This invention is concerned with the supporting of mine roofs, a practice commonly called rock bolting.

The practice is well known and is one which utilises a rod which is anchored at one end by a chemical anchor in a hole drilled deep into the mine roof and is threaded at the exposed other end. A retaining plate is passed over the threaded rod end and the plate is clamped against the mine roof by a nut on the threaded end of the rod. The purpose of rock bolt is to support the roof of a mine thereby minimising the chance of rock falls and cave-ins.

The chemical anchor is conventionally provided by inserting a cartridge containing two reactive materials in separate cartridge compartments into the hole in the mine roof and then rupturing the cartridge compartments and mixing the two reactive materials by rotating the rod in the hole. The rod nut is used in many installation practices as the means for transferring torque from a power source to the rod to cause it to rotate.

Over the years several connections between the rod and the nut have been devised in which the drive connection between the nut and the rod is released when a predetermined resistance to the torque applied to the rod is developed in the chemical anchor. One example is the use of a plug which is pushed aside when the predetermined resistance develops, see for example

Australian patents numbered 539083 and 538338.

This invention provides an economic and simple alternative to the forgoing torque resisting plug arrangement. One important feature of the present invention is that when the predetermined resistance is achieved the nut will not be suddenly released, as is the case when the plug of the "plug type" arrangements yields to the applied forces.

In very general terms, the present invention involves the concept of providing a rod which has the cross-sectional shape of a zone of a threaded portion deformed sufficiently to impede the free passage of the nut along the threads in the deformed zone. When torque is applied to the nut on the threaded portion of the rod the nut will pass freely over the rod threads until it encounters the deformed zone. The binding action results in the torque applied to the nut being transferred to the rod.

When the resistance to rotation of the rod exerted by a chemical anchor exceeds the force needed to overcome the binding action of the nut on the threads in the deformed zone the nut will be forced to pass over the threads in the deformed zone. After the nut has passed over the threads in the deformed zone there will be a normal free threading relationship between the nut and the threads on the rod until the nut encounters a roof plate against which it is tightened by means applying a desired torque to the nut.

The present invention can be said broadly defined

as a rock bolt including an elongated metal body, an external thread extending over a portion of the body from one end of the body to a termination point spaced from said one end, said threaded portion including an interference zone spaced from said termination point, the external body thread except for that in said interference zone being for free threaded passage of a nut and the external body thread in the interference zone being for forced threaded passage of a nut. The preferred method of manufacturing a rock bolt as set forth above includes the steps of providing the body with the whole of the threaded portion for free threaded passage of a nut and the subsequent step of applying pressure to opposite surfaces of the threaded body portion to locally deform the body threaded portion to provide the interference zone.

Several presently preferred arrangements and embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig.l is a perspective view of a representative threaded rod of the type currently used as a rock bolt, Fig.2 is a schematic exaggerated front view to show the deformation of the rod threaded portion to provide an interference zone,

Fig.3 is a view similar to that of Fig.2 with the bolt rotated 90 degrees to show a side view of the deformed portion of the bolt, Fig.4 is an enlarged side view of part of a threaded

portion of a rod as shown in Fig.l after working in a first manner to provide an interference zone in the threaded portion,

Fig.5 is a schematic view showing tools for working the rod threaded portion to achieve the deformation shown in Figs.2 and 3

Fig.6 is a schematic view showing other tools for working the rod threaded portion to achieve the deformation shown in Figs.2 and 3 and Fig.7 is a sectional end view on the section line 7-7 of Fig.2 to further illustrate the slightly oval cross-sectional shape of the rod in the deformed zone.

In Fig.l there is a rod 1 having a body 3 provided with a threaded portion 2. The rod of Fig.l is worked, preferably cold worked, to provide an interference zone indicated 12 in the threaded portion 2 and the interference zone will have the characteristics of cross-sectional shape shown (in very exaggerated form) in Figs.2 and 3. The Figs.2 and 3 represent elevations of the rod in positions 90 degrees rotational spaced apart.

Fig.4 shows one method of achieving the Figs.2 and 3 result and involves the provision of depressed portions 4 in diametrically opposed positions. The depressions 4 are shown as being rectangular in shape but this is optional and other shapes are possible, for example diamond or oval or round. With the different shapes slightly different (but generally the same) cross-sectional shape for the bolt in the deformed zone

can be obtained.

Figs.5 and 6 show two forms of tool for achieving the Figs.2 and 3 result without forming a depression. The workheads of the tool of Fig.5 identified 5 and 6 are provided with concave working surfaces 7 and those surfaces are provided with surface contours (not shown) which are a match for the surface contours of the thread of the portion 2 of the rod 1.

The workheads 8 and 9 of the tool of Fig.6 have convex working surfaces 10 provided with surface contours which are a match for the surface contours of the thread of the portion 2 of the rod 1.

Rod deformation is achieved by placing a zone of the rod threaded portion 2 to be deformed between two workheads which can be in the form of punches (the Fig.4 and Fig.6 methods) or concave tool faces (the Fig.5 method) and then applying pressure. The pressure can be applied as a blow or as a squeeze.

The pressure applied is of a magnitude sufficient to deform the zone 12 of the threaded portion of the rod to the general shape shown in Figs.2 and 3. In reality the end result is the provision of a deformed zone 12 which is very slightly oval.

The Fig.7 view illustrates the outline of the cross-sectional shape of the deformed zone 12 in full lines 11 and the outline of the cross-sectional shape of the undeformed threaded rod portion in broken lines 12. The ovality is again illustrated in exaggerated

form for the purpose of illustration. In practice the ovality would be difficult to identify with the eye and is only such as to impede but not prevent the passage of the nut through the deformed thread zone.

In a representative example of a rod according to the invention, the rod would have a nominal outside diameter for the threaded portion of 16mm (which would slightly greater than the minor dimension of the oval) and the major dimension of the oval would slightly greater than the nominal diameter. The torque which can be applied to the rod through the nut engaging with the threads in the deformed zone is a function of the deformation and the resultant impedance to the passage of a nut along the thread.

The impedance can be readily varied by varying the major dimension of the oval. It has been found through experiment that the major dimension of the ovality can be closely controlled. As a result quite small and readily predicted variations in maximum torque which can be delivered through the nut to rod engagement can be achieved.

The foregoing is a description of preferred embodiments of the invention and it is to be understood that changes can be made to details given without departing from the inventive concept disclosed and hereinafter claimed.