Page 115 - Shale Shakers Drilling Fluid Systems
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98 SHALE SHAKERS AND DRILLING FLUID SYSTEMS
and mechanically reliable, but were generally lim-
ited to 20-mesh and coarser screens.
In an unbalanced elliptical motion shaker (Fig-
ure 3-1), the movement of the shaker deck/bas-
ket is accomplished by placing a single vibrator
system above the shaker deck. In other words, the
mechanical system of spinning counterweights (or
an elliptical-shaped drive shaft) is installed above
the center of gravity of the deck. The resulting
motion imparted to the bed is a combination of
elliptical and circular. Directly below the vibrator
the motion of the basket is circular, while at ei-
ther end of the deck the motion is elliptical.
The orientation of the major axes of the ellipses
formed at the feed end and solids discharge end
FIGURE 3-3. Linear motion.
of the basket has a major impact on solids con-
veyance. Specifically, it is desirable for the major
axis of the ellipsoidal trace to be directed toward
the solids discharge end. However, the orientation
of the major axis of the ellipse formed at the
solids discharge end is exactly the opposite; it is
directed backward toward the feed end. This dis-
charge end thrust orientation is undesirable since
it makes discharging solids from the shaker more
difficult (Figure 3-5). To assist in solids convey-
ance, the deck or last screen is tilted downward
(Figure 3-6), or the vibrator is moved to the dis-
charge end. Moving the vibrator toward the dis-
charge end significantly reduces the fluid capac-
ity and the screen life of the end screen. This also
reduces the residence time of the feed slurry on
the screening surface.
Early elliptical motion shale shakers used hook
strip screens that were manually tensioned. A
series of tension rails and tension bolt spring
assemblies were used to pull the screens tightly
over the support bars to ensure proper tightening.
Pretensioned screens and pretensioned screen
panels were not introduced until the 1970s and
even then were not commonly used on elliptical
motion units.
FIGURE 3-4. Balanced elliptical motion.
Unbalanced Elliptical Motion
Shale Shakers
In the 1930s, elliptical (unbalanced) shale
shakers were adapted to the oil field. These first
shakers originated from the mineral ore dressing
industries (e.g., coal, copper, etc.) with little or no FIGURE 3-5. Undesirable discharge end thrust
modifications. These machines were basic, rugged, orientation.
