For major corporations like U.S. Steel, the ability to minimize unnecessary repairs on production equipment isn’t a matter of preference; it’s a matter of survival. With organizations of all kinds facing tremendous pressure to keep productivity and profitability high, the interruption of any production process can create a serious hardship and even impact the company’s competitive position.

   At U.S. Steel’s Gary, Indiana plant, the #1 continuous caster plays an important role in the steel production process. As a result, the ability to keep the caster running reliably is paramount to the company’s long-term success. Unfortunately, the caster’s ball screws – small, but vital components – were requiring a replacement about every five weeks. While the replacement didn’t cause a catastrophic loss, the material and labor costs were adding up quickly.

   “The average cost of repair was running over $ 30,000 for each incident,” says Scott Colvin, Mechanical Expediter. “The labor cost alone was expensive, since each repair required anywhere from 60 to 84 man-hours of time.” Clearly, U.S. Steel needed a better alternative. And the company found that alternative in a patented product from SKF.

Let it flow
The #1 continuous caster at U.S. Steel is an integral machine in the process of producing particularly large slabs of steel for use in a variety of applications. The caster processes 23 heats per day, yielding an estimated 155,000 tons of steel monthly.

   A key step in the process is the raising and lowering of huge ladles that carry hot, molten steel from a furnace building for continuous casting. At the bottom of the ladle is a tube that allows molten steel to flow into the “tundish.” The molten steel then flows into an oscillating mold for continuous casting. In the mold the steel develops a “skin” and the slab continues to solidify as it travels through the continuous caster’s segments. At the end of the strand, the continuous cast slab is cut to length.

   For the steelmaker, the ball screw repair problem was originating in the ladle car – the device that raises and lowers the heavy ladles during production. For each heat, the ladle car must lift a full ladle that averages 350 tons. U.S. Steel was using limit torque ball screw actuators in its ladle cars, but the ball screw was failing, on average, every five weeks.
   With a repair bill of over $ 30,000 each, those failures quickly caught the attention of the maintenance management staff.

   In exploring a more cost-effective, reliable alternative, U.S. Steel took a look at SKF’s roller screw jack actuator – a component that is fast making an impact in the global steel industry. In fact, more than 60 continuous casters in 23 countries currently use the SKF roller screw jack actuator. Their average life is 12 years, though some have experienced operating lives of as long as 22 years.

   “The use of roller screws in steelmaking is a growing trend, but still a relatively unknown approach,” says Roger Kahle, SKF Sales. “You might say it’s the ‘best kept secret’ of the industry.”

Better by design
Based on patented technology, the SKF roller screw jack actuator provides superior performance over an extended operating life. The actuator’s rugged design enables it to operate reliably for extended periods of time – even in applications where contaminants are abrasive and temperatures are high.

   Integral to the reliability of the actuator is the design of the SKF roller screws themselves. Their design allows for a “planetary motion” of the rollers, as opposed to recirculating balls in a ball screw. By eliminating a rolling element re-circulation system and the associated failures, the roller screw design helps to improve reliability significantly.

   In addition to their excellent reliability, the SKF roller screw jack actuators can provide an overall efficiency as high as 70% – allowing them to move loads and apply forces more efficiently and reliably than any other type of actuator. Again, the design of the roller screws comes into play, as they are capable of handling two to three times the capacity of a similarly-sized ball screw.

   U.S. Steel installed SKF roller screw jack actuators in its west ladle car in 1990. By 1994, the ladle car had experienced only one screw failure – an isolated incident resulting from a lubrication problem early in the project. Based on that tremendous track record of success, U.S. Steel decided to install SKF roller screw jack actuators on its east ladle car and both of its tundish cars at the #1 continuous caster. Since that time, none of the four cars has experienced a roller screw failure.

   “The roller screws from SKF have been great – we haven’t had any problems with them,” says Al Payne, Maintenance Manager. “We’ve had limit switches fail during that time, but we haven’t had any trouble with the roller screws. They’re extremely reliable.”

A superior alternative
In many instances, SKF roller screw jack actuators are being selected as a preferred option over hydraulic systems. Industry studies show that a hydraulic system costs considerably more than one based on roller screws. At the same time, roller screws offer higher reliability and better efficiency than a hydraulic system.

   In addition, hydraulic systems require a system of sumps, pumps, compressors, cylinders and other ancillary equipment – resulting in more components subject to failure. For instance, a typical hydraulic system uses as many as a dozen or more cylinders, at an average cost of $ 12,000 to $ 15,000 each. Unfortunately, those cylinders are prone to failure as a result of seal problems, which typically requires a full replacement.

   SKF roller screw jack actuators are proving a reliable, cost-effective alternative for steel producers around the world, who rely on these components to help assure the smooth and continued operation of their casters. For more information about SKF roller screw actuators, request a copy of SKF publication #960-612.