Patent Application
20060180079
The present invention relates to an apparatus for scraping a roll surface to remove surface deposits that cause surface defects in a molten metal coating process.
Continuous hot-dip galvanizing lines are known in the art. A cleaned strip of steel is heat-treated and passed from the furnace into a coating bath without being exposed to air. The coating bath contains molten zinc or zinc-aluminum (Zn—Al) alloy. As the strip emerges from the coating bath, an air knife is directed at both sides of the strip to control the weight and thickness of the coating.
After the strip enters the coating bath from the submerged furnace snout, the strip is held under the surface of the liquid metal by a submerged roll called a sink roll. Inter-metallic particles and oxides form in the bath and create undesirable substances known as dross. Dross occurs in several forms, and each form has several causative factors. The primary causes of dross are impurities in the bath (primarily iron) and temperature differentials between the molten bath, the entering substrate steel, and the sink roll equipment. Dross can form on the sink roll, causing degradation in the quality of the coated strip metal in the form of dents, resulting in defective product that fails to meet product specifications. By successfully removing the dross from a sink roll, it is possible to increase the yield and the quality of the coating process.
A sink roll assembly with accumulated dross must be replaced periodically for machining of the surface within acceptable tolerances. In zinc-aluminum continuous coating lines, replacement of a sink roll frequently takes two to four hours, and sometimes longer, during which the continuous production line is idle. When there is no dressing or scraping of the sink roll, a typical sink roll assembly in a high-speed coating line operates for three to five days, (or nine to fifteen operating shifts). The roll assembly must be disassembled, machined and reassembled, at considerable time and expense, before it can be placed back into the coating line.
In some cases, dross is removed manually by a worker manipulating a pole-mounted scraper, requiring the worker to stand directly above the pot containing molten metal 55% Zn—Al at 1100° F. For worker safety and for environmental reasons, it is desirable to avoid handling manual tools directly above liquid metal.
Mechanical scrapers have also been employed to solve the problem of dross buildup. There are two types of mechanical scrapers for cleaning the dross from sink rolls. A full-width blade is a stationary blade contacting the rotating sink roll. The blade extends the entire width of the sink roll. The full width blade wears to the profile of the sink roll over time due to the constant friction. Sink rolls are periodically removed for resurfacing, and may be machined with a crowned profile. Also if dross does appear on a sink roll despite the use of the full width mechanical scraper, the defect creates a wear spot on the scraper blade, thus permanently transferring a defect to the finished steel product.
A second type of mechanical scraper blade employs a short blade, approximately ¼ or less of the roll width. The short-blade scraper device is disposed above the molten metal bath and traverses the entire width of the sink roll by a worm drive, from which the scraper blade depends. The pressure applied by the scraper blade against the sink roll is adjusted by various means, such as by a system of weights and flotation device appended to the scraper arm to counter the weight of the blade; or by the use of a scraper blade drive unit responsive to a torque sensing device to regulate the pressure of the traversing blade. The use of a torque sensor unit in combination with a scraper blade driver is complex and expensive. The floatation device, however, is cumbersome and inflexible, requiring the operator to physically add or remove weights or floats for adjustment. Moreover, the positioning of a worm drive above a molten metal bath introduces corrosion and bending of the worm drive member in the hot environment.
Controlling the force applied to the sink roll by the scraper is critical, since the sink roll rotates by the frictional force between the steel and the sink roll as the strip passes under the sink roll. The application of excessive force may cause the sink roll to slip against the steel strip, creating scratches and other defects. By contrast, application of insufficient force may result in accumulation of dross.
Thus, there is a need for an improved sink roll scraper blade system with automatically controlled scraping pressure, and a traversing means disposed away from exposure to the molten metal bath.
Essentially, the preferred embodiment of the apparatus comprises a pair of independently controlled, twin-bladed articulated scraper heads mounted on movable arms, with pneumatic pressure control, that permits a methodical wiping of the entire surface of a roll as the submerged roll rotates in a pot, and scraper blades traverse the roll along its axis.
According to one aspect of the invention, there is disclosed an apparatus for scraping a roll surface in a molten metal coating process comprising a support member having a pair of linearly movable arms supported thereon. The support member depends from a bridge structure spanning a continuous metal coating line, and a pair of arms, the arms being disposed on opposite sides of said support member. Each arm has a scraper assembly portion attached thereto. In a preferred embodiment, each scraper assembly portion has two blades affixed thereto, a forward scraper blade and a rear scraper blade, with a connecting portion connecting the two blades. The connecting portion has a first pivot point for attaching the connecting portion to the arm associated with the scraper assembly portion, the first pivot point being disposed between said forward and rear scraper blades for following the radial contours of the roll surface. Connecting portion also includes a second pivot point to allow the blades to pivot along the crowned axis of the roll. There is provided a means for advancing the arms such that at least one scraper blade presses against the roll surface under pressure. There is also control means for controlling the pressure of the scraping force of said blades applied to the roll surface. A traversing means provides for communicating lateral movement of said blades laterally along the axis of the roll while scraping against the roll.
Means for advancing arms comprises a cylinder operatively connected to and responsive to a source of pressurized gas. Scraper assembly connecting portion also has a pair of limit portions to restrict the angle of rotation about the pivot point of said blade assembly in relation to said arm. There may also be included means for pivotally connecting each said scraper assembly to the associated arm to create a second degree of rotation for said scraper assembly relative to said arm, to allow said blades to pivot along the crowned axis of the roll.
In a preferred embodiment the means for advancing arms comprises a cylinder operatively connected to and responsive to a source of pressurized gas, the pressure in said cylinder being is variably controlled by said control means.
A control means comprises a digital controller in electronic communication with an analog device, such as a proportional control valve, such that the pressure may be varied over a predefined range corresponding to zero pressure up to full line pressure. Traversing means comprises a motor, an actuator portion, and a cylinder portion, the cylinder portion being operatively connected to the support member, such that the motor drives said actuator portion, thereby imparting linear motion to the support member through said cylinder portion, causing said blade or blades to traverse the horizontal axis of the roll in contact with the surface of the roll.
A speed control interface in electronic communication with said motor portion and said control means controlling the speed of said motor from a speed reference point communicated from said control means.
In another aspect of the invention, a method is disclosed for scraping a roll surface rotating in a molten metal coating process comprising traversing a support assembly to one side, lifting the arms attached to said the assembly to a fully retracted position to disengage scraper blades from contact with a roll surface; initiating a scraping cycle by means of a trigger signal; lowering the arms and scraper blades in the mid-point of the sink roll; increasing the pressure reference from lifting pressure to approximately zero; energizing one or more directional valves; increasing the pressure value gradually to a preselected pressure; extending the cylinder in a controlled manner to engage the scraper head gently on to the sink roll; moving the engaged scraping head at a controlled, predetermined speed from the sink roll mid point to the fully traversed out position; stopping the traversing means upon reaching the fully traversed out position; and lifting the scraper head from the roll surface by removing pressure from the cylinder; then repeating the sequence until a predetermined number of cycles are completed.
In another aspect of the method, a counter is incremented in the control means after each cycle, comparing said counter after each repetition of a cycle, repeating another cycle until a pre-selected number of cycles is completed, and then traversing the support assembly to one side of the sink roll and lifting the arms away from the surface of the roll. The trigger may be selected from one or more of the following: a timer which activates the sequence on a regular time based interval; a weld signal from the weld tracking logic in the PLC; or an operator initiated “Cycle Now” pushbutton.
It is an object of the invention to provide two or more independently operated arms that allow one or more scrapers to independently scrape against the roll to remove dross and other surface imperfections.
It is further object of the invention to provide twin-blade articulated scraper heads in pressurized contact against the roll and transverse the blades across the roll during rotation.
A further object of the invention is to provide a methodical wiping of the entire roll face during roll rotation through traverse motion of the scraper heads, allowing every point on the roll face to contact the front and back blades at least one time during roll rotation and traversal of the scraper heads.
Yet another object of the invention is to provide optional independent control of the scraper heads.
Another object of the present invention is to provide pivotal motion of each scraper head in two directions to conform to roll radius or curvature, and to the roll axis or crown.
It is still another object of the invention to provide a pressurized cylinder coupled to each scraper head for advancing and retracting the associated scraper head under controlled pressure, which is adjustable by the operator to apply more or less pressure as required.
Another object of the present invention is to provide a digital processor system for controlling the pressure applied by scraper blades against the roll, and for controlling the speed and travel of the scraper heads traversing the roll surface.
Another object of the present invention is to provide a structural support and integrity, with minimal weight, for the scraper heads and movement arms.
A further object of the invention is to provide a transport drive mechanism that is remote from the heat of the molten metal pot, to either side and not directly above the pot.
Further objects of the invention will be made apparent in the following Detailed Description and the appended claims.
Referring to
A sink roll scraper assembly is generally designated as 10. A sink roll 16 is submerged in a molten metal pot 12. The sink roll scraper assembly 10 includes a support member 14 having a pair of scraper arms 18, 20. An arm 18 or 20 is disposed at either side of support member 14. Each scraper arm 18, 20 has a scraper head assembly 22, 24 attached at an end adjacent to the sink roll 16. Scraper arm 20 at one side of the support member 14 is captured in a lower guide sleeve 26, 28 and an upper guide sleeve 30, 32, to align the scraper arm when advancing and retracting the scraper head assembly 24. Similarly, on the opposite side of support member 14, scraper arm 18 is captured at two points on the support member at guide sleeve 26 and guide sleeve 30. Guide sleeves have an annular opening coaxially aligned with the associated arm to permit lower telescoping movement for advancing and retracting the scraper head assembly 22. Support member 14 is preferably comprised of a rigid, durable plate material capable of withstanding high temperature. To minimize the weight of the support member, in the preferred embodiment, support member includes horizontal top and bottom arms 14a, 14b, connected by a web portion 14c. The side edges of the web 14c are cut away, to form an hourglass shaped backing plate. Other structural configurations for the support member may be employed, within the scope of the appended claims.
Cylinder 34, is operatively connected to scraper arm 18 and cylinder 36 is operatively connected to arm 20. Each cylinder advances or retracts the arm with which it is associated. Arms 18, 20 may operate independently of one another. In normal operation, both arms operate so as to cover the entire width of the roll in a single cycle. Preferably, the cylinders are pneumatically pressurized with direction action pressing the scraper head assembly into contact with the roll; and pressurized in the opposite direction so as to retract scraper head assemblies 22, 24 away from contact with the roll when not scraping.
A traversing cylinder 38 is driven by an electromechanical actuator motor 40, which acts on transport shaft 42 to impart lateral movement to scraper assembly 10, such that scraper head assemblies 22, 24 traverse back and forth to cover the entire surface of sink roll 16. Traversing cylinder 38 and electromechanical actuator motor 40 are positioned off to one side of pot 12, removed from the heat radiating directly above the pot. A lower transport rail 44 is provided to both support the scraper assembly and to guide the lateral movement of the lower portion of scraper assembly 10. Rail 44 preferrably employs a wear coating to reduce friction. Rail 44 prevents the arms 18, 20 from kicking back, and positions support member 14 in an inclined plane directed at roll 16.
As can be seen in
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Control of Scraper Arms
Control of the linear arm movements and the traversing movements may be accomplished in several ways, including direct manual operation. In the preferred embodiment, Referring to
The scraper operates cyclically in automatic mode, scraping the roll surface for a portion of every hour. For example, in a Zn—Al coating line, the cycle setting in the preferred embodiment is approximately ten minutes per hour of operation. The portion of time may be varied by the operator according to factors such as coating hardness or line speed. While the blades are scraping the roll, transport shaft 42 oscillates horizontally above the pot 12, moving the support member 14 and the blades transversely, so that the entire width of the roll 16 is covered by the dual blade configuration. Generally, the lateral stroke of the transport shaft 42 and traversing cylinder 38 does not exceed one-half the width of the roll, and may be less than one half the roll width, depending on the width of the blades. The short stroke reduces cycle time for scraper blade assemblies 22,24 to pass back and forth across the roll surface.
The electromechanical actuator motor 40 for the transport shaft 42 is preferably a variable speed motor, controlled through the PLC. Control may be automatic or manual, as indicated above.
Control signals are derived from a combination of Operator requests and System Interfaces, processed through the programmable controller. The pneumatic cylinders are controlled by a combination of an electronic pressure regulator in line with a spring return single solenoid operated directional valve, preferably, although a dual directional valve configuration is also capable of operating the cylinders.
An electronic pressure regulator valve (not shown) is controlled by an analog signal over its operating range of control pressure. The analog signal is generated from the PLC using the Operator Requested pressure setting from the human machine interface (HMI), which is described in greater detail below. Any changes in pressure set point are ramped in the PLC for smooth operation. Directional valves 102, 104 transfer the controlled pressure from the regulators 106,108 to stroke the cylinders up or down. The downstroke engages and upstroke disengages the scraper heads from the sink roll. The signal to energize the directional valves solenoids is derived from an operator request input to the PLC 100 to engage the scraper head assemblies 22,24, when in manual control, or from an HMI when the PLC is set in automatic mode. In the preferred embodiment, the directional valves are proportionally controlled via the PLC. It is noted that control of the directional valves via the PLC is disclosed by way of example and not by limitation. Other, less sophisticated means may be employed within the scope and spirit of the present invention, to control the operation of the cylinders. For example, direct manually operated directional valves, or relay-operated valves may be employed
The PLC 100 transmits a speed control signal to variable frequency drive 110 for the frequency set point. PLC 100 also transmits separate control signals for Run, Forward and Reverse operation. The drive 110 transmits signals to PLC 100 to indicate drive running, speed and motor current controller, for control processing. PLC logic determines when the limit of travel is reached by reference to drive running, speed and current signals. When the limit of travel is detected in one direction, further movement in that direction is inhibited by the PLC.
Human Machine Interface (HMI) Description
There is an interactive human machine interface (HMI) 200 provided with the scraper unit, in electrical communication with the PLC or other digital controller. The HMI 200 comprises a graphical screen as shown in
The traverse movement of the scraper (referred to as “East” and “West”) is controlled from the traverse motor interface 216. The traverse motor interface button 216 allows the operator to change the speed of traverse via the speed reference operator input, and to jog the scraper unit East and West via the jog pushbuttons 218, 220. Jog East and jog West buttons are visible only in manual mode. Indications of motor running, direction, speed and current are displayed to the operator 222. Up and down movement of the scraper blade assemblies is controlled from the scraper head interface 224.
The East and West scraper heads interfaces allow the operator to change the scraping pressure via the pressure reference operator inputs 226, 228 and to lift and lower the heads to engage and disengage them from the sink roll via the lift/lower buttons 230-236. Lift and lower buttons 230-236 are visible only in manual mode. In automatic mode, the arms and scraper blade assemblies are interlocked to operate in unison, and raising and lowering is done through the PLC according to the selected time cycle. Optionally, other indicators on the screen indicate nitrogen pressure; traverse fully east and fully west; and cylinder directional valve commands.
The scraper heads scraping pressure references may be adjusted by the operator input from 0 to line pressure, which in the disclosed embodiment is approximately 70 psi. These signals are interpreted by the PLC and converted to an analog signal for the electronic pressure regulators.
Automatic System Control
In automatic mode the scraper directional movements are controlled entirely by the PLC using operator input speed and pressure reference values.
The automatic sequence is enabled when the scraper is selected for auto mode and the process line is running. The automatic method for scraping the roll while the roll is rotating in the pot of molten metal is as follows:
The support assembly is traversed by the transport shaft to one side and the arms attached to the support assembly are lifted to a fully retracted position, disengaging the scraper blades from contact with the roll surface. A scraper cycle is then initiated by means of a trigger signal. Next, the arm that is positioned above the mid-point of the roll is lowered against the sink roll, the pressure reference is increased from lifting pressure to approximately zero, and directional valves are energized. The pressure value is gradually increased to a preselected pressure suitable for cleaning dross from the roll surface. The cylinder is then extended in a controlled manner to engage the scraper head gently on to the sink roll. The scraper head is then moved laterally by the transport shaft acting on the support member at a controlled, predetermined speed from the sink roll mid point to the fully traversed out position. The opposing arm is now positioned at the mid-point of the sink roll and the arm is lowered to engage the roll surface, while the first arm is retracted from the roll surface. The traversing means stops upon reaching the fully traversed out position. Then the scraper head is lifted from the roll surface by removing pressure from the cylinder, returning the arms to the retracted position. The transport shaft is returned to the starting position and the sequence is then repeated until a predetermined number of cycles are completed. A counter in the control means is incremented after each cycle. The counter value is compared with the number of selected cycles after each repetition of a cycle. The cycle is repeated until the counter value matches the number of cycles selected. The support assembly then traverses to one side of the sink roll and lifts the arms away from the surface of the roll. The trigger may be 1) a timer within the PLC which activates the sequence on a regular time based interval; 2) a weld signal from the weld tracking logic in the PLC; or 3) an operator initiated “Cycle Now” pushbutton.
The automatic sequence is enabled when the scraper is selected for auto mode and the process line is stopped. The sequence executes the following steps:
Return the traverse to the fully west side (unless already at fully east side) and both scraper heads are commanded to lift. Wait for a sequence trigger. The trigger originates from a timer in the PLC which activates the sequence on a regular time based interval, or from an operator initiated “Cycle Now” button. The traverse drive is commanded to run at the operator input speed reference to move from one end of travel to the opposite end of travel. When the PLC detects that the traverse motor has reached end of travel the command to run is released. Both scraper heads are commanded to lower. The PLC commands the pressure references to ramp from lifting pressure to approximately zero psi. When the pressure references have reached approximately zero pressure the directional valves are energized and the pressures are ramped to the scraping pressure reference. This causes the cylinders to extend in a controlled manner to engage the scraper heads gently on to the sink roll.
Scraper heads are then commanded to disengage or lift away from the roll surface. The PLC transmits a signal to the directional valves to de-energize, causing the cylinders to retract and the heads to lift. The PLC checks whether the pre-selected number of cycles have been completed, if not the cycle is repeated, and if so, the scraper assembly returns to the resting position.
Referring next to
According to the provisions of the patent statutes, we have explained the principle, preferred construction, and mode of operation of the present invention, and have illustrated and described what we now consider to represent its best embodiments. However, it should be understood that within the scope of the appended claims and the foregoing description, the present invention may be practiced otherwise than as specifically illustrated and described.
