Many devices are known where a force is used to extend a workpiece from a resting position to a position where it has its effect be it impacting on a surface or another object. The force may be exerted directly or indirectly and can be pneumatically, hydraulically, electrically or mechanically mediated. For example, pneumatic drills rely on rapid repetitive extension of a workpiece driven by a pneumatic actuator.

Devices of the foregoing type usually include a mechanism for retraction of the workpiece from its extended position to its rest position and can also include a mechanism for retaining the workpiece in the rest position. An example of a device including both mechanisms is the fish stunning device described in International Patent Application No. PCT/AU01/00745 (Publication No. WO 01/97621). The workpiece of this device comprises a piston which extends from a nosepiece wherein the workpiece is extended by a pneumatic actuator. Retraction of the workpiece is effected by the momentary supply of air to the underside of the actuator piston. However, some slippage of the workpiece from its rest position can occur if the device is positioned with the workpiece downwards (which is usual). Any vibration transmitted to the device contributes to such slippage.

To help maintain the workpiece of the PCT/AU01/00745 device in the rest position, the workpiece piston has an o-ring about is proximal end-that is, the end closest the actuator-which resides in a complementary groove in the nosepiece when the workpiece is in the rest position. This solution is not 100% effective however and adds to the complexity of manufacture of the device.

There are alternative mechanisms to resist slippage of a workpiece like that of the PCT/AU01/00745 device other than the solution described above. For example,

the actuator piston of such devices can be held in a retracted position by a spring, the biasing of which is overcome by the pressure exerted on the actuator piston on firing of the device. However, such mechanisms can begin to fail with extended use of a device.

It would therefore be desirable to have available a mechanism for preventing slippage of workpieces of devices referred to above which are reliable but nevertheless relatively simple.

SUMMARY OF THE INVENTION In a broad format, the invention provides a retention mechanism for retaining a workpiece in a rest position, the mechanism comprising at least two permanent magnets positioned about the proximal end of said workpiece and a ferromagnetic zone at at least the proximal end of said workpiece, wherein said at least two permanent magnets are of the rare earth type and said magnets are radially positioned with respect to an axis of said workpiece with the opposed poles being of opposite polarity.

The term"workpiece"is used in the foregoing definition of the invention to denote any article that is extended by an applied force-be it a pneumatic, hydraulic or otherwise-from its rest position to a position where the workpiece has its effect. The workpiece can thus be a tool that impacts on a surface and extends from a support structure or a piston that extends from a cylinder.

The term"proximal"is used above and hereafter to denote the end of the workpiece to which force is applied to extend the piece from its rest position. The distal end of the workpiece is thus its working end.

It will be appreciated from the above definition of the invention that the at least two permanent magnets hold the workpiece in the rest position. However, on application of a force to the workpiece, the retaining effect is overcome and the workpiece is extended. If necessary, a retraction mechanism is activated to return the workpiece to the rest position where it is retained by the mechanism of the invention until the device is next operated or"fired".

The rare earth type magnets are typically based on samarium and neodynium and have BBo/o values of greater than about 150 Kj/m3. Specific magnets of this type comprise the following alloys : SmCo5 ; Sm2Co17 ; and, Nd2Fe14B. A preferred magnet is the neodynium-based magnet.

The magnets are conveniently housed in a member that surrounds the proximal end of the workpiece or is at least partially about the proximal end. The member can be a simple framework for the magnets or a machined or shaped member that includes channels for the magnets. In many instances where the workpiece is a piston or at least piston-like at its proximal end, the member can be part of a cylinder for the workpiece.

The retention mechanism of the invention preferably has four radially- positioned, equally-spaced magnets. The magnets are thus positioned every 90° about the proximal end of the work piece when in the rest position. In such a configuration, the magnets are orientated so that the poles adjacent the work piece alternate between north and south.

Having broadly described the retention mechanism according to the invention, the mechanism will now be exemplified with reference to the accompanying drawings briefly described hereafter.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a piston and cylinder arrangement including a retention mechanism where the cross section is along the axis of the piston and cylinder.

Figure 2 is a cross-sectional view through the piston and cylinder as shown in Figure 1 at the position indicated at A in the latter Figure.

Figure 3 is a side view of a device like that disclosed in PCT/AU01/00745 but incorporating a retention mechanism according to the present invention.

BEST MODE AND OTHER MODES OF CARRYING OUT THE INVENTION The piston and cylinder arrangement of the figures is configured for use in a stunning device according to PCT/AU01/00745, the entire content of which is incorporated herein by cross-reference.

Referring to Figure 1, there is shown cylinder 1 comprising a nosepiece from which piston 2 can extend when acted on by second piston 3 which is connected to piston 2 by rod 4. Piston 3 is within its own cylinder (not shown in the drawing) which in combination act as a pneumatic actuator. For convenience, piston 3 and rod 4 are not in cross section. An o-ring seal 5 located in a channel 6 in piston 3 is in cross section however.

With further reference to cylinder 1, it can be seen to include a liner 7 and a plurality of neodymium magnets, one of which is item 8. Piston 2 is hollow and has curved distal end 9 which acts as a striker. Piston 2 further includes a ferromagnetic liner 10.

Turning to Figure 2, cylinder 1 can be seen with piston 2 therein. Also visible are cylinder liner 7 and ferromagnetic liner 10. It can be appreciated from Figure 2 that there are four neodymium magnets, equally positioned about the bore of cylinder 1 with north south poles alternating.

The magnets installed in cylinder 1 have fields that are adequate to retain piston 2 in the position shown in Figure 1-the retracted rest position. The force of the magnets can nevertheless be overcome by the pneumatic pressure (typically 5 bar) applied to piston 3 of the pneumatic actuator when the device is fired. After firing, air pressure is momentarily applied to the underside of piston 3 of the pneumatic actuator to return piston 2 to its rest position. Once ferromagnetic liner 10 falls within the fields of the magnets, the field strength also aids retraction of piston 2.

A stunning device according to PCT/AU01/00745 incorporating the piston and cylinder arrangement of Figures 1 and 2 is shown in Figure 3. With reference to Figure 3, there is shown device 11 comprising piston assembly 12 and support assembly 13.

The support assembly is described in detail in PCT/AU01/00745. In Figure 3, only base member 14 is visible. In Figure 3, side members been omitted so that other features of the device can be seen. Base member 44 has fixed thereto a chin plate 15 which aids proper positioning of the head of a fish 16 in the device.

Turning to piston assembly 12, this is pivotally mounted to an axle 17 extending between the side members of the support to which rails at each side of the assembly are connected. One of these rails is item 18 of Figure 3 which is the rail furthest the viewer. The rail at the other side of the piston assembly has been omitted to again allow features of the assembly to be seen. Pivoting of piston assembly 12 is limited in movement away from base member 14 by an adjusting screw 19 in cross-piece 20 which contacts a tab 21 extending from the assembly. Item 21 comprises the nosepiece of the device illustrated in Figures 1 and 2. The minimum distance between nose piece 21 and chin plate 15 is set by an adjusting screw 22 on the piston assembly which contacts a tab 23 projecting inwardly from a side member. Adjusting screw 22 and tab 23 are shown in phantom in Figure 3. It will be appreciated from

Figure 3 that with adjusting screws 19 and 22 set as shown, some pivoting of piston assembly 12 is still possible. That is, the piston assembly can pivot until tab 24 contacts the end of adjusting screw 19. This allows adjustment of the distance between nosepiece 21 and chin plate 15 to compensate for differently sized fish.

Item 25 is a standard pneumatic gun, the particular gun in the example being a <BR> <BR> Max Model No. NF510 manufactured by Max Co. , Ltd, Tokyo, Japan. The bores of the gun cylinder and the nosepiece are indicated by the dashed lines in Figure 3. The nosepiece has mounted at its open end a head plate 26, with an appropriate aperture therein to allow passage of the nosepiece piston, to further aid positioning of a fish in the device.

Device 11 includes a trigger comprises a contact plate 27 adjustably linked to a pivot frame 28 which has a pushrod 29 extending therefrom for contacting the activating button 30 of pneumatic gun 25. Pushrod 29 is also acted on by a pneumatic pushrod 31 that disengages pushrod 29 from button 30 during operation of the device.

The position of contact plate 27 relative to the head 16 of a fish positioned in the device can be set by adjusting the angle between the contact plate and pivot frame 28 via adjusting screw 32. Pivot frame 28 is biased to a pre-firing position as shown in Figure 3 by spring 33 with the position adjustable via a stop screw 34. The movement of pushrod 29 is limited by a stop screw 35 and the pushrod is biased to the button 30 contact position shown in Figure 3 via spring 36. The piston assembly also includes a five-way valve 37.

Device components such as the support assembly 3 are typically fabricated from a corrosion resistant metals material. A preferred material is stainless steel. The support assembly of the exemplified device has a length of about 370 mm, a width of about 185 mm, and a height of about 205 mm. The pneumatic nail gun component 25 of the piston assembly utilizes a normal air supply of 3.0 to 5.0 bar and typically operates at bar. The device is suitable for stunning fish such as salmon of 2 to 7 kg.

Operation of device 11 is as described in for the devices described in that document.

It will be appreciated that many changes can be made to the retention mechanism exemplified above and it can be utilised in many other situations and devices without departing from the broad ambit and scope of the invention.

The term"comprise"and variants of the term such as"comprises"or "comprising"are used herein to denote the inclusion of a stated integer or stated integers but not to exclude any other integer or any other integers, unless in the context or usage an exclusive interpretation of the term is required.

The reference to the publication cited above is not an admission that the disclosure constitutes common general knowledge in Australia.