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Wall Friction Effects
by Joseph Marinelli
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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
Locus |
Description |
Result |
| 1 |
Straight line through origin |
Wall friction constant and same
no matter what the pressure |
| 2 |
Straight line with an intercept |
Wall friction changes as
pressure increases |
| 3 |
Concave downward,
no intercept |
Wall friction changes as
pressure increases, higher at low pressures. |
| 4 |
Concave downward, intercept |
Wall friction changes as
pressure increases, higher at low pressures. |
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.
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