Page 215 - Shale Shakers Drilling Fluid Systems
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CENTRIFUGAL PUMPS 197
Shaft deflection. When not in motion, the shaft Proper lubrication is essential to bearings. Oil
is straight and centered in the stuffing box. When levels must be maintained, contamination pre-
rotating, any unbalanced radial load on the impel- vented, and regreasing performed properly. In gen-
ler deflects the shaft rotation from that of a true eral, grease is preferred as a lubricant when:
circle. Under these conditions, the shaft will run
in an elliptical or oval pattern. Packing in the stuff- • Temperatures are moderate—usually not above
ing box can be adjusted with the packing gland to 200°F
allow minimal leakage. • Rotating speed does not exceed bearing manu-
facturer's recommendations
Shaft whip. A cone-shaped pattern of rotation • Extra protection from dirt or fumes is needed
characterizes shaft whip. This creates a hole in the • Prolonged periods of operation without quali-
packing larger than the shaft diameter and exces- fied maintenance are anticipated
sive leakage will occur. A properly balanced im-
peller will always cause whip because the heavy In general, oil is the preferred bearing lubricant in
side of the impeller is always on the same side of cases where:
the shaft as it rotates. It is also possible to have
a combination of shaft deflection and shaft whip • Operating temperatures are consistently high
at the same time.
• Speed exceeds bearing manufacturer's limit
for grease
Shaft runout. Shaft runout is the amount that • Dirt and dust are not excessive and oil-tight
the shaft power end section is out of true when seals can be used
measured as the shaft is slowly rotated. Defects • Bearing design does not lend itself to grease
such as an out-of-round shaft, eccentricity be- lubrication
tween the shaft and shaft sleeve, or permanent
bend in the shaft produce an oversized hole in the
shaft packing, thereby making the stuffing box Bearing assembly checklist:
hard to seal.
1. Does the shaft meet manufacturer's specifi-
Bearings and bearing life. Bearings are one of cations for machined tolerances?
the most important components of a centrifugal 2. Are locating shoulders at 90° angles to the
pump. When properly mounted, lubricated, and shaft centerline so that the bearing will be
protected from dirt, a ball bearing will carry axial squared with the shaft?
and radial loads at high rotary speeds for years. 3. Are housing bores in straight alignment so
For troublefree service, bearings should: that the bearing will not operate in a twisted
position?
• Allow the shaft to rotate with negligible 4. Are grease and/or oil of proper grade and
friction, keeping power requirements to a viscosity at correct levels?
minimum 5. Do lubricant seals "seal"?
• Hold the shaft assembly radially and axially
in the proper position relative to the station-
ary parts of the pump, preventing contact be-
tween moving and nonmoving parts VIBRATION
• Absorb forces transmitted to them from im-
peller loading Vibration damages the bearings and other parts
of the pump. It is measured in the horizontal,
Bearings operate for a specific number of revo- vertical, and radial directions and expressed in
lutions under specified loads before evidence of terms of peak-to-peak displacements.
failure appears. At higher loads, the total number Vibration may originate within the pump and
of revolutions decreases, and at lighter loads the motor or be transmitted from an outside source.
number increases. Bearings that control axial move- If large enough, it will damage some of the pump
ment are designed to limit end movement below parts. The change in vibration magnitude between
0.002. Generally, end-play above 0.002 has been inspection periods is at least as important as the
found to be detrimental to packing and mechani- amount of vibration itself. An increasing vibration
cal seal integrity. Bearing temperatures should not level, even at low values, may indicate an impend-
exceed 225°F. Excessive noise, heat, or vibration ing problem. An unchanged vibration level, even
are signs that bearings have reached the end of though relatively high in absolute value, gives
their service life. evidence of a stable operating condition.
