Expansion Bolt types
Expansion bolts are mechanical fixings that work by expanding and compressing against the wall of the hole drilled into the rock. They are easy to install, requiring less equipment for the installation and knowledge to do so.
Expansion bolts are grouped into two principle types; torque controlled (most common) or displacement, with the former using friction and the latter physical rock displacement.
Torque controlled - common
- Wedge (bolt-in)
- Sleeve bolts
Displacement - not common
- Self drilling
- Nail drive
Torque Controlled - Wedge Bolts
The typical bolt used for climbing is a torque-controlled wedge bolt (otherwise referred to as a bolt-in) that consists of a threaded bolt shaft with a single or double conical end wrapped in a clip and a hanger placed over the end of the bolt shaft where it protrudes from the rock. A nut and washer secure the hanger.
Standard diameters in Europe are commonly 10mm or increasingly 12mm, and in North America 3/8inch (9.525mm) or ½ inch (12.7mm).
Wedge bolts can be purchased in a variety of lengths but the embedment depth must be 5 times the bolt diameter in order to conform with European standards. In North America 2 to 5 inch lengths being the most commonly used by climbers.
The vast majority of wedge bolts are stainless steel 304 or increasingly 316 grade steel with some areas of North America continuing to use plated or galvanised bolts in desert areas, although this is actively discouraged. Fixe and Petzl now produce wedge bolts from higher grade steels for improved corrosion resistance; Fixe in duplex 2304 steel or Petzl using a combination of 926 for the wedge bolt and 904L externally for the hanger.
Wedge bolts are reliable, strong and relatively easy to place one-handed, on lead, and they are unlikely to result in botched placements. Whilst undoubtedly the most common fixing in use, wedge bolts are weaker than other types in both pullout and shear strength, and they also have a more limited expansion range than sleeve bolts. The mechanical expansion is very localised around the wedge and this results in a narrow range of expansion in which to achieve strength without the clip moving too far over the wedge. The clip on the end must be resisted by rock and not allowed to rotate otherwise a wedge bolt can pull out under these conditions, and fatalities have occurred where wedge bolts were placed in soft rock and falls taken (e.g. Blue Mountains, Australia). Single and double expansion clip versions are available, the former for normal or hard rock conditions and the latter where cavities may result in the clip not engaging over the wedge. Double clip wedge bolts can also provide improved holding power in soft rock however sleeve or glue-in bolts are better suited and generally recommended instead.
Wedge bolts are common throughout Europe and overcome the issues associated with ensuring that removable sleeve bolts always have sufficient applied torque. Throughout North America, Fixe, Powers and Hilti KB3 are common alternatives, used with Fixe or equivalent brand hangers.
Torque Controlled - Sleeve Bolts
Sleeve bolts are the other form of torque control bolts these being instantly recognisable by a sleeve that extends the full length of the bolt and a hexagonal bolt head. The standard bolt in North Americais made by Powers (formerly Rawl) and is commonly called a “five-piece” because of the five pieces that comprise the bolt itself: the bolt, a large washer, the sleeve, the blue plastic bushing, and a threaded cone.
Originally developed as a construction bolt, they are available in many different sizes and materials. These have an advantage over the typical wedge bolt in that the nut cannot drop off and they are easier to replace. The longer versions have two sleeves, but are still called five-pieces. Upon tightening the hex head of the bolt, the cone pulls into the sleeve, causing the sleeve to expand and exert pressure on the walls of the hole. These bolts have more surface contact inside the hole than a wedge bolt, making it stronger in both pullout and shear strength.
Displacement Bolts - Self Drilling
Displacement bolts are less common these days because of the insufficient embedment depth (5 times the diameter) means they do not conform to either UIAA 123 or EN 959 standards.
Two types likely to be encountered by climbers are the Petzl Longlife, which is a nail drive, or varieties of self-drilling bolt typically used by cavers with a spreader cone inserted prior to being hammered back into the hole.
In both cases, the sleeve of the anchor is forced outwards producing compression (displacement) against the rock around the end of the bolt.
During the 80’s and early 90’s, climbers initially used self-drilling caving bolts, consisting of a self-driving anchor into which a hanger was screwed after the sleeve had been drilled sufficiently deep. The threaded sleeve has teeth cut at one end, is screwed onto a handled driver then hit and rotated to ‘self-drill’ the bolt hole. Upon ‘drilling’ the hole, a spreader cone of metal is placed in the rear of the anchor and the assembly hit back into the hole causing an expansion cone to form. The hanger is generally made of aluminium (this system was developed for expedition caving) and never intended to be attached permanently to the plated steel anchor due to issues of dissimilar metal corrosion.
A 10mm version designed for climbing was produced by Petzl and driven into the rock in the same manner using an adaptor fitted to the hand driver. Due to the significant time in hand drilling a single placement these never proved popular.
Hand drilling for this type of displacement style bolt is not to be confused with other handled drivers into which a normal drill bit is fixed for deeper holes that take wedge or sleeve style bolts. Hand drilling in this situation is only encountered where ethics prohibit the use of cordless power drills e.g. Yosemite.
Displacement - Nail Drive Bolts
The Petzl Long life nail drive bolt is a 12mm diameter, 46mm long stainless steel bolt with a steel pin set in the shaft. Driving the pin into the bolt expands the shaft in the hole. They are easy to place and are actually very strong in hard granite. Because they’re stainless steel—and so is the hanger they come with—they are strong and will last a long time. However, these bolts are not safe in medium to soft rock and no longer manufactured due to non-compliance with European standards.
Key point: displacement expansion anchors do not provide the installer with any indictation of the placement quality that is apparent when tightening a torque-controlled anchor.
Other types of displacement bolts found predominently in the United States are button heads because the end of the bolt looks like a smooth button. The majority of these bolts were made by Rawl and range in length from 30mm to 50mm long and are usually only 6mm in diameter.
Regardless of the expansion bolt type some form of hanger is necessary to provide an attachment point. The generic expansion bolt hanger is made from steel plate, bent approximately perpendicular with sufficient eye clearance for 2 karabiners.
Traditional expansion bolt hangers come with several problems:
- Visually obtrusive
- Sharp hanger edges damage karabiners
- Abseiling or lowering directly from the hanger is not safe due to the sharp edges
- Hangers never truely sit flat against the rock surface and the induced stresses contribute to corrosion should the fixing become affected.
- Additional problems include the risk of broken karabiners should one become jammed in-between the hanger and the bolt shaft during a lead fall.
- Spinning hangers...
Hangers are manufactured to suit 10mm and 12mm bolts so it is important to match the appropriate hanger to the bolt otherwise the ability of the hanger to spin or move can develop damage in the bolt shaft.
Typical hanger strengths are 25kN for 10mm bolts and 30kN for 12mm bolts although in reality the hangers are more than likely able to sustain higher loading before failure.
Bolt Products produce a bolt hanger fabricated in marine grade (316) stainless steel bar that is less obtrusive and solves all the previously mentioned issues associated with traditional bolt hangers. Fabricated from 8mm marine grade stainless steel bar using cold bending ensures the fabrication cost remains acceptable and avoid the need for heating the steel to prevent cracking.