US3128954A

US3128954A - davidson

Info

Publication number: US3128954A
Authority: US
Priority date: 1961-03-17
Filing date: 1962-02-26
Publication date: 1964-04-14
Anticipated expiration: 1981-04-14
Also published as: GB917743A; DE1219774B

Description

April 14, 1964 D. J. DAVIDSON 3,128,954

JAW CRUSHERS Filed Feb. 26, 1962 S Sheets-Sheet l D. J. DAVIDSON 3,128,954

JAW CRUSHERS April 14, 1964 3 Sheets-Sheet 2 Filed Feb. 26, 19

April 14, 1964 D. J. DAVIDSON JAW CRUSHERS 3 Sheets-Sheet 3 Filed Feb. 26, 1962 United States Patent 3,128,954 HAW QRUSHERS Donald J. Davidson, Ravenstone, England, assignor to hegson Limited, Coalville, England, a British company Filed Feb. 26, 1962, Ser. No. 175,406 lairns priority, application Great Britain Mar. 17, 1961 6 Claims. (til. 241-269) The present invention is concerned with crushers, i.e. machines for crushing rock, stone, brick, or like material and comprising a pair of confronting jaws defining be tween them a downwardly-tapering gap constituting a crusher chamber through which the material to be broken up is crushed as it progressively descends through the gap.

Moreover, the invention is concerned with such crushers of the kind wherein one of the crushing jaws is stationary whilst the other jaw is carried by a movable jaw stock and is arranged to be periodically thrust towards the other and fixed jaw by a Pitman through the medium of a pair of toggles. In a crusher of this kind the Pitman is mounted upon and depends from an eccentric; a toggle, hereinafter called the front toggle, usually in the form of a plate, extends between the lower end of the Pitman and the movable jaw stock and is articulated between these parts, and a further toggle, hereinafter called the rear toggle, is articulated between the Pitman and a backing member located at the side of the Pitman remote from the jaw stock, i.e. at or adjacent to the rear of the crusher. The arrangement is such that, upon rotation of the eccentric by a suitable prime mover, the movable jaw stock is oscillated by virtue of the eccentric and the action of the toggles, so that the lower end of the movable jaw carried by the movable jaw stock is reciprocated towards and away from the stationary jaw.

Each toggle is articulated between the said parts by reason of the fact that the opposite front and rear edges of each toggle are rounded and are turnable in toggle seats provided in or on the relevant parts, i.e. the movable jaw stock, the Pitman and the backing member.

Crushers as defined in the preceding paragraphs will be hereinafter referred to as crushers of the kind concerned.

The aforesaid toggle seats may be formed in the relevant parts, although usually each seat is provided in an elongated block which is received in a groove in the relevant part, i.e. movable jaw stock, Pitman or backing member, the block being recessed or channelled and the bottom of the recess or channel constituting the seat for the relevant rounded edge of the toggle plate. Thus, in one arrangement the part-circular section of each of the rounded front and rear edges of each toggle is of a diameter approximately equal to the thickness of the toggle and is located in a complementary curved seat so that when the toggle plate turns relatively to the seat the relevant rounded edge of the plate slides within the complementary seat. In another arrangement the seat is plane, although it may be curved, and the rounded edge of the plate, the diameter of which in this case is approximately equal to the length of the toggle from front to rear, is arranged to roll along the seat. It will be appreciated that the second form of seat allows only very limited turning or inclination of the toggle plate relatively to the seat.

In crushers of the kind concerned it is necessary to effect adjustments of the movable jaw stock. Thus, it is necessary to adjust the crusher setting, i.e. gap size, that is to say, the maximum distance between the lower edges of the movable and fixed jaws, to produce crushed material of the required size. Moreover, adjustments are also required to overcome the effect of wear of the jaws.

In conventional machines of the pind concerned these adjustments are effected in two ways. Large adjustments,

i.e. adjustments in gap size, are effected by substituting for the front toggle plate another toggle plate of different length, a plurality of different sized toggle plates being made available for this purpose. Slight adjustments, such as are necessary to overcome the effect of wear of the jaws, are produced by moving the rear seat block of the rear toggle plate, i.e. that located in or on the said backing member. This backing member is, in conventional machines, usually in the form of a toggle block located Within the crusher adjacent the rear wall of the latter, and the adjustment of the rear seat is effected by moving this toggle block. This adjustment of the rear seat block may be efiected either by placing shims between the toggle block and the rear wall of the crusher or by means of an adjustable wedge arranged between the said toggle block and the rear wall. As will be appreciated placing of shims at this location or appropriate adjustment of the wedge will move the rear toggle into a more forward position, swing the Pitman forward, and thus swing the movable jaw stock and hence the rear and movable jaw forwards towards the fixed jaw, thus producing a smaller setting of the crusher, and vice versa.

However, the arrangement is such that the Pit-man is movable to either side of a normal mean, i.e. mid or halfway, position, and this rear position is the most advantageous position for the transmission of movement to the movable jaw stock. The mean position of the Pitman is, in fact, usually vertical. It is accordingly undesirable to move the Pitman, i.e. the mean position thereof, for adjustment to any substantial extent from its original mean position, more particularly because this inclination produces greater inclination between the toggle plates in their mean positions and the associated seats, which in clination should be kept to a minimum. That is to say the mean, or intermediate, position of each toggle plate is preferably normal to each seat and the arrangement should be such that the toggle plate deviates as little as possible from this position.

The principal object of the present invention is to provide, in a jaw crusher of the kind concerned, a novel arrangement whereby the crusher setting may be conveniently adjusted without displacing the mean position of the Pitman and incurring the aforesaid variations in the relative angles of the toggle plates and the associated toggle seats.

Another object is to provide a jaw crusher of the kind concerned which avoids the trouble and time customarily incurred in substituting different-sized toggle plates and then resetting the machine.

A further object is to provide a jaw crusher in which all adjustments of the jaw stock, both large adjustments for varying the crusher setting and minor adjustments for overcoming the effect of wear of the jaws are effected by a single means. Thus, there is provided, in accordance with the present invention, a jaw crusher of the kind concerned wherein the front seat of the front toggle plate, i.e. the seat provided in or on the movable jaw stock, is provided by a member which is provided on, and adjustable relatively to, the movable jaw stock and means are provided for retaining this seat member in an adjusted position relatively to the movable jaw stock.

As will be appreciated, the idea is that the toggle seats in or on the Pitman shall be non-movable relatively to, and movable with, the Pitman and the toggle seat provided in or on the aforesaid backing member shall be nonmovable relatively to the latter, as in a conventional jaw crusher. Moreover, it is the intention that the backing member itself shall be non-movable, as distinct from a conventional crusher in which this backing member is in the form of an adjustable toggle block. Thus, the backing member in the crusher of the invention may, in fact, be constituted by the rear wall of the crusher. Thus,

3 in the jaw crusher of the invention the only toggle seat which is movable relatively to the part in or on which it is provided, is the front toggle seat, i.e. that provided by the said seat member on the movable jaw stock, which seat, in a conventional crusher, is usually non-movable relatively to the movable jaw stock.

It is to be noted that where, as will usually be the case, the toggle seats in or on the Pitman and backing member are provided by elongated blocks located in recesses in the Pitman and the backing member, the blocks may, of course, be detachable for maintenance or replacement.

Thus, in the jaw crusher provided by the invention, any adjustment of the movable jaw stock is carried out at the front of the machine adjacent the movable jaw stock as distinct from a conventional machine in which minor adjustments only are carried out at the rear of the machine. Thus, whereas in the prior arrangement the Pitman, together with the toggle plates, is thrust forwardly or rearwardly from its normal mean position by adjustment of the backing member, in the arrangement according to the invention the mean position of the Pitman remains unchanged, irrespective of any adjustment of the movable jaw stock. Thus, only the movable jaw stock is adjusted.

The said adjustable toggle seat member, which is conveniently constituted by an elongated recessed or channelled block of like form to those which may provide the other toggle seats, may be retained in an adjusted position by the insertion of shims between this adjustable toggle seat block and the rear of the movable jaw stock. Thus, a rearwardly-extending step may be provided upon the rear of the movable jaw stock adjacent the lower end thereof and the adjustable toggle seat block can rest upon this step and be movable towards and away from the rear face of the jaw stock upon the step. Shims in the form of elongated flat metal strips may be placed between the seat and the rear of the jaw stock by sliding these shims longitudinally into a gap between the seat and the jaw stock.

The aforesaid retaining means may be constituted by a clamping plate or block which is disposed above the aforesaid step and is arranged to be urged downwardly towards the step, so that, when the adjustable toggle seat block has been moved to a desired position upon the step relatively to the jaw stock and shims have been placed between the toggle seat block and the rear of the movable jaw stock, then the clamping plate may be moved downwardly towards the step to rigidly clamp the toggle seat block in the adjusted position.

In order that the invention may be more clearly understood and readily carried into practical effect one specific constructional example thereof will now be described with reference to the accompanying drawings, wherein:

FIGURE 1 is a sectional side elevation of the complete jaw crusher taken along the line 11 of FIG. 3.

FIGURE 2 is a sectional side elevation, on a larger scale, of the movable jaw stock taken on the line 22 of FIG. 3.

FIGURE 3 is a view of the rear of the movable jaw stock as viewed from the plane A-A in FIGURE 2, and

FIGURE 4 is an elevation of one of the shims.

The crusher illustrated in FIGURE 1 has a frame, indicated generally at 1, comprising two spaced side walls, one of which is shown at 2, a front end wall 3 and a rear end wall 4. Secured to the front end wall 3 is a fixed jaw 5 clamped in position by a conventional mechanism 6.

The frame 1 carries a cross shaft 7 providing a pivotal mounting for the depending swing jaw stock 8, to which is attached the movable jaw 9. The confronting fixed and movable jaws 5, 9 define between them a downwardly tapering gap G through which material to be broken up is crushed. The frame 1 also carries a further cross shaft having an eccentric portion 10 providing an eccentric mounting for the Pitman 11, which depends therefrom. Articulated between the lower portion of the Pitman 11 and the rear wall as will be hereinafter described, is a rear toggle plate 12 and articulated between the lower end portion of the Pitman and the swing jaw stock 8 is a front toggle plate 13. Secured to the cross shaft having an eccentric portion 10 is a fly wheel 14 which is arranged to be rotated by a prime mover (not shown). Thus, the arrangement, as in a conventional jaw crusher, is such that, upon rotation of the eccentric 10 about the axis of the fly wheel 14, the swing jaw stock 8 is oscillated by virtue of the action of the eccentric and the toggle plates 12 and 13 so that the lower end 9a of the movable jaw 9 is reciprocated towards and away from the fixed jaw 5.

The opposite ends of each of the toggle plates 12 and 13 are rounded or curved, the diameter of curvature being equal to the length of the toggle plate, and these rounded ends are seated and turnable in toggle seats 15, 16, I7 and 18. These toggle seats 15-18 are each in the form of an elongated block of a length commensurate with the width of the toggle plates 12 and 13 and of initially oblong cross-section but recessed or channelled at one side to provide the seat for the relevant edge of a toggle plate. The actual seating surfaces of the blocks are plane and the round front and rear edges of the toggle plates 12 and 13 are arranged to roll along the seating surfaces of the blocks. These toggle seat blocks 15-18 are each located in or on the relevant parts 4, 8 and 11. Thus the toggle seat blocks 15 and 17 are located in grooves in the Pitman 11 and additionally located against longitudinal movement by pins or spigots 15a and 17a provided thereon intermediate their ends and entering blind holes formed in the inner ends of the grooves. The rear toggle seat block 16 is similarly non-adjustably located in a groove formed in the rear wall 4 of the crusher, a pin or spigot 16:: is provided on the block 16 intermediate its ends and enters a blind hole in the inner end of the groove, to prevent longitudinal movement of the seat block. As will be appreciated the rear wall 4 in this example constitutes the backing member hereinbefore referred to.

The front seat block 18 is not, however, received in a recess but is arranged to be secured in adjustable positions upon the rear of the swing jaw stock 8. Thus, as will be seen in FIGURES 1, 2 and 3, the swing jaw 8 is provided at its rear side adjacent the lower end thereof with a rearwardly-projecting step 19 which extends across its full width. The upper surface of the step 19 is plane and is inclined from the horizontal, as will be seen in FIG- URES 1 and 2 to an extent dependent upon the inclination of the jaw stock. The front seat block 18 rests upon this step 19 and is movable, towards and away from the portionSb of the rear face of the swing jaw stock 8, upon the step. Shims 20 in the form of elongated fiat metal strips each of the form shown in FIGURE 4 may be placed between the front seat block 18 and the portion 8b of the rear face of the swing jaw stock 8 to locate the seat block 18 in a desired position upon the step 19. As will be seen from FIGURE 2 the shims 20 can be of different thicknesses so as to allow for optimum combinations to make up fine variations of thicknesses. As will be appreciated the requisite number of these shims 20 of appropriate thicknesses may be placed between the seat block 18 and the portion 8b of the rear face of the swing jaw stock 8 to locate the front seat block 18 in the desired position, these shims being inserted longitudinally. Thus, the forward thrust of the front toggle plate 13 is transmitted to the swing jaw through the shims 20.

The front seat block 18 is arranged to be rigidly clamped in position upon the step 19 of the swing jaw stock 8 by means of a clamping plate or block 21 which extends above the step 19 across the full width of the swing jaw stock 8 as will be clearly seen from FIGURE 3. The clamping plate or block 21 is adapted to be urged downwardly towards the step 19 by means of two parallel rods 22 extending through holes formed in the upper and lower pairs of

lugs

23, 24 formed integrally upon the rear of the swing jaw stock 8.

The lower ends of the rods 22 are reduced at 22a and enter spaced recesses or blind holes 25 formed in the upper part of the clamping plate or block 21. Threaded onto the lower reduced end 22a of each rod 22 is a rectangular plate 26 formed with a hole to receive the reduced end of the rod. The plates 26 are welded to the rods 22. and are each secured to the plate or block 21 by two screws 27 extending through holes in the plate at respectively opposite sides of the rod 22 and engaged in complementarily tapped holes in the plate and block 21. Thus the latter is attached to the rods 22 and may be raised or lowered by the latter.

The upper end portions of the rods 22 extending through the upper pair of lugs 23 are screw-threaded and correspondingly-tapped adjusting nuts 28 are provided upon these screw-threaded portions and are adapted to abut against the undersides of the lugs 23. Similar corresspondingly tapped nuts 29 are engaged upon the upper extremities of the screw-threaded portions of the rods 22 and are adapted to abut against the upper edges of the lugs 23. As will readily be appreciated when the shims 20 have been located upon the step 19 and the front seat block 18 has been pushed forwardly into abutting relation with the rear-most shim 20 so as to be located in the desired position, the clamping block or plate 21 is moved downwardly to clamp the front seat block 18 between the block or plate 21 and the step 19 in the desired position by rotation of the nuts 28 in the appropriate direction against the undersides of the lugs 21. The locking nuts 29 are then rotated and tightened downwardly against the lugs 23 to rigidly lock the rods 22 in the adjusted position and prevent movement of the rods 22 and unclamping of the block or plate 21 during operation of the crusher.

It will be observed form FIGURE 2 that the height of the shims 20 is less than the height of the toggle seat block 18 so that there is a clearance 30 between the upper edges of the shims 20 and the underside ofthe clamping block or plate 21 when the latter is in its clamping position, to ensure that the latter is certain to engage the toggle seat block 18.

As will be seen from FIGURES 3 and 4 each of the shims 20 is formed at its lower edge intermediate its opposite ends with a downwardly depending lug or spigot 20a which, when the shim 20 is placed in position enters a transverse, i.e. rearwardly-extending, recess 31 formed intermediate the ends of the step 19 so as to prevent longitudinal movement of the shims 20 during operation of the crusher. The toggle seat block 18 is also formed at its lower edge intermediate its opposite ends with a depending spigot 18a (FIGURE 3) which also enters the recess 31 to similarly prevent longitudinal movement of this block. The depth of the downwardly depending lugs r spigots 20a of the shims 20 is slightly less than the aforesaid clearance 30 between the upper edges of the shims 20 and the lower edge of the clamping plate or block 21 so that the shims may be removed from between the toggle seat block 21 and the surface 8b by simply lifting the shims to clear the lugs or spigots 26a from the recess 31 and withdrawing the shims longitudially. To this end each of the shims 20 is formed in its lower edge adjacent the opposite ends with recesses 2% with which hook-shaped tools may be engaged to lift the shims. For this purpose the step 19 is cut away at its opposite ends at 32, see FIGURE 3, to enable the tools to be inserted into the recesses 20b.

Manifestly, any desired adjustment of the swing jaw stock 8, i.e. adjustment of the mean position of the latter, may be achieved by locating the front toggle seat block 18 in the desired position, inserting the requisitie number of shims 20 of appropriate thicknesses between the block 18 and the rear surface portion 8b of the jaw stock 8, and clamping the block 18 by means of the plate or block 21. Where minor adjustments of the jaw stock 8 are required, e.g. to overcome the effect of wear of the jaws and 9 to restore a particular crusher setting a shim of small thickness will be inserted. Where it is desired to produce a larger adjustment of the jaw stock 8, e.g. to produce a smaller crusher setting one or more shims 20 of substantial thickness may be inserted. Conversely when it is desired to increase the crusher setting one or more of the inserted shims 20 will be removed. The seat block 18 is, of course, clamped in the new position after each adjustment.

It is to be noted that with this arrangement any adjustment of the swing jaw stock 8 will not displace the Pitman from its mean position, which in the present example is a vertical position.

A further advantage of the jaw crusher provided by this invention is that since the complementary wedge blocks or shim clamping means conventionally provided at the rear of a crusher adjacent the toggle block are eliminated, the jaw crusher of the invention may be of shorter length than a conventional crusher.

I claim:

1. A jaw crusher comprising, in combination, a frame having front and rear walls, a fixed jaw mounted within and adjacent the front end of said frame, a first shaft mounted on said frame, a movable jaw stock pivotally mounted upon and depending from said first shaft for oscillation within said frame towards and away from said fixed jaw and formed at the rear side adjacent the lower end thereof with a rearwardly-extending step, a jaw mounted upon the front side of said jaw stock and confronting said fixed jaw, a second shaft mounted on said frame for rotation and having an eccentric portion intermediate its ends, a Pitman rotatably mounted upon said eccentric portion of said second shaft and located between said jaw stock and the rear end of said frame, a first elongated block received in a recess at the rear side of said Pitman and formed with a first toggle seat, a second elongated block received in a recess in the rear wall of said frame and formed with a second toggle seat, a rear toggle interposed between said Pitman and said rear wall and seating at its opposite ends in said first and second toggle seats, a third elongated block received in a recess at the front side of said Pitman and formed with a third togg e seat, a fourth elongated and channelled block formed with a fourth toggle seat resting upon said rearwardly-extending step and adjustable upon said step towards and away from the rear side of said movable jaw stock, a clamping plate provided above said step and having an engaging surface substantially parallel to the upper surface of said step, means for urging said clamping plate downwardly toward said step to clamp said fourth toggle seat block in an adjusted position relatively to said movable jaw stock, and a front toggle interposed between said Pitman and said movable jaw stock and seating at its opposite ends in said third and fourth toggle seats.

2. A jaw crusher according to claim 1, wherein shims are provided between said fourth toggle seat block and the rear side of said movable jaw stock to locate said fourth toggle seat block in an adjusted position relatively to said movable jaw stock.

3. A jaw crusher according to claim 2, wherein said shims are constituted by elongated strips interposed between said fourth toggle seat block and the rear side of said movable jaw stock, these shims being of a height slightly less than the height of the said fourth toggle seat block so as to provide a clearance between the upper edges of the shims and the clamping plate to ensure that when the clamping plate is moved into its clamping position it is certain to engage the fourth toggle seat block.

4. A jaw crusher according to claim 3, wherein each of the said shims is provided intermediate its ends with a downwardly-depending spigot adapted to be received within a recess formed in the upper edge of the said step and extending rearwardly from the rear side of the movable jaw stock, whereby longitudinal shifting of the shims during operation of the crusher is prevented.

5. A jaw crusher comprising, in combination, a frame having front and rear walls; a fixed jaw mounted within and adjacent the front end of said frame; a first shaft mounted on said frame; a movable jaw stock pivotally mounted upon and depending from said first shaft for oscillation within said frame towards and away from said fixed jaw and formed at the rear side adjacent the lower end thereof with a rearwardly-extending step; a jaw mounted upon the front side of said jaw stock and confronting said fixed jaw; 21 second shaft mounted on said frame for rotation and having an eccentric portion inter mediate its ends; a Pitman rotatably mounted upon said eccentric portion of said second shaft and located between said jaw stock and the rear end of said frame; a first elongated block received in a recess at the rear side of said Pitman and formed with a first toggle seat, a second elongated block received in a recess in the rear wall of said frame and formed with a second toggle seat; a rear toggle interposed between said Pitman and said rear end and seating at its opposite ends in said first and second toggle seats; a third elongated block received in a recess at the front side of said Pitman and formed with a third toggle seat, a fourth elongated and channelled block formed with a fourth toggle seat resting upon said rearwardly-extending step and adjustable upon said step towards and away from the rear side of said movable jaw stock; at least one elongated shim interposed between said fourth toggle seat block and the rear side of said movable jaw stock, a clamping plate provided above said step; a pair of spaced upper lugs provided upon the rear side of said movable jaw stock adjacent the upper end thereof, a

0 pair of spaced lower lugs provided upon the rear side of said movable jaw stock adjacent the aforesaid step and aligned with the upper lugs; two rods which are axially slidable in respective axes of the upper and lower pairs of lugs, the lower ends of which abut the clamping plate and the upper end portions of which are screw threaded; correspondingly tapped nuts on said screw-threaded portions adapted to abut against the undersides of the upper lugs whereby the rods and the clamping plate may be moved downwardly towards said step to clamp said fourth toggle seat block in an adjusted position relatively to said movable jaw stock, and a front toggle interposed between said Pitman and said movable jaw stock and seating at its opposite ends in said third and fourth toggle seats.

6. A jaw clusher according to claim 5, wherein each of the said two rods is furnished at the lower end thereof with a plate rigidly secured thereon, and the clamping plate is detachably-attached to the said plates by means of screws, whereby the clamping plate can be raised and lowered by the rods.

References Cited in the file of this patent Telesmith Jaw Crushers, published by Smith Engineering Works, East Capitol Drive at North Holton Street, Milwaukee, Wisconsin in November 1937.

Claims

1. A JAW CRUSHER COMPRISING, IN COMBINATION, A FRAME HAVING FRONT AND REAR WALLS, A FIXED JAW MOUNTED WITHIN AND ADJACENT THE FRONT END OF SAID FRAME, A FIRST SHAFT MOUNTED ON SAID FRAME, A MOVABLE JAW STOCK PIVOTALLY MOUNTED UPON AND DEPENDING FROM SAID FIRST SHAFT FOR OSCILLATION WITHIN SAID FRAME TOWARDS AND AWAY FROM SAID FIXED JAW AND FORMED AT THE REAR SIDE ADJACENT THE LOWER END THEREOF WITH A REARWARDLY-EXTENDING STEP, A JAW MOUNTED UPON THE FRONT SIDE OF SAID JAW STOCK AND CONFRONTING SAID FIXED JAW, A SECOND SHAFT MOUNTED ON SAID FRAME FOR ROTATION AND HAVING AN ECCENTRIC PORTION INTERMEDIATE ITS ENDS, A PITMAN ROTATABLY MOUNTED UPON SAID ECCENTRIC PORTION OF SAID SECOND SHAFT AND LOCATED BETWEEN SAID JAW STOCK AND THE REAR END OF SAID FRAME, A FIRST ELONGATED BLOCK RECEIVED IN A RECESS AT THE REAR SIDE OF SAID PITMAN AND FORMED WITH A FIRST TOGGLE SEAT, A SECOND ELONGATED BLOCK RECEIVED IN A RECESS IN THE REAR WALL OF SAID FRAME AND FORMED WITH A SECOND TOGGLE SEAT, A REAR TOGGLE INTERPOSED BETWEEN SAID PITMAN AND SAID REAR WALL AND SEATING AT ITS OPPOSITE ENDS IN SAID FIRST AND SECOND TOGGLE SEATS, A THIRD ELONGATED BLOCK RECEIVED IN A RECESS AT THE FRONT SIDE OF SAID PITMAN AND FORMED WITH A THIRD TOGGLE SEAT, A FOURTH ELONGATED AND CHANNELLED BLOCK FORMED WITH A FOURTH TOGGLE SEAT RESTING UPON SAID REARWARDLY-EXTENDING STEP AND ADJUSTABLE UPON SAID STEP TOWARDS AND AWAY FROM THE REAR SIDE OF SAID MOVABLE JAW STOCK, A CLAMPING PLATE PROVIDED ABOVE SAID STEP AND HAVING AN ENGAGING SURFACE SUBSTANTIALLY PARALLEL TO THE UPPER SURFACE OF SAID STEP, MEANS FOR URGING SAID CLAMPING PLATE DOWNWARDLY TOWARD SAID STEP TO CLAMP SAID FOURTH TOGGLE SEAT BLOCK IN AN ADJUSTED POSITION RELATIVELY TO SAID MOVABLE JAW STOCK, AND A FRONT TOGGLE INTERPOSED BETWEEN SAID PITMAN AND SAID MOVABLE JAW STOCK AND SEATING AT ITS OPPOSITE ENDS IN SAID THIRD AND FOURTH TOGGLE SEATS.