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As part of a predictive maintenance plan, 16 rugged accelerometers were installed in a cold rolling mill. The associated 1-wire Sensor Highway facilitates data collection and reduces the amount of sensor cabling required.

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John Judd, Vibra-Metrics, Inc.

I/N Tek, a joint venture between Inland Steel and Nippon Steel, operates a cold rolling mill in New Carlisle, Indiana, that supplies high-quality steel to U.S. automotive plants. The 2000 ft continuous cold-rolling process is typically performed in five separate operations throughout the steel industry, but at I/N Tek the five are combined into one.

To be made into a sheet, steel must first be hot rolled. A slab with a thickness of >9 in. is heated to 2000ºF and passed through rollers. At the end of this process the sheet is < ¹/₄ in. thick and furled around a hollow center.

Cold rolling further reduces the thickness of the steel, hardens its surface, and improves its resistance to fatigue—qualities requisite for hoods and other vehicle body parts. This procedure can reduce a sheet of steel ¹/₈ in. thick, for example, to a thickness of ¹/₆₄ in.; its length, logically enough, increases by a factor of 8. I/N Tek's facility is a tandem mill, a rolling mill consisting of two or more stands in succession and synchronized so that the metal passes directly from one to another (see Figure 1).

Figure 1. The primary components of a tandem mill are shown here greatly simplified. The steel slab enters on the right and passes first through a 3-roll cluster that provides back-tensioning. It is then fed through a succession of four 6-high stands, where three rollers over three reduce its thickness. Tensiometers and pass line rollers measure the tension on the steel and control its wrap angle to prevent strip breaks.

The steel passes first through a 3-roll cluster whose first and third rollers are lower than the middle one. The cluster exerts 2000 psi pressure, providing the back-tensioning neccessary to thread the sheet through the entire mill. The steel then travels through four six-high stands, 3 rollers over 3, that continue to reduce the thickness of the sheet. Interspersed with the stands are tensiometers and pass line rollers, whose function is to set the strip's elevation so as to obtain the desired wrap angle. The wrap angle is there to ensure an accurate tension reading and prevent strip breaks in the mill.

Photo 1. As part of a predictive maintenance program, accelerometers from the Guardian Series were selected to monitor the behavior of the roller bearings. The sensors were installed at 16 locations in the I/N Tek tandem mill.

Guardian Accelerometers I/N Tek's unsatisfactory experience with conventional accelerometers led Mid America Dynamics, a vibration consultant, to recommend the 8002 Guardian sensors. Design was a strong argument. The accelerometers I/N Tek used initially had MIL-style top two-pin connectors that leaked when doused with coolant spray, and eliminating the spray was not an option. The Guardian sensors feature a stainless steel housing that is welded and injection molded to the case assembly for a hermetic seal against environmental assaults. The injection-molded design reduces risks of conductive fluids' interfering with vibration signal integrity. Conveniently small at 1 in. by 1.2 in., the sensors are quickly and easily mounted by means of a center through-bolt. The basic Guardian architecture is available in several models. The 10 mV/g version can measure up to 500 g; and the high-temperature type can withstand temperatures up to 680ºF. All accelerometers in the 8000 Series are equipped with Tufkem cable that resists most industrial chemicals and combinations of chemicals. Other features of the Guardian line include: NIST calibration Low noise Internal electronics design with built-in shock and reverse voltage protection An isolated shear sensing element design that is unaffected by blasts of hot and cold air Sensing element that withstands the mechanical bending that is often present in the large machinery used in steel production, allowing reliable low-frequency data reproduction All sensor materials conservatively rated for the 250ºF specified Hermetically sealed stainless interior housing that is electron beam welded with glass fused connections, creating a Faraday RFI shield for excellent noise rejection Interior sensor housing connections and cable termination completely sealed in a high-impact composite shell, forming a protective, totally isolated, chemically resistant enclosure with no connections exposed to corrosive chemicals Superior cable pull strength and strain relief Partnership lifetime warranty

The 16 accelerometers were installed in the axial direction of the rollers to eliminate the very large vibrations produced in the radial direction by the rollers pinching the steel, or making small reductions in its thickness. Axial vibration data thus more accurately represent the behavior of the bearing, rather than that of the steel.

Photo 2. The 16 accelerometers are connected to the 1-wire Sensor Highway by a transducer adapter module, which relays the sensor signals to the digital control system room. This configuration eliminates the need for individual sensor cables.

Each accelerometer is connected to the 1-wire Sensor Highway, also from Vibra-Metrics, by a transducer adapter module (see Photo 2). The sensor signals are transmitted by the TAM to the digital control system room. The Highway's 1-wire design kept the installation's initial cost and complexity to a minimum by eliminating the need for individual cables from each sensor to the control room. The system's modularity permits expansion to accommodate additional sensors that might in future be placed throughout the mill. The Highway can also be easily upgraded to include Vibralarm continuous surveillance software. By obtaining real-time snapshots of the vibration signatures, Vibralarm can detect the possibility of failures between readings on the roller bearings.

Before the Sensor Highway was installed, John Lorentson of Mid America Dynamics used a portable data logger to take vibration measurements every

Photo 3. Before the Guardian and Sensor Highway scheme was implemented, accelerometer data had to be collected directly from the switchboxes near each roller stand. The new system permits data collection from the control system room, a safer and more convenient location.

4-6 weeks at the switchbox locations near each roll stand. The stands are difficult to access and are in potentially dangerous areas. The Highway now permits data collection from the adjacent digital control system room (see Photo 3).

The Sensor Highway also helps the accelerometers and cables stand up to constant spray cooling, strip breaks (ruptures in the steel), and the routine cleaning performed during mill shutdown. Moreover, roller bearing failures that would once have caused unscheduled downtime are now being accurately predicted so that appropriate maintenance procedures can be carried out.

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