Getting the Best of Both Worlds in Reversing Mill Recoilers at Nucor

Getting the Best of Both
Worlds in Reversing Mill
Recoilers at Nucor Steel–
Berkeley
This article is available online at www.aist.org for 30 days following publication.
This project won the AIST Maintenance Processes Operating Committee’s 2006 Gold Reliability Achievement Award, which
recognizes iron and steel producing companies for reliability improvements and achievements.
P
roductivity improvements have become
a joint effort, as mills are unleashing
the expertise within their supplier base to
identify and implement the most promising
changes. These changes in equipment or
In a recent equipment upgrade, parts from two cold
mill coilers at Nucor Steel–Berkeley were combined to
5,000 horsepower. Other characteristics of the
mill are as follows:
•Strip width is 32–66 inches.
•Incoming gauge is 0.050–0.240 inch.
•Finish gauge is 0.010–0.110 inch.
•Maximum strip speed is 4,000 feet per
minute.
•Maximum strip tension is 40,000
pounds.
form a highly productive hybrid machine. This paper
discusses the adaptation process and the productivity
Figure 1
gains that resulted.
working procedures are contributing measurable dollar impact. In one recent equipment upgrade, two separate cold mill coilers
at Nucor Steel–Berkeley effectively donated
major subunits into a promising combination,
ultimately forming a highly productive hybrid
machine. This modified coiler is now installed
and working well in the mill. This paper looks
at the way team members from the producer
mill and equipment suppliers banded together to capture the productivity gains through
design innovation and a dedicated implementation plan.
Mill Features
The No. 2 RCM at Nucor Steel–Berkeley is a
4-high reversing cold mill with axial work roll
shifting. The left and right tension reel drives
are powered by single AC motors rated at
Hybrid coiler in service at Nucor Steel–
Berkeley.
Authors
Jason B. Curtis, meltshop mechanical supervisor, Nucor Steel–Berkeley, Huger, S.C. ([email protected]); Charles
P. Bender (pictured), senior application engineer, NSK Corp., Ann Arbor, Mich. ([email protected]); and Steven
J. Matovic, manager — drives and services, Horsburgh & Scott, Cleveland, Ohio ([email protected])
42 ✦ Iron & Steel Technology
Figure 2
OEM mandrel set for right-hand side.
Figure 3
OEM mandrel.
Figure 4
Figure 5
Illustration of entire end with nine of 13 bolts
sheared.
•Maximum coil diameter is 80 inches.
•Maximum coil weight is 80,000
pounds.
Series of Events, Trials and
Results
The mill began operation on April 26, 2001.
The supplied mandrel design was a standard
mechanical design with a pull rod, expand
cylinder and wedge tree design to expand the
mandrel. It had dual gripper slots (Figure 2),
so that the mandrel could be converted for
use in the left or right position. One somewhat
unique feature was that the nose cone, which
sits in the bearing support arm, was bolted
to the wedge bar alignment plate (WBAP),
which was bolted to the mandrel shaft forging
(Figure 3). A more common design is to have
the nose cone and the mandrel shaft as one
Close-up view of Figure 4.
forging. The bolted connection between the
WBAP and the mandrel shaft was the initial
design problem that manifested itself.
On Oct. 5, 2001, the first failure occurred
on the left tension reel mandrel when several
bolts sheared (Figures 4–5). A similar failure
occurred on the right tension reel mandrel
on Oct. 6, 2001. Fretting corrosion was also
noted on various parts on both mandrels,
which indicated movement between the parts
and brought the bolt specifications into question. The original equipment manufacturer
(OEM) was contacted to recalculate the bolt
specification; they verified that the bolts were
adequate for the application.
The left tension reel mandrel bolts failed
again on Nov. 3, 2001. In this case, the mill
ran for some time before the operator noticed
that the end of the mandrel was moving
around and the broken bolts were found. Due
December 2007 ✦ 43
Figure 7
Figure 6
View of the cracked WBAP from the inside.
View of the cracked WBAP from the back side.
to running with the broken bolts, the WBAP
had breaks caused by cyclical fatigue (Figures
6–7).
Due to mill requirements at the time, it
was decided to bolt the cracked WBAP to the
mandrel shaft to realign everything as close
as possible and weld a piece of metal into the
gripper slot of the WBAP (surprisingly, this
“repair” lasted eight weeks). The only negative
effect would be that the mill could not run an
even-pass coil, which rarely happened.
The OEM asked that the left mandrel be
changed so that the operating mandrel could
be sent back to the machine shop that had
been contracted to manufacture them. The
OEM’s QC department wished to inspect
the mandrel and review assembly tolerances;
they had already reviewed the supplied spare
mandrel. The mandrel was changed on Jan.
10, 2002; the review did not produce any
findings.
During the period between Jan. 10, 2002,
and Jan. 20, 2003, the WBAP bolts failed on
either the left or right tension reels 27 times,
Figure 8
Modified mandrel with load-bearing bore.
44 ✦ Iron & Steel Technology
or approximately every two weeks, involving
608 man-hours and 164 hours of mill downtime. At that time, Nucor decided to pursue
alternative designs independent of ongoing
trials by the OEM.
Modified Conventional Design — Nucor
came to an agreement with a company other
than the OEM. Nucor decided to try a design
that changed the WBAP into a non-loadbearing component. In order to do this, the
wedge adapter was modified to be supported
in the bore of the mandrel shaft by increasing the bore for the pull rod. The design
also required increasing the thickness of the
flange of the wedge adapter to compensate
for the shorter WBAP (Figure 8). This would
transfer the load created by the weight of the
coil and operating tension into the mandrel
shaft via a solid forging rather than relying on
a bolted connection. The nose cone remained
a bolted connection to the wedge adapter.
This allowed the nose cone to be shimmed as
necessary to achieve proper engagement with
the bearing support arm. The nose cone now
moved axially with the pull rod as the mandrel
operated.
The first modified mandrel was installed in
the right-hand position on April 29, 2003. Due
to some additional design modifications that
were incorporated in the right-hand mandrel,
the modified design was not installed in the
left-hand position until Oct. 2, 2003. During
the next year, the two mandrels combined
resulted in 11 delays, accounting for 182 manhours and 63 hours of mill downtime. This
was a substantial improvement over the original mandrels’ performance.
Hydraulic Mandrel — The modified version
of the conventional mandrel created a much
more reliable piece of equipment than was
originally supplied, but it still didn’t provide
the results that were desired. As with most
conventional mandrels, repair costs were consistently in excess of $30,000 due to all the
moving/sliding parts in this style of mandrel.
Nucor did not have the resources in-house to
do all the necessary work, so the mandrels had
to be sent to a repair shop. This led to turnaround times that were undesirable at times.
The mandrel was still not reliable enough. On
average, it required 12 hours of mill downtime
and 48 man-hours to change the mandrel
when necessary. The most significant downside to the conventional mechanical design
is the yield loss incurred on each mandrel of
every coil that is run. This is due to having to
take one to two wraps, depending on product,
before run tension could be established, due
to the insufficient gripping force inherent
with this design.
The No. 1 reversing cold mill at Nucor
Steel–Berkeley was originally supplied with
fully hydraulic mandrels. This design allows
the operator to “stick” the head of the strip
into the gripper slot, expand the mandrel,
apply full run tension and start the mill ongauge. This leads to worse yield than a closedgap threading tandem mill, but it is the best a
reversing mill can do. There are also several
maintenance benefits to the hydraulic design
over the conventional style. First, a mandrel
can be changed in approximately 90 minutes,
as opposed to 12 hours. Second, a complete
rebuild is less than $5,000, versus $30,000 or
more, and can be rebuilt in-house. There were
concerns, particularly with capital costs and
design considerations, but it was decided that
the project was worth investigating.
Design Considerations — The objective was
to design a fully hydraulic mandrel that would
maximize current spare parts and keep major
components interchangeable with the No.
1 mill’s mandrels. There were four major
obstacles that had to be overcome before this
could happen:
•The length of the mandrel drum block,
due to mill No. 2 being designed for a
wider strip than mill No. 1.
•To minimize changes to the bearing
support arm to minimize capital costs.
•To overcome the different gear housing designs used on each mill, which
included bearing designs.
•The cost implication presented if the
existing gear could not be used with the
hydraulic design.
Nucor discussed the options with several
engineering companies and decided on a
design that accomplished all or most of these
four concerns (Figure 9).
Mandrel and Support Arm — Finalized solutions for the first two concerns listed above
actually worked out quite well. The drum
block for the No. 2 mill was required to be longer in order to compensate for the wider strip
design on the No. 2 mill versus the No. 1 mill
and to eliminate major modifications to existing support equipment. The increased drum
block length requirements allowed for the
tapered nose cone that sits in the bearing support at the No. 2 mill to be added to the end
of the No. 1 mill’s current drum block design.
In doing so, the tapered nose cone arrangement for the No. 1 mill was incorporated into
Figure 9
Hydraulic mandrel assembly.
December 2007 ✦ 45
Figure 10
Picture of shaft assembly.
the drum block of the No. 2 mill’s design.
This is significant because the diameter of the
support bearing on mill No. 1 is significantly
larger than the support bearing on mill No. 2.
This allows clearance for the support bearing
at the No. 1 mill to swing shut and seat properly without the No. 2 mill nose cone interfering, and keeps the same bearing-to-bearing
distance of the original mandrels on both
mills. The mandrels on the No. 1 mill will be
modified to match the No. 2 mill so that the
same spare can be used at either mill.
The third concern was addressed by installing an “extended” hollow shaft to allow for
the same drum block to work on both mills.
This also allowed the use of the same expand
cylinder that is used in mill No. 1. Due to
bearing center differences in the gear housings on each mill, the hollow shafts are not
interchangeable between the mills. This was
deemed an acceptable condition versus the
alternatives.
Hollow Shaft and Geardrive Housing — The
difference in bearing centers between the two
mills required the use of two different hollow
shafts. The bearing spread across the housing
in the No. 2 mill is 43.50 inches (1,104.9 mm),
while the bearing spread across the housing
in the No. 1 mill is 70.87 inches (1,800 mm).
Both hollow shafts had to have the same
dimensions internally and end-to-end so as to
accommodate the same drum block, mandrel
assembly and expand cylinder. However, the
dimensions on the outer journals that interface with each gearbox had to be different.
The objectives for the new No. 2 mill hol46 ✦ Iron & Steel Technology
low shaft were: (a) to duplicate the internal
dimensions from the No. 1 mill hollow shaft,
(b) to reuse the existing low-speed gears and
(c) to make the new hollow shaft assembly as
close to a drop-in replacement as possible.
The existing internal dimensions and housing bore diameters served as dimensional
constraints for the new hollow shaft. The
design process went through a few iterations
before Nucor settled on a shaft configuration
that accomplished the goal and provided the
structural integrity necessary to support the
mill loads. Given the relative size of the shaft
and location of the bearing supports, the
calculated shaft stresses and deflections were
well below typical design limits and did not
pose a concern for the long-term success of
the design.
Since the conventional No. 2 mill mandrel
design utilized a smaller shaft diameter under
the low-speed gear than what was required
for the new design, the existing low-speed
gear could be removed and bored out to
accommodate the new, hollow shaft design.
Reusing the low-speed gears saved capital cost
and helped minimize the lead time for the
overall project. To complete the shaft assembly, a new set of parts had to be designed.
The most critical of these were the bearings
(see “Bearing Selection” below). However,
other components — such as shaft spacers,
locknuts, retaining plates and oil seals — also
had to be redesigned. All of these changes fit
in the existing No. 2 mill housing, with the
exception of the mill side bearing. This housing bore had to be opened up approximately
0.50 inch to fit the new, larger spherical roller
Table 1
Bearing Selection Chart
Gearbox
position
Front fixed
Distance between
bearing center-
Gearbox
Bearing
Bore
OD
Width
lines (mm)
OEM
part no.
(mm)
(mm)
(mm)
1,104.9
Catalog
Radial
speed limit dynamic load
(rpm, oil) rating Cr (kN)
No. 2 mill
23088CAME7P55S11U22
440.0
650.0
157.0
530
3,150
Rear float
No. 2 mill
23076CAME4S11
380.0
560.0
135.0
630
2,500
Front float
No. 1 mill 23096CAME4P55S11U22
480.0
700.0
165.0
480
3,800
Rear fixed
No. 1 mill
431KBE5751GE+L
431.8 571.5 146.05/
192.088
500
2,610
Front fixed
No. 2 mill
final hybrid
500
2,580
Rear float
No. 2 mill
431KBE5751GE+L
431.8 571.5 146.05/
final hybrid
192.088
500
2,610
1,800.0
bearing selected for this location. With the use
of a bearing spacer and new retaining plate,
the distance through this bore remained the
same. All other housing features remained
the same.
Bearing Selection — Early in the design process, it became clear that most of the equipment could be reused from the existing coilers.
This was true except for the bearing selection.
The role of these bearings is to transmit the
applied dynamic forces from the moving pieces of machinery into the stationary support of
the external gearbox walls. The primary operating characteristics involved with this type of
coiler project made it imperative to develop a
careful understanding of the mechanical reactions involved, and these fell into three main
areas: (a) uniquely shifting load magnitudes,
(b) speeds at minimal loading conditions and
(c) the flexing of mating components inside
the coiler. Each of these areas warranted a full
sub-review to screen out potential problems
before finalizing the hybrid coilers’ bearing
selection and specifications.
A summary of existing and final bearing
selections (Table 1) highlights the necessary
bearing changes that made it possible to
squeeze the two coilers’ components into the
final “hybrid” coiler. For each shaft, one bearing is designated as a “fixed” position, and this
keeps the shaft from walking out of the housing due to applied forces acting in the axial
direction (i.e., along the shaft centerline).
This makes the bearing loading magnitude
higher because multiple loads are superimposed onto the same spot in the machinery. In
the No. 1 mill coiler, a tapered roller bearing
reacted with these “fixed position” loads quite
successfully, and is found in the rear of the
mandrel.
The other position is considered a “float
position.” At the front of the No. 1 mill coiler
mandrel, a spherical bearing supports the
applied radial loads, as well as shifting to
23996CAME4S11
480.0
650.0
128.0
accommodate things like thermal expansion
or when machine components are bending
under load. This arrangement makes full use
of the core of the spherical type of bearing
design to self-align under dynamic shaft conditions.
In the No. 2 mill coiler, both original
bearings are spherical roller products. Upon
review of the new set of conditions, the
favored design copied the arrangement in
the No. 1 mill coiler. This meant converting
one position to a tapered roller bearing while
maintaining the spherical roller bearing in
the other position. There was an end-for-end
swap in the machine to change which bearing
position took the combined radial and axial
loading of the fixed position. By reusing the
same bearing from existing mill inventory, the
hybrid coiler became conveniently linked to a
common warehouse item already in-house for
the No. 1 mill coiler.
The operating conditions of an active coiler
are dominated by the need for the equipment
to continuously cycle through light loads at
high speeds in the beginning of the coiling
operation, then finish with heavier loads at
slower speeds in the end of the coiling operation. Shaft deflection of the new components
needed to be reviewed early in the engineering study because this was a critical step in
advancing to the rest of the project. When
conditions causing excessive shaft bending are
left unresolved, bearing components react by
transmitting uneven loading within the individual bearing components, creating unexpectedly short service campaigns. The first
elements of the engineering study quickly
qualified these deflections to be within safe
limits, even with the changing rigidity of the
new component dimensions. This is true for
the complete range of loads produced in the
total coiling process.
Beyond assuring the rigidity of the new
design, developing a suitable service interval
between maintenance outages also played
December 2007 ✦ 47
a key role in the final bearing selection.
While the first step succeeded in physically
positioning all the donated components into
a new and relatively unique hybrid coiler
arrangement, it was still important to determine the total amount of space that could
be utilized for commercially available bearings. This dedicated space then needed to be
qualified for the predicted lives of individual
bearing proposals to establish suitable meantime durations between component wear-out.
Individual industry preferences have been
consolidated through associations like AGMA
to formalize generally accepted life predictions. Calculations provided by the bearing
manufacturer were cross-referenced against
applicable AGMA guidelines to choose the
final bearings.
Physical inspections of the operating
equipment are encouraged, and are often
factored into redesign recommendations.
This opportunity became available when the
mandrel was removed from service in the
Jan. 10, 2002, outage. On-site inspection of
the bearing components revealed no permanent damage to the bearings, even though
the bolts had sheared and physically cracked
the WBAP. The inspection also provided an
early familiarization with the equipment and
quickly proved invaluable when beginning
the expanding redesign project.
Project timing remained tight, with the
initial review starting in August 2003, and the
bulk of the redesign completed by October
2003. Bearing inventory proved useful, as the
recommended spherical part needed to be
on-site and ready for installation within the
scope of the overall project critical path.
Shaft Assembly Conversion and Installation
— Once the new components were manufactured, each mandrel shaft assembly (right, left
and a spare) could be converted. This process
was completed in approximately two-week
intervals, beginning with the spare mandrel
assembly. After receipt of the spare mandrel
assembly, the existing low-speed gear was
removed from the mandrel shaft (this proved
to be more difficult than expected, given the
large amount of interference between the
gear and mandrel shaft). The bore was then
machined to accommodate the new hollow
shaft diameter, and the new components and
remachined low-speed gear were assembled
onto the new hollow shaft. Upon receipt of
the new hollow shaft assembly at the mill, the
geardrive was removed from service and was
machined to accommodate the new, larger
mill-side bearing. The new hollow shaft assembly was then easily installed into the housing
in place of the existing mandrel assembly. The
mandrel shaft assembly removed from service
was then sent out for the same conversion and
eventual reassembly into the modified housing on the opposite side. That mandrel assembly was then sent out to have the low-speed
gear removed and remachined. The third
hollow shaft, the modified low-speed gear, and
a full set of bearings and components were
returned to the mill as unassembled spares.
Results
The right mandrel was installed on Nov. 7,
2004, and the left was installed on Dec. 13,
2004. Each installation took only 60 hours
from the time that the lockout/tagout process
began until the mill was ready to run. In 2005,
there were 24 delays involving 37 man-hours
resulting in 25 hours of mill downtime. There
was not a delay from Dec. 21, 2004, through
Feb. 2, 2006. The most significant gain is
that, in 2005, Nucor realized a yield savings of
2.5%, equivalent to more than $1.5 million.
The return on investment was 0.31 years. F
This paper was presented at AISTech 2007 — The Iron & Steel Technology
Conference and Exposition, Indianapolis, Ind., and published in the AISTech 2007 Proceedings.
48 ✦ Iron & Steel Technology