FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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ACOUSTIC SORTING OF GRINDER BALLS
F. Na.de.au. and J.F. Zixalnfin
national Re.itax.ch Council o i Canada, Boucke.n.v±lle., Qntan-io
An automated non destructive testing device for ball mill grinder balls
was developed using acoustic flaw detection. An initial study of the sound
generated in air by the impact of grinder balls has shown that the changes
in ball velocity upon impact produce an initial low frequency pressure
pulse and that various resonances exited in the impacted spheres produce the
higher frequency oscillations that follow. Resonances of uncracked balls
were related to a particular spheroidal mode of vibration while resonances
of cracked balls are attributed to "tuning fork" modes of vibrations.
Both a laboratory and an industrial prototype that perform the sorting
of cracked and uncracked grinder balls using the differences in their
acoustic properties were built and tested. A 97% success rate was achieved
for 3" diameter balls. The device is being fitted to the production line.
INTRODUCTION
Many of the more sophisticated Non Destructive Testing (N.D.T.)
techniques are expensive. This often restricts their use either to the
aerospace or the nuclear industries. When an N.D.T. problem involves a low
mark-up non critical product, emphasis is placed on the per unit cost of the
test. Automation becomes important while 100% reliability is no longer
required. This paper presents such an example where a cheap, fast,
automated N.D.T. method was developed using audio range acoustics to detect
flaws in grinder balls. Grinder balls are hardened steel balls used in
stone grinding "ball mills". Those used in this study are forged from high
carbon steel rods to roughly spherical shapes and waterquenched to a
hardness of about 60 Re. A non négligeable proportion of the production
exhibits deep running cracks and therefore tend to rupture under operating
conditions inside the ball mill. This problem is familiar to ball mill
operators who often have to manually remove the broken parts from inside the
mill to ensure smooth operating conditions and prevent exercise wear of the
mill's inner sleeve. Testing of the production is done on a statistical
basis (samples of each batch) by manual inspection. Cracks can be detected
visually and also acoustically: two balls are knocked together and a
characteristic high pitch sound is heard if one of them is cracked.
Acoustic, or "sonic" inspection is probably one of the oldest N.D.T.
method. The sound of a body's natural resonances has been used to evaluate
material properties (1, 2, 3), size and shape (4), and also for flaw
detection (5). In the case of grinder balls, a study of the acoustical
properties of good and cracked balls confirmed that repeatabledifferences in
their natural sounds could be electronically detected. These results are
presented in the first part of this article.
An automatic grinder ball tester was designed and both a laboratory and
an industrial prototype were built and tested. The second part of this
article contains description of the laboratory machine along with some test
results.