US3817474A

US3817474A - Retractable safety belt

Info

Publication number: US3817474A
Application number: US00444658A
Authority: US
Inventors: R Board; N Shapiro
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-04-01
Filing date: 1965-04-01
Publication date: 1974-06-18
Anticipated expiration: 1991-06-18

Abstract

A seat belt retractor having an automatic clamp for preventing extension of seat belt employing a moving clamp part which also acts as a buffer to prevent the retraction reel from taking up increments of slack which occur while the belt is fastened due to abnormal movements of the user.

Description

' United States Patent [191 Board et a1.

[11] 3,817,474 i451 June 18, 1974 1 1 RETRACTABLE SAFETY BELT [76] Inventors: Richard G. Board, 3000 Connecticut Ave., Washington,

DC. 20008; NelsonH. Shapiro, 640

Washington Bldg., Washington, DC 20005 221 Filed: Apr. 1, 1965 211 App]. No.: 444,658

[52] US. Cl. 242/ 107.2, 242/107 SB, 297/388 [51] Int. Cl A62b 35/00 [58] Field of Search... 297/388; 242/107 SB, 107.2,

[56] References Cited UNITED STATES PATENTS 3,205,004 9/ l 965 Spouge 297/388 3,227,489 1/ 1966 Stubblefield 297/388 3,246,929 4/1966 Taggart 297/388 FOREIGN PATENTS OR APPLICATIONS 7/191 1 Switzerland 242/107.2

Primary Examiner-Wemer 1-1. Schroeder Attorney, Agent, or Firm-Shapiro and Shapiro 1 1 ABSTRACT A seat belt retractor having an automatic clamp for preventing extension of seat belt employing a moving clamp part which also acts as a buffer to prevent the retraction reel from taking up increments of slack which occur while the belt is fastened due to HQXQEBFQLQflhEEQ:

2 Claims, 13 Drawing Figures PM'ENYEDJUN I 8 I974 SHEU 1 BF 3 INVENTORS RICHARD G. BOARD NELSON H. SHAPIRO BY Ska 00b a/lci ka 0170 ATTORNEYS PMENTEDJUN 1a 1914 snmzora- FIG. 5

FIG. 5

v INVENTORS RICHARD G. BOARD NELSON H. SHAPIRO BY \S IOPI IO and SAQIO ATTORNEYS INVENTORS RICHARD 6- BOARD NELSON H. SHAPIRO BY SA 0H0 'cmci Ska aim ATTORNEYS r 1 'RE'I'RACT ABLE SAFETY BELT This invention relates to retractable safety belts and more particularly to so-called retractable seat belts for use in automotive vehicles.

Copending applications, Ser. No. 417,924, filed Dec. 14, I964, Ser. No. 409,266, filed Nov. 5, 1964, and Ser. No. 377,344, filed June 23, 1964, all entitled RE- TRACTABLE SAFETY BELTS disclose and claim retractable seat belt apparatus in which a strap having a fastener member at a free end thereof may be grasped by the user and extended from a retraction device to place the strap in user-restraining position, whereupon further extension of the strapis prevented automatically in response, for example, to the cessation of extensile movement or to slight retractile movement of the strap. At the user-restraining position the strap may have any selected length within a wide range of lengths, so as to accommodate varying conditions of user size and clothing. The fastener member is engaged with a mating fastener member in order to complete the belt. When the fastener members are disengaged, the strap retracts automatically. The present application discloses and claims improvements in and modifications of such apparatus.

It is accordingly a principal object of the invention to further. extension, and retracted automatically when released.

Still another object of the invention is to provide im-. proved apparatus for retractable seat belts and the like which is responsive to changes in tension of the strap.

The applicants have previously proposed retractable seat belt apparatus and'the like having a member past which a strap is trained, the member being moved automatically to a first position when the strap is retracted, remaining in that position during extension of the'strap', and moving automatically to a second position when extensile movement of the strap ceases. At the second position the member serves to prevent further extensile movement of the strap. It is a principal object of the present invention to provide improved retractable seat belt apparatus and the like employing this principle.

Still another object of the invention is to provide retractable seat belt apparatus of the foregoing type whichutilizes variations in the forces in the strap extending from the aforesaid member to provide the desired operation.

Yet another object of the invention is to provide an improved retractable'seat belt and the like employing a feeler mechanism for sensing the amount of strap wound upon a retraction reel and for controlling the operation of the aforesaid member.

still another object of the invention is to provide retractable seat belt apparatus and the like having im proved means for preventing the extensile movement of a strap after the strap has been placed in userrestraining position.

Applicants have previously proposed retractable seat belt apparatus in which extensile movement of a strap is prevented by the closing of a clamp after the strap has been extended to the desired user-restraining position and in which the clamp opens automatically during retraction of the strap, remains open until the strap has been extended to the desired length, and then closes automatically.

It is a principal object of the present invention to provide improved apparatus of the foregoing type.

Another principal object of the invention is to provide apparatus of this type which requires only one simple moving part.

A possible objection to the use of automatic clamps for preventing extension of a seat belt is that the normal movement of the user, particularly as the vehicle rides over bumps on the road, may tendto create increments of slack which are immediately taken up by the retractor, progressively tightening the belt to an undesirable extent. It is accordingly a principal object of the invention to provide apparatus which alleviates this problem.

More specifically, it is an object of the invention to provide apparatus employing a moving clamp part which also acts as a buffer to prevent the retraction reel from taking up increments of slack which occur whil the belt is fastened.

Briefly stated, but without limitation, the present invention utilizes a clamp having a moving clamp part and a fxed clamp part. A strap is trained from a retraction reel between the clamp parts and about the moving clamp part in such a way that forces exerted by the user upon the belt after the belt has been fastened close the clamp to prevent extension of the belt, and forces exerted upon the moving clamppan by the reel open the clamp during retraction. In addition to pivotal movement tov provide the clamping function, the moving clamp part is supported for translational movement to provide a disabling function, that is, to prevent the closing of the clamp during extension of the strap to the desired user-restraining position Furthermore, by virtue of the manner in which the strap is trained about the moving clamp part, a buffering functionis provided which prevents the reel from taking up increments of slack when the belt is fastened.

The foregoing and other objects, advantages, and feature of the invention, and the manner in which the same are accomplished, will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments, and wherein:

FIG. 1 is a fragmentary prespective view illustrating the external appearance of housing, strap, and fastener components of a typical embodiment of retractable seat belt apparatus in accordance with the invention;

FIGS. 2 6 are longitudinal sectional views illustrating different positions assumed by a moving clamp part of the invention in the course of the operation of the invention;

FIG. 7 is a sectional view taken along line 7-7 of FIG. 6;

FIG. 8 is a perspective view of a moving clamp part employed in the invention;

FIGS. 9 12 are longitudinal sectional views illustrating different positions of a moving clamp part in accordance with a modification of the invention; and

FIG. 13 is a perspective view of a modified moving clamp part.

Referring to the drawings, and initially to FIG. 1 thereof, reference numeral 10 designates a conventional seat belt fastener couple comprising a buckle l3 and a tongue 14. The fastener parts may have the usual construction, the buckle comprising a housing having an access opening at the front for reception of the tongue and including the usual locking abutments which enter the openings 16 of the tongue so as to retain the tongue in the buckle until the buckle release lever 18 is actuated. A strap 20 affixed to the buckle may have its distal end anchored to the floor of the vehicle at one side of the seat, for example. This strap may be kept quite short, may be rigidified, and need not be adjustable. Strap 22, such as nylon webbing, is affixed to the tongue and is arranged to be retracted and extended with respect to a retraction device 24, which may be a spring-wound retraction reel supported for rotation in a housing or casing 26 anchored to the floor of the vehicle at the opposite side of the seat, for example. The housing may be appropriately contoured to minimize space requirements and for aesthetic reasons and it may have an opening 28 through which the strap 22 moves and at which the tongue 14 is located when the strap is fully retracted. This opening may, in effect, be extended toward the user by the use of a conventional boot or tube which guides the strap to the rest position of the tongue.

As is well known in the art, the reel 24 (FIG. 2) may comprise a tubular hub 30 having bearings (not shown) at each end for supporting the hub for rotation upon a pin 32 extending through the hub and fixed between side plates 34 of a frame, which may be part of the housing 26. A helical torsion spring 36 is received loosely within the hub between the hub and the pin 32 and has one end fixed to the pin and the other end fixed to the hub, so that the spring is wound when the reel is turned in a direction to extend the strap. Other types of reels, such as those using spiral clock springs at the ends, may also be used. The side plates 34 may be joined by spacers 38. It will thus be apparent that the tongue 14 may be grasped to extend the strap 22 from the housing 26, turning the reel 24 and winding the spring 36, and that when the tongue is released the reel will be turned by the spring, winding up the strap so as to retract it into the housing.

In accordance with the invention a clamp is employed for preventing extension of the strap when the strap has been extended to the desired user-restraining position. The clamp comprises a moving clamp part 40 and a fixed clamp part 42. The fixed clamp part may simply be a bar or rod extending between and mounted at its ends upon the side plates 34. As shown in FIG. 8, the moving clamp part may be a suitably shaped plate of generally rectangular outline provided with a pair of laterally extending pivot pins 44, which may be integral with the plate. The pivot pins extend into slots 46 of the side plates 34 with sufiicient play to permit pivotal and translational movement of the pins. In the form shown in FIG. 2, for example, each slot is L-or chevronshaped having legs forming a right angle the bisector of which is substantially horizontal. One of the legs extends generally toward the center of the reel.

In the form shown the moving clamp plate has a generally concave-upward curvature, although the shape of the plate may be varied somewhat, as will be described more fully hereinafter. Both the lower or leading edge and the upper or trailing edge of the plate are shown smoothly curved, the strap being trained from one side of the reel past the upper edge of the plate, between the plate and the fixed clamp part 42, along the outer surface of the plate, around the lower edge of the plate, and then upwardly at an appropriate angle past the other side of the reel. The generally upwardly directed surface at the lower edge of the clamp plate may be flattened (at least at the lateral extremities of the plate) as shown at 48 in FIG. 8. These surfaces are adapted to engage stop pins 50 which project laterally inward from the side plates (See FIG. 7).

In the position of the clamp plate shown in FIG. 5 the pivot pins 44 are at the top of the slots 46, and the pres sure exerted upon the lower end of the clamp plate by the tension in the strap leading to the tongue 14 turns the clamp plate so that it clamps the strap against the fixed clamp part 42, as shown in FIG. 6. The clamping pressure is determined by the relative length of the lever arms between the pivot pins and the lower edge of the clamp plate and between the pivot pins and the portion of the clamp plate adjacent to the fixed clamp part. This may be a favorable ratio of the order of 2 to l, 3 to l, or even more. The clamping pressure is also determined by the angle of pull of the portion of the strap extending to the tongue with respect to the lever arm extending between the lower edge of the clamping plate and the pivot pins. This angle may be of the order of for example. The harder the pull upon the strap, the tighter the clamping. The actual clamping surfaces may be smooth, or one or both of them (preferably just the fixed part, which does not normally engage the strap during movement thereof) may be roughened or knurled. The upper edge of the clamp plate indents the strap, as shown in FIG. 6, at least when the clamp is closed.

If the belt is unfastened, the pressure of the strap upon the upper edge of the clamp plate due to the spring force of the reel will turn the clamp plate so as to open the clamp, as shown in FIG. 5, and the strap will retract. If when the strap is substantially fully retracted the strap tension is substantially removed, the clamp plate will be free to slide along the slots under the influence of gravity to the position of FIG. 2. This may be facilitated by weighting the lower end of the plate, by having the reel spring fully unwound when the strap is fully retracted or by having the webbing on the reel press against a portion of the housing when the strap is fully retracted, so as to stop the movement of the reel. FIG. 2 illustrates the fully retracted condition, it being noted that the clamp plate 40 has slipped to the bottom of the slots 46 and that the webbing is relatively slack.

If the strap is grasped and pulled from the housing, the pressure on the clamp plate 40 will cause it to turn to the position of FIG. 3, at which the flattened portions 48 engage the stop pins 50. The pressure of the flattened portions 48 against the stop pins and the pressure of the pivot pins 44 on the edge of the slots 46 will immobilize the clamp plate in an open clamp position while the tension in the strap is maintained during extension of the strap to the user-restraining position.

If now the strap is relaxed slightly, as occurs during the fastening of the belt, the clamp plate will turn slightly more open because of the pressure of the strap upon the upper edge of the plate. The pivot pins 44 will roll slightly upon the upper edges of the slots and will move slightly up the slots, engaging the curved lower edge of the clamp plate with the stop pins 50 and camming the clamp plate about the stop pins as shown in FIG. 4, so as to accelerate the upward movement of the clamp plate. The upward pressure exerted by the strap on the clamp plate causes the pivot pins 44 instantaneously to slide along the slots to the position of FIG. 5, any slack in the belt created by this movement of the clamp plate being taken up by the reel. Any further extensile pull exerted upon the strap causes the clamp to close immediately as shown in FIG. 6, the clamp plate being freeof obstruction by the stop pins.

If now the belt is unfastened, the pressure exerted upon the upper edge of the clamp plate by the webbing causes the clamp plate to turn to the open position of FIG. 5, permitting the strap to be retracted upon the reel. It is only when the tension in the strap is substantially removed, as shown in FIG. 2, that the clamp plate will again travel underv the influence of gravity down the slots. Otherwise, the strap tension is sufficient to maintain the clamp plate at the top of the slots.

FIGS. 5 and 6 illustrate another useful characteristic of the invention, whereby a buffering action is provided so as to avoid taking 'up increments of slack created during normal use with the belt fastened. If the belt is tightened sufficiently to move the clamp to the closed position of FIG. 6, the upper edge of the clamp plate will substantially indent the webbing laterally, the reel turning in a clockwise direction in FIG. 6 to provide the webbing for the indentation. If now the belt is relaxed somewhat, as when the user moves more deeply into the seat, the clamp plate will turn toward the position of FIG. 5, and the reel will turn in a counterclockwise direction to absorb the slack created as the indentation is removed. However, there will be no actual movement of the strap across the surface of the clamp plate, because the turning movement of the clamp plate will cause the lower edge of the clamp'plate to indent the strap more deeply. it is only after the clamp plate has turned to the open position of FIG. 5 that further relaxation of the belt will cause slack to be taken up by the reel. Otherwise, when the user moves back up relative to the seat, the clamp plate will merely turn back to the position of FIG. 6 withdrawing some webbing from the reel to .create indentation at the upper edge.

The configuration of the slots 46 is such as to ensure that the clamp plate moves behind (under) the stop pins 50 as it moves down the slots and clears the stop pins as it moves up the slots. The upper leg of the slots is generally radial to the reel, and the lower leg is generally tangential. The upper edge of the lower leg assists the stop pins in maintaining the position of the clamp plate during extension of the strap. The effect of the slots is to permit the clamp plate to move between a first position, at which turning is obstructed by stop pins 50, and a second position at which turning is unobstructed. The clamp plate thus becomes its own latch." The pivot pins may be enlarged for strength and the slots enlarged accordingly. However; it is not necessary that the upper leg of the slots be long enough to accommodate the whole pivot pin. The pin may protrude below the bend in the slot when the pin is at the top of the slot. Moreover, the pins which ride the slots may be mere guide pins; the actual pivot pins may be larger and concentric with the guide pins, engaging suitable bearing surfaces fixed to the side plates when the guide pins are at the top of the slots.

FIGS. 9 l3 illustrate a modification in which the moving clamp part 52 is moved to its rest position by the pressure of the webbing wound upon the reel, rather than by gravity. It will also be observed that the moving clamp part, while a generally rectangular plate, has a different configuration from the part 40 previously described in that the lower portion of the clamp is upwardly convex and the upper portion of the clamp is essentially flat. Pivot pins 54 fixed to the sides of the clamp plate are mounted for free movement in slots 56 in the side plates. The slots have a different configuration from those previously described, being generally crescent shaped. The outer slot edge is smoothly curved, while the inner edge (toward the reel) has a steppedconfiguration to provide a shelf on which the pins rest during extension of the strap. The orientation of the slots is somewhat different in this embodiment, the bisector of the angle formed by the slots being directed upwardly at about thirty degrees in the general direction in which the strap is pulled. The upper surface of the clamp plate has a generally cylindrical feeler or pusher member 58 afiixed thereto slightly below the pivot pins 54 and adapted to engage the webbing wound upon the reel when the strap is retracted. Adjacent to its lower edge the clamp plate is formed at its lateral extremities with shallow notches 60, which are adapted to engage and receive stop pins 62 projecting slightly inwardly from the side plates 34. The clamp plate has an enlargement 64 at its upper extremity for engaging the webbing wound upon the reel when the reel is almost full, as shown in FIG. 9. The fixed clamp part 66 in this embodiment is shown as a cylindrical pin extending between the side plates.

When the strap is fully retracted, the movable clamp part will have the position shown in FIG. 9. When the strap is extended, the lower edge of the clamp plate will move slightly, due to the drag of the strap over the clamp plate on its outer surface as well as on the surface of pusher member 58, so as to seat the stop pins 62 in the notches 60. In this movement of the clamp plate the pivot pins 54 will move to the bottom of the slots 56 as can be seen from FIGS. 9 and 10. The clamp plate will remain in the position of FIG. 10, as long as the strap is pulled during extension of the strap to the user-restraining position. If now the strap is relaxed slightly, during the fastening of the belt, the movable clamp part will snap around the stop pins, as shown in FIG. 11 and as described in connection with the previous embodiment, and will slide upwardly to the top of the slots at which any further pull upon the strap will cause the clamp to close as shown in FIG. 12, preventing any extension of the strap. If now the belt is unfastened, the clamp will move to the open position of FIG. 11, as previously described. As the webbing winds upon the reel, it will finally engage the pusher member 58 and will press the clamp plate downwardly along the.

slots almost to the bottom thereof. At this time the enlargement or pusher element 64 of the upper edge of the clamp plate will engage the webbing upon the reel, turning the clamp plate slightly clockwise as seen in FIG. 9 so that the notches approach the stop pins 62.

The same buffering action described previously is also available in the embodiment of FIGS. 9 l3, pres sure on the upper edge of the clamp plate by the strap acting as a return spring to turn the clamp plate to its open position. It is also to be noted that in this embodiment if only a slight amount of webbing is withdrawn from the reel before the clamp is permitted to close, the pusher member 58 may engage the webbing upon the reel. In this event, the pusher member, riding upon the webbing, may serve as the fulcrum for the clamp until the webbing upon the reel is compressed sufiiciently to permit the pivot pins to reach the tops of the slots, but clamping of the strap nevertheless occurs.

The undersurface of the lower edge of the clamp plate at its lateral extremities may be contoured, as may the upper surface of the stop pins to facilitate camming of the movable clamp part around the stop pins to the rest position of FIG. 9 in the event the clamp is not fully open during retraction of the strap. This is significant in this embodiment, because the lower edge of the clamp plate must be close enough to the stop pins to engage them promptly when the strap is initially pulled, in order to prevent undesired pressure of the pusher member against the strap on the reel. Furthermore, instead of having stop pins projecting inwardly from the side plates of the frame, the lower edge of the clamp plate may have laterally projecting stop pins which move in suitably shaped windows in the side plates, the windows having shelf portions to engage the upper surface of the stop pins at the initial position of the clamp plate.

If the reel has end discs, as shown at 68, the upper edge of the clamp plate may rub gently against the end discs during retraction of the strap until the last turn expands beyond the end discs to engage the upper edge of the clamp plate as in FIG. 9. If the upper edge of the clamp plate has an enlargement fitting between the end discs, the same engagement can be obtained without expansion of the last turn of webbing beyond the end discs. Moreover, in addition to or instead of end discs on the reel, the strap may be guided by the side plate 34 or bosses thereon.

The amount of translational movement of the clamp plate (broadly, movement in addition to the pivotal clamping movement) need be only sufficient to distinguish between the obstructed and unobstructed positions of the clamp plate. The amount of turning or pivotal movement of the clamp plate is influenced by the amount of buffering action desired and may be quite small in the absence of buffering. The total apparatus may be kept quite compact by suitably shaping the clamp plate and by suitably positioning the clamp plate in the housing. The clamp plate may be made as thick as necessary to take collision loads.

The pusher member in FIGS. 9 13 has been located to ensure that the clamp plate is pushed in an open position, but other locations of the pusher member are possible, if correlated with other parameters, such as the slot orientation relative to the reel.

The configuration of the clamp plate influences the friction drag applied to the strap, and, of course, the reaction of the clamp plate to the strap tension forces, which provide clamp opening and closing pivotal movement as well as latching and unlatching action. Friction can be reduced by the use of appropriate contours or by the use of rollers, but the latter introduces complication.

Slot shape and orientation are determined by the requirements of translational movement of the clamp plate between obstructed (latched) or unobstructed (unlatched) positions. Slot components radial to the reel can be utilized to provide translational movement in response to expansion of the wound webbing, while tangential components may be utilized to provide translational movement in response to drag exerted by the strap (which may also provide turning movement). The edges of the slots may provide cam surfaces for guiding the translational movement or to support tuming or rolling movement of the pivot pins. Slot shelves are useful in maintaining the position of the pivot pins during strap extension, although the engagement of the clamp plate with stop pins or thelike may be sufficient in itself to prevent closing of the clamp during extension of the strap.

While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. For example, other forces or combinations of forces may be used to move the clamp plate to its initial position and/or to hold it there. Drag of the strap across the clamp plate, when the strap is slightly retracted after fastening, may be used to release the latch. The clamp plate may be formed with a channel to guide the strap across its surface, or may have lateral extensions passing into receptacles in the side plates to engage recessed stops. Accordingly, the foregoing embodiments are considered to be illustrative, rather than restrictive of the invention, and those modifications which come within the meaning and range of equivalents of the claims are intended to be included therein.

The invention claimed is:

1. In a retractable belt having a retraction device with a strap connected thereto and adapted to be retracted, extended to a desired length and then prevented from substantial further extension, the im rovement which comprises a strap guide supported or pivotal movement relative to said device and having leading and trailing edges spaced on opposite sides of its pivotal axis, and means training said strap from said retraction I device and reversely around said guide, with indentations at said edges, for exerting forces on said guide and causing it to turn in one direction in response to the retracting force of said retraction device exerted upon the trailing edge of said guide and to turn in the opposite direction in response to a force exerted upon the leading edge of said guide through the strap beyond said leading edge, the indentation at one edge increasing as the indentation at the other edge decreases, and vice versa, whereby the effective length of the strap beyond said leading edge may decrease and then increase agan by an amount determined by the turning of said gui e.

2. In a retractable belt having a retraction device with a strap connected thereto and adapted to be retracted, extended to a desired length and then prevented from substantial further extension, the improvement which comprises a strap guide supported for pivotal movement relative to said device and having leading and trailing edges spaced on opposite sides of its pivotal axis, and means training said strap from said retraction device and reversely around said guide, in frictional engagement therewith, for exerting forces on said guide and causing it to turn in one direction in response to the retracting force of said retraction device exerted upon the trailing edge of said guide and to turn in the opposite direction in response to a force exerted upon the leading edge of said guide through the strap beyond said leading edge, the guide being pivotally movable through a predetermined distance with belt movement in one direction, and vice versa, whereby the effective length of the strap beyond said leading edge may decrease and then increase a ain by an amount determined by the tuming of saic? guide.

Claims

1. In a retractable belt having a retraction device with a strap connected thereto and adapted to be retracted, extended to a desired length and then prevented from substantial further extension, the improvement which comprises a strap guide supported for pivotal movement relative to said device and having leading and trailing edges spaced on opposite sides of its pivotal axis, and means training said strap from said retraction device and reversely around said guide, with indentations at said edges, for exerting forces on said guide and causing it to turn in one direction in response to the retracting force of said retraction device exerted upon the trailing edge of said guide and to turn in the opposite direction in response to a force exerted upon the leading edge of said guide through the strap beyond said leading edge, the indentation at one edge increasing as the indentation at the other edge decreases, and vice versa, whereby the effective length of the strap beyond said leading edge may decrease and then increase again by an amount determined by the tUrning of said guide.

2. In a retractable belt having a retraction device with a strap connected thereto and adapted to be retracted, extended to a desired length and then prevented from substantial further extension, the improvement which comprises a strap guide supported for pivotal movement relative to said device and having leading and trailing edges spaced on opposite sides of its pivotal axis, and means training said strap from said retraction device and reversely around said guide, in frictional engagement therewith, for exerting forces on said guide and causing it to turn in one direction in response to the retracting force of said retraction device exerted upon the trailing edge of said guide and to turn in the opposite direction in response to a force exerted upon the leading edge of said guide through the strap beyond said leading edge, the guide being pivotally movable through a predetermined distance with belt movement in one direction, and vice versa, whereby the effective length of the strap beyond said leading edge may decrease and then increase again by an amount determined by the turning of said guide.