If the material is granular (less than 25 mm size), then there would not be any advantage by providing impact idlers (tubular rollers fitted with rubber disc). However, provision of these idlers is often mentioned in enquiry specifications irrespective of material size. So, supplier / manufacturer while quoting and signing for contract do not enter into argument with the buyer / consultant on such point, because there is no serious objection in providing impact idlers. Thus, the tradition continues because of inertia in the industrial system.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

Rubber impact idlers are used to cushion impact and protect the belt from cutting and tearing of the top cover and damage of the tensile member due to impact.

Key factors in protecting against belt damage are understanding:

1. Particle size and density (mass),

2. Sharpness (ability to cut rubber)

3. Kinetic energy impulses ( drop height and impact angle)

4. Ore structural strength (friable vs hard and strong)

5. Flow pressure gradients (high tonnage fluctuations)

Chute design thus becomes as significant or more so than ore size and its properties. With a proper chute design, and dry ore, the ore size can easily exceed 100mm for most installations before rubber impact rolls need be considered.

Rubber impact rolls build up heat with high pressure ( high tonnage and belt speed). I have recommended to remove rubber impact rolls that were been destroyed by heat buildup. THe pressure (12,000 t/h; > 5m/s) and speed were not sufficient to cut the rubber only enough to cause it to melt from hysteresis). The steel rolls eliminated the problem. ■

Once again the book is being questioned for the sake of it. If you don't think you need impact idlers then don't fit them. But if you knacker the belt then don't blame the book. It boils down to gambling the cost of impact idlers against the cost of belt damage.

We only know what we can measure - Kelvin. ■

"Dear John ".... as has been said before:

Which book are you refering to? We certainly don't want to blame the wrong book.

Some engineer because that is what they are taught.

Some copy others because they are not taught.

Sometimes the generalities need revision as progress teaches us.

I believe you are confusing diligence with gambling.

Some uses of impact rolls are not justifiable.

What we measure we know - Nordell ■

In applications where impact is not an issue. Guard-A Seal by Martin Engineering is used in the load zone to prevent belt sag and increase sealing. This systems asists standard idlers and has performed well. Web site www.martin-eng.com ■

There are times when lots of items should not be used. Using impact idlers on narrow belts is courting disaster as many a poor soul has found. With belt wander the discs hook the belt preventing it from returning to its central position resulting in lots and lots of spillage.

Refer photo:

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