US2201210A - Strip mill - Google Patents

Description

- 2 Sheets-Sheet 1 STRIP MILL.

Filed Aug. 28, 1936 F. H. SMITH ET AL May 21, 1940,

Frank H. Smith Hen7yA.Strin fellow & 7c

ATTORNEYS M y 1940- F. H. SMITH ET AL 2,201,210

STRIP MILL Filed Aug. 28, 1936 2 Sheets-Sheet 2 /0 Z Q!) v lla I L/Z Ii] I IZ l l 9 I L i l5 /Z STRESS STRAIN 6 Fiy. 7 I 9 /5' i I /z 2 F1 H INVENTORS Frank H- Smith 5 Henry A-Stringfellow ATTOR N EYS Patented May 21, 1940 UNITED STATES PATENT OFFICE STRIP BULL fellow,

and one-half to George Downing Hartley, Worcester, Mass. Application August 28, 1936, Serial No. 98,366

Claims. (o1. 80-32) This invention relates to cold rolling strip mills and particularly to reversing mills of the type in which opposite ends of the strip being rolled are attached to winding reels positioned on opposite sides of a roll stand and inwhich the strip stock is wound up on one reel and unwound from the other and passes alternately back and forth between the squeeze rolls on the roll stand.

In our co-pending application Serial No. 24,666, filed June 3, 1935, we have disclosed and claimed a method of reducing the cross-section of an elongated metal body by pulling the body through a device which applies lateral pressure thereto, maintaining a back pull on the body rearwardly of the pressure applying means which is at all times a substantially predetermined fraction of the forward pull. The tension existing in the material during the reducing operation depends upon'the drag imposed upon the material by the reducing means, and, since the load imposed upon the pressure applying reducing means is dependent upon the physical characteristics of the material, the back pull exerted on the material is also dependent upon the physical charac teristics of the material and proportioned to the strength of the material instead of being a substantially fixed amount such as the case with ordinary retarding devices. Since the backward pull is proportioned to the strength of the material, the tension automatically may be kept close to the elastic limit of the material regardless of what the tensile strength of the material may be and the danger of applying a tension in excess of the strength of the material is greatly lessened.

It is the purpose of the present invention to provide improved instrumentalities for performing the method of our co-pending application which are particularly suitable for applying the proportional pull method to cold rolling strip mills.

In our co-pending application the method is shown applied to wire and rod drawing operations. Two types of devices for regulating back pull are there disclosed. In one of the devices, the field of the retarding motor is controlled by the field of the pulling motor in such a way so as to maintain a substantially constant ratio between the torques of the two motors, and in the other device the desired ratio is maintained by means of a differential driving mechanism.

In applying the proportional pull method disclosed in our co-pending application to reversing strip mills, account must be taken of the fact that the torque of each of the two reel shafts is constantly changing during travel of the strip from one reel to the other. The pulling motor should be operated to normally maintain a substantially constant pull on the strip and a substantially constant speed. The torque of this pulling motor must constantly increase proportionally to the increase in the effective diameter of the reel as the strip winds thereon to maintain a uniform pull on the strip. Since the unwinding reel constantly decreases in diameter during operation the torque of the retarding motor should be decreased proportionally to maintain a uniform back pull.

If a, controlling device such as disclosed in our co-pending application were applied without modification to a reversing mill, there would be a progressive variation from the desired ratio between forward and backward pull due to the progressive variation of the torque in the pulling reel shafts.

It is an important object of the present invention to provide means for maintaining a substantially fixed ratio between forward and backward pull which is not affected by the variations in the torque of the reel shafts, and which permits operation of the machine in such manner so as to maintain a substantially constant strip speed. In order to maintain an accurate control of tensions in a reversing strip mill it is highly desirable that means be provided for accurately proportioning the torque of the unwinding reel to its effective diameter throughout the unwinding operation, so that a constant ratio is maintained between the reel diameter and applied torque.

We are aware that various methods have been heretofore proposed for varying the torque, of the retarding motor of a reversing strip mill to compensate for the reduction in effective diameter due to the unwinding of the strip from the reel. However, none of the devices of which we have knowledge is capable of effecting the accurate compensation desirable for operation with tensions near the elastic limit of the material.

One method that has been proposed is to regulate the back pull by controlling the field of the retarding motor from the field of the pulling motor to decrease the torque of the retarding motor proportionally to the increase in torque of the pulling motor. There is, however, an inherent error in such a method of control due to the fact that the rate at which the wind-up reel increases in diameter is not the same as the rate at which the unwinding reel decreases in diameter. As a strip winds up on a reel the rate of increase of diameter gradually decreases as the diameter increases, and, conversely, the rate of decrease in diameter in unwinding gradually increases as the diameter decreases. The variation in the torque of the pulling motor is not a correct measure of the changes in torque which should be introduced into the retarding motor to maintain a constant back pull. The variation in the pull exerted on the strip by a retarding motor so controlled is substantial and very undesirable when the strip tension is near the-elastic limit of the material.

Another type of regulating mechanism proposed has been a balanced controlling element responsive to variations from a predetermined tension in the strip back of the rolls which operates to control the torque of the retarding motor by means of a motor actuated controller which operates until the balance of the controlling element is reestablished. The purpose of the above methods of control is simply to maintain a predetermined amount of back pull, during the rolling operation. What the amount of back pull is to be is determined arbitrarily and the judgment of the operator must be depended upon to keep that pull within safe limits.

The present invention not only provides means for changing the torque of the retarding motor to compensate for changes in diameter but also makes it possible to increase the back pull to near the elastic limit by providing an additional control which increases or decreases the torque of the retarding motor in accordance with changes in tension of the strip between the squeeze roll and wind-up reel so as to maintain a substantially constant ratio between the forward pull and the back pull.

A further object of the invention is to so interconnect a tension sensitive device and a device for compensating for changes in the diameter of the retarding reel as to accurately regulate the increases and decreases in back pull due to changes in tension and maintain the correct ratio between the forward and backward pull.

A further object of the invention is to provide a shiftable tension responsive element which engages with the strip on opposite sides of the roll stand and which is operable to measure the ratio between the tension existing in the strip on opposite sides of the squeeze rolls.

A further object is to utilize the tension ratio measuring device as a motor controlling element to maintain proportional pulls, this device being applicable quite generally in the practice of the proportional pull method disclosed in our copending application.

A further object of the invention is to provide a strip mill in which the reel motors are so controlled that they will maintain a tension upon the strip during reversal of the movement as well as during travel of the strip.

A further object is to provide a back tension controlling mechanism for reversing mills which does not require adjustment during operation of the mill. I

A further object of the invention is to provide a method of control which makes possible maintenance of the strip substantially at itsproportional elastic limit during the rolling operation.

With the above and other objects in view the invention may be said to comprise the machine as illustrated in the accompanying drawings hereinafter described and particularly set forth in the appended claims, together with such variations and modifications thereof as will be apparent to one skilled in the art to which the invention appertains.

Reference should be had to the accompanying drawings forming a part of this specification in which:

Figure 1 is a diagrammatic view showing the mill in side elevation together with a diagram showing the motor control circuits;

Fig. 2 is a view: similar to Fig. 1 in which thediagram shows in full lines the circuits which are active during the travel of the strip in the direction indicated by the arrows, the then inactive circuits being indicated in dotted lines;

Fig. 3 is a side elevation showing the tension ratio measuring. device;

Fig. 4 is an end elevation of the strip guiding and tension ratio, measuring device;

Fig. 5 is a side elevation of one of the rocker,

Fig. 7 is a stress-strain diagram showing stresss-train characteristics typical of strip steel; and

Fig. 8 is a detail View showing adjustable control contacts forming part of the motor control system.

Referring to the accompanying drawings in which the invention is shown applied to a reversing strip mill of the type in which the metal strip is passed alternately back and forth between a. pair of squeeze rolls, the roll stand is indicated by reference numeral l and may be a two-high or four-high stand as desired. For convenience of illustration the stand is shown as a two-high stand having a pair of squeeze rolls 2 of medium size. Heels 3 and 4 are mounted at substantially equal distances on opposite sides of the roll stand and these reels are alternately operated in opposite direction, winding the strip 01f of one reel, winding it up upon the other and drawing the strip through the squeeze rolls first in one direction and then the opposite. The strip passes from the upper sides of the reels beneath a pair of fixed guide rollers 5 located between the reels and the roll stand and the guide rollers 5 are positioned with their under sides below the tops of the barrels of the reels so that the strip is delivered downwardly atan angle from the unwinding reel to the guide roller and from the other guide roller upwardly to the barrel of the wind-up reel. Beneath the inclined portions of the strip adjacent each of the reels there is mounted a follower consisting of a roller 6 carried by pivoted lever 1 which is yieldingly pressed against the strip by a spring 8. The function of each of the followers is to measure the diameter of its reel and to actuate motor controlling elements, which will be hereinafter described.

An important feature of the present invention is a device which engages the strip on opposite sides of the roll stand and measures the ratio between the tensions, this device serving as an actuator for motor controlling elements which act to oppose variations from a fixed ratio. On the roll stand there is mounted a floating frame 9 which has movements in a straight line horizontally and carries guide rollers I ll on opposite sides of the roll stand, the tops of which are level with the top of the lower working roll. The frame 9 serves the purpose of supporting the portion of the strip passing through the squeeze rolls in the plane of the roll pass and also as a motor controlling device responsive to variations in strip tensions. In order to provide the sensitiveness desirable for motor control suitable supports are provided for the frame which offer very slight frictional resistance to horizontal movements of the frame As herein shown, the frame 9 is mounted upon upright rocker supports H, a pair of which are provided at each end of the frame and which serve to support the floating frame on the roll stand. Each of the rocker supports H has arcuate end faces I! which have a common center midway between the center points of the arcuate faces and which have rolling contact with flat horizontal faces I8 and M on the base of the roll stand, and on the under side of the frame 9, respectively. During movements of the frame 9 the arcuate end faces l2 of the rocker supports roll upon the flat, horizontal faces I3 and M of the frame and roll stand. Sliding movements of the frame or roll stand with respect to the rocker supports is prevented by means of short racks l5 fixed to the sides of the frame 9 and to the ends of the roll stand l which mesh with gear segments l5 fixed to the ends of the rocker supports. Suitable springs ll may be provided 'mtween lugs on the frame 9 and the roll stand i to absorb the unbalanced weight of the rockers as they are moved in opposite directions from their vertical position. The purpose of the floating frame is to measure the ratio of the tensions existing in the strip on opposite sides of the roll stand and to operate suitable electrical controls for maintaining a substantial constant ratio between these tensions. I

As shown in Figs. 1 and 2, the strip passes from the under sides of the fixed guide rollers 5 upwardly at an incline over the guide rollers it! on the floating frame. Since the strip is maintained under tension on opposite sides of the roll stand the force exerted on each of the guide rollers ill due to the tension of the strip will have a horizontal component acting in a direction toward the roll stand. If the tension on one side of the roll stand is made greater than the tension on the opposite side the horizontal component on the side where the pull is the greatest will exceed the horizontal component on the opposite side of the roll stand and the floating frame will shift toward the side where the tension is less. This shifting movement of the floating frame increases the angle of the strip to the horizontal between the rollers 5 and m on the side toward which the frame has been moved and decreases the angle of the strip between the flxed guide roller 5 and the guide roller M on the opposite side of the roll stand. The increase in the inclination ofthe strip on the side where the tension is less increases the horizontal component of the tension on that side and the decrease in inclination on the side where the tension is greater will decrease the horizontal component of tension. The inherent tendency of the frame 9 is to move to the position at which the horizontal components of the strip tensions are equal, each position of the frame providing a measurement of the ratio of the tensions existing in the strip on opposite sides of the roll stand.

It is apparent that unequal tensions in the strip on opposite sides of the frame will cause the floating frame 9 to shift toward the side where the tension is less, the actuating force on the side having the higher tension gradually decreasing and the opposing force on the side where the tension is less exerting a gradually increasing resistance to such movement so that the frame will assume a new position in which the horizontal components of the two tensions are "equal. The position of the floating frame 9, therefore, indicates the ratio between the tensions existing in the strip on opposite sides of the roll stand. The tension ratio measuring device,

floating frame 9, is utilized to actuate motor controlling elements which maintain a substantially fixed ratio between the forward and backward pulls and which also compensates for any inaccuracies in the control provided by the, follower 6. To actuate motor controlling devices the frame 9 is provided with a rack it! which meshes with a gear segment l9 fixed to a shaft 20 journalled in the roll stand, the shaft 20 serving as an actuator for suitable control elements.

Assuming that the retarding motor is controlled so as to substantially compensate for changes in diameter of the unwinding reel, the back pull would be maintained substantially uniiorm but there would be fluctuations in the tension between the squeeze rolls and the pulling reel due to variations in the load imposed upon the squeeze rolls due to variations in thickness otfri the strip and variations in the density of the s p.

The intent of the present invention is to maintain at all times a substantially constant ratio between the tensions on opposite sides of the roll stand, relieving the back pull when the load imposed upon the squeeze rolls is reduced, and add,- ing to the back pull when the load on the squeeze rolls is increased.

In the stress-strain diagram shown in Fig. '7, the point P which is known as the proportional elastic limit is the point at which tension applied to the strip begins to cause permanent deformation of the strip. Below the tension indicated by the point P the strip of metal will return to its original length when the tension is released.

Y is the yield point which is known as the commercial elastic limit. S indicates a point where there is a predetermined permanent elongation of the strip. In producing the elongation of the strip indicatedby the point S on the diagram bymeans of squeeze rolls without back tension in the strip, the work done by the rolls is proportional to the area of the surfaces 0, P, Y, S, N. If, however, the strip is rolled at the proportional elastic limit the work done by the rolls is proportional to the area of the surface P, Y, S, M. It is apparent from inspection of the diagram that the work required to be done by the squeeze rolls may be very materially decreased by applying tension to the strip and that-as the tension is increased the work required of the rolls is decreased. This aiding of the squeeze rolls also reduces wear, decreases the generation of heat which is highly advantageous and permits greater reductions in thickness during each pass. The desirability of operating with a high back tension is apparent.

An important aim of the present invention is to provide a tension'control which is sufllciently sensitive to variations in tension to enable the squeeze rolls to act upon the strip which is under an initial tension, at substantially the proportional elastic limit, thereby effecting the maximum practicable reduction of the load on the rolls, reducing the wear on the rolls and making it possible to employ rolls of medium 'size instead of small rolls backed by large rolls as heretofore has been common practice.

By maintaining a substantially fixed ratio between the pulls existing forwardly and rearwardly of the squeeze rolls accurate determination of the tensile strength of the strip being acted upon is made unnecessary, since the tension applying devices accommodate themselves to the strength of the strip being acted upon. This is highly important since as the strip is reduced in thickness by successive rolling operations the tensile strength of the strip is reduced and it is a difficult matter to determine just how many pounds back pull can safely be applied to the strip, and it is also difiicult to make an adjustment of retarding devices such as heretofore used which will efi'ect the desired changes in the amount of back pull. With the control provided by the present invention, however, the amount of pull is immaterial since the control devices maintain a fixed ratio regardless of the amount of pull, the amount of pull being automatically regulated in accordance with the load imposed upon the squeeze rolls during successive operations.

During operation the wind-up reel is driven by a suitable motor while the unwinding reel is braked dynamically by a suitably controlled dynamo electric machine. While it is entirely feasible to provide a retarding mechanism independent of the driving mechanism for each reel, we prefer to employ electric motors each provided with the coils, switches and other controlling elements necessary for effecting the changes necessary to adapt the motor for dynamic braking.

Each of the motors is provided with field coils, controlling elements and switches in addition to its series field coil by means of which its characteristics may be suitably altered to enable it to act as a dynamic brake and which may be controlled to impart a torque to the shaft of the unwinding reel which is varied automatically in' the manner necessary to maintain a back pull proportional to the forward pull.

Referring to Figs. 1 and 2 of the drawings, each motor has an armature 2|, a. series field coil 22, a compensating shunt field coil 23 which is active only when the motor is operating the windup reel, and a shunt field coil 24 which is active only when the motor is acting as a retarding motor. When the motor is acting as a pulling motor the fleld coil 22 is in series with'the armature 2! and is connected to the power line through a lead 25 while the armature is connected to the line through a lead 26. Field coil 23 is connected by the leads 2! and 28 to the line through leads 25 and 26, a switch 29 being provided to render the coil 23 inactive when the motor is retarding. A variable resistance 30 is connected in parallel with the armature 2| by means of a lead 3| connecting with the lead 26, a rheostat arm 32 is mounted to move along the resistance 30 and is connected by a lead 33 to the motor circuit between the series coil 22 and armature 2l, a switch 34 being provided to render the circuit through the resistance 30 inactive while the motor is driving the wind-up reel. The rheostat arm 32 is actuated by means of a suitable operating connection indicated by the line 35 to the follower lever 7, whereby it is given a movement proportional to the reduction in the diameter of the unwinding reel. As the reel unwinds, the resistance 32 is decreased to shunt more and more of the current from the armature 2i, the value of the resistance being so proportioned that the torque of the retarding motor is decreased substantially in proportion to the movement of the rheostat arm. The retarding field coil 25 is connected on one side to the line through a lead 36 and on the other side to a variable resistance 31 by a lead 38. A rheostatarm 39 actuated by the follower lever l moves over the resistance 31, and is connected by a lead 40 to a movable rheostat arm M which moves over a resistance 42 which is connected to line through a lead 43, a switch 44 being provided to render the field coil 24 inactive except when the motor is acting as a brake. The rheostat arm All is actuated from the shaft 20 so that its movements are proportional to movements of the floating frame 9.

The amount of current flowing through the retarding field coil 24 is determined by the position. of the rheostat arms 39 and dl. A change in the position of the floatng frame 9 due to a change in the ratio of tensions will shift the rheostat arm 4| to increase or decrease the amount of current flowing through the coil 24, increasing the amount of current flowing through the coil 2% and increasing the torque of the retardingmotoruponan increase in tension between the squeeze rolls and the wind-up reel and decreasing the amount of current and the torqueof the retarding motor when the pull between the squeeze rolls and wind-up reel is decreased. The motion of the rheostate arm ti is proportional to the movement of the floating frame 9 so that it causes an increase or decrease in the current flowing through the field coil 26 which is proportional to the amount of movement of the frame 9.

The effect of the increase or decrease in back pull caused by movements of the rheostat arm 4! is to reduce the amount of movement of the frame 9 for any initial change in the pull ratio and reestablish a new ratio which differs from the original ratio by an amount which is a fraction of the difference between the original ratio and the ratio initially established by an increase or decrease in the load on the rolls. For example, if the maximum variations in roll load would normally efiect changes of ten percent in the pull ratio, the rheostat control can be made to reduce the variation to any desired fraction of the normal variation within reasonable limits.

A given increase or decrease in the torque of the retarding motor will effect less increase or decrease in the actual pull on the strip when the reel is full than it will when the reel is nearly empty due to the reduction in leverage as the reel unwinds. In order to compensate for this variation in torque pull ratio the resistance 31 is increased proportionally to the reduction in diameter of the unwinding reel by a movement of the rheostat arm 39 controlled by the follower lever 1, so that the variation in the torque of the retarding motor due to the variations in current through the coil 24 are so proportioned to the reel diameter that equal changes in the amount of current flowing through the coil 24 effects substantially equal changes in the pull exerted upon the strip regardless of the diameter of the unwinding reel.

The coils, switches and variable resistances so far described are identical in the two motors and are identified in the drawings by the same reference numerals. The rheostat arms actuated by the shaft 20 are designated 4| and lla and the resistances in the two motor circuits controlled by the arms 4| and filo, are designated 42 and 42a. The rheostat arms ll-Ma operating over resistances 42 and 42a. of the two motors may be mounted upon a single shaft operated from the shaft 20. During operation, the automatically actuated rheostats of the motor acting as a retarding motor will be active and the circuits of the rheostats of the pulling motor will be dead. The two resistances 42, 42a are so arranged that increase or decrease of resistance is effected by movement of the rheostat arms in opposite directions.

Resistances 42 and 420. are preferably mounted upon a rotatable support 45 which may be given a turning movement in either direction, by suitable means. As herein shown, the support 45 is operated by a small reversible pilot motor 48 which is controlled by the movable frame 9. The control through the pilot motor 46 provides a convenient means for adjusting the ratio between forward and backward pulls and also to rectify the action of the other controlling elements.

The tendency of the movable frame 9 is to establish a slightly different ratio between forward pull and back pull after each actuation thereof. Although variations in pull ratio due to movements of the frame 9 are normally very slight, it is desirable to provide means for preventing any substantial variation from the correct ratio. The control of the pilot motor 45 is effected by a centrally located contact 41 carried by the frame 9 which is engageable with either of two fixed contacts 48 and 49 when the frame is shifted to one side of the center plane of the squeeze rolls and with either of two fixed contacts 58 and 5| when the frame is shifted to the opposite side of the center plane. Contact 41 is connected to line through a lead 52, contacts 48 and 5| are connected to line through a lead 53, field coil 54 and armature 55 of the motor 48, contacts 49 and 58 are connected to line through a lead 56, field coil 51 and the motor armature 55. Engagement of the contact 41 with contact 48 or 5| operates the motor 48 in one direction and engagement of the contact 41 with contact 49 or 58 operates the motor 45 in the opposite direction. The engagement of the contact 41 on the frame with the contact 48 operates the pilot motor in a direction to decrease the current through the field coil 24 and reduce the torque of the retarding motor. Engagement of the contact 41 with the contact 49 operates the motor 46 in a direction to increase the current through the field coil 24 and increase the torque of the retarding motor. Engagement of the contact 41 with the contact 48 causes the back pull to be relieved until the contact 41 has passed the contact 48 and is positioned between the contacts 48 and 49. Thereafter the pilot motor is actuated only when variations in the pull ratio are sufflcient to shift the contact 41 into engagement with either of the contact 48 or 49. The action of the contacts 58 and 5| is exactly the same as that of the contacts 48 and 49 when the direction of strip movement and the pull ratio are reversed. The contacts 48, 49, 58 and 5| are preferably mounted on the roll stand for adjustment toward and away from the center plane of the squeeze rolls and toward and away from each other. The distance of the pairs of fixed contacts 48, '49 and 58, 5| from the center plane of the squeeze rolls determine the ratio of the backward pull to forward pull and the distance between the contacts of each pair determines the amount of variation from the predetermined ratio which is permitted during operation of the machine.

As shown in Fig. 8, the contacts 48 and 58 are simultaneously adjustable in opposite directions by means of a screw 58 which has end portions with threads of opposite hand in threaded engagement with the contacts 48 and 58. By

means of the screw 58 the contacts 48. and 58 may be simultaneously adjusted toward or away from the center plane of the squeeze rolls. The contacts 49 and 5| may be independently adjusted by means of screws 59 and 58. By means of the adjusting screws the distances of the contacts from the center plane of the rolls may be varied and the space between the contacts of each pair may be varied as desired.

In starting the movement of the strip in either direction, the forward pull exceeds the backward pull and theframe 9 is shifted toward the unwinding reel. Upon engagement of the contact 41 with the contact 48 or 58 on the side from which the strip is moving, the pilot motor 46 is started in operation and reduces the current through the coil 24 of the retarding motor, reducing the torque until the contact 41 passes the contact 48 or 58 depending on the direction of movement. When the contact 41 is between the contacts of either pair 48 and 49 or 58 and 5|, the predetermined pull ratio has been established and thereafter engagement of the contact with either of the contacts between which it is located simply operates the motor 45 to sufficiently increase or decrease the back pull to shift the frame 9 to a position where the contact 41 is between and out of engagement with both fixed contacts. The distance between the contacts 48 and 49 and 58 and 5| determines the amount of variation permitted from the predetermined ratio between the backward and forward pullson the strip. The distances of the pairs of contacts from the center plane of the squeeze rolls determines the ratio between backward and forward pulls during operation. The movement of the contact 41 over contact 48 or contact 58 operates the motor 46 to adjust the resistance 42 or 42a to establish the ratio between the motor torques which will create the desired predetermined ratio between the backward and forward pulls. Thereafter the motor 46 is actuated only when the pull ratio varies sufliciently to cause the contact 41 to touch one of the two contacts between which it is located and then only momentarily to correct the ratio sufficiently to disengage the contacts. Tension is maintained on the strip during the reversing operation and the shifting of the frame 9 from one off-center position to the opposite off-center position is accomplished automatically by changes in the tension on opposite sides of the roll stand occasioned by the shifting of the motor connections. Shifting across the center line the contact 41 engages the fixed contact 48 or 58 which causes the pilot motor 45 to operate in a direction to decrease the tension on the retarding side which facilitates the movement of the frame 9 to its new controlling position. When the contact 41 passes the first fixed contact the pilot motor is shut off and the frame 9 is in its new controlling position.

In Fig. 2 of the drawings the machine is assumed to be operating to advance the strip to the right as indicated by the arrows, the active circuits being shown in full lines and the inactive circuits in dotted lines. The motor A is acting as a pulling motor having a predominating series fieldwhich makes the motor a substantially constant horsepower motor whose speed varies automatically inversely with the load so that as the wind-up reel increases in diameter the speed is automatically decreased proportionally to the increase in torque and the forward pull on the strip and the speed of travel of the strip are maintained substantially constant. The circuit of the motor B has been reset by opening the switch 29 and closing the switches 34 and 44 and the rheostat arms 32 and 39 operated by the follower lever I vary the torque of the motor proportionally to the decrease in diameter of the unwinding reel as the strip unwinds so that except for variations due to variations in the load on the rolls there is substantially a fixed ratio between the forward and backward pulls. When operating in the opposite direction the motor B is connected exactly as motor A is shown in Fig. 2 and motor A is connected as motor B is shown.

Whenever there is an increase in the tension of the strip between thesqueeze rolls and the wind-up reel a thrust will be exerted upon the floating frame 9, tending to shift it further toward the unwinding reel. The first efiect of this thrust is the mechanical application of additional tension between the squeeze rolls and the unwinding reel and a decrease in the tension between the rolls and the wind-up reel due to the lengthening of the path of the strip between the guide roll 5 and thesqueeze rolls on the side toward the unwinding reel and the shortening of the corresponding path on the wind-up side. If tension is decreased between the squeeze rolls and the wind-up reel the frame 9 moves toward the wind-up reel and relieves tension in the strip between the squeeze rolls and the unwinding reel and increases tension on the opposite side of the squeeze rolls. This action of the floating frame is instantaneous and, although the change in tension ratio due to this movement is slight it occurs during the lag between the actuation of the rheostat arm and the reaction of the retarding motor so that the readjustment of tensions begins instantly and is eifected gradually.

Any tendency of the floating frame 9 to creep away from its correct ratio controlling position is counteracted by the action of the pilot motor 46 which automatically resets the rheostats to establish the correct ratio upon a predetermined variation from that ratio.

It will be understood that the starting and stopping of the motors will be controlled by conventional motor controllers (not shown) which will be provided with the usual controlling elements which are necessary for gradually starting and stopping motors of the type employed.

In reversing, the pulling motor switches are first operated to convert the pulling motor into a retarding motor and then the switches of the other motor are shifted to convert the motor into a pulling motor. The resetting of the pulling motor reduces the pull between the reel of this motor and the squeeze rolls and the resetting of the other motor then create an excess pull on the opposite side of the roll stand which causes the frame to shift to its new controlling position after which the motors are brought up to speed by operation of the main controllers.

While the circuit of the variable " resistance 42 or 42a of the pulling motor is open during the operation of its motor as a pulling motor the cooperating rheostat arm 4| or Ma is shifted to a position where it cuts a greater resistance into the circuit than when the frame 9 is in its center position so that when its motor is reset for retarding, the initial retarding effect is reduced providing a greater initial thrust on the frame 9 to shift the same to its opposite controlling position.

The squeeze rolls may or may not be provided with a driving motor. If provided with such 9.

motor, its connections are independent of the controls herein shown and this motor is preferably used simply as a booster and retarder to increase the acceleration of a strip while it is being brought up to full speed and to increase the rate of deceleration while the motors are being brought to a stop.

While it is considered to be preferable to provide the floating frame 9 with both the direct rheostat operating connection and the pilot motor control, the use of both controlling devices is not essential. The pilot motor control alone may be used or the pilot motor may be omitted. With the pilot motor control alone, there would be a slightly greater lag between movements of the frame 9 and the motor reaction. If the pilot motor were omitted the rheostat could be reset manually for different pull ratios and there might be somewhat greater variations from the predetermined pull ratio.

It will be apparent that the present invention provides very accurate and sensitive motor controls compensating for variations in reel diameters by direct measurement of the reel and for variations in the ratio between forward and backward pulls due to variations in load on the rolls by means of a device which measures the ratio between the forward and backward pulls and constrip enables the amount of tension applied to the strip during operation to be maintained at or near the proportional elastic limit of the material being rolled and also maintains the proper amount of tension rearwardly of the squeeze rolls regardless of the amount of pull exerted by the wind-up reel, making it unnecessaryto accurately determine the tensile strength of the material at each stage of the operation, since the adjustments are adjustments of proportion instead of adjustments effecting definite changes in the amount of backward pull.

Furthermore, it is tobe understood that the particular form of apparatus shown and described, and the particular procedure set forth, are presented for purposes of explanation and illu stration and. that various modifications of said apparatus and procedure can be made without departing from our invention as defined in the appended claims.

What we claim is:

l. A strip mill comprising a roll stand provided with reducing rolls, strip reels to which opposite ends of a strip extending between the reducing rolls are attached, a motor geared to each reel,

each motor being adapted to drive its reel to wind the strip thereon and to be driven by its reel while the strip is unwinding therefrom, each motor having a field winding through which torque is applied which tends to rotate the motor shaft in a direction tov wind the strip on its reel, a controlling element movable in response to a variation in the tension of the strip on either side of the roll stand, and means operated by said controlling element for controlling the motor torque ratio.

2. A strip mill comprising a roll stand provided with reducing rolls, strip reels to which opposite 75 end of a strip extending betweenthe reducing rolls are attached, a motor geared to each reel, each motor being adapted to drive its reel to wind the strip thereon and to be driven by its reel while the strip is unwinding therefrom, each motor having a field winding through which torque is applied which tends to rotate the motor shaft in a direction to wind the strip on its reel, means for measuring the ratio of the strip tensions on opposite sides of the roll stand, and means controlled by said measuring means for controlling the motor torque ratio.

3. A strip mill comprising a roll stand, strip' pulling means at one side of the roll stand, strip retarding means atthe opposite side of thestand, means for measuring the ratio of the strip tensions on opposite sides of the roll stand, and means operated by said measuring means for controlling said strip-retarding means.

4. A strip mill comprising a roll stand, strip pulling means at one side of the roll stand, strip retarding means at the opposite side of the stand, a movable frame having strip guiding rollers on opposite sides of the roll stand, and fixed rollers spaced outwardly of said guiding rollers and positioned to deflect the strip outwardly of the guide rollers at an angle to the portion of the strip between said rollers, so that a thrust varying with the tension in the strip is exerted upon each of the guide rollers and the position of the frame is dependent upon the ratio of the tensions in the strip on opposite sides of the roll stand.

5. A strip mill comprising a roll stand, strip pulling means at one side of the roll stand, strip retarding means at the oppositeside of the stand, a pair of movably mounted strip guiding rollers mounted one on each side of the roll stand and connected for movement in unison, fixed guide rollers positioned to deflect the strip outwardly of the guide rollers at an angle to the portion of the strip between'said rollers, so that variations in the ratio of strip tensions on opposite sides of the roll stand causes the guide rollers to be shifted, and means controlled by movements 01' said guide rollers for varying the tension in a portion of the strip.

6. In a strip mill, a roll stand, strip reels on opposite sides of said stand, motors having driving connections with the reels, each motor adapted to alternately drive its reel and be driven by its reel, means responsive to variations in strip tension occurring on either side of the roll stand for controlling one of said motors to vary the torque thereof, and means controlled by one of said reels and responsive to changes in the diameter of said reel for controlling one of said motors to vary the torque thereof.

7. In a strip mill, a roll stand, strip reels on opposite sides of said stand, motors having driving connections with the reels, each motor adapted to alternately drive its reel and be driven by its reel, means responsive to changes in the diameter of the unwinding reel for controlling the motor of said reel to vary the torque thereof, and means responsive to changes in the ratio of the strip tensions on opposite sides of the roll stand for controlling the motor of the unwinding reel to vary the torque thereof.

8. In a strip mill, a roll stand, strip reels on opposite sides of said stand, motors having driving connections with the reels, .each motor adapted to alternately drive its reel and be driven by its reel, means responsive to changes in the diameter of the unwinding reel for controlling the motor of said reel, means responsive to changes in the ratio of the strip tensions on opposite sides of the roll stand for controlling the motor of the unwinding reel, and means responsive to changes in the diameterof the unwinding reel for modifying the action of the tension responsive means.

9. In a strip mill a roll stand, strip reels on opposite sides of said stand, a motor for retarding the unwinding reel, a motor for driving the windup reel, means for varying the torque of the retarding motor proportionally to changes in the diameter of the unwinding reel, and means for increasing or decreasing the torque of the retarding motor as the tension of the strip between the rolls and wind-up reel increases or decreases.

10. In a machine for reducing the cross-sectional area of elongated metal bodies, means for applying lateral pressure to said body, means forwardly of the pressure applying means for applying a forward pull to said body, means for applying a backward pull to said body rearwardly of the pressure applying means, a tension ratio measuring device engaging with sat-i body ahead of and to the rear of said pressure applying means, and means controlled by said measuring device for increasing or decreasing the backward pull to maintain a substantially fixed ratio between the forward and backward pulls.

11. In a machine for reducing the cross-sectional area of elongated metal bodies, means for applying lateral pressure to said body, means forwardly of the pressure applying means for applying a forward pull to said body, means for applying a backward-pull to said body rearwardly of the pressure applying means, fixed roller guides for said body spaced forwardly and rearwardly of sa1d pressure applying means, a movable frame having a roller engaging the elongated body between each of the fixed guides and the pressure applying means and positioned to maintain bends in said body between the fixed rollers and movable frame rollers, and means connected to said movable frame for controlling the ratio between gheiyforward and backward pulls exerted on the 12. In a reversing strip mill, a roll stand, strip reels mounted on opposite sides of said stand, an electric motor for each reel, each motor having windings and connections by which it may be operated as a substantially constant horsepower pulling motor and windings and connections by which its characteristics may be altered to adapt it for dynamic braking, means associated with each motor for varying the torque thereof while braking, and means operated by the strip and responsive to variations in the ratio of strip tensions on opposite sides of the roll stand for con trolling said torque varying means to maintain a back pull on the strip which bears a substantially fixed ratio to the forward pull.

13. In a reversing strip mill, a roll stand, strip reels mounted on opposite sides of said stand, an electric motor for each reel, each motor having windings and connections by which it may be operated as a substantially constant horsepower pulling motor and windings and connections by which its characteristics may be altered to adapt it for dynamic braking, means associated with each motor for varying the torque thereof while braking, means for measuring changes in the effective diameter of the unwinding reel and for controlling said torque varying means, and means for measuring the ratio of the tensions in the strip on opposite sides of the roll stand and for controlling said torque varying means.

pulling motor and windings and connections by which its characteristics may be altered to adapt it for dynamic braking, means associated with each motor for varying the torque thereof while braking, means for measuring changes in the effective diameter of the unwinding reel and for controlling said torque varying means, means for a measuring the ratio of the tensions in the strip on opposite sides of the roll stand, and for controlling said torque varying means, and means operated by the reel diameter measuring means for modifying the action of the tension ratio measuring means.

15. A strip mill comprising a roll stand provided with reducing rolls, strip reels to which opposite ends of a strip extending between the reducing rolls are attached, a motor geared to each reel, each motor being adapted to drive its reel to wind the strip thereon and to be driven by its reel while the strip is unwinding therefrom, means responsive to variations in the ratio of the strip tensions on opposite sides of said rolls for varying the torque of one of said motors to maintain a substantially constant ratio between said strip tensions, and means responsive to variations in the efiective diameter of one of said reels for modifying the action of said torque varying means.

16. A strip mill comprising a roll stand provided with reducing rolls, strip reels to which opposite ends of a strip extending between the reducing rolls are attached, a motor geared to each reel, each motor being adapted to drive its reel to wind the strip thereon and to be driven by its reel while the strip is unwinding therefrom, means responsive to variations in the ratio of the strip tensions on opposite sides of said rolls for varying the torque of the unwinding reel motor to maintain a substantially constant ratio between said strip tensions, and means responsive to variations in the efiective diameter of the unwinding reel for modifying the action of said torque varying means.

17. A strip mill comprising a roll stand proanemic vided with reducing rolls, strip reels to which opposite ends of a strip extending between the reducing rolls are attached, a motor geared to each reel, each motor being adapted to drive its reel to wind the strip thereon and to be driven by its reel while the strip is unwinding therefrom, means operated by said strip and connected to one of said motors to vary the torque thereof upon a variation in the ratio of the strip tensions on opposite sides of the rolls to maintain asubstantially constant ratio between said strip tensions, and means operated by said strip and operably connected to said torque varying means to modify the action thereof in accordance with changes in the eiTective diameter of one of said reels.

18. A strip mill comprising a roll stand, strip pulling means at one side of the roll stand, strip retarding means at the opposite side of the stand, a member controlled by the tensions in the strip on opposite sides of the roll stand and movable in response to a change in the ratio of said tensions, and means controlled by said member for varying the tension of the strip to the rear of .the roll stand to maintain a substantially fixed ratio between said tensions.

19. In a strip mill, the combination with the roll stand and devices on opposite sides of the roll stand for applying tension to the strip, of means for maintaining a substantially fixed ratio between the tensions to which the strip is subjected on opposite sides of the roll stand comprising a device for measuring the ratio of the strip tensions on opposite sides of the roll stand and means operatedby said measuring means for controlling one of said tension applying devices.

20. In a strip mill, the combination with the roll stand and devices on opposite sides of the roll stand for applying tension to the strip, of means for maintaining a substantially fixed ratio between the tensions to which the strip is subjected on opposite sides of the roll stand comprising a control for one of said tension applying devices sensitive to variations in the tension of the strip on the side of the roll stand opposite to that of the controlled tension applying device FRANK H. SMITH.

HENRY A. S'I'RINGFEILOW.

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