Costs Reduced, Quality Sustained with Drive System Disc Coupling Installation at Wisconsin Mini-Mill
 Charter Steel eliminated a casting process variable, which was contributing
to steel billet surface quality imperfections, and drastically decreased
drive system maintenance time and costs by replacing gear couplings
with non-lubricated all-steel disc couplings.
The problem at Charter was contained in the area of the four strand billet-caster
tangent-roll drive system. According to Mike Sauer, melting division
maintenance manager at Charter Steel, "The hot, moist, corrosive environment
in the continuous caster area makes it very difficult to maintain gear
couplings. Plus, we were concerned that the gear couplings' backlash
might be contributing to billet surface imperfections." Established
in 1978, the steel division of Charter Manufacturing Company Inc. is
located in Saukville, Wis. Charter Steel's production facilities contain
a melt shop, rolling mill, and processing plant.
The majority of Charter Steel's 300,000 ton annual production of top
quality specialty and alloy steels are employed in cold-heading and
recessed-heading applications. Nearly 60 percent of the steel is utilized
in the automotive industry. The remaining percentage is used in the
production of fasteners, bolts, chains, steel wool, wire rope, and tool
steels.
Charter Steel's process begins with the melting of scrap metal in a DC
furnace. The metal is further processed in a vacuum arc degasser which
draws impurities out of the steel while achieving final temperature
and chemistry. The molten steel proceeds to the four strand continuous
caster, where 4-½-inch square billets are cast and then cut to 32 foot
lengths.
It is in the area of the bend zone of the caster, that Charter faced
some particularly difficult operational challenges related to the tangent-roll
drive system. The harsh environment in the area of the caster, which
included high ambient temperatures (resulting from 1,800˚ F [982˚ C]
billet temperatures) coupled with moisture from the water cooling of
the steel, was wreaking havoc with some tangent-roll drive system components.
In particular, the systems' gear couplings were falling victim to the harsh
environment. The hot conditions led to premature seal and lubrication
breakdown, which in turn resulted in frequent maintenance intervals
and a shorter than normal life for the gear couplings. In most applications,
gear couplings can operate six months to one-year between lubrications,
but not under Charter Steel's extreme conditions.
"The gear couplings that were previously installed required constant lubrication
which challenged us in achieving our goal of continuous operations,
said Paul Bares, lubrication technician at Charter Steel." "The need
to frequently monitor, lubricate, and change-out the couplings, along
with the hard to get at location of the equipment, made it a difficult
maintenance problem, at best."
Additionally, Charter suspected that the gear couplings were also a
possible contributing factor in billet-surface imperfections. "Along
with the large amounts of maintenance time demanded by the gear couplings,
we had concerns about product quality related to the couplings," Sauer
said. Charter Steel engineers theorized that the backlash inherent with
gear couplings may have been adversely affecting the drive system and
introducing billet surface quality problems. "Ensuring a smooth billet
surface is critical because a defect on the billet could result in a
defect in the end product, "explained Sauer. "Backlash is a type of rotation
allowed by the rotational clearances between coupling parts. Gear couplings
contain a small amount of this clearance between hub teeth and sleeve
teeth. A gear coupling's backlash is believed to be a contributing factor
in, what Sauer labels, a "lap" problem. "A lap is a fold in the billet's
surface. It's as if a piece of paper was folded over. There would be
a raised surface, after you unfolded it," Sauer said. "A gear coupling's
natural backlash could cause a lap on the billet's surface ¼ something
we need to avoid. We wanted to eliminate coupling backlash as a possible
source of variance in the casting process." Disc coupling stainless
steel disc blades are lubrication- and corrosion-free.
After researching coupling replacement possibilities, Charter determined
that the Falk Freedom® disc coupling would best meet the quality and operational
performance demands required by Charter Steel's production process.
According to Sauer, both major problems encountered by Charter Steel
-- excessive maintenance time and cost, and backlash -- were eliminated
by the switch to disc couplings.
The Falk disc couplings, equipped with high-strength fasteners and one-piece
stainless steel disc-packs, are torsionally stiff with zero backlash,
are lubrication-free and are virtually maintenance-free. These tough,
corrosion resistant disc couplings can easily withstand the operational
conditions of the continuous billet caster environment.
"The disc coupling packs, because they're constructed from stainless steel,
don't corrode under the hot and wet conditions in which they operate,"
Sauer said. The disc coupling components are unaffected by temperature,
moisture, sunlight or ozone extremes. And, by design, they are lubrication-free,"
Sauer noted. Because there are no moving parts, the non-lubricated disc
couplings are virtually maintenance-free and easy to install. When inspecting
the disc coupling for maintenance needs, all that's required is a visual
examination. "We found that the disc couplings are virtually problem-free
compared to the gear-type couplings. We can basically put them in and
forget about them," Sauer added.
The disc couplings also eliminated any possible problems associated with
the gear coupling backlash. A disc coupling has no backlash because
its components are tightly held together. The disc couplings provide
a flexible connection that can accommodate the normal misalignment innate
to most shaft-to-shaft connections.
Since installation of the disc couplings, Sauer has noted a significant
reduction in drive system maintenance costs and time, and an improvement
in billet surface quality. Installation of the four new disc couplings
occurred during a scheduled maintenance interval. "The retrofit installation
went smoothly," said Sauer. "The disc couplings were easy to install.
And since then, we've had to change out one of our drive system motors.
It was easy to reconnect the new motor with the disc coupling."
Lubrication technician Paul Bares summed up the advantages of a Falk
disc coupling: "The disc couplings have worked excellent for us. They
are lubrication-free, can take the high temperatures and withstand the
corrosive water. My life has definitely been made easier, since the
installation of the disc couplings."
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