Material: Recycled
Glass
Issue: Thyxotropic or slurried castable materials often need to be vibrated
after they are poured into molds to get the particles to level and
to pack in the most efficient manner.
Variable speed, variable displacement vibrating tables are
available through equipment manufacturers for the casting industry. However, OEM equipment may be too expensive
for small shops or labs experimenting with castables.
Best
Practice: A
simple, inexpensive, and effective vibratory compaction table can
be built from materials available at any hardware store.
The design is self-leveling so that the castable maintains
uniform thickness. It also
can be made to have a relatively large displacement at relatively
slow speeds. The frequency suggested for vibratory compaction
of concrete is 3000 rpm.
The following equipment is necessary to build the compaction
table:
·
A fractional
(1/3-1/2) horsepower motor (variable speed if possible) with a pulley.
·
One short
(1 to 1 1/2”) machine bolt as heavy as possible, but thin enough to
mount through a hole drilled in the motor pulley, along with several
spare nuts and washers.
·
Four heavy
springs such as those used in door closures. The springs should be six to ten inches long with connection loops
on each end and be designed for extension rather than compression. If total weight of the table, motor, and expected
load will be 40 pounds, the springs should each extend about one-half
inch with a load of 10 pounds.
·
Eight lag
wood bolts at least three inches long.
·
Table top
material, preferably with a cleanable surface like Formica, and a
thickness and density great enough to firmly anchor the motor beneath.
·
Wood for
the frame, possibly 2x4’s.
1.
Drill a hole
in the motor pulley and mount the machine bolt through it. This gives an eccentric weight to the pulley.
Check the eccentric by plugging in the motor while holding
it in the hands. The eccentric
should cause the motor to shake. Add washers and nuts to increase the eccentric.
2.
Mount the
motor on the bottom of the tabletop.
The bolt on the pulley must turn freely, clear of obstructions. The center of gravity of the motor and pulley
should coincide with the center of the table.
3.
Build a frame
around the motor, mounted to the bottom of the tabletop. This will be the inner frame. Remember, there will be two frames, an inner
frame and an outer frame. Ideally,
the outside of the outer frame should be inside the outer edge of
the table. That way, any liquids running off the table
will run directly onto the floor rather than onto the frame.
4.
Build the
inner frame. This frame will
support the unit on the bench or floor.
There should be at least 1” clearance between the inner and
outer frames on the sides to allow room for the lag bolt heads and
spring bodies. Clearance on the ends should be at least 1/2”.
5.
Mount four
lag bolts on the sides toward the bottom of the inside of the outer
frame and four more toward the top of the outside of the inner frame. Some experimentation will determine mounting
points where the table with the motor mounted to it hangs by the springs
from the outside of the outer frame without touching any side.
6.
Hang the
inner table on the outer frame with the springs.
7.
Test the
table by running the motor. The
spring tension should be such that the table hangs with only slight
spring extension with no load, and does not bottom out on the outer
frame under maximum load. The
table should be always be fully suspended by the springs.
8.
The whole
assembly can be placed on a counter.
If properly isolated, no “walking” should occur during operation.
Because the table is mounted on springs, it will be
somewhat self-leveling. However,
it is useful to mount a leveling eye on the tabletop so the tabletop
can be kept both level and centered.
Since this description was written, it has been discovered
that ball bearing “Glides” can be purchased at woodworker supply stores.
The glides, mounted under each corner, can be a great aid to
prevent tipping.
The displacement can be adjusted by adding eccentric
weight to the pulley. If a
variable speed motor is being used, the frequency changes with motor
speed.
Many kind of vibrators can be purchased. Some electric and pneumatic vibrators fasten
to the bottom of suspended tables to generate vibration for leveling
materials. Finding the best
frequency, displacement, and direction of vibration are all critical
factors. The equipment described in this Best Practice
primarily generates vibration in the plane of the table. Some vibrators generate vibration perpendicular
to the plane of the table. It
has been found by experience that leveling calcium aluminate cement
works better with vibration in the plane of the table.
Implementation: Before building the vibratory
compactor, estimate as closely as possible the size and weight of
casting to be compacted. That
judgment will inform the size of the table, strength of the springs,
and power of the motor.
Benefits:
This
compaction table can be made inexpensively from used or new hardware
materials and is a necessary piece of equipment for experimentation
with castables.
Application
Sites: Small
shops or laboratories experimenting with thyxotropic or slurried castables. Contact: For more information, contact CWC, info@cwc.org Issue
Date / Update: November 1998 |
