Direct answer: Choose a friction bolt (split set) by matching four things to your ground conditions and drill plan — diameter (33, 39, 42, or 47 mm, selected to suit the hole you drill), length (long enough to anchor into stable rock beyond the failure zone), steel strength (enough yield and tensile capacity for the expected load), and the bearing plate and coating (plate type and galvanizing for the surface and corrosion environment). Get those four right and the bolt will perform; get any one wrong and support can be inadequate. The rest of this guide walks through each decision with the numbers and rules of thumb our engineers use.
A friction bolt — also called a split set — is a C-shaped, slotted steel tube formed from high-tensile steel strip. One end is tapered for insertion; the other carries a welded ring to hold a bearing plate. When driven into a drill hole of slightly smaller diameter, the slot partially closes, the tube is compressed, and it presses against the rock along its full length, generating frictional anchorage immediately.
The model number tells you the nominal diameter, which is the first selection decision:
| Model | Typical drill-hole Ø | Min. yield | Typ. tensile | Typical use |
|---|---|---|---|---|
| FB-33 | 30–32 mm | 80 kN | 120 kN | Light to moderate ground, small headings, mesh support |
| FB-39 | 35–38 mm | 90 kN | 135 kN | General underground mining, tunnels, slopes |
| FB-42 | 38–40 mm | 100 kN | 150 kN | Moderate to high-stress rock |
| FB-47 | 43–45.5 mm | 120–135 kN | 180–200 kN | Deep mines, high-stress ground, jumbo headings |
These are representative industry figures for split-set friction bolts; exact values vary by steel grade and manufacturer. Always confirm the supplier's certified test values for the specific product you order.
Diameter selection is a system decision, not a bolt decision in isolation: the bolt must match the hole your drill rig produces.
Match the bit size. The drill-hole diameter must be slightly smaller than the bolt's nominal diameter so the slot can close and generate friction. If the hole is too large, the bolt will not grip; if too small, it cannot be driven.
Bigger is not automatically better. A larger diameter gives more anchorage per metre, but it also requires a bigger hole and more driving energy. Use the smallest diameter that delivers the required capacity for your ground.
Consistency matters. Friction bolts are unforgiving of inconsistent hole size — a worn bit that over-drills the hole will reduce anchorage. This is why installation and bit discipline are part of selection.
As a rule of thumb: light ground and mesh work start at FB-33; general mining and tunnelling commonly use FB-39; deep, high-stress, or seismic ground steps up to FB-47. Browse the full friction bolt range to compare diameters and capacities.
The bolt must extend far enough to anchor into competent rock behind the failure zone. If it ends inside loose or fractured rock, it will not hold.
Anchor into sound ground. The installed length should reach beyond the anticipated failure or stress-relief zone into intact rock.
Account for the exposed end. Allow for the bearing plate and the portion of the bolt that stands proud of the rock face.
Drill deeper than the bolt. The hole should be at least 150 mm longer than the bolt so the full bolt can seat without bottoming out.
Standard lengths. Split sets are commonly produced from 450 mm up to 3,000 mm (and beyond, to order). Confirm the available length range with your supplier.
A bolt that is too short is the most common cause of a pull-test failure — and it is a specification error, not a product defect.
Strength is set by the steel grade and the bolt's geometry. The key figures to check are yield strength (the load before permanent deformation) and tensile strength (the load before failure), both expressed in kilonewtons (kN).
Yield governs long-term support. You want the bolt to stay in its elastic range under the design load, so yield strength is the number that matters most for a permanent support design.
High-stress and seismic ground demands higher yield. In squeezing or burst-prone ground, bolts are loaded well beyond static conditions — specify a higher grade and a larger diameter.
Ask for certified values. A serious manufacturer provides certified test data for yield, tensile, and elongation for each diameter, not just a catalogue number.
For a deeper look at how steel grade affects performance in high-stress rock, see our article on steel grade selection for friction bolts.
Coating is a corrosion decision, and getting it wrong shortens service life even when the bolt is mechanically correct.
Black (bare) steel — the economical choice for dry, non-corrosive conditions.
Cold galvanized — a thin zinc layer for mild corrosion environments.
Hot-dip galvanized — a thick, durable zinc coating for wet, humid, or chemically active underground conditions where corrosion is a real risk.
In damp or acidic mine water, an uncoated bolt can corrode and lose capacity prematurely — a safety risk, not just a durability nuisance. Match the coating to the water chemistry and expected service life of the opening.
A friction bolt works as part of a system. The bearing plate transfers the bolt's load onto the rock surface, and the accessories complete the installation.
Bearing plate. Standard plates are typically 150×150 mm in 4–5 mm steel. Dome, combi, duo, and strata plates provide higher capacity or better surface contact for specific ground conditions. Choose the plate to match the surface and the load.
Mesh. In broken or blocky ground, combine bolts with weld mesh to retain loose rock between bolts.
Driver and pull collar. The correct installation accessories — driver and pull collar — ensure the bolt is driven straight and to the correct depth without damaging the end.
Selecting the bolt and plate together, from one supplier, avoids the compatibility gaps that cause installation delays.
Here is a practical starting point that ties the four decisions together:
| Ground condition | Typical selection |
|---|---|
| Light to moderate, competent rock | FB-33 or FB-39, shorter lengths, black or cold-galvanized, standard flat or dome plate |
| General mining / tunnelling | FB-39, standard lengths, hot-dip galvanized if wet, dome or combi plate |
| High-stress, deep, or seismic ground | FB-47, longer lengths to reach stable rock, high steel grade, high-capacity plate, hot-dip galvanized |
| Blocky or heavily fractured ground | Bolt + mesh combination, appropriate diameter and length, careful plate selection |
These are starting points. Ground support is site-specific, and a good manufacturer will help you refine the selection using your drill plan, hole size, and load requirements — rather than just quoting a part number.
Tanrimine Metal Support Co., Ltd (TRM) is a professional manufacturer of friction bolts and bearing plates, plus a full range of accessories and related components. We combine manufacturing with engineering know-how to help customers solve ground-support problems in mining, tunnelling, and dam projects.
Production capacity: an annual capacity of 30,000 tons of friction bolts and related products, with large stock of regular items for fast delivery and urgent requirements.
Quality: production is managed under ISO 9001, with a double-track quality control system monitoring every process from raw material to finished product. See our quality control and capacity & service pages.
R&D: our R&D team can design a new bolt or plate type and put it into production quickly, so you are not limited to catalogue items.
Mission: safety is our top priority — because a ground-support product is a life-safety product.
Underground mining: friction bolts are widely used for roof and wall support in gold, copper, and other metal mines — including the high-stress underground operations where Australian mines prefer friction bolts for immediate support in seismic and squeezing ground.
Tunnelling: split sets provide immediate support behind the advancing face during excavation.
Slopes and dams: friction bolts stabilize rock slopes and dam abutments where immediate, full-length anchorage is required.
Custom projects: our R&D team has designed and tested new bolt types for specific ground conditions, so non-standard projects are handled as engineering work, not off-the-shelf sales.
We serve ground-support projects across mining, tunnelling, and civil works markets worldwide. When you evaluate any supplier — including us — ask for specifics: diameters, certified test values, coatings, and where the product has actually been used.
Even a correctly selected bolt performs badly if installed incorrectly. These points are part of getting the specification right in the field.
Drill the right hole. Use the correct bit for the bolt diameter, and keep bits in good condition — a worn bit over-drills and reduces anchorage.
Drill deep enough. The hole should be at least 150 mm longer than the bolt.
Install perpendicular. Drive the bolt as perpendicular to the rock surface as practical, with the plate seated against the surface.
Stop at the plate. Once the plate is firmly against the rock, stop driving — continuing will damage the bolt end.
Pull-test to confirm. Perform pull tests after installation to verify initial anchorage, as specified in your ground-control plan.
Monitor critical ground. In high-stress areas, combine bolts with convergence monitoring to confirm support is holding.
Protect against corrosion. Match the coating to the environment at the specification stage; retrofitting corrosion protection is rarely practical.
Re-verify with pull tests. Scheduled pull testing confirms anchorage over time in dynamic ground.
Store correctly. Keep uninstalled bolts dry and off the ground, and protect galvanized surfaces from handling damage before installation.
Selection does not end at the specification — how you buy, and what support follows, matter just as much.
Lock the specification in writing. Diameter, length, steel grade, coating, plate type, and quantity — documented, so the delivered product matches the selection.
Confirm MOQ and lead time. Standard sizes usually ship from stock; custom diameters, lengths, or plates carry a higher MOQ and longer lead time. TRM's 30,000-ton capacity and regular stock support quick and urgent delivery.
Request certified test values. Yield, tensile, and elongation for the exact product, not a catalogue number — this is what makes a selection defensible on site.
Confirm packaging and delivery. Friction bolts are bundled for transport; confirm bundling and export packing for sea freight, and agree delivery terms.
Start with a trial. For a new supplier, order a trial batch and pull-test on site before committing to a full programme.
After-sales support. Choose a supplier that helps with selection, installation, and pull-test interpretation — not just shipping. TRM's engineering team supports design and selection, and our ISO 9001 double-track quality system lets us trace any issue back to its production batch. Reach us at jimwang@cnort.com.
Match the bolt diameter to the drill-hole size your rig produces, then check the bolt's yield and tensile capacity against your design load. Light ground starts at FB-33; deep or high-stress ground typically steps up to FB-47.
Long enough to anchor into competent rock beyond the failure zone, plus allowance for the plate and exposed end. The hole should be at least 150 mm longer than the bolt.
It depends on the environment. Use black steel in dry conditions, cold galvanizing in mild conditions, and hot-dip galvanizing in wet, humid, or chemically active ground where corrosion is a risk.
Yes. TRM has an in-house R&D team that can design a new bolt or plate and put it into production quickly. Custom items typically involve a higher MOQ and longer lead time, which we confirm up front.
Request certified test values for yield, tensile, and elongation for the exact diameter and steel grade you are ordering — and confirm the supplier's quality system (TRM operates under ISO 9001 with double-track quality control).
TRM's annual capacity is 30,000 tons, with large stock of regular items for quick delivery and support for urgent requirements. Confirm lead time for your specific size, length, coating, and quantity.
Need help selecting the right friction bolt?
Send your drill plan, hole size, and load requirements to jimwang@cnort.com, or explore our friction bolts, bearing plates, and accessories.
