US3285650A

US3285650A - Hook

Info

Publication number: US3285650A
Application number: US426081A
Authority: US
Inventors: Perry L Dukes
Original assignee: Somaeco D Inc
Current assignee: Somaeco D Inc
Priority date: 1965-01-18
Filing date: 1965-01-18
Publication date: 1966-11-15
Anticipated expiration: 1983-11-15

Description

Nov. 15, 1966 P. L. DUKES 3,285,650

noon I Filed Jan. 18, 1965 5 Sheets-Sheet 1 INVENTOR PERRY L. DUKES BY M ATT'YS.

Nov. 15, 1966 P. L. DUKES 3,285,650

HOOK

Filed Jan. 18, 1965 5 Sheets-Sheet 2 INVENTOR. PERRY L. DUKES ATTYS.

Nov. 15 1966 Filed Jan. 18, 1965 P. L. DUKES 3,285,650

HOOK

5 Sheet 3 V VENTO P E R RY L. DU K E 8 Nov. 15, 1966 P. L. DUKES 3,235,650

HOOK

Filed Jan. 18, 1965 5 Sheets-Sheet INVENTOR. PERRY L. DUKES ATTYS.

Nov. 15, 1966 P. 1.. DUKES 3,285,650

INVENTOR. PERRY L. DUKES BY% W) AT TYS- United States Patent corporation of Washington Filed Jan. 18, 1965, Ser. No. 426,081 7 Claims. (Cl. 29483) This invention relates to a novel hook for use on a hoist adapted to lift an article having a rigid upright bail.

The present invention is concerned with a hook for carrying heavy objects, such as buckets of concrete, by means of a mobile hoist or crane. The hook is adapted to automatically engage the ball of an article and to automatically release the bail when the article has been safely lowered into ground contact. It is specifically designed to prevent the possibility of disengagement of the article while in a lifted position.

It is a first object of this invention to provide such a hook that can fit down upon a bail With a limited amount of precision, the jaws of the hook being adapted to receive the bail over a wide latitude of positions.

Another object of this invention is to provide a hook whose jaws are automatically closed due to their own weight at all times except when the hook has been lowered onto the bail so that the weight of the hook itself rests on the bail. In this manner, accidental disengagement of the hook while the article is in a lifted position is entirely eliminated.

Another object of this invention is to provide such a device that can be manufactured with a rigid protective housing so that the interior moving elements of the hook are not subject to accidental damage.

Another object of this invention is to provide such a device that is rather simple from a mechanical standpoint and which can be subjected to heavy usage without the danger of accidental damage. If damage should occur to any part, the load carried by the hook cannot disengage accidentally while in a lifted position.

These and further objects will be evident from a study of the following disclosure which relates to the illustrated embodiment of the invention. It is to be understood that this embodiment of the invention, while exemplary of the apparatus desired, is not intended to restrict or limit the scope of the invention defined in the appended claims.

In the drawings:

FIGURE 1 is a side elevation view of the hook;

FIGURE 2 is an end view of the hook as seen from the right in FIGURE 1; i

FIGURE 3 is a top view of the hook shown in FIG- URE 1;

FIGURE 4 is the bottom view of the hook shown in FIGURE 1;

FIGURES 5, 6 and 7 are enlarged sectional views of the hook as seen along line 55 in FIGURE 2, illustrating the operating positions of the hook;

FIGURE 5 shows the entry of a bail past the movable J aws;

FIGURE 6 shows the carrying of a bail by the hook and jaws;

FIGURE 7 shows the jaws in their fully retracted positions prior to removal of the bail from the hook;

FIGURE 8 is an enlarged sectional view taken along line 88 in FIGURE 1; and

FIGURE 9 is an enlarged sectional view of the hook as seen along line 99 in FIGURE 7.

The hook shown in the attached drawings was designed principally for use on heavy construction projects for carrying large buckets of concrete on a conveying hoist. The sheer weight of such buckets makes imperative the safe transporting of the loads over a work area. They "ice must be carried by a hook which is incapable of accidentally dropping the bucket. The present book was designed principally with that purpose in mind, also taking into account the usage made of such a hook on a construction project, and the requirement for simple repair procedures when required.

The hook basically comprises a pair of upright side plates 10 having a general rectangular configuration. The upper ends of the side plates 10 are connected by a cover 11 which serves as a structural brace to rigidly space the plates 10 from one another. The plates 10 are also connected by protective end plates 12 that join the plates 10 at their respective ends. The cover 11 and plates 12 are connected to the side plates 10 by a series of releasable bolts so that they can be removed when necessary. I

The two side plates 10 are identical to one another. Each includes a tapered slot 13 that is enlarged at its lower end. The slots 13 extend upwardly from the bottom end of each side plate in registry with one another and terminate short of the upper end of each side plate 10 as shown in FIGURE 1. Along the upper portion of the sides of each slot 13 are connecting inside plates 14 joining the two side plates 10 for additional rigidity. Plates 14 also serve to protect the interior of the hook from possible damage. The slots 13 and the boundary plates 14 define a throat in the rigid structure just described within which can be received the bail of the article to be hoisted. The enlarged taper at the bottom end of the slot 13 permits the plates 10 to be lowered onto the bail with a minimum of accuracy, since the hook will be. automatically guided on the bail as it is lowered into direct engagement with it.

The hook has a pair of heavy jaws 15 pivoted between the side plates 10 adjacent to their lower ends. The two jaws 15 are located at each side of the slot 13 respectively. Jaws 15 are complementary to one another and each includes an upper surface 16 adapted to cradle a bail 17 for lifting purposes as shown in FIGURE 6. The two jaws 15 are identical. Each terminates at an overlapping extended hook portion 18 capable of carrying a bale alone .if necessary. The respective portions 18 are each half the thickness of the respective jaws 18 and are formed at an abutting surface 21 on each of the jaws 15, the abutting surfaces 21 being in actual contact with one another when the jaws 13 .are fully closed (see FIGURE 6).

Each jaw 15 is pivotally mounted between the two side plates 10 about a horizontal axis defined by a thin axle 22 rotatably journalled by each side plate 10. Since the width of the jaws 15 is less than the separation between the side plates 10, pins 23 received within apertures in each jaw 15 protrude to the sides of the jaws. Pins 23 rest on welded bars 20 fixed to the inner surfaces of the plates 10 to limit downward motion of the respective jaws, which do not always drop closed in unison. It is important to note, as can be seen in FIGURE 7, that the center of gravity of each jaw 15 is always located between I the central axis of the pin axle 22 and the throat formed by slots 13 and is above the elevation of the axis of the pin axle 22. Thus, each jaw 15 is biased to its closed position as shown in FIGURE 6, due to its own weight. The jaws 15 are freely piv-ote-d about the pin axles 22 and upward motion of each jaw 15 is resisted only by the weight of that jaw.

The only mechanical operating device contacted by the bail 17 in the operation of this hook is an abutment beam 24 that is located between the hook 10 and above the jaws 15. The abutment beam 24 has a reinforcing contact block 25 in the throat formed by the slots 13. The block 25 extends into the respective slots 13 and tenminates flush with the outer surfaces of the side plates 10.

Fixed to each end of the abutment 24 are downwardly extending end plates 26 formed as rigid extensions of beam 24. Mounted at the top and bottom portions of end plates 26 are rollers 27 adapted to roll along the inside surfaces of the respective side plates 10 contacted thereby. The rollers 27 provide the necessary guidance and frictional reduction for proper vertical movement of beam 24 relative to the plates 10.

Fixed to the inside surface of the side plate 10 shown in the background in FIGURES through 7 are a pair of laterallyspaced vertical guide plates 28. The guide plates 28 are contacted by upper and lower rollers 30 mounted on brackets 31 fixed to the abutment beam 24. The vertically aligned rollers 30 assure proper vertical guidance for the abutment beam 24 as it moves relative to the plates 10.

The abutment beam 24 is biased in a downward direction both by its own weight and by a compression spring 32 wrapped about a telescoping rod 33 whose upper section is fixed to the cover 11 and Whose lower section is fixed to the beam 24. At each end of the beam 24 are a pair of hydraulic retarding cylinders 34. The upper end of each cylinder 34 is mounted on a cross shaft 35 between the plates 10. The lower end of each cylinder 34 is connected to a bracket 36 on the beam 24. The retarding cylinders 34 slow downward movement of the beam 24 relative to plates and also limit the extent of such downward movement by defining the lowermost position of the beam 24. This position is that illustrated in FIGURES 5 and 6.

The jaws must be mechanically opened in order to release the bail 17. This is accomplished by means of a roller chain 37 on each jaw 15. Since the various mechanical devices that control each jaw 15 are identical, although being entirely independent, the apparatus concerned with a single jaw will be described below, it being understood that an identical apparatus is utilized to control the remaining jaw 15.

One end of the roller chain 37 is fastened at 38 to the upper end of the jaw 15. The remaining end of the roller chain 37 is fastened to an anchor bolt 44 slidably received within a roller 43 and spring loaded in a longitudinal direction to absorb shock in the chain 37. The roller 43 is rotatably carried between the side plates 10 by a pair of spacing collars 41 and stub shafts 40 that extend into the side plates 10 as we'll as the roller 43. The spacers 41 are non-rotatably carried between the plates 10.

The chain 3 7 is entrained over a roller 45 journalled on a shaft 46 which in turn is rotatably carried by bearings 47. The bearings 47 are located on mounting plates 48 fixed to the abutment beam 24. Thus, upward movement of the beam 24 and roller 45 will cause the lower end of the chains 37 to lift the jaws 15 to retract the jaws 15 between the plates 10 at positions located beyond the throat formed by the slots 13.

Each jaw 15 is provided with a pair of retaining pins 50 fixed to and protruding outwardly along each side surface thereof. Each pin 50 is adapted to be engaged by a movable latch member 51 that is pivoted to the side plates 10 by a shaft 52. The latch members 51 are U-shaped, and straddle the intermediate portions of the jaws 15. Each latch member 51 is provided with a pair of notches 53 in registry with one another repectively adapted to receive the retaining .pins '50 when the jaws 15 are fully opened by action of the chain 37 (FIG- URE 7).

The latches 51 are biased upwardly by means of tension springs 54 connected between the latches 51 and an adjustable anchor bolt 55 on each of the spacers 41 previously described.

At the lower end of each plate 26 on the abutment beam 24 is an adjustable release bolt 56 adapted to contact the top outer surface of the latch member 51 when the beam 24 is in its lowermost position (FIGURES 5 and 6). When the beam 24 has been raised by contact 4 with a bail 17, the bolts 56 will be disengaged from the latch members 51 and the upward movement of the latch members 51 will be limited only by engagement with the retaining pins 50 on the jaws 15.

The operation of the hook, suspended from a hoist by means of top apertures 57, is completely automatic and requires no external manipulation other than operation of the hoist itself. As shown in FIGURE 5, initial lowering of the hook over a bail 17 will cause one or both jaws 15 to be freely pivoted upwardly so that the bail 17 can pass their respective abutting surfaces 21. The t-apered nature of the throat formed by slots 13 will .permit the hook to accommodate a bail 17 without extremely accurate placement of the hook relative to the bail. As the hook is lowered over the bail 17, the slot 13 will guide the hook into the proper position and the jaws 15 will close due to their own weight when they are no longer contacted by the bail 17.

The hook and load is lifted with the jaws in their closed position as shown in FIGURE 6. The article is then transferred to the position where it is desired and it is impossible for the jaws 15 to release since their own weight, as well as the force exerted by the 'bail 17, will serve to maintain the jaws 15 in their respective abutting positions.

The jaws 15 can only be released when the weight of the bail 17 has been removed from them. This is accomplished when the article carried by the bail 17 has again been provided with vertical support, so that the weight of the hook can cause the beam 24 to be raised due to downward movement of the plates 10 over the bail 17 as shown in FIGURE 7. The upward movement of the beam 24 relative to the downwardly moving plates 10 will cause the chains 37 to pivot the jaws 15 to the position shown in FIGURE 7 where the retaining pins 50 are received within the notches 53 of the latch members 51. At this time, the hook can be raised from the bail 17 since the jaws 15 are restrained from falling back across the throat formed by the slots 13 due to the mechanical engagement of the pins 50 by the latch members 51.

When the weight of the beam 24 no longer rests on the bail 17 due to lifting of the hook, the beam 24 will begin to move downwardly due to its own weight. The downward movement of beam 24 is slowed by the hydrulic retarding cylinders 34, but eventually the latch members 51 will be contacted by the downwardly moving bolts 56 at the lower ends of the plates 26 and the latch members 51 will be pivoted downwardly to release the respective pins 50. The jaws 15 will then pivot back into the positions shown in FIGURE 6 and the apparatus will again be in condition for reception of another bail prior to lifting it. I

-In actual practice, the retarding cylinders 34 are designed to permit the downward movement of beam 24 over a distance of six inches in approximately 5 seconds. This is suflicient time to lift the hook from its position over the bail 17 and permits the hook to be immediately used to lift another article.

It is important to note that the pins 50 cannot be engaged by the latch member 51 at each side of the hook unless the beam 24 has been raised by lowering the hook into contact wit-h the bail in the manner shown in FIG- URE 7. As shown in FIGURE 5, a jaw 15 can be urged by a bail 17 to a position flush with the slot 13, without accidentally engaging the .pins that would hold it in its open position. This is insured by the fact that additional movement, as shown in FIGURE 7, is required in order to engage the pins 50 and also by the fact that the beam 24 will be in its lowermost position so that the latch members 51 are below the path of movement of the pins 50. There is no way in which the jaws 15 can be accidentally locked in an open position except by raising the beam 24.

The jaws 15 abut one another in their closed position as shown in FIGURE 6 during the time in which a load is carried by them. If a connection in one of the chains 37 should break, the jaws would not move and the load would not be dropped. The jaw 15 support each other and provide a rigid bridge across the throat that can safely carry large loads without danger of disengagement.

Modifications could be made in this basic structure without deviating from the basic concept of an efiicient, safe and strong hook for construction hoisting purposes. For these reasons only the following claims are intended to limit or define the scope of my invention.

Having thus described my invention, I claim:

1. A hook for a hoist adapted to carry an article having an upstanding bail comprising:

a pair of upright side plates fixed relative to one another in a spaced relation, each of said plates being provided with a vertically disposed slot extending from the lower end thereof and terminating short of the upper end of said plate, the slots being in registry with one another and providing an open throat to receive a bail of an article to be lifted;

a pair of movable jaws pivotally mounted between said plates about parallel transverse horizontal axes, said jaws being located adjacent the lower ends of said plates :at each side of the slots forming said throat, said jaws each being biased to a first position protruding into the throat and being each movable to a second position fully recessed beyond said throat;

a vertically reciprocable abutment member mounted on said plates above said jaw, said abutment member being in a location bridging said throat;

motion limiting means operatively connected between said abutment means and said plate to limit the extent of downward motion of said abutment member relative to said plates;

lifting means operatively connected between said abutment member and said jaws to cause said jaws to be moved to their second positions in response to predetermined upward motion of said abutment member relative to said plates;

latch means on said jaws and said plate engageable when said jaws attain their second positions to selectively prevent motion of said jaws relative to said plates;

and means on said abutment member operatively engageable with said latch means to release said latch means when said abutment member is in its lowermost position relative to said plates and allow said jaws to return to their first positions.

2. A hook for a hoist adapted to carry an article having an upstanding bail, comprising:

a pair of parallel upright side plates fixed relative to one another in a spaced relation, each ofsaid plates being provided with a vertically disposed slot extending from the lower end thereof and terminating short of the upper end of said plates, the slots being in registry with one another and providing a throat to receive a bail of an article to be lifted;

a pair of complementary movable jaws pivotally mounted between said plates about parallel transverse horizontal axes, said jaws being located adjacent the lower end of said plates at each side of the slots forming said throat, each jaw including a complementary abutting surface adapted to contact one another at the center line of the throat formed by the slots to thereby bridge the area formed by the slots, the center of gravity of each jaw being located between its pivotal axis relative to said plates and the throat formed by said slots and being at an elevation above that of its pivotal axis on said plates, said jaws each being biased to a first position protruding into the throat due to the location of their centers of gravity and being movable to a second position recessed beyond the edges of said plates defining said slots;

a vertically receprica ble abutment member mounted on said plates above said jaw, said abutment member being in a location bridging said throat;

latch means on said jaws and said plates engagea ble when said jaws attain their second positions to selectively prevent motion of said jaws relative to said plates;

3. A hook for a hoist adapted to carry an artcle having an upstanding bail, comprising:

a pair of parallel upright side plates fixed relative to one another in a spaced relation, each of said plates being provided with a vertically disposed slot extending from the lower end thereof and terminating short of the upper end of said plates, the slots being in registry with one another and providing a throat to receive a bail of an article to be lifted;

a pair of complementary movable jaws pivotally mounted between said plates about parallel transverse horizontal axes, said jaws being located adjacent the lower end of said plates at each side of the slots forming said throat, each jaw including a complementary abutting surface adapted to contact one another at the center line of the throat formed by the slots to thereby bridge the area formed by the slots, the center of gravity of each jaw being located between its pivotal axis relative to said plates and the throat formed by said slots and being at an elevation above that of its pivotal axis on said plates, said jaws each being biased to a first position protruding int-o the throat due to the location of their centers of gravity and being movable to a second position recessed beyond the edges of said plates defining said slots;

spring means operatively connected between'said plates and said abutment member urging said abutment member downwardly relative to said plates;

hydraulic retarding cylinder means connected between said plates and said abutment member to retard the rate of downward movement of said abutment member relative to said plates and to limit the amount of said downward movement to thereby define the lowermost position of said abutment member relative to said plates;

a pair of roller chains respectively anchored to the upper end of said jaws, the remaining ends of said chains being anchored between said plates in a fixed pivotal connection;

' a pair of rollers mounted on said abutment member and respectively having said chains entrained over them intermediate the ends of said chains, whereby upward movement of said abutment member relative to said plates will cause said chains to lift said jaws and pivot them about their pivotal axes relative to said plates;

latch means on said jaws and said plates engageable when said jaws attain their second positions to se-' lecti-vely prevent motion of said jaws relative to said plates;

and means on said abutment member operatively engageable with said latch means to release said latch means when said abutment member i in its lowermost position relative to said plates and allow said jaws to return to their first positions.

. 4. A hook for a hoist adapted to carry an article having an upstanding bail, comprising:

,a pair of complementary movable jaws pivotally mounted between said plates about parallel transverse horizontal axes, said jaws being located adjacent the lower end of said plates at each side of the slots forming said throat, each jaw including a complementary abutting surface adapted to contact one another at the center line of the throat formed by the slots to thereby bridge the area formed by the slots, the center of gravity of each jaw being located between its pivotal axis relative to said plates and the throat formed by said slots and being at an elevation above that of its pivotal axis on said plates, said jaws each being biased to -a first position protruding into the throat due to the location of their centers of gravity and being movable to a second position recessed beyond the edges of said plates defining said slots;

yieldable means operatively connected between said plates and said abutment member urging said abutment downwardly relative to said plates;

a hydraulic retarding cylinder connected between said plates and said abutment member to retard the rate of downward movement of said abutment relative to said plates and to limit the amount of said downward movement to thereby define the lowermost position of said abutment member relative to said plates;

a pair of rollers mounted on said abutment member and respectively having said chains entrained over them intermediate the ends of said chains, whereby upward movement of said abutment member relative to said plates will cause said chains to lift said jaws and pivot them about their pivotal axes relative to said plates;

a pair of fixed pins protruding outwardly respectively from each of said jaws toward the adjacent side plate;

a pivoted latch carried by said side plates intermediate the jaws and said side plates and located directly below said pins, said latch members each having upwardly open notches to selectively receive said pins;

yieldable means operatively connected between said latch members and said side plates to bias the latch members upwardly against said pins;

and an abutment formed at the lower portion of said abutment member engageable with each of said latch members when said abutment member is in its lowermost position to thereby pivot said latch members about their axes relative to said side plates so as to position the latch members below the path of movement of the pins.

5. A hook for a hoist adapted to carry an article having an upstanding bail comprising:

a rigid frame; movable j-aw means pivotally mounted on said frame about a horizontal transverse axis, said jaw means comprising a first section including a protruding upwardly directed bail engaging surface adapted to support the lower surface of a bail carried thereby, said jaw section being biased to a first position relative to said frame wherein the bail engaging surface thereof extends across a plane on said frame in vertical alignment with a bail to be supported thereby, said jaw section being selectively movable about said axis to a second position relative to said frame wherein said bail engaging surface is retracted beyond said plane;

retracting means on said jaw selectively activated by downward motion of the frame relative to a bail aligned with said plane on said frame beyond the elevation at which the bail engaging surface of said jaw section is in contact with the bail to selectively shift said jaw from said first position to said second position;

and retarding means operatively connected to said jaw means to delay return of said jaw section to said first position subsequent to shifting thereof to said second position.

6. A hook for a hoist adapted to carry an article having an upstanding bail, comprising:

a pair of upright side plates fixed relative to one another in a spaced relation, each of said plates being provided with a vertically disposed slot extending from the lower end thereof and terminating short of the upper end of said plate, the slots being in registry with one another and providing a throat to receive a bail of an article to be lifted;

a movable jaw pivotally mounted between said plates about a transverse horizontal axis, said jaw being located adjacent the lower ends of said plates at one side of the slots forming said throat, said jaw being biased to a first position protruding into the throat and being movable to a second position fully recessed beyond said throat;

a vertically reciprocable abutment member mounted on saidplates above said jaw, said abutment member being in a location bridging said throat;

motion limiting means on said abutment means and said plates to limit the extent of downward motion of said abutment member relative to said plates;

lifting means operatively connected between said abutment member and said jaw to cause said jaw to be moved to its second position in response to predetermined upward motion of said abutment member relative to said plates;

latch means on said jaw and said plates engageable when said jaw attains its second position to selectively prevent motion of said jaw relative to said plates;

and means on said abutment member operatively engageable with said latch means to release said latch means when said abutment member is in its lowermost position relative to said plates;

said motion limiting means comprising hydraulic retarding cylinders connected between the upper ends of the plates and said abutment means to control the rate of downward movement of said abutment member relative to said plates as well as the extent of such movement.

7. A hook for a hoist adapted to carry an article having an upstanding bail, comprising:

a pair of upright side plates fixed relative to one another in a spaced relation, each of said plates being provided with a vertically disposed slot extending from the lower end thereof and terminating short of the upper end of said plate, the slots being in registry with one another and providing a throat to receive a hail of an article to be lifted;

motion limiting means on said aboutment means and said plates to limit the extent of downward motion of said abutment member relative to said plates;

said lifting means comprising a roller chain anchored at its respective ends to said plates and said jaw, said chain being entrained over a roller rotatably journalled about a fixed axis on said abutment member.

References Cited by the Examiner UNITED STATES PATENTS 15 HUGO O. SCHULZ, Primary Examiner.

Claims

1. A HOOK FOR A HOIST ADAPTED TO CARRY AN ARTICLE HAVING AN UPSTANDING BAIL COMPRISING: A PAIR OF UPRIGHT SIDE PLATES FIXED RELATIVE TO ONE ANOTHER IN A SPACED RELATION, EACH OF SAID PLATES BEING PROVIDED WITH A VERTICAL DISPOSED SLOT EXTENDING FROM THE LOWER END THEREOF AND TERMINATING SHORT OF THE UPPER END OF SAID PLATE, THE SLOTS BEING IN REGISTRY WITH ONE ANOTHER AND PROVIDING AN OPEN THROAT TO RECEIVE A BAIL OF AN ARTICLE TO BE LIFTED; A PAIR OF MOVABLE JAWS PIVOTALLY MOUNTED BETWEEN SAID PLATES ABOUT PARALLEL TRANSVERSE HORIZONTAL AXES, SAID JAWS BEING LOCATED ADJACENT THE LOWER ENDS OF SAID PLATES AT EACH SIDE OF THE SLOTS FORMING SAID THROAT, SAID JAWS EACH BEING BIASED TO A FIRST POSITION PROTRUDING INTO THE THROAT AND BEING EACH MOVABLE TO A SECOND POSITION FULLY RECESSED BEYOND SAID THROAT; A VERTICALLY RECIPROCABLE ABUTMENT MEMBER MOUNTED ON SAID PLATES ABOVE SAID JAW, SAID ABUTMENT MEMBER BEING IN A LOCATION BRIDGING SAID THROAT; MOTION LIMITING MEANS OPERATIVELY CONNECTED BETWEEN SAID ABUTMENT MEANS AND SAID PLATE TO LIMIT THE EXTENT OF DOWNWARD MOTION OF SAID ABUTMENT MEMBER REALTIVE TO SAID PLATES; LIFTING MEANS OPERATIVELY CONNECTED BETWEEN SAID ABUTMENT MEMBER AND SAID JAWS TO CAUSE SAID JAWS TO BE MOVED TO THEIR SECOND POSITIONS IN RESPONSE TO PREDETERMINED UPWARD MOTION OF SAID ABUTMENT MEMBER RELATIVE TO SAID PLATES; LATCH MEANS ON SAID JAWS AND SAID PLATES ENGAGEABLE WHEN SAID JAWS ATTAIN THEIR SECOND POSITIONS TO SELECTIVELY PREVENT MOTION OF SAID JAWS RELATIVE TO SAID PLATES; AND MEANS ON SAID JAWS ABUTMENT MEMBER OPERATIVELY ENGAGEABLE WITH SAID LATCH MEANS TO RELEASE SAID LATCH MEANS WHEN SAID ABUTMENT MEMBER IS IN ITS LOWERMOST POSITION RELATIVE TO SAID PLATES AND ALLOW SAID JAWS TO RETURN TO THEIR FIRST POSITIONS.