US3627214A

US3627214A - Jaw crusher

Info

Publication number: US3627214A
Application number: US844150*[A
Authority: US
Inventors: Jan Willem Hilbrands
Original assignee: Maschinenfabrik Koeppern GmbH and Co KG
Current assignee: Maschinenfabrik Koeppern GmbH and Co KG
Priority date: 1966-03-23
Filing date: 1969-10-09
Publication date: 1971-12-14
Anticipated expiration: 1988-12-14
Also published as: GB1169593A; DE1296936B

Abstract

A jaw crusher consists of two opposing plate-shaped jaws, one of which is axially immovable but which may be rotated, the other jaw being hydrostatically mounted for both axially reciprocatory motion as well as rotary motion.

Description

United States Patent int. Cl E021: 1/02,

B02c 7/08, B021; 19/90 [50] Field 01 Search 241/205, I 256, 259,285,286

[56] References Cited UNITED STATES PATENTS 1,066,218 7/l9 l 3 Newhouse 241/205 2,216,6l1 10/1940 Dimm 24l/256X 2,646,728 7/1953 Coghill 241/259 X 2,852,203 9/1958 Ditting 241/259 Primary Examiner Donald G. Kelly Attorney-Christen & Sabol ABSTRACT: A jaw crusher consists of two opposing plateshaped jaws, one of which is axially immovable but which may be rotated, the other jaw being hydros'tatically mounted for both axially reciprocatory motion as well as rotary motion.

.IAW CRUSHER This application is a continuation-impart of my copending application, Ser. No. 625,383, filed Mar. 23, 1967, now abancloned.

This invention relates to jaw crushing apparatus of the type where two round rotation symmetric jaws face each other and enclose an annular outwardly narrowing crushing slot; one of the jaws is driven through an oscillating path and the other is rotatable. The material is fed coaxially to the rotary axis through one of the jaws into the crushing chamber.

The machine of German Pat. No. 1 12,059 discloses a rotatably mounted lower conical jaw. The upper jaw is provided with an aperture which is coaxial to the axis of rotation of the lower jaw, and is axially reciprocatable. The rotatable jaw is periodically driven so as to receive a short rotary impulse whenever the upper jaw is raised; said impulse exerts a centrifugal force on the material which is moved at a moderate rate of advance outwardly towards the discharge slot at the periphery.

It is the object of the invention to provide a jaw crusher of the character described which is capable of obtaining increased capacity over previously known devices.

In accordance with the invention, the jaws are plate shaped, are both rotatably mounted, and at least one of the jaws is equipped with a drive which is continuously at such a high speed that the material at the entrance to the crushing slot receives an acceleration which is greater than 1 g preferably greater than 53 (g =acceleration due to gravity).

By adjusting the angular velocity of the jaws, the rate of advance of the material in the crushing slot can be considerably increased over the obtainable in the conventional machines. This provided for shortened opening periods, which must be considered idling times, which in turn provides for a larger number of crushing strokes per unit of time. A jaw crusher according to the invention has the further advantage that the surface of the jaws increases outwardly with the square of the diameter; this provides an increasing surface of the jaws for an increasing surface of the crushed material and ensures an even wear of the jaws.

The invention will be best understood from the following illustrative description of two specific embodiments when read in connection with the accompanying drawings, in which FIG. 1 is a sectional view of an embodiment of the invention using a hydraulic drive, and

FIG. 2 is a sectional view of another embodiment.

Referring first to FIG. 1, the machine comprises a stationary frame which is firmly secured to a foundation and consists ofa lower portion 1 and an upper portion 3, screwed together. A cylinder 2 ofa hydraulic drive is secured in said lower portion 1. A ring bearing 4 is fixedly arranged at the upper portion 3 of the frame.

On the ring bearing 4, there is rotatably supported an upper crushing disc 5 which has a feed opening at its axis. A flange 7 with a V-belt pulley is screwed onto the upper side of the crushing disc, whereby the outer portion of said flange overlaps the ring bearing 4. The conically shaped lower portion of the crushing disc 5 is provided with a lining 8 for the crushing chamber, which serves as wear protection.

The upper portion 3 of the frame carries a motor 9 with a V- belt pulley 10. Said motor starts, via the schematically indicated V-belt pulley, the rotary movement of the upper crushing disc 5.

Below the crushing disc 5, there is a second crushing disc 11, whose upper surface is, in the illustrated embodiment, flat; also this disc is provided with a lining 12 for the crushing chamber. A hydraulic plunger 13 is operated in the hydraulic cylinder 2 and firmly screwed to the underside of the crushing disc 11. The upper crushing disc 5 is coupled with the lower crushing disc 11 through pins M which engage claws at the lower disc and start the rotary movement thereof.

The hydraulic drive, which consists of the cylinder 2 and the plunger 13, receives the hydraulic medium from a source of pressurized hydraulic fluid which consists of a motor pump aggregate 15, a control valve 16, and an oil reservoir 17, which is preferably equipped with cooling means for the oil return.

The control valve 16 is reversible and may be operated electrically or hydraulically. The control pulses are supplied to the control valve through a line 21 into which they are fed from a hydraulic or electric reversing switch operated by the movement of the lower crushing disc 11; said switch may be disposed, for instance, at the underside of the crushing disc.

Hydraulic oil is fed alternately into the annular space 19 and in the space 18 below the plunger 13 through the control valve 16, said arrangement being typical of a so-called pressureless fluid return means" because the fluid is returned to the reservoir at atmospheric pressure only.

Therefore, the plunger 13 starts a translatory motion in the crushing disc 11, which transmits the crushing force onto the material to be crushed.

A stationary feed hopper 6, which extends almost into the crushing chamber, is arranged at the upper portion 3 of the frame.

The material is charged through the stationary hopper 6 into the annular crushing slot. On contact with the crushing discs, the material receives a centrifugal acceleration which forces it radially outwardly into the crushing slot between the crushing discs. Said centrifugal force acts on the material until it is discharged at the outer periphery from the crushing slot. The number of revolutions of the crushing jaws is so adjusted that the acceleration at the entrance to the crushing slot, i.e. on the outer diameter of the charge opening in the upper crushing jaw, is greater than lg preferably greater than 5g. The greater said acceleration the smaller is the time which the material requires for the advance between two crushing strokes, and the greater is the throughput capacity of the crusher.

The crushed material emerges at the periphery from the slot between the two crushing discs. As this discharge takes place with high energy, it is possible to arrange, in front of the slot at the inside of the upper housing portion 3, a baffle ring 22 which, on the one side, prevents wear of the housing, and, on the other side, provides for a further disintegration of the material, particularly of the already broken grains.

It is also possible to classify the emerging material by arranging one or more partitions through which the material, as a function of its trajectory, is passed to different outlets.

In order to prevent damage of the crusher due to the introduction of unbreakable objects, e.g. the wood or metals, the hydraulic system contains a high-pressure relief valve 23.

As will be seen on viewing the drawing, the bearing ring 4, on which the upper crushing disc is rotatably mounted, must take up considerable axial forces. Suitable for this purpose are particularly hydrostatic bearings which should be arranged at the side impinged by the pressure. The axial bearings may be hydrostatic or mechanical bearings.

The lower crushing disc does not require a special pivot bearing as it is easily possible to use the hydraulic plunger 13 as such bearing.

In the drawing, the crusher is shown with a vertical axis of rotation. It is also possible to design the crusher with a horizontal axis of rotation whereby the material is laterally introduced into the hopper 6. Also, with this mode of charging, the material will be distributed evenly over the periphery of the crushing chamber, due to the high speed of rotation.

It is possible to do without a mechanical coupling between the upper and the lower crushing disc as the material effects such coupling anyhow and as the rotating masses are so large as to eliminate the risk of a noticeable drop of the number of revolutions of the crushing disc 11 during the short opening stroke.

It is also possible to drive directly the lower crushing disc as well as the upper crushing disc. Deviatiing from the design as shown in the drawing, the side of the lower crushing disc facing the crushing chamber may have a surface which becomes depressed towards the middle.

FIG. 2 is a modified embodiment of the disc crusher. The crusher has a cylindrical casing 24 with an annulus 26 welded in the upper part of said casing, the annulus bearing upwardly against a flange 28. On its inner periphery the annulus is provided with a thread 30 into which an annular bearing support 32 is screwed. The crushing gap, i.e. the axial distance between the crusher jaws is adjustable by means of the thread 30. The bearing support 32 is secured relative to the annulus 26 through conventional locking means to prevent rotation. A hollow pin 42 is rotatably mounted in said bearing support 32 by means of bearings 34, 36, 38 and 40, a crushing disc 44 being antirotatingly secured to the lower end of said hollow pin 42. A cover plate 46 is mounted on the front end of said bearing ring 32, said cover plate 46 carrying in its center a feed hopper 48 which extends down beyond the central opening 52 of the crusher disc 44 by a tube 50.

A bush 56 is mounted in the lower art of the casing 24 by means of bars 54. The annular chamber 55 between the casing 24 and said bush 56, which chamber is interrupted only by the bars 54, serves as a down shaft for the discharging of the crushed material, A hydraulic cylinder 58 is mounted in the bush 56, the said cylinder bearing against a shoulder 60 against axial downward thrust. The lower end of the cylinder is closed by an end cover 62. A hydraulic piston 64 is rotatably and axially slidably mounted in the cylinder 58. On its upper end the said hydraulic piston 64 carries a crusher jaw 66 coacting with the crusher disc 44, the said jaw 66 being firmly connected with the cylinder 64.

An upper hydrostatic bearing 68 and a lower hydrostatic bearing 70 are provided which serve as a pivot bearing for the piston 64. The upper bearing 68 has a bearing ring 72 mourited in the cylinder 58, and the lower bearing 70 has a bearing ring 74 mounted in said cylinder 58. Opposite the two bearing rings pressure pockets 76 and 78 are provided in the piston 64. The pressure medium is supplied to each of the pressure pockets through axially parallel bores 80 in the piston 64, the said bores 80 being closed at their lower ends by means of plugs 82. Branch bores 84 connect the bores 80 with the pressure pockets 76 and 78.

Furthermore, the axially parallel bores 80 are in communication with an annular recess 88 through a radical bore 86, into which recess 88 a line 90 issues which is passed through the cylinder 58 and the bush 56.

The drive in working direction (upwards) is effected via an annular space 92, the pressure acting against the annular surface 94. The backstroke of the piston is effected via the annular cylinder chamber 96 including the annular surface 98. A connection line 100 issues into the annular space 92 and a connection line 102 issues into the annular chamber 96. The rotary drive for the piston 94 is a hydraulic motor 104 mounted coaxially with the cylinder 58, the said motor extending with its driving shaft 106 through a bore [08 in the end cover 62 of the cylinder and engaging by means of splines a splined axial bore 110 in the lower end of the piston 64.

To seal the cylinder against the ingress of dust a labyrinth packing is provided between the underside of the crusher jaw 66 and the upper side of the cylinder 58, the said labyrinth packing being represented by a ring 112 mounted on the underside of the crushing disc 66 which ring engages into a slot 114 on the upper side of the cylinder 58. Of course, if necessary, said labyrinth packing may be multiplied. and so could flexible sealing means be provided as well.

The working medium for the hydraulic motor 104 is supplied through a controllable pump 116 which feeds the hydraulic motor through a line 118. Said line is protected by means ofa pressure relief valve 120. The driving medium leaving the hydraulic motor 104 issues into the tank 124 through oil return pipe 122.

A pump 126 is provided for the supply of the medium driving the hydraulic drive of the crusher, said pump 126 being connected with the connection line 100 through a line 128, a reversing valve 130 and a line [32. The line 128 is protected by means of a pressure relief valve 134. Further, a pressure reservoir 136 is connected to the line 128.

A branch line 138 connects line 128 with line 90. Connected to line 138 is a line 140 which communicates with line I02 through a pressure-reducing valve 142. A pressure relief valve 144 and a pressure reservoir 146 are connected to the line 140, which are arranged behind the pressure-reducing valve 142.

Furthennore, a controllable pump 148 is provided, which feeds a line 150, the latter being protected by a pressure relief valve 152 and being connected with an hydraulic motor 154 which drives a sequence switch cam accomplishing the reversing motion of the reversing valve constructed as an axially movable piston valve, or which directly drives a reversing valve constructed as a rotary slide valve.

The respective oil return pipes are shown by dotted lines and are not particularly described, and, as in the case of the previous embodiment, the fluid returns to the reservoir under atmospheric pressure only.

The crusher described above operates as follows: Rotational motion is imparted by the hydraulic motor 104 to the piston 64 and the lower crusher disc 66. The rotational speed can be adjusted by means ofthe pump 116.

The pockets 76 and 78 of the hydrostatic bearings 68, 70 are under full pump pressure by line 138 and the connection line 90, uniformity of the pressure being secured by the reservoir 136. The lower cylinder space 92 is connected alternately with the pump and the reservoir by means of the reversing valve 130. The upper annular cylinder chamber 96 is always under the pressure reduced by the pressure-reducing valve 142. Thus, the pressure in the cylinder space 92 acts, on the one hand, against the pressure in the cylinder chamber 96 and, on the other hand against the load by the material to be crushed and being in the crushing gap between the crusher jaws 44 and 66. The pressure in the cylinder chamber 96 need only be adjusted as high as to secure the backstroke of the piston in time. With working frequencies up to 70 c.p.s. possibly a backstroke by hydraulic means may be entirely omitted if the casing 24 is of a flexibility sufficient enough as to effect the backstroke of the piston.

The working frequency of the crusher, i.e. the number of the strokes made by the piston 64 per unit of time is adjusted by the pump 148.

Hence, an independent adjustment of the rotational speed by the pump 6 and of the stroke frequency through the pump 148 is possible.

Instead of providing the pump 116 adjustable in delivery there may also be provided a pump having a constant delivery and a series-connected valve, the amount of flow of said valve being adjustable.

In using the crusher as a fine crusher it has been found that with a stroke of a few millimeters working frequencies in the order of I00 c.p.s. can be reached.

The transmission of the rotational speed to the upper crushing disc can be achieved by the friction present in the crushing chamber. Of course, it is also possible to provide a coupling between the two chamber discs as such is shown and indicated at 14 in FIG. 1.

lclaim:

l. A disc jaw crusher apparatus comprising frame means, first disc jaw means mounted on the frame means for rotational movement about a vertical axis but axially fixed with respect to the frame means, second disc jaw means mounted on a hydraulically operable piston, said piston being axially and rotationally movably supported in a cylinder having a working chamber for the piston, said cylinder being fixedly secured to the frame means for positioning the second jaw means in spaced concentrically opposed operative relationship with the firstjaw means, said firstjaw means having a central feed hopper to receive material to be crushed, the crushed material being discharged from the peripheral space between said first and second disc jaw means, means to rotate at least one of said disc jaw means to urge material toward said peripheral space at an acceleration of at least 1 g, a source of pressurized hydraulic fluid including pressureless fluid return means and valve means connected with said working chamber for reciprocating said piston and second disc jaw means.

2. The invention defined in claim l, wherein motor means is secured to said frame means, and means is provided for connecting said motor means to rotate the firstjaw means.

3. The invention defined in claim 2, which includes coupling means connecting said first and second jaw means for transmitting rotary motion from one jaw means to the other jaw means.

4i. The invention defined in claim 3, wherein said coupling means includes a pin secured to one of said jaws adjacent the periphery thereof and extending parallel with the axis of said jaws for operative slidable engagement with a portion of the other of said jaw means.

5. The invention defined in claim 5, wherein said cylinder includes hydrostatic bearing means for radially supporting and axially guiding said second piston.

6. The invention defined in claim ll, wherein said piston includes at least one annular chamber at one end of the piston for actuating said piston in an axial direction, said chamber being defined by a radially projecting surface spaced from one end of the piston and in sliding contact with the cylinder.

7. The invention defined in claim l, wherein said source of pressurized hydraulic fluid includes motor means for actuating said valve means.

ill. The invention defined in claim '7, wherein said motor means includes adjustable speed control means.

9. The invention defined in claim 1, wherein said valve means comprises an axially movable piston valve.

110. The invention defined in claim 11. wherein said valve means comprises a rotary slide valve.

111. The invention defined in claim l, which includes a second annular working chamber means for supplying fluid to the first chamber at a substantially constant pressure to move said piston on one direction, and means for intermittently supplying fluid to the second chamber at a pressure greater than said first pressure to move the piston in the opposite direction.

12. The invention defined in claim llll, wherein the axis of said piston is vertical and said piston is positioned below said second disc jaw means, whereby the weight of said jaw means and material being crushed produces force acting to move the piston in a downward direction, and said first annular working chamber is positioned for urging the piston in a downward direction. said second annular working chamber being positioned for urging the piston in the upward direction, and said intermittent fluid supply means includes means for supplying said fluid at a pressure sufficient to produce upward force greater than the downward force ofthe first working chamber. the jaw means and material being crushed.

13. The invention defined in claim l, which includes motor means supported by the cylinder for rotating said piston.

M. The invention defined in claim l3, wherein said motor means includes a splined shaft rotatably supported by the motor means concentrically with the piston. said piston including a splined recess for axially movable nonrotational connection with the splined shaft.

15. The invention defined in claim 113, wherein said motor means includes a hydraulically actuated rotary element concentrically connected with said piston, and includes means for supplying fluid under pressure to said rotary element.

116. The invention defined in claim 15, wherein the supply of fluid to said rotary element may be varied to regulate the speed of rotation thereof.

37. The invention defined in claim 1, wherein said frame means includes means for adjusting the fixed axial position of said first jaw means, comprising an axial thrust bearing having an annular bearing support means, said frame means including an annulus surrounding said bearing support means in coacting threaded engagement therewith.

lid. The invention defined in claim l, wherein said frame means includes cylindrical casing means enclosing both said jaw means and said actuating cylinder.

19. The invention defined in claim 118, wherein said cylinder is axially slidably received within a cylindrical bush, said bush being secured within said casing by a plurality of radially extendin bars.

20. jaw crushing apparatus comprising a pair of opposed circular rotatably mounted plate-shaped jaws enclosing an annular outwardly narrowing crushing slot, one of said jaws being axially reciprocatable, feed inlet means for charging material through one of said jaws coaxially to the axis of rotation, drive means rotating at least one of said jaws continuously at a speed imparting to material entering said crushing slot an acceleration exceeding 1 g, at least one of said jaws being supported concentrically on a piston means having opposed working surfaces mounted for reciprocatory and revoluble movement in a fixed working cylinder means, means to intermittently supply hydraulic fluid. under pressure to one of the opposed surfaces of said piston means in said working cylinder means to move the piston means and the jaw supported thereon toward the other said jaws, and means to urge the piston means in the opposite direction when said fluid is not supplied under pressure.

211. A jaw crusher as claimed in claim 20, wherein drive means for both said jaws are provided.

22. Ajaw crusher as claimed in claim 21, wherein both jaws are adapted to be driven at the same angular speed.

23. Ajaw crusher as claimed in claim 20. comprising means coupling said jaws at their circumference.

M. A jaw crusher as claimed in claim 20, wherein the jaw facing the material inlet has a plane crushing surface.

25. Ajaw crusher as claimed in claim 20, wherein said feed means is a stationary chute.

26. The invention defined in claim l, wherein said first disc jaw means is mounted on the frame means for free rotational movement.

27. The invention defined in claim 20, wherein the jaw other than said one of the pair ofjaws is mounted for free rotational but axially fixed movement.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated December 1 4 .1971

Patent No. 3 627 21 4 Inventor(s) Jan Willem Hilbrands It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In column 5, line 15 of the specification, cancel "second" Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOfITSCHALK Commissioner of Patents Attesting Officer USCOMM-DC 60376-1 69 u.s. GOVERNMENT PRINTING OFFICE I969 o-ase-azu FORM PO-105O (10-69)

Claims

1. A disc jaw crusher apparatus comprising frame means, first disc jaw means mounted on the frame means for rotational movement about a vertical axis but axially fixed with respect to the frame means, second disc jaw means mounted on a hydraulically operable piston, said piston being axially and rotationally movably supported in a cylinder having a working chamber for the piston, said cylinder being fixedly secured to the frame means for positioning the second jaw means in spaced concentrically opposed operative relationship with the first jaw means, said first jaw means having a central feed hopper to receive material to be crushed, the crushed material being discharged from the peripheral space between said first and second disc jaw means, means to rotate at least one of said disc jaw means to urge material toward said peripheral space at an acceleration of at least 1 g, a source of pressurized hydraulic fluid including pressureless fluid return means and valve means connected with said working chamber for reciprocating said piston and second disc jaw means.

2. The invention defined in claim 1, wherein motor means is secured to said frame means, and means is provided for connecting said motor means to rotate the first jaw means.

4. The invention defined in claim 3, wherein said coupling means includes a pin secured to one of said jaws adjacent the periphery thereof and extending parallel with the axis of said jaws for operative slidable engagement with a portion of the other of said jaw means.

5. The invention defined in claim 1, wherein said cylinder includes hydrostatic bearing means for radially supporting and axially guiding said second piston.

6. The invention defined in claim 1, wherein said piston includes at least one annular chamber at one end of the piston for actuating said piston in an axial direction, said chamber being defined by a radially projecting surface spaced from one end of the piston and in sliding contact with the cylinder.

7. The invention defined in claim 1, wherein said source of pressurized hydraulic fluid includes motor means for actuating said valve means.

8. The invention defined in claim 7, wherein said motor means includes adjustable speed control means.

10. The invention defined in claim 1, wherein said valve means comprises a rotary slide valve.

11. The invention defined in claim 1, which includes a second annular working chamber means for supplying fluid to the first chamber at a substantially constant pressure to move said piston on one direction, and means for intermittently supplying fluid to the second chamber at a pressure greater than said first pressure to move the piston in the opposite direction.

12. The invention defined in claim 11, wherein the axis of said piston is vertical and said piston is positioned below said second disc jaw means, whereby the weight of said jaw means and material being crushed produces force acting to move the piston in a downward direction, and said first annular working chamber is positioned for urging the piston in a downward direction, said second annular working chamber being positioned for urging the piston in the upward directioN, and said intermittent fluid supply means includes means for supplying said fluid at a pressure sufficient to produce upward force greater than the downward force of the first working chamber, the jaw means and material being crushed.

13. The invention defined in claim 1, which includes motor means supported by the cylinder for rotating said piston.

14. The invention defined in claim 13, wherein said motor means includes a splined shaft rotatably supported by the motor means concentrically with the piston, said piston including a splined recess for axially movable nonrotational connection with the splined shaft.

15. The invention defined in claim 13, wherein said motor means includes a hydraulically actuated rotary element concentrically connected with said piston, and includes means for supplying fluid under pressure to said rotary element.

16. The invention defined in claim 15, wherein the supply of fluid to said rotary element may be varied to regulate the speed of rotation thereof.

17. The invention defined in claim 1, wherein said frame means includes means for adjusting the fixed axial position of said first jaw means, comprising an axial thrust bearing having an annular bearing support means, said frame means including an annulus surrounding said bearing support means in coacting threaded engagement therewith.

18. The invention defined in claim 1, wherein said frame means includes cylindrical casing means enclosing both said jaw means and said actuating cylinder.

19. The invention defined in claim 18, wherein said cylinder is axially slidably received within a cylindrical bush, said bush being secured within said casing by a plurality of radially extending bars.

20. A jaw crushing apparatus comprising a pair of opposed circular rotatably mounted plate-shaped jaws enclosing an annular outwardly narrowing crushing slot, one of said jaws being axially reciprocatable, feed inlet means for charging material through one of said jaws coaxially to the axis of rotation, drive means rotating at least one of said jaws continuously at a speed imparting to material entering said crushing slot an acceleration exceeding 1 g, at least one of said jaws being supported concentrically on a piston means having opposed working surfaces mounted for reciprocatory and revoluble movement in a fixed working cylinder means, means to intermittently supply hydraulic fluid under pressure to one of the opposed surfaces of said piston means in said working cylinder means to move the piston means and the jaw supported thereon toward the other said jaws, and means to urge the piston means in the opposite direction when said fluid is not supplied under pressure.

21. A jaw crusher as claimed in claim 20, wherein drive means for both said jaws are provided.

22. A jaw crusher as claimed in claim 21, wherein both jaws are adapted to be driven at the same angular speed.

23. A jaw crusher as claimed in claim 20, comprising means coupling said jaws at their circumference.

24. A jaw crusher as claimed in claim 20, wherein the jaw facing the material inlet has a plane crushing surface.

25. A jaw crusher as claimed in claim 20, wherein said feed means is a stationary chute.

26. The invention defined in claim 1, wherein said first disc jaw means is mounted on the frame means for free rotational movement.

27. The invention defined in claim 20, wherein the jaw other than said one of the pair of jaws is mounted for free rotational but axially fixed movement.