After many years of service, the gears in a hot-strip rolling mill must be replaced. But, the new gears face a dual challenge
In the early 1980s, the United States Steel Corp. (USS) found the gears in their Gary Works, Ind. hot strip mill to be in poor condition. A number of these large gears failed, causing downtime and production delays. Though these gears performed adequately for approximately 20 years, recent higher loads and throughput had exacted a toll in damage and failures.
In cooperation with Drive Systems Technology Inc., a power transmission consulting engineering firm, USS started a program to replace the deteriorated gears with advanced technology gears that fit the existing housing, but accommodate increased loading and production rates. Other steel companies have faced similar problems and implemented similar solutions.
The first step in the replacement process was to analyze the mill operation. A typical strip mill consists of a series of “stands” that progressively reduce the thickness of a hot steel slab by passing it between successive roll pairs, Figure 1.
Each stand has an electric motor (3,000 to 12,000 hp), coupled to a mainreduction gearbox containing single or double helical gears. The main gearbox output shaft is flexibly coupled to a pinion stand that drives the rolls. Each pinion stand has a gearbox, also with helical gears, that splits the power between upper and lower mill rolls.
As Figure 2 shows, the gears are huge. Pinions range from 2 to 4-ft pitch diameter and 4 to 10 ft long. A pinion, with integral shaft, weighs 10,000 to 20,000 lb. The gears range from 5 to 15-ft pitch diameter and 2 to 5-ft face width. A typical gear and shaft assembly weighs 80,000 to 190,000 lb.
The power required to roll steel varies, depending on the type of steel, plus its thickness and width. Most of the time, the gearbox operates at moderate load, usually less than 15% of the motor rating. But, when a bar enters the mill, the drive is subjected to a sudden, large momentary torque caused by squeezing the bar between upper and lower rolls. This peak torque sometimes exceeds four times the rated motor torque.
Evaluating the gear system
Next, engineers evaluate the condition of both gears and gearbox as a prelude to upgrading the system.
• Gears. Highly loaded mill gears typically exhibit surface distress, such as tooth wear, pitting, and spalling, as well as cracking in tooth areas, Figure 3, and in gear blanks. Despite extensive maintenance, catastrophic failures, such as a broken pinion shaft, Figure 4, have occurred.
• Gearbox. Because these drives are so heavy, their supports sometimes settle, causing substantial misalignment of the housing bores. Alignment is best evaluated by performing a detailed analysis.
• Lubrication. Usually, all gear drives (and supporting bearings) in a mill are lubricated by a common system. Inspectors check for proper filtration, which is often poor in old systems, plus flow and alignment of oil spray bars.
To obtain the greatest improvement in gear capacity, engineers implemented a host of advanced design and manufacturing techniques. They incorporated hardened pinions with modified tooth profiles and leads, and fabricated through-hardened gears. Special materials and gearblank fabrication techniques were applied and gearboxes were refurbished.
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Initial efforts focused on improving surface durability and wear resistance of the gear teeth while maintaining or improving their bending strength. This is accomplished by modifying the tooth geometry to reduce contact stress, decrease slip, and improve contact ratio. Other actions include shifting tooth proportions and changing pressure or helix angles.
Engineers must implement these modifications carefully to ensure compatibility with the existing system. For example, shifting the profiles increases the pinion OD, so that housing clearances must be checked. Changes in helix or pressure angle affect the loads.
Most mill gear sets have large backlash, which gets larger with wear. Because of an impact when a bar enters the mill, the pinion “bounces” within the gear tooth space, causing high dynamic tooth loads. To reduce bounce and the resultant loads, backlash on the new gears was held to relatively low values (0.020 to 0.040 in. for a main drive gear set of 0.75 to 1.0-in. diametral pitch) .
Other design changes
The large face width of these gears often leads to poor load distribution across the tooth face, which causes premature failure. Poor load distribution is attributed to three factors: gear-tooth inaccuracy, bearing-bore misalignment, and gear blank and shaft deflections.
Because of inaccuracies in tooth geometry, older mill gears commonly have lead mismatch between pinion and gear. Though some gears start as lead-matched sets, steel mills frequently mix gears indiscriminately after a failure, causing mismatches, further exacerbating the problem. Improved accuracy (described later) in the new gears makes them truly interchangeable, thus eliminating the mix-and-match problem.
Engineers develop tooth lead and profile modifications to accommodate system deflections and improve load distribution, both across the tooth face and among the teeth in contact.
Profile modifications consist of tip relief on both pinion and gear to eliminate hard contact during entrance and exit phases of tooth meshing.
Lead, the most significant modification, must account for system deflections, especially in the pinion shaft, and alignment variations in the gearbox.
Defining the lead and profile modifications is complicated by several factors. First, the housing size and difficulty in modifying it on site makes perfect gear alignment impossible. Thus, an allowance for misalignment must be provided by superimposing a crown on the lead modification.
Second, load varies during a typical rolling cycle from near-zero to several times the motor rating. This situation is complicated by load variations due to different sizes and grades of steel. Moreover, a given gear may be used in different mill stands.
It is tempting to calculate deflections for the worst combination of factors, then determine the required modifications. But, this usually yields unsatisfactory system performance and life. If gears are modified for worst-case loads, the tooth contact pattern is significantly less than full-face and the gears are noisy in operation.
Conversely, if gears are modified for low-loads, the contact pattern is likely to run off the teeth ends or tips at peak loads, causing early failure due to high localized stresses.
The only workable solution is a complete study of the mill operation applied in combination with experience in designing large gears to define the optimum modifications.
Because optimum modifications are unique to each gear set, it is impossible to give specific guidelines. For reference, however, typical lead and profile modifications range from a few thousands to over 0.010 in.
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Rounding and peening
As simple as it sounds, edge rounding makes an important contribution to gear capacity. All tooth edges, both at the tips and ends of the face width, are rounded to reduce stress concentrations when the contact runs off or near the edges.
By eliminating these high stresses, rounding enables the gears to operate over a wider load range. The key is to modify the profile for a load that is well below the peak value and round the tooth edges to minimize stresses during peak loads.
As before, specific guidelines are impossible, but, typical edge radii range from 0.030 to over 0.090 in.
To enhance fatigue life, all tooth roots are radiused, polished and shot-peened, and all critical shaft fillet radii are polished. Shot peening produces a compressive stress in the steel surface that improves fatigue life. But, it can also degrade fine surface finishes (15 to 25 min. rms for hard-finished pinions). This degradation is avoided by using “soft” shot and preventing impingement on tooth flanks.
Building gear blanks
The new gears usually consist of forged rims and hubs connected by fabricated web plates. The rims, into which teeth are cut, are rolled ring forgings of high hardness, generally 340 to 400 Bhn. The forging process improves and refines grain flow, thus improving fatigue properties.
To ensure sufficient gear-blank stiffness, manufacturers use cross stiffeners to connect rim, hub, and web sections. Some cases call for a triple-web design and stiffeners along the face width.
This approach yields heavier gears with higher inertia than those which they replace. The heavier weight may shorten bearing life slightly. However, smoother operation and lower bearing reaction loads offset the weight disadvantage so bearing life is more likely to increase. Further, the higher inertia increases the gear’s flywheel effect, which can provide some of the increased torque demand when a bar enters the mill, thereby reducing loads on gear teeth.
Materials and heat treatment
Pinions, plus gear rims and hubs, start as vacuum-degassed forgings. For pinions that are to be carburized, hydrogen content is monitored to enhance fatigue properties.
To increase load capacity, pinions are usually carburized, case hardened to HRC 58, and then hard finished. Heat treatment of these pinions is challenging because they have thick sections that are slow to heat and cool, and difficult to quench. And, heat treatment must be closely controlled (usually by computer) to obtain good core properties. Best results have been obtained with a modifiedchemistry version of AISI 4320 steel.
Gear rims and noncarburized pinions are produced from either AISI 4340 or 4350 steel, typically hardened to 363-401 Bhn. Low fatigue stresses in the web allow the use of common AISI 1020 or A-36 structural steels. AISI 1020 steel is also used for some hubs, but AISI 4340 or 4140 is generally used for maximum capacity.
Manufacturers use several methods to generate teeth in these gears, depending on their size and hardness. Most pinions are carburized and hardened, then finished, either by grinding or by using Cubic Boron Nitride (CBN) cutter inserts. CBN is also used to finish-cut teeth on larger gears, Figure 5, because of their hardness, generally 340 to 400 Bhn. CBN hard-finishing produces pinion tooth accuracies of AGMA Quality Class 10 to 12 (compared to Class 7 or lower for the original gears), especially for profile and lead control. In addition, the fine tooth surface finishes — carburized pinions range from 15 to 25 min. rms; throughhardened gears, 25 to 35 min. rms — help to develop a full elastohydrodynamic (EHD) film thickness in the lubricant, so important in limiting tooth wear.
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New gears are usually introduced to a mill in stages, so new and old gears must coexist. Therefore, technicians adjust the lubrication system before gear replacement so that debris from the old gears is filtered from the oil before it reaches the new gears. They also carefully realign the housing bores.
Next, the pinion and gear assemblies are replaced with the new gear sets. Then, backlash and contact pattern are checked to ensure proper installation and alignment.
After installation, technicians check the lubrication system, then start the gearbox and monitor both noise level and temperature. Contact patterns are visually checked after the box has been in service for several days to several weeks.
The most commonly recognized measures of gear capacity are the strength and durability ratings calculated according to AGMA Standard 2001. These ratings show a substantially improved capacity for the new gears, Figure 6.
However, the actual improvement is far greater because of two related factors. First, the use of a fine-finish, carburized pinion with a through-hardened gear of optimized tooth proportions yields little wear. Second, lower backlash and improved accuracy of the new gears reduces peak load, thus improving mill capacity over that indicated by Figure 6.
After six years of service, the new gears show virtually no wear and no distress. And, a full contact pattern is evident on all gear teeth. Conversely, conventional mill gears begin to wear shortly after start-up and continue to do so throughout their life.
Due to the combination of improved load capacity and reduced peak load, the mill can handle wider steel plates, tougher materials, and larger thickness reductions. An additional benefit is a surprising reduction in gear noise and vibration.
For more details, see American Gear Manufacturers Association (AGMA) paper 92 FTM 4, “The Design, Development and Manufacture of Advanced Technology Gearing for Hot Strip Rolling Mill Applications,” by Raymond J. Drago and Laurence E. Scott. The paper can be obtained from the AGMA in Alexandria, Va.
For information from Drive Systems Technology Inc., call 215-358-0785 or FAX 215-358-2776.
Raymond J. Drago is chief engineer of Drive Systems Technology Inc., Glen Mills, Pa.
Laurence E. Scott is process manager for sheet and tin at United States Steel Corp., Gary Works, Gary, Ind.