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If an economizer is installed, check the unit operating
balance with the economizer at full outside air and at
minimum outside air. Upon completion of the air flow
balancing, we recommend replacing the variable pitched
motor sheave with a properly-sized fixed sheave. A
matching fixed sheave will provide longer belt and bearing
life and vibration free operation. Initially, it is best to have
a variable pitched motor sheave for the purpose of airflow
balancing, but once the balance has been achieved, fixed
sheaves maintain alignment and minimize vibration more
effectively.
NOTE: Never run high stage CFM below 300
CFM per ton evaporator freezing or poor unit
performance is possible.
For gas heat units, the airflow must be adjusted so that the
air temperature rise falls within the ranges given stated on
Data Plate.
Evaporator Fan Rotation Check
Check that fan rotates clockwise when viewed from the
drive side of unit and in accordance with rotation arrow
shown on blower housing. If it does not, reverse any two
incoming power cables at Single Point Power Block. In this
case, repeat bearing check.
Do not attempt to change load side wiring. Internal wiring
assures all motors and compressors will rotate in correct
direction once evaporator fan motor rotation check has
been made.
Electrical Input Check
Make preliminary check of evaporator fan ampere draw
and verify that motor nameplate amps are not exceeded. A
final check of amp draw should be made upon completion
of air balancing of the duct system.
Set Evaporator Fan Rpm
Actual RPM’s must be set and verified with a tachometer
or strobe light. Refer to Appendex A for basic unit fan
RPM. Refer also to “Airflow” section of this manual. With
disconnect switch open, disconnect thermostat wires
from terminals Y and W. This will prevent heating and
mechanical cooling from coming on. Place a jumper wire
across terminals R and G at TB1 terminal block. Close
disconnect switch; evaporator fan motor will operate so
RPM can be checked.
BELT DRIVE MODELS ONLY
Refer to “Motor Sheave Adjustmens” section.
Bearing Check
Prior to energizing any fans, check and make sure that all
setscrews are tight so that bearings are properly secured
to shafts.
Tension and Alignment Adjustment
Correct belt tension is very important to the life of your belt.
Too loose a belt will shorten its life; too tight, premature
motor and bearing failure will occur. Check you belt
drive for adequate “run-in” belt tension by measuring the
force required to deflect the belt at the midpoint of the
span length. Belt tension force can be measured using
a belt tension gauge, available through most belt drive
manufacturers.
SPAN LENGTH t
*DEFLECTION
FORCE
h
C
d
H
D
*Apply force to the center of the span.
t = Span length, inches
C = Center distance, inches
D = Larger sheave diameter, inches
d = Smaller sheave diameter, inches
h = Deflection height, inches
DRIVE BELT TENSION ADJUSTMENT
BELT DRIVE
Used
New
7.5 Ton
2.6 to 3.6 4.5
±
.5 5.5
±
.5
1/4
±
1/16
8.5 Ton
2.6 to 3.6 4.5
±
.5 5.5
±
.5
1/4
±
1/16
10 Ton
2.6 to 3.6 4.5
±
.5 5.5
±
.5
1/4
±
1/16
12.5 Ton
2.8 to 3.8 4.5
±
.5 5.5
±
.5
1/4
±
1/16
TYPE
SHEAVE
DIAMETER
(in)
MODEL
DEFLECTION
FORCE (lbs) DEFLECTION
(in)
A
Standard
Recommended Pounds of Force Per Belt
When new V-belts are installed on a drive the initial
tension will drop rapidly during the first few hours. Check
tension frequently during the first 24 hours of operation.
Subsequent retensioning should fall between the minimum
and maximum force. To determine the deflection distance
from the normal position, use a straightedge or stretch a
cord from sheave to sheave to use as a reference line. On
multiple belt drives, an adjacent undeflected belt can be
used as a reference.
MOTOR SHEAVE ADJUSTMENTS
Vl, Vm & 2vp Variable Pitch Key Type Motor
Sheaves
The driving and driven motor sheaves should be in
alignment with each other and the shafts parallel.
Summary of Contents for DBG Series
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