US3738189A

US3738189A - Tension releasing device

Info

Publication number: US3738189A
Application number: US00153173A
Authority: US
Inventors: Chao Chen Kun
Original assignee: Burroughs Corp
Current assignee: Unisys Corp
Priority date: 1971-06-15
Filing date: 1971-06-15
Publication date: 1973-06-12
Anticipated expiration: 1990-06-12

Abstract

A device for releasing tension in a flexible tape or the like having a movable tape-engaging member that is guided in its displacement by a first camming surface. Excessive tension exerted in the tape causes the tape-engaging member to displace in opposition to a biasing means along said first camming surface, the camming surface guiding the tape-engaging member pivotally and translationally to describe a desired path of tapeslackening displacement.

Description

il'nited States Patent i191 Chen [54] TENSION RELEASING DEVICE [75] Inventor: Kun-Chao C. Chen, Plymouth, Mich.

[73] Assignee: Burroughs Corporation, Detroit,

Mich.

[22] Filed; time 1s, 1971 [21] Appl. No.: 153,173

[52] U.S. Cl. 74/242.15 R, 74/227 [51] int. Cl Flh 7/12 [58] Field of Search 74/227, 242.15 R,

74/242.1l R, 242.11 C, 242.11 P, 242.11 A, 242.9; 197/16; 37/42 VL; 192/56, 10, 1l,

[56] References Cited UNITED STATES PATENTS 3,295,383 l/l967 Allen 74/227 [111 3,738,139 [451 .lune 12, 1973 2,389,433 11/1945 Hough 74/242.l5 R X 3,631,734 5/1970 Wagner.... 74/242.l5 R X 2,458,767 l/l949 Cooper 74/242.l5 R

Primary Examiner-Leonard H. Gerin Assistant Examner-Frank H. McKenzie, Jr. Attorney Kenneth L. Miller and Edwin W. Uren [57] ABSTRACT A device for releasing tension in a flexible tape or the like having a movable tape-engaging member that is guided in its displacement by a first camming surface. Excessive tension exerted in the tape causes the tapeengaging member to displace in opposition to a biasing means along said first camming surface, the camming surface guiding the tape-engaging member pivotally and translationally to describe a desired path of tapeslackening displacement.

8 Claims, 6 Drawing Figures -SIEU'I if 2 INVENTOIL KUN-CHAO C. CHEN. BY

BACKGROUND OF THE INVENTION This invention relates generally to mechanical release devices and more particularly concerns devices for slackening tension in an over-stressed tape or the like.

Although the present invention finds applications in many varieties of mechanisms, an appreciation of its advantages and unique operation may best be gained when it is viewed in association with a certain type of printing apparatus. Such printing apparatus ulitlizes a tape and pulley system to interconnect a character selection unit to a rotatable print head, wherein movement of one end of the tape by the character selection unit causes the print head to correspondingly displace, bringing a desired character or group of characters into a print position. A more detailed description of a tape and pulley system of this variety is contained in U.S. Pat. No. 3,493,090 having common ownership herewith.

The character selection unit that is coupled to the rotatable printing head via the tape and pulley system is intricately constructed and thus sensitive to excessive forces that may be transmitted back to it through the tape and pulley systems. A jammed printing head, for example, could cause such a problem. The present invention, when incorporated in the tape and pulley system, serves to respond to excessive tension in the tape by releasing the tension and thus breaking the coupling chain between a jammed printing head and the character selection unit.

Alternative devices for releasing excessive tape tension have been contemplated and tried, but all have been found wanting in one respect or another. For example, a tape-engaging member in the form of a lever arm may be maintained in a tensioned position by a shear pin, the shear pin being selected so that a predetermined amount of tape tension would collapse the pin and thus slacken the tape. The primary problem with a shear pin biased device is that small differences in size, material and finishing of the pin, as well as the force applying conditions (slowly or suddenly) result in an undesirably wide range of collapsing forces. Once the shear pin has collapsed, resetting the device in the proper position to tension the tape requires pivoting the lever arm with a force at least as great as the tape tension, and aligning holes into which a new shear pin is inserted. This is both a difficult and time-consuming procedure.

A tape-engaging member provided with only the capability of pivotal displacement presents other difficulties when incorporated in a printing apparatus, such as described above. During a slackening displacement the pivotal tape-engaging member traverses a circular path toward the printing head carrier. The portion of the circular path traversed by the pulley that is above the level ofthe print head carrier, would be a restricted zone into which the carrier could not be moved without the possibility of a collision with the displacing tapeengaging member.

A pivotal tape-engaging member may be biased in a tensioned position by means other than a shear pin, for example, a spring or a friction coupling may be substituted. ln addition to the problem of interfering with the printing head carrier due to its purely pivotal displacement, a tape-engaging member in the form of a pivotal lever arm that is biased by a spring tends to amplify any vibratory action of the tape during a normal printing operation. In addition the spring biases the tapeengaging member with an ever increasing force as the member is displaced by excessive tension in the tape so that the tape tension is never actually released by such a device but only yielded to. A spring or frictionally biased tape-engaging member requires a difficult resetting procedure after the device has collapsed, since the biasing force of the frictional coupling or spring as well as the force of the desired tape tension cumulatively resist resetting of the tape-engaging member. A frictionally biased member presents a further problem in that wear of its friction surfaces tends to reduce the biasing force imparted to the tape with continued use of the device.

SUMMARY OF THE INVENTION With the foregoing considerations and problems of the prior art in mind, it is an object of the present invention to provide a tension releasing device that uniformly responds to a predetermined tension in the tape notwithstanding the number of previous collapses the device has undergone.

It is another object of the present invention to provide for a path of slackening displacement of a tapeengaging member which creates a minimum interference with associated elements in the apparatus.

It is further an object of -the present invention to provide a tension-releasing device that is easy to reset after a collapse.

It is still further an object of the present invention to provide a tension-releasing device that is responsive to various rates of tension build-up in the tape to release the tension therein with corresponding speed.

It is still further an object of the present invention to provide a tension-releasing device that absorbs momentary increases in tape tension without collapsing.

It is another object of the present invention to provide a design for a tension-releasing device that may be readily modifiable to achieve various paths of tape slackening displacement and various threshholds of collapse using the same biasing means.

In one of its aspects the present invention utilizes a single biasing means to both yieldably maintain the tape-engaging member in opposition to tape tension and to accelerate the collapse thereof after a predetermined degree of slackening displacement.

In accordance with the invention a device has been conceived in which a tape-engaging member is pivotally and translationally engaged with a stationary support. The tape-engaging member is guided in its pivotal and translational displacement by a first camming surface that defines a particular displacing path during the slackening movement of the tape-engaging member. Biasing means are coupled with the tape-engaging member to provide a predetermined biasing force that serves to tension the tape. The coupling of the biasing means with the tape-engaging member has the effect of reversing the force of the biasing means after a predetermined amount of displacement of the tape-engaging member thus to speed the slackening displacement thereof.

BRIEF DESCRIPTION OF THE DRAWINGS In order tofacilitate a more complete understanding of the present invention, reference may be had to the following detailed description presented with reference to the drawings in which:

FIG. 1 is a perspective view partly in schematic and broken away form showing a portion of a printing apparatus in which the invention is incorporated;

FIG. 2 is a fragmentary front elevational view of a preferred form of the tension releasing device;

FIG. 3 is a side elevational view of the device shown in FIG. 2;

FIG. 4 is a view similar to FIG. 2 showing an initial stage of the tension-releasing displacement of the device;

FIG. 5 is a view similar to FIG. 2 showing an advanced stage of the tension-releasing displacement of the device; and

FIG. 6 is a view similar to FIG. 2 showing the extreme tension-releasing position of the device.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the drawings wherein like reference characters designate identical elements in each of the Figures, and particularly to FIG. 1, there is shown apparatus for connecting a rotatable printing head 2 to a character selection device 4. The printing head 2 is displaced laterally along a cylindrical platen 6 by a worm 8 engaged with a printing head carrier 10. The printing head 2 is rotated to bring a desired raised character, as at 12, into a printing position by vertical displacement of an output arm 14 of the character selection device 4. A system of tapes 15 and pulleys 17 convert the vertical displacement ofthe output arm 14 into rotational displacement of the printing head 2. In the illustrative apparatus, upward displacement of the output arm 14 rocks a tape-engaging lever 16 leftwardly which in turn rotates a tape reel 18 axially connected to the printing head 2. Downward displacement of the output arm 14 rocks the tape-engaging lever 16 rightwardly which rotates the tape reel 18 in the opposite direction. The amount of rotation of the printing head 2, hence the particular character 12 which is brought into the print position, is determined by the amount of vertical displacement ofthe output arm 14 of the character selection device 4. During a normal printing operation the output arm 14 moves between various vertical positions at high rates of speed with the tape-engaging lever 16 rocking at a corresponding rate.

The invention, as incorporated in the illustrative apparatus (generally designated at 20 therein), functions primarily to avoid undue strain on the character selection device 4 that would result in damage thereto if the printing head 2 were rotatably restrained during a printing operation. For example, if a foreign object fell into the apparatus and jammed the printing head 2, the output arm 14 of the character selection device 4 would vainly attempt to rock the tape-engaging lever 16. Since the tape reel 18 in such instance would be held motionless by the restrained printing head 2, the displacing force imparted to the output arm 14 by the character selection device 4 would s'erve only to increase the tension in the tapes l5. Without some provision for releasing this tension, a damaging force would be transmitted to the fragile elements of the character selection device 4. The present invention, as illustrated at 20 in FIG. l, serves to slacken the tape l5 upon the exertion of a predetermined tension therein.

In the preferred form of the invention a tapeengaging member 22 journals an axially rotatable pulley 24. Engaged on the circumference of the pulley 24 is one extreme end or bight of the tape 15. Biasing means, such as a tension spring 26 coupled to the tapeengaging member 22 provides a tape-tensioning force of predetermined magnitude to the tape-tensioning member. When the rotation of the printing head 2 is restrained, displacement of the output arm 14 of the character selection device 4 would exert an excessive tension in the tape 15 as heretofore described. In response thereto the tape-tensioning member 22 initially displaces in the direction of tape tension, and in opposition to the biasing force of the spring 26. If the restraint on the printing head 2 is of sufficient magnitude and duration, the tape-engaging member 22 would pivot about a stationary support or post 28 until the pulley 24 assumes a position below the level of the printing head carrier 10. The tape-engaging member 22 would then translate an additional distance below the carrier to thereby slacken the tape l5 against even the most extreme displacement of the output arm 14. In the latter part of its slackening movement the tape-engaging member 22 would be assisted in its movement in the slackening direction by the same spring 26 that initially provided the biasing force to tension the tape 15.

To obtain the capability of moving both pivotally and translationally, the tape-engaging member 22 has a slot 36 that engages the stationary post 28 so that the tapeengaging member 22 may either slide along the post or pivot thereabout. For controlling the pivotal and translational motion of the tape-engaging member 22, a first camming surface 30 is formed by the contoured edge of a plate 32, the plate being disposed adjacent to the tape-engaging member and parallel with a plane defined by the displacement thereof. A lug 34 projecting from one side of the tape-engaging member 22 rides along the first camming surface 30 during displacement of the tape-engaging member. Means for maintaining the lug 34 in abutment with the first camming surface, in this instance a linking member 38, a pivotally connected to the opposite end of the tape-engaging member 22 from the pulley 24. A second camming surface 40 formed by a V-shaped opening in a stationary support plate 42 serves to cammably engage a stud 44 projecting from the linking member 38. The stud 44 in addition to its camming function provides a connecting point for a free end 4S of the spring 26, the linking member 38 thus serving to couple the spring with the tape-engaging member 22.

During normal operation of the associated printing apparatus the spatial relationship between the elements of the tension-releasing device remain substantially as shown in FIG. 2 with the stud 44 of the linking member 38 abutting the rightmost side of the second or V- shaped camming surface 40 with a very slight clearance between the stud and the leftmost side of the V-shaped camming surface. In such a configuration the resultant force of the spring 26 upon the tape-engaging member 22 creates a couple about the stationary post 28 which maintains the tape-engaging member in equilibrium with the tension in the tape l5. A decrease in the tension of the tape 15 causes a slight clockwise rotation of the tape-engaging member 22 to close the gap between the stud 44 of the linking member 38 and the leftmost side of the second camming surface 40. When the tension in the tape 15 is increased due, for example, to a jam of the printing head 2, the spring 26 yield" to allow the lug 34 of the member 22 to move rightwardly along the first camming surface 30 and the stud 44 of the linking member 38 to move along the rightmost side of the second camming surface 40. As the displacement of the tape-engaging member 22 continues, the pivot point of the linking member 38 represented by the lug 34 moves upwardly and to the right while the stud 44 of the linking member moves downwardly and to the right an additional distance along the second camming surface 40, as shown in FIG. 4. During the initial stage of this movement, the tape-engaging member 22 merely pivots about the stationary post 28 due to the substantially circular configuration of the utilized sector of the first camming surface 30. As the camming portion or lug 34 of the tape-engaging member 22 moves upwardly along the first camming surface 30 and the stud 44 of the linking member 38 moves downwardly along the rightmost camming surface 40, the component of restraining force imparted by the spring 26 to the tape-engaging member 22 diminishes, as does the resistance applied by the tape-engaging member 22 to the tension of the tape 15, thereby also diminishing tape tension itself, the component of tape tension effecting the pivotal action of the tape-engaging member also being diminished such that the restraining force of the spring 26 is reduced in correspondence with the diminishing components of the opposing force of tape tension. The the force required to collapse the tapeengaging member 22 of the device is accordingly maintained substantially constant throughout the initial portion of its displacement.

As the lug 34 of the tape-engaging member 22 continu'es moving upwardly on the first camming surface 30, and the stud 44 of the linking member 38 continues moving downwardly on the rightmost side of the second camming surface 40, the resultant force of the spring 26 upon the linking member 38 is so offset from the pivot point of the linking member at lug 34 that the linking member is pivoted by the spring away from the rightmost side of the second camming surface and into the leftmost side thereof, as shown in FIG. 5. With the linking member 38 in this position, the spring 26 no longer has any restraining effect upon the further displacement of the tape engaging member 22. On the contrary, the action of the spring 26 in pivoting the stud 44 of the linking member 38 from the rightmost side to the leftmost side of the second camming surface 40 acts to accelerate the slackening displacement of the tapeengaging member 22. At any time before the stud 44 is so moved, a relaxation in the tension in the tape will permit the tape-engaging member 22 to be pivoted back to its home position by the biasing force of spring 26. The jam, therefore, that would cause the total collapse of the tension releasing device must be of a duration long enough to complete the movement of the stud 44 of the linking member 38 from the rightmost to the leftmost side of the second camming surface 40. lt is also apparent that an instantaneous and powerful jamming restraint of the printing head 2 would cause the tape-engaging member 22 to pivot more quickly and to thereby accelerate the movement of the stud 44 of the linking member 38 from the rightmost to the leftmost side of the second camming surface 40, the collapse of the tension releasing device being accordingly also accelerated. In this manner the tension-releasing-action of the device corresponds with the particular type of loading placed on the tape i5 due to various jam conditions at the printing head 2.

When the spring 26 becomes ineffective to oppose further slackening displacement of the tape-engaging member 22, due to the change in the position of the linking member 36 on the second camming surface 4t), as illustrated in FIG. 5, the spring 26 force then acts to advance the lug 34 of the tape-engaging member 22 along the first camming surface 30 in the tensionreleasing direction. This force of spring 26 in addition to the tension remaining in the tape l5 and the inertia of the tape-engaging member 22, serves to force the lug 34 of the tape-engaging member around an abrupt curve 46 in the first camming surface 30. Up to the point that the lug 34 of the tape-engaging member 22 reaches the abrupt curve 46, the displacement of the tape-engaging member 22 is substantially pivotal about the stationary post 23. As the member begins to round the abrupt curve 46, the pulley 24 is carried substantially below the level of the printing head carrier l0 and a translational motion of the tape-engaging member occurs, such translational motion being accommodated by the slot 36 formed therein. An increased slackening displacement of the pulley 24 it is accordingly permitted, at the same time minimizing the clearance necessary between the rightmost position of the print carrier 10 and the pulley.

After a jam of the printing head 2 has been cleared, the tension-releasing device may be reset to bring the tape 15 back to its preestablished tension. Resetting is accomplished by sliding the tape-engaging member 22 back along the slot 36, and by then pushing the tapeengaging member downwardly to bring the stud 44 of the linking member 38 down and along the leftmost side of the second camming surface 40. The force of the spring 26 then takes effect and restores the tapeengaging member 22 to its home position wherein the tape l5 is properly tensioned. Resetting of the device does not require a great amount of force since the spring 26 acts to apply the biasing tension to the tape l5 as the tape-engaging member 22 approaches its home position in the resetting procedure.

The tension releasing device lends itself to a great variety of modifications and alternatives. The shapes and positions of the first and second camming surfaces (30 and 40 respectively) may be altered to achieve variations in the relative pivotal and translational distances of the tape-engaging member 22 displacement, and to vary as well as varying the tension required to move the member in the slackening direction. Changing the position of the fixed end of the spring 26 may increase the tensioning force which the device applies to the tape l5, and may change the position at which the linking member 33 pivots to release the tape-engaging member 22 from the restraining effect of the spring. A satisfactory device could also be constructed by removing the linking member 38 altogether, and by providing some other means for maintaining the tape-engaging member 22 in abutment with the first camming surface 30. A slot in the place of the first camming surface 30 might be used to engage the tape-engaging member 22 and to guide it over a the desired path.

These are but a few of the many alternatives and modifications that may be apparent to one skilled in the art in light of the foregoing description. It is intended therefore to include in the appended claims all such alternatives and modifications that do not truly depart from the inventive concept disclosed herein.

What is claimed is:

l. A device for collapsibly releasing tension in a flexible tape or the like in response to the exertion of excessive tension therein comprising:

a stationary support;

a tape-engaging member coupled to said stationary support for pivotal and translational displacement with respect thereto, said tape-engaging member being cooperably disposed relative to said flexible tape and movable in a first tension-applying and a second tension-releasing direction, said tensionapplying movement of said tape-engaging member being pivotal in character and said tensionreleasing movement being both pivotal and translational in character;

a first camming surface associated with said support and abuttably engaged by said tape-engaging member, said camming surface being contoured for guiding said tape-engaging member over a predetermined tension-releasing path; and

biasing means normally effective for pivotally urging said tape-engaging member in said first direction to thereby apply tension to said tape, said biasing means being initially yieldable to excessive tension in said tape and effective thereafter for pivotally and translationally urging said tape-engaging member in said second direction to slacken said tape and to effectuate a tensional collapse in said tapeengaging member.

2. A device as defined by claim 1 wherein said stationary support is characterized by a post and said tapeengaging member is provided with a slot engaged with said post for pivotal and translational displacement thereon, said post and said slot in response to said urging of said biasing means serving to maintain said tape-engaging member in engagement with said first camming surface during said tension-applying and said tension-releasing movements thereof.

3. A device as defined by claim 2 wherein said first camming surface is contoured to pivotally guide said tape-engaging member in its said tension-releasing movement, the surface thereof abruptly curving at a predetermined point to thereby translationally guide said tape-engaging member in its said tensional collapse.

4. A device as defined in claim 3 wherein said biasing means resistingly yields to the pivotal motion of said tape-engaging member in its said tension-releasing movement, said biasing means becoming effective to aid in the translational displacement of the tapeengaging member as said tape-engaging member is guided around the abrupt curve of said first camming surface.

5. A device as defined by claim 4 wherein said biasing means is a spring having a free end coupled to said tape-engaging member and a fixed end stationarily supported at a predetermined distance from said free end to apply a desired tape-tensioning force to said tapeengaging member.

6. A device as defined by Claim 1 wherein said tapeengaging member is maintained in engagement with said first camming surface byl a link pivotally coupled to said tape-engaging member and connected to said biasing means for transmitting a biasing force to said tape-engaging member, said link being guided by a second camming surface during said tension-applying and said tension-releasing movements of said tape-engaging member and maintained by said second camming surface in opposition to the biasing force of said biasing means to define a home position for said tape-engaging member, said guidance provided said link by said second camming surface being effective to initially apply the biasing force of said biasing means in opposition to said tension-releasing movement of said tape-engaging member, and to thereafter adapt said biasing force to resiliently and translationally motivate said tensional collapse in said tape-engaging member.

7. A device for releasing tension in a flexible tape or the like in response to the exertion of excessive tension therein comprising:

a stationary support;

a tape-engaging member having a slot couplable with said stationary support for pivotal and translational displacement thereon, a portion of said tapeengaging member being adapted for engagement with said tape;

a first camming surface for cammably guiding the displacement of said tape-engaging member during a tension-releasing movement thereof;

a linking member pivotally coupled to said tapeengaging member;

a second camming surface for cammably guiding the pivotal motion of said linking member with respect to said tape-engaging member; and

a spring having a free end connected to said linking member and a fixed end stationarily supported at a predetermined distance from said free end for transmitting a desired tape-tensioning force to said tape-engaging member, whereby the joint action of said spring and said linking member serves to yieldably resist said tension-releasing movement of said tape-engaging member while maintaining said tape-engaging member in abutment with said first camming surface throughout its said tensionreleasing movement.

8. A device as defined by claim 7 wherein said second camming surface is substantially V shaped in configuration, the linking member being cammable along one leg of the V shaped camming surface to thereby apply the biasing force of said spring in yieldable opposition to the tape-tension, a predetermined displacement of said tape-engaging member along said first camming surface under the influence of excessive tension in the tape being effective to position said spring with respect to the pivot point of said linking member so that said linking member is pivoted by said spring into camming engagement with the other leg of said V shaped camming surface to thereby apply the biasing force of said spring to said tension-releasing movement of said tape-engaging member.

19K Ik

Claims

1. A device for collapsibly releasing tension in a flexible tape or the like in response to the exertion of excessive tension therein comprising: a stationary support; a tape-engaging member coupled to said stationary support for pivotal and translational displacement with respect thereto, said tape-engaging member being cooperably disposed relative to said flexible tape and movable in a first tension-applying and a second tension-releasing direction, said tension-applying movement of said tape-engaging member being pivotal in character and said tension-releasing movement being both pivotal and translational in character; a first camming surface associated with said support and abuttably engaged by said tape-engaging member, said camming surface being contoured for guiding said tape-engaging member over a predetermined tension-releasing path; and biasing means normally effective for pivotally urging said tapeengaging member in said first direction to thereby apply tension to said tape, said biasing means being initially yieldable to excessive tension in said tape and effective thereafter for pivotally and translationally urging said tapeengaging member in said second direction to slacken said tape and to effectuate a tensional collapse in said tape-engaging member.

2. A device as defined by claim 1 wherein said stationary support is characterized by a post and said tape-engaging member is provided with a slot engaged with said post for pivotal and translational displacement thereon, said post and said slot in response to said urging of said biasing means serving to maintain said tape-engaging member in engagement with said first camming surface during said tension-applying and said tension-releasing movements thereof.

4. A device as defined in claim 3 wherein said biasing means resistingly yields to the pivotal motion of said tape-engaging member in its said tension-releasing movement, said biasing means becoming effective to aid in the translational displacement of the tape-engaging member As said tape-engaging member is guided around the abrupt curve of said first camming surface.

5. A device as defined by claim 4 wherein said biasing means is a spring having a free end coupled to said tape-engaging member and a fixed end stationarily supported at a predetermined distance from said free end to apply a desired tape-tensioning force to said tape-engaging member.

6. A device as defined by Claim 1 wherein said tape-engaging member is maintained in engagement with said first camming surface by a link pivotally coupled to said tape-engaging member and connected to said biasing means for transmitting a biasing force to said tape-engaging member, said link being guided by a second camming surface during said tension-applying and said tension-releasing movements of said tape-engaging member and maintained by said second camming surface in opposition to the biasing force of said biasing means to define a home position for said tape-engaging member, said guidance provided said link by said second camming surface being effective to initially apply the biasing force of said biasing means in opposition to said tension-releasing movement of said tape-engaging member, and to thereafter adapt said biasing force to resiliently and translationally motivate said tensional collapse in said tape-engaging member.

7. A device for releasing tension in a flexible tape or the like in response to the exertion of excessive tension therein comprising: a stationary support; a tape-engaging member having a slot couplable with said stationary support for pivotal and translational displacement thereon, a portion of said tape-engaging member being adapted for engagement with said tape; a first camming surface for cammably guiding the displacement of said tape-engaging member during a tension-releasing movement thereof; a linking member pivotally coupled to said tape-engaging member; a second camming surface for cammably guiding the pivotal motion of said linking member with respect to said tape-engaging member; and a spring having a free end connected to said linking member and a fixed end stationarily supported at a predetermined distance from said free end for transmitting a desired tape-tensioning force to said tape-engaging member, whereby the joint action of said spring and said linking member serves to yieldably resist said tension-releasing movement of said tape-engaging member while maintaining said tape-engaging member in abutment with said first camming surface throughout its said tension-releasing movement.

8. A device as defined by claim 7 wherein said second camming surface is substantially ''''V'''' shaped in configuration, the linking member being cammable along one leg of the ''''V'''' shaped camming surface to thereby apply the biasing force of said spring in yieldable opposition to the tape-tension, a predetermined displacement of said tape-engaging member along said first camming surface under the influence of excessive tension in the tape being effective to position said spring with respect to the pivot point of said linking member so that said linking member is pivoted by said spring into camming engagement with the other leg of said ''''V'''' shaped camming surface to thereby apply the biasing force of said spring to said tension-releasing movement of said tape-engaging member.