US 3226934 A
Description (OCR text may contain errors)
C- L. EMERY ROCK BOLTS Jan. 4, 1966 2 Sheets-Sheet 1 Filed May 16. 1961 A Homeyg Jan. 4, 1966 c. 1.. EMERY 3,
RQCK BOLTS Filed May 16. 1961 2 Sheets-Sheet 2 Attorney United States Patent Office 3,226,934 Patented Jan. 4, 1966 3,226,934 ROCK BOLTS Charles L. Emery, Kingston, Ontario, Canada, assignor to C-I-M Consultants Limited, Kingston, Ontario, Canada Filed May 16, 1961, Ser. No. 110,574 2 Claims. (CI. 61-45) This invention relates to rock bolts of the type comprising an expandible anchor adapted to fit the bore of a hole drilled in rock, a stem extending from the anchor, 21 plate through which the stem is passed and adapted to bridge the mouth of the hole, and means both to engage the stem and to bear on the outside of the plate to hold the plate to the rock under tension applied to the stem.
The stem of one form of rock bolt is a length of round steel rod screw-threaded at both ends, one end to screw into an anchor and the other to receive a tensioning nut. One variant of this has a bolt as its stem, with its inner end screwing into an anchor.
These, and other variants utilizing screw-threaded stems, require the application of torque to put the stem under tension. The simultaneous subjection of the screw-threaded stem to torque and tension calls for a minimum crosssection greater than is required by the final desired tensioned stress. Even then, thereis risk of failure during tensioning, because of this combination of stresses. Moreover, particularly because of uncertainty as to the tightening efficiency of screw threads because of surface condition and as to the character of lubrication (if any), and of the. error encountered in practice in theme of torquewrenches, there is risk of serious deformation of the parts of the rock bolt, and of substantially reducing the yield point of the stem by the application of excessive torque. This risk is increased if nut-runners or like machines are used to apply the tensioning torque, when the chance of error is greater because the operator cannot feel what he is doing, notably in the overhead or other position of poor accessibility in which a rock bolt is so frequently applied.
According to the present invention, a rock bolt comprises a round stem, an expandible anchor to be engaged by the inner end of the stem, and a wedging unit to grip the surface of the stem at the outer end of the latter and to bear on a bridging plate for the mouth of the hole in which tr e bolt is to be applied.
With the anchor pushed by the stem to its location in a hole and a bridging plate placed over the protruding outer end of the stem, the wedging unit is applied to that end, and the stem itself may then be gripped and put under simple tension, as by means of a hydraulic jack that bears on the bridging plate and has a gripping device for the stem connected to its ram. When the wedging unit is pushed along the protruding part of the tensioned stem until it bears on the bridging plate, the stem is held under tension on disconnection of the tension-applying means.
The wedging unit may consist of a shell with a conical bore to receive wedges that fit the bore externally and the stem internally, with serrations to bite frictionally on the stem, the unit being applied to the stem so that the tension in the stem urges the wedges towards the smaller end of the bore. The shell may be integral with a bridging plate, or may be a separate member to bear on a bridging plate inserted between itself and the mouth of the hole. The serrations on the wedges should fit the bolt stem closely, so that the wedges initially grip the stem, and are drawn tighter to the stem on the slight retraction of the stem that takes place upon disconnection of the stem from the tension-applying means. I
The anchor preferably includes a wedging unit, operable by the simple tension in the stem, to grip the inner end of the stem. In this case, the anchor conveniently consists of a shell to grip the Wall of the hole, split lengthwise into two or more parts, to be expanded by wedging action exerted by the tensioning of the stem. Anchor shell parts separate from each other may be spring-urged apart to bring their external surfaces, suitably serrated, into contact with the wall of the hole.
Thus, with the internal surfaces of the anchor shell parts conical, to fit a wedging cone at the inner end of the stem, the tension in the stern draws the cone along the parts to expand them into tight gripping contact with the wall. The internal surfaces may have lengthwise grooves to permit each shell part to be expansively distorted as the larger end of the wedging cone is drawn into contact with progressively smaller diameters of the conical internal surfaces. As a result, the external surfaces of the parts, which necessarily have an initial diameter less than the hole to permit them to enter it, are forced tightly to the wall over a substantial part of their area.
The small diameter possible with high-tensile material used under torque-less conditions enables a correspondingly small diameter anchor to be used, with correspondingly quicker and cheaper drilling of smaller holes in the rock.
Because the bolt stem is stressed by simple tension, and is merely held by the contact between the wedge unit or units and its cylindrical surface, full use is made of its tensile strength. Moreover, high ten-sile material may be used for the main length of the stem, notably rope wire, with a tensile strength of the order of 300,000 p.s.i. This may be achieved by a single strand of 7 mm. dia. rope wire, but a stranded wire rope of like small diameter may also be used.
This invention will now be further described with reference to the accompanying drawings, in which FIGURE 1 is a longitudinal section showing a rock bolt in position in a hole drilled in rock, with a tensioning jack applied to it;
FIGURE 2 is an underneath plate of a combined bridging plate and wedge shell used in FIGURE 1;
FIGURE 3 is a plan of the bridging plate, showing its gripping faces;
FIGURE 4 corresponds to the bottom of FIGURE 1, but shows a bridging plate separate from the wedge shell;
FIGURE 5 is a side elevation of one of a pair of anchor shell parts used in FIGURE 1;
FIGURE 6 is an elevation of the shell part of FIGURE 5 as viewed on the line VIVI;
FIGURE 7 is a plan of FIGURE 6;
FIGURES 8 to' 11 are sections taken at the lines A-A, B-B, CC, and DD of FIGURE 1, showing progressive application of the two anchor shell parts to the gripping of the hole in the rock; and
FIGURE 12 shows a rock bolt stem consisting essentially of stranded Wire cable.
In FIGURE 1, a hole l is drilled from the rock face 2, to any depth necessary by the nature of the rock, Into the hole is introduced the stem 3 of a rock-bolt, the stem being of a high-tensile rope wire. Butt-welded coaxially at 4 to the inner end of the stem 3 is a terminal wedging cone5. Alternatively, the cone may be formed integrally with the stem by an upsetting operation. Two anchor wedge parts 6 (one only shown in FIGURE 1), details of which are given in FIGURES 5 to 7, surround the cone. The stem 3 is of such a length that, when the anchor reaches the bottom of the hole, a length 8 of the stem protrudes beyond the rock face 2.
A bridging plate 9 with a central conical hole 10 is placed over the end 8, with the smaller end of the hole 10 directed towards the hole 1. The plate 9 has three 3 arms 11, stiffened by ribs 12, with roughened pads 13 to grip the rock face 2. Into the conical hole are fitted two wedge parts 14, with serrated inner surfaces conforming to the diameter of the protruding end 8 of the stem 3, to grip the stem frictionally.
The anchor wedge parts 6 are gripped to the wall of the hole 1 simply on application of tension to the stem 3 by gripping the protruding end 8 between Wedges 16 of a clamping device 17 of a hydraulic jack shown diagrammatically at 18. The upper portion 19 of the jack 18 is simply applied against the lower end of a boss of the bridging plate 9 containing the hole 10 for the wedges 14. The jack 13 is supplied from a pump (not shown) at any convenient location, connected by a flexible pipe 21. The jack may be held in position by hand, when its operation causes the wedges 16 to grip the protruding end 8 to draw the stem 3 and its terminal cone 5 somewhat along the hole 1, for the cone to force the wedges 6 apart into gripping contact with the hole.
During pulling by the jack 18, the wedges 14 slightly relax their hold on the end 8, but as soon as the jack starts a return stroke the tension now existing in the stem 3 causes the end 8 to draw the wedges 14- into self-tightening engagement with the end 3. Further tension may then be applied by the jack 1%, in as many stages as are necessary to bring the stem 3 up to working load, the wedges 14 holding the load at the end of each stage, including the final stage, the load being transmitted to the rock face through the bridging plate 9. The stem 3 is thus put under full tension without being subjected to torque.
In FIGURE 4, a bridging plate 9A is separate from a shell or boss 20A containing the wedges 14.
The wedge parts 6 of the anchor must necessarily occupy a slightly less overall cross-section than the hole 1, to enable them to he slid into the hole. Their lower ends are urged to the wall of the hole by a circular spring 21 (FIGURE 1) that fits into internal grooves 22 (see also FIGURES 5 and 6). Expansion by lengthwise movement of the cone 5 brings the whole length of each part 6 into contact with the hole. Each part 6 has a lengthwise internal V-groove 23 (FIGURES 5 and 7) to enable the part 6 to distorted into particularly effective contact, as will now be explained with reference to FIGURES 8 to 11.
Since the cone 5 is movable along the wedge parts 6, it follows that at only one relative position can the cone fit exactly the conical internal surfaces of the parts. It is arranged that this precise fit take place when the cone 5 has moved somewhat along the parts 6, this being the position shown in FIGURE 1. Initially, the larger end 26 of the cone 5 was at the level AA of FIGURE 1, against retaining cars on the parts 6. As FIGURE 8 shows, there was then clearance 27 between it and the adjoining parts 28A of the internal walls, and clearance 29 between the outside of the parts 6 and the hole 1.
When the cone 5 has moved to the level B-B (see shown-by FIGURE 9), the larger end 26 finds itself at parts 28B of the internal walls that match its diameter, and so on over the whole length of the cone towards its smaller end. The parts 6 are now in contact with the wall of the hole 1 over their whole length. The clearance 29 has disappeared at opposite position 30, but still exists where the gaps 31 between the parts 6 have merely become wider.
Further movement of the larger end 26 to level CC produces the situation shown by FIGURE 10. The end 26 finds itself at parts 280 of the internal walls that are smaller than its diameter, so that contact is transferred to points 32. The increasing wedging action of the cone 5 has now compressed the parts 6 tightly into the sides of the hole 1, as indicated at 33, and the widening of the gaps 31 has lessened the length of the clearance 29.
Now, with both parts 6 making very substantial area contact with the hole 1, further (and final) movement of the larger end 26 towards level DD, the cone pressure concentrated at points 32 in FIGURE 10 causes each part 6 to bend lengthwise, starting about the apex 34 of its V-groove 23. As shown by FIGURES 5 and 6, each V-groove becomes wider and deeper towards the smaller end of the conical surfaces, so that the bending tendency increases as level DD is approached. The result is shown in FIGURE 11. The larger end 26 now bears on four arcs 28D, which conform reasonably closely to its curvature, and the distributed pressure of the cone 5 on each part 6 forces the whole exterior of each part into biting contact with the wall of the hole 1, to eliminate the clearance 29 at each side of the gaps 31. The actual amount of penetration or crushing of the rock by the serrations 35 of course depends upon the nature of the rock; the amount as shown in FIGURES 10 and 11 would be an exaggeration in the case of hard rock.
It will be seen, therefore, that exceedingly firm anchoring is effected by the combined action of the cone 5 and the wedge parts 6 as a result of pure tension applied to the stem 3. Since that stem is not subjected to torque, its diameter can be reached to the minimum required to withstand the high tensioned load, and this reflects directly on the diameter of the wedge parts 6 and of the hole 1 to receive them. Consequently, very high holding power is applied at the rock face 2 with rock-bolt equipment that calls for a small-diameter hole, capable of being very quickly drilled.
In FIGURE 12, a stem 3A is shown as consisting of stranded wire cable. A wedging cone 5A is butt-welded to it at 4A and at 8X there is welded to it a solid wire strand 8A for engagement by the holding wedges 14 and also by the tensioning wedges 16 of the jack 18 (FIGURE 1).
What I claim is:
1. A rock bolt comprising a round stem to be put in tension over a length extending inwardly from the mouth of a hole to an anchoring location within the hole, in combination with an anchoring unit connected to said inwardly extending length of the stern, a bearing unit consisting of a bridging member to span the mouth of the hole and formed with a conical aperture converging toward said anchoring unit and receiving said stem, together with a plurality of wedges operatively disposed about the stern within said conical aperture, said wedges being formed externally to fit the conical aperture and formed internally with gripping surfaces to embrace the round stern, said bridging member having gripping surfaces for frictionally engaging the face of the rock surrounding said mouth of said hole to position said inwardly extending length of said stem within sai dhole in response to said tension, said inwardly extending length of the stem being provided with the said anchoring unit consisting of an inner member formed by an aligned conical extension of the stem, said extension diverging from a narrower end integrally connected to the stem to a wider terminal end so as to provide a conical surface, and a plurality of outer hole-gripping members in the form of longitudinal shells together substantially surrounding the conical extension, said outer gripping members being formed externally with serrations for gripping the wall of the hole and internally with conical surfaces shaped to engage and fit the conical surface of the extension when said extension is displaced lengthwise by part of its length relatively to the shells in the direction of said narrower end, whereby said tension in said stem along the length between said mouth of said hole and said anchoring location is maintained by the cooperation of said wedges with said stem.
2. A rock bolt as claimed in claim 1, wherein each of said conical surfaces of said outer shells is divided by a longitudinal groove internally of said shells, said grooves increasing in size toward the end of the shell engaged by said narrower end of said conical extension.
FOREIGN PATENTS 642,589 9/1950 Great Britain. References Cited by the Examiner g fi g fiigg UNITED STATES PATENTS 5 OTHER REFERENCES Brady 852'4 German printed application, Forsell, Serial NO. F. Bell 7444 v/37b Se tember 13 1956 Palmer 61--45 I P Thomas et al. 61-45 Brandt 50*128 10 CHARLES E. OCONNELL, Przmary Examzner.
peter 61 45 WILLIAM I. MUSHAKE, HENRY C. SUTHERLAND, Dempsey RICHARD W. COOKE, JR., Examiners.
Olivier 50-129 C. G. MCBRIDE, I. L. RIDGILL, Assistant Examiners.