MOORE FANS LLC, Marceline, MO 64658 Phone (660 ) 376-3575 FAX (660) 376-2909
Page 11
TMC-647-(Rev E) - 01/06
3.3.7 FIELD BALANCING
Unbalance in older fans may develop because of some
structural change or by installing one new blade on an old fan
where the existing blades had changed in weight in the
course of operation.
Use wire to attach a small weight in succession to each of
the clamp studs until the best location for the weight is found.
The weight should then be increased or decreased until the
best balance is achieved. The permanent weight may then be
secured to the clamp stud, clevis, or hub tube, whichever is
the most convenient for the type and shape of weight to be
used. One or more pieces of metal shaped like a washer could
be placed over the clamp stud, on the hub tube, behind the
clamp stud, or over the threaded portion of the clevis. Alumi-
num or stainless weights should be used and weights should
not be attached to the blade skin.
3.3.6 THROAT FLUTTER
Any fan that is effectively moving air at the tips of the
blades will develop a reduced pressure area (or suction) on
the fan throat or ring at the tip of the blade. This suction tends
to draw the throat toward the tip of each blade, which means
that a four blade fan would tend to draw the throat into
something approaching a square while a six blade fan would
draw it into something resembling a hexagon, etc. Since the
fan is rotating, the effect on the throat is that of continually
drawing it into a rotating polygon. The resulting throat
flutter is frequently mistaken for fan unbalance.
A substantial throat or ring will be sufficiently rigid that
flutter will not exist. A weak or flexible throat, particularly
when used with a fan of a low number of blades, will be
greatly affected by this type of vibration. Throat flutter is
easily detected due to the fact that it is invariably of a
frequency of the fan RPM times the number of blades on the
fan.
Throat flutter will cause no damage to the fan so long as
the throat does not disintegrate and fall into the fan blades. It
may be eliminated by stiffening or bracing the throat.
If in doubt that throat flutter is the cause of vibration,
reduce the angle of the blades until the fan is doing little or no
work. If the vibration ceases under this condition, it is certain
that throat flutter is present when the blades are loaded.
3.3.5 ROUGH GEARS
Continued operation on rough gears and bearings is
almost certain to develop cracks in the blade skins. Rough
gears may be of two types:
3.3.4 DRIVE SHAFT OR MOTOR UNBALANCE
Fans are often driven by 1200 or 1800 RPM motors
which drive a right angle gear through a long drive shaft
equipped with two flexible couplings. This type of drive is
subject to several possible causes of vibration:
1. A long drive shaft with too much flexibility causes a
whip in the drive shaft which will cause severe unbalance.
2. In the case of carbon steel shafts, uneven corrosion
may throw the shaft out of balance.
3. Flexible couplings may become misaligned, throw-
ing the drive shaft off-center.
4. Motor bearings or speed reducer input shaft bear-
ings may become worn and loose, allowing the drive shaft
to slip off-center or rotate eccentrically around its normal
axis.
5. Supports too limber connecting the gear to the motor
may result in misalignment.
Drive shaft unbalance may be easily distinguished
from fan unbalance because the frequency of the vibration
is far higher than the RPM of the fan. This may be detected
with a frequency meter or it may be felt by standing on the
frame supporting the motor and gear. If vibration of this
type is present, it will be much too high a frequency to be
counted but may be felt as a pulsation in the soles of the feet.
This type of vibration is not only detrimental to the fan
but will also result in eventual damage to the bearings in the
motor and gear. Repair or replacement of the malfunction-
ing or unbalanced parts should be accomplished immedi-
ately.
3.3.3 BELT DRIVE UNITS
The more common causes of vibration in belt drive
units are not the drives themselves but the result of shafts
that are too flexible or non-rigid supporting members. Vi-
bration can be caused by misalignment of the sheaves or
poorly adjusted belt tension. Consult the manufacturer of
the drives for information. The quickest way to identify the
cause of vibration in belt drive units is to operate the fan
with the blades removed.
1. Rough or failed bearings in the drives or gears will
result in a high frequency vibration being transmitted into
the fan where some areas of the skin will respond to the
frequencies applied. Cracks will appear in the blade skin and
eventually, in some areas, the skin may actually fall away.
2. The other type of rough bearing occurs when the
output shaft accelerates and decelerates with each pinion
tooth engagement. With a six tooth pinion and a motor speed
of 1800 RPM, or 30 cycles per second, this gear misalignment
impresses upon the fan a vibrating frequency of 30 x 6 = 180
cycles per second. If the engagement of teeth is also included,
the frequency is 360 cycles per second. This type of high
frequency vibration is at least as serious as that caused by bad
bearings.
of perhaps 25 HP, would be supported on bearings rated
less than 2000 or 3000 pounds radial load. For higher
horsepowers, the bearing capacity would be correspond-
ingly increased. From this it is evident that speed reducer or
drive shaft bearing failure could never be caused by moder-
ate or even objectionable fan unbalance.
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