US3791420A

US3791420A - Gripping mechanism for strapping machine

Info

Publication number: US3791420A
Application number: US00281054A
Authority: US
Inventors: J Beach
Original assignee: Signode Corp
Current assignee: Signode Corp
Priority date: 1972-08-16
Filing date: 1972-08-16
Publication date: 1974-02-12
Anticipated expiration: 1991-02-12
Also published as: NL180298C; SU513613A3; CA987582A; DE2329844B2; JPS5226200B2; DE2329844C3; FR2196271A1; NL180298B; SE400246B; DE2329844A1; CH562126A5; NL7310783A; JPS49133192A; FR2196271B1; GB1393838A

Abstract

A gripping device for holding the free end of a strap that is being tensioned about an article is disclosed in this application. The gripping device is designed so that the gripping action on the strap is directly proportional to the tension on the strap which results in a self-energizing gripping device. The gripping device consists of a pair of gripping jaws that are pivoted around openings receiving pins that define spaced parallel axes extending longitudinally of the strap. The openings that receive the pins defining the pivot axes are tapered from the opposite sides of the gripping jaws towards the center so that the gripping jaws may pivot between extreme positions about an axis extending generally normal to the length of the strap when tension is applied to the strap. The surfaces which engage the strap also have notches that define teeth and the teeth are configured so that the engaging surface is generally parallel to the parallel axes defined by the pins.

Description

United States Patent 1191 Beach GRIPPING MECHANISM FOR STRAPPING MACHINE [75] Inventor: John R. Beach, Elmhurst, Ill.

[73] Assignee: Signode Corporation, Glenview, 111..

[22] Filed: Aug. 16, 1972 2] Appl. No.: 281,054

Primary Examiner-Lowell A. Larson Attorney, Agent, or FirmDressler, Goldsmith, Clement & Gordon, Ltd.

n] 3,791,420 [4 1 Feb. 12, 1974 [57] ABSTRACT Q A gripping device for holding the free end of a strap that is being tensioned about an article is disclosed in this application. The gripping device is designed so that the gripping action on the strap is directly proportional to the tension on the strap which results in a self-energizing gripping device. The gripping device consists of a pair of gripping jaws that are pivoted around openings receiving pins that define spaced parallel axes extending longitudinally of the strap. The openings that receive the pins defining the pivot axes are tapered from the opposite sides of the gripping jaws towards the center so that the gripping jaws may pivot between extreme positions about an axis extending generally normal to the length of the strap when tension is applied to the strap. The surfaces which engage the strap also have notches that define teeth and the teeth are configured so that the engaging surface is generally parallel to the parallel axes defined by the pins.

18 Claims, 7 Drawing Figures TENSiON MECH.

- v rcalzlsm PAIENTED SHEET 2 2 TENSION MECH- FIG. 2

GRIPPING MECHANISM FOR STRAPPING MACHINE BACKGROUND OF THE INVENTION This invention relates to an apparatus for tensioning a strap around an article, such as a stack of bricks and lumber, where high tension is needed. In tensioning of straps about articles, it is customary to hold one end of the strap with a suitable gripping device and apply tension adjacent an overlapping portion of the strap. When a predetermined amount of tension has been applied to the strap, one or more seals are crimped to the overlapping portions of the strap by crimping jaws and the strap is severed from the strap supply. Automatic strapping machines or tools for performing these functions are disclosed in Orban et al. U.S. Pat. No. 3,493,014, and Ericsson et al. U.S. Pat. No. 3,198,218, both of which were granted to the assignee of this invention. Both of these patents disclose motor-driven tension mechanisms wherein a high tension may be exerted on the strap before overlapping portions are connected by seals.

In utilizing devices of this type, some problems have been encountered in holding the strap while a high degree of tension is being applied thereto. One method of holding the free end of the strap consists of a pair of gripping jaws that are pivoted about spaced parallel axes on opposite sides of the strap and the free ends of the jaws force the end of the strap against a support or anvil.

In order to provide the necessary gripping force between the jaws and the anvil, the jaws usually have teeth defined on the strap engaging surface. However, it has been found that in employing prior art devices, when a high degree of tension is applied to the strap, the strap may be weakened in the area where the gripping jaws are applied and may ultimately rupture while the package is being transported and handled.

SUMMARY OF THE INVENTION The present invention contemplates an improved gripping device for holding the free end of the strap while it is being tensioned and the gripping device is constructed so that the retaining force or holding force is increased as the tension on the strap is increased. Stated another way, the gripping device is selfenergizing so that the gripping force applied to the strap increases and is directly proportional to the amount of tension applied'to the strap.

The gripping device or mechanism consists of a pair of gripping jaws that are pivoted on pins carried by the support or frame of the strapping tool or machine and the openings that receive the pivot pins are tapered from the opposite sides of the gripping jaws towards the center thereof, so that the jaws may be pivoted between two gripping positions about an axis generally normal to the longitudinal direction of tension being applied to the strap.

According to another aspect of the invention, the gripping or engaging surfaces have notches that define gripping teeth for engaging the strap. The gripping teeth on each engaging or gripping surface are arranged so that successive teeth progressively engage the strap as the jaws are pivoted about the normal pivot axis. This results from increasing the dimensions between the tips of the respective teeth and the axis of the opening as the distance increases between the respective tooth tips and a plane running through the normal pivot axis and parallel to the leading edge of the jaw or tension side of the strap.

With the gripping jaws as above described, the gripping jaws will be pivoted in the direction of strap tension from one extreme position toward another extreme position as the tension on the strap increases, which causes the distance from the strap engaging surface to the normal pivot axis to become shorter and position the teeth to apply more gripping force to the strap. The arrangement of the gripping teeth on the gripping surface is such that no single tooth will penetrate deeply enough to damage the strap to any substantial extent. This arrangement allows for producing the required gripping force for the tension applied to the strap without seriously weakening the strap.

According to another aspectof the invention, the jaws are pivoted between open and gripping positions about the pivot pins through means that cooperate with the sides of the jaws that are adjacent the tension side of the strap so that the jaws are always rotated about the normal pivot axis, as well as the pins when the jaws are opened. The position of the connection of the pivot means on the jaws with respect to the normal pivot axis insures that the jaws will return to the de-energized position each time the jaws are moved to the open position.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS FIG. 1 of the drawings shows a fragmentary sectional view of a power strapping machine having the gripping mechanism of the present invention incorporated therein;

FIG. 2 is a side elevation of the gripping mechanism in an open position;

FIG. 3 is a view similar to FIG. 2 showing the gripping mechanism in its closed position before tension is applied to the strap;

FIG. 4 shows a sectional view of the gripping mechanism in the closed position, as viewed above line 44 of FIG. 5, before tension is applied to the strap;

FIG. 5 is a view similar to FIG. 4 showing the'gripping mechanism after tension has been applied to the strap and as viewed along line 5--5 of FIG. 6;

FIG. 6 is a view similar to FIG. 3 showing the closed gripping mechanism after tension has been applied to the strap; and

FIG. 7 (appearing with FIG. 1) is an enlarged sectional view, as viewed generally along lines 7-7 of FIG. 2, showing the details of the gripping surface of the gripping jaws.

DETAILED DESCRIPTION While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one embodiment, with the understandingthat the present disclosure is to be considered as an exemplification of the principles of disclosure of either of said patents for a more detailed description of the structure that is common to this application and the aforesaid patents, such as the tension mechanism and the sealing mechanism.

Referring to FIG. 1 of the drawings, strapping mechanism consists of housing or support 12 that supports gripping or holding mechanism 14, tensioning mechanism 16, and sealing and shearing mechanism 18. The sealing and shearing mechanism 18, as well as tensioning mechanism 16 have only illustratively been shown and for a more detailed description, reference may be had to either of the above mentioned patents, particularly to the Orban et al. patent.

According to the present invention, strap gripping mechanism or device 14 consists of left and right

hand gripping jaws

20 and 22 that are supported in slot 24 which extends crosswise of the longitudinal direction of strap 26 in housing or support 12.

Gripping jaws

20 and 22 are pivoted on pins 30 that define spaced parallel axes extending longitudinally of the direction of the strap, and each of the

jaws

20 and 22 defines an opening 32 for receiving a pin 30. The lower ends 34 of

jaws

20 and 22 extend below anvil 38 and have inwardly directed portions that define strap engaging surfaces 40. Strap engaging surfaces 40 are adapted to engage the lower surface of strap end 42 to force the strap end against the lower surface of anvil 38. Anvil 38 also has an elongated opening 44 defining a guide for the strap.

The strap gripping device also includes jaw actuating means 50 for pivoting

gripping jaws

20 and 22 about pins 30 between open and gripping positions. Jaw actuating means 50 consists of a pair of

links

52 and 54 that are respectively pivotally connected at 59 and 58 to the upper ends of

jaws

20 and 22. The opposite ends of

links

52 and 54 are pivotally interconnected and also connected to actuating member 60 through a common pivot pin 62. Actuating member 60 may be in theform of a piston rod connected at its upper end to a piston not shown) and mounted in a cylinder (not shown) for reciprocal movement therein.

Links

52 and 54 are in the form of a stepped toggle linkage that cooperates with gripping

jaws

20 and 22 on the strap tension or leading side of the jaws, for a purpose that will be described later.

According to the present invention, the

gripping jaws

20 and 22 are connected to pivot pins 30 in a manner to accommodate pivotal movement about an axis that extends transversely of, or generally normal to the longitudinal direction of strap 26 to produce the selfenergizing effect described above. This results in increasing the gripping force in direct proportion to the tension applied to the strap. Thus, when the tension on the strap is low, the gripping force is low, while high tension on the strap will produce high gripping force. The result is that the gripping force is at all times sufficient to hold the strap, but is never excessive when considered in relation to the strap tension. For this purpose, openings 32 in gripping

jaws

20 and 22, each has first and second reduced

tapered portions

70 and 72 leading from each side of the associated jaw with the reduced

tapered portions

70 and 72 merging with each other in a plane (2:) which extends through the transverse pivot axis. Each jaw also has

opposed surfaces

74 and 76 defining opposite sides and surface 74 has a reduced taper leading from the opening downwardly to the gripping portion, while surface 76 has a reduced taper extending away from the opening toward thedrive mechanism. The reduced tapers divide the respective surfaces into

first surface segments

74a and 76a and

second surface segments

74b and 76b.

Surface segments

74a and 76b define spaced parallel planes, while surface segments 74b and 760 also define spaced parallel planes and plane (1:) extends parallel to surface segments 76b.

With the tapering of the openings and the taper of opposed surfaces or opposite sides of the gripping jaws as described above, the closed jaws may rotate from a first gripping position about an axis extending generally normal to the longitudinal direction of the strap, while the gripping jaws are in the closed or gripping position. The first gripping position is shown in FIG. 4 of the drawings. In the first gripping position shown in FIG. 4, surface segment 74a is in engagement with planar wall 77 of slot 24 while surface segment 76b is in engagement with planar wall 78 of slot 24. In this position, the leading edge or surface segment 74b of each jaw defines a small acute angle (a) with respect to wall 77, which extends normal or perpendicular to the longitudinal direction of strap 26. This acute angle is determined by the taper on the opposite ends of the jaw. As the tension on the strap increases, the jaws will pivot about the normal pivot axis to decrease the small acute angle (a). The pivotal movement of

jaws

20 and 22 about the normal pivot axis is accomplished by relative movement between the jaws and

links

52,54 from the position shown in FIG. 4 to that shown in FIG. 5. This relative movement may be accommodated by having the pivotal connections in the form of pins received in enlarged openings in the jaws and/or the links or having integral pins or

jaws

20, 22 received in openings in the links which will allow axial movement of the pins relative to the openings. As will be discussed below, the gripping force rapidly increases as the angle (a) approaches zero. The maximum second extreme gripping position is shown in FIG. 5, wherein the angle (a) is zero. The jaws are preferably. designed so that the gripping jaws approach, but never reach the second extreme position. With this design, the gripping force is proportional to. and increases as the tension on the strap is increased.

In the specific embodiment illustrated, the combined taper of the tapered

portions

70, 72 is equal to the angle (a) described above. For example, by having a combined taper of 2% for the

tapered portions

70, 72 of the opening in the jaw and having an equal taper of 25? on opposite ends of the jaws, the maximum pivotal movement about the normal pivot axis would be 2%". It will be appreciated that if the angles are not equal, the smallest angle will determine the maximum amount of pivot for the jaws about the normal pivot axis.

The gripping force for the gripping jaws is provided by teeth on the gripping surface of the jaws that are dimensioned to provide increased gripping force in response to increased strap tension, but will not penetrate a sufficient amount to materially weaken or damage the strap. With particular reference to FIG. 7, it will be noted that portions 34 of gripping

jaws

20 and 22, each has a plurality of notches 80 that define a plurality of teeth 82. The teeth 82 are of progressively decreasing height from plane (2:) which extends through the normal axis extending transversely of the longitudinal direction of the strap or parallel to trailing surface segment 76b, to the leading edge or surface segment 74b of each gripping jaw, while the teeth between plane (x) and surface segment 76b are of equal height. Stated another way, plane (x) divides gripping surface 40 into first and

second surface portions

40a and 40b and surface portion 40a is planar, while surface portion 40b defines a generally arcuate surface that has its axis located a substantial distance below the jaw. The generally arcuate surface portion is produced by progressively decreasing the dimension between successive tooth tips 84 and the axis of opening 32 as the dimension from leading side 74 of the jaw to the plane (x) is increased. The arrangement is such that in the first gripping position shown in FIG. 4, first planar surface portions 40a are located parallel to the axes of pins 30 and all teeth on surface portions 40a are in equal engagement with the strap.

As the jaws are pivoted from one extreme pivoted position the successive teeth on surface portion 40b progressively grip the strap to substantially increase the gripping force without having any particular tooth penetrate sufficiently to damage the strap to any material extent. The arrangement of the teeth on the gripping surface produces the self-energizing feature or increased gripping force that is proportional to the tension applied to the strap.

The two surface portions on each gripping jaw are of considerable advantage over a single continuous planar surface. With a single planar surface, the teeth on' the trailing edge of each jaw would penetrate more than the teeth adjacent the leading edge in the first extreme pivoted gripping position while the reverse would be true in the second extreme pivoted gripping position.

The gripping force for each tooth that is in gripping engagement with the strap at any given pivoted position is again a function of the tension applied to the strap. Each tooth has what may be referred to as an energizing angle which is defined by-a plane extending perpendicular to the strap through the normal pivot axis and a second plane extending through the particular tooth tip and the normal pivot axis. The gripping force applied by each tooth is approximately equal to the force induced by the strap tension times the cotangent of the energizing angle for that tooth.

It will be appreciated that the energizing angle for each tooth between the plane (1:) and the leading side of the jaw will be increasing and the gripping force for a given tooth will be decreasing. However, because additional teeth are gripping the strap, the total gripping force will be increasing.

Actual tests have shown that the arrangement of the gripping teeth as described above will insure that the strap is not significantly weakened in the area where the strap has been gripped during the tensioning and strapping operation.

Returning now to jaw actuating means 50 discussed above, as was indicated above, drive links 52 and 54 cooperate with the normal pivot axis, as well as the pivot axes defined by the pins, while moving from the closed and energized position shown in FIG. 5 to the open position shown in FIG. 1. By having the slipped

toggle linkage

52, 54 on the leading edges or tension sides of the jaws, the jaws are easily disengaged even when fully energized, because the jaws will be pivoted about the normal pivot axis at the same time they are pivoted about the pin axes. This pivotal movement will result from the fact that the upward opening force will actually be applied to the left of the normal pivot axis, as viewed in FIG. 5, so that the jaws will pivot about the normal pivot axis as well as pivot pins 30 during the opening movement. The effectis that the jaws will always move to the de-energi'zed position shown in FIG. 1, when they are moved to the fully open position.

While an illustrated embodiment of the invention has been shown and described, it will be appreciated that numerous modifications come to mind without departing from the spirit of the invention. For example, the openings 30 could have a continuous taper between the leading and trailing edges of the gripping jaw. This would locate the normal pivot axis along the trailing edge of the gripping jaw. Stated another way, the jaw would essentially pivot about the rear side of the gripping jaw. With a reduced tapered opening from one side to the other, the tooth height would be progressively increased from the leading to the trailing edge of the gripping jaw. Of course, the reduced taper could be in the opposite direction from the trailing edge to the leading edge and this would reverse the normal pivot axis placing it along the leading edge of each gripping It should also be noted that, while the gripping mechanism has been specifically shown and describedin connection with the power strapping machine, the gripping mechanism could be utilized in any area where increased gripping force is desired in response to tension being applied to a member that is being gripped.

I claim:

1. A gripping mechanism for holding one end of a strap and increasing the gripping force as tension is being applied to said strap comprising, a support; an anvil on said support; a pair of gripping jaws; pivot pins fixed with respect to said support adjacent said anvil and having their axes extending parallel with the longitudinal direction of the strap, each of said gripping jaws having an opening for receiving a pin; and means for pivoting said gripping jaws about said pins between open and gripping positions to grip the strap between said jaws and said anvil, said openings being tapered to allow said jaws to bepivoted about an axis generally normal to the longitudinal direction of said strap while in a gripping position.

2. A gripping mechanism as defined in claim 1, in which said gripping jaws are initially in a first gripping position and said jaws are pivoted about said normal axis from said first gripping position as a function of the tension applied to the strap.

3. A gripping mechanism as defined in claim 2, in which each gripping jaw has a gripping surface engaging said strap and said gripping surface has teeth, said teeth being arranged so that the teeth gripping said strap in any pivoted position about the normal axis will not penetrate the strap a sufficient amount to materially weaken the strap.

4. A gripping mechanism as defined in claim 1, in which said openings each has a reduced taper leading from opposite sides of the gripping jaw and merges at a reduced portion intermediate the opposite sides.

5. A gripping mechanism as defined in claim 1, in which said gripping jaws each has a leading side adjacent the tension side of each strap, and'said gripping jaws have notches defining teeth on a gripping surface for engaging the strap and in which said teeth are arranged so that the gripping surface between the leading side of each jaw and a plane extending longitudinally of the jaw through the normal pivot axis is generally arcuate so that a different portion of said generally arcuate surface is in engagement with said strap as the jaws are pivoted from a first gripping position with respect to said normal axis.

6. A gripping mechanism as defined in claim 5, in which said openings in the jaws have a reduced taper leading from the leading and trailing sides of said gripping jaws and said reduced tapers merge at the plane extending longitudinally of the jaws and in which said gripping surfaces each has a planar portion between the trailing edge of the jaw and the longitudinal plane of the jaw so that the teeth in the planar portions of the gripping surface are in equal engagement with the strap when the jaws are in the first gripping position.

7. A gripping mechanism as defined in claim 6, in which said means cooperates with said jaws between the leading edge and the longitudinal plane so that said jaws are automatically returned to the first gripping position as the jaws are returned to an open position.

8. A strapping device for tensioning a strap loop about anarticle comprising a support having an anvil defining a guide for said strap and gripping means for holding a free end of said strap with respect to said anvil and applying increased gripping force as tension is applied to said strap, said gripping means including a pair of jaws; fixed pivot pins on said support and having spaced parallel axes extending longitudinally of the direction of said strap with said jaws respectively having openings receiving said pins, said openings being tapered between opposite sides of said jaws, said jaws each having a portion extending below said anvil adjacent opposite edges of said strap; and means for pivoting said portions of said jaws about said pins between open and gripping positions to grip the free end of said strap between said portions and said anvil, said tapered openings accommodating pivotal movement of said portions of said jaws from a first gripping position about an axis generally normal to the longitudinal direction of said strap to increase the gripping force on said strap as tension is applied thereto.

9. A strapping device as defined in claim 8, in which each opening has a reduced taper leading from each side of the associated jaw.

10. A strapping device as defined in claim 9, in which said support has an elongated slot extending across the longitudinal direction of said strap and said jaws are pivoted between the open and the first gripping position in said elongated slot by said means, and in which said jaws have a reduced taper between opposite sides thereof and below the normal axis to accommodate the pivotal movement about the normal axis from the first gripping position.

11. A strapping device as defined in claim 10, in which said means for pivoting includes a pair of links pivoted to said jaws adjacent the tension side of the strap for each jaw at the upper end and pivoting means connected to said links, said links insuring that said jaws are returned to said first gripping position in said slot when the jaws are moved to the open position.

12. A strapping device as defined in claim 8, in which the free ends of said jaws each has a gripping surface engaging ,said strap and each gripping surface has notches extending perpendicular to the longitudinal direction of said strap, said notches defining teeth on the gripping surface, said teeth being arranged so that the teeth gripping the strap in any pivoted position about said normal pivot axis will not damage the strap.

13. A strapping device as defined in claim 12, in which said teeth have tips which have a decreasing dimension from the axis of the opening in the associated jaw as the dimension between the tooth tip and the leading side of the jaw adjacent the tension side of the strap is increased.

14. A strapping device as defined in claim 13, in which each opening has a first and second reduced taper respectively extending from the ieading side and a trailing side of the associated jaw with said reduced tapers merging along a plane extending longitudinally of the jaw through the normal pivot axis and in which said teeth define a planar surface portion between the longitudinal plane and trailing side of the jaw, said planar surface portion extending parallel to the lower edge of said second taper so that the teeth of said planar surface portion are in equal engagement with said strap in thefirst gripping position of said jaws.

15. A strapping device as defined in claim 13, in which said means for pivoting includes a pair of links respectively connected to the leading sides of the jaws adjacent the tension side for the strap so that said jaws are pivoted about said normal pivot axis while being pivoted from the gripping position to the open position.

16. A gripping jaw for use in gripping a strap while tension is being applied thereto comprising a member having an upper portion and a lower portion, said upper portion having a tapered opening with the axis of the opening extending between opposite sides of the upper portion, said lower portion having a gripping surface directed toward said axis with said gripping surface having a plurality of teeth, said teeth having tips spaced from said axis by different dimensions to accommodate pivotal movement of the jaw about an axis extending generally normal to the axis of the opening so that the teeth gripping the strap in any pivoted position will not damage the strap.

17. A gripping jaw as defined in claim 16, in which the tips of said teeth are of decreasing dimensions from the opening axis as the distance from one side of the jaw is increased.

18. A gripping device as defined in claim 17, in which the opening has a reduced taper from each side of the jaw with the reduced tapers merging along aplane extending through the normal pivot axis and in which the. tooth tips between the opposite sides of said jaws and the plane are equally spaced from the opening axis.

Claims

1. A gripping mechanism for holding one end of a strap and increasing the gripping force as tension is being applied to said strap comprising, a support; an anvil on said support; a pair of gripping jaws; pivot pins fixed with respect to said support adjacent said anvil and having their axes extending parallel with the longitudinal direction of the strap, each of said gripping jaws having an opening for receiving a pin; and means for pivoting said gripping jaws about said pins between open and gripping positions to grip the strap between said jaws and said anvil, said openings being tapered to allow said jaws to be pivoted about an axis generally normal to the longitudinal direction of said strap while in a gripping position.

5. A gripping mechanism as defined in claim 1, in which said gripping jaws each has a leading side adjacent the tension side of each strap, and said gripping jaws have notches defining teeth on a gripping surface for engaging the strap and in which said teeth are arranged so that the gripping surface between the leading side of each jaw and a plane extending longitudinally of the jaw through the normal pivot axis is generally arcuate so that a different portion of said generally arcuate surface is in engagement with said strap as the jaws are pivoted from a first gripping position with respect to said normal axis.

8. A strapping device for tensioning a strap loop about an article comprising a support having an anvil defining a guide for said strap and gripping means for holding a free end of said strap with respect to said anvil and applying increased gripping force as tension is applied to said strap, said gripping means including a pair of jaws; fixed pivot pins on said support and having spaced parallel axes extending longitudinally of the direction of said strap with said jaws respectively having openings receiving said pins, said openings being tapered between opposite sides of said jaws, said jaws each having a portion extending below said anvil adjacent opposite edges of said strap; and means for pivoting said portions of said jaws about said pins between open and gripping positions to grip the free end of said strap between said portions and said anvil, said tapered openings accommodating pivotal movement of said portions of said jaws from a first gripping position about an axis generally normal to the longitudinal directiOn of said strap to increase the gripping force on said strap as tension is applied thereto.

12. A strapping device as defined in claim 8, in which the free ends of said jaws each has a gripping surface engaging said strap and each gripping surface has notches extending perpendicular to the longitudinal direction of said strap, said notches defining teeth on the gripping surface, said teeth being arranged so that the teeth gripping the strap in any pivoted position about said normal pivot axis will not damage the strap.

14. A strapping device as defined in claim 13, in which each opening has a first and second reduced taper respectively extending from the leading side and a trailing side of the associated jaw with said reduced tapers merging along a plane extending longitudinally of the jaw through the normal pivot axis and in which said teeth define a planar surface portion between the longitudinal plane and trailing side of the jaw, said planar surface portion extending parallel to the lower edge of said second taper so that the teeth of said planar surface portion are in equal engagement with said strap in the first gripping position of said jaws.

18. A gripping device as defined in claim 17, in which the opening has a reduced taper from each side of the jaw with the reduced tapers merging along a plane extending through the normal pivot axis and in which the tooth tips between the opposite sides of said jaws and the plane are equally spaced from the opening axis.