EP3018365A1

EP3018365A1 - Pneumatic apparatus

Info

Publication number: EP3018365A1
Application number: EP15192685.4A
Authority: EP
Inventors: Enzo Landi
Original assignee: Elleprogetti Di Enzo Landi
Current assignee: Elleprogetti Di Enzo Landi
Priority date: 2014-11-03
Filing date: 2015-11-03
Publication date: 2016-05-11
Anticipated expiration: 2035-11-03
Also published as: EP3018365B1

Abstract

A pneumatic apparatus (1) comprises:
- a linear pneumatic actuator (20) comprising a casing (21) and a piston (22) slidable inside the casing (21) and defining in the latter a first chamber (23) and a second chamber (24),
- fluid distributing means (51) arranged for distributing pressurized fluid alternatively in the first chamber (23) and in the second chamber (24),
- vacuum generating means (60), and
- an economizer device (100; 100'; 200; 200'; 300) comprising:
a body (102) provided with a first opening (103) connected to the fluid distributing means (51), with a second opening (104) connected to one of the chambers (24), and with a third opening (105) connected to the vacuum generating means (60), the first, second and third opening (103, 104, 105) being connected together by a main conduit (106) made in the body (102);
flow controlling means (110) that is movable between a first operating configuration (C1) wherein the flow controlling means (110) permits the connection between the second opening (104) and the third opening (105) so as connect the vacuum generating means (60) to the chamber (24) and prevents the connection between the second opening (104) and the first opening (103), and a second operating configuration (C2) wherein the flow controlling means (110) closes the connection between the second opening (104) and the third opening (105) and permits the passage of fluid between the second opening (104) and the first opening (103).

Description

The invention relates to a pneumatic apparatus comprising one or more pneumatic actuators, for example cylinders or suction cups, and an economizer device that can be associated with each pneumatic actuator in order to reduce the consumption of compressed air by each pneumatic actuator whilst maintaining the performance of each pneumatic actuator unvaried.
Known linear pneumatic actuators are typically pneumatic cylinders that comprise a hollow outer casing or container (cylinder) inside which a piston stem slides that divides the inside of the cylinder into two chambers. In single-action cylinders only one of the chambers is supplied with compressed air so as to exert a thrust force on the piston in just one direction and during one of the strokes of the latter (forward stroke). In dual action cylinders, both chambers are supplied selectively with compressed air to exert respective thrust and traction forces on the piston during the strokes (forward and return). Pneumatic cylinders are generally used in operating apparatuses and machines, inserted inside pneumatic plants or circuits comprising, in addition to other components (rotating actuators, valves, distributors, etc), compression means that is able to supply compressed air at the required supply pressure, typically comprised between 1 and 7 bar (0.1-0.7 Mpa). The compression means comprises one or more compressors provided with electric motors or internal combustion engines.
In the analysis of the operating costs of an operating machine and, more in general, of a manufacturing company provided with a plurality of operating machines and pneumatic plants, the cost of producing compressed air is a rather significant percentage of total operating costs. This cost comprises not only the energy expense (e.g. electricity) that is required to supply the compression means, but also the routine and extraordinary maintenance thereof, the use of cleaning and filtration systems, the elimination of condensate, refrigeration of air, etc. In particular, the quantity of energy necessary for producing compressed air is directly proportionally to the value of the working or operating pressure required in the plant (typically 6-7 bar).
Consequently, lower working pressure enables the expense of the electric energy necessary to maintain this pressure to be reduced. Further, by working at lower pressure the compression means, being subject to less stress, would require less routine and extraordinary maintenance.
Working pressure is calculated in such a manner as to ensure correct operation of the apparatuses and of installations or work points, in particular in work points in which higher pressure is required. Using lower working pressure in a pneumatic plant would thus allow an energy saving but would cause a drop in performance or even a malfunction at the work points in which higher pressure is required.
In order to remedy these drawbacks, international patent application WO 2013/042044 made known an economizer device that is associable with a linear pneumatic actuator that enables a significant quantity of compressed air to be saved. This is obtainable because the economizer device makes it a lower working pressure possible whilst maintaining performance unaltered in the various work points. In other words, owing to the economizer device the force dispensed by the linear pneumatic actuators remains substantially unvaried although the linear pneumatic actuators are supplied at a lower working pressure.
One drawback of this economizer device is that during the forward stroke of the piston the economizer device has to be supplied with compressed air to operate correctly. Another drawback of this device is that, during the return stroke, leaks may occur that cause a certain quantity of compressed air to be dispersed.
Both these drawbacks reduce the efficiency of the economizer device.
One object of the invention is to make available a pneumatic apparatus comprising improved economizer devices.
Another object of the invention is to make available a pneumatic apparatus that is able to work at lower working pressures so as to reduce significantly energy consumption (in particular, electric energy) necessary for supplying the air compression means and thus obtain a significant financial saving in the costs of running and maintaining the apparatus. A further object is to make available an economizer device that is associable with a linear pneumatic actuator that enables the consumption of compressed air to be reduced maintaining unaltered the performance (thrust and traction force on the piston, speed, acceleration) of the latter.
Still another object is to make an economizer device that is compact, with moderate bulk and dimensions and which is easily installable on or integrable into, a linear pneumatic actuator.
A further object is to make an economizer device that is simple and cheap and does not increase the total costs of the pneumatic apparatus.
Another further object is to make available a pneumatic apparatus that has reliable operation owing to the economizer device that operates completely automatically, adopting autonomously the correct operational configuration in response to the pressure present in the elements of the pneumatic apparatus that are connected thereto.
According to the invention a pneumatic apparatus according to claim 1 is provided.
In a first embodiment, an economizer device (100; 200) is disclosed, which is in particular associable with a chamber (23, 24) of a linear pneumatic actuator (20), comprising:

a body (102) provided with a first opening (103) that is connectable to fluid distributing means (51), with a second opening (104) that is connectable to said chamber (23, 24), and with a third opening (105) that is connectable to vacuum generating means (60), said first, second and third opening (103, 104, 105) being connected together by a main conduit (106) made in said body (102);
first shutter means (111) that is movable between an opening position (A) in which the first shutter means (111) permits the connection between said second opening (104) and said third opening (105), and a closure position (B) in which the first shutter means (111) closes the connection between said second opening (104) and said third opening (105);
second shutter means (112; 212) arranged for preventing the connection between said second opening (104) and said first opening (103) when said first shutter means (111) is in said opening position (A) and for permitting the passage of fluid between said second opening (104) and said first opening (103) when said first shutter means (111) is in said closure position (B).

In a second embodiment, an economizer device according to the first embodiment is disclosed, comprising a closure element (114) in which said first opening (103) is made and provided with supporting means (115) that is arranged for supporting said second shutter means (112; 212), said supporting means (115) comprising passage means (116) for permitting the pressurized fluid entering through said first opening (103) to flow to said second opening (104).
In a third embodiment, an economizer device according to the second embodiment is disclosed, in which said second shutter means (212) comprises:

a first seal zone (220) that is slidable axially inside a seat (221) made in said supporting means (115),
a second seal zone (222) arranged for engaging in a sealed manner with abutting means (223) made in said supporting means (115), said first and second seal zones (220, 222) being at opposite ends of said second shutter means (212) so as to define therebetween an intermediate zone (224) into which said passage means (116) leads, said second seal zone (222) having a radial extent that is greater than the radial extent of said first seal zone (220),

In a fourth embodiment, an economizer device according to the third embodiment is disclosed, in which the maximum stroke performed by said second shutter means (212) between said first position (D) and said second position (E) is less than the maximum stroke performed by said first shutter means (111) between said opening position (A) and said closure position (B).
In a fifth embodiment, an economizer device according to any of the first four embodiments is disclosed, in which said second shutter means (212) comprises a head element (227), in particular that is of tubular shape, that is arranged for engaging with a corresponding portion (118) of said first shutter means (111), said head element (227) being provided with a plurality of passages (227a) that permit the passage of pressurized fluid when said first shutter means (111) is in said closure position (B).
In a sixth embodiment, an economizer device according to any of the first five embodiments is disclosed, in which said first shutter means comprises a first tubular shutter (111) that is provided with a through longitudinal hole (113) and said second shutter means (112; 212) is arranged in said through longitudinal hole (113).
In a seventh embodiment, an economizer device according to any of the first fix embodiments is disclosed, in which said first shutter means (111) comprises spacer means (107) that protrudes to a stroke end (108) made in said main conduit (106) and is arranged for abutting on said end stroke (108) at least when said first shutter means (111) is in said closure position (B), said spacer means (107) comprising one or more passage openings (109) that permit a pressurized fluid entering said economizer device (100) through said second opening (104) to reach said main conduit (106).
In an eighth embodiment, an economizer device according to the sixth or seventh embodiment is disclosed, in which said second shutter means (112) comprises a head element (117), in particular shaped as a pin, which is provided with first seal means (117a) and is arranged for sealed coupling with a corresponding portion (118) of said first shutter means (111) when the latter is in said opening position (A) to close in a sealed manner the connection between said first opening (103) and said second opening (104). In a ninth embodiment, an economizer device according to the sixth or the seventh or the eighth embodiment is disclosed, in which said second shutter means (112) is fixed and comprises check valve means (119) arranged for preventing the passage of fluid from said second opening (104) to said first opening (103) when said first shutter means (111) is in said opening position (A) and for permitting the passage of fluid from said first opening (103) to said second opening (104) when said first shutter means (111) is in said closure position (B).
The invention can be better understood and implemented with reference to the attached drawings that illustrate some embodiments thereof by way of non-limiting example, i which:

Figure 1 is a schematic view of a pneumatic apparatus according to the invention in which an economizer device and a linear pneumatic actuator are visible that are both sectioned according to a longitudinal section plane with flow controlling means of the economizer device illustrated in a first operating configuration;
Figure 2 is a schematic view of the pneumatic apparatus of Figure 1 in which the flow controlling means is illustrated in a second operating configuration;
Figures 3-5 are schematic views of a pneumatic apparatus according to the invention in which a version of the economizer device and a linear pneumatic actuator are visible, both sectioned according to a longitudinal section plane, with flow controlling means of the economizer device positioned in different configurations;
Figures 6 and 7 are schematic views of the pneumatic apparatus according to the invention which another version of the economizer device and a linear pneumatic actuator are visible, which are both sectioned according to a longitudinal section plane, with flow controlling means of the economizer device illustrated in two different operating configurations;
Figures 8 and 9 are schematic views of a pneumatic apparatus according to the invention in which a version of the economizer device illustrated in Figures 1 and 2 and a linear pneumatic actuator are visible, both sectioned according to a longitudinal section plane, with flow controlling means of the economizer device shown respectively in the first and in the second operating configuration;
Figures 10 and 11 are schematic views of a pneumatic apparatus according to the invention in which a version of the economizer device illustrated in Figures 3-5 and a linear pneumatic actuator are visible, both sectioned according to a longitudinal section plane, with flow controlling means of the economizer device shown respectively in the first and in the second operating configuration.

With reference to the Figures, a pneumatic apparatus 1 is illustrated that may, in particular, be included in pneumatic plants and circuits for apparatuses and operating machines, such as, for example, automatic packaging machines.
The pneumatic apparatus 1 comprises a linear pneumatic actuator 20, in particular a pneumatic cylinder, comprising an outer casing 21 (cylinder) and a piston 22 provided with a stem 25 and slidable inside the casing 21, defining in the latter a first chamber 23 and a second chamber 24.
The pneumatic apparatus 1 further comprises fluid distributing means 51 that is arranged for distributing pressurized fluid alternatively in the first chamber 23 and in the second chamber 24. The fluid distributing means 51 may comprise, for example, a four-way and three-position pneumatic solenoid valve or a four-way and two-position pneumatic solenoid valve, as in the illustrated embodiments.
The pneumatic apparatus 1 comprises vacuum generating means 60 that may include, for example, a vacuum pump of known type.
The pneumatic apparatus may comprise different embodiments of an economizer device 100, 100', 200, 200', 300 that is associated with one of the two chambers 23, 24 of the cylinder.
In the different embodiments of the economizer device 100, 100', 200, 200', 300, elements or components that have substantially the same structure or function have been indicated by the same reference numbers.
In the illustrated embodiments, the economizer device 100, 100', 200, 200', 300 is associated with, and flowingly connected with, the second chamber 24. It is understood that, in alternative embodiments that are not shown, the economizer device may be associated with the first chamber 23.
The economizer device 100, 100', 200, 200', 300 comprises a body 102 provided with a first opening 103 that is connected to the fluid distributing means 51, with a second opening 104 connected to the second chamber 24 of the pneumatic cylinder 20, and with a third opening 105 connected to the vacuum generating means 60.
The openings 103, 104, 105 are connected together by a main conduit 106 made in the body 102.
It should be noted that the economizer device 100, 100', 200, 200', 300 may optionally be removably connected to the casing 21, with the second opening 104 that may, for example, be connected to the second chamber 24 by pneumatic connecting means of known type.
Alternatively, the economizer device 100, 100', 200, 200', 300 may be integrated into the casing 21.
The economizer device 100, 100', 200, 200', 300 comprises flow controlling means 110, that is movable between a first operating configuration C1 and a second operating configuration C2. In the first operating configuration C1, the flow controlling means 110 permits the connection between the second opening 104 and the third opening 105 to place the vacuum generating means 60 in communication with the second chamber 24, and prevents the connection between the second opening 104 and the first opening 103. In the second operating configuration C2, the flow controlling means 110 closes the connection between the second opening 104 and the third opening 105 and on the other hand permits the passage of fluid between the second opening 104 and the first opening 103.
With reference to the embodiments of the economizer device 100, 100', 200, 200' shown in Figures 1-5 and 8-11, the flow controlling means 110 comprises a first tubular shutter 111. The economizer device 100, 100' (Figures 1, 2, 8 and 9) comprises a fixed second shutter 112. The economizer device 200, 200' (Figures 3-5, 10 and 11) on the other hand comprises a movable second shutter 212.
The first tubular shutter 111 is provided with a through longitudinal hole 113, inside which the second shutter 112, 212 is arranged.
The first tubular shutter 111 is movable between an opening position A, in which the first tubular shutter 111 permits the connection between the second opening 104 and the third opening 105 so as to place the vacuum generating means 60 in communication with the second chamber 24, and a closure position B in which first tubular shutter 111 closes the connection between the second opening 104 and the third opening 105. The second shutter 112, 212 is arranged for preventing the connection between the second opening 104 and the first opening 103 when the first tubular shutter 111 is in the opening position A.
The economizer device 100, 100', 200, 200' comprises a closure element 114, for example shaped as a cap, that engages with the body 102 to close the main conduit 106 at an end of the latter. In the closure element 114 the first opening 103 is made.
The closure element 114 is provided with supporting means 115 that is arranged for supporting the second shutter 112, 212. The supporting means 115 comprises passage means 116 for permitting the passage of pressurized fluid between the first opening 103 and the through longitudinal hole 113.
The fixed second shutter 112 comprises a head element 117, that may be shaped as a pin and is provided with first seal means 117a (Figure 1). The head element 117 is arranged for sealed coupling with a corresponding portion 118 of the first tubular shutter 111 when the latter is in the opening position A, so as to close in a sealed manner the connection between the first opening 103 and the second opening 104. In other words, in the first operating configuration C1, the sealed coupling between the head element 117 and the portion 118 prevents the passage of pressurized fluid into the through longitudinal hole 113 and, consequently, closes the connection between the first opening 103 and the second opening 104. The portion 118 is for example defined by an inner wall portion of the first tubular shutter 111 that bounds the through longitudinal hole 113.
On the fixed second shutter 112 check valve means 119 is mounted that may, for example, be configured as an annular seal lip that surrounds the second shutter 112 and contacts the inner wall of the first tubular shutter 111. In the first operating configuration C1, the check valve means 119 assists the first seal means 117a to close in a sealed manner the through longitudinal hole 113 and thus prevent the passage of fluid from the second opening 104 to the first opening 103. In the second operating configuration C2, the check valve means 119 on the other hand permits the pressurized fluid that enters the economizer device 100 through the first opening 103 to flow through the through longitudinal hole 113 to direct itself to the second opening 104.
In the embodiment illustrated in Figures 8 and 9, the economizer device 100' comprises elastic means 130, for example configured as a coil spring.
With reference to the economizer device 200, 200' illustrated in Figures 3-5, 10 and 11, the second shutter 212 is on the other hand movable between a first position D and a second position E.
The movable second shutter 212 comprises a first seal zone 220, which is slidable inside a seat 221 made in the supporting means 115.
In the seat 221 there is made a vent hole 226 that connects the inside of the seat 221 to the outside of the body 102 in order to prevent a vacuum being created in the seat that could cause a malfunction of the economizer device 200, 200'.
The second shutter 212 further comprises a second seal zone 222 that is arranged for engaging in a sealed manner with abutting means 223 made in the supporting means 115. The first seal zone 220 and the second seal zone 222 are at opposite ends of the second shutter 212 so as to define therebetween an intermediate zone 224 in which the passage means 116 leads. The intermediate zone 224 may, for example, be shaped like an annular chamber that surrounds a portion of the second shutter 212 and is bounded by the first seal zone 220, by the second seal zone 222 and by the supporting means 115.
The second seal zone 222 has a radial extent (diameter) that is greater than the radial extent (diameter) of the first seal zone 220. Radial extent is understood to means the extent in the direction that is substantially perpendicular to a longitudinal axis X around which there extend, in a substantially coaxial manner, the main conduit 106 and the through longitudinal hole 113. It should be noted that also the first tubular shutter 111 and the second shutter 212 may be, for example, coaxial to the longitudinal axis X.
As the radial extent of the second seal zone 222 is greater than the radial extent of the first seal zone 220, it is clear that the pressurized fluid that reaches the intermediate zone 224 from the passage means 116 exerts on the second shutter 212 a resulting force that is oriented according to the direction indicated by the arrow F (Figures 4 and 5). Thus this force F exerted by the pressurized fluid tends to move the second seal zone 222 away from the abutting means 223.
In the first position D, the second seal zone 222 abuts in a sealing manner on the abutting means 223 to close in a sealed manner the connection between the first opening 103 and the second opening 104 when the first tubular shutter 111 is in the opening position A.
In the second position E, on the other hand, the second seal zone 222 does not contact the abutting means 223 so as to permit the passage of pressurized fluid between the first opening 103 and the second opening 104 when the first tubular shutter 111 is in the closure position B.
The movable second shutter 212 comprises a respective head element 227 that is of tubular shape. The head element 227 is provided with a plurality of passages 227a that are arranged for permitting the passage of pressurized fluid through the through longitudinal hole 113 when the first tubular shutter 111 is in the closure position B.
In the embodiment shown in Figures 10 and 11, the economizer device 200' comprises check valve means 219.
The check valve means 219, which may be for example configured as an annular sealing lip, is mounted on the movable second shutter 212. The check valve means 219 surrounds a portion of the movable second shutter 112 that is interposed between the second seal zone 222 and the head element 227, and contacts the inner wall of the first tubular shutter 111. In the first operating configuration C1, the check valve means 219 assists the second seal zone 222 to close in a sealed manner the through longitudinal hole 113 and thus prevent the passage of fluid from the second opening 104 to the first opening 103. In the second operating configuration C2, the check valve means 219 on the other hand permits the pressurized fluid that enters the economizer device 200' through the first opening 103 to flow through the through longitudinal hole 113 to direct itself to the second opening 104.
The economizer device 200' further comprises elastic means 230, for example configured as a coil spring.
The first tubular shutter 111 of the economizer device 100, 100', 200, 200' comprises spacer means 107. The latter protrudes from the first tubular shutter to a stroke end 108 made in the main conduit 106. The stroke end 108 may, for example, be shaped as a shoulder obtained in the main conduit 106.
The spacer means 107 is arranged for abutting on the stroke end 108 at least when the first tubular shutter 111 is in the closure position B. The spacer means 107 comprises one or more passage openings 109 that permit a pressurized fluid entering the economizer device 100, 100', 200, 200' through the second opening 104 to reach the main conduit 106. The spacer means 107 functions as a movement promoting means to ensure that the first tubular shutter 111 starts to move from the closure position B to the opening position A. In fact, in the absence of the spacer means 107, the pressurized fluid entering the economizer device 100, 100', 200, 200' through the second opening 104 might not find a suitable area on which to exert a thrust action to take the first tubular shutter 111 from the closure position B to the opening position A. The spacer means 107 thus supplies a spacer element that enables the pressurized fluid to be able to press on an annular area of the first tubular shutter 111 of suitable dimensions, so as to ensure the movement of the first tubular shutter 111 from the closure position B to the opening position A.
The first tubular shutter 111 further comprises seal means 125, for example one or more 0-rings of known type, which are associated with the outer wall of the first tubular shutter 111 facing the main conduit 106. In the embodiments shown, the seal means 125 comprises two seal rings that are positioned on the aforesaid outer wall in suitable positions that enable the third opening 105 to be sealingly isolated when the first tubular shutter 111 is in the closure position B. In other words, in this position the third opening 105 faces a zone of the outer wall interposed between the two seal rings.
In the embodiment shown in Figures 10 and 11, the movable second tubular shutter 212 is provided with seal means 225, for example a plurality of O-rings. In particular, the seal means 225 may comprise a first seal ring 225a that is positioned at the second seal zone 222 facing the abutting means 223, and a second seal ring 225b adjacent to the head element 227.
The second seal ring 225b enables a hydraulic seal to be made between the second tubular shutter 212 and the first tubular shutter 111 (when the latter is in the opening position A) so as to close in a sealed manner the connection between the first opening 103 and the second opening 104. In the first operating configuration C1, the seal means 225, in particular the first and the second seal ring 225a and 225b, and the check valve means 219 thus prevent the passage of pressurized fluid in the through longitudinal hole 113 and consequently close the connection between the first opening 103 and the second opening 104.
In the economizer device 300, shown in Figures 6 and 7, the flow controlling means 110 is shaped as shuttle means 131, that is movable, and slidably housed, in the main conduit 106.
The shuttle means 131 is provided with first conduit means 132 (Figure 6), which is arranged for connecting the second opening 104 to the third opening 105, and second conduit means 133 (Figure 7), which is arranged for connecting the first opening 103 to the second opening 104.
The first conduit means 132 and the second conduit means 133 comprise a plurality of conduits that are arranged around the longitudinal axis X, for example according to a configuration in which the conduits of the first conduit means 132 alternate with the conduits of the second conduit means 133.
In the first operating configuration C1, the shuttle means 131 is positioned in a position that is such that the first conduit means 132 permits the connection between the second opening 104 and the third opening 105 to place the vacuum generating means 60 in communication with the second chamber 24 and, at the same time, the connection between the second opening 104 and the first opening 103 is closed. In the second operating configuration C2, the shuttle means 131 is positioned in a manner that is such that the first conduit means 132 does not connect the second opening 104 and the third opening 105 and, at the same time, the second conduit means 133 permits the connection between the second opening 104 and the first opening 103.
The shuttle means 131 comprises an annular throat 134 into which the first conduit means 132 leads. The annular throat 134 is made in a position that is such as to face the third opening 105 when the shuttle means 131 is in the first operating configuration C1.
The shuttle means 131 further comprises a tubular portion 135 that is substantially coaxial with the longitudinal axis X and in contact with the walls of the main conduit 106.
The tubular portion 135 is arranged for obstructing the third opening 105 when the shuttle means 131 is in the second operating configuration C2.
In the first configuration C1, the annular throat 134 faces the third opening 105 so that the latter is connected to the second opening 104 by the first conduit means 132 (Figure 6). In the second configuration C2, the tubular portion 135 faces the third opening 105 and the annular throat 134 faces the walls of the main conduit 106 (Figure 7).
In this manner, this conformation of the shuttle means 131 means that it acts as a shutter element that is selectively able to open or close the third opening 105 respectively when the latter is faced by the annular throat 134 or the tubular portion 135.
The shuttle means 131 further comprises throttle means 136, interposed between the first conduit means 132 and the second opening 104 and arranged for adjusting the flowrate of the fluid entering the economizer device 300 from the second opening 104.
The operation of the pneumatic apparatus 1 will now be disclosed. In the figures, the flow of the pressurized air inside the pneumatic apparatus 1 has been indicated by arrows in order to facilitate the understanding of the reader.
The pneumatic actuator 20 can be typically used to control an element of the plant or operating machine, such as for example to move a slide.
If the pneumatic actuator 20 is a pneumatic cylinder, it typically has forward stroke (operating stroke) during which the controlled element moves to perform the intended task, and a return stroke for returning the controlled element to the initial position.
The fluid distributing means 51 is movable between a first work configuration P1 and a second work configuration P2.
In the first work configuration P1, the fluid distributing means 51 connects the first opening 103 to an environment at atmospheric pressure and sends air at working pressure p in the first chamber 23, to move the piston 22 in a forward or work stroke. Movement of the piston 22 reduces the volume of the second chamber 24 so as to cause pressurized air to exit the second chamber 24. The pressurized air that exits the second chamber 24 enters the economizer device 100, 100' through the second opening 104.
In the economizer device 100, 100' the pressurized air reaches the main conduit 106, where it meets the first tubular shutter 111 arranged in the closure position B.
The pressurized air traverses the spacer means 107 and pushes the first tubular shutter 111 in the direction indicated by the arrow H until the first tubular shutter 111 is taken to abut on the closure element 114, that acts as a stroke end. The first tubular shutter 111 is then taken to the opening position A so as to permit the connection between the third opening 105 and the second opening 104. In this manner the vacuum generating means 60 is flowingly connected with the second chamber 24 so as to suck compressed air from the latter. Consequently, to a thrust force generated on the piston 22 by the pressurized air in the first chamber 23 a further thrust force is added that is generated by the vacuum made in the second chamber 24. At the same time, owing to the first seal means 117a and to the check valve means 119 a sealed closure is created between the first tubular shutter 111 and the second shutter 112 that prevents the connection between the first opening 103 and the second opening 104 (Figures 1 and 8).
In the embodiment illustrated in Figures 8 and 9, the elastic means 130 acts on the first shutter 111 and cooperates with the pressure of the fluid entering from the second opening 104 to maintain the first tubular shutter 111 in the opening position A. Although the first tubular shutter 111 can remain in the opening position A only owing to the pressure of the fluid, the elastic means 130 makes the economizer device 100' more reliable.
The vacuum created by the vacuum generating means 60 could in fact in certain operating conditions generate a sucking force on the first tubular shutter 111 that is opposed to the force exerted on the latter by the pressure of the operating fluid entering through the second opening 104, i.e. a sucking force that tends to take the first tubular shutter 111 to the stroke end 108.
Owing to the elastic means 130 that assists the pressurized fluid, the first tubular shutter 111 remains in the opening position A even if the aforesaid sucking force is generated.
When it is desired to return the piston 22 to the original position, the fluid distributing means 51 adopts the second work configuration P2, in which it connects the first chamber 23 to the environment at atmospheric pressure and sends air at working pressure p to the first opening 103. The pressurized air enters the latter, traverses the passage means 116 and reaches the first tubular shutter 111, which is in the opening position A. The pressurized air pushes the first tubular shutter 111 in the direction indicated by the arrow K until the spacer means 107 reaches and abuts on the stroke end 108. In this manner, the first tubular shutter 111 compresses the elastic means 130, if it is present, and is taken again to the closure position B, it which it obstructs third opening 105 and closes in a sealed manner the connection between the latter and the second opening 104, i.e. between the vacuum generating means 60 and the second chamber 24. At the same time, the pressurized air that enters from the first opening 103, traverses the through longitudinal hole 113 and exits from the second opening 104 to discharge into the second chamber 24 so as to push the piston 22 in the return stroke (Figures 2 and 9).
Subsequently, each time that it is necessary to repeat a work cycle of the element controlled by the pneumatic actuator 20, it is sufficient to make the fluid distributing means 51 adopt in sequence the work configurations P1 and P2 so that the economizer device 100, 100' operates in the manner disclosed in the preceding paragraphs.
With reference to the economizer device 200, 200' (Figures 3-5, 10 and 11), in which the second shutter 212 is movable, when the fluid distributing means 51 is in the first work configuration P1, the pressurized air that enters the economizer device 200, 200' through the second opening 104 pushes both the first tubular shutter 111 and the second shutter 212 in the direction indicated by the arrow H.
In the embodiment illustrated in Figures 10 and 11, the elastic means 230 cooperates with the pressure of the fluid entering from the second opening 104 to maintain the first tubular shutter 111 in the opening position A, in a similar manner to what has been explained previously with reference to Figures 8 and 9.
The first tubular shutter 111 and the second shutter 212 move together until the second seal zone 222 comes to abut on the abutting means 223. The through longitudinal hole 113 and, consequently, the connection between the second opening 104 and the first opening 103 is sealingly closed owing to the cooperation between the second seal zone 222 and the abutting means 223, and also owing to the seal means 225 and to the check valve means 219, if it is present (Figure 10). When the second seal zone 222 abuts on the abutting means 223, the first tubular shutter 111 is in the opening position A and the movable second shutter 212 is in the first position D.
When the piston 22 has terminated the forward stroke and has to perform the return stroke, the fluid distributing means 51 adopts the second work configuration P2.
The pressurized air that enters the economizer device 200, 200' from the first opening 103, traverses the passage means 116 and leads into the intermediate zone 224. As explained previously, it is here that the force F originates that tends to move the second seal zone 222 away from the abutting means 223 so as to take the movable second shutter 212 to the second position E. In this manner, between the second seal zone 222 and the abutting means 223 a passage opens for the pressurized air, which can thus enter the through longitudinal hole 113. From here, i.e. from inside the first tubular shutter 111, the pressurized air exerts a thrust action on the first tubular shutter 111 in the direction indicated by the arrow K so as to take the first tubular shutter 111 to the closure position B, in which the elastic means 230 (if it is present) is compressed and the spacer means 107 reaches and abuts on the stroke end 108 (Figures 4 and 11).
The second position E of the movable second shutter 212 is in a position of equilibrium in which the shutter is positioned and depends, on the one hand, on the pressure of the fluid (air) that enters the economizer device 200, 200' through the first opening 103, and on the other side, on the pressure of the fluid in the second chamber 24.
In Figures 4 and 11 the movable second shutter 212 is shown in a second position E in which the second seal zone 222 does not abut on the abutting means 223 and the head element 227 does not abut on the portion 118 of the first tubular shutter 111.
In Figure 5 the first tubular shutter 111 is shown in the closure position B and the movable second shutter 212 in a second position E in which the head element 227 abuts on the portion 118. In other words, Figure 5 shows the second shutter 212 in a stroke end position.
With reference to Figure 5, it is also possible to note that the maximum stroke performed by the movable second shutter 212 between the first position D and the second position E is less than the maximum stroke performed by the first tubular shutter 111 between the opening position A and the closure position B.
This means that when the fluid distributing means 51 moves from the second work configuration P2 to the first work configuration P1, in a first moment the movable second shutter 212 moves from the second position E to the first position D, and only subsequently does the first tubular shutter 111 move from the closure position B to the opening position A. In other words, this ensures correct operation of the economizer device 200, 200', owing to the fact that the flow connection between the second opening 104 and the third opening 105 opens only after the flow connection between the second opening 104 and the first opening 103 has been already closed.
The maximum stroke of the movable second shutter 212 is defined by the maximum distance M (taken in a longitudinal direction) between the second seal zone 222 and the abutting means 223.
The maximum stroke of the first tubular shutter 111 is defined by the maximum distance N between the bottom of the first tubular shutter 111 and the closure element 114.
In this embodiment, as will be explained better below, the economizer device 200, 200' also enables the supply pressure to be reduced in the second chamber 24 and thus enables air consumption to be reduced in the movement of the piston 22 in a return stroke.
The movable second shutter 212 in fact acts as a pressure-reducing element that is known in the prior art that enables the working pressure p to be reduced of the air entering from the first opening 103 to a reduced pressure p_r that is less than the working pressure p. Owing to the second shutter 212 it is thus possible to adjust the air pressure inside the second chamber 24, in particular in the case of a return stroke of the piston without a load.
Experimental tests have shown that a difference between the radial extensions (diameters) of the first and the second second seal zone 220, 222 of about 10% enables reduced pressure p_r to be obtained that is approximately the same as 50% of working pressure p.
It should be noted that if in the return stroke the piston 22 has to exert a force (load movement), the value of the reduced pressure p_r increases proportionately to the size of the load.
Also the economizer device 300 (Figures 6 and 7) operates in a similar manner. Nevertheless, unlike the other embodiments, as said, the flow controlling means 110 is shaped as shuttle means 131.
When the fluid distributing means 51 is in the first work configuration P1, the piston 22 performs the forward stroke and the pressurized air (exiting the second chamber 24) enters the economizer device 300 through the second opening 104 to push the shuttle means 131 in the direction indicated by the arrow H. The shuttle means 131 thus adopts the first operating configuration C1, in which the first conduit means 132 connects the second opening 104 and the third opening 105 to place the vacuum generating means 60 in communication with the second chamber 24 (Figure 6).
When it is desired to return the piston 22 to the original position, the fluid distributing means 51 adopts the second work configuration P2, in which it delivers pressurized air to the first opening 103. The pressurized air pushes the shuttle means 131 in the direction indicated by the arrow K until the shuttle means 131 reaches and abuts on the stroke end 108 (shoulder). Further, the pressurized air enters the second conduit means 133 to reach the second opening 104, and, consequently, the second chamber 24 (Figure 7). Experimental tests have been run to demonstrate the obtaining of energy saving that the various embodiments of the economizer device 100, 100', 200, 200' and 300 of the invention permit when they are applied to a linear pneumatic actuator.
With reference to the pneumatic apparatus 1, the value of the force developed by the cylinder and measured on the stem 25 depends on the actual thrust section of the piston 22 (i.e. the area of the piston, for example in mm²) on which the pressurized air can act, and the pressure (bar) of the air delivered to the chambers 23, 24.
If it is wished to assess the cost of an operating cycle of the linear pneumatic actuator 20 (forward stroke and return stroke) it is necessary to calculate the air consumption (N1 - normal litre) for running this operating cycle.
Considering, purely by way of non-limiting example, a cylinder having a bore D = 80 mm, stroke c = 200 mm and stem with a diameter d = 25 mm, supplied with air at working pressure p = 6 bar (relative pressure), in the event of a pneumatic cylinder devoid of economizer devices the theoretical thrust force F_s(6) (at 6 bar working pressure) in the forward stroke is: $F_{s (6)} = area \times p = π \frac{{0.08}^{2}}{4} \times 6 \times 10^{5} = 3.014 N$
Theoretical area consumption in the thrust phase V_s(6) (forward stroke) and in the traction phase V_t(6) (return stroke) is: $V_{s (6)} = \frac{π D^{2} (p + 1)}{4 \cdot 10^{6}} c = \frac{π \times 80^{2} \times 7}{4 \cdot 10^{6}} \times 200 = 7, 04 Nl$
$V_{t (6)} = \frac{π (D^{2} - d^{2}) (p + 1)}{4 \cdot 10^{6}} c = \frac{π \times (80^{2} - 25^{2}) \times 7}{4 \cdot 10^{6}} \times 200 = 6, 35 Nl$
Total consumption V_tot(6) of compressed air at working pressure p = 6 bar is: $V_{tot (6)} = V_{s (6)} + V_{t (6)} = 13.39 Nl .$
A pneumatic apparatus 1 is now considered that is provided with the economizer device according to the invention associated with the linear pneumatic actuator.
Purely by way of non-limiting example, it is considered that the vacuum generating means 60 supply a vacuum equal to 0.9 bar (90 KPa).
Reference is now made to a pneumatic apparatus 1 comprising the embodiment of the economizer device 100, 100' specified in Figures 1 and 2, 8 and 9 or the embodiment of the economizer device 300 specified in Figures 6 and 7.
If a pneumatic cylinder is considered that is provided with the economizer device 100, 100', 300 and supplied with compressed air at working pressure p = 5 bar, a theoretical thrust force F_s(5) (at 5 bar working pressure) in the forward stroke is obtained that is generated by the air pressure in the first chamber 23 (Figures 1, 6 and 8) equal to: $F_{s (5)} = Area \times p = π \frac{{0.08}^{2}}{4} \times 5 \times 10^{5} = 2.512 N$
to which a further thrust force F'_s(5) has to be added that is generated by the vacuum (in this hypothesis, p'= -0.90 bar) created by the vacuum generating means 60 connected to the second chamber 24 and equal to: ${Fʹ}_{s (5)} = Area \times p = π \times ({0.04}^{2} - {0.0125}^{2}) \times 0.90 \times 10^{5} = 409 N$
The total theoretical thrust force F_stot(5) is thus equal to: $F_{stot (5)} = F_{s (5)} + {Fʹ}_{s (5)} = 2.921 N$
i.e. a value very near the value (3.014 N) obtained by supplying 6 bar pressure to the pneumatic cylinder that is devoid of the economizer device.
It should be pointed out that owing to the economizer device 100, 100', 300 of the invention it is thus possible to obtain substantially the same performance as the pneumatic actuator (thrust force) by reducing working pressure by 1 bar.
It is clear that, alternatively, at the same working pressure, the economizer device 100, 100', 300 enables a thrust force on the actuator to be obtained that is much greater than that available for an actuator devoid of the economizer device. In other words, if the pneumatic apparatus 1 works at a working pressure of 6 bar, the thrust force available to the actuator is much greater owing to the presence of the economizer device 100, 100', and 300.
Theoretical area consumption in the thrust phase V_s(5) (forward stroke) and theoretical area consumption in the traction phase V_t(5) (return stroke) is: $V_{s (5)} = \frac{π D^{2} (p + 1)}{4 \cdot 10^{6}} c = \frac{π \times 80^{2} \times 7}{4 \cdot 10^{6}} \times 200 = 6, 03 Nl$
$V_{t (5)} = \frac{π (D^{2} - d^{2}) (p + 1)}{4 \cdot 10^{6}} c = \frac{π \times (80^{2} - 25^{2}) \times 7}{4 \cdot 10^{6}} \times 200 = 5.4 Nl$
Total consumption V_tot(5) of compressed air at working pressure p = 5 bar is: $V_{tot (5)} = V_{s (5)} + V_{t (5)} = 11.43 Nl .$
The economizer device 100, 100', 300 thus enables 1.96 N1 (V_tot(6) - V_tot(5) = 13.39 - 11.43) of pressurized air to be saved for each operating cycle of the linear pneumatic actuator 20. Thus the economizer devices 100, 100' and 300, in which during the forward stroke of the piston 22 the vacuum generating means 60 supplies thereto a further thrust force F'_s(5) (Figures 1, 6 and 8), enables about 15% of pressurized air to be saved compared with a similar pneumatic apparatus devoid of these economizer devices.
With reference to the pneumatic apparatus 1 comprising the embodiment of the economizer device 200, 200' shown in Figures 3-5, 10 and 11 it is noted that even in this case during the forward stroke of the piston 22 the vacuum generating means 60 supplies thereto a further thrust force F'_s(5) calculated previously (Figures 3 and 10). Further, in this embodiment, the economizer device 200, 200' enables a further pressurized air saving to be obtained owing to the fact that the second shutter 212 acts as a pressure reducer and thus enables working pressure p, equal to 5 bar, to be reduced as far as reduced pressure p_r, equal to about 50% of working pressure p, i.e. 2.5 bar (Figures 4, 5 and 11).
In this embodiment, theoretical air consumption in the traction phase V_t(5) (return stroke) is: $V_{t (pr)} = \frac{π (D^{2} - d^{2}) (p_{r} + 1)}{4 \cdot 10^{6}} c = \frac{π \times (80^{2} - 25^{2}) \times 3, 5}{4 \cdot 10^{6}} \times 200 = 2.7 Nl$
Total consumption V_tot(5) of compressed air at working pressure p = 5 bar is thus equal to V_tot(5) = V_s(5) + V_t(pr) = 8.7 N1.
It is thus observed that, owing to the economizer device 200, 200' of the invention it is possible to save 4.69 N1 (V_tot(6) - V_tot(5) = 13.39 - 8.7) during each operating cycle of the pneumatic cylinder 20 (in the case of an unloaded return stroke of the piston 22).
The economizer device 200, 200' that is able to reduce working pressure during the return stroke of the piston, enables about 35% of pressurized air to be saved compared with a similar pneumatic apparatus devoid of the economizer device.
If it is hypothesised, purely by way of example, that the pneumatic cylinder 20, installed on an operating machine, runs 10 cycles a minute for 10 hours a day for 230 days/year, 1,380,000 cycles will be run in a year. Multiplying this number of cycles/year by the litres saved at each cycle (4.69 Nl), the annual compressed air saving is obtained, which will be about 6,500 m³ of compressed air at 5 bar.
It should be noted that the greater the bore dimensions and the stroke of the pneumatic cylinder the greater will be the compressed air consumption saving.
Owing to the economizer devices 100, 100', 200, 200', 300 of the invention, comprised in a pneumatic apparatus, it is thus possible to obtain a considerable saving of compressed air consumption for the same performance supplied by the linear pneumatic actuator 20 (thrust and traction force on the piston, speed, acceleration). In particular, as has been seen, it is possible to obtain the same performance from the cylinders by supplying the pneumatic actuators found in a plant with a working pressure reduced by 1 bar.
According to the scientific literature specialised in the sector of pneumatics, for each variation of 0.1 bar, there is a variation of one percentage point in electricity consumption, starting from the compressors unit and continuing by cascade along the entire set of equipment involved in the production of compressed air. Consequently, owing to the economizer device 100, 100', 200, 200', 300, which enables pressure to be reduced by 1 bar, it is possible to reduce energy consumption (e.g. electric energy) by about 10%.
In addition to saving on the use of energy that is necessary for supplying the air compression means, the economizer device 100, 100', 200, 200', 300 of the invention also enables wear to the aforesaid compression means routine and extraordinary maintenance costs thereof to be reduced. Further, the lower air consumption enables the costs of cleaning, filtering, air filtration and condensation elimination to be reduced. The lower energy consumption enables CO₂ emissions into the atmosphere to be reduced, with evident benefits for the environment.
By using lower working pressure, the compression units are not only subjected to less stress but are also less noisy, this causing a significant reduction in existing noise pollution (decibels/hour) in the work environment.
Another advantage of the economizer device 100, 100', 200, 200', 300, if used at standard working pressure standard, and thus at non-reduced pressure, is that it supplies a significantly increased thrust force to the actuator. It is clear that in this case the economizer device does not enable the same compressed air consumption savings to be made but does enable to a considerable increase in the performance supplied by the linear pneumatic actuator (in particular thrust force) to be obtained for the same working pressure.
The economizer device 100, 100', 200, 200', 300 of the invention is moreover particularly compact, with moderate bulk and dimensions and is thus easily installable on a linear pneumatic actuator. It should be noted that a pneumatic apparatus can be equipped with the economizer devices according to the invention even subsequently to the installation of the plant or of the operating machine.
Further, the economizer device 100, 100', 200, 200', 300 of the invention has a structure that is simple and therefore rather cheap to make. Consequently, the economizer devices 100, 100', 200, 200', 300 according to the invention do not significantly increase the total costs of the pneumatic apparatus 1.
The pneumatic apparatus 1 according to the invention has reliable operation thanks to the economizer device 100, 100', 200, 200' 300 that operates completely automatically. In fact, as is clear from the preceding description, the flow controlling means 110 adopts each time the first or the second operating configuration independently, on the basis of the pressure of the fluid (compressed air) present in the second chamber 24 and of the working pressure p.
In an alternative embodiment, the economizer devices 100, 100', 200, 200', 300 of the invention may also be associated with a single-effect pneumatic cylinder provided in the chamber traversed by the stem of a spring for performing the return stroke.

Claims

Pneumatic apparatus (1) comprising:
- a linear pneumatic actuator (20) comprising a casing (21) and a piston (22) that is slidable inside said casing (21) and defining in the latter a first chamber (23) and a second chamber (24),

- fluid distributing means (51) arranged for distributing pressurized fluid alternatively to said first chamber (23) and to said second chamber (24),

- vacuum generating means (60),

- an economizer device (100; 100'; 200; 200'; 300) comprising:
a body (102) provided with a first opening (103) connected to said fluid distributing means (51), with a second opening (104) connected to one of said chambers (24), and with a third opening (105) connected to said vacuum generating means (60), said first, second and third opening (103, 104, 105) being connected together by a main conduit (106) made in said body (102);

flow controlling means (110) that is movable between a first operating configuration (C1), wherein the flow controlling means (110) permits the connection between said second opening (104) and said third opening (105) so as to place said vacuum generating means (60) in communication with said chamber (24) and prevents the connection between said second opening (104) and said first opening (103), and a second operating configuration (C2),

wherein the flow controlling means (110) closes the connection between said second opening (104) and said third opening (105) and permits the passage of fluid between said second opening (104) and said first opening (103).
Apparatus according to claim 1, wherein said flow controlling means (110) comprises a first tubular shutter (111) that is movable between an opening position (A) wherein the first tubular shutter (111) permits the connection between said second opening (104) and said third opening (105) so as to place said vacuum generating means (60) in communication with said chamber (24), and a closure position (B) wherein the first tubular shutter (111) closes the connection between said second opening (104) and said third opening (105).
Apparatus according to claim 2, wherein said flow controlling means (110) comprises a second shutter (112; 212) arranged for preventing the connection between said second opening (104) and said first opening (103) when said first tubular shutter (111) is in said opening position (A).
Apparatus according to claim 3, wherein said first tubular shutter (111) is provided with a through longitudinal hole (113) wherein said second shutter (112; 212) is arranged.
Apparatus according to claim 3 or 4, wherein said economizer device (100; 100'; 200; 200') comprises a closure element (114) wherein said first opening (103) is made and provided with supporting means (115) that is arranged for supporting said second shutter (112; 212).
Apparatus according to claim 5, wherein said supporting means (115) comprises passage means (116) for permitting the passage of pressurized fluid between said first opening (103) and said through longitudinal hole (113).
Apparatus according to any one of claims 3 to 6, wherein said second shutter (112) comprises a head element (117), in particular shaped as a pin, which is provided with first seal means (117a) and is arranged for sealed coupling with a corresponding portion (118) of said first tubular shutter (111) when the latter is in said opening position (A) to close in a sealed manner the connection between said first opening (103) and said second opening (104).
Apparatus according to any one of claims 3 to 6, wherein said second shutter (212) comprises a head element (227) that is of tubular shape and is provided with a plurality of passages (227a) that permit the passage of pressurized fluid when said first tubular shutter (111) is in said closure position (B).
Apparatus according to any one of claims 3 to 6 or according to claim 8, wherein said second shutter (212) comprises:
- a first seal zone (220) that is slidable inside a seat (221) made in said supporting means (115),

- a second seal zone (222) arranged for engaging in a sealed manner with abutting means (223) made in said supporting means (115), said first and second seal zones (220, 222) being at opposite ends of said second shutter (212) so as to define therebetween an intermediate zone (224) into which passage means (116) leads of said supporting means (115), said second seal zone (222) having a radial extent that is greater than the radial extent of said first seal zone (220),
wherein said second shutter (212) is movable between a first position (D), wherein said second seal zone (222) abuts on said abutting means (223) to close in a sealed manner the connection between said first opening (103) and said second opening (104) when said first tubular shutter (111) is in said opening position (A), and a second position (E), wherein said second seal zone (222) does not contact the abutting means (223) so as to permit the passage of pressurized fluid between said first opening (103) and said second opening (104) when said first tubular shutter (111) is in said closure position (B).
Apparatus according to claim 9, wherein the maximum stroke (M) performed by said second shutter (212) between said first position (D) and said second position (E) is less than the maximum stroke (N) performed by said first tubular shutter (111) between said opening position (A) and said closure position (B).
Apparatus according to any one of claims 2 to 10, wherein said first tubular shutter (111) comprises spacer means (107) that protrudes to a stroke end (108) made in said main conduit (106) and is arranged for abutting on said end stroke (108) at least when said first tubular shutter (111) is in said closure position (B), said spacer means (107) comprising one or more passage openings (109) that enable a pressurized fluid entering said economizer device (100; 100'; 200; 200') through said second opening (104) to reach said main conduit (106).
Apparatus according to any one of claims 2 to 11, wherein said economizer device (100; 100'; 200; 200') comprises elastic means (130; 230) that acts on said first shutter (111) and cooperates with the pressure of the fluid entering from said second opening (104) to maintain said first tubular shutter (111) in said opening position (A).
Apparatus according to any preceding claim, wherein said economizer device (100; 100'; 200; 200'; 300) is removably connected to said casing (21), in particular said second opening (104) being coupled with said second chamber (24) by pneumatic connecting means.
Apparatus according to any one of claims 1 to 12, wherein said economizer device (100; 100'; 200; 200'; 300) is integrated into said casing (21).
Use of an economizer device (100; 100'; 200; 200') comprising:
- a body (102) provided with a first opening (103) that is connectable to fluid distributing means (51), with a second opening (104) that is connectable to said chamber (23, 24), and with a third opening (105) that is connectable to vacuum generating means (60), said first, second and third opening (103, 104, 105) being connected together by a main conduit (106) made in said body (102);

- first shutter means (111) that is movable between an opening position (A) wherein the first shutter means (111) permits the connection between said second opening (104) and said third opening (105), and a closure position (B) wherein the first shutter means (111) closes the connection between said second opening (104) and said third opening (105);

- second shutter means (112; 212) arranged for preventing the connection between said second opening (104) and said first opening (103) when said first shutter means (111) is in said opening position (A) and for permitting the passage of fluid between said second opening (104) and said first opening (103) when said first shutter means (111) is in said closure position (B);
wherein said first opening (103) is connected to said fluid distributing means (51), said second opening (104) is connected to a chamber (23, 24) of a linear pneumatic actuator (20) and said third opening (105) is connected to vacuum generating means (60).