Flow Properties Testing - Jenike
Shear Tester
by Joseph Marinelli, Solids Handling TechnologyThe picture that you see of me as
"Ask Joe" on Powder and Bulk Dot Com's home page and
above, shows me holding something. Some people have told me it is a
coffee cup, dish, etc. In fact, it is the bottom half of a Jenike
shear cell filled with corn grits. The Jenike Shear Tester is the
device that is recognized as one of the standards for testing bulk
solids by ASTM (D-6128-97) in the US and in Europe.
Although there are additional means
to characterize solids flow properties, the Jenike method has been
scrutinized and perfected over the years and remains the preferred
approach. It is classified as a direct shear tester that is capable
of providing information on a solid's "cohesive strength"
as well as its "wall friction" properties. These are the
two main considerations when designing a bin or hopper to ensure
reliable material flow.
Our first photograph (above) shows a
Jenike Shear Tester, which is composed of a strip chart recorder
that records a signal from a load cell located in the blue
rectangular box (shear unit), mounted on the front. This unit also
contains a small motor that drives the load cell horizontally into a
sample located on the stainless steel circular base (shear cell not
shown).
 The
second photograph shows a shear cell in place. Notice the protrusion
from the box that is the load cell stem. The shear cell is in two
parts that allow the top to be sheared across the bottom, while a
normal load is applied by the weight hanger. A shear cover comes in
contact with the load cell stem to provide the linear motion.
This test allows us to measure the
strength of a bulk solid as a function of pressure applied to it.
This information is then used to determine the opening size required
to prevent arching and ratholing in a bin or hopper. In subsequent
articles we will describe this evaluation in more detail. This test
can be run to simulate the affect of several environmental
conditions on material flowability, namely, the affect of moisture,
particle size reduction, temperature, and time of storage at rest.
All these factors come into play when evaluating a material's
flowability.
The Jenike Shear Tester is also used
to measure wall friction properties. From previous articles, you
will remember that friction is developed between a solid and the
walls of a bin or hopper. This friction determines whether the solid
will slide on the wall (mass flow) or adhere to the wall and
preferentially slide on itself (funnel flow).
 Our
final photograph shows that the bottom "cell" of the shear
cell arrangement has been replaced with a sample of wall material.
The weight hanger applies the load simulating pressures acting in a
bin and shear force is recorded as a function of this pressure. The
information gathered during this test, results in a wall friction
angle that is subsequently converted to a hopper slope for mass
flow.
The test procedure should not be
considered simple, because it requires constant practice to maintain
skills. As well, the testing process can be time consuming and as
such, you should evaluate your testing requirements before adding
this type equipment to your laboratory.
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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