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Air Balancing
Air balancing should be performed by a qualified air balanc-
ing technician. Note that the supply fan motors are usually
shipped with variable pitch sheaves which are typically set at
the low end of the drive’s fan rpm range. See “Mounting and
Adjusting Motor Sheaves” below for more information. The
return fan motors are usually shipped with fixed pitch sheaves.
Moving machinery hazard. Can cause severe personal
injury or death.
Use a strobe tachometer to measure the speed of return
fans. Safety considerations prohibit the use of a mechani-
cally driven tachometer on this fan arrangement.
The following should be performed as part of the air
balancing procedure:
1. Check the operating balance with the economizer damp-
ers positioned for both full outdoor air and minimum
outdoor air.
2. Assure that the total airflow will never be less than that
required for operation of the electric heaters or gas
furnace.
3. For VAV units that have fan tracking control, adjust the
supply/return fan balance by using the MicroTech con-
troller’s built-in, automatic capability. For complete infor-
mation on using this feature, see the “Return Fan Airflow
Control: Fan Tracking” section in Bulletin No. OM 108,
“MicroTech Applied Rooftop Unit Controller: VAV Control.”
4. When the final drive adjustments or changes are com-
plete, check the current draw of the supply and return fan
motors. The amperage must not exceed the service factor
stamped on the motor nameplate.
5. Upon completion of the air balance, replace variable pitch
motor sheaves (if any) with comparably sized fixed pitch
sheaves. A fixed pitch sheave will reduce vibration and
provide longer belt and bearing life.
Mounting and adjusting motor sheaves
VM and VP variable pitch sheaves
Mounting:
1. All sheaves should be mounted on the motor shaft with
setscrew “A” toward the motor (see Figure 33).
2. Be sure both the driving and driven sheaves are in align-
ment and that the shafts are parallel.
3. Fit internal key “D” between sheave and shaft, and lock
setscrew “A” securely in place.
Adjusting:
1. Slack off all belt tension by moving the motor toward the
driven shaft until the belts are free from the grooves. For
easiest adjustment, remove the belts.
2. Loosen setscrews “B” and “C” in the moving parts of the
sheave and pull out external key “E” (see Figure 33). This
key projects a small amount to provide a grip for removing.
3. Adjust the sheave pitch diameter for the desired fan speed
by opening the moving parts by half or full turns from
closed position. Do not open more than five full turns
for “A” belts or six full turns for “B” belts.
Adjust both halves of two-groove sheaves by the same
number of turns from closed to ensure that both grooves
have the same pitch diameter.
4. Replace external key “E” and securely tighten setscrews
“B” over the key. Tighten setscrews “C” into the keyway
in the fixed half of the sheave.
5. Put on belts and adjust the belt tension. Do not force
Figure 33. VM and VP variable pitch sheaves
C
C
A
One Groove
Two Groove
B
B
B
E
E
D
D
belts over grooves. Loosen the belts by adjusting the
motor base closer to the fan shaft.
6. Be sure that all keys are in place and that all setscrews are
tight before starting the drive. Check the setscrews and
belt tension after 24 hours of service.
LVP variable pitch sheaves
Mounting:
1. For single-groove sheaves, slide the sheave onto the
motor shaft so that the side of the sheave with setscrew
“A” is next to the motor (see Figure 34).
For two-groove sheaves, slide the sheave onto the
motor shaft so that the side of the sheave with setscrew
“A” is away from the motor (see Figure 34).
2. To remove the flange and locking rings:
a. Loosen setscrews “D”.
b. Loosen but do not remove capscrews “E”.
c. Remove key “F”. This key projects a small amount to
provide a grip for removing.
d. Rotate the flange counterclockwise until it disengages
the threads on the shaft barrel.
3. Be sure that the driving and driven sheaves are in align-
ment and the shafts are parallel. When aligning two-
groove sheaves, allow room between the sheave and
motor to get to capscrews “E”.
4. Insert key “C” between the sheave and the shaft and
tighten setscrew “A” securely.
Adjusting:
1. Slack off all belt tension by moving the motor toward the
driven shaft until the belts are free from the grooves. For
easiest adjustment, remove the belts.
2. Loosen setscrews “D”.
3. Loosen but do not remove capscrews “E”.
4. Remove key “F”. This key projects a small amount to
provide a grip for removing.
5. Adjust the pitch diameter by opening or closing the
movable flange by half or full turns. Note that two-groove
sheaves are supplied with both grooves set at the same
pitch diameter. Both movable flanges must be moved
the same number of turns to ensure the same pitch
diameter for satisfactory operation. Do not open
sheaves more than five turns for “A” belts or six turns
for “B” belts.
6. Replace key “F”.
7. Tighten setscrews “D” and capscrews “E”.
8. Put on the belts and adjust the belt tension. Do not force
belts over grooves. Loosen the belts by adjusting the
motor base closer to the fan shaft.
Key “E” projects to provide a grip for removal.
Do not operate sheaves with flange projecting beyond the hub end.
!
WARNING
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