As important as knowing the "slide-back" conveying angle, which is very influenced by dynamics, sag, material gradation, etc, etc, etc, is determining an appropriate safety factor against slide back. This term "S F against slide back" is not known in the literature. Rather, conveying angle is determined by failure, just a little less.

After enduring numerous slide-backs (with great cost implications), at their 17 dgrees coal conveyors, a major South African company decided to addressed the issue at length. The result was a company policy that set max conveying angle at 12 degrees for their coal conveyors. The capital cost of their next coal conveyor project was so high that they abandoned the 12 degrees max policy after only one project.

This is the best argument that I can make for DSI Snake-Sandwich High-Angle conveyors, where the slide-back safety factor is well known and set to be safe. No slide-back has ever occured at a commercial unit.

Joseph A. Dos Santos, PE ■

There are a number of test companies that can perform this type of work. Three of which are in the Akron, Ohio area. Smithers is probably the largest. In all cases, although their primary focus is on tires, they conduct specialty test ing to meet the needs of clients.

A more cost effective and practical approach, however, would be to contact a research lab at a university that specializes in bulk material handling. The University of Hanover in Germany has a very sophisticated lab set up.

If you have trouble obtaining contact information, feel free to e-mail me.

Good luck with your trials. ■

Todd

There are a number of factors that play on the angle. Per CEMA, the surcharge angle or dynamic angle of repose is about 25 degrees. This is the measured angle resulting from an aggitated stockpile. CEMA says nothing about the coal particle shape, moisture, chute loading configuration and belt surface texture, speed, transport length, lump size and distribution, sag and idler spacing. Load stability on the trailing charge, to control roll-back, is usually the governing factor. Thus, the lab test criterion and procedures are also factors which must test for the above points. Conventional conveyors can handle slopes of 20 degrees with granulated structure and light moisture. Some labs that can help are:

1. Jenike & Johanson USA

2. TUNRA (Alan Roberts legacy) Australia

3. Univeristy of Wollongong Australia

4. Wits University South Africa

5. Penn St probably has a coal lab that classifies propertiies

6. Univ. of West Virginia does coal characterization

7. Delft Univ. and Twente Univ. in Holland

Lawrence Nordell

President

Conveyor Dynamics, Inc.

1111 West Holly St.

Bellingham, Wa 98225

ph 360/671-2200

fx 360/671-8450

email: nordell@conveyor-dynamics.com

web: www@conveyor-dynamics.com ■