NORD Drivesystems B1000 - Manual (Page 13)

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INTRODUCTION
Selection
Information
Gearbox Selection
A number of factors are considered when selecting
a gear unit, including gearbox rating, service factor,
speed and speed variation, horsepower, thermal ca-
pacity, ratio, physical size, ambient conditions and
cost. Below are some guideline steps to help aid in
the gear unit selection.
1. Determine the speed and/or gear ratio
2. Determine the required power or torque
3. Determine Service Factor
4. Select the basic gearbox type and input
5. Determine the required mounting position
6. Select options
7. Checks – overhung load, thrust load, NEMA motor
weight, thermal considerations, and other
application considerations
1. Speed and Gear Ratio
The fi rst step in selecting a gear unit is determin-
ing the fi nal output speed or speeds you need. This
speed is normally described in revolutions per min-
ute (rpm). This output speed or speeds is deter-
mined by the input speed to the gear unit divided
by its gear ratio. Their relationship is described by
the following formulas.
i (gear ratio) =
Input speed [rpm]
Output speed [rpm] =
Input speed [rpm]
To specify a gear unit, you can identify either gear
ratio needed or the output speed (rpm) if the input
speed is known.
2. Power and Torque
The second step for selecting a gear unit is the re-
quired power or torque needed to power the load.
Torque in this catalog is normally expressed in
pound-inches [lb-in].
Power [hp] =
Torque [lb-in] x speed [rpm]
Torque [lb-in] =
Power [hp] x 63025
For a proper selection you must ensure that the
motor or other prime mover can produce enough
torque or power and that the gear unit has ade-
quate torque or power capacity. You must also con-
sider if the power or torque is specifi ed at the input
or output of the gear unit. The Helical-worm gear
units have lower effi ciency than in-line or bevel gear
units, therefore helical-worm products effi ciency may
need to be considered in the selection.
To specify a gear unit you can identify torque or power.
3. Service Factor or Service Class
In addition to power or torque, service factor must
also be considered. A service factor is essentially
the ratio of extra capacity in a gear unit compared
to the power or torque that is needed to run that
application. The goal of selecting a gear unit with
extra capacity (service factor) is to provide adequate
service life in operation.
One reason to apply a larger service factor is if a unit
operates more hours per day. If a unit runs 24 hours
per day it should normally have a higher service
factor than a unit that runs 8 hours per day if you
expect the same calendar life.
A second reason for applying a larger service factor
is to cope with a more diffi cult application. Even
if it takes the same power and speed to operate a
rock crusher as a fan, the rock crusher needs a stron-
ger gearbox (higher service factor) to give the same
operating life as the gear unit powering the fan.
The real question is how to determine the proper
service factor for a gear unit in an application. Fol-
lowing are four possible methods.
Customer or User Specifi cation
Many customers will have their own service factor
guidelines or specifi cations.
AGMA Service Factoring
American Gear Manufacturers Association (AGMA)
publishes lists of recommended service factors for
different applications. These service factor recom-
mendations have been determined form the experi-
ence of many gear manufactures and are in AGMA
standard 6010. See page 68 for additional detail.
AGMA Service Classes
American Gear Manufactures Association (AGMA) has
another method for selecting gear units service fac-
tors. AGMA standard 6009 lists many applications by a
service class (I, II, III) with class I being the simplest
applications and class III being the hardest. These
application service classes are associated with a
range of service factors by the following table.
AGMA Service Class Service Factor
I 1.00 to 1.39
II 1.40 to 1.99
III 2.00 and above
In the gearmotors selection table each unit is also
classifi ed by an AGMA service class. See page 64 for
additional detail.
Output speed [rpm]
i (gear ratio)
63025
speed [rpm]