can be found in two-(2) forms, either as a wear related
particle or a contaminant related particle. Wear related spheres form as
fatigue cracks in a loaded surface. As the micro-cracks work through the
surface and a spall begins to form, the spall will isolate, generating small
(>5mm) spheres with oxidized
surfaces. Spheres are found in large concentrations and consistent sizes.
Spheres form in much of the same manner as having glue on your hands and
rubbing your hands together. The result is little balls of material.
Equipment related spheres
are tektites formed not specifically by a wear mechanism but by equipment
related factors. These factors include excessive aeration of lubricants in
hydraulic systems. Such aeration or restricted inlet flow to pumps cause
fluid vacancies. These vacancies or air bubbles then implode causing shocks
which crack the surface in the load zone thus removing material. Similar
occurrence can be seen in cavitation of impellers in high speed operation.
Contaminant related spheres
are typically the resultant tektites from such activities as welding,
grinding, sand-blasting, mold dust, the result of combustion of coal or
electrical shorts passing though the component. Contaminant spheres are
comparatively few in concentration and inconsistent in size. Size can range
from 6mm-12mm.
Regardless of the origin
of these unique particle types, it is to be understood that these particles
are hard and abrasive. The doughnut shaped appearance results form
the way bi-chromatic light is reflected back to the observer. The
bright center returns a higher concetration of reflect light. While
progressing down the curved surface of the sphere returns less light
directly back up to the observer.