US-B-6311950 describes a fluid metering device in which a sealing element is provided in the form of metal bellows. The sealing element is provided between a valve needle, moving axially through a chamber for delivering a metered dose of a pressurised fluid from the chamber, and a housing. In this document, weld joints are proposed to join the metal bellows to the valve needle and the housing. However, a problem of welding the bellows to the valve needle is that the valve needle is hardened and tends to crack in the fusion zone when the welding is carried out, affecting the long term stability of the device.

In accordance with a first aspect of the present invention, a metering device for a pressurised fluid comprises a housing, a valve needle and a bushing element; wherein the bushing element is crimped to the valve needle.

The benefit of crimping is that it is not done at elevated temperatures, so it does not cause the needle to crack, as occurs with welding, for example laser welding as proposed by the prior art, but it does have the necessary mechanical strength and sealing effect to maintain a bond between the bushing and the needle even when exposed to vibration.

Preferably, the metering device further comprises an adhesive sealant layer.

This additional layer provides additional sealing of the joint and is able to withstand the typical temperatures of operation of the valve. Although, adhesive alone would not have the necessary mechanical strength, the combination of a crimp bond and adhesive sealant allows the beneficial features of a hermetic sealing element without causing damage to the needle when bonding the sealing element to the needle, as occurs in the prior art device. The resulting device has long term stability and the joint is resistant to vibration and elevated temperature.

Preferably, the adhesives are able to withstand temperatures up to 250°C.

Preferably, the adhesives comprise silicone, polyimide or ceramic based adhesives.

In accordance with a second aspect of the present invention, a method of forming a metering device for a pressurised fluid comprises providing a housing, a

valve needle and a bushing element; and crimping the bushing element to the valve needle.

Preferably, the method further comprises applying an adhesive sealant to the crimped bushing element.

Preferably, the bushing element comprises metal bellows.

Preferably, the method further comprises welding the housing to the metal bellows.

An example of a metering device and a method of forming such a device in accordance with the present invention will now be described with reference to the accompanying drawing in which: Figure 1 illustrates an example of an armature assembly of a metering device according to the present invention.

For high pressure direct injection (HPDI) injectors a bellows has to be fixed to a needle. Conventionally, this is done by welding the bellows to a ring and subsequently welding the ring to the needle. As the needle is made out of hardened material, cracking of the needle occurs when it is welded.

Fig. 1 shows an example of an HPDI armature assembly for a metering device according to the present invention. The armature assembly comprises a housing 1 surrounding a valve needle 2. A bushing element, comprising a set of metal bellows 3 having a carrier ring 4 at one end, is provided between the needle 2 and the housing 1 to guide the valve needle as it moves through a chamber 5 filled with pressurised fluid, without allowing the fluid pressure to influence the movement of the needle. The carrier ring 4 is joined to the needle and the opposite end 6 of the bellows is joined to the housing 1. This join may be an adhesive join or a weld.

It is necessary to provide a hermetic seal between the needle and the bellows to prevent fluid leakage. As discussed above, the conventional technique for joining the carrier ring and the needle is to weld the ring to the needle, but typically, the material of the needle is hardened and so it tends to crack in the fusion zone when subjected to the elevated temperatures required for welding. Another, problem with joints in such a device is that the joint is exposed to vibration and elevated temperature (up to 250 C) during operation. At this elevated temperature most adhesives lose their mechanical

strength, so it would not be suitable to use an adhesive bond to overcome the problems of fracturing caused by welding.

The needle 2 is provided with a groove 8 into which a crimp notch, formed when the carrier ring 4 is crimped, engages. The present invention solves the problems described by crimping the carrier ring 4 to the needle 2, thereby preventing cracking of the fusion zone and overcoming the lack of long term stability. Crimping the ring 4 to the needle 2, provides mechanical integrity. In addition, a layer of adhesive 7 may be applied to the join between the carrier ring and the needle to further improve the sealing effect of the join. Several classes of adhesives can withstand the required temperatures of operation without becoming brittle, which is a prerequisite for vibration resistance, for example silicones such as those used for moulding a piezo stack or polyimides, generally used for high temperature insulation of wires. Polyimides, in particular, have a very low tendency to outgas. Furthermore, the solid crimping joint is appropriate for ceramic adhesives, which are prone to be brittle, so this type of adhesive can be used, which would not otherwise be possible.

In the example given above, the bushing element is provided by a metal bellows, but the invention is equally applicable to any hermetic seal where a weld has to be replaced due to thermal constraints or the material type being unsuitable for welding, such as a membrane on a shaft. The present invention circumvents the thermal load problems suffered by the needle due to welding and also fulfils the requirements of fixing the bellows to the needle hermetically.