WO2007042780A2

WO2007042780A2 - Actuator assembly

Info

Publication number: WO2007042780A2
Application number: PCT/GB2006/003743
Authority: WO
Inventors: Adrian Keith Postles; Andrew David Salter
Original assignee: Norgren Limited
Priority date: 2005-10-08
Filing date: 2006-10-09
Publication date: 2007-04-19
Also published as: WO2007042780A3; GB0520497D0

Abstract

An actuator assembly is used for breaking a crust that forms during the smelting of aluminium. The assembly (1) comprises a fluid-pressure- operated cylinder (2) in which works a piston (3) attached to a piston rod (4). The piston rod (4) has a piercing tip at its free end for breaking the crust. The piston (3) is adapted to be driven to a retracted position by supplying fluid to a front side (11) and to an extended position by supplying fluid to a rear side (10). The assembly (1) also includes sensing means (9) to determine when the piston (3) is in the extended position and cause it to return to its retracted position, and automatic retraction means (15) causing the piston (3) to move to the retracted position after a predetermined time if the extended position has not been reached following the supply of fluid to the rear side (10). The assembly may also include monitoring means to monitor the position of the piston (3), and retention means (63) for retaining the piston (3) in the retracted position.

Description

ACTUATOR ASSEMBLY

This invention relates to an actuator assembly and, in particular, to a crust breaking actuator assembly for breaking the crust that forms during the production of aluminium.

Alumina is reduced to produce aluminium metal in electrolytic cells commonly known as pots. The alumina is dissolved into an electrolytic bath of molten cryolite (sodium aluminium fluoride) . The alumina is fed in to the pots during electrolysis along with other compounds, which are added to aid the electrolytic process. However, during production, a solid crust forms on the surface of the molten electrolyte, preventing the addition of the alumina and other compounds. A crust-breaking device having a chisel attached thereto is used to penetrate and physically break this crust.

Crust breaking systems are known and typically comprise a piston and cylinder arrangement that is mounted above a smelting vessel. The piston is connected to a piston rod that extends from the cylinder. The free end of the piston rod has a crust-breaking chisel attached thereto for piercing and breaking the crust. In operation, pressurised air is supplied to a first side of the piston to drive the piston and chisel of the piston rod downward to break the crust. Pressurised air is then supplied to the other side of the piston to retract the tip from the molten electrolyte. It is important that the tip is retracted as soon as possible to minimize heat transfer through the piston rod and degradation of the chisel itself. The transfer of heat through the piston rod can affect the integrity of various seals between the cylinder and the piston rod, for example.

However, as crust breaking cylinders are relatively large, they displace a large volume and require a substantial amount of pressurized air to operate. A typical aluminium production plant will have many such electrolysis pots operating constantly and therefore the pressurized air consumption is very large.

According to a first aspect of the invention we provide a crust breaker actuator assembly comprising a cylinder in which works a piston attached to a piston rod, the piston rod having a piercing tip at its free end, the piston being adapted to be driven to a retracted position by supplying fluid to a front side and to an extended position by supplying fluid to a rear side, the assembly including sensing means to determine when the piston is in the extended position and thereby cause the piston to return toward the retracted position, and automatic retraction means that causes the piston to move toward the retracted position after a predetermined time if the extended position has not been reached following the supply of fluid to the rear side.

This is advantageous as the sensing means ensures that the piston rod is retracted as soon as it reaches the extended position. The piston rod is therefore only in contact with the molten electrolyte for a short period, thereby reducing the degradation to the rod and the conduction of heat through the piston rod to the cylinder. Further, if the crust is particularly thick and the assembly is unable to break through, the automatic retraction means ensures that the piston rod is not held against the crust for an extended period. This reduces the heat transfer to the actuator assembly and also allows the piston to be returned to the retracted position so that further attempts to break the crust can be made. Thus, the present invention will require less maintenance as it reduces the wear to seals in the cylinder and deterioration of the piercing tip.

Preferably the piercing tip is in the form of a crust breaking chisel. The chisel can easily be replaced as necessary. The assembly may operate using hydraulic fluid. However, preferably the assembly operates using pressurized air.

Preferably the sensing means comprises a switch mounted in the cylinder. The piston may be adapted to contact the switch when it reaches the extended position. Preferably the switch includes a valve that causes fluid to be supplied to the front side of the piston to return it to the retracted position.

Preferably the automatic retraction means comprises a reservoir that receives a proportion of the fluid supplied to the rear side of the piston, wherein when the pressure in the reservoir reaches a predetermined amount it actuates a valve means to cause the piston to return toward the retracted position.

Preferably, the reservoir receives a proportion of the fluid supplied to the rear side of the piston through a reservoir restriction. The reservoir can therefore be small and will not substantially affect the force with which the piston is driven toward the extended position.

Preferably, the fluid is supplied to the rear side of the piston through a supply restriction. Preferably, the fluid is supplied to the front side of the piston through a restriction. Most preferably the fluid is supplied to each of the front and rear sides of the piston through a common restriction.

Preferably, a monitoring means monitors the automatic retraction means and actuates a warning means when it detects that the automatic retraction means has been actuated. The warning means may be an alarm and, in particular, an audible alarm. This is advantageous as workers in an aluminium production factory can be warned when the crust is not being broken. If a mechanical problem, such as a defective seal, is preventing the piercing tip penetrating with sufficient force, this can be repaired quickly to ensure the electrolysis plant operates as effectively as possible. If the automatic retraction means is being actuated due to an abnormally thick crust, workers can be alerted to take action to ensure that the chemical composition of the molten electrolyte does not become dangerous, for example.

The monitoring means may increment a counter each time it detects actuation of the automatic retraction means and then activate the warning means when the counter reaches a predetermined value.

The monitoring means may be used to monitor the performance of the crust breaker actuator assembly.

The monitoring of the crust breaker assembly is important to ensure it operates safely, reliably and effectively. If the crust is not broken during electrolytic aluminium production then alumina and other chemicals cannot be added to the pot. This can upset the chemical balance of the molten alumina affecting its conductivity. As such large currents flow through the alumina during electrolysis, a change in the conductivity can be hazardous as the electricity may arc across the surface in a occurrence known in the art as "the anode effect" . Further, production of aluminium by electrolysis is very expensive and it is therefore important that the crust breaker operates reliably.

According to a second aspect of the invention we provide a crust breaker actuator assembly comprising a cylinder in which works a piston attached to a piston rod, the piston rod having a piercing tip at its free end, the piston being adapted to be driven to a retracted position by supplying fluid to a front side and to an extended position by supplying fluid to a rear side, the assembly including initiation means to initiate the supply of fluid to the rear side, rear sensing means to determine when the piston is in the retracted position and monitoring means adapted to at least monitor the initiation means and rear sensing means and generate a signal for a warning means when either the monitoring means detects actuation of the initiation means and no change in state of the rear sensing means or the monitoring means detects actuation of the initiation means and a change in state of the rear sensing means and then does not detect a further change in state of the rear sensing means after a predetermined time.

This is advantageous as the monitoring means can monitor the initiation means and rear sensing means to determine if the assembly is operating safely. The monitoring means monitors when the initiation means is actuated and when the output from the rear sensing means changes state. Thus, after actuation of the initiation means, if there is no change in state of the rear sensing means then the piston rod is static in the retracted position and is therefore not piercing the crust. This may mean that there is a malfunction with the initiation means, or the piston or piston rod may be jammed. If this occurs a crust could form on the surface of the aluminium affecting the electrolytic process, which could become dangerous. The monitoring means can detect this and send signals to the warning means indicating the state of the assembly. Further, if after actuation of the initiation means and a change in state of the rear sensing means, a further change in state of the rear sensing means is not detected after a predetermined time, then the piston has not returned to the retracted position. The piston rod may be stuck in the molten electrolyte or crust, or there may be a loss of fluid pressure. Similarly, the warning means receives a signal from the monitoring means so that appropriate action can be taken to ensure the assembly operates safely, efficiently and reliably, as possible problems can be identified quickly. The warning means may be an alarm, preferably audible, adapted to alert users when it receives a signal from the monitoring means .

Preferably, the monitoring means monitors the assembly and sends a signal to the warning means when pre-programmed criteria are met. The monitoring means may increment a counter each time the above conditions are met and activate the warning means when the value of the counter exceeds a predetermined amount.

The monitoring means may be used to monitor the performance of the crust breaker actuator assembly. This performance data could be useful in predicting when the crust breaking assembly requires maintenance. Thus, the warning means may simply be a log of the data from the monitoring means that can be reviewed by a user.

The signal sent by the monitoring means may be a pneumatic signal. Alternatively, it is an electrical signal.

Preferably the assembly also includes front sensing means to determine when the piston is in the extended position, which is also monitored by the monitoring means. Accordingly, the monitoring means may be adapted to send a signal to the warning means when, following actuation of the initiation means and a change in state of the rear sensing means, it does not detect a change in state of the front sensing means before it detects a further change in state of the rear sensing means. In this case, the piston rod was not driven to its fully extended position and was returned by an automatic retraction means, for example. This may indicate that the crust was not successfully broken. Preferably, the assembly also includes automatic retraction means that causes the piston to move toward the retracted position after a predetermined time if the extended position has not been reached following the actuation of the initiation means causing the supply of fluid to the rear side.

The monitoring means may detect the actuation of the initiation means, front sensing means and rear sensing means by way of sensors attached thereto. Preferably the monitoring means detects the actuation of the initiation means, front sensing means or rear sensing means by monitoring the change in signals output from these components. The signals may be pneumatic outputs or electrical outputs.

According to a third aspect of the invention we provide a crust breaker actuator assembly comprising a cylinder in which works a piston attached to a piston rod, the piston rod having a piercing tip at its free end, the piston being adapted to be driven to a retracted position by supplying fluid to a front side and to an extended position by supplying fluid to a rear side, the assembly including front sensing means to determine when the piston is in the extended position, initiation means comprising a pair of control valves, the first control valve being adapted to control the flow to the front side and the second control valve being adapted to control the flow to the rear side, the pair of control valves themselves being actuated by a bi-stable pilot/pilot valve, wherein the front sensing means is adapted to reset the pilot/pilot valve to cause the flow of fluid to the front side of the piston when it determines the piston is in the extended position and manual reset means is provided to move the pilot/pilot valve from a position in which fluid is supplied to the rear side to a position in which fluid is supplied to the front side. The manual reset means is advantageous as it enables a user to reset the assembly to an operational condition if the assembly malfunctions. For example, if the pilot/pilot valve is actuated and there is insufficient fluid pressure to drive the piston rod to the extended position it may not reach the fully extended position and the front sensing means will not be actuated to reset the pilot/pilot valve. The manual reset means allows a user to reset the pilot/pilot valve if the assembly is in a condition in which the front sensing means has failed to reset it.

The manual reset means may be a push button.

The pilot/pilot valve may be a spool valve. The push button may act directly on the spool of the pilot/pilot valve to reset its position. Alternatively, the push button may act on a plunger that in turn acts on the spool. Preferably, the manual reset means can only be actuated against the force of a spring. The spring reduces the possibility of the manual reset means being actuated inadvertently.

According to a fourth aspect of the invention we provide a crust breaker actuator assembly comprising a cylinder in which works a piston attached to a piston rod, the piston rod having a piercing tip at its free end, the piston and piston rod being adapted to be driven to a retracted position by supplying fluid to a front side and to an extended position by supplying fluid to a rear side, the assembly having retention means to hold the piston in the retracted position, the retention means being adapted to release the piston when fluid is supplied to the rear side of the piston.

The retention means ensures that the piston is held in the retracted position even if there is a loss of air pressure at the front side. This is advantageous as the retention means prevents the piston rod from falling from the retracted position where it may contact the molten alumina thereby causing damage to the piercing tip and conduction of heat along the piston rod to the rest of the assembly. Further, as the retention means is released when fluid is supplied to the rear side of the piston, the retention means does not interfere with the operation of the assembly.

Preferably, the retention means comprises at least one latch. Preferably, the retention means comprises two latches.

The retention means may engage directly with the piston, or with a groove in the piston rod. Preferably, the retention means engages with a piston-retaining nut, which secures the piston to the piston rod and, in particular, with a groove in the nut.

Preferably, the retention means is urged into engagement with the piston by spring means.

Preferably, the fluid supplied to the rear side acts on the retention means to disengage it from the piston.

There now follows by way of example only a detailed description of the present invention with reference to the accompanying drawings in which;

Figure 1 shows a perspective view of the actuator assembly of the invention;

Figure 2 is a pneumatic diagram of the actuator assembly in the retracted position at a first time;

Figure 3 is a pneumatic diagram of the actuator assembly in the retracted position at a second time; Figure 4 is a pneumatic diagram showing the state of the actuator assembly when a signal is supplied to cause it to actuate;

Figure 5 is a pneumatic diagram showing the state of the actuator assembly while it is being driven to the extended position;

Figure 6 is a pneumatic diagram showing the actuator assembly in the extended position;

Figure 7 is a pneumatic diagram showing the state of the actuator assembly when the piston rod is unable to reach the extended position and is returned by automatic retraction means; and

Figure 8 shows the latches of the fourth aspect of the invention.

The embodiment of the actuator assembly shown in Figures 1 to 7 is a crustbreaker actuation assembly for the breaking of the crust that forms on the surface of molten electrolyte during the production of aluminium by electrolysis. The assembly is mounted over a pot in which molten electrolyte is undergoing electrolysis. The assembly is adapted to penetrate the crust so that additional raw material or other chemicals can be added.

With reference to Figures 1 and 2, the actuator assembly 1 comprises a pneumatic cylinder 2 in which works a piston 3. The piston is attached to a piston rod 4, which is adapted to extend from the cylinder 2 toward the crust. The piston rod 4 has a crust breaking chisel (not shown) attached to its free end 5 for penetrating the crust. The cylinder 2 has two end- caps; a forward end-cap 6 and a rear end-cap 7. Thus, the piston rod 4 of the actuator assembly 1 has an extended position, in which the piston rod 4 is fully extended from the cylinder 2, and a retracted position, in which the piston rod 4 is withdrawn into the cylinder 2. The end-caps 6,7 include sensing means 8,9 for determining when the piston 3, and therefore the piston rod 4, is in the extended position or the retracted position.

The piston 3 is driven toward the extended position by the supply of pneumatic air to a rear side 10 and is driven toward the retracted position by the supply of pneumatic air to a front side 11.

An automatic retraction means 15 is located in a valve body 16, which is mounted to the rear end-cap 7. The automatic retraction means 15 comprises a reset valve means 17, a reservoir 18 and a reservoir restriction 19. The reservoir 18 is connected, via a conduit 20 having the reservoir restriction 19 therein, to a rear air line 22, which supplies the rear side 10 of the piston 3 with air. The reset valve means 17 comprises a pilot valve having a priority to the side connected to the reservoir 18.

Pressurized air is supplied to the assembly through a main air supply 25, which is connected to a control valve 26. The control valve 26 is a 2 port, 2 position (commonly represented in the art as 2/2) normally open valve. The control valve 26 is acted on by a rear sensing valve 27 that forms part of sensing means 9. The sensing valve 27 has a silencer 28 through which air may be exhausted to atmosphere. The rear sensing valve 27 is physically mounted in the rear end-cap 7.

Pressurized air is supplied to the cylinder 2 via a supply conduit 29 having a supply restriction 30 therein. The supply conduit 29 splits into a front supply conduit 31 and a rear supply conduit 32 downstream of the supply restriction 30. The front and rear conduits 31 , 32 are each connected to one of a pair of control valves 36. The pair of control valves 36 comprise two 3/2 valves 33, 34 arranged to be acted on through a common input signal line 37. The front supply conduit 31 is connected to the valve 33 and the rear supply conduit 32 is connected to the valve 34. The pair of control valves 36 are such that only one of them is open at any one time. Thus, either the rear supply conduit 32 is in communication with the rear air line 22 and a front air line 35 is connected to atmosphere via a silencer 38; or the front supply conduit 31 is in communication with the front air line 35 and the rear air line 22 is connected to atmosphere via a silencer 39.

The common input signal 37 is provided through a pilot/pilot valve 40. The valve 40 is of 3/2 pilot/pilot type. The pilot/pilot valve 40 has a connection to atmosphere via a silencer 41 and a subsidiary air supply 42. The pilot/pilot valve 40 is actuated in response to a main trigger signal 43. When trigger signal 43 is supplied, the common input signal line 37 is connected with the subsidiary air supply 42 to cause the pair of control valves 36 to actuate. The pilot/pilot valve 40 and pair of control valves 36 form an initiation means. The trigger signal 43 also acts on the control valve 26 via a conduit 44. The pilot/pilot valve 40 is also acted on by the reset valve means 17 of the automatic retraction means 15 via a conduit 45. The reset valve means 17 is connected to a further subsidiary air supply 46. When actuated, reset valve means 17 connects the further subsidiary air supply 46 with conduit 45 such that the air acts on pilot/pilot valve 40.

Manual reset means (not shown) may also be adapted to act on the pilot/pilot valve 40. The manual reset means may take the form of a push button that physically acts on the pilot/pilot valve 40 to reset it to the position shown in Figure 2. The valve 40 is a spool valve and therefore the manual reset means acts on the spool. As discussed above the air supplied to the rear side 10 of the piston 3 via rear air line 22 is also supplied to the automatic retraction means 15 via the branch conduit 20. The conduit 20 also delivers air to a front sensing valve 47, which forms part of the sensing means 8. Thus, the front sensing valve 47 is physically mounted in the front end-cap 6. The front sensing valve 47 also has a connection to atmosphere via a silencer 48. Further, the valve 47 controls communication between the conduit 20 and a conduit 50, which is connected to the reset valve means 17.

Similarly, the front air line 35 also includes a branch conduit 49. The branch conduit 49 delivers air to the rear sensing valve 27. The rear sensing valve 27 controls communication between the branch conduit 49 and a conduit 51, which is connected to the control valve 26.

The piston rod 4 is held in the retracted position by a preset holding pressure, which is much less than the supply pressure and may be varied, on the front side 11 of the piston 3. Figure 2 shows the crust breaking actuator assembly 1 in a state where the piston rod 4 is in the retracted position and the pressure of the air applied to the front side 11 of the piston 3 is less than the holding pressure or has fallen below the holding pressure. The pair of control valves 36 are set to deliver air to the front side 11 of the piston 3. The piston 3 is adjacent the end-cap 7 and has thereby actuated the rear sensing means 9. The rear sensing valve 27 has therefore connected the branch conduit 49 with conduit 51, therefore causing the air pressure acting on the front side 11 of the piston 3 to act on and open the control valve 26. Thus, the control valve 26 supplies more air from the main air supply 25 to the front side 11 of the piston 3, via conduits 29, 31 and 35 through restriction 30.

Figure 3 shows the crust breaking actuator assembly 1 in a state where the piston rod 4 is in the retracted position and the pressure of the air applied to the front side 11 of the piston 3 is greater than the holding pressure. The state of the various valves is similar to that shown in Figure 2, except that the pressure acting on control valve 26 through conduit 51 is such that the control valve 26 is closed. Thus, no further air from the main air supply 26 is supplied to the front side 11 of the piston 3 and the piston 3 is held in the retracted position by the pressure of the air presently in the cylinder 2. The construction and arrangement of conduits 49, 50, the sensing valve 27 and the main control valve 26 in particular is such that the main air supply 25 is cut-off when the pressure in the cylinder is above the holding pressure.

Thus, as the air supplied to the front side 11 of the piston is fed back via conduits 49 and 50, through rear sensing valve 27, to the control valve 26, the air pressure applied to the front side 11 of the piston 3 is regulated and automatically compensates for any loss of pressure.

Figure 4 shows the condition of the assembly when a trigger signal is received at 43. The pilot/pilot valve 40 is moved to a position in which the subsidiary air supply 42 is connected with input supply line 37. This causes the pair of control valves 36 to be actuated, such that control valve 33 connects front air line 35 to atmosphere via silencer 38 and control valve 34 connects the rear supply conduit 32 to rear air line 22. The trigger signal also acts on control valve 26, via conduit 44 to connect supply conduit 29 to the main air supply 25.

Thus, the front side 11 of the piston 3 is exhausted to atmosphere, while air is supplied to the rear side 10. The piston 3 will therefore begin to move toward the extended position.

Figure 5 shows the assembly 1 mid-stroke moving from the retracted position to the extended position. The trigger signal 43 has been removed but as the pilot/pilot valve 40 is bistable, the subsidiary air supply 42 remains connected to the input supply line 37. The air supplied to the rear side 10 of the piston 3 passes through the restriction 30 and therefore the pressure in the cylinder 2 rises relatively slowly as the piston 3 is driven forward. This arrangement ensures that only the pressure required is used, thereby saving air.

In Figure 5 the piston 3 has moved from adjacent the rear end-cap 7 and therefore no longer actuates the rear sensing means 9. The rear sensing valve 27 therefore closes communication between branch conduit 49 and conduit 51. Conduit 51 is connected to atmosphere via silencer 28.

While air is being supplied to the rear side 10 of the piston 3, a proportion of it passes from the rear air line 22 along branch conduit 20. The flow along branch conduit 20 is restricted by restriction 19. The restriction is such that, in normal operation, the majority of the air will flow into cylinder 2 at the rear side 10 of the piston 3.

Figures 6 and 7 show two different scenarios to illustrate how the assembly 1 behaves when either the piston rod 4 successfully breaks the crust (Figure 6) and when an unusually thick crust prevents the piston rod 4 reaching the extended position (Figure 7) .

In Figure 6, the piston rod 4 is shown in the extended position having successfully broken through the crust. The piston 3 is therefore adjacent the front end-cap 6 and has actuated the front sensing means 8. The actuation of the sensing means 8 causes front sensing valve 47 to actuate thereby connecting the branch conduit 20 to conduit 50, which is in communication with the pilot/pilot valve 40. The air supplied to the rear side 10 of the piston 3 is therefore used to reset the pilot/pilot valve 40.

Thus, the air supply 42 to input supply line 37 is cut-off. This, in turn, causes the pair of control valves 36 to return to their default position in which the front supply conduit 31 is in communication with the front air line 35, and the rear air line 22 is connected to atmosphere via the silencer 39. The main air supply 25 will therefore supply the front end 11 of the cylinder 2 with pressurized air to return it toward the retracted position. Any pressure that has built up in reservoir 18 is exhausted to atmosphere through branch conduit 20 and rear air line 22, via silencer 39. The actuation of the manual reset means (not shown) would also reset the pilot/pilot valve 40 as described above.

In the retracted position, the piston 3 will contact the rear sensing means 9 in the end-cap 7. The assembly 1 will then return to a state as described in relation to Figures 2 and 3.

In Figure 7 the piston 3 and piston rod 4 is shown stalled mid-stroke during movement from the retracted position to the extended position. This may occur when the piston rod 4 strikes an unusually thick crust, which it is unable to break through. When the piston 3 stalls the pressure at the rear side 10 within the cylinder 2 will increase steadily as more pressurized air is supplied by the main air supply 25 through restriction 30. The pressure will ultimately reach full supply pressure.

As described above, a proportion of the pressurized air supplied to the rear side 10 is also received by the reservoir 18 through restriction 19. If the front sensing means 9 is not actuated by the piston 3, the piston rod 4 will be automatically retracted toward the retracted position after a predetermined time by the automatic retraction means 15. The predetermined time is dependent on the time it takes the air received by the reservoir 18 to reach a pressure great enough to actuate the reset valve means 17. The time period that the air pressure can build in the cylinder 2 when attempting to break through the crust will be affected by the size of the restriction 19 and the size of the reservoir 18. Thus, the size of the restriction 19 and reservoir 18 can be chosen such that sufficient force is applied to the crust in an attempt to break it, while the piston rod 4 is automatically withdrawn after a period of time so that damage to the chisel and conduction of heat through the piston rod 4 is kept to a minimum.

The piston rod 4 is automatically retracted when the reset valve means 17 is actuated as conduit 45 is connected to the further subsidiary air supply 46. As described in relation to Figure 6, this will reset the pilot/pilot valve 40 thereby cutting-off the air supply 42 to input supply line 37. This, in turn, causes the pair of control valves 36 to return to their default position in which the front supply conduit 31 is in communication with the front air line 35, and the rear air line 22 is connected to atmosphere via the silencer 39. The main air supply 25 will therefore supply the front end 11 of the cylinder 2 with pressurized air to return it toward the retracted position. The pressure that has built up in reservoir 18 is exhausted to atmosphere through branch conduit 20 and rear air line 22, via silencer 39.

Further, when the automatic retraction means 15 has caused the piston rod 4 to return to the retracted position, the piston 3 will contact the rear sensing means 9 in the end-cap 7. The assembly 1 will then return to a state as described in relation to Figures 2 and 3.

The pneumatic signals output by the rear sensing means 9, front sensing means 8 and initiation means, which in this embodiment is represented by the trigger signal 43, are used by a monitoring means (not shown) . Thus, the conduit 51, conduit 50 and trigger signal 43 have branches (not shown) that direct a proportion of the pneumatic air to a monitoring means. The monitoring means uses the presence or absence of pressurized air to determine whether the front or rear sensing means 8,9 or the initiation means 43 has been actuated. In response to the pneumatic signals, the monitoring means actuates a warning means to inform users of how the assembly is performing. It will be appreciated that the monitoring means may generate a signal for the warning means after predetermined criteria, relating to the number and order of the pneumatic signals, are met. For example, the monitoring means may increment a counter each time it detects the actuation of the trigger signal 43 and does not detect a loss of pressure from conduit 51, indicating that the rear sensing means 9 has failed to actuate. Thus, when the counter determines that the assembly has made three, for example, attempts to drive the piston to the extended position it will send a signal to the warning means. Appropriate action can then be taken.

Figure 8 shows the cylinder 2 of the assembly 1 in detail. The piston 3 is shown in the retracted position. The piston rod has a piston-retaining nut 60, which secures the piston 3 to the rod 4. The piston 3 is secured to the piston rod 4 by a threaded connection and by the piston-retaining nut 60. A port 61 is shown which conveys air to the rear side of the piston 3 and is thus connected with the rear air line 22. The nut 60 has a circumferential groove 62 in spaced separation from the piston 3. The cylinder 2 includes retention means in the form of a pair of latches 63. The latches 63 comprise a body portion 64 and an engagement portion 65. The engagement portion 65 is adapted to engage with the groove 62, thereby holding the piston 3 and piston rod 4 in the retracted position. The latches 63 are urged into engagement with the groove 62 by springs, that abut the body portion 64.

Thus, the latches 63 will retain the piston 3 and piston rod 4 in the retracted position even if there is a loss of air pressure to the front side 11 of the piston 3. The latches 63 are constructed such that their engagement with the groove 62 is released on application of pressurized air to the rear side 10 of the piston 3. This is advantageous, as additional means are not required to release the latches 63, which are automatically released when the initiation means initiates a driving stroke.

Claims

1. A crust breaker actuator assembly (1) comprising a cylinder (2) in which works a piston (3) attached to a piston rod, the piston rod (4) having a piercing tip at its free end, the piston (3) being adapted to be driven to a retracted position by supplying fluid to a front side (11) and to an extended position by supplying fluid to a rear side (10) , characterised in that the assembly includes sensing means (9) to determine when the piston (3) is in the extended position and thereby cause the piston (3) to return toward the retracted position, and automatic retraction means (15) that causes the piston (3) to move toward the retracted position after a predetermined time if the extended position has not been reached following the supply of fluid to the rear side (10) .

2. A crust breaker assembly according to claim 1, in which the piercing tip is in the form of a crust breaking chisel.

3. A crust breaker assembly according to claim 1 or claim 2, in which the assembly (1) operates using hydraulic fluid.

4. A crust breaker actuator according to claim 1 or claim 2, in which the assembly (1) operates using pressurized air.

5. A crust breaker assembly according to any preceding claim, in which the sensing means includes a switch (8) mounted in the cylinder (2) .

6. A crust breaker assembly according to claim 5, in which the piston (3) is adapted to contact the switch (8) when it reaches the extended position.

7. A crust breaker assembly according to claim 5 or claim 6, in which the switch includes a valve (47) that causes fluid to be supplied to the front side (11) of the piston (3) to return it to the retracted position.

8. A crust breaker assembly according to any preceding claim, in which the automatic retraction means (15) includes a reservoir (18) that receives a proportion of the fluid supplied to the rear side (10) of the piston, wherein when the pressure in the reservoir (18) reaches a predetermined amount it actuates a valve means (17) to cause the piston (3) to return toward the retracted position.

9. A crust breaker assembly according to claim 8, in which the reservoir (18) receives a proportion of the fluid supplied to the rear side (10) of the piston through a reservoir restriction (19) .

10. A crust breaker assembly according to any preceding claim, in which the fluid is supplied to the rear side (10) of the piston through a supply restriction (30) .

11. A crust breaker assembly according to any preceding claim, in which the fluid is supplied to the front side (11) of the piston through a restriction (30) .

12. A crust breaker assembly according to any preceding claim, in which the fluid is supplied to each of the front and rear sides of the piston through a common restriction (30) .

13. A crust breaker assembly according to any preceding claim, in which a monitoring means monitors the actuation of the automatic retraction means (15) .

14. A crust breaker assembly according to claim 13, in which the monitoring means actuates a warning means when it detects that the automatic retraction means (15) has been actuated.

14. A crust breaker assembly according to claim 13, in which the warning means is an alarm.

15. A crust breaker assembly according to claim 13, in which the monitoring means increments a counter each time it detects actuation of the automatic retraction means (15) and then actuates the warning means when the counter reaches a predetermined value.

16. A crust breaker assembly according to any of claims 13 to 15, in which the monitoring means is used to monitor the performance of the crust breaker actuator assembly.

17. A crust breaker actuator assembly (1) comprising a cylinder (2) in which works a piston (3) attached to a piston rod (4) , the piston rod (4) having a piercing tip at its free end, the piston (3) being adapted to be driven to a retracted position by supplying fluid to a front side (11) and to an extended position by supplying fluid to a rear side (10) , characterised in that the assembly includes initiation means (43) to initiate the supply of fluid to the rear side (10) , rear sensing means (9) to determine when the piston (3) is in the retracted position and monitoring means adapted to at least monitor the initiation means (43) and rear sensing means (9) and generate a signal for a warning means when either the monitoring means detects actuation of the initiation means (43) and no change in state of the rear sensing means (9) or the monitoring means detects actuation of the initiation means (43) and a change in state of the rear sensing means (9) and then does not detect a further change in state of the rear sensing means (9) after a predetermined time.

18. A crust breaker assembly according to claim 17, in which the warning means is an alarm adapted to alert users when it receives a signal from the monitoring means.

19. A crust breaker assembly according to claim 17 or claim 18, in which the monitoring means monitors the assembly and sends a signal to the warning means when pre-programmed criteria are met.

20. A crust breaker assembly according to any of claims 17 to 19, in which the monitoring means is adapted to increment a counter each time the monitoring means generates a signal and to actuate the warning means when the value of the counter exceeds a predetermined amount.

21. A crust breaker assembly according to any of claims 17 to 20, in which the monitoring means is used to monitor the performance of the crust breaker actuator assembly.

22. A crust breaker assembly according to claim 17, in which the warning means is a log of the data from the monitoring means that can be reviewed by a user.

23. A crust breaker assembly according to any of claims 17 to 22, in which the signal sent by the monitoring means is a pneumatic signal.

24. A crust breaker assembly according to any of claims 17 to 22, in which the signal sent by the monitoring means is an electrical signal.

25. A crust breaker assembly according to any of claims 17 to 24, in which the assembly also includes front sensing means (8) to determine when the piston (3) is in the extended position, which is also monitored by the monitoring means.

26. A crust breaker assembly according to claim 25, in which the monitoring means is adapted to send a signal to the warning means when, following actuation of the initiation means (43) and a change in state of the rear sensing means (9) , it does not detect a change in state of the front sensing means (8) before it detects a further change in state of the rear sensing means (9) .

27. A crust breaker assembly according to any of claims 17 to 26, in which the assembly also includes automatic retraction means (15) that causes the piston (3) to move toward the retracted position after a predetermined time if the extended position has not been reached following the actuation of the initiation means (43) causing the supply of fluid to the rear side (10) .

28. A crust breaker assembly according to any of claims 17 to 27, in which the monitoring means detects the actuation of any of the initiation means (43) , front sensing means (8) and rear sensing means (9) by way of sensors attached thereto.

29. A crust breaker assembly according to any of claims 17 to 28, in which the monitoring means detects the actuation of any of the initiation means (43), front sensing means (8) and rear sensing means (9) by monitoring the change in signals output from these components.

30. A crust breaker actuator assembly (1) comprising a cylinder (2) in which works a piston (3) attached to a piston rod (4), the piston rod (4) having a piercing tip at its free end, the piston (3) being adapted to be driven to a retracted position by supplying fluid to a front side (11) and to an extended position by supplying fluid to a rear side (10) , characterised in that the assembly includes front sensing means (8) to determine when the piston (3) is in the extended position, initiation means comprising a pair of control valves (36) , the first control valve being adapted to control the flow to the front side (11) and the second control valve being adapted to control the flow to the rear side (10) , the pair of control valves (36) themselves being actuated by a bi-stable pilot/pilot valve (40) , wherein the front sensing means (8) is adapted to reset the pilot/pilot valve (40) to cause the flow of fluid to the front side (11) of the piston when it determines the piston is in the extended position and manual reset means is provided to move the pilot/pilot valve (40) from a position in which fluid is supplied to the rear side (10) to a position in which fluid is supplied to the front side (11) .

31. A crust breaker actuator assembly according to claim 30, in which the manual reset means is a push button.

32. A crust breaker actuator assembly according to claim 30 or claim 31, in which the pilot/pilot valve (40) is a spool valve.

33. A crust breaker actuator assembly according to claim 32, in which a push button acts directly on the spool of the pilot/pilot valve (40) to reset its position.

34. A crust breaker actuator assembly according to claim 32, in which a push button acts on a plunger that in turn acts on the spool.

35. A crust breaker actuator assembly according to any of claims 30 to 34, in which the manual reset means can only be actuated against the force of a spring.

36. A crust breaker actuator assembly (1) comprising a cylinder (2) in which works a piston (3) attached to a piston rod (4) , the piston rod (4) having a piercing tip at its free end, the piston (3) and piston rod (4) being adapted to be driven to a retracted position by supplying fluid to a front side (11) and to an extended position by supplying fluid to a rear side (10) , characterised in that the assembly has retention means (63) to hold the piston (3) in the retracted position, the retention means being adapted to release the piston (3) when fluid is supplied to the rear side (10) of the piston (3) .

37. A crust breaker actuator assembly according to claim 36, in which the retention means comprises at least one latch (63) .

38. A crust breaker actuator assembly according to claim 36 or claim 37, in which the retention means comprises two latches (63).

39. A crust breaker actuator assembly according to any of claims 36 to 38, in which the retention means (63) engages directly with the piston (3) .

40. A crust breaker actuator assembly according to any of claims 36 to 38, in which the retention means (63) engages with a groove in the piston rod (4) .

41. A crust breaker actuator assembly according to any of claims 36 to 38, in which the retention means engages with a piston-retaining nut (60) , which secures the piston (3) to the piston rod (4) .

42. A crust breaker actuator assembly according to claim 41, in which the retention means (63) engages with a groove (62) in the piston- retaining nut (60).

43. A crust breaker actuator assembly according to any of claims 36 to 42, in which the retention means (63) is urged to hold the piston (3) in the retracted position by spring means.

44. A crust breaker actuator assembly according to any of claims 36 to 43, in which the fluid supplied to the rear side (10) acts on the retention means (63) to disengage it from the piston (3) .