Non-destructive Examinations (NDE) is a predictive maintenance technique that investigates the
condition of machinery such as extrusion presses in order to determine whether
and when maintenance and repairs should be completed.NDE methods can detect cracks and other flaws
deep within a metallic component before they become lead to its failure.Unexpected component failure can completely
shut down a machine.Unexpected failure
of major components can potentially disable a press for very long periods of
time.Detecting problems before they
lead to failure can also prevent unnecessary physical dangers to
employees.NDE methods include: Visual Inspection, Ultrasonic, Magnetic
Particle, and Liquid Penetrant Examinations.
Types of Inspection Techniques
Oil Leak
Visual Inspection Visual Inspection is the primary press examination
method.During the visual inspection one
can see problems such as loose or missing components such as detached die slide cylinders.Other visual indications, such as oil leaks
and rust steaks, can lead an experienced examiner to serious underlying flaws
Cracks in thread of tie rod
Ultrasonic
Inspection One of the most sensitive
predictive technologies for detecting mechanical problems is ultrasonic
monitoring.Frequencies ranging from .5
to 50 MHz are used on metal objects to detect flaws such as cracks or to
evaluate the thickness of an object to determine whether there is
corrosion.Not only can ultrasonic testing (UT)
detect extremely small flaws and at very great depths within machinery, but one
only needs one accessible surface to identify these flaws.Also the ultrasonic frequencies do not affect
the machines adversely.A transducer is
placed on the metal material and the screen on the diagnostic machine shows the
amplitude and the time it takes for the wave to return to the transducer.From this information, the inspector can
determine where there might be cracks, how long they are, and at what depth.
(Wikipedia, 2007, September 2).
For example, tie rods are examined
with UT straight and angle beam techniques.The examinations are applied from both ends, and where necessary, the
center spans of each tie rod.The
examinations are expanded to cover the complete length of each tie rod.The rod ends have to be prepared to an
adequate surface condition. Because there are several mechanisms that affect
the conductance of an ultrasonic beam within a tie rod, UT straight beams
applied from the ends of rods can result in false indications.Ultrasonic instruments with special functions
must be used to discriminate between false indications and true cracks.Tie rods are susceptible to cracking in their
threaded areas.Cracks usually initiate
in the last threads behind the end nuts, but can also be found in the inner
threaded sections.Any surface
anomalies, such as welds or lifting lugs should be examined with Magnetic testing.
The image to the left show an ultrasonic straight beam indication of a crack in the threads of a tie
rod.The size of the crack is marked up
on the end of the tie rod. The crack is in the third thread from the end where
the nut bears on the rear platen.
Magnetic particle indications of cracks in hot container housing
Magnetic Particle Testing
Magnetic Particle Testing shows indications of flaws
on accessible surfaces of ferrous materials based on “the principle that the
magnetic susceptibility of a defect is markedly poorer (the magnetic resistance
is greater) than that of the surrounding material” (Wikipedia, 2007, September
8).With Magnetic Particle testing
colored magnetic particles are scattered over a surface that the inspector then
magnetizes externally.If there is a
crack or other flaw, they become attracted to and build up along the area of
defect.The image to the right shows the
magnetic particle indications of cracks in a hot container housing.
Red dye residual from liquid penetrant application shows a crack
Liquid
Penetrant Testing
Liquid Penetrant testing is often utilized on
non-ferrous materials or ferrous materials when magnetic particle examination
is difficult to use (Wikipedia, 2005, September 7).Before work can begin, the area needs to be
thoroughly cleaned.A red dye is placed
on the machinery and within 10 to 30 minutes any excess fluid is removed with a
cloth doused with solvent remover.Then
a white developer is sprayed on the surface that draws the dye in defects to
the surface.If one sees a colored dye
appear, that indicates the position of the flaw (Wikipedia, 2005, September
7).Once the area has been inspected,
the surface area is cleaned.Oftentimes
leaking oil from the machinery also acts as a Liquid Penetrant. This image indicates a crack as highlighted with the red dye.
