An Analysis of Active Anchors Used by Lead Climbers

Nicholas J. Huber

The sport of rock climbing developed from a sport where thrill seekers would see how close to death they could come while climbing a rock cliff, into a booming sport of the '90's that is pursued by millions [Adler, 1994]. Part of this conversion is due to recent advances in safety equipment and especially anchors. This report will analyze one of the most technologically advanced anchors, the active camming device. Active anchors will be evaluated in several categories including the design, material composition, and motion. In addition, this report will summarize findings on the strength and use of these anchors.

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Active Anchor Design

The revolutionary design of the active anchor by Ray Jardine in the late 1970's helped changed the definition of a climbable cliff. Before his design, climbers could not safely climb cliffs with very large cracks. The active anchor, with its cams that would expand under loading, let climbers use one piece of equipment in various ranges in crack sizes and once the anchor was placed, it was almost guaranteed not to fail. This section intends to describe the cams and the other components of an active anchor as well as how they work.

Anchoring Device Components

By definition, an active anchoring device has moving parts. Generally, these devices have spring loaded cams that are retractable by pulling back on a trigger. These cams rotate on an axle that is connected to the stem of the anchor. The stem may be manufactured from a rigid shaft or a flexible cable. A lead climber can attach a sling to the stem of the anchor and then attach a rope to that sling. Figure 1 depicts the components of an active camming device.

Cams. The cams of an active anchor are curved pieces of metal that rotate on an offset axis. This offset axis is the key aspect of design that allows the anchor to redirect a force. A downward force is redirected horizontally to the crack of the rock as the cams rotate on their offset center. (For a complete explanation of the force redirection see Active Anchor Motion.) This offset rotation is the same that occurs in a combustion engine. As an engine cam shaft rotates, the outer edges displace different lengths for different positions of the rotating shaft. The pistons are then connected to the outer edges of the cams and are displaced different heights during a cam shaft rotation.

Figure 1. The components of an camming device [Long, 1993a].

Active anchors are manufactured with several different styles of cams. Figure 2 shows the different camming devices and their variations in design. Note that the left anchor in the figure has four cams while the others have three. The variation in the numbers of cams have been incorporated into different models of anchors because of the wide range of placements in which they will be used. Four-cam devices generally work well in larger cracks and are more stable while three-cam anchors are better for placements in smaller cracks.

Springs. The springs provide the staying force for the anchor. These springs play an important role in the usability of the anchor. When placing an anchor, the climber pulls back on the trigger which retracts the cams. Placing the anchor in the crack, the climber releases the trigger and the springs force the cams into the surface of the rock. These springs become very important when a climber is transversing a cliff because the climber's rope moves back and forth. These movements of the rope can cause the anchor to "walk" which means that the anchor will either move outward and fail to support the climber or move inward and be impossible to remove. A stiff spring is one of the keys in keeping these anchors in place.

Figure 2. Some of the different types of active anchors available [Long, 1993a].

Stem. Since the invention of the first camming device, several manufactures have created different variations in the original design. One of the most significant changes has come in the area of the stem. The original camming device had a rigid aluminum shaft that worked well in vertical placements. Problems came when the device was place horizontally on a ledge and the edge of the frame extended over the ledge (see figure 3a). A large downward force on an anchor in this orientation could cause the frame to shear. New models now have a flexible stem made out of cable as shown in figure 3b. These models can withstand a placement that would have sheared rigid stems. Figure 2 shows a rigid stemmed model and several different flexible stemmed models that are available on the market today.

Figure 3. Part (a) illustrates the unsafe positioning of a rigid shafted camming device. A large downward force would shear the shaft apart. Part (b) illustrates a flexible shafted camming device, safely positioned in a similar crack [Long, 1993a].

Sling. The final component of an active anchor is the sling. The sling is important because it offsets the anchor from the rope. Just as the springs play a key role in the stability of the placement, so do the slings. These slings absorb some of the wiggling movements caused by the rope while the climber is moving on the cliff. Again, absorbing these movements are important in keeping the anchors from walking.

Active Anchor Motion

The camming device that is depicted in figure 1 is similar to the original design developed in the late 1970s. Due to the curvature of the cams and the offset placement of the axle, these devices have the ability to redirect a force. When a climber falls, the anchor experiences a downward force of up to 2600 lb-ft caused when the rope tightens. When this occurs, the stem of the anchor displaces downward. As the stem moves downward, the cams rotate on the offset axis and are forced into the surface of the rock. The camming device holds its position because of the frictional force between the edge of the cams and the rock. The redirection of the downward force into a horizontal force against the rock approximately doubles the downward force, thus making a very safe anchor. The cam rotation not only creates the holding force that supports a falling climber, but it also makes the anchor more versatile. A single middle-size camming device, a #3 Wild Country Friend for example, can expand to fit a crack 1.9 inches across to 2.4 inches across [Luebben, 1994]. In comparison, a climber would have to have 3-4 passive, hex type anchors to fit the same difference in crack sizes. Obviously, climbers prefer these anchors because they need to carry less hardware while climbing. Unfortunately, most climbers only carry a few camming devices for those places so a passive anchor just will not work because of the high cost of active anchors.

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Compositional Materials of Active Anchors

If you are a climber, the last thing that you want to have is added weight limiting your movements and slowing you down. On the other had, you want to be able to carry enough equipment to complete the climb and you want that equipment to be safe. These three requirements have made anchor manufacturers use some of the latest in material science advances to create strong but lightweight active anchors. In the following paragraphs, the compositional materials of the camming device components shown in Figure 1 will be examined.

The Axle and Cams

The axle of the camming device is 6061 cast aluminum. This alloy mixes small percentages of chromium, magnesium, silicon, and copper with aluminum. This alloy has a maximum strength of 45,000 psi and a strength to weight ratio of .35x106 in [Shackelford, 1992]. Some of the other common uses of this alloy are the support members of commercial airplanes and other lightweight structures [Van Horn, 1987].

Some cams on the market are made out of the same material as the axle, 6061 Al, but some manufactures use a more expensive 7075 Al alloy. The switch to the 7075 alloy is due the higher strength of 83,000 psi and a comparable strength to weight ratio to that of 6061 Al. The choice of switching to a stronger material might not be as cut and dry as one might think. First of all, the likelihood of breaking a cam of either alloy is pretty low and second, there are some instances were a softer alloy is better. An example of this is if someone is climbing on a granite cliff (a very hard stone), a softer cam might deform slightly upon loading, give a better bite, and therefore a better hold. The choice of what kind of camming device to buy and what material cams it has is one that must be determined by the climber and the type of rock that he or she will be climbing.

The Stem and Sling

The anchor's stem may be composed of two types of materials as well. Flexible stems are generally made out of steel cable and rigid stems are made out of 6061 Al. The choice of aluminum alloy is because of the characteristics that were discussed above but the choice of steel may seem as obvious. Although steel is heavier that aluminum, manufactures use this material because its superior strength of 110,000 psi [Shackleford, 1992]. Most flexible-stemmed camming devices are used in positions as seen in figure 2b, with a portion of the stem protruding off a ledge. As discussed before, this placement intensifies the stress on the point where the frame leaves the ledge but because of steels superior strength, those forces will not damage the camming device.

The other parts of the camming device are made up of materials with the same characteristics. The nylon sling, when professionally sewn together, is extremely strong and lightweight. The trigger is usually made up of a thin steel wire or a sliding 6061 Al bar. For the holding reasons previously discussed, the springs are make out of steel with a strong spring constant.

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Strength Testing Results

One of the foremost rock climbing magazines, Rock and Ice, independently tested the major types of active camming devices. The magazine tested the anchors in two different types of rock to compare the different materials and how those materials fair in the real world. The following section will overview the results of their testing that was published in the May-June 1994 issue of Rock and Ice.

The Testing Controls

The tests were performed in two different types of rock, soft Lyons Sandstone and a harder Dakota Sandstone. The testers used a hand operated winch with a crane scale to inflict and measure the force. This winch setup induced a static loading on the anchors being tested. This is not the type of force that one produce on an anchor during a fall but because of the duration that the force was induced on the device, the components would actually fail sooner and at a slightly lower force that what would occur under normal circumstances. Because of this, the author of the article claimed that the test results were more conservative that what would actually occur while climbing.

There are several limitations of this study. First and foremost, the number of camming devices tested was small. In this study there was only one or maybe two devices of a specific type and model tested. This is a very large limitation. Generally when testing, one has a large test group of the same models and repeats the test several times. Averaging the results of several specimens reduces the errors that could be incurred from variations in the compositional materials and variations caused by manufacturing. The environment caused the other limitation of this study. The anchors were placed as they would be when used, in the crack of a rock face. However, the same crack, and thus the exact same composition of rock, could not be used. Since different cracks were used, the findings may not be as accurate as one would see in a strict laboratory test.

These findings are important to look at even though they have several limitations. They are still useful because they show how a random anchor will perform in a random placement. If one were to purchase a camming device from a manufacturer, they would have just as good of chance as getting an anchor that will perform as the anchors tested did. In addition, if a trained climber places a camming device in a crack, they will have just as good of a chance to find a composition of rock that will perform as well as the tests. For these reasons, I feel that these results are useful and important.

The Testing Results

Rock and Ice published its findings in a tabular format that listed the force applied and the result on the anchor of that force. The results listed whether the failure was due to the rock or the anchor and if the anchor failed, what caused that failure. Figures 4 and 5 are a collection of the important findings from their results. These figures show a wide range of forces at which the active anchors failed. Some of the anchors withstood large force, over 4000 lbs while others failed at 2300 lbs. It is also important to note that although the camming devices did not fail, some of the units pulled out a very low forces. One of the camming devices actually pulled out at 115 lbs. Anchors should be rated to withstand a force of 2640 lbs which is to be the largest force that on can attain while falling. These results are important for climbers because they dramatically show how vulnerable their anchor my be depending on the type of rock that they are climbing in.

Figure 4. Results of strength testing of active camming devices in soft sandstone performed by Rock and Ice [Luebben, 1994].

Figure 5. Results of strength testing of active camming devices in soft sandstone performed by Rock and Ice [Luebben, 1994].

There are several significant other findings to notice in these results shown in Figure 4 and Figure 5. First of all, note that in the hard sandstone, with strength common to many of the climbable rock types in the United States, many of the camming devices failed at about 2800 lbs. They failed by axle deformation and breaking of the cables. Second, note that if one is climbing in a softer stone, most of the units began to track or pull out at very low forces, near 1600 lbs. Although these active anchors have been heavily researched with lots of time and money, climbers must remember that anchors are only metal and the environment of the anchor placements may be variable so they must make sure that they have more than one or two anchors to back them up if one fails.

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Conclusion

In the past several decades, the sport of rock climbing has developed from a few thousand fanatics who wanted to see how close to death they could come into one of the fastest growing sports of the 90's. Today, an estimated 1.6 million men and women trust their strength and teamwork on the cliffs and mountains of the world [Sussnan, 1990]. One of the major reasons for the explosion of interest, were advances in the sport's safety. New designs in anchors were one of the key aspects in making this sport safer. In this report I have reviewed the active anchors that lead climbers use to attach themselves to a cliff. In reviewing them, I looked at the anchors' design, compositional materials, and testing results.

When analyzing this class of anchors, I discovered that there are many different variations in design and composition. One of the major differences was that there were two types of stems: rigid and flexible. I found that each type had it advantages and drawbacks including less stability or greater usability. Another difference I found was in the composition of the cams. Some manufactures use 7075 Al alloy simply because it is stronger yet lightweight, while other use softer metals like 6061 Al alloy because the softer metal gives a better hold.

Overall, one can not claim that active anchors are better that any other anchor on the market. If every style of anchor holds when a climber falls, who can say that one is substantially better? It is true that active anchors may be more versatile because of their adjustability for different size cracks but a full rack of passive anchor may be just as useful. Active anchors may also be more technologically advanced than other anchors but if all the anchors work, is that technology really necessary? It is certainly true that there are certain circumstances where one anchor will out perform the others. For active anchors, their niche is in large cracks. Without the advancement in this class of anchors in the last two decades, some mountains would still be unclimbable and the sport would still be very dangerous.

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Appendix
Removable Passive Anchoring Devices

Before 1970, securing a climber to a cliff was a very labor intensive and environmentally damaging practice. It involved taking a steel piton, a large nail, and literally hamming it into the side of a cliff. Some types of these pitons would permanently stay in the cliff while the removable ones would leave large holes or "piton scars" in the rock upon their removal. In the early seventies, wide spread use of a removable passive anchor became common place. Through a little research and development, nuts became safer anchors than pitons and using them was a common practice world wide.

As the name implies, passive anchors are anchors that do not have any moving parts. This does not imply that the anchors must have a simple design or geometry. There has been a considerable amount of research in both the design aspects and the compositional material aspects of these devices which have made this class of anchors the most common anchor on the racks of the world's climbers.

Passive anchors are designed to fit into cracks that are narrowing. Due to gravity and the freezing and thawing of cliffs, most cracks in nature are larger at the top and taper downward toward the bottom. Figure A1 depicts a passive anchor in a narrowing crack. One can see that if a climber fell, a downward force would be induced to the bottom of the cable, thus forcing the metal wedge to lodge into the crack and support the falling climber. There are some instances where the force may not be directed downward. Because of this, climbers must be aware of what direction they will pull the anchor when they fall. Climbers generally take several minutes analyzing the situation and the cliff before placing an anchor.

In addition to the direction of the pull, climber must worry about the size of the crack to which he or she is going to attach. Through advances in metallurgy, manufactures, are able to create anchors from 8 inches across to 1/4 of an inch across. Obviously the smaller ones will not support as much, generally only up to 1000 lbs, but a good placed large piece will hold more than any climb related force could inflict. The smallest passive anchors use several different metals in both the wedge and the wire to create a safe anchor.

Figure A-1. A tapered passive anchor in a narrowing crack. The arrows depict the contact points. [Long, 1993b]

The wedge end of a passive anchor, commonly called a nut, is the key aspect of the anchor. The anchor's overall stability, strength, and safety is centered around this part. There are many different manufactures creating these anchors but there are usually only three materials they use. Aluminum alloys are the most commonly used material for their high strength and low weight. Most aluminum alloys perform very well under the forces caused by a falling climber. They usually do not deform and there is little chance of shearing. In fact, if anything was to fail in an anchor setup, it would probably be the rock in which the anchor was placed. For smaller anchors, less than one inch across, steel and brass are commonly used for the material of the nut. For anchoring in small cracks in a hard granite, a steel nut is used because there is little chance of the rock breaking away. In a softer stone, steel anchors have a tendency to pull through the stone. To compensate for this, brass nuts are used because they are more malleable and will deform upon loading thus giving a better hold.

Advances have also come in the material that connects the nut to the climber. Thin swaged steel wire has replace the older, thicker wire, making the anchor lighter and less costly. These types of wires are almost always used on the smaller anchors. For larger anchors there are two major choices. Some manufactures have stayed with the steel wire while other have gone to a special rope. Spectra line, "is pound for pound, stronger than steel" [Long, 1993a] and many of the larger anchors come pre-drilled for a 5.5 mm Specra cord. Figure A-2 shows a passive anchor strung with spectra cord.

The first passive anchors were railroad nuts with steel cable strung through the hole. The nuts were stuck into narrowing crack and the climbers would clip their ropes into the steel cable. These primitive anchors worked all right but there was one problem that quickly arose with the use of railroad nuts: climbers were limited to finding big cracks that a RR nut would fit into. Soon after the railroad nuts, two different styles of passive anchors became common, tapers and hexentrics

Tapers are six sided metal (aluminum, steel, or brass) wedges. As the name implies, they are trapezoidal shaped with the edges tapering toward the bottom of the anchor. Almost, all of the climbing gear manufactures produce some sort of taper, all with a slightly different twist on design. Almost all original tapers had straight edges, but today a large percentage of tapers incorporate some type of curvature to their design. This curvature helps the anchor rotate slightly when under load and creates three contact points instead of two. Tapers narrow going from top to bottom on both sets of the vertical faces. The taper has a different width than length and therefor can fit twice as many cracks.

The other common type of passive anchor on the market is the hexentric. There are several manufactures of hexentrics but almost all look like the one in figure A-2. Their six sides are all on a little different angle. The result generally is will have full surface contact on one side and a least an edge contact of the other side. Hexes come in several sizes and work well in more parallel cracks where a taper placement my be questionable. They work well in large crack and before the invention of active camming devices, they were the only anchor for large cracks.

Passive anchors are generally less expensive than active anchors. Generally they cost around $8-$10 per piece in comparison to a $50 camming device. In addition, a correctly sized, well-placed anchor will be as safe or safer that any active anchor. Because of their safety and lower cost, passive anchors are much more commonly used by amateur climbers.

Figure A-2. A hexentric strung with spectra cord in a parallel crack.

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References

Alder, Jerry and Glick, Daniel, "No Room, No Rest," Newsweek (August 1, 1994),

pp. 46-51.

Luebben, Craig, "Camming Gear Review," Rock and Ice, no. 61 (1994), pp. 53-59.

Long, J., Rock Climb! (Evergreen, CO.: Chockstone Press, 1993).

Long, J., Climbing Anchors (Evergreen, CO.: Chockstone Press, 1993).

Shackelford, James F., Introduction to Materials Science for Engineers, edition 3 (New York:

Macmillan Publishing Company, 1992).

Sussnan, Vic, "Health Guide: A Sporting Life," U.S. News and World Report

(June 18, 1990), pp. 64-67.

Van Horn, Kent R., Aluminum (Metals Park, Ohio: American Society for Metals, 1967).

Author's Note: Nick Huber is a senior in Mechanical Engineering Department at the University of Wisconsin. May 9, 1995. (Back to Beginning)