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.

Strip-mill operation

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.

Gear-improvement strategy

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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