US2338276A - Tension control device - Google Patents

Description

1944- v T. J. AAMLAND EI'AL 2,333,276

TENSION CONTROL DEVICE Filed March 21, 1941 3 Sheets-Sheet l INVENTORS THO/7 d. lA'MLl NAT/M11062 H. (U/f 7/66 A 770 mvt Y 1944- T. .1. AAMLAND ETAL 2,338,276

TENS ION CONTROL DEVICE Filed March 21, 1941 3 Sheets-Sheet 2 NV ENTORS M I'M? 3 A T ORNEY T. J. AAMLAND ETAL TENSION CONTROL DEVICE Jan. 4, 1944.

3 Shee'ts-Sheet 3' Filed March 21, 1941 INVENTORS 7/905 (1 IIMAl/Vfl ATTORNEY Patented Jan. 4, 1944 2,338,278 TENSION con'raor. nnvroa Thor J. Aamland, Leonia, and Nathaniel 11. Cartiss, Clifton, N. 1., assignors to United States Rubber Company, New York, N. Y., a corporation of New Jersey Application March 21, 1941, Serial No. 384,510

Claims. (Cl. 242 158) This invention relates to tension control devices, particularly but not exclusively adapted to be used in the braiding of metal strands or wires under high tension, for'example in braiding machines shown in the patent to J. A. Muller, No. 2,197,111. This application is a continuation-inpart of our co-pending application, Serial No. 262,696, filed March 18, 1939, now abandoned.

Heretofore in tension control devices in general use it has been found diflicult to maintain a predetermined substantially constant and uniform tension on a strand of thread, wire, or similar material, while unwinding it from a carrying spool. The feeding of the strand from the spool to a knitting, weaving, braiding, sewing or like industrial machine at a uniform, predetermined tension is. in many cases, very desirable, but has not been obtainable using control devices of former constructions.

Various types of devices have been previously employed to control the tension in the strand taken from the spool or reel but it has not been possible with any of these devices to maintain the proper predetermined and substantially uniform tension desired under all conditions of operation, and accordingly many diiilculties and undesirable conditions have been encountered. When it was attempted to use such devices for supplying a strand to industrial machines mnning at high, low and varying speeds such objectionable conditions as overrunning of the spool, slackening, twisting and breaking of the strand, and unevenness or braiding, weaving, sewing, or knitting, or the like, of the article being formed has occurred. Such conditions have also resulted in requiring extra power in the operation of the machine and excessive wear and breakage in the parts thereof and parts of the tension control devices. Some of these devices have operated for certain conditions, for example, when a spool is substantially full or when the strand taken therefrom is perpendicular to the axis of the spool, but have failed materially to give anything like the same results when these conditions have beenvaried.

Our invention has largely eliminated the above objections and diiliculties since we have provided a device that is substantially constant and uniform in its operation, allowing the strand to be taken from the spool at substantially uniform tension and at any predetermined value desired regardless of the amount of material remaining upon the spool, the speed at which the strand is taken, or the angle of the strand being fed from the 8900 Our invention comprises a constant tension control device in conjunction with a spool or reel so arranged and constructed as to be responsive to a predeterminedtension in a strand being unreeled therefrom, to yield and allow unwinding of the strand from the spool when the desired tension has been exerted on the strand, and to hold the spool against rotational movement at all times except when such predetermined tension exists in the strand. This predetermined tension is governed by the strength of a particular spring employed in the device and the device is so arranged that it is immediately and instantaneously responsive to variations 01' tension that may oc our. The unreeling action continues as long as suflicient pull is maintained on the strand but should this pull be lessened, even a very slight amount, our device is immediately responsive to retard the unreeling action or to prevent further feeding of the strand therefrom as may be required to maintain a constant tension.

It will be seen from an inspection of the accompanying drawings and from the description that follows that our device is not only effective in its operation, constant and direct in its response to predetermined tension, but also simple in operation, economical in construction, and composed of parts easily and readily adjustable and replaceable.

In the drawings:

Fig. 1 shows a front elevational view of our invention with a spool partly broken away and exposing a strand of material thereon and a brake shoe in engagement with the spool rim.

Fig. 2 is a view of the device taken substantially along the line 2-2 of Fig. 1.

Fig. 3 is a side elevation of our preferred brake shoe.

Fig. 4 is a plan view of the brake shoe shown in Fig. 3.

Fig. 5 is a side elevation of the device shown in Fig. 1 but having the lower part thereof broken away along line 5-5 of Fig. 1, and the strand of the flexible material removed to more clearly show parts of the device.

Fig. 6 is a top plan view of the structure shown in Fig. 1 but having the strand of flexible ma terial removed therefrom. A

Fig. '7 is a partial front elevational view of our invention taken approximately along the line 1-1 of Fig. 6 but showing parts of the control mechanism in place thereon and in an inoperative position.

Fig. 8 is a partial elevational view, somewhat reduced and partly in section, of a modified form of our invention showing a brake controlled rotatable disc in engagement with a removable spool.

Fig. 9. is a side elevation of a modified form of brake shoe having an eccentrically located bore. Fig. 10 is a side elevation of another modified form of brake shoe, and

Fig. 11 is a plan view similar to Fig. 2 showing the brake shoe of Fig. 10, and mounting means therefor, substituted for the brake shoe shown in Figs. 3 and 4 and its mounting means.

I As illustrated herein our device comprises a frame having the base I, upstanding support standard 2, and top cross member 3. Carried by support standard 2 is a fixed, laterally projecting spool supporting arm 4, on which a spool 5 may be rotatably mounted. Positioned above an end flange 5a of the spool is a fixed shaft 6 on which the swinging lever I is pivoted at its lower end. The lever I has a laterally projecting eccentric circular boss or cam 8 on which the brake shoe 9 is supported in position to be moved into and out of contact with the periphery of spool end flange 5a.

The upper end of lever I is secured by pin I to the lower end of a rod II, the upper end of which projects through a slot l2 extending vertically through the frame top cross member 3. As illustrated in Fig. the upper end of lever l is bifurcated for the reception of the lower end of rod ll.

Encircling rod II is a compression spring l3 which has at its upper end the washer l4 which bears against the downwardly projecting edge l4a of a contact member l5, centrally bored for the passage of rod I I. secured to top cross frame member 3 around slot I2. The lower end of spring l3 abuts against shoulder l6 formed by the enlargement of the lower end of rod l l.

The pulley I1 is mounted for free rotation on an enlarged central portion of shaft I! fixedly secured in any suitable manner in top cross frame member 3, and is provided with a downwardly extending guide bracket l9 which fits over the threaded end portion of shaft I8 and is held against the shoulder 18a of the enlarged portion by a nut lBa.

A similar pulley 20 is mounted for free rotation on an enlarged central portion of a shaft 2|, which is fixed in any suitable way to lever 1 below its connection to rod II, and is provided with an upwardly extending guide bracket 22 which fits over the threaded end portion of shaft 2| and is held against the shoulder Ila of the enlarged central portion by a nut 22a.

The guide brackets I8 and 22 are provided with laterally extending arms curved back to form guides, and arranged to overlie the central portion of the bracket as best seen in Figs. 1 and 11. These arms are so arranged that the strand of material S can easily be slipped into either slot formed thereby and when so positioned will remain securely in place as long as any tension is maintained in the strand S.

As shown in Fig. 1, a strand S of flexible material such as thread, wire, or the like, is led from spool 5, over pulley ll around pulley 20 and again upwardly through the guide slot 25, extending vertically through the frame top crossv member 3, to an external location where it is to be utilized for any general industrial purpose. It will be understood that any tension on strand S will tend to rotate lever I in a clockwise direction, against the force of spring l3 which tends to rotate lever l in a counterclockwise direction.

Various strength springs or additional washers about the rod H at the ends of the spring may be utilized to obtain various degrees ofpressure and accordingly various amounts of resistance may be offered to the strand being unreeled from our device.

The spring I3 is positioned with respect to lever I at an obtuse angle such that the length of the force arm through which the force of the spring is transmitted to the braking element varies substantially in inverse proportion to the force exerted by the spring throughout the range of angular movement of lever 1.

Thus the spring l3 exerts a constant moment of force tending to rotate the lever 1 counterclockwise.

As illustrated in Figs. 3 and 4, the brake shoe 9 is a rectangular member having four similar concave contact surfaces So, each of which is curved along an arc concentric with the periphery of the end flange 5a of spool 5. Each contact surface 8a of brake shoe 9 is also grooved longitudinally to fit over, the laterally rounded edge of end flange 5a of the spool. The brake shoe 9 has a central bore c adapted to fit loosely over the periphery 8a. of cam member 8 on lever 1. As illustrated herein (Fig. l), cam member 8 is eccentrically mounted so that its leading edge m, as it rotates in a clockwise manner, raises brake shoe 9 out of contact with the end flange 5a of the spool, and as it rotates in a counterclockwise direction it lowers the selected contact surface 9a of the brake shoe into contact with the end flange Ed. I

If desired the contact surfaces 8a may be spaced at different distances from the bore 0 to compensate for wear on the peripheral surface of flange 6a. Thus, for example, if the surface of spool flange 5a became worn, and lever arm I dropped too far as a result, the brake shoe could be turned so that a contact surface in spaced farther from bore 0 than the contact surface previously employed, would rest on flange 5a, thus maintaining a more or less uniform setting for the spring.

The spool 5 is rotatably and removably mounted upon shaft 4, which i firmly secured in a boss formed in the intermediate portion of the upstanding support arm 3, so that the spool may be removed laterally therefrom whenever it is desired to replace it by another. A lock-bar 26 is positioned between bifurcated ends 21 of the shaft 4 and is retained in place by a pin 28 extending through said bifurcated ends and through a key hole slot 29 in lock-bar 26, A spring actuated plunger 30 is arranged in the shaft 4 and is adapted to bear against the lock-bar 26 and hold it either in a vertical operative or an extended horizontal inoperative position, the latter position being shown by a dash and dot line in Fig. 5. The mere sliding of the lock-bar upwardly from its full line vertical position, shown in Fig. 5, and then swinging it to its extended dash and dot line position, allows the spool 5 to be easily slipped over the lock-bar and onto the shaft 4. The lock-bar is then returned to a full line position and as such serves to prevent.

the spool from moving ofl the shaft 4. The

plunger 30 tends to maintain the lock-bar in When it is desired to remove a spool from the shaft 4 and replace it by another, it is only necessary. as best seen in Figs. 1 and 'I, to move the lock-bar to its extended position by means of the curved end of work tool 32 and then insert the straight end of the work tool into the bore b in the lever I and rotate the lever about the shaft 3 against the action of the spring I3. The lever 1 and shoe 3 will be moved from the operative position shown in Fig. 1 to the inoperative position shown in Fig. 7. This rotation of lever I raises the brake shoe 3 and lifts the entire contact face So away from peripheral edge or flange a of the spool 8, asindicated in Fig. 'I, so that the spool is free therefrom and may be moved laterally along and off the shaft 4.

The slot I2 is shaped to allow the swinging lever I and rod II to be moved past the dead center position, and to stop the movement just after the dead center position has been passed. The spring then holds regulating device in the inoperative position shown in Fig. 'l.

The flanges 5a of the spool 5 are formed of case hardened steel or other suitable material to stand the abrasive wear occurring between them and the brake shoe 9 while the device is in operation. The brake shoe 3 is similarly constructed of case hardened steel or like material in order to also resist excessive wear that would otherwise occur between these parts. A stop member 33 is threaded or otherwise fastened into the side of the frame member 2 and is provided to limit the downward swinging movement of the lever I. When the spool 5 is in its operative position upon the shaft 4 the peripheral edge or flange So will limit the downward swinging movement of the lever I by frictionally engaging the brake shoe 3. But should the lever I be moved downwardly when the spooli not in its proper place upon the shaft to limit this movement, the stop member 33 is effective to provide an abutment for the device and prevent the upper end, Ila, of the guide rod II from becoming disengaged from guide slot l2 as will be apparent. from Fig. 7. If it is desired to remove the regulating device, the stop 33 maybe removed and the spool 5 removed to permit the arm I to rotate counter-clockwise until the end of the rod II is withdrawn from the slot l2. All of the parts may then be slid axially from the shaft 6.

The operation of our device is as follows:

When it is desired to place the device in operation the spool of thread, wire or like material is moved onto the shaft 4 whilethe look-bar is extended and the lever 'I and brake shoe 3 are in inoperative position. The lock-bar 26 i then moved to its vertical position and lever 1 and brake 3 are swung to their operative position at which time the flanges of the brake shoe 3 will straddle the peripheral edge of a flange 5a of the spool 5 and retain the spool against lateral movement along the shaft 4. The pressure exerted downwardly upon the swinging lever I by the spring I3 transmits a predetermined pressure, through cam 3, to the brake shoe 9 in contact with the flange 5a of the spool 5, and thereby holds the spool against rotational movement. As shown in Fig. 1, the free end of the strand S is passed upwardly through one slot in the guide bracket I 9, over the pulley I1 and then downwardly through the oppostie slot in the guide bracket I3. It is then passed downwardly through the slot in one side of the guide bracket 22, around the pulley 20 and hence upwardly through the other slot in the bracket 22 and through the guide or eye member 25 in the top frame member 3, from which it extends to the place or machine where it is to be utilized. Thu the pulley 23 serves to transmit to lever arm I the force of tension on strand 8.

As long asthe brake shoe 3 is in contact with the flange of the spool, the braking force equals the moment of force of the spring I3 minu the moment of tension in the strand acting at the center of the shaft 2| of the pulley 20. Since the moment of the spring I3 is constant, the braking force varies inversely with the tension in the strand. As the tension increases the braking force decreases until it reaches a value which permits the spool to rotate. The strand then unwinds. tension of the strand (for example, friction or binding of the strand as it is being unwound from the spool) immediately reduces the braking force, and any, tendency to reduce the tension (for example by too free running of the spool will immediately increase the braking force). Thus a balanced condition will be maintained and the braking force will continuously and automatically be adjusted to maintain a constant tension in the strand. It is only necessary that the strand of material on the spool be held or secured against slipping relative to the spool in order that our device function properly. Any common manner or means of preventing such slipping action from occurring will be satisfactory for the operation of our device.

In Fig. 8 is shown a modified form of our invention arranged with the brake shoe 9 operating against a rotating disc 35 which is similar in form to one of the flanges 5a of the spool 5 of our preferred embodiment, but is operatively mounted upon the same shaft 36 which supports the spool. The shaft 36 is removably secured in support 2 in any well known manner and is provided with a shoulder 31 to rotatably retain the disc 35 in place thereon. In such a construction it will only be necessary to provide the disc with suitable clutch elements, such as a laterally extending finger 38, to engage an aperture formed in theflange of any conventional spool 39 placed upon the shaft 36. Thus, an operative drive between the spool and the rotating disc is formed which is simple in construction and readily allows the removal of said spool from the shaft. Such an arrangement would not limit the control device to any particular size or type of spool but would be useful for any conventional spool as will be readily apparent. It would only be necessary for such a spool to have formed therein apertures for alignment with the finger 38 extending laterally from the rotating disc.

The operation of our device occurs under substantially uniform conditions regardles of whether we employ such a rotating disc 35, in conjunction with the brake shoe 9, or the flange 5a of the spool 5 in conjunction with such brake shoe.

In Figs. 9 and 11 a modified brake shoe 40 and supporting means therefor are shown. In this embodiment of the invention. a swinging lever or arm 4| is used instead of lever I, and is mounted on the outer end of sleeve 42 which is rotatably supported on shaft 43, which, like arm 2|, is fixedly supported by frame member 2. Brake shoe 40 is threadedly engaged on the inner end of shaft 43. The arm ll and brake shoe 40 are held together by the clamping action of shoulder 4311, on .the outer end of the sleeve 42, when the inner end thereof is in threaded engagement Any tendency to increase the with the brake shoe 40 as clearly shown in Fig. 11.

The swinging arm 4| is provided with means to prevent relative rotation between it and the brake shoe 40. For this purpose a pin P extends laterally through the arm 4| and has its end arranged and adapted to engage in one of a series of apertures 44 upon a side of the brake shoe 40.

This arrangement is such that when the swingcondition thereof is produced and the brake shoe may be reversed and an opposite edge used in a like manner for braking purposes when considerable wear hasdeveloped along one edge. This brake shoe 40 is eccentrically bored, thus forming an eccentric cam, having a groove 46 in its 7 elements.

peripheral edge or rim. This groove forms flanges 41 at either side thereof and these are cut away or beveled at portions adjacent the center of oscillation of the cam, as indicated by the reference numerals 48, Fig. 9.

The outer end of the sleeve 42 is squared as shown at 49 so that the sleeve may be easily gripped while the threaded inner portion 50 is being turned into threads of the brake shoe 40. It will be seen by inspection of Fig. 11 that when the sleeve 42, swinging arm 4|, brake shoe 40 and pin P have been fitted together a rigid but readily detachable and adjustable assembly is effected.

The bore 52, illustrated in the swinging lever 4| of Fig. 11, like the bore b of Fig. 1, is for receiving the end of tool 32 by which lever 4| and brake shoe 40 may be swung to inoperative position.

In Fig. we show another modified brake shoe 53 which may be employed in place of the brake shoe previously described. The brake shoe 53 is circular and has a peripheral edge 54 which is concentrically arranged relative to the center of oscillation of the shoe. This edge 54 preferably although not necessarily, has a groove 55 formed therein for centering and retaining the flange 5a of the spool 5 in place therebeneath.

Either face 56 of brake shoe 53 may be used to engage the flange 5a of the spoo1 5 for braking 5 purposes. A series of holes 51 are provided in one side of the brake shoe to allow relative adjustment between the shoe and the swinging arm 4| (Fig. 11) by detachable engagement with the end of the pin P carried in the swinging lever 4| (Fig. 11). A similar series of holes (not shown) are located in the opposite side of the brake shoe 50 and allow for similar adjustment of the shoe when it is being used in its reversed position.

When this brake shoe 5!! is being used upon our device a small area of contact is formed between the face 53 and the flange 5a. When wear has resulted at this area of contact, the shoe may be readjusted and it will be seen that the area of contact remains substantially constant even though a considerable amount of wear may have resulted. The two series of holes on the opposite sides of the brake shoe 50 make it possible to utilize a large amount of the peripheral edge of the brake shoe for braking purposes by merely place this shoe by another shoe of similar construction. Even after considerable wear has resulted along the edge of thisbrake shoe the braking resistance oifered by the device may be maintained substantially constant by a mere readjustment of the shoe relative to the swinging lever 4|.

Throughout the description reference has been made, for the sake of brevity and avoiding repetition,-generally to the term strand in connection with our invention. However, it should be clearly understood that the word strand appearing herein is intended in its generic sense and that our invention is equally useful and intended to serve in conjunction with various materials such as thread, yarn, wire, cord, tape and the like either in single filament form or when twisted or braided together as composite flexible It is only necessary when a certain material is to be used to proportion the parts of our control device for the particular characteristics of the material and such can be readily done without involving more than changes in size or shape of the parts described herein and without departing from the spirit of our invention.

While we have described certain present preferred embodiments of our invention, it will be readily apparent to those skilled in the art, that certain changes may be made therein without departing from the essence thereof or the spirit and scope of the appended claims.

Having thus described our invention, what we claim and desired to protect by Letters Patent is:

1. In a tension control device for a strand of flexible material comprising a support, a spool from which a strand of flexible material is being unwound mounted on said supp rt, a lever arm pivoted to said support adjacent said spool, means tending to rotate said lever arm in one direction around its pivot, means responsive to tension in the strand being unwound from said spool for opposing said first mentioned means, a braking element having a number of contact surfaces of substantial length each of which is shaped so as to contact said spool throughout its entire length, said braking element being movable into and out of contact with said spool by a cam member mounted on said lever arm.

2. A tension control device comprising a support, a spool rotatably mounted thereon and adapted to carry a strand of flexible material, and control means carried by said support comprising a movably mounted brake element for normally holding said spool against rotational movement and spring means for normally urging said brake element into holding position, said control means being normally responsive to predetermined tension in said strand to release said brake element and allow said spool to rotate under the influence of said predetermined tension, and also being responsive to the release of said predetermined tension in the strand to hold said spool against rotational movement, said brake element being movable from its holding position to an inoperative position, said spring means being automatically operable to releasably retain the brake element in said last named position.

3. A tension control device comprising a support, a spool rotatably mounted thereon and adapted to carry a strand of flexible material, a

pulley and a guide mounted upon said support and arranged to receive the strand. extending from said spool, control means pivotally carried by said support comprising brake means normally holding said spool against rotational movement and having a braking surface, a second pulley and a lever for supporting said pulley, said lever being connected to said brake element for controlling the angular position thereof, and a spring urging said lever and said brake means into holding position, said guide and pulleys being so arranged that an elongated loop is formed in the strand extending from the spool successively around the first and second pulleys and through said guide, and being so positioned that said loop extends in a general direction which is substantially parallel to said spring and is at a substantial obtuse angle relative to said lever, said braking surface normally engaging said spool at a substantially constant predetermined location, whereby said lever and brake means are responsive to predetermined uniform tension in the loop to release said spool to rotate under the influence of the tension in the strand, said control means being pivotally movable to an inoperative position so as to retract said brake element from said spool, said spring means being automatically operable to retain said control means in said retracted position.

4. A tension control device comprising a sup porting frame having an aperture formed in an end portion thereof, a shaft and a pivot carried by said frame in spaced relation to said end portion, a spool rotatably mounted upon said shaft and adapted to carry a strand of flexible material, means upon said pivot for controlling the rotation of said spool, said means comprising a brake element normally in engagement with said spool and a control arm secured to said brake element for pivotal movement therewith, said control means also comprising a guide rod hingedly connected to said arm at a point remote from said pivot and having an end thereof slidably fitting within the aperture in said frame, a coil sprin surrounding said rod and having its opposite ends in engagement with the end portion of the frame and with said rod so as to force said brake element into engagement with said spool, a pulley upon said arm remote from said pivot and adapted to be actuated by the strand extending from said spool, said control means being normally responsive to predetermined tension in said strand for releasing said brake and allowing the spool to rotate, said control means being swingable when said spool is removed from said shaft into an inoperative position retracting said rod from said aperture and thus allowing the control means to be moved axially as a unit and disengaged from said pivot.

5. A tension control device comprising a supporting frame having an aperture formed in an end'portion thereof, a shaft and a pivot carried by said frame in spaced relation to said end portion, a spool rotatably mounted upon said shaft and adapted to carry a strand of flexible material, means swingably carried by said pivot for holding said spool against rotational movement and for controlling the rotation thereof, said means comprising a semi-circular brake element concentrically mounted for movement about said pivot and having a contacting surface disposed upon the periphery-thereof and normally in engagement with said spool, said means also comprising a control arm releasably secured to and holding position, said control means being nor--.

mally responsive to predetermined tension in the strand extending from said spool for releasing said brake element, said contacting surface upon said semi-circular brake element remaining at a constant distance from said pivot regardless of wearing away of the brake element during operation of said device:

6. Apparatus for maintaining a constant tension in a strand of flexible material comprising in combination means for rotatably supporting a coil of flexible strand, a brake having a brake surface and being of substantial thickness in a direction normal to said surface, said brake being movable to place the braking surface in a predetermined position to prevent rotation of the coil, control means exerting a predetermined force urging the brake to braking position, ,the' control means including a pivoted brake arm supporting the brake and a spring urging the brake arm to braking position, the angle of action of the spring, length of brake arm and distortion of the spring being so related as to maintain said force substantially constant throughout movement of the brake arm due to wear of the brake, and means responsive to the tension in a strand being un- 'wound from said coil for urging the brake in the opposite direction against the force of the control means.

7. Apparatus for maintaining a constant tensurface is in braking position regardless of change of position of the brake arm due to wear between the brake and spool and means responsive to the tension in a strand being unwound from said spool for urging the brake against the force of the control means.

8. Apparatus for maintaining a constant tension in a strand of flexible material comprising in combination a rotatable member adapted to be associated with a coil of flexible strand, a brake having a braking surface and being of substantial thickness in a direction normal to said' surface, control means exerting a. predetermined constant force urging the brake against the member, the control means including a pivoted brake arm supporting the brake and a spring urging the brake arm to braking position, the angle of action of the spring, length of brake arm and distortion of the spring being so related as to exert said predetermined force when the surface is in contact with the member, regardless of wear between the surface and the member, and means responsive to the tension in a strand being unwound from said coil for urging the brake against the force of the control means.

9. Apparatus for maintaining a constant tensiori in a strand of flexible material comprising in combination means for rotatably supporting a coil or flexible strand, a brake and a rotatable brake arm adapted to place the surface of the brake in a predetermined position to prevent rotation of the coil, control means including a spring exerting a predetermined constant moment tending to rotate the arm in one direction to place :the brake surface in said predetermined posiin combination means for rotatably supporting a coil or flexible strand, a-brake and a rotatable brake arm adapted to place the surface of the brake in a predetermined position to prevent rotation of the coil, a pivot on the brake arm, a spring acting on the pivot to urge the brake to braking position, the angle of action of the spring with the brake arm, the length of the brake arm and the distortion of the spring being so related that as the force exerted by the spring decreases due to'movement of the brake the eiiective length of the lever arm through which the spring acts to rotate the brake arm increases proportionately preserving a constant moment tending to rotate the brake arm regardless of the angular position of the brake arm, and means responsive to the tension on a strand being unwound from said coil for urging the brake against ,the force or the spring.

THOR J. AAMLAND. NATHANIEL H. CURTISS.

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