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