Corp. announced it has expanded its Airflex®
clutch line by creating the world's largest clutch for the mineral
processing industry. The Airflex 76VC2000 dual clutch system transmits rotary
torque from high-speed electric motors that drive gearboxes or low-speed
synchronous motors to high-performing mills up to 14,750 hp per pinion. The
clutch's 76-inch-diameter friction drum enables it to transmit adequate torque
for large grinding mill-drive applications.
The clutch offers a new motion control
solution for higher horsepower grinding mills, which have been driven primarily
by clutchless low-speed motors, by offering a very slow startup rate that keeps
component wear to a minimum and reduces power consumption.
"High-speed motors using gearboxes or
low-speed synchronous motors have been available for grinding mills, but
mismatched clutches resulted in a startup rate that was too fast, causing
increased stresses on mechanical systems," says Eaton's Don Keck, Airflex
global market development manager - mining.
"The new VC clutch will enable
low-speed synchronous motors or high-speed motors driving through a gearbox to
achieve full engagement at a slower rate not achievable with previous clutch
solutions, thereby protecting the power train by slowing the acceleration of
the entire drive system."
Keck says Eaton has gradually increased the
size of clutches over the years as application requirements have grown, and the
availability of new gearing has enabled systems offering larger horsepower. Until
recently, for example, a single pinion was limited to 9,000 hp across the gear
faces, but with the advent of new gearing technology and grinding alloys, systems
can handle up to 15,000 hp per pinion.
Working with GE's Large Motor
Division, Eaton is supplying systems that often use two large synchronous
motors and two pinions to drive a common bull gear. This approach saves the end
user on power consumption because a low-speed synchronous motor offers a power
factor where the system can actually put power back on the grid. "With the availability of 15,000 hp
per pinion and dual pinion drives, systems can produce a combination of 30,000
hp which is the current state-of-the-art," says Keck. "Our idea was to make the
existing 76VC1600 wider to increase torque capacity by 25 percent per element."
Because low-speed synchronous motors
need to start "no loaded" when coming up to speed, a key advantage is very low
power consumption at start-up compared to mills that are direct drive and have
very large power consumption on start-up. Keck says that nearly all of the
power companies in the U.S. charge based on peak power rather than average
power, so power spikes can cost millions of dollars per month.
"With a low speed synchronous solution, we
use 70 percent less power just by providing the low-speed start," says Keck. "Start-up
times can be extended over seven, eight, nine or 10 seconds which allows the
gearing, motors and mill to come up-to-speed and keep the power spike much
lower. Until now, solutions were to the 9,000 horsepower range but now we have
made a quantum leap and moved into the 14,000 to 16,000 horsepower range
depending on system speed requirements." The clutch technology also offers
benefits in terms of protecting the power train by cushioning the impact of the
motor. The clutch is the "fuse" in the system and, even though the clutches are
costly, motors and gearing are multi-million dollar pieces of equipment. The
clutch functions as a torque limiter to protect the bearings and gearing, and also
protects customers from excessive downtime due to equipment damage.
Larger horsepower systems are also important
as customers look for higher throughput. The trend is larger mills and, rather
than using concentrators, solutions that offer twice as much processing
"The key to the clutch technology is
that it is ultimately simple," says Keck. "There are no wear adjustments
required, and no significant downtime is required to repair and replace the
unit. The only moving parts are the actuating air tubes that inflate as we
supply air from the air tank group."
Advancements in auxiliary systems
include complete air tank systems and electrical controls. The latest
innovation is a "slip detect" control which monitors rotating components and
feeds information back into a PLC. The system can be programmed for an optimum
start time, for example, so if the customer sets the equipment to start too
fast, the system will flash a light indicating to slow the system down. If the
system is set up to start too slowly and exceeds the pre-programmed limit, it
aborts the clutch rather than letting it slip.
"It monitors the system during rotation so
that if something goes wrong in the mechanical or electrical system and we see
a speed differential between the motor and gearing, the system also abort the
clutch to protect the clutch, motor and grinding mill,"
says Keck. "The 76VC may be the world's largest clutch but it is also the
world's simplest. The beauty is in its simplicity, and it's been this way for many
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