FR2888138A1 - PNEUMATIC ASSEMBLY FOR DESSAVING FOUNDRY CORES - Google Patents

Abstract

The invention relates to a grinding device for casting cores, comprising: - a barrel (1) comprising first compressed air supply means (20) and means (25) for discharging pulsed air; an approach cylinder (3) sliding in the drum under the action of the compressed air, between a retracted position and a working position where a rivet (5) is in contact with a work piece; and- striking means (50) for printing repeated shocks on the rivet.This device comprises: - means for closing (219) said air evacuation means, said closure means being able to inhibit said striking means during the stroke of the approach cylinder from its retracted position to its working position; and timing means (225) for opening said shutter means at the end of a predetermined time after occurrence of a given event.

Description

PNEUMATIC ASSEMBLY FOR INSTALLATION

  OF DESSABLAGE DES CORES DE FONDERIE

  The present invention relates to a pneumatic assembly for grit removal of foundry cores, and more particularly to such an assembly comprising: - a barrel comprising means for supplying compressed air and means for exhausting air; - An approach cylinder slidably mounted in the drum under the action of compressed air, between a retracted position and a working position where a rivet mounted at the free end of the approach cylinder is in contact with a workpiece working; and striking means for printing, under the action of compressed air, repeated shocks on the end of the rivet opposite its end of contact with the workpiece.

  Such a pneumatic assembly is known from document FR-A-2742365.

  In the installation described in this document, the drum is attached to a support vis-à-vis a workpiece to be de-sanded, itself attached to the support. At the start of the grit cycle, the approach cylinder is in the retracted position with the rivet knife away from the workpiece.

  The supply of compressed air is then fed so that the approach cylinder is advanced from its retracted position to its working position. The stroke of the cylinder is longer or shorter depending on the original distance between the free end of the rivet and the workpiece.

  When the free end of the rivet comes into contact with the workpiece, the striking means enter into operation. These striking means consist of a piston performing reciprocating movements in a bore of the approach cylinder, under the effect of the air pressure.

  Finally, the supply of compressed air is cut off so that the strikes are interrupted. The approach cylinder is returned to the retracted position under the effect of a return spring.

  Although generally satisfactory, this device has the disadvantage that the strike begins immediately after the arrival of the cylinder in the working position, without any possibility of control.

  The present invention aims to overcome this disadvantage.

  For this purpose, the subject of the invention is a pneumatic assembly for a grinding installation of foundry cores, comprising: - a barrel comprising means for supplying compressed air and means for exhausting air; - An approach cylinder slidably mounted in the drum under the action of compressed air, between a retracted position and a working position where a rivet mounted at the free end of the approach cylinder is in contact with a workpiece working; and - striking means for printing, under the action of compressed air, repeated shocks on the end of the rivet opposite its end of contact with the workpiece; means for closing said air evacuation means, said closure means being able to inhibit said striking means during the stroke of the approach cylinder from its retracted position to its working position; and delay means for opening said shutter means at the end of a predetermined time after the occurrence of a given event.

  Thus, the keystroke begins only after the expiry of the aforementioned period, which can be set to any suitable value and controlled by a PLC. This time can run for example from the departure of the approach cylinder from its retracted position, or its arrival in the working position.

  In a particular embodiment: the outer surface of the approach roll comprises a thickened part, said part having sealing means with a bore of the drum in which the approach roll is slidably mounted, said part delimiting between said outer surface and the inner surface of said bore a first compression chamber into which the compressed air supply means open to bring the approach cylinder from its retracted position to its working position; said air evacuation means comprise a hole in the wall of the barrel disposed in such a way that it opens into said first compression chamber; said closure means comprise a venting solenoid valve connected to said bore.

  More particularly, said thickened portion may form an annular bead in the middle portion of the approach cylinder, said bead delimiting between said outer surface of the cylinder and the inner surface of said bore said first compression chamber located on the side of the approach cylinder opposed to its free side and a second compression chamber located on the free side of the approach cylinder, and second compressed air supply means being provided to bring compressed air into the second chamber to move the cylinder of approaching his working position to his retracted position.

  The devices of the prior art had the disadvantage that the return of the approach cylinder from its working position to its retracted position was effected under the action of a helical return spring housed in an annular chamber between the wall of the bore and the approach cylinder.

  The first result was a limitation of the maximum stroke of the approach cylinder for a given overall length of the assembly. Indeed, the return spring must, when the cylinder is at its maximum stroke and is therefore compressed, exert on the cylinder a force less than that exerted by the supply pressure. But it must also exert sufficient effort to bring the cylinder to its retracted position while it is decompressed. The compression ratio was therefore limited, and therefore the maximum stroke of the approach roll was also limited.

  Another disadvantage was spring vibrations which could lead to premature deterioration of the bore wall.

  With the aforementioned arrangement, the movements of the approach cylinder in the bore of the barrel are fully pneumatically controlled in both directions. The spring is removed, which increases the maximum stroke of the cylinder and eliminates causes of premature deterioration.

  This provision can also find application independently of the other characteristics mentioned above.

  Also in a particular embodiment, the assembly according to the invention comprises: detection means for detecting the presence, respectively absence, of the compressed air supply pressure when the stroke of the approach cylinder to from its retracted position exceeds a maximum value; and inhibiting means for inhibiting said striking means when the supply pressure is, respectively, not detected.

  The devices of the prior art had the disadvantage that, when the cylinder had reached its maximum approach stroke, the striking means would come into action even if the free end of the end-of-stroke cutter had not encountered no work piece. This resulted in a vacuum operation not in accordance with the normal use of the device. This abnormal operation could cause damage to the device that could go as far as destruction.

  With the aforementioned arrangement, in one embodiment, in the case where the riveter has encountered no workpiece in its approach stroke, it exceeds its maximum stroke. The supply pressure is detected and typing is inhibited. Alternatively, the supply pressure is not detected when the stroke of the approach cylinder from its retracted position exceeds a maximum value and the striking is inhibited. Vacuum strikes are rendered impossible.

  This provision can also find application independently of the other characteristics mentioned above.

  The assembly according to the invention may also comprise blowing means for creating an air curtain around the approach cylinder at the location where it enters a body comprising the approach cylinder.

  The devices of the prior art had the disadvantage that sand from the parts being sanded had a tendency to enter the apparatus between the guide and retaining flange and the approach cylinder. This sand disrupted the operation of the tire assembly, and caused premature wear. It also caused an increase in the frequency of maintenance operations.

  With this last arrangement, the compressed air that leaks between the edge of the lip and the approach cylinder causes a blow around the cylinder, this blowing pushing the sand outside the device.

  This provision can also find application independently of the other characteristics mentioned above.

  Also in a particular embodiment, the assembly according to the invention comprises: detection means for detecting the presence or absence of a compressed air supply pressure greater than a predetermined threshold when the cylinder approach is in its retracted position and a rivet is mounted on the approach cylinder; and - inhibiting means for inhibiting said air supply means when the supply pressure is not, respectively is detected.

  The devices of the prior art had the disadvantage that they could be put into operation even in the case where, inadvertently, no rivet had been mounted on the approach cylinder. Such operation is obviously abnormal and can even be dangerous in case of ejection of the striking means.

  It is also desirable to be able to control the degree of wear and tightness of the striking means, which the known tire assembly is not capable of.

  With this last arrangement, in one embodiment, in the case where the rivet is not mounted on the approach cylinder, the supply pressure is not detected and the air intake means are inhibited so that the approach cylinder and the striking means are not actuated. Alternatively, the supply pressure is not detected when the rivet is not mounted on the approach cylinder, in which case the inhibition of the air inlet means occurs in the absence of detection.

  By detecting a predetermined threshold for the supply pressure, it is possible to control a characteristic leak of a leakage resulting from wear of the striking means.

  This provision can also find application independently of the other characteristics mentioned above.

  The invention also relates to a grit blast installation comprising a pneumatic assembly as described above.

  A particular embodiment of the invention will now be described, by way of nonlimiting example, with reference to the appended diagrammatic drawings in which: FIG. 1 is an external perspective view of a grit hammer according to the invention ; Figure 2 is an elevational view along the arrow II of Figure 1; FIG. 3 is an exploded perspective view of the grit hammer of FIG. 1; Figure 4 is an enlarged view of detail IV of Figure 3; Figure 5 is an enlarged view of detail V of Figure 3; Figure 6 is a simplified axial sectional view, particularly with respect to the section planes, of the grit hammer of Figure 1 with the approach cylinder in the retracted position; Figure 7 is a partial view similar to Figure 6 with the approach cylinder in the working position; Figure 8 is an enlarged view of Detail VIII of Figure 7; Figure 9 is a sectional view along the line IX-IX of Figure 2; - Figure 10 is a sectional view along the line X-X of Figure 2; FIG. 11 is an enlarged view of detail XI of FIG. 10 FIG. 12 is an enlarged view of detail XII of FIG. 9. FIG. 13 is a view on a larger scale of detail XIII of FIG. FIG. 14 is an enlarged view of detail XIV of FIG. 10 FIG. 15 is a view on a larger scale of detail XV of FIG. 9 FIG. 16 is a sectional view along the line XVI-XVI of Figure 2, with the approach cylinder in the retracted position; - Figure 17 is a view similar to Figure 16, with the approach cylinder in the working position; Figure 18 is a view similar to Figure 16, with the approach cylinder in over-stroke position; FIG. 19 is an enlarged view of detail XIX of FIG. 17; FIG. 20 is an enlarged view of detail XX of FIG. 16; FIG. 21 is a pneumatic diagram of the grit hammer of FIG. 1; and FIGS. 22a, 22b and 22c are diagrams illustrating the operation of the grit hammer of the preceding figures.

  The grit hammer shown in the figures comprises firstly a barrel 1 provided with fastening means 2 for mounting the hammer in a grit removal facility. An approach cylinder 3 is slidably mounted in a bore 4 of the barrel 1 from which it protrudes, and supports at its free outer end to the barrel a rivet 5. The end of the barrel opposite the exit of the approach cylinder is closed.

  The term "front" of the hammer will be designated hereinafter the direction of the free end of the approach cylinder and the rivet 5, and "backward" in the opposite direction, namely that of the closed end of the barrel 1.

  The approach roll 3 slides in the drum 1 from a retracted position (FIGS. 6, 9 and 16) to a working position (FIGS. 10 and 17) in which the free end of the rivet is in abutment with a workpiece. desilt. It will be seen hereinafter that, if nothing limits the stroke of the approach cylinder, it comes into over-stroke position (Figures 7 and 18).

  The outer surface of the approach cylinder 3 bears on the surface of the bore 4 in two sealing and guiding zones. A first zone is formed by a median bead 6 carrying a seal 7 at the front with the bore 4, behind which is formed an annular groove arranged to receive a guide segment 8. A second zone is formed by a hammer head 9 screwed to the rear of the approach cylinder 3, and whose outer surface carries a seal 10 behind a guide segment 11.

  At the front of the barrel 1, a guide and retainer flange 12 is mounted on the barrel by means of screws 13. The flange 12 receives in a suitable annular groove a seal 14 with the barrel. approach 3, and maintains against a shoulder of the bore 4 a resilient stop before 15. The stop 15 is provided to come into contact with a shoulder 16 before the bead 6 when the approach cylinder 3 is in the position of over-stroke aforementioned (Figure 8).

  Thus, the O-rings 7, 10 and 14 delimit two chambers between the outer surface of the approach roll 3 and the surface of the bore 4, respectively a rear chamber 17 between the seals 7 and 10 and a front chamber 18 between the joints 7 and 14. Another chamber 19 is formed at the rear of the approach cylinder 3, between the seal 10 and the bottom of the barrel 1. It will be seen below that, when the chambers 17 and 19 communicate, they form a first compression chamber, while the chamber 18 forms a second compression chamber.

  The barrel 1 is provided at its closed end with a port 20 for the admission of compressed air, capable of connecting the chamber 19 to a source of compressed air via a connector 21. The air source compressed is also capable of being connected to the chamber 18 via a connector 22 mounted on an orifice 23 formed in the wall of the barrel 1 immediately behind the elastic abutment 15. Finally, the chamber 17 is capable of be vented through a fitting 24 mounted on an orifice 25 formed in the wall of the barrel 1 immediately behind the bead 6 when the approach cylinder 3 is in its retracted position and forward of the seal 10 when the approach cylinder 3 is in its over-stroke position.

  The wall of the barrel 1 is pierced by a hole 26 which is behind the seal 7 when the approach cylinder 3 is in over-stroke position abutting the elastic abutment 15. The hole 26 thus opens into the chamber 17 when the approach cylinder 3 is in this position. A connector 27 is mounted on the hole 26 to connect the hole 26 to a pressure switch.

  The bore 28 of the guide and retainer flange 12 is provided with two annular grooves in front of the mounting groove of the seal 14 (Fig. 8).

  The front groove forms an annular chamber 29 connected by a bore 30 in the wall of the flange 12 to a coupling 31 which can be connected to a source of high pressure air. The annular chamber 29 is separated from the front face 32 of the flange 12 by an annular lip 33 forming a clearance with the outer surface of the approach cylinder 3, this clearance determining an annular blow orifice around the cylinder 3. A wiper seal 34 is disposed in the annular chamber 29 (Figure 15).

  The rear groove forms an annular chamber 35 connected by a bore 36 (Figures 6 and 15) in the wall of the flange 12 to a connector 37 may be connected to a source of high pressure air. A bore 38 is furthermore formed in the wall of the approach cylinder 3 so as to open on the one hand into the bore 39 of the approach cylinder, and on the other hand into the annular chamber 35 when the cylinder of approach is in its retracted position. The longitudinal location of the bore 39 of the approach cylinder 3 where the piercing 38 opens is normally occupied by the tail 40 of the rivet 5 when the rivet is mounted on the hammer.

  For the mounting of the rivet 5, the tail 40 of the rivet has an annular flange 41. A flange mounting bracket 42 is made in two parts separated by an axial plane and held together by a retaining spring 43 engaged in a groove outer 44 of the flange 42.

  The flange 42 is held axially on the approach cylinder 3 by means of an insert 45 also in two parts separated by an axial plane. The insert 45 is partially engaged radially in two opposite annular grooves 46 and 47 respectively formed on the outer surface of the approach cylinder and on the inner surface of the flange 12. The annular flange 41 is clamped between the front end face of the approach cylinder 3 and a washer forming a snap damper retained by an annular flange 49 of the flange 12.

  A piston 50 is also slidably mounted in the bore 39 of the approach cylinder 3. This piston 50 comprises a rear portion 51 of diameter corresponding to that of the bore 4 and a front portion 52 of smaller diameter. Thus, the front portion 51 of the piston defines with the bore 39 of the approach cylinder 3 an annular volume 53.

  When the grit hammer is in action with the rivet in contact with a workpiece, the piston moves back and forth between a rear position to be exposed hereinafter and a forward position in contact with the tail 40 5. A valve 54 acting as a distributor moving in a valve box 55, associated with conduits, which will be described below, formed in the walls of the approach cylinder 3 and other organs also described below, makes it possible to make these movements to the piston under the action of the compressed air admitted through the orifice 20 and escaping through the orifice 25.

  The valve 54 has a generally cylindrical outer casing in which two annular grooves 56 and 57 respectively are formed respectively. The outer casing therefore forms three ranges: a trailing pad 58, a median pad 59 between the two grooves, and a front pad 60. Inwardly, the valve 54 comprises a generally conical axial separation web 61, having at its apex a seat 62 substantially in the rear end plane of the valve, for a calibrated orifice which will be described hereinafter.

  Valve box 55 is a generally cylindrical annular member whose outer surface 63 is substantially cylindrical.

  The surface 64 of the bore of the valve box 55 comprises a set of annular milling forming grooves in circular arcs. From the rear towards the front, these milling forms the grooves 65, 66 and 67 cooperating with the grooves 56 and 57 of the valve 54. Another groove 68 cooperates with the groove 56, holes 69 passing through the wall of the box with a valve 55 opening at the bottom of the groove 68. Finally, another groove 70 is formed foremost in the surface of the bore 64.

  The front face 71 of the valve box 55 has a cavity 72 (Figures 8 and 12) communicating with holes 73 which open into the grooves 65 and 66 for the return of the cylinder.

  The valve 54 is retained in the bore 64 of the valve box 55 forwards by the rear face 90 of the approach cylinder 3 and rearwardly by a valve box cover 91.

  The cover 91 has an axial bore 92 whose front end forms a calibrated orifice 93 which has been discussed above. This orifice 93 is able to bear on the seat 62 by sliding the valve 54 in the bore 64 and thus be closed. A set of peripheral holes 94 also generally pass axially through the cover 91, however, moving away from the rear towards the front. The bores 94 communicate with axial bores formed in the valve box 55, these axial bores opening into the groove 70.

  The rear part of the hammer head 9 comprises a plastic plug 100 forming a damper. Holes 101 allow the admission of the high pressure into the apparatus.

  In its middle part, the wall of the hammer head 9 has holes 110 opening into grooves 111 (Figure 8) for connection to the exhaust.

  Bores 120 distributed circumferentially and converging forwardly through the wall of the approach cylinder 3 to the bore 39, from the rear shoulder 121 of the bead 6 where they thus open into the chamber 17. A radial bore 122 through which the function will be explained below, is also formed in the wall of the approach cylinder 3, opening into the annular space 53 when the approach cylinder 3 is in its retracted position. A ring 123 engaged in an annular groove formed in the bore 39 guides the front portion 52 of the piston 50 while in this position.

  Finally, a blind hole 124 opens into the bore 39 in the extension of the bore 122, which is plugged by a plug 125. The bore 124 communicates with radial bores in the wall of the approach cylinder 3 for feeding and venting.

  Referring now to Figure 21, the grit hammer is shown with its connections 21, 22, 24, 27, 31 and 37 described above. Pipes are connected to these connections and out of a grit chamber 200 in which is mounted the hammer, to connect it to pneumatic or electropneumatic equipment which will be described below, but which are all located outside the cabin 200. This equipment is therefore not subject to aggressive conditions in the cabin. It is also easy to ensure their maintenance.

  Compressed air is fed through a pipe 201 of a compressor not shown to a filter 202 and then to a pressure regulator 203. From there, a portion of the compressed air is fed through a pipe 204 to a lubricator 205, while the other part is used non-lubricated as will be described below.

  Lubricated compressed air is fed through line 206 to a two-way electropneumatic distributor 207. One channel 208 is connected to the connector 21 and the other channel 209 is connected to the connector 22 to respectively provide forward and reverse movements of the approach cylinder 3.

  The non-lubricated compressed air is led on the one hand by a pipe 210 to a blowing valve 211 and thence to the coupling 31. It is, on the other hand, led via a pipe 212 to a valve 213 and from there to the connection 37 via a pipe 214. A pressure switch 215 is connected to the pipe 214 for detecting the presence of the rivet 5 on the approach cylinder 3.

  The connector 27 is connected by a line 216 to a pressure switch 217 for the over-travel detection of the approach cylinder 3.

  Finally, the connection 24 is connected by a pipe 218 to a valve 219 and from there to a venting pipe 220.

  The pneumatic or electropneumatic equipment that has just been described is electrically connected by lines not shown to a programmable controller 225 which controls the equipment so as to ensure the programmed operation of the grit hammer.

  The sandblasting hammer which has just been described operates in the following manner.

  An operating cycle starts with the retracted approach cylinder 3, by detecting the presence of the rivet 5 on the approach cylinder 3. In this position of the approach cylinder 3, the bore 38 is in communication with the valve 213 through the annular chamber 35, the bore 36, the connector 37 and the pipe 214. The valve 213 is then open. If the rivet is absent, the pressure switch 215 detects no pressure. The cycle is interrupted and an alarm is generated. The cycle can also be interrupted if the detected pressure is below a certain threshold, which indicates an abnormal level of leakage.

  If the rivet is present, the bore 38 is plugged so that the pressure switch 215 detects a pressure. The valve 213 is then closed and the cycle continues.

  Firstly, the valve 219 is closed, which has the effect of blocking the escape of the chamber 17. This blocking has the effect of inhibiting the drive of the piston 50 until the valve 219 is subsequently opened . The distributor 207 is then controlled so as to bring the lubricated compressed air to the connection 21 and thence to the chamber 19 and then to the chamber 17. The approach cylinder 3 advances under the effect of the compressed air until it comes into abutment, either against a workpiece via the rivet 5, or by contacting its shoulder 16 with the abutment 15.

  The next step is to detect if a workpiece is absent, that is to say, if the approach cylinder 3 comes into over travel abutting against the stopper 15. In this case, the seal 7 exceeds the hole 26 which has the effect of putting the pressure switch 217 in communication with the high pressure of the chamber 17 via the connector 27 and the pipe 216. When this high pressure is detected, the cycle is interrupted, the compressed air supply is cut by the distributor 207 and an alarm is generated.

  If the high pressure is not detected by the pressure switch 217 after a predetermined time, the blast valve 211 is opened to blow an air curtain around the approach cylinder 3, and the vent valve 219 free of the chamber 17 is opened so as to start the striking of the rivet by the piston 50.

  The typing step is broken down into three-phase cycles illustrated respectively in Figures 22a, 22b and 22c.

  In FIG. 22a, the piston 50 is in the advance phase. The high pressure is exerted on the rear face 230 of the piston via the groove 70. The groove 66 is exhausted through the annular groove 56 and the holes 69. The valve 54 is held in position. rear position by the high pressure in the groove 70, with the calibrated orifice 93 resting on the seat 62.

  When the front face 231 of the piston 50 exceeds the bores 120 of the exhaust (via the chamber 17), the air at the front of the piston is driven by the groove 66, the groove 56 and the holes 69. rear 230 of the piston exceeds the holes 120, there is a depression at these holes. Valve 54 which experiences high pressure through port 93 tends to move downward.

  In FIG. 22b, the valve 54 has been brought to the advanced position by the high pressure. The piston 50 hits the rivet 5 and bounces. Groove 65 is under pressure and valve 54 is pressed forward.

  When the rear face 230 of the piston exceeds the bores 120, there is compression on the valve 54 for its return to the rear position. The groove 67 creates a leak between the grooves 70 and 65 so as to damp the stroke of the piston between the bores 120 and the valve box 55. When the front face 231 of the piston exceeds the bores 120, there is depression on this face before and the valve 54 returns to the rear position.

  In FIG. 22c, when the front face 231 of the piston exceeds the bores 120, there is a depression on this face. The valve 54 moves backwards and closes the intake in the groove 67. The groove 66 is at the exhaust. When the valve arrives in the rear position, we end up at the beginning of the first phase.

  At the end of a delay corresponding to the desired striking time, the distributor interrupts the compressed air supply of the coupling 21 to supply the connection 22 via the pipe 209. The striking stops and the chamber 18 is fed, which has the effect of pushing the approach cylinder 3 back while the air in the chambers 17 and 19 escapes via the connector 24 and the valve 219.

  The cycle is complete when the approach roll 3 has returned to its retracted position and the blow valve 211 is closed.

  It will be observed that all the sensors necessary for the operation of the device are electropneumatic, which makes it possible to avoid the use of fragile optical components such as fibers, cells, lasers, etc.

Claims (3)

  1 - Pneumatic assembly for sandblasting installation of foundry cores, comprising: - a shaft (1) comprising means (20) for supplying compressed air and means (25) for exhausting air; an approach cylinder (3) slidably mounted in the drum under the action of the compressed air, between a retracted position and a working position where a rivet (5) mounted at the free end of the approach cylinder is in contact with a work piece; and - striking means (50) for printing, under the action of compressed air, repeated shocks on the end of the rivet opposite its end of contact with the workpiece; characterized in that it comprises - shutter means (219) of said air discharge means, said shutter means being adapted to inhibit said striking means during the stroke of the approach cylinder from its position retracted to his working position; and timer means (225) for opening said shutter means after a predetermined time after the occurrence of a determined event.   2 Apparatus according to claim 1, wherein said event is the departure of the approach cylinder from its retracted position.   3 Apparatus according to claim 1, wherein said event is the arrival of the approach cylinder in its working position.   4 Apparatus according to any one of claims 1 to 3, wherein: - the outer surface of the approach cylinder comprises a portion of extra thickness (6), said portion having sealing means (7) with a bore (4). ) of the barrel in which the approach cylinder is slidably mounted, said portion delimiting between said outer surface and the inner surface of said bore a first compression chamber (17, 19) into which the compressed air supply means for bringing the approach cylinder from its retracted position to its working position; said air evacuation means comprise a bore (25) in the wall of the barrel arranged so that it opens into said first compression chamber; said closure means comprise a venting solenoid valve connected to said bore.     Device according to claim 4, wherein said thickened portion forms an annular bead (6) in the middle part of the approach cylinder, said bead delimiting between said outer surface of the cylinder and the inner surface of said bore said first compression chamber located on the opposite side of the approach cylinder to its free side and a second compression chamber (18) located on the free side of the approach cylinder, and second compressed air supply means (23) being provided to bring compressed air in the second chamber to move the approach cylinder from its working position to its retracted position.   6 - Device according to any one of claims 1 to 5, comprising: - detection means (26) for detecting the presence or absence of the compressed air supply pressure when the cylinder stroke of approach from its retracted position exceeds a maximum value; and - inhibiting means (225) for inhibiting said striking means when the supply pressure is respectively not detected.   Pneumatic assembly according to any one of claims 1 to 6, comprising blowing means (29) for creating an air curtain around the approach cylinder at the location where it enters a body (1, 12) comprising the approach cylinder.   8 pneumatic assembly according to any one of claims 1 to 7, comprising: - detection means (36, 38) for detecting the presence or absence of a compressed air supply pressure greater than a threshold predetermined when the approach cylinder is in its retracted position and a rivet is mounted on the approach cylinder; and - inhibiting means (225) for inhibiting said air supply means when the supply pressure is not, respectively is detected.   9 - Sandblasting installation, characterized in that it comprises a pneumatic assembly according to any one of claims 1 to 8.