Steering Units (strip center sensing systems)

Due to imperfect shape of strip and minor misalignment or instability of equipment, there will be some mistracking of the strip through a processing line.

Steering units work at points throughout the line to bring the strip back to center on the line, preventing damage to the strip and the equipment if the strip wanders too far. Steering is also done at the uncoiler and recoiler.

Operating principles:

Strip will tend to align itself 90° to a roll, so when a roll is pivoted the strip will tend to turn with it. This is used to turn the strip back towards center if it is off center.

A sensor is used immediately after each steering unit which detects where the strip is in relation to the line center.

This information is used by the steering unit controller which causes the steering unit to respond accordingly by pivoting. For effective steering, the strip after the steering unit should be perpendicular to the plane of pivot of the steering roll(s).

Parameters:

Steering unit type – Steering units can be single roll, double roll, or three roll, with various arrangements. Generally, these are determined by the line layout and steering requirements in each area.

Single roll units usually wrap the strip 90° and require 3-5 times the strip width at the entry span and 1-2 times the strip width at the exit span. 180° wrap can also be used in cases like horizontal accumulators.

Double roll units usually wrap 180° total and only require 1-2 times the strip width at the entry and exit spans.

Three roll units maintain passline, but require up to 10 times the strip width at the entry span. They are usually used for thicker or higher tension strip.

Diameter of steering rolls – Larger rolls are more expensive, but cause less bending of the strip. Thicker strip usually requires a larger roll diameter to reduce the stress caused by wrapping around the rolls. Roll diameter also depends on the line section.

For instance, larger rolls are normally used after process section. (e.g. Entry – Ø800, Intermediate – Ø1250, Exit – Ø1000 / Ø1250) Rolls are generally no smaller than Ø800 and can be up to Ø1800 for heavier lines. Three roll steering units use smaller rolls (Ø500 or Ø800).

Length of steering rolls – Dependant on maximum strip width. Generally, use the strip width plus the steering range.

More length is required for a vertical accumulator (strip width + approx. 420) than a horizontal accumulator (strip width + approx. 220).

Material of steering rolls – Standard: use chrome coated rolls. For surface sensitive strip (as in pickling lines), use PU coating (also rubber).

Shape of steering rolls – Standard: use crowned profile for chrome coated rolls and cylindrical profile for PU coated rolls.

Steering units already provide steering, so there is no advantage to using crowned rolls (which improve strip tracking, but not predictably)?

Bearing diameter on rolls – The bearing diameter must be large enough to withstand the load on the roll due to the strip tension. Sizes are determined through the TKSolver drive calculation program.

Standard sizes are Ø140 for light duty, Ø190 for medium duty, and Ø280 for heavy duty.

Steering type – Proportional steering units use a pivot point near the entry side of the frame. Generally, double roll units use this type.

Proportional plus integral (PI) type steering employs two linkage arms to pivot the frame as well as moving it laterally, creating a theoretical pivot point somewhere in the entry span.

This point should be at ½ to ⅔ the distance of the entry span upstream of the steering guide. Generally, single and three roll units use PI type steering.

For PI type steering, the pre-entry span should not be longer than the entry span to avoid detrimental steering effects in the pre-entry span.

Steering actuation – Standard: use hydraulic cylinders to actuate the steering frame. Electromechanical cylinders can be used in some cases, especially for faster steering response, lower loads, and where it is preferred not to use hydraulic fluids.

Pivoting plane of steering unit – Usually the steering units are installed with the pivoting plane horizontal (0°) or vertical (90°). Some designs pivot at an angle of 5° to provide more wrap on the second roll, especially in normally-full vertical accumulators.

Other angles are also possible on some units to keep the pivot plane perpendicular to the exit span.

Strip flow – What direction the strip needs to go and whether there is vertical or horizontal travel has an effect on whether the pivot plane is horizontal or vertical, which side of the unit has the pivot point, and the distance between rolls if it is a 2-roll unit.

The pivot plane should always be perpendicular to the exiting strip, and the pivot point should always be at the entry side of the steering unit.

Distance between rolls – This does not apply to single roll units.

For double roll units, the distance depends on the vertical or horizontal change in strip passline, according to the line layout. Larger distances require a larger, heavier frame.

For three roll units, the distance depends on the roll size used.

Cam follower size – The cam follower diameter must be large enough to withstand the load of the pivoting frame on the fixed frame due to weight and strip tension.

Sensor type – For standard applications it is recommended to use BMI2 type sensor from EMG.  

For higher precision steering, BMIH type sensor from EMG. Both types should be sized according to strip width. Generally, the sensors should be guarded against accidental damage from the strip.

Snubber rolls – Occasionally, snubber rolls are used on steering units. Usually they are driven pinch rolls, cylindrical, PU coated, Ø300, and the same roll face as the steering rolls.

Mounting – Standard: the steering unit is mounted on the concrete foundation. Depending on the line layout, some units may be mounted on steel structure.

Polishing Devices – Polishing devices are used to clean the rolls as they transport the strip. Generally, they are used on rolls that are after the zinc pot or skin pass mill in a line.

Polishing devices use counterweights to keep them engaged and may be manually disengaged, or pneumatically disengaged.

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