FORM 2THE PATENT ACT 1970(39 OF 1970)&The Patent Rules, 2003COMPLETE SPECIFICATION(See Section 10 and Rule 13)
1 TITLE OF THE INVENTION :
A COUNTERWEIGHT MAGNETIC LIMIT SWITCH
2 APPLICANT (S)
Name : STEEL AUTHORITY OF INDIA LIMITED
Nationality : A Govt, of India Enterprise
Address : Bhilai Steel Plant, Bhilai, State of Chattisgarh, India
3 PREAMBLE TO THE DESCRIPTION
COMPLETE
The following specificatior l particularly descibes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION:
The present invention relates to a counterweight magnetic limit switch for limiting the movement of a drive shaft and, in particular, to the provision of a counterweight magnetic limit switch in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow with possible linear limit settings and avoid the limitations and complexities of existing rotary limit switch used in such systems. The counterweight magnetic limit switch of the invention is thus directed to avoid hazards of harsh and unsafe environmental conditions of installation of the conventional rotary limit switches presently in use. Moreover, the system of the invention which is simple to obtain and operate is user friendly and would not require any expertise or specialized training to operate and use and would thus benefit wide scale application and use of the system for variety of drive limiting applications/end uses.
BACKGROUND AND PRIOR ART:
It is a well known process of making steel in basic oxygen furnaces in the steel melting shop of a steel plant by suppressed combustion of liquid hot metal by oxygen blowing. The skirt mechanism that exist below the hood, is vertically movable outside the lower section of the hood surrounding the furnace mouth during the blow, to ensure the draft regulation and the useful carbon monoxide which is produced during the decarburization process of the hot metal inside the converter, being used as a constituent fuel gas in different other sections of the steel plant. The skirt movement is effected by using a motor that is operatively connected by a vertical shaft with the skirt, rotation of which is stopped automatically on either direction, clockwise or anticlockwise, by rotary limit switches used in the control circuit. The extreme limits of vertical movement of the skirt mechanism are controlled by the rotary limit switches by limiting the rotation of the shaft in either direction. The rotational movement of the shaft is converted to vertical movement of the skirt by operative mechanical means.
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The present invention originates from and concentrates on the consequences of the failure in performance of these rotary limit switches used to stop the skirt movement at the intended positions. The failure of the rotary limit switch on one side cause the skirt to move beyond the safe limit while moving in the upward direction and in the extreme case it crashes against the equipments and portions above the furnace causing severe damage to these. As a consequence of this, the entire steel melting unit is subjected to downtime extending from 6 hours to 3 days, depending on the severity and extent of damages caused. In the event of a melting furnace breakdown for hours, it leads to financial losses to the tune of a few lacs and possibly crores of rupees.
The different types of limit switches used in the existing systems to restrict the movement of the equipment e.g. the skirt mechanism, including the rotary limit switch in particular, are thus not failsafe in their performance especially in the adverse environmental conditions like the heat, dust flames and hazardous fumes. Moreover, difficult to access required location of the system, makes occurrence of a failure of a limit switch much more inconvenient to be repaired within a reasonable time. The preventive maintenance measures and necessary adjustments for the said switches in critical to approach locations at the furnace top are also difficult, time consuming, unsafe and expensive. The rotary switches presently in use also need critical care and associated skill of the related personnel for coupling and alignment of the limit switch with the shaft during installation and also the requisite level of expertise necessary for the adjustments while the rotary limit switch is in service, which is usually done by trial and error method. The rotary limit switches capable to work satisfactorily in harsh environmental condition like that of a zone in proximity to top of a melting furnace is a complex and cost-extensive gadget and normally imported at a high cost.
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OBJECTS OF THE INVENTION:
It is thus the basic object of the present invention to provide for a counterweight magnetic limit switch for limiting the movement of a drive shaft, which would avoid the aforediscussed limitations and disadvantages of the conventional rotary limit switches both in terms of complexities of manufacture/use and also cost.
Another object of the present invention to provide for a counterweight magnetic limit switch for limiting the movement of a drive shaft, which would be totally safe and would be simple to install and use requiring only a simple rope connections to the drive shaft whose movement is required to be limited by the limit switch of the invention.
Another object of the present invention to provide for a counterweight magnetic limit switch for limiting the movement of a drive shaft, which would not require affecting the running of the equipment /drive shaft in case of any problems in the limit switch and such maintenance /installation would thus be free of any loss in valuable productivity/equipment functioning.
Further object of the present invention is directed to a counterweight magnetic limit switch for use in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow which would avoid the aforediscussed limitations and disadvantages of the presently in use rotary limit switches both in terms of complexities of manufacture/use and also cost.
A further object of the present invention is directed to a counterweight magnetic limit switch for use in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow which can be located in safe and easy to operate position free of the hazards of unsafe working environment.
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A further object of the present invention is directed to a counterweight magnetic limit switch for use in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow which would not be exposed to the problems of maintenance and use and would have no effect of any dust or heat due to its possible favourable locations away from the direct vicinity of the working environment.
A further object of the present invention is directed to a counterweight magnetic limit switch for use in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow which would be simple and user friendly and based on linear movement adjustments which is very simple.
A further object of the present invention is directed to a counterweight magnetic limit switch for use in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow which would be simple to manufacture and would also be cost-effective and can thus be locally and readily manufactured.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a counterweight magnetic limit switch for limiting the movement of a drive shaft comprising:
rope means connected to the drive shaft whose movement need to be limited at one end and to a counter weight at its other end via pulley means;
said counter weight means having magnet;
a lower limit magnetic switch means and an upper limit magnetic switch means selectively adjusted to favour controlled desired movement of the
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drive shaft based on the counter weight movement limited by said lower and upper limiting switches.
According to a preferred aspect of the present invention there is provided a counterweight magnetic limit switch in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow comprising :
rope means connected to the skirt mechanism whose movement need to be limited at one end and to a counter weight at its other end via pulley means;
said counter weight means having magnet;
a lower limit magnetic switch means and an upper limit magnetic switch means selectively adjusted to favour controlled desired movement of the skirt mechanism based on the counter weight movement limited by said lower and upper limiting switches.
Advantageously, in the above embodiment of the counterweight magnetic limit switch of the invention, the said rope of proper size is laid through a bottom guide pulley and a top guide pulley and fixed with counter weight at said one end and said drive shaft/skirt mechanism at the other end. A clamp means is provided on said drive shaft/skirt mechanism for operative fixing of an end of said rope thereto.
The said counter weight comprises iron piece with a magnet fixed therein. The weight of the counter weight, diameter of the clamp, and the length of the rope are selectively determined based on the desired end use/application.
In accordance with a preferred aspect the above disclosed counterweight magnetic limit switch comprises a structure housing said counter weight means adapted to move up and down in said structure, said top guide pulley fixed on the top of said structure and said bottom guide pulley fixed on the outside of the structure; said
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upper limit magnetic switch means fixed on the upper outside of said structure; said lower limit magnetic switch fixed on the lower outside of the said structure.
In the above counterweight magnetic limit switch the said upper limiting magnetic switch and lower limiting magnetic switch are manually adjustable.
It is thus possible by way of the above counter weight magnetic limit switch of the invention to attend to the known problems caused due to failure of the rotary limit switches, which are presently in use in the control of skirt movement in basic oxygen furnaces. Advantageously, the system makes use of a counterweight magnetic limit switch enabling simple linear movement adjustment for setting upper and lower limits of skirt movement instead of the expertise needed for the difficult alignment and coupling with the drive shaft and rotational movement adjustment for the rotary limit switch.
By way of the present invention, the coupling of the skirt mechanism with the moving shaft is achieved by simply attaching a rope with a simple type of clamp being mounted on the said shaft, so that the rotational movement of the drive shaft is converted to the linear movement of the rope being operatively connected with the counterweight with embedded permanent magnet, the rope being guided by a pair of pulleys suitably supported preferably by a fabricated structure. The linear movement of the counterweight magnet vertically up and down operates the upper and lower magnetic limit switches with preset positions mounted on to a slot on the supporting structure, which in turn stops/starts the motor, driving the shaft on either direction and that again controls in turn the vertical movements of the skirt mechanism.
Advantageously, the above disclosed counterweight magnetic limit switch of the present invention provides means for eliminating the complex and expert means for alignment and conventional coupling and also the need for difficult (usually trial and error) process of rotational adjustments of a rotary limit switch for setting the limits. The upper and lower limits for the linear movement of counter weight
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operating the magnetic limit switches can be easily and promptly fixed, corresponding to the intended upper and lower bounds for the skirt mechanism.
According to yet another preferred aspect of the present invention counterweight magnetic limit switch is adapted to be used at a safe distance from the hazardous operating conditions of the furnace top and at a convenient accessible location, enhancing the operating life of the limit switch with improved reliability and maintainability, at much less cost as compared to existing rotary limit switches in terms of operation, service/maintenance.
Also the counterweight magnetic limit switch of the present invention is also having means to locate the Limit Switches at safe convenient locations making routine servicing or adjustments and also breakdown maintenance easy, safe and cost effective, and thereby avoiding the unwanted impact of loss of production, ensuring safe operation and long term reliability of the system. Accordingly also the limit switch of the invention makes installation easier, free from any need for coupling and alignment with the drive shaft involving very simple adjustments and requiring no high level expertise and/or any specialized training of the operator.
It is thus possible, as evident from the afore discussed various aspects of the present invention, by adopting the simple to obtain counterweight magnetic limit switch in steel melting furnaces/converters, a better and efficient method of controlling the vertical movements of the skirt mechanism is achieved substituting the existing imported rotary control switches, ensuring ease, safety and hazard free environment in operation and maintenance with longer life of the device at lesser cost.
The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustrations as per the accompanying figures wherein:
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BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES:
Figure 1: is a schematic illustration of the arrangement of a rotary limit switch located in the hazardous and unsafe working location of a basic oxygen furnace in a steel plant.
Figure 2: schematically illustrates an embodiment of the counterweight magnetic limit switch of the present invention.
DETAILED DESCRIPTION WITH REFERENCE TO FIGURE:
Reference is first invited to accompanying Figure 1, which illustrates the arrangement of a rotary limit switch presently in use.
As shown in said Figure 1 in the well known process of making steel in basic oxygen furnaces in the steel melting shop of a steel plant by suppressed combustion of liquid hot metal by oxygen blowing, the skirt (1) mechanism that exist below the hood (2), is vertically movable outside the lower section of the hood surrounding the furnace mouth (4) during the blow, to ensure the draft regulation and the useful carbon monoxide which is produced during the decarburization process of the hot metal inside the converter, being used as a constituent fuel gas in different other sections of the steel plant. The skirt (1) movement is effected by using a motor (8) that is operatively connected by a horizontal shaft (7) with the skirt, rotation of which is stopped automatically on either direction, clockwise or anticlockwise, by rotary limit switches (9) used in the control circuit. Thus under such system the extreme limits of vertical movement of the skirt mechanism (i) are controlled by the rotary limit switches by limiting the rotation of the shaft in either direction. The rotational movement of the shaft is converted to vertical movement of the skirt by operative mechanical means.
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Importantly, the disposition and use of such conventional rotary limit switch (9) presently in use are subject to failure in performance of these rotary limit switches to stop the skirt movement at the intended positions. Any such failure of the rotary limit switch on one side cause the skirt to move beyond the safe limit while moving in the upward direction and in the extreme case it crashes against the equipments and portions above the furnace causing severe damage to these. As a consequence of this, the entire steel melting unit is subjected to downtime extending from 6 hours to 3 days, depending on the severity and extent of damages caused. In the event of a melting furnace breakdown for hours, it can lead to financial losses to the tune of a few lacs and possibly crores of rupees.
Importantly also as would be well apparent from the schematic illustration in Figure 1, in case of such conventional rotary limit switches (9), the same is essentially required to be installed in the hazardous and unsafe working environment of the furnace which not only affects its performance due to the heat and dust in the region but also is a serious problem to access the limit switch in case of maintenance and repair job.
Moreover, apart from complex and unsafe location of the rotary limit switch, the same also has certain inherent limitations. In particular, in such rotary limit switches, the rotary movement adjustment is extremely difficult and is attended by trial and error. Also, the coupling and aligning with the drive shaft (7) involves expertise to do so which again adds to complexities in its installation and use.
Moreover, apart from the location of the rotary limit switch in harse conditions, which lead to occasional failure of the system, it is difficult for the operator and maintenance personnel to access such installations due to the extremely unsafe and hazardous location of the rotary limit switch unit.
Reference is now invited to figure 2, which schematically illustrates the arrangement of the counter weight magnetic limit switch of the invention directed to avoid the aforediscussed limitations of the conventional rotary limit switches. As
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shown in the figure the device basically comprises a rigid steel fabricated structure (8) such as obtained of 25mm leg length angle sections, assembled to assume a shape of parallelepiped, large enough to accommodate the limiting arrangements of the system and their linear variability in operation. A top guide pulley (6) and the bottom guide pulley (2) are mounted on the said structure at suitable locations at the top and bottom respectively by means of brackets mounted outside of the structure to facilitate smooth sliding of the rope (5) through the grooves of the pulleys. The rope (5) of proper size in diameter and length is operatively connected with the drive shaft (4) imparting intended motion to the skirt mechanism and whose movement is required to be limited in either direction, by means of simple clamp (1) of suitable diameter mounted on to the said shaft (4) so that the rope winds on to the shaft as it rotates, whether clockwise or anticlockwise. At the other end of the same rope, that is laid over through the grooves of the top guide pulley(6) and the bottom guide pulley(2) as shown in the referred figure, it is tied/fixed with the counterweight(9) that is suspended inside the structure with a suitable stretch of overhang of the rope (5) . As the shaft rotates, being driven by a motor (not shown) supply power to which is controlled by the limit switches, the rope gets wound up on to the shaft (for either direction of rotation) sliding on and guided by the pair of pulleys. This causes the Counterweight (9) to move up or down, along with the permanent magnet (3) of suitable size and strength mounted on it, depending on the direction of rotation and number of turns it rotates, which in effect is a parameter dependant on the requirement of the limits of vertical movements of the skirt mechanism. Once defined by the safe operating range for the skirt mechanism, the limits of the movements of counterweight in upward and downward direction is set and marked as per this requirement. The upper limit magnet switch (7) and the lower limit magnet switch (10), mounted on to slots provided on the outside of the structure, are adjusted to required positions by sliding manually through the slot and then fixed at these required locations. Thus the upper and lower limits for the linear vertical movement of the counterweight are set. This in turn limits the vertical movement of the skirt mechanism by controlling power supply to the motor, driving the shaft connected to the same mechanism, via the upper and lower magnetic limit switches being part of the
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control circuit. As the counter weight embedded with the permanent magnet reaches at the control points, the make/break for the limit switches are actuated by magnetic force. The linear adjustment for the invented device is thus implemented with minimum time and simplicity, without any need for any special expertise.
It is thus possible by way of the present invention to provide for a limit switch, which can be advantageously located away from the unsafe and hazardous working environment of furnaces in steel plants and also be totally safe for the people attending to the installation and maintenance of the limit switch controlling skirt movement. The limit switch of the invention would favour avoiding any costly breakdown time of furnace operation due to limit switch failure and thereby make the operation and use of limit switches effective, user-friendly and also cost-effective.
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We claim:
1. A counterweight magnetic limit switch for limiting the movement of a drive
shaft comprising :
rope means connected to the drive shaft whose movement need to be limited at one end and to a counter weight at its other end via pulley means;
said counter weight means having magnet;
a lower limit magnetic switch means and an upper limit magnetic switch means selectively adjusted to favour controlled desired movement of the drive shaft based on the counter weight movement limited by said lower and upper limiting switches.
2. A counterweight magnetic limit switch in a basic oxygen furnace for limiting
the vertical movement of a skirt mechanism outside the lower section of
hood surrounding the furnace mouth during the blow comprising :
rope means connected to the skirt mechanism whose movement need to be limited at one end and to a counter weight at its other end via pulley means;
said counter weight means having magnet;
a lower limit magnetic switch means and an upper limit magnetic switch means selectively adjusted to favour controlled desired movement of the skirt mechanism based on the counter weight movement limited by said lower and upper limiting switches.
3. A counterweight magnetic limit switch as claimed in anyone of claims 1 or 2 wherein said rope of proper size is laid through a bottom guide pulley and a top guide pulley and fixed with counter weight at said one end and said drive shaft/skirt mechanism at the other end.
4. A counterweight magnetic limit switch as claimed in anyone of claims 1 to 3 wherein said counter weight comprises iron piece with a magnet fixed therein.
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5. A counterweight magnetic limit switch as claimed in anyone of claims 1 to 4 comprising clamp means provided on said drive shaft/skirt mechanism for operative fixing of an end of said rope thereto.
6. A counterweight magnetic limit switch as claimed in anyone of claims 1 to 5 comprising a structure housing said counter weight means adapted to move up and down in said structure,
said top guide pulley fixed on the top of said structure and said bottom guide pulley fixed on the outside of the structure;
said upper limit magnetic switch means fixed on the upper outside of said structure;
said lower limit magnetic switch fixed on the lower outside of the said structure.
7. A counterweight magnetic limit switch as claimed in anyone of claims 1 to 6 wherein the diameter of the clamp, length of the rope and the weight of the counter weight are selectively determined.
8. A counterweight magnetic limit switch as claimed in anyone of claims 1 to 7 wherein said upper limiting magnetic switch and lower limiting magnetic switch are manually adjustable.
9. A counterweight magnetic limit switch for limiting the movement of a drive shaft and a counterweight magnetic limit switch in a basic oxygen furnace for limiting the vertical movement of a skirt mechanism outside the lower section of hood surrounding the furnace mouth during the blow substantially as herein described and illustrated with reference to the accompanying figure.
Dated this day of 15th November, 2006 Anjan Sen
Of Anjan Sen & Associates Applicant's Agent
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ABSTRACT
A COUNTERWEIGHT MAGNETIC LIMIT SWITCH
A counterweight magnetic limit switch advantageously used to limit the vertically up and down movement of the skirt mechanism surrounding the throat of a basic oxygen furnace in a steel melting shop, suitably located at a safe distance ensuring ease, safety, reliability in operation and maintenance, replacing an existing rotary limit switch, frequent failure of which caused substantial financial loss and damage of auxiliary equipments above skirt. The device makes use of a rope(5), operatively connected with the driving shaft(4) at one end by a clamp(l) and at other end with a counterweight(9) with magnet(3), rope being laid over guide pulleys- top(6) and bottom(2), supported on a steel structure(8), so that the rotation of shaft causing the skirt movements is converted to linear up and down movement of the counterweight magnet that actuates the upper(7) and lower(lO) magnetic limit switches adapted to limit skirt movements through a drive motor in a control circuit. Fig. 2
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