Rawl-Drive Climbing Anchor
Rawl-Drive Climbing Anchor. In recent years, some of the climbers in the Northwest have begun carrying and using rock anchors as standard equipment in rock climbing. Usually the anchors have been used either for safety or for direct aid in situations where natural aids or piton cracks are not available, but recently the use of anchors has come to be regarded as practical even in some places where pitons could have been employed. This procedure can be adopted because the anchor is superior in strength and reliability to most other aids.
At present there are many types of rock anchors available, and several which have been or are being used in rock climbing.1 A survey of several types, considering cost, weight, strength and foolproofness, led to the decision that the Rawl-drive anchors, manufactured by the Rawl Co., were very practical for rock-climbing purposes in most types of rock. Like most of the commercial anchors, however, Rawl-drives require a hanger before they can be used to hold the karabiner or rope slings. Therefore, the two features that one must consider in preparing anchor equipment are, first, the diameter, length and type of Rawl-drive and, secondly, the construction of the hanger.
Rawl-drives used in rock climbing would probably fail (1) by shearing or tearing in two by forces normal to the shaft or (2) by forces pulling the Rawl-drive out of the hole in the rock. In this latter case, the strength of the rock would probably be the controlling factor and would determine the length of the anchor required. Usually, however, the large forces produced by a fall would be approximately parallel to the rock or normal to the shaft—which would determine the diameter of the anchor necessary.
Although most of the work on Rawl-drives has been qualitative, Porter Varney reported that a ¼-in. Rawl-drive tested for Charles Wilts failed at 3520.2 Other tests by Mr. Varney, using 5/16-in. anchors, gave no indication of weakness in forces applied either normal to the shaft or at an angle of 30° from normal up to 3350. These tests, along with the many qualitative tests made in the Northwest, seem to indicate that the use of ¼-in. Rawl-drives is satisfactory for direct aid and rappelling and on the borderline for safety purposes, and that the 5/16-in size is satisfactory for all general uses.3 Quantitative information is lacking, however, for the specific applications. The chance of material flaws and metal upsets due to pounding limits the absolute reliability of any result.
Recently, the use of stud-head Rawl-drives has been adapted to some extent. These have two distinct advantages over the roundhead type. First, the stud-heads allow removal of the hanger, making the hanger cost and weight less important. Second, they allow the hanger eye to fit snugly over the shaft of the Rawl-drive, thus giving an even distribution of the applied forces over the surface of the shaft.
In designing the hanger, one must consider cost, weight, strength and transmission of the applied force to the anchor. Usually the optimum design is obtained by some balance of these features along with the type of Rawl-drive employed. A good illustration is the angle hanger vs. the eye or ring hangers. The cost of the angle hanger is low, but its tendency to lever the anchor out when the force is supplied is very undesirable. The cost of eye or ring hangers is higher; but the transmission of a force parallel to the rock would be very close to normal to the shaft of the Rawl-drive. Probably the angle hanger would be satisfactory with the roundhead Rawl-drive, and the eye and ring hangers better with the stud-head anchor, since removal would be possible.
Although these hangers have been constructed, no strength information is known, so actual dimensions will not be reported except to say that 1/8-in. stock was used in most cases. Some 1020 mild steel was used in preliminary work, but the most recent constructions have been made with stainless steel.4
1A. Nelson, Sierra Club Bulletin, XXXIII (March 1948), 103; R. Widrig, Mountaineer, XXXIX (Dec. 1948), 55; “Contraction Bolts,” A. A. J., VII (Jan. 1949), 230; C. Wilts, Sierra Club Bulletin, XXXIV (June 1949), 123.
2Letter from Mr. Porter Varney, Equipment Testing Association, P.O. Box 1507, Santa Barbara, Calif.
3R. M. Leonard and A. Wexler, Sierra Club Bulletin, XXXI (Dec. 1946), 68-100; A. Wexler, A. A. /., VII (1950), 379-405.
4Nelson, op cit.