Tuesday 13 August 2013

Hot steel, cool gears!

The steel industry is one of the harshest industrial environments and the technical demands on plant and machinery are extremely high. Watt Drive (part of the WEG Group) helical bevel gearboxes are being used in a new straightening unit at steel producer ArcelorMittal’s plant in Belval, Luxembourg. The project was carried out by engineering company TBR casting technologies (part of the Inteco Group). The Type K139 gearboxes prove that compactness and robustness can deliver with high performance.

Watt Drive Type K139 gearboxes drive the straightening rollers

TBR Casting Technologies planned and supplied the new straightening unit

TBR opted for the Watt Drive K139 helical bevel gearbox, mounted on the plant shaft by means of a hollow shaft

With annual steel production of 130 million tonnes and around 245,000 employees, ArcelorMittal is one of the world's largest steel producers. At its plant in Belval, Luxembourg, the industrial giant uses an electric arc furnace and large continuous casting equipment to produce steel products such as light and medium girders and sheet piles. The annual capacity of the plant is approximately one million tonnes of steel.

They wanted to convert one strand on a six strand caster to a new casting format, the Beam Blank 4 (BB4). The requirements for this strand included a new straightening unit in order to be able to cast the BB4 format, which is around 30% larger than the previously biggest format, which will continue to be produced on the other five strands (BB3). The new casting format is intended mainly for the production of girders as well as special profiles in the downstream rolling mill. The annual production of the BB4 strand is planned to be around 100,000 tonnes of steel.

ArcelorMittal contracted engineering specialist TBR (previously Technisches Büro Rumpler) Casting Technologies to modify the plant. TBR, an innovative engineering company for customer-specific, high-end solutions for the metallurgical industry, took care of the engineering, planning and delivery of the components such as the mould, segment comprising secondary cooling and straightening unit. TBR called on Watt Drive to drive the straightening rollers. The decision was made to install the K139 helical bevel gearbox, which can be easily mounted on the plant shaft simply by means of a hollow shaft. Watt Drive delivered five of these models as geared motor units, four of them for use on the straightener and one as a spare.

Little space and high temperatures
As steel production is exposed to constant high temperatures, special materials and seals are required on the gearbox in order to ensure heat resistance. Another particular challenge in the design of the straightening unit was the limited space available – as a result of the larger casting format – together with a strand separation of only 1,400 mm. In addition, all straightener frames had to be very stable, easy to maintain and interchangeable within the unit.

Because of the demanding requirements, Watt Drive customised the standard K139 helical bevel gearbox. The geared motors must withstand a high ambient temperature on the BB4 strand while at the same time delivering a high torque, and in the most compact possible housing. In addition to the high ambient temperatures, harsh environment and large control range of the self-ventilated motor, Watt Drive also took into account ArcelorMittal’s requirement for an integrated water cooling system when designing the motor and gearbox.

Compact, robust design essential
The drives are designed for a maximum ambient temperature of +100°C without additional oil cooling. At the client's wish, Watt Drive's developers also fitted a water cooling system providing the gearbox with additional thermal protection. The water cooling system ensures a lowering of the oil temperature by means of internal gear pipework. At these high temperatures, special motor and gearbox seals are also required. Here, Watt Drive used temperature-resistant shaft seal rings (Viton) on hardened running surfaces.

The almost 100 mm wider BB4 format results in extremely cramped conditions between the strands with the strand spacing kept unchanged. Thanks to the especially compact, slim design of the Watt Drive gearboxes, it was possible to implement the new straightener alongside the existing five strands. The gear housing is a uniblock design (“Multimounting”) and can be fastened on all sides. The Watt MAS (modular drive system) principle enables the standard gearbox to be quickly customised to application-specific requirements.

Each of the K139 helical bevel geared motors delivers a drive power of 5 kW, a torque of 17,580 Nm and a rotational speed of 2.2 rpm at 50 Hz. The drives are designed for frequency inverter operation, which adjusts the rotational speed of the geared motor, thereby regulating the strand speed – the larger the beam blank formats produced, the slower the speed at which the strand must to be driven. An integrated 100 Nm brake with manual release – also specially designed for the high ambient temperature – serves to hold the cold strand in a fixed position at the point of entry because at the start of the casting process, a “dummy bar” closes the mould exit outlet. Thanks to the manual release, the brake can be released by hand in an emergency, enabling the strand to be moved.

To improve ease of maintenance, Watt Drive designed the oil level indicator to be more prominent and easier to read. The gearboxes also feature a special expansion tank or oil expansion chamber. The vertical construction requires more oil in the gearboxes, which expands at the high operating temperatures. The expansion tank accommodates the expansion of the oil such that no pressure increase occurs, thereby extending the longevity of the seals and contributing to greater operational reliability.

“The comparatively slim design of the K139 gearbox was a fundamental prerequisite for creating a reliable and easy to maintain straightener,” Leonhard Paar, Engineering Manager at TBR Casting Technologies explained. “Thanks to its particular compactness and robustness, the larger BB4 format at the modified plant can be cast to the required quality, while at the same time reducing the servicing overhead.”

A gearbox for all occasions
The steel industry demands high performance as well as special robustness and temperature resistance. With its application-specific, customised helical bevel gearboxes for the driving rollers in ArcelorMittal's new straightening unit, Watt Drive has demonstrated that its gearboxes can be quickly and precisely customised to extremely demanding applications. Thanks to their robust, compact design, the K139 models supplied meet all application-specific requirements while being especially easy to maintain. The partnership with TBR Casting Technologies could soon be repeated as it is anticipated that a further strand at ArcelorMittal Belval will be converted.

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