CA1157327A - Method of automatic operation of a percussion apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The invention provides a method of automatic operation of a percussion apparatus of the type consisting in letting fall or driving a percussion element onto an object to be struck such as a pile, a nail, a tube or the like, in relifting the said percussion element by means of a cable passing round a pulley and connected to a drum driven in rotation by driving means at least in one direction of rotation, characterized in that the said percussion element is connected by a mechanical means to the said drive drum to form a closed circuit in such a manner as to cause the stoppage of the drum to substantially coincide with the moment the strike of the said percussion on the said object ends.

Description

The present invention relates to a method of automatic operation of a percussion apparatus and the percussion apparatus itself.
The present invention relates more particularly to a percussion apparatus, a pile driver, especially of the free-fall type.
Free-fall-type pile drivers normally comprise a platform on which are mounted a winch and a mast. A pulley is fixed at the upper end of the mast, a cable wound around lQ the winch drum passing round the pulley and descending verti-cally down to a hammer or percussion element supported thereby.
The hammer is of course aligned with the pile or the object to be struck in order to be driven into the ground. During the operation, the winch motor operates permanently and the ~perator controls a clutch and a drum brake. Thus, when the hammer is at the desired height, the clutch is disengaged, thus allowing the hammer to fall freely and resulting in a free rotation of the drum. At the moment the hammer contacts the pile, the operator actuates the brake so as to prevent 2Q continued unwinding of the cable upon sudden stoppage of the hammer. Immediately after, he disengages the brake and actuates the clutch so that the winch can drive the drum in the opposite direction and thus relift the hammer.
There are also known other free-fall-type pile drivers using for example a crane, the stages of the operation remaining however the same.
This conventional system suffers from certain draw-backs, .~

ll~7327 for the whole of the operation depends on the operator's skill. In the first place, the frequency of the operation, that is, the number of strikes per minute applied to the pile, depends not only on the possibilities of the machine, but also on the operator's experience. Moreover, any inattention on the part of the operator in actuating the drum brake causes the unwinding of the cable after the stoppage of the hammer, thus resulting in a highly dangerous si-taation. Likewise, should the operator actuate the brake too early, the hammer will be braked before the completion of the strike on the pile, since the precise moment for stopping the drum is at the end of the strike, when the pile has accomplished its pene-tration into the ground.
Only a highly experienced operator can actuate the brake at the ideal moment, which moment varies according to the machine, since its action is not instantaneous.
Moreover, the actuation of the brake while the hammer is still at a certain height of the pile may result in complete deterioration, through deterioration of the brake or even through rupture of the cable, which constitutes a great hazard.
Consequently, in order to obtain optimum operation of the machine, the operator must be highly experienced and skilled in order to choose the precise moment when he must discontinue applying the brake and actuate the clutch to relift the hammer without losing time.
Lastly, the operator must keep the fall height constant, which height must always be measured with respect to the top of the pile and, of course, is every time at a lower level.
In practice, it often occurs that, as a result of in-sufficient brake efficiency or even for the sake of operator's convenience, the drum is not stopped by actuating the brake but by actuating the clutch a few moments before the hammer strikes the pile. This of course results in increased wear of the clutch, the life of which is thus considerably reduced.
One of the purposes of the present invention is to avoid one or several abovementioned drawbacks, by providing a per-cussion apparatus whose operation is entirely automated, thus making the presence of a highly experienced operator un-necessary.

, L 1~7327 To this end, the invention provides a method of automatic operation of a percussion apparatus of the type consisting in letting the percussion element fall, or in driving the same, onto an object to be struck such as a pile, a nail, a tube or the like, in order for example to drive it into the ground, in relifting or displacing the said percussion element by means of a cable passing round a pulley and connected to ~a drum, in driving the said drum in rotation by driving means at least in one direction of rotation, characterized lQ in that the said percussion element is connected by a mechanical means to the said drum to form a closed circuit in such a manner as to cause the stoppage of the drum to substantially caincide with the moment the strike of the said percussion element on the said object ends.
Advantageously, the aforementioned mechanical means of connection are constituted by the cable referred to as a "return cable".
Thus, at the moment the hammer ends its strike on the object to be struck and stops, the winch drum is auto-matically stopped by the return cable.
When such a system is used manually, the operator'sw~ark consists merely in actuating the clutch upon stopping the drum and in disengaging it when the hammer is suspended at the required height before its following fall onto the object to be struck.
Preferably, according to the invention, the operation of the percussion apparatus is completely automated by the fact that the stoppage of the drum automatically results in starting the means for driving the latter in order to relift the per-cussion element to a predetermined height, and that the heightat which said percussion element is relifted is controlled by measuring the number of revolutions of the said drum.

-: 1 1 57327 Under such conditions, the operation of the percussion apparatus is completely automatic and it is sufficient for the operator to start the operation and then stop it when the struck object is completely driven in.
Moreover, the invention is also directed to a percussion apparatus for carrying out the above-described method of lQ

- 3a -l 157327 operation, of~etype comprising a driving drum, driving means for causing the said drum to rotate at least in one direction, a pulley, a cable referred to as an "operating cable" passing round the pulley connected to the drum, and to which is attached a percussion element, characterized in that the per-cussion element is connected by a second cable, referred to as a "return cable", to the said driving drum so as to form a closed circuit in such a manner as to cause the stoppage of the drum to substantially coincide with the moment the strike of the percussion element on the object to be struck ends.
According to another charac-terizing feature of the in-vention, the said closed circuit is obtained by means of a single cable both ends of which are attached to the said driving drum, the percussion element being attached to an intermediate portion of the said cable.
Advantageously, the apparatus according to the invention comprises means for tensioning the cable or cables. Moreover, it comprises devices for connecting the aforesaid driving means for causing the aforesaid driving drum to rotate in a first direction and for disconnecting the said driving means after a predetermined rotation of the driving drum in the said first direction. Thus, the percussion element is relifted to a predetermined height above the object to be struck. More-over, the aforementioned connecting and disconnecting means connect the aforementioned driving means to the driving drum substantially at the moment the percussion element contacts the object to be struck.
Advantageously, said driving means and driving drum are mounted on a winch, and comprise a clutch device allowing the driving means and the driving drum to be coupled together, the said clutch device being preferably controlled by the said connecting and disconnecting means.
Other characterizing features, advantages and details of the invention will appear more clearly from the following explanatory description made with reference to the appended diagrammatic drawing illustrating several forms of embodiment of the present invention and given solely by way of example and wherein, j.~.,~ ....

;.~.,.,':',, - Figure 1 is a diagrammatic view illustrating a per-cussion apparatus according to -the present invention, in its most usual form ;
- Figures 2 to 5 are diagrammatic views illustrating various forms of embodiment of a free-fall-type pile driver according to the present invention ; and - Figure 6 is a diagramma-tic view of a device for auto-mating the driving of the drum of the percussion apparatus according to the present invention.
The device in the Figure illustrates the application of the method of the invention to any percussion apparatus, which is not necessarily a pile driver. Indeed, the method of automatic operation according to the present invention can be applied to all types of percussion apparatus, whether hydrau-lic, mechanical or electrical, provided with at least one winch.
As shown in Figure 1, according to the invention, the closed circuitof a percussion apparatus comprises a drum 1, a pulley 2 and a cable 3 wound several times around the drum 1 thereafter passing round the pulley 2 to lastly return onto the drum 1. At a given point of the cable 3 extending between the drum 1 and the pulley 2, a percussion element 4 is fixed so as to strike any body designated by the reference numeral 5.
The cable or the chain 3 may be continuous, it may have both its ends connected to a percussion element 4, or it may have both its ends attached to the drum 1. Whatever t~e case, according to the invention, the circuit formed by the cable is closed, and any movement of the percussion element 4 results in a corresponding automatic and forced rotation of the drum 1 and of the pulley 2, and vice-versa.
Both the drum 1 and the pulley 2 are connected to the apparatus and the pulley 2 is advantageously acted upon by resilient means (not shown) which keep the cable 3 under per-manent tension.
In case the movement of the percussion element 4 is sub-stantially horizontal, as illustrated in Figure1, andtherefore there is no free fall of the percussion element 4, the drum 1 ,-;~....~.~...;

, is provided with reversible driving means. Thus, during the operation, the drum 1 is operated so as to rotate in the direction of arrow S1, the percussion element 4 being dis-placed leftward in Figure 1, until a strike is applied to the body 5. This strike on the body 5 stops the displacement of the percussion element 4, thus automatically resulting in the stopping of the drum 1 and the disconnection of the means for driving the drum 1 in the direction S1. Immediately after, the drum 1 is actuated by the driving means in the direction S2 to again move the percussion element 4 away from the body 5. The distance over which the percussion element 4 is moved away from the body 5 is controlled by measuring the number of revolutions of the drum 1, and when this number reaches a pre-determined value, other adequate means reverse the rotation of the drum 1 to thereafter begin a new cycle.
It should be noted that, in view of the configuration of the closed circuit, the reaction between the percussion element 4 and the body 5 produces a braking effect on the drum 1, thus making completely unnecessary the use of a conventional brake.
Moreover, since the distance over which the percussion element 4 automatically moves away from the body 5 is determined by the number of revolutions of the drum 1, from the moment the per-cussion element is stopped in contact with the body 5, the said distance is constant with respect to the position of the body 5. Stated in other words, the impact of the percussion element 4 always results from its displacement over a pre-determined distance.
The apparatus illustrated in Figure 7 has many applications such as horizontal nailing of iron or other elements, driving in of tubes, piles or the like into the ground. However, its main application is a percussion apparatus or pile driver of the free fall type such as the one illustrated diagrammatically in Figure 2.
Referring to Figure 2, a pile driver comprises conven-tionally a platform 6 on which is mounted a winch carrying a drum 1 and a mast 7 provided at its upper end with a pulley 2 to ensure the rotation of the cable 3.
According to the invention, a second pulley 8 is mounted .: . , - .: ., .;:
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at the upper portion of the mast 7 to ensure the rotation and guiding of the return cable 3 in such a manner as to maintain vertical the cable portion located below the percussion element or hammer 9. In the case illustrated in Figure 1, the appara-tus comprises two cables 3, 3a to form the closed circuit according to the invention.
The cable 3, referred to as the "operating cable", having one end attached to the drum 1, is wound several times round this drum then passes round the pulley 2, has its other end attached to the upper end of the hammer 9. Likewise, the cable 3a having one end attached to the drum 1 is wound round the drum in the contrary direction to the winding direction of the cable 3, and then passes round the pulley 8 and is attached by its other end to the lower end of the hammer 9.
It is understood that the points of attachment of the cable to the drum are given by way of example, thus the end of the cable 3a may be attached to any portion of the hammer without however departing from the scope of the invention.
Moreover, it is obvious that, as described previously, use may be made of a single cable 3 wound round the drum 1 and having both its ends attached to the hammer 9.
Advantageously, the pulley 9 is acted upon by resilient means (not shown) so as to maintain the cable 3a under tension.
From a mere analysis of Figure 2 it appears that when the hammer 9 strikes on a pile 10 to be driven for example into the ground, and when the latter, after having been driven to a certain depth into the ground, stops the displacement of the hammer, drum 1 also is simultaneously stopped by the return cable 3a. In the preferred form of embodiment of the invention, this stoppage of the drum 1 actuates the clutch of the winch, whose motor is in permanent opèration, so that the drum 1 is driven to relift the hammer 9 to the desired height to allow the following stage of the operation to be carried out. In other words, after a predetermined number of revolutions of the drum 1, corresponding to the desired lifting height of the hammer 9, the clutch is again actuated to disconnect the means driving the drum 1 to allow the latter to rotate freely. At that moment, the hammer 9 falls by gravity onto the head of the . . ..........
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pile 10, and so on.
Thus, owing to the con-trol of the number of revolutions of the drum 1 during the relifting of the hammer 9, -the fall-ing height of the latter is constant with respect to the top of the pile, independently of the degree of penetration thereof into the ground. There is thus obtained a pile driver whose operation is completely automated, which continues to apply strikes to the pile 10 until the operator stops the system when the pile has been sufficiently driven into the ground.
One example of the means allowing automatic control of the r.1eans for driving the drum 1 is illustrated in Figure 6.
It is understood that there exist many means or devices allowing such automation, the system described hereunder serving merely to indicate one of those means. The device com-prises a case 11 within which is mounted shaft 12 supported by a bearing 13. The end 14 of the shaft 12 is connected by any appropriate means to the winch of Figure 2, so as to be ro-tatable jointly with the shaft of the drum 1. The medial portion of the shaft 12 is provided with a thread15000perating with a nut 16, the latter being provided with a projection 17 engaged into a straight guiding slot 18 formed on one of the inner walls of the case 11, in parallel relationship to the shaft 12. When the shaft 12 rotates, the nut 16 is displaced axially. The nut 16 is also provided with a second projection or stop 19 whose function will be explained hereafter.
Moreover,ablock 20 subjected to the action of a spring 21 is mounted in the case 11 on the shaft 12 between the end 14 of the latter and the thread 15. The distance between the block 20 and the thread 15 is slightly smaller than the thiokness of the nut 16, so that when the shaft 12 rotates and causes the nut 16 t~leavethethread 15, the block 20 prevents the nut from continuing its horizontal displacement and always keeps it against the starting point of the first turn of the thread 15, under the action of the spring 21. When the rotation of the shaft 12 is reversed, the nut 16 can thus again engage the thread 15 and be displaced leftward (Figure 6).
The case 11 is provided with two electric connections 22 and 23. Connection 23 is connected to the electric contact 24 .~ .. :..:..,:
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" 1157327 of the spring type, whereas the connection 22 is electrically connected to a shaft 25 parallel with the shaft 12 and to a second electric contact 26 at the end of the shaft 25. The shaft 25 can thus perform small displacements towards the left or the right, so as to separate or join together the contacts 24 and 26, respectively. The end of the shaft 25 is also pro-vided with a groove 27 which, in the closed position of the contacts, bears upon a steel ball 28 supported by a spring, the shaft 25 thus being resiliently maintained in the circuit closing position.
On the other hand, substantially below the block 20, a member 29 in the form of a nut adapted to co-operate with the stop 19 of the nut 17 is mounted on the thread 30 provided on the shaft 25. This thread also receives a second nut 31 pro-vided with a lateral projection 32 engaged into a slit 33 formed in another wall of the case 11 in parallel relationship to the shaft 25.
The end 25a of the shaft 25 is provided with an extension passing through the wall of the case 11 thus allowing the po-sition of the nut 31 of the shaft 25 to be controlled, that is, the axial position of the nut 31 to be modified by merely ro-tating the button 34.
Likewise, the end 12a of the shaft 12 is provided with an extension which passes through the wall of the case 11 and ends with a button 35, so that when the button is pulled outward the end 14 of the shaft 25 is disconnected from the winch or from the drum 1, so that it is possible, by rotating this button, to adjust the initial position of the nut 16.
Thus, with the device illustrated in Figure 6, mounted in co-operation with the pile driver of Figure 2, the hammer is first positioned above the pile 10 and bears upon the latter.
By rotating the button 35, the nut 16 is displaced towards the right in Figure 6 until it leaves the thread 15 and comes into contact with the member 29, pushing the shaft 25 into the closed circuit position of the contacts 24 and 26, the sphere 28 being located in the groove 27. By connecting the winch or the drum 1 and a manual key of the clutch circuit, the winch begins to wind the operating cable 3, supporting the hammer 9 and simul-.... ....
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taneously causes the shaft 12 to rotate, thus causing the dis-placement of the nut 16 towards the left until the stop 19 comes into contact with the nut 31 of the shaft 25. This con-tact results in the displacement of the shaft 25 towards the lef~, ~s s~ating the contacts 24 and 26 and disconnecting the clutch. The hammer 9, having reached the desired height, then freely falls onto the pile. Its fall caE;es the shaft 12 to rotate in the opposite direction and the nut 16 is quickly displaced towards the right until it comes into contact with the member 29 at the moment it leaves the thread 15. This contact on the member 29 again displaces the shaft 25 towards the right in Figure 6, thus interconnecting the contacts 24 and 26 and again connecting the clutch. It is obvious that, even with the clutch thus connected, the hammer descends additionally over a small distance equivalent to the penetration of the pile into the ground, as a result of the action of the hammer on its head. However, this distance is relatively reduced and does not result in an abnormal wear of the clutch. It is of course obvious that once the nut 16 is adjusted by means of the button 35, at the beginning of the driving in of a given pile, and the block 20, owing to the spring 21, always maintains the nut 16 applied against the initial turn of the thread 15, the lifting height of the hammer 9, with respect to the pile head, is always determined, from the position of the said pile head at the end of the strike, by the distance between the member 29 and the nut 31 on the shaft 25. This height is of course ad-justable by rotating the button 34, as was described previously.
It should also be noted that the device described above and shown in Figure 6 illustrates only one of a number of man-ners of performing the automation of a pile driver by using the present invention. It is also possible, for instance, to de-termine the moment of disconnection of the clutch so as to make it accurately coincide with the stoppage of the winch drum 1, i.e. the moment the pile 10 ends its penetration into the ground, thus completely stopping the hammer 9.
Referring to Figures 3, 4, 5, there will now be described other forms of embodiment for forming the closed circuit con-stituted by the operating and return cables of a percussion mr~..Y....~
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element or hammer, according to the invention, and usable no-tably in pile drivers of the free fall type.
In the form of embodiment illustrated in Figure 3, hammer 9 is connected by its upper end to tl~e operating cable 3 passing round a pulley 2 mounted at the top of the mast (not shown) of the pile driver, the other end of the said operating cable being connected to the shaft of a pulley 35 so as to support the latter. Moreover, the cable 3 passes round a guide pulley 36. The hammer 9 is also connected to the return cable 3a which passes round the pulley 8 attached to the bottom portion of the mast, and after passing round the guiding pulley 37, is connected to the shaft of the pulley 38 so as to support the latter. As appears from this Figure, pulleys 38 and 35 are therefore movable and their displacement depends upon the dis-placement of the end of the operating and return cables 3 and 3a, respectively, connected to the hammer. The displacement of these pulleys is adjusted and controlled by a third cable 39 and a fourth cable 40, respectively, having one of their ends connected to fixed points 41 and their other end attached to the driving drum 1 of the pile driver. In the preferred form of embodiment of the invention, the drum 1 is connected to a device for automating the operation of the pile driver. On the other hand, according to the invention, the third and fourth cables ~9 and 40 are wound round the drum 1 in opposite direc-tions. Thus, the closed circuit according to the invention is constituted by the cable 39, the operating cable 3, the return cable 3a-and the cable 40. This form of embodiment is par-ticularly advantageous when the hammer 9 is of great weight.
In another form of embodiment illustrated in Figure 4, the pile driver comprises two drums 1, 42. Drum 1 is referred to as the "driving drum" for it is driven and connected to the driving means controlled by the automating device according to the invention, whereas the drum 42 is a conventional drum for winding the hammer operating cable. In this form of embodiment, the operating cable 3 connected to the hammer 9 passes round a pulley 35a so as to support the latter, and round guiding pulleys 36, and is wound onto the drum 42 The shaft of the pulley 35a is connected to the shaft of the pulley 35, in such a manner that these two pulleys are movable jointly. In other 7 ! ~ "*~
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~:::..:, words, when the pulley 35 is displaced upwards, the pulley 35a is also displaced upwards, or vice-versa. The thus assembled pulleys 35, 35a define a movable block 43. In a manner similar to the foregoing form of embodiment, a third cable 39 having one end attached to a fixed point 41 and its other end attached to the automated drum 1 passes round the pulley 35 so as to control the displacement of the movable block 43. The circuit is closed by the return cable 3a wound in the opposite direction to the direction of winding of the cable 39 on the drum 1 and connected to the hammer 9.
In another variant of embodiment illustrated in Figure 5, and usable notably for bottom striking during the driving of a tube 44 into the ground, and comprising a hammer 9 penetrating into the tube in order to compact the earth at the lower por-tion of the latter and a striker 45 striking on a damper 46 placed at the end of the tube 44 to drive the latter into the ground. The movable block 43 comprises two pulleys 35a round which pass the cables 3 for operating the hammer and striker 45. In this apparatus, it is obvious that the return cable 3a is attached to the striker and not to the hammer 9. However, since the striker and the hammer 9 moye in synchronism, the operation of this form of embodiment is in conformity with the present invention. Advantageously, the operating cable 3 of the striker and the return cable 3a may be constituted by a single cable, and in this case the striker is attached to an intermediate portion of the said cable.
Thus, the present invention provides a percussion apparatus whose operation is automated, allowing the drum 1 to be stopped when the percussion element is stopped on the element to be struck. Such automatic stoppage of the drum allows the wear of the elements of its clutch mechanism to be reduced.
Moreover, the productivity of a percussion apparatus ac-cording to the present invention is considerably increased even when the latter is used by fairly skilled operators.
Another advantage of the invention resides in the fact that the time necessary for the installation and starting of a per-cussion apparatus according to the invention is very short, thus reducing investments, compared notably to the diesel ham-.. I : 1 . 1 . '' .::.: :...
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``` 1157327 mer used heretofore.
Of course, the invention is by no means limited to theforms of embodiment described and illustrated which have been given by way of example only. Thus, thoæ various forms of em-bodiment may be combined according to the use of the per-cussion machine.

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Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for the operation of a percussion apparatus comprising letting fall or driving a percussion element onto an object to be struck, lifting said percussion element again by a cable passing round a pulley and connected to a drum rotated by a driving means at least in one direction of rotation, said percussion element being connected by mechanical means to the said drum to form a closed circuit so as to cause stoppage of the drum to substantially coincide with the moment the striking of said percussion on the said object ends.

2. A method according to claim 1, in which the mechanical connecting means is a cable, a chain.

3. A method according to claim 1, in which the object to be struck is a pile, nail or tube.

4. A method according to claim 1, 2 or 3, in which the stoppage of the drum automatically starts the operation of the drum driving means so as to move the percussion element away from the object to be struck, and the distance from the said percussion element to the said object to be struck is controlled by controlling and counting the number of revolutions of the drum.

5. A percussion apparatus comprising a drive drum, means for driving the said drum in rotation in at least one direction, a pulley, an operating cable passing round the pulley and connected to the drum, and to which is attached a percussion element, the percussion element being connected by a return cable to the drum to form a closed circuit as to cause the stoppage of the drum to substantially coincide with the moment that striking of the percussion element on the said object to be struck ends.

6. An apparatus according to claim 4, in which the closed circuit comprises a single cable, the percussion element being attached to an intermediate portion of the said cable.

7. An apparatus according to claim 5 or 6, including means for maintaining the cable or cables in rotation.

8. An apparatus according to claim 5, including devices for connecting the driving means for rotating the drive drum in a first direction, and disconnecting said driving means after a predetermined rotation of the drive drum in the said first direction.

9. An apparatus according to claim 8, in which said devices connect the driving means to the drive drum substantially at the moment the percussion element contacts the object to be struck.

10. An apparatus according to claim 5, in which said driving means and said drive drum are mounted in a winch.

11. An apparatus according to claim 8, including a clutch device for coupling together the driving means and the drive drum.

12. An apparatus according to claim 11, in which said devices control the clutch device.

13. An apparatus according to claim 5 of the free-fall pile-driver type comprising a pulley mounted at the top of a mast or pole, a hammer as a percussion element and an operating cable passing round the pulley and connected to the said hammer and a drive drum, a second pulley being mounted at a lower level than that of the first pulley, the hammer being connected to the said cables forming the closed circuit, between the two pulleys.

14. An apparatus according to claim 13, in which the closed circuit is a single cable wound several times round the drive drum and having its ends attached to the hammer.

15. An apparatus according to claim 13, in which said closed circuit comprises two cables, each having a first end attached to the drive drum and being thereafter wound round said drum, in the opposite direction, and their other end being attached to the hammer.

16. An apparatus according to claim 13, in which the displacements of the hammer are adjusted and controlled by a third cable having one end connected to a fixed point and the other end connected with said drive drum, the said third cable passing round a pulley with a movable axis pin, the axis pin of the said pulley being connected directly to said cable for operating the said hammer. the said return cable and the said third cable being wound on the said drive drum in opposite directions.

17. An apparatus according to claim 16, in which the movable pulley is mounted in a movable assembly or block comprising at least one other pulley with a movable axis-pin, round which passes the hammer operating cable, the axis-pins of the said pulley being jointly movable, the end of the said operating cable being attached to a second, freely rotating drum.

18. An apparatus according to claim 16 or 17, in which the end of the return cable not connected to the hammer is connected to the axis-pin of a movable pulley round which passes a fourth cable having one end connected to a fixed point and its other end connected to the drive drum, the third cable and the fourth cable being wound round the drive drum in opposite directions.