A clever engineering solution has renewed a wrecked steel
mill coupling for half the cost of replacement, as Dan Kocel,
global metals industry manager at Emerson Power
Transmission Solutions explains

When the bore on the rigid half of a
high-torque steel mill coupling
was badly galled during a
maintenance teardown, the East Chicago,
Indiana, mill put the damaged part in the
plant’s bone yard. However, engineers at Kop-
Flex – part of the Power Transmission
Solutions business of Emerson Industrial
Automation – developed a solution that saved
the customer almost half the cost to replace
the $112,000 part, that transmits about
1,130,000Nm of torque to a roughing stand.

The CM-30 cast steel gear coupling
connects the pinion on a 10,000hp bull gear
motor to a roughing stand on an 84-in
(2.1m) hot strip mill. To transmit the
required torque, the coupling’s rigid half
attaches to the pinion with a heavy
interference fit of 0.028in (0.7mm), plus two
massive split-taper keys 4 x 5.5 x 47in (100 x
140 x 1194mm), each weighing about 340lb
(154kg).

According to the manager of
maintenance/spares, the rigid half of the
coupling, which weighs about 31,000lb
(14,061kg) and has a bolt-hole diameter of
almost six feet (1.8m), had to be removed
from a damaged bull gear shaft after the
gearbox lost a tooth. During removal, the
coupling’s bore was badly gouged.

“We considered the damaged part
hopeless,” the manager said. “We were going
to replace the bull gear shaft, so could not
simply bore out the coupling to a larger size
which would then not fit. A standard
coupling repair procedure would involve
enlarging the bore, shrink fitting and
welding a plug in the bore, and then reboring
the center diameter to correct size.

However, the rigid half is ASTM A27 cast
steel, and the diametral force of a heavy
interference fit for the plug would create
internal stresses exceeding the limits for this
part, and welding would not provide the
strength to transmit the torque. Our solution
was to scrap the rigid half and replace it.

“Months later when assessing our mill’s
inventory of critical spare parts, we realised
we had no spares for this part, so asked Kop-
Flex for a quote on one,” he added. “The
Kop-Flex manager suggested we consider
repairing the damaged part, so we agreed to
have them assess the engineering options.”
To create a new bore in the part without
using a welded plug, Kop-Flex engineers
proposed a new solution using a shrinkfitted,
splined-in bushing, because splined
connections can transmit high torque loads.

Engineers designed the splines to withstand
the bending and compressive forces with an
adequate safety factor. They also designed
the splines to achieve an interference fit
(negative backlash) of 0.015 to 0.020in (0.4
to 0.5mm), along with the bushing’s
diametral interference. The repair bushing
itself is 39in (1m) in diameter, with an
approximate weight of 8000 lb (3628kg).

The bore of the salvaged rigid half was
enlarged and cut with mating splines, as well
as locating surfaces for the bushing. After
shrink fitting the two parts, the faces were
machined flush with the flange surface.

The repaired part has since been installed
and tested on a herringbone bull gear at a
vendor’s plant and determined to meet
specifications. The repair saved the customer
$54,000 over the cost of a new rigid half, and
the coupling was due to be reinstalled in the
mill in the fourth quarter of 2012.

A splined repair is also an efficient way to
replace worn gears on large gear coupling
hubs. The existing gears are cut off the OD
and replaced with a spline section. A repair
ring with gear teeth on the OD and splines
on the ID is then installed on the modified
hub – an economical way to repair large gear
couplings with excessive tooth wear.