I assume we are speaking about the idler frames, with the difference in transom bolt centers between stringers.

No change in performance. The idler rolls are the same. Only the support transom is altered. Standard base idlers follow CEMA norms for small in-plant conveyors to something like bolt centers at:

belt width + 12 inches.

Wide base idlers have the bolt centers at:

belt width plus 15 inches.

This is if my memory is still there.

The extra bolt center clearance provides for greater belt movement of the return strand. This also goes together with the orientation of the stringer C section with toes in so you can fasten the vertical support on the outer surface along with the return idler frame. This maximizes the clearance between belt and steel to accomodate belt irregularities, bad installation, settlement, increasing belt width in the future, et al. ■

Here in South Africa, our main iron ore producer only uses wide frame idlers. This is to get wider steelwork and better belt clearance. A belt rubbing in iron ore dust is like a knife.

I only use the wider rolls on the wings for horizontally curved conveyors, otherwise you loose out on shaft deflection.

Cheers

LSL Tekpro ■

Per CEMA, standard base idlers have bolt centers of BW+9", wide base idlers have bolt centers of BW+15".

In my early career we used wide base idlers at the bucketwheel boom conveyor of stacker/reclaimers. These are reversing belts that are dificult to train and these are quite crowded by the steel structure especially at the boom tip where the material is transfered from the bucket wheel thru a hinged chute that is lowered into place for the reclaiming.

We have used wide base idlers almost exclusively at our sandwich belt-high angle conveyors. These tend to be bounded by the verticals of a box truss or uprights that support the return top belt. The wide base idlers tend to spread the steel away from the belt edges and tend to better align the intermediate structure with the end framing, simplifying the details.

Joe Dos Santos ■

Dear Mr. Andrea,

The query is not very clear, so interpretation can vary.

The earlier respondents have considered that the query is applicable to the idle mounting hole centers along conveyor width. This interpretation is perfectly in order. But there is also on more interpretation, which can refer to bolt hole centers in base plate for each side. If the base plate is put below the transom then its bolt centre would be somewhat larger i.e. transom width + clearance for the bolt or nut head + margin on the outer side of the bolt or nut head. Sometimes to reduce the base plate length, it is in the form of piece of angle welded at the end of transom. This gives narrow width for the idler and thereby very close spacing for idlers at impact zone, if need be. In this case transom ends just before the stringer inner side. Such arrangement is often necessary for belt feeder where very close spaced idlers are needed.

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 25882916 ■

For what it is worth, my advice is to never use close spaced idlers on a belt feeder.

Normally the initial head load is high and precludes the use of conventional idler rolls, as they get horribly overloaded.

This means they don't last.

This means you have to change them out frequently, which is almost impossible as they are closely spaced and inaccessible, and as a result special provision has to be made to change them.

I would suggest that for reliable easy to maintain belt feeders, one should rather go the more modern and extremely successful route for belt feeder design, and lets not advocate something horrible from the bad old days.

Regards

LSL Tekpro ■

Referring to my earlier reply, I add following information for the benefit of readers.

The idler for belt feeder may look the same as for conveyor, but constructionally they are quite stronger compared to the idler used for belt conveyors, on belt width basis.

In case of belt feeder, the material discharge pressure at hopper outlet is to be calculated as per DIN or other formula used by the designer for the hoppers. This pressure could be quite high for certain applications and the load to be considered on idlers accordingly. The idler selected based on this load will function without any difficulty and give the required life. The idlers provide rolling support, which is never outdated, as it is synonymous to wheel.

For example, for a 1200 mm wide belt feeder under a particular hopper, has material column load height equal to 2.5 times the skirt board width. This means material column height imposing the load is about 2 m. If the material happens to be sand of bulk density 1500 kg/m3, then it amounts to about 3000 kgf/m2 of pressure. In this particular case, the idlers spaced at about 210 mm pitch will have a loading of nearly 500 kgf on central roller (which is longer than the skirt board width and bearing the material load). The idler particulars are 15 degree incline side roller of 139.7 mm OD tube rubber lagged to 165 mm. This roller has 6205 bearing and 28 mm dia spindle because this is not carrying any material load. However, the middle roller is of 950 mm length, 139.7 mm OD tube rubber lagged to 165 mm, fitted with 6208 bearing and 45 mm dia spindle. Therefore, I draw attention of the reader that the idlers for the belt feeder happen to be of heavier construction matching to their load. The idlers so selected will work satisfactorily with appropriate life.

For exceptionally heavy class of belt feeder, rollers happen to be of flat type (without troughing) and this can have live shaft and outboard bearing plummer blocks.

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 25882916 ■

Sorry Mr Mulani,... but I really must strongly advise that people tuned into this forum should never attempt to design a belt feeder using the above principles.

For a start the 2.5m x skirt width rule of thumb is too risky, and modern technology defines the initial and flow conditions based on flow test results for the material being handled and outlet geometry. Some of these results indicate very hign loadings, especially on a time basis. Only if you are able to break the stress field will the rule of thumb be anywhere near accurate.

Secondly, our standard bracket bolt centres for a 1200mm idler are 240mm therefore making the bracket about 300mm wide on top of which you will need clearance to say 350mm minimum. 210mm idler spacing is therefore not practical.

Thirdly, such an arrangement is not maintenance friendly, to such an extent that I have previously been asked to modify existing belt feeders done like this to be as per my new design, which does not suffer from and of the above faults.

LSL Tekpro ■

Not only belt feeders. Also high impact and high tonnage with moderate to high speed causes excessive forward thrust on the idler frame that result in an axial collapse with the direction of belt conveyor motion.

Also normal chutes with build up from differential coasting.

The frames must be have knee bracing. Usually there is not sufficient room.

Then owners brace between frames to make a composite assy. Consequently, the idler rolls become hard to remove. ■