Why? ■

I have only limited experience with magnetic head pulleys and my observations are :

1) extra weight

2) they impare belt cleaning as the chute for the ferrous, even if made from stainless steel, also gets gunked up with fines. And as these chutes are "only to collect a few bits of steel" they are under designed for the fines that fall into them.

I would tend to agree that a magnet seperator over the discharge trajectory is to be preferred, but as a retro-fit it can be difficult to install them, so a magnetic head pulley can be better than nothing! ■

Dear Mr. Dave Morgan,

The issue is the effective removal of the tramp iron from the material / coal on the conveyor. The equipment available are cross belt magnetic separator, over band inline magnetic separator and magnetic pulley. The magnetic pulley also has its area of application as for the other items.

In case of cross belt magnetic separator or over band magnetic separator, the tramp iron embedded within the material is to pass under the magnet. In case of high speed conveyor running at say 2.5 mps or more, the separation effect would be in action may be say for a second or a fraction of second. During this small time interval, the tramp iron gets magnetised in a polarity opposite to the magnets of the magnetic separator, then it gets attracted towards the magnet and then it has to wriggle out of the material bed to reach the magnet face. If the tramp iron is well within the material cross section and overlaid with the coal big lump then it will not be able to come out of the material. In case of over band inline magnetic separator, the tramp iron is subjected to attraction initially well within the material but immediately on discharge the material is in a free falling suspension condition and therefore, its effectiveness for removal of the tramp iron is superior. Because in this case tramp iron is not under the hold of material but it has to only clear the obstruction of the free falling material.

The good practice is to use the cross belt magnetic separator or over band inline magnetic separator on comparatively slow speed and wide belt conveyor instead of installing on regular high speed main line conveyors. If there is choice one can restrict the speed to about 1 mps for such slow speed conveyor. For example, if the main line conveyor is 1200 mm belt running at 3.5 mps, then it should discharge on to say 1600 mm belt conveyor running at slower speed with lesser troughing angle, and then put the cross belt magnetic separator or over band inline magnetic separator on the same. Such arrangement will effectively remove the tramp iron. The effectiveness of removal will get enhanced by 2 to 3 times due to slower speed and wider area of material exposed to the magnetic flux.

The cross belt magnetic separator and magnetic pulley are sometimes used to have the tramp iron separation from upper zone of the material as well as from lower zone of the material. Obviously, the effectiveness of tramp iron removal is superior compared to the cross belt magnetic separator or over band inline magnetic separator. In this case also, the tramp iron separation efficiency can be substantially increased by using a small length (12 / 15 m long) slow speed wide belt conveyor. In general one should not use magnetic pulley as a drive pulley for high capacity heavy duty conveyor to transmit large power / torque. The pulley failure has been observed when it is used in this fashion.

The above observations are based on extensive experience and also suggestions from Europe.

Often, the buyers are fixing the location of magnetic separator on main line conveyors running at quite high speed, and in this situation it becomes difficult to introduce the better system due to commercial reasons, and thereby system remains unchanged inspite of wide spread dissatisfaction in this matter. The introduction of wide belt and slow speed small conveyor is likely to make the price of magnetic separators double but still its performance will be far superior. In case of magnetic pulley, the use of separate slow speed conveyor will have a low belt tension and thereby will not increase the price of magnetic pulley. The carcass for such conveyor will also not demand large diameter of pulley. Contrary it will reduce its price and it may compensate for the price addition due to small length conveyor. There are also certain confusion / difficulty about the technical parameters being mentioned about the trial operation and commitments of removal of tramp iron.

There is also practice to install the metal detector prior to magnetic separator. This metal detector will boost up the excitation current in the magnetic separator, when tramp iron is likely to reach its zone. This arrangement certainly increases the separation capability, coupled with economical magnetic separator because the magnetic separator more heating occurs at time intervals instead of continuously. This means same magnetic separator will have more capability to deal with the tramp iron. Such arrangement failed at one of the power stations in India because its survival depend upon the frequency of incoming tramp iron / metal. By misfortune if the tramp iron or metal objects come successively for say half hour or so, and if the climatic condition is unfavourable then it will get overheated. The damage due to such misfortune can be prevented by putting thermal sensors in the magnetic separator and trip the system before there is damage.

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 25871916 ■

Hi all

What I would like to know is the experiences of others relating to the selection of pulley materials of construction when in use with overband magnets.

I have heared conflicting opinions i.e. for and against stainless steel pulleys, as well as how much of the pulley should be stainless if you do go that route. I have my experiences, but I would be delighted to get yours.

LSL Tekpro ■

Wood won't get magnetised. ■

Is that it?

Is that the sum total of the expertise and experience out there?

"Wood won't magnetise"

I really would like to know experiences on this one, as I am about to order a head drive pulley which will operate under an overband magnet, and I am not sure what to fabricate the pulley out of.

South Africa's pulley expert Max Schenk, once indicated to me that in his opinion, stainless steel in this application is a bit of a waste of time.

Any comments anyone (other than wood not being magnetic..)

Regards

LSL Tekpro ■

Sorry, I wasn't being flippant but thinking more of the chute to take away the ferrous. Having had one made in stainless steel that had to be modified on site (not easy with stainless) and finding that there was still a tendancy for the ferrous to stick to it I concluded that maybe in future it would be easier and cheaper to buy some wood, screws and even a power saw and make it on site! ■

Nothing wrong with a wooden hood on the chute as long as they are well designed and painted to match the rest of the equipment. Have done a couple of installations like this but had the hood made from a drawing so that you don't end up with a backwoods look to it. Below the pulley where the material has sliding contact we will go back to steel chutes.

We have always used stainless shells & end discs on head pulleys under a magnet. You may get some magnetism induced but it will not be as strong as you would get with a carbon steel construction. We always use at least 1/2" thick rubber lagging to provide some more distance between the belt & the pulley shell (as well as providing the usual benefits of grooved lagging)

As an aside, don't laugh about the benefits of wood in select locations. Years ago, prior to the widespread use of plastics on idlers, I quoted hardwood idler rolls to a plant handling highly acidic material on a slow speed belt. They claimed wood worked better than stainless. ■

Sometimes you have to go backwards to go forwards :-) ■

Trevor Jenkins at Edward Latrobe Bateman once told me that when they made things wooden they wouldn't work & now they make em in steel & they still don't work.

We had a big overband magnet on C21 at Gasgoigne Wood in N Yorkshire cleaning 4,000 tph at 4.5m/s. It ripped out safety boots. oil drums, nuts & bolts, braddish clips & shovels. Mind you it scraped the hardwood handles off the shovels so they were no use for gardening anyway & the boots were too grubby to put on a car boot sale. Dear me.

It just dependsd on how much brass you've got for the magnet. Our pulleys were Babcock Jeknins, made from steel. Max Schenk was quite correct about stainless drums, a waste of dosh in any situation. ■

No deep thought is necessary anyway.

BUT. ..........

What are you making the steel belt cords out of? ■

solid woven ■

OK for you then Graham.

I just thought I'd give someone the opportunity to write a thesis about the power consumption effects of separating a magnetised belt from a magnetised pulley. I'm not going to write it! ■

Stop encouraging them, get them out in the field working with real machines and then writing a thesis relating theoretical work to practical end results.

Ah. snag -- they might get their hands dirty and not be able to sit at a computer screen all day. :-)) ■