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How to Design a Volumetric Screw Feeder
(Part 2)
by Joseph Marinelli
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In our previous article, we discussed the WRONG way to design
a mass flow screw feeder. This article will discuss three possible approaches to providing
uniform discharge with a mass flow screw feeder that will maintain mass flow from our bin.
If you will remember from our last article, the key to
successful mass flow screw feeder design was to increase the screw’s capacity in the
discharge direction. We showed how not to do it! Here is the proper way to provide
increased screw capacity to ensure uniform withdrawal.
Fig. 1 shows a tapered shaft mass flow screw feeder. This
conical shaft is either wound in between the ½ pitch flights or formed and the flights
fitted around the shaft. This approach will provide increased flight capacity; however,
the length of outlet over which you can use this is limited to three (3) times the screw
diameter. In other words, a 12" screw with a conical shaft can be used with a
36" long outlet. The reason for this limit is simply fabrication tolerances. The
screw feeder fabricator can only maintain the flight tolerances over this particular
diameter and length of screw.
Fig. 2 shows an increasing pitch mass flow screw feeder.
Notice that the flights increase from ½ pitch to full pitch over the length of the screw.
This approach will as well, provide increased flight capacity; however, the length of
outlet over which you can use this is also limited to three (3) times the screw diameter.
Fig. 3 shows a combination of conical shaft and increasing
pitch to maintain mass flow. The two approaches stated previously are joined together to
provide a longer screw. Instead of being limited by a 3 to 1 outlet length to screw
diameter ratio, we can use a 6 to 1ratio. This allows us to use larger screw lengths to
accommodate larger slot openings and/or more cohesive materials.
Notice the following:
- There is U-shaped trough to contain the screw. vee-shaped
troughs will not work because material usually only feeds directly above the screw
diameter. A vee-shaped trough will create stagnant material and destroy our mass flow
pattern.
- There no hanger bearings to support the screw; therefore, the
screw shaft has to be sized to withstand deflection. Hanger bearings will interfere with
the full capacity screw trough. Material will back up into the hopper, effecting flow.
Your hanger bearings will also wear out quickly.
- The screw flights should be smooth while the screw trough
(walls) should be rough. If they are both smooth, the material will simply spin in the
trough and not convey.
- The screw speed should be kept between 3 and 40 rpm. Three on
the low side to minimize motor reducer size and 40 on the high side to maintain screw
efficiency.
- There is ½" clearance between the screw and screw
trough.
In upcoming articles we will discuss other volumetric feeder
designs. See you at the Powders Show in Chicago !!!
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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