Wednesday, April 27, 2011

But...What is wall friction anyways? Part 2

Now, Jenike proposed a measuring device to determine wall friction, that device is now known as the Jenike tester, other testers have sprung up which could be used to determine wall friction, but the Jenike cell has become the industry standard. The basic method consists of taking a sample of the powder and a sample piece of the hopper or silo wall, the powder is then poured into a ring (usually metallic) and then a normal load is applied (by placing dead weights on the cell), then the ring is pushed by an actuator and the shear force is measured. The ring displacement can be measured too. This is done for different loads so as to cover a wide range of normal loads.

If one plots shear force (or shear stress by dividing over the area of the cell) against normal force (or normal stress), a line is usually formed, but this is not always the case. For solids this line is straight, most of the times, but not for powders (at least not that often). I have seen more than one publication where the shear stress is plotted against the normal stress and a line is created for each point (the second point being the origin), and an envelop of lines is presented. Other times, the researchers have reported their wall friction by fitting a line to the data, which could produce a low linear correlation value.

A better way to understand the data is by plotting the wall friction against the applied normal stress. The wall friction angle is calculated by taking the arc tangent of the ratio of shear stress to normal stress. Usually, a decaying curve is obtained so that at low normal stresses, a high friction angle is obtained. This plot is then used to determine the wall friction at a certain normal stress.

To be continued...

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