Nykanen: Couplings or belts are used to drive industrial
rotary blowers. Small radial fans such as regenerative
blowers are often direct-coupled with inexpensive, rigid
couplings. This is possible through precision casting and
machining in large production quantities which allows for
pilot mounting motor and blower — essentially eliminating
the labor of alignment. Medium-to-large centrifugal fans
are either belt- or coupling-driven. These systems often use
inexpensive, elastomeric coupling or belt drives that result
in fairly easy installation, with rudimentary alignment
typically necessary. Economy- and mid-cost-range positive
displacement blowers often use v-belt drives. The belt
drive allows for flow and pressure adjustment by changing
the size of the pulleys. Low-to-mid-cost systems often use
mass-produced, low-precision cast metal or molded plastic
parts. Most of these blowers are operated in an openloop
electrical system with a start/stop switch being the
only control. High-end screw and helical lobe machines
use either direct-drive or belt drives, depending on system
requirements. Expensive air-movers often employ sophisticated
electronics such as variable frequency drives, sensors,
and controls that operate in a closed-loop system
that dynamically adjusts to operating conditions using PID
controls. High-cost systems often demand precision-made
coupling systems that tend to be machined from quality
metals custom-specified for the application.
- Click image to enlarge
Indirect-drive belts are economical and easily allow for
connection of a motor (shafts in parallel) in many orientations
to a piece of driven equipment. Due to the multiple
connection options, belts allow options for many space
constraints. Elastomeric belting also adsorbs mechanical
vibration and takes up misalignments. V-belts are a
common choice with positive-displacement blowers and some centrifugal blowers. In a blower system, flow and pressure
are controlled by driven speed. Belt sheaves function
like gears by increasing or decreasing motor speed ratio to
achieve desired system parameters. After a belt-driven machine
and the motor are in place and lined up, removing the
belts can normally be done without unbolting anything but
the sheaves, making maintenance easy. A no-load motor test
or belt and sheave change can take as little as a few minutes
once the guarding is removed. However, belt sizing can be
tedious using manual calculations; computer selection software
offered by major manufacturers can alleviate this. With
belt drives, overhung shaft loads must always be considered
and manufacturers of economy blowers often do not allow
for enough bearing loads to give the drive designer many options
in choosing belts. This often leads to custom guarding,
which can add considerable cost to a system. Due to a constant
radial load on bearings with belts, there is a decrease
in lifespan vs. direct-driven machines. Elastomeric belts are
also a wear item that essentially stretch out over time and
need to be manually or automatically tensioned throughout
the life of the belt — creating a need for maintenance or purchase
of an additional automatic belt tensioner.
Direct-drive (shaft-to-shaft) couplings for blowers are normally
fairly simple to size, often by simply looking at a catalogue
page and evaluating torque, speed and safety factors.
Often, standard off-the-shelf guarding can be used, resulting
in a bit of cost and labor savings. A well-aligned coupling
places little radial pressure on bearings, thus increasing their
life. Also, losses in mechanical efficiency can be somewhat
reduced via direct-drive by saving some electricity. Although
most couplings offer a 1:1 speed ratio, limiting speed to
number of motor poles (in the case of standard AC motors),
they are often used in conjunction with variable frequency
drives that offer many electrical and mechanical advantages
for blower systems. A common example of this is programming
a soft start and constant torque, or current limiting.
Many VFDs also can be wired with sensors that provide PID
speed control to maintain constant pressure in a blower system;
this is very common when variable orifices in pipework
are present, e.g., filters and valves.
Another advantage with direct-drive couplings for blower
systems is that several prominent manufacturers offer ATEX-
certified couplings that provide electrical continuity between
shafts and prevent sparking. Many blowers and motors
are specifically designed to operate in ATEX or NEMA
hazardous locations and are name-plated accordingly. This
is especially important and is often specified by system designers
in the gas and fuel processing industries. Couplings
used in high-end equipment can be precisely machined and
balanced, which can impact budgets and lead times. Thus
great care must be taken in shaft alignment and in planning
the installation projects for these systems. Custom-woundand-
name-plated electric motors can have lead times of
several months; the same applies for blowers and drive components
— especially when material certificates and specific
countries of origin become a factor.
- Click image to enlarge
In conclusion, a designer behind a gas blower system likely
would choose a direct-coupling over belt drive for the reasons
given above. A belt drive usually requires more maintenance
than a coupling. Given the highly competitive nature
of processing a commodity product, downtime for maintaining
the system is a huge factor in determining the parts to be
used. Also, there are more options for couplings operating in
hazardous locations then there are for belts in most markets,
affording the system designer a wider variety of choices.
Labriolla: Full-disclosure — I do not typically work with
blowers, but in general motion the two main issues between
couplers are usually 1) geometry and 2) speed ratio. (System
stiffness may be a third, but is not likely related to blowers.)
Geometry of the layout: With a belt the motor can be in
the same width as the blower, sometimes making a more
compact configuration, whereas the coupler forces an inline
configuration.
