Pressure vessels and tanks in general are provided with lids which typically are circular and are pivotally attached to akneck ring which is attached to the vessel. Swing bolt assemblies secured to the neck ring at pivot brackets engage with lugs secured to the lid so as to clamp the lid in sealing engagement with the neck ring.

Such swing bolt assemblies typically comprise a bolt which is pivotally mounted about a horizontal axis which is also tangential to the exterior of the pressure vessel or tank. A hand nut is threadedly mounted on the swing bolt which may be pivoted upwardly into the slot of a lug bracket attached to the lid.

By screwing the hand nut towards the pivotal axis of the swing bolt the hand nut can be made to bear upon the slotted lug bracket, thereby securing the lid of the vessel to the vessel itself. Usually, several swing bolt assemblies are used to provide a secure attachment.

A typical known arrangement is shown in GB 2098302.

The standard swing bolt assembly is formed from a stainless steel swing bolt and a stainless steel slotted lug bracket, whereas the hand nut is made of Naval brass. As the hand nut is tightened against the stainless steel bracket, considerable wear takes place on the seat of the nut. Very often, the seat of the nut is part-spherical and is adapted to engage with part-spherical recesses in the lug bracket, in order to produce a secure fixing. The wearing of the seat of the brass hand nut has a detrimental effect on the engagement of the nut and the lug bracket, thereby reducing the effectiveness of the fixing.

In GB-A-2225825, the hand nut is provided with a captive washer which is rotatably mounted on the hand nut and which is made of a harder material than the

hand-nut material. The washer bears against the lugs in use, allowing the hand nut to rotate inside the washer with lull face contact on a complementarily-shaped internal seat.

However, in some circumstances, it is undesirable, or not permitted, to use a brass hand nut but instead to form the hand nut from stainless steel, either with or without a captive washer.

However, when the hand nut and the swing bolt are both made from stainless steel the contact between the screw threads can result in galling which can compromise the security of the swing bolt assembly and which normally necessitates replacement or repair of the swing bolt assembly or its components.

In accordance with a first aspect of the present invention, a securing nut of a first material comprises an internally threaded insert secured to the nut and formed from a second material which differs from the first material.

In one embodiment, the second material from which the insert is made is softer than the first material from which the nut is made.

In one embodiment, the insert is made from brass, for example, Naval brass.

The insert is preferably generally tubular and preferably fits into a generally tubular recess.

The walls of the recess preferably taper, either inwardly or outwardly, from each end.

The outer walls of the insert and the inner walls of the through recess in which the insert is received are preferably complementarily shaped.

Preferably, the insert and the securing nut comprise one or more interengaging recesses and projections to prevent rotation of the insert relative to the nut. For example, there may be a plurality of interengaging recesses and projections around the periphery of the insert and around the wall of the recess which receives the insert.

Preferably, the insert is hot forged into the recess and is preferably subsequently bored and tapped.

The nut assembly may further comprise a captive washer which is rotatably mounted with respect to the nut and the insert.

The nut and/or the insert on the one hand and the captive washer on the other hand are preferably complementarily-shaped.

Preferably, the captive washer is harder and/or more wear resistant than the material from which the insert is made.

In accordance with a second aspect of the present invention, a swing bolt assembly comprises a securing nut in accordance with the first aspect and a pivotally mounted swing bolt, the insert of the nut assembly being formed from a material different from that of the swing bolt.

Preferably, the material from which the swing bolt is made is harder and/or more wear resistant than the second material from which the insert is made.

Preferably, the swing bolt and the first material from which the nut is made comprise the same material.

By way of example only, a specific embodiment of the present invention will now be described, with reference to the accompanying drawings, in which: Fig. 1 is a plan view of an embodiment of hand nut in accordance with the present invention; Fig. 2 is a side elevation of the hand nut of Fig. 1; Fig. 3 is a perspective view of the hand nut of Fig. 1, with the insert removed; Fig. 4 is a plan view of the hand nut as shown in Fig. 3; Fig. 5 is a cross-sectional view of the hand nut as shown in Fig. 3, looking in the direction of the arrows V-V; Fig. 6 is a cross-sectional view of the hand nut of Fig. 1, with the captive washer removed, looking in the direction of the arrows VI-VI; and

Fig. 7 is a view of the portion marked A in Fig. 6, to a larger scale.

Referring to the Figures, a hand nut assembly 10 for a swing bolt assembly comprises a hand nut 12 having a tubular central portion 14 and two identical, dog-legged arms 16,18 extending upwardly from diametrically opposite sides of the tubular central portion 14. The internal wall of the tubular central portion 14 is not cylindrical but instead is formed into a narrowed waist 20 from two frusto- conical wall portions 22,24 each of which tapers from a respectve one end of the tubular central portion to the narrowed waist. The internal wall of the tubular central portion is also provided with six elongate recesses 26, equally angularly spaced around the tubular central portion and whose base walls 27 extend parallel to the longitudinal axis of the tubular central portion 14.

The tubular central portion 14 receives a complementarily-shaped tubular insert 28 made from a material which is softer and/or less wear resistant than the stainless steel of the hand nut 12, in this case Naval brass. The tubular insert 28 is hot-forged into the stainless steel tubular central portion 14 of the hand nut 10 and is subsequently drilled and tapped to produce an internal screw thread 30 which is engageable with the screw thread of a swing bolt 40 (shown in dotted lines in the Figures) forming part of a swing bolt assembly. It will also be noted that the lowermost portion of the central tubular portion 14 is provided with a peripheral recess which is to receive the intumed peripheral edge of a captive stainless steel annular cup or washer 32 which is thereby free to rotate with respect to the central tubular portion 14. It will also be seen from Figs. 4 and 5 that the insert 28 projects downardly beyond the lowermost end of the central tubular portion 14 and that the projecting end 34 of the insert and the adjacent periphery of the central tubular portion 36 are shaped to form a part-spherical surface 38 which is shaped complementarily to the inner face of the captive washer.

Thus, when the hand nut is screwed onto a swing bolt 40 of the swing bolt assembly (shown in dotted lines in the Figures), the brass internal screw threads

30 of the insert 28 engage with the stainless steel thread of the swing bolt, which greatly reduces the occurrence of galling since the insert is made of material which differs from that of the swing bolt. In addition, as the hand nut is tightened into frictional contact with the lugs of a lug bracket, the primary frictional contact is between the projecting end 34 of the insert and the inner face of the captive washer, which again reduces the likelihood of galling. However, this allows the swing bolt 40 and the hand nut 12 to be formed from the same material.

The invention is not restricted to the details of the foregoing embodiment.

For example, although the tubular insert 28 is described as being made from Naval brass, other materials could be used instead, both softer and harder than stainless steel. For example, it would be possible to form the insert from plastics material.

Moreover, although the internal wall of the tubular portion 14 is shown and described as having a narrowed waist portion, the diameter of the wall may instead increase from each end, i. e. the frusto-conical wall portions 22,24 may instead taper towards the respective ends of the tubular central portion.