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Compression Bolts   

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Best Use: Remote or alpine environments; temporary placement during first-ascents
Rock Type: Hard rock
Pros: Easily removed; light, cheap, and easy to replace
Cons: Weak; unreliable

The bolts originally used in large numbers were compression bolts known as button-heads. These primarily came in ¼” and 5/16” diameter. These bolts have dual splits cut into the shaft running long ways down the axis and gain their ability to hold the rock by utilizing the elastic effects of the metal. The splits causes the body of the bolt to be a larger diameter than the hole, but when hammered in place the bolt compresses laterally. The resultant outward force applied within the drilled hole in the rock provides resistance against pulling out in the event of a fall.

While the ¼” and 5/16” diameter compression bolts provided shear and tensile strengths considered acceptable to early bolters, the margin for safety was narrow. Corrosion quickly renders the strength of these bolts (mostly carbon steel) unpredictable, resulting in their more common failure than other bolt types.

Rock Climbing Photo: Rawl spike

Rawl spike
Another type of less common compression bolt is the rivet. Used in aid climbing for progression up blank sections of rock, rivet style bolts have also been used as fall protection in decades past. A rivet is basically a plain threaded machine bolt hammered into a hole that is 1/16” smaller in diameter than the bolt. Sometimes the tip of the machine bolt is made into a conical shape in order to facilitate easier placement. As the bolt is hammered in, the threads deform and exert outward force, acting much like a compression bolt. These rivet style bolts are inherently weak in sheer and axial strength.

Compression bolts do have their place in modern climbing. Light, cheap, and easier to place quickly and one handed than other bolts, button heads are still used by first-ascensionists in remote and alpine environments or when establishing routes with the aim to temporarily protect sections of a climb before returning to replace the compression bolts with more reliable hardware. Because compression bolts are just held in place by the elasticity of the metal when hammered into a smaller hole, they can be removed with a device such as a specially adapted crow bar with little trouble, making them fairly easy to replace with a stronger, permanent bolt. Compression bolts should only be used in hard rock, as soft rock is more likely to compress and deform during placement or in the event of a fall.

There are also larger (i.e. 3/8” or 10mm), stainless compression bolts still in use in places such as Yosemite, where they have been used to replace other aged compression bolts. These are stronger and longer lasting than their skinnier, high-carbon predecessors. However, the axial strengths of these bolts remain below those offered by mechanical bolts.

Placement: Today, compression bolts are considered specialized, niche bolts that should only be used for temporary placements or on very remote routes that are not likely to see a repeat in the near future. They are also only suited for hard rock. The beauty of these bolts is that placement is simple and straightforward. A hole with a diameter equal to the stated diameter of the bolt is drilled and cleaned of dust and then the bolt is hammered into the hole. The most important consideration when using these bolts is to avoid deforming the rock at the surface of the hole. The opening of the hole can be reamed out slightly in order to facilitate placement of the bolt without chipping or otherwise deforming the hole (which could compromise the axial strength of the bolt).

Rock Climbing Photo: Rawl split drive bolt

Rawl split drive bolt
Removal: Compression bolts are generally the easiest anchor bolts to remove. Absent corrosion, they require the same amount of force to remove as to place. The difficulty lies in constructing a way to apply outward force without damaging the rock. The most popular device for pulling compression bolts is a wedge device called a “tuning fork”. These are often made from lost arrow or knife blade pitons with the center milled out longitudinally and a width equal to the diameter of the bolt. Many variations have been used successfully. Tuning forks act as a wedge, slowly forcing the bolt from the hole until it can be pulled by hand or a funkness device. Successful removal often requires a couple of tuning forks, which are used individually and then together in order to achieve the necessary amount of extraction for the bolt to pull free.

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