Extremely robust sensor technology
Looper encoders with an extraordinarily long service life for heavy industry
- Objective: Maximize the availability and operational lifetime of the encoder required to measure the angle of rotation of the looper – despite extreme operating conditions
- Implementation: Installed the looper encoder ALS 40 K, which was specifically developed for this application with special ball bearings and an extra-robust construction
- Result: Looper encoder proven in the field to have a two to three fold longer operational lifetime in comparison with encoders available from other manufacturers
In a hot rolling plant, encoders operate in a harsh environment. Nevertheless, they need to run without wear for as long as possible because a mill stoppage is very costly for the operator. After trying out different encoders, voestalpine Stahl GmbH now relies on the looper encoders of Johannes Huebner Giessen. Thanks to their special design features, they meet the requirements of heavy industry.
With dirt and extreme temperatures, cooling liquids and oil, vibrations and shock, hot rolling plants are an inhospitable environment. Technical devices that operate in these conditions, such as loopers, which ensure correct mass flow control on a wide hot strip mill, must be able to withstand these harsh conditions. Six looper encoders ALS 40 K from Johannes Huebner Giessen have been operating in the hot rolling plant of voestalpine Stahl GmbH in Linz since early 2012. Reliability, a long service life and high operational safety are the features of the ALS 40 K that convinced the directors of the Steel Division in the voestalpine Group. “We tested several encoders from other manufacturers and were always unhappy with them. None of the products we used before lived up to the requirements of our plant”, said Alfred Pichler, electronics engineer at voestalpine Stahl GmbH.
The “Steel Division” is the biggest unit of Austria’s voestalpine Group, producing high-quality flat steel products for the automobile, construction, home appliances and energy industries, among others. The steel strips produced at the hot rolling plant form the basis for car and lorry bodies, mobile cranes, agricultural machinery, and containers such as gas cylinders or sheet metal claddings. Along the production line in the hot rolling plant, the slabs are heated in special furnaces to around 1200 degree Celsius and then passed on to the wide hot strip mill. The mill is some 450 meters long and features a pusher furnace and walking beam furnace, an edger mill and roughing stand, a cooling line and coiler as well as a finishing train with seven successively arranged roll stands. The Huebner looper encoders are installed along the finishing train of the wide hot strip mill. The 35 mm thick strips are passed through the roll stands where the hot strip is reduced to a final minimum thickness of 1.4 mm. According to voestalpine, the wide strip mill produces a steel strip measuring up to 1.7 kilometers in length and up to 1.75 m in width in less than three minutes. In this way, the wide hot strip mill produces some 4.5 million tons of hot-rolled steel strip each year.
Between the seven roll stands, a strip lifter or looper lifts the steel strip up high. This creates a certain tensile force between the stands. The rotation angle and the rotation movement of the looper must be constantly captured by an absolute encoder known as a looper encoder. By adjusting the sling length, it can measure and offset mass flow disturbances in the milling process. The looper measures the rotation angle of the strip so that the control can maintain a constant strip tension on the basis of the measured angle. The hot strip enters the roll stand with precision – an important process for the productivity in the plant.
Specially desgined for heavy industry applications
For the looper encoder, the conditions at the installation site along the finishing train are critical. The encoder absorbs the vibrations of the wide strip, and these vibrations subject it to high shock loads; the strip temperature of approximately 1,000 °C generates great heat, which is combined with high humidity from cooling water used in this area. The environment of the roll stand is also heavily loaded with scale dust.
The way the encoder is deployed also generates further load. “Since the working angle doesn’t exceed some 45 degrees, the encoder is subjected to only a partial load as well as inadequate bearing lubrication,” explains Frank Tscherney, member of the Huebner Giessen executive board. In looper applications, the ball bearing generally swings back and forth in a 45-degree rotating movement and never completes a 360-degree rotation. In a classic encoder, the load is always on only one part of the bearing and the ball bearing is inadequately lubricated.
These factors directly affect the service life of the looper encoder. Standard devices previously used did not reach the service life required by voestalpine. “We had to replace some of the encoders after six months. They had mechanical flaws and were not sufficiently leakproof,” explained Pichler. Because of its special structural design, the Huebner Looper Encoder ALS 40 K is ideal for heavy industry applications. It achieves two to three times the usual service life for encoders. The benefit for the client is that the plant is continuously available because the looper encoder requires less maintenance and does not have to be replaced as often. This reduces plant downtimes – particularly unscheduled ones. Alfred Pichler agrees: “Interruptions on the finishing train cost us several thousand euros an hour when the plant comes to a standstill, and they delay production.”
Reinforced shaft, shock-resistant bearings
An important parameter for the service life of bearings is the load rating. The Huebner looper encoder ALS 40 K has a 40 per cent higher dynamic load rating than standard encoders. It features specially dimensioned bearings with a steel bush and bearing housing suitable for the load. In order to ensure a long service life, the bearings are designed not primarily for high rotation speeds, but for high shock loads of up to 450 g. The permitted temperature range is up to 100 °C. To ensure a wear-free 45-degree rotation movement of the ball bearing, Huebner uses a solid bearing grease with a viscosity adapted to the particular movement conditions. The grease is firmer so that the ball bearings are embedded in the grease, even when the rotation movement is one-sided. This prevents friction along the outer ring of the bearing, which would speed up wear and power loss.
The encoder boasts a reinforced shaft with a diameter of 11mm or 14mm made from high-strength stainless steel – a further design feature that prolongs the service life of the encoder. The ALS 40 K also has a radial shaft seal with an additional protective lip. The seal of the encoder runs directly on the drive shaft near the hot strip. Spray water, oil or cooling liquid could penetrate the encoder. As the seal ring is in compliance with one of the top protection classes, IP 67, it guarantees protection from the plant’s liquids and dirt. The high protection class IP66/IP67 applies to the whole device, including the terminal box and plug connections. “The higher the protection class, the longer the service life of the device. This proves that our special encoder is ideal for heavy industry,” said Tscherney. At voestalpine, the ALS 40 K operates as B5 flange variant and with an SSI interface. Besides the flange-mounting option, the looper encoder is also available as construction type (B35) with flange and foot. It also has all common interfaces such as SSI, incremental, PROFIBUS and EtherCAT and others.
The new encoder comes with a guaranteed high signal quality. The supply voltage is 12 to 30 V (DC) and a 13-bit singleturn resolution (8,192 steps per revolution). Alfred Pichler sums it up: “The looper encoders we used before did have the same technical specifications as the Huebner device, such as the resolution. However, the difference lies in their operating safety. The Huebner looper encoder is one of the most reliable products in this unforgiving environment.”