US2285654A - Tension regulator - Google Patents

Description

June 9, 1942. c. R. HANNA ETAL TENSIONfiEGULATOR Filed April 30, 1941 I INVENTORS C/mzon f #02074 (Illa xggan/ey 1% Mk/flg.

M 53 kmmukmkm Patented June 9, 1942 TENSION REGULATOR Clinton R. Hanna, Pittsburgh, and Stanley J.

Mikina, Wilkinsburg, Pa., assignors to Westinghouse Electric a Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 30, 1941, Serial No. 391,082

10 Claims.

This invention relates to regulators, and particularly to tension regulators.

In the winding of coils of insulated wire such as are used in distribution transformers and the like, the coils are usually formed of a rectangular or irregular shape on mandrels or the like having the predetermined shape of the desired coil. The wire is wound on the mandrel at the rate of several hundred feet per minute, being unwound from a standard round supply reel that is restrained by a friction brake, to produce a tightly wound coil due to the wire tension required to overcome the friction of the reel brake.

Because of the irregular shape of the mandrel and the coil being wound, and the rapid winding rate, it is found that the wire supply reel is subjected to a harmonic acceleration that causes large variations in the wire tension. For example, as the mandrel turns, and assuming that the mandrel is rectangular in shape, as the wire is wound on the fiat side of the rectangle, there is a sudden increase in the wire speed as it approaches the corner of the mandrel. The force for accelerating the wire reel is supplied by the wire as it is wound on the mandrel, with the result that the wire is sometimes jammed between the turns on the reel by the excessive tension introduced, thereby causing damage to the insulation of the wire and unduly stretching the wire. This condition is also encountered in starting the winding operation, the tension introduced tending to jam the wire between the turns of wire on the supply reel.

An object of this invention is to provide in a regulator for controlling the tension of a wire extending between a winding mandrel and a supply reel for compensating for any changes in tension encountered, and to so control the compensation as to maintain the tension substantially constant.

A further object of this invention is to provide in a. regulator for controlling the tension of a wire as it is wound on a mandrel from a reel for compensating for any changes in tension encountered to maintain the tension substantially constant, and to provide for. so controlling the compensation as to prevent over-compensation,

' and to maintain the tension substantially constant even when the mandrel is at rest.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing, in which:

Figure 1 is a diagrammatic illustration of the apparatus and circuits embodying the teachings of this invention;

Fig. 2 is a perspective view of a cam employed in a part of the apparatus embodying this inven- I tion; and

Fig. 3 is a graph, the curve of which illustrates characteristics of the generator field current obtained by employing this invention.

Referring to the drawing, and particularly Fig. 1 thereof, this invention is illustrated by reference to the winding of a wire ill on an irregularly-shaped mandrel I2 from a supply reel i4. The mandrel i2 illustrated is of rectangular shape such as is employed in industry for the winding of coils for distribution transformers or the like, and is disposed to be driven at a predetermined speed by any suitable motor IS. The motor I6 comprises an armature winding I8 and a field winding 20 which are connected to any suitable source of supply (not shown).

As illustrated, a motor 22 is provided for cooperating in controlling the speed of the supply reel l4 and the tension of the wire l0 between the reel l4 and the mandrel l2. The motor 22 comprises an armature winding 24 and a field winding 26, the field winding being separately excited from any suitable source of power (not shown). The supply reel I4 is carried on the armature shaft 28 and is, therefore, responsive to the speed of the motor 22.

A generator 30, comprising the armature winding 32 and the field winding 34, is disposed for controlling the speed of the motor 22. As illustrated, the armature winding 32 of the generator 30 is directly connected across the armature winding 24 of the motor 22 for impressing a voltage across the motor, depending upon the excitation of the generator 30.

As illustrated, one end of the field winding 34 is connected through a resistor 36 and a variable resistor 38 to one side of the source of supply (not shown), the other end of the field winding 34 being connected through the resistor 40 to the other side of the source of supply (not shown). An iron core reactor 42 is also connected across the field winding 34 of the generator III for cooperating to control the exciting current therein, as will be explained more fully hereinafter.

In order to further control the exciting current in the field winding 34 of the generator 30, provision is made for bucking and reversing the exciting current in the field winding 34 under predetermined conditions. Thls is effected by impressing a direct-current voltage across the resistor 40, the impressed voltage being of a polarity opposite to the polarity of the voltage of the normal source of supply of the exciting current (not shown). The voltage impressed across the resistor 48 is supplied in the present embodiment from any suitable source of alternatingcurrent supply (not shown) through the transformer 44 and the rectifying device 48, as illustrated.

In the particular embodiment illustrated, the variable resistor 88 is formed of a plurality of resistor sections which are graduated in values of resistance increasing from one end of the resistor toward the other. The graduated resistor 88 illustrated and described gives the regulator of this invention predetermined characteristics, as will be explained more fully hereinafter.

In order to control the number and sequence of resistor sections of the resistor 38, which are to be connected in series circuit relation with the generator field winding 34, a primary control device 48 is provided and disposed to be operated in a predetermined manner. The primary control device 48 is formed of a plurality of elongated conducting leaf spring members 58,

having one end of each member secured in fixed spaced relation to each other and extending through suitable insulating means, such as the block 52, so that their free ends are self-biased against a sloped stop 54 of insulating material. The contact members 58 carried on the free ends will be in spaced relation to each other when the free ends seat against the sloped stop 54. Each of the fixed ends of the stack of elongated conducting members 58 is so connected by a conductor 58 to a different tap of the resistor 38 that the conducting spring members 58 of the stack are progressively connected to progressive taps of the resistor 38. An insulated driving member 88 is disposed to be actuated under predetermined conditions to progressively actuate the spring members 58 away from the sloped stop 54 to progressively shunt the resistor sections of the graduated resistor 38, beginning with the resistor sections of small value. Any suitable means for mounting the driving member 88 may be utilized, or, as illustrated, the driving member can be carried by the free ends of the leaf springs 82 which are secured at their other ends to the insulating block 52 by any suitable means (not shown). The primary control device 48 referred to hereinbefore is disclosed and claimed in the copending application of C. R. Hanna, et a1. Serial No. 203,876, filed April 23, 1938, and assigned to the assignee of this invention.

The driving member 88 of the primary control device 48 is disposed to be actuated in accord ance with the tension of the wire l8 between the supply reel l4 and the winding mandrel I2. As illustrated, the wire I8 is supplied to the mandrel l2 over an idler pulley 84, which is carried in a substantially non-friction manner at the end of an arm 88 mounted for pivotal movement. In practice, although only one arm 88 is shown, two of the arms 88 are provided, being disposed in spaced relation, the idler pulley 84 being carried in a ball bearing mounting (not shown) on the shaft 88 disposed between the ends of the arms 88. The other ends of the arms 88 are fixedly mounted on a shaft 18 which extends between and through suitable brackets 12, only one of which is shown, the shaft 18 being disposed to be rotated therein. The bracket 12 is mounted on a base plate I4, which can be supported in any suitable manner such as on a motor housing, or otherwise (not shown), the base member 14 also carrying elongated arms 18, I8 and 88 disposed in spaced relation to each other. The arms 18 and 88 carry stop members 82 and 84, respectively, at their free ends disposed in the path of movement of the arm 88 for limiting the extent of movement thereof.

As illustrated, the movement of the arms 88 carrying the pulley 84 in a clockwise direction about their pivotal shaft 18 is restrained by a tension spring 88 carried between the free end of the elongated arm 18 and the pulley carrying arms 88. The spring 88 is positioned to give a toggle action, so that uniform tension is maintained in the wire I8 over the entire range of the pulley travel. This is evident for as the arms 88 rotate in a clockwise direction, the spring 88 stretches and the center line of the spring approaches the pivotal shaft 18 of the pulley carrying arms 88 by just the amount suflicient to keep the wire tension constant. Changes in the tension desired to be controlled in the wire l8 may be made simply by changing the spring 88 which restrains the pulley arms 88. The toggle action referred to hereinbefore is unimpaired, if the free length of the spring is kept the same in all cases. However, where it is desired to adjust the tension of any individual spring, the adjustment may be made by means of the nut 88 and the bolt 88 which secures one end of the spring 88 to the free end of the arm 18 to change the effective length of the spring 88.

A cam 82 is carried on the end of the shaft 18 for controlling the movement of the driving member 88 in accordance with a movement of the pulley carrying arms 88. As more clearly illustrated in Fig. 2, the cam 82 is in the form of a ring member, with the slope of the cam surface progressing from the small end to the large end of the cam as the cam is rotated in a clockwise direction. The ring cam 82 is carried on the end of the rotatable shaft 18 in any suitable manner, or, as illustrated, is secured by means of the bolt 84 to the end of a block 88, which is secured to the shaft 18 as by means of brazing, or otherwise (not shown). In practice, the elements of the apparatus described are so associated that the end of the driving member 88 rides on the cam surface of the cam 82, and is directly responsive to a movement of the pulley carrying arms 88.

In operation, assuming that the wire I8 has been threaded over the idler pulley 84 to the mandrel l2, and that the motor I8 is driving the mandrel at some predetermined speed, a tension is developed in the wire l8 as it is being wound. This tension causes rotation of the pulley arms 88 and their pivotal shaft I8 against the restraining force of the spring 88 to position the arms 88 at some intermediate position between the stops 82 and 84. The rotation of the pulley arm 88 rotates the cam 82 to force the driving member 88 against the elongated conducting members 58 of the primary control device 48 to control the number of resistor sections of the resistor 88 in circuit with the exciting field 84 of the generator 88 to control the voltage impressed across the motor 22, and thereby control the speed of the reel l4.

As the mandrel I2 is driven to wind the wire l8 thereon, its irregular shape tries to change the tension of the wire l8. At the same time,

, because the wire is being removed from the supthat the speed of the reel I4 is constantly changing as the wire is unwound therefrom regardless of the shape of the mandrel 12.

Forgetting for the time being the eifect of the irregular shape of the mandrel l2 upon the tension of the wire I8, and assuming that the winding of the mandrel has just started, the pulley arm 66 will be in a position adjacent the stop 82, so that substantially all of the resistor sections of the graduated resistor 38 are in circuit with the field winding 34 of the generator 38, and the exciting current therein is at a minimum for impressing a very low voltage across the motor 22 to drive the reel l4 at a very slow speed.

As the wire is unwound from the reel I4, more tension is developed in the wire III to effect a rotation of the pulley arms 66 in a clockwise direction about their pivot 18, and progressively drive the contact members 56 into circuit closing position to progressively shunt the resistor sections of the graduated resistor 38 from series circuit with the field winding 34. The progressive shunting of the resistor sections of the graduated resistor 38 eifectively increases the excitation of the generator 38 to increase the speed of the motor 22, and consequently increases the speed of the reel H to maintain a constant tension on the wire [8 as the diameter of the supply of wire on the reel I4 diminishes.

With the wire l8 threaded over the pulley 64 between the supply reel l4 and the mandrel I2, and before actual winding of the wire on the mandrel I2 is started, or while the mandrel is at rest, a negative torque is developed in the motor 22 to maintain the tension in the wire l8 substantially constant. This negative torque is developed by impressing a polarity across the resistor 40 which is opposite to the polarity of the voltage of the normal exciting current supplied to the field winding 34 of the generator 30. The effect of the opposite polarity supplied through the transformer 44 and the rectifying device 46 is to buck the exciting current in the field winding 34 and provide a negative generator field current.

The cooperative efiect of the bucking of the exciting current, and the variation of the resistor sections of the graduated resistor 38 through a change in position of the pulley arm 86 is illustrated in Fig. 3 of the drawing, in which the curve 98 represents the generator field current developed, starting with a full reel at rest and progressing until the reel is empty. As illustrated, because of the bucking of the normal exciting current, a negative field current is actually developed to knock down the generator residual and insure a zero speed of the motor 22, as well as develop a negative motor torque for maintaining the tension of the wire In at a desired value, even at zero winding speed. As the resistor sections of the graduated resistor 38 are progressively varied, beginning with the sections of small value, it is seen that the exciting current increases progressively in accordance with the amount of resistance shunted from series circuit with the field winding by the action of the driving member 68 in response to a movement of the pulley arms 66 in a clockwise direction. The graduated resistor thus compensates for the varying regulator sensitivity as the diameter of the supply of wire on the reel becomes smaller.

At the same time that the resistor sections of the graduated resistor 38 are varied, it is seen that the iron core reactor 42 cooperates with the resistor 38 for influencing the exciting current in the field winding 34 of the generator 38, and consequently affects the speed developed in the motor 22. If the change in the resistor sections of the graduated resistor 38 is a large change, shunting out large sections of the resistor 38, it is found that the impedance of the iron core reactor 42 is so great as to cause more of the current from the source of supply to immediately flow in the generator field winding 34, giving an approximate rate of change correction for a change in the position of the pulley arm 86. If, however, the change in the resistor sections of the graduated resistor 38 is to cut in large sections of the resistor 38, the iron core reactor 42 functions to effect a discharge through the field winding 34 to boost the normal exciting current and prevent a quick decay in the exciting current. The iron core reactor 42 thus functions to stabilize the system of this invention, giving a generator excitation proportional to the rate of change of the idler pulley arm position and compensating for system delays.

As the wire I8 is wound on the mandrel l2, the tension of the wire I, of course, varies depending upon the position of the mandrel l2 as it is rotated by the motor I6. For example, as the wire is wound on the flat side of the rectangular-shaped mandrel 12, there is a sudden increase in the wire speed as the mandrel is rotated and the wire approaches the corner of the rectangle. The increase in the wire speed efiects an increase in the tension of the wire being unwound from the reel I4, so that to prevent damage to the turns of the wire on the reel M, or the turns of the wire being wound upon the mandrel l2, it is necessary to increase the speed of the motor 22 to increase the speed of the supply reel I4 to decrease the tension of the wire.

The increase in the tension of the wire l8 effects a rotation of the pulley arm 66 in a clockwise direction about its pivot 18 to operate the driving member 68 to shunt more of the resistor sections of the graduated resistor 38 from series with the field winding 34 of the generator 30, and if the increase in tension to be compensated is a large increase, large sections of the resistor 38 will be immediately shunted from the circuit. The shunting of the resistor sections 38 causes a larger current to flow in the field winding 34, and if the sections shunted are of large value, the reactor 42 cooperates, as explained hereinbefore, to permit most of the current to flow through the field winding 34 for effecting an immediate increase in the excitation of the generator 38. This immediate increase in the excitation of the generator 38 effects an increase in the voltage impressed across the motor 22 to immediately increase the speed of the motor 22, and consequently increase the speed of the reel l4 to compensate for the increase in the tension of the wire ID as it passes the corner of the rectangular mandrel l2.

Of course, the tension of the wire I8 is immediately decreased as the mandrel I2 is rotated to wind the wire ID on its fiat side, so that the reel l4 tends to run faster than is necessary for maintaining the desired tension in the wire ID. The decrease in the tension of the wire I!) permits the spring 86 to actuate the pulley arms 66 in a counter-clockwise direction about their pivot 18 to so move the cam 92 as to permit the spring members 62 to bias the driving member 68 away from the elongated conductor members 50 to cut in progressively smaller sections of the graduated resistor 38 in circuit with the field winding 34. The cutting in of the resistor sections of the graduated resistor 38 decreases the exciting current in the field winding 34, but if the resistor sections cut in are of such large value as would normally tend to immediately effect a large decrease in the exciting current, the iron core reactor 42 functions to prevent such immediate large decreases in the exciting current as it discharges through the generator field winding 34 to give a'substantially gradual decrease or decay in the exciting current. The decrease in the exciting current in the field winding 34 eflects a decrease in the voltage impressed across the generator 22, and efifectively decreases the speed of the supply reel H to maintain the tension substantially constant on the wire III as it is being wound on the mandrel i2.

With the regulator of this invention, it is evident that it is possible to compensate for variations in the amount of wire on the supply reel and variations in the tension developed in the wire as it is being wound on a mandrel of irregular shape, as well as maintaining the tension substantially constant even when the mandrel is at rest. The change in the speed of the reel motor is effected immediately upon a change in the tension of the wire between the supply reel and the winding mandrel. Further, provision is made for obtaining a rate of change of correction in the speed of the motor depending upon the rate of change of the idler pulley arm position, thus compensating for system delays. The regulating action is quick and accurate, and because of the construction of the pulley arm assembly, which is usually a light, still frame of aluminum or other suitable light-weight metal or alloy, can follow the changes in velocity of the reel due to the irregular shape of the man drel on which the wire is being wound with a minimum of tension disturbance due to low inertia forces.

We claim as our invention:

1. Apparatus for controlling the tension of a wire extending between a winding mandrel and a supply reel comprising, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a pivotally mounted idler pulley associated with the wire and disposed for movement about its pivot as the wire tension changes, an over-center spring so associated with the pivotally mounted idler pulley as to rest-rain movement of the pulley about its pivot as the tension of the wire between the mandrel and the reel increases, and means comprising a variable graduated resistor responsive to a movement of the idler pulley about its pivot for controlling the speed of the motor to control the speed of the reel and the tension of the wire.

2. Apparatus for controlling the tension of a wire extending between a winding mandrel and a supply reel comprising, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a pivotally mounted idler pulley associated with the wire and disposed for movement about its pivot as the wire tension changes, an over-center spring so associated' with the pivotally mounted idler pulley as to restrain movement of the pulley about its pivot as the tension of the wire between the mandrel and the reel increases, and means including a variable graduated resistor and a reactor cooperative therewith responsive to a movement of the idler pulley about its pivot and to the rate of change of the motion of the pulley about its pivot to control the speed of the motor to control the speed of the reel and the tension of the wire.

3. In a regulator for controlling the tension of a wire as it is wound on a mandrel from a reel, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a generator having a field winding connected to impress a voltage across the motor to develop a torque, a variable resistor having a plurality of resistor sections disposed to be connected in circuit relation with the generator field winding to control the excitation of the generator, means responsive to changes in the tension of the wire for progressively, varying the number of resistor sections connected in circuit with the generator field winding to control the energization of the field-winding and thereby control the torque of the motor to maintain substantially constant tension on the wire as it is being wound, and means connected in circuit relation with the generator field winding to buck the exciting current, thereby so controlling the torque of the motor as to maintain the tension of the wire substantially constant when the mandrel is at rest.

4. In a regulator for controlling the tension of a wire as it is wound on a mandrel from a reel, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a generator having a field winding connected to impress a voltage across the motor to develop a torque, a resistor having a plurality of resistor sections disposed to be connected in circuit relation with the generator field winding to control the excitation of the generator, means disposed to be operated to progressively vary the number of the resistor sections connected in circuit with the field winding, means responsive to changesin tension of the wire disposed to operate said resistor varying means to control the energization of the generator field winding and thereby control the torque of the motor to maintain substantially constant tension on the wire as it is being wound, and means connected in circuit relation with the generator field winding to buck the exciting current, thereby so controlling the torque of the motor as to maintain the tension of the wire substantially constant when the mandrel is at rest.

5. In a regulator for controlling the tension of a wire as it is wound on a mandrel from a reel, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a generatorhaving a field winding connected to impress a voltage across the motor to develop a torque, a variable resistor having a plurality of resistor sections disposed to be connected in circuit relation with the generator field winding to control the excitation of the generator, the resistor being graduated with the sections progressively increasing in resistance from one end to the other, means responsive to changes in the tension of the wire for progressively varying the number of resistor sections connected in circuit with the generator field winding to control the energization of the field winding and thereby control the torque of the motor to maintain substantially constant tension on the wire as it is being wound, the varying means progressively connecting smaller sections of the resistor in circuit with the generator field winding as the wire is removed from the reel to maintain a sensitive control of the excitation of the generator field winding as the reel is unwound, and means connected in circuit relation with the generator field winding to buck the exciting current, thereby so controlling the torque of the motor as to maintain the tension of the wire substantially constant when the mandrel is at rest.

6. In a regulator for controlling the tension of a wire as it is wound on a mandrel from a reel, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a generator having a field winding connected to impress a voltage across the motor to develop a torque, a resistor having a plurality of resistor sections disposed to be connected in circuit relation with the generator field winding to control the excitation of the generator, the resistor being graduated with the sections progressively increasing in resistance from one end to the other, means disposed to be operated to progressively vary the number of the resistor sections connected in circuit with the field winding, means responsive to changes in the tension of the wire disposed to operate the resistor varying means to control the energization of the generator field winding and thereby control the torque of the motor to maintain substantially constant tension on the wire as it is being wound, the operation of the resistor varying means progressively connecting smaller sections of the graduated resistor in circuit with the generator field winding as the wire is removed from the reel to maintain a sensitive control of the excitation of the generator field winding as the reel is unwound, and means connected in circuit relation with the generator field winding to buck the exciting current, thereby so controlling the torque of the motor as to maintain the tension of the wire substantially constant when the mandrel is at rest.

7. In a regulator for controlling the tension of a wire as it is wound on a mandrel from a reel, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a generator having a field winding connected to impress a voltage across the motor to develop a torque, a variable resistor having a plurality of resistor sections disposed to be connected in circuit relation with the generator field winding to control the excitation of the generator, means responsive to changes in the tension of the wire for progressively varying the number of resistor sections connected in circuit with the generator field winding to control the energization of the field winding and thereby control the torque of the motor, means comprising a reactor connected across the generator field winding for cooperating with the resistor as the sections connected in circuit are varied to so control the torque of the motor as to maintain substantially constant tension on the wire as it is being wound, and means connected in circuit relation with the generator field winding to buck the exciting current, thereby so controlling the torque of the mo tor as to maintain the tension of the wire substantially constant when the mandrel is at rest.

8. In a regulator for controlling the tension of a wire as it is wound on a mandrel from a reel,

in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a generator having a field winding connected to impress a voltage across the motor to develop a torque, a resistor having a plurality of resistor sections disposed to be connected in circuit relation with the generator field winding to control the excitation of the generator, means disposed to be operated to progressively vary the number of the resistor sections connected in circuit with the field winding, means responsive to changes in tension of the wire disposed to operate said resistor varying means to control the energization of the generator field winding and thereby control the torque of the motor, a reactor connected across the generator field winding, the reactor being cooperative as the resistor sections connected in circuit with the generator field winding are varied to give a sensitive control of the excitation of the generator field winding and consequently the torque of the motor to maintain a substantially constant tension on the wire as it is being wound, and means connected in circuit relation with the generator field winding to buck the exciting current, thereby so controlling the torque of the motor as to maintain a substantially constant tension on the wire when the mandrel is at rest.

9. In a regulator for controlling the tension of a wire as it is wound on a mandrel from a reel, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a generator having a field winding connected to impress a voltage across the motor to develop a torque, a variable resistor having a plurality of resistor sections disposed to be connected in circuit relation with the generator field winding to control the excitation of the generator, the resistor being graduated with the sections progressively increasing in resistance from one end to the other, means responsive to changes in the tension of the wire for progressively varying the number 01' resistor sections connected in circuit with the generator field winding to control the energization of the field winding and thereby control the torque of the motor to maintain substantially constant tension on the wire as it is being wound, the varying means progressively connecting smaller sections of the resistor in circuit with the generator field winding as the wire is removed from the reel, a reactor connected across the generator field winding, the reactor cooperating with the resistor as the sections connected in circuit are varied to so control the excitation of the generator field winding as to maintain a sensitive control of the torque of the motor and thereby maintain a substantially constant tension on the wire as it is being wound, and means connected in circuit relation with the generator field winding to buck the exciting current, thereby so controlling the torque of the motor as to maintain the tension of the wire substantially constant when the mandrel is at rest.

10. Apparatus for controlling the tension of a wire extending between a mandrel and a supply reel comprising, in combination, means for driving the mandrel to wind the wire thereon, a motor cooperative to control the speed of the reel and the tension of the wire, a pivotally mounted idler pulley associated with the wire and disposed for movement about its pivot as the wire tension changes, an overcenter spring so assomounted idler pulley disposed for translating the pivotal movement or the pulley about its pivot to a straight line movement to actuate the contact members progressively.

CLINTON R. HANNA. STANLEY J MIKINA.

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