Chris,

Over the period of my employment in the materials handling industry there have been many ocasions when I have had the need of assorted factors to carry out calculations "from first principles". However I have always preferred to establish these myself for the simple reason I then know EXACTLY the nature of the materials I have used.

While this was before the internet and being able to ask others on discussion forums I believe it still holds good. Would you rather put your faith in measurements you have taken yourself or in information picked up from whereever and that you have no idea of the origin of?? ■

Which is why you should do your own tests on the materials you will be using!

That's life in materials handling ■

Chris and the designer are certainly right; databases are dangerous places in bulk solids design. Wall friction is dependent on the nature of the bulk material and the type and finish of the contact surface. It should be measured with a long-stroke tester, as made by Ajax Equipment, for the specific conditions of the application over the total bounds of bulk material conditions to be experienced.

More surprising is that the enquirer does not express interest in the internal friction of the product or the overpressures applicable in feeders. These are much more awkward values to determine, but the first can be measured in shear cell equipment and the second controlled to a large extent by the geometry and construction of the feeder hopper. The properties of the bulk material are a fact of life, the real challenge to the designer is to secure the required performance at optimised overall cost, bearing in mind that the hopper and discharge device are inter-related items requiring an integrated design. There are many factors to consider that influence the width and length of outlet to the feeder but, after determination of the critical arching span, probably the most important is to develop a taper shear plane matching the incremental extraction capacity of the feeder to minimise the drag-out forces required. ■

My fear would be of someone "plugging" numbers into a formula without being clear about the numbers or the formula. ■

It is very true that understanding must preceed the application of a formula and that the overall situation under investigation is complicated.

However, looking at the original interest in comparing metal surfaces with a rubber belt for a similar duty, our original advice stands; that the contact friction values must be measured to have validity and enable the user to draw a specific conclusion. The detailed choice and design of the equipment is much more the domain of the specialist. ■

Then I really think if you are doing "research" then part of that research should be to measure your own coefficients of friction. ■

I misunderstood that you were working for "free" given your inital statement.

It's a poor state of affairs when a silo company cannot afford the small amount of money necessary for their researcher to do the measurements! ■