance beat frequency caused ore volcanoes where, within 200m of the load station, mountains and valleys developed along the belt longitudinal axis. This was later eliminated by stiffing idler stringer and transom structures.

A version of this also occurred at the Channar 2 km intermediate feed point about 12 km from the overland tail station. Again, structural stiffening eliminated the behavior. The design report noted the possibility, if certain design aspects were not followed. You may know the contractor did not install the conveyor in a conventional manner. He eliminated the thermal expansion joints, which in turn caused the structure to tilt.....

The belt wear pattern, top cover, is not uniform across the width. It will usually be highest in the center area. This may not be true depending on the chute loading. The wear on top is confined to the skirt width. However, as the same belt is returned without belt turnovers, the same zone may have contaminates, that act like a grinding paste. The grinding paste like surface abrades both belt and idler roll surfaces.

Usually, the ore tends to move toward the belt center and the load is agitated across repeated idler motions. We have photos of the 20 km Channar overland particle migration backward and moving toward the center as traced by their dust trail from loading station to discharge. As this happens, the fines percolate to the bottom of the ore and coarsest rock migrate to the top. This behavior is similar to placing a big rock into a bucket of dry sand and then vibrating the bucket. The big rock will be lifted by the sand particles.

Have you solved the problem? ■

Originally Posted by SteveDavis

Thanks for the detailed response

Eccentric, i.e. off centre. The belt loads quite well from the transfer point and then appears to move across the belt in transit giving erratic belt drift at times, similar to results from off centre loading.

I agree that the ore on a troughed belt tends to centralise, but on this conveyor fines seem to move, and this can be to either side. They don't move much but sufficient to push the belt in the opposite direction.

I can't see what could mechanically cause the movement. The belt, mechanicals and structure are in resonable condition. Speed is fixed. Movement does not appear to be impacted by temperature variation (day/night) or temperature differential (from sun on one side).

I have seen many belts where loading matches the description given. Like an inverted surcharge. Many belts load at much reduced surcharge angles over those anticipated from testwork and tabular data (for info, not that I would use this), so we often get between zero to 10 surcharge in iron ore. Good thing we never design above 80% full. Obviously this is a result of the chute operation.

I have occasionally seen bauxite ore where the surcharge angle increases noticeably during transport, but not noticeably on iron ore.

I used to see hourglassing or bunching on longer conveyors from resonance. Changing idler spacing so there are minor variations seems to reduce this significantly - not random spacing.

Our belt wear is generally higher in the centre as noted, but often at the edges from skirt issues. Chute design could be better than it is and seems to be a large contributor to wear through expecting the belt to accelerate the abrasive ore.

Is it fixed, not really but we have it controlled. Will we have the issue after the belt is changed? Have to wait and see - not too long with iron ore!

Dear Steve,

Can you send me the chute geometry feeding this conveyor, including skirt boards and their alignment?

Is the tracking behavior consistent, is it more cyclic, ...? This later point was demonstrated during commissioning of the Channar 20 km.

ContiTech built the first 6 or so belt reels (600 m each) as single width, then got permission from someone in Rio to build 2 x wide in the plant press. When they were installed on site, these later 2xwide rolls (separated at factory), mal tracked about 150 mm off center. We applied a digital wand to the belt edges and demonstrated the cyclic pattern. It was repeatable to 0.8 mm over the 20 km of belt length. We cut out these mal-tracked belt reels and reversed their orientation. The belts then did track within about +/- 30 mm. I believe in simple physics. Is it chute and ore loading, belt construction, idler alignment, sun, moon, ...... ?

Are you sufficiently confident of the idler trough roll position, orientation and their alignments to be accurate?

Have you made a video of this phenomenon?

You note about ore and surcharge angle can be dependent on the chute flow control. Channar did have consolidation of ore from tail to head. The edge clearance increased about 40 mm. Each dusty stone left a clear track, at each edge, on the migration.

Highly curious to know the outcome. Its the mysteries that make engineering exciting.

My email is: Nordell@conveyor-dynamics.com

phone if you are inclined: west coast USA: 360-671-2200 office, cell 360-201-1237 ■

Originally Posted by nordell

....... the fines percolate to the bottom of the ore and coarsest rock migrate to the top. This behavior is similar to placing a big rock into a bucket of dry sand and then vibrating the bucket. The big rock will be lifted by the sand particles....

One of my previous threads asked how, in a jar of coffee on the supermarket shelves, the larger grains were very often seen in the top of the jar contrary to segregation expectations. The first reply was Ozzie, obtuse and dismissive but this was soon followed by the reply from Lynn Bates who explained the Brazil Nut Effect. ■