Wall Friction Effects
by Joseph Marinelli

As I was contemplating this week's article, it dawned on me that I have not written an article about the importance of wall friction properties. I am amazed that I have not covered this important topic until now.

Outside of knowing the opening size required to prevent arching and ratholing, wall friction values are the most important consideration for ensuring mass flow. The reason is that once wall friction values are known, mass flow hopper angles, as well as wall loads can be determined.

Wall friction values are expressed as a wall friction angle or coefficient of sliding friction. The lower the coefficient of sliding friction, the less steep the hopper walls need to be to ensure mass flow. The coefficient of sliding friction can be measured using the Jenike Shear Tester by determining the force it takes to slide a sample of solid across a stationary wall surface. See Figure 1. The friction that develops between the wall surface and bulk solid resists this force.

Figure 1.  Wall Friction Test

For a given bulk material and wall surface the wall friction angle is not necessarily a constant but often varies with normal pressure, usually decreasing as normal pressure increases.

The results of a typical wall friction test are shown in Figure 2. Notice that at low consolidation pressures, the wall friction angle is higher than at high pressures. This results in lower wall friction values at higher pressures and as a result, lower hopper angles required for mass flow (to be discussed in our next article).

Figure 2:  Example, Wall Yield Locus

Wall friction tests result in a wall yield locus that can take one of four different configurations, as shown in Figure 3.

Figure 3.  Wall Yield Locus Configurations

Wall Yield

Once wall friction angles are measured, hopper angles for mass flow can be determined.

In my next article I will discuss how mass flow hopper angles are determined from wall friction angles. As I stated in the beginning, wall friction values are critical to proper mass flow hopper design.

Help others by posting your comments, suggestions and experiences with bulk solids feeding or any other materials handling concerns you may have on our On-Line Help Forum.  For past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.