An Anchor rod extension that fails at the eye-clevis joint is, in practical terms, a guyed tower that fails. The forged eye loop and the forged clevis-yoke carry the full guy tension, and the clevis pin that joins them is the single hardware interface that decides whether the load path stays intact for the design life.
On a guyed communications tower, a transmission pole, or a wind-measurement mast, the anchor rod extension is a short length of threaded rod with a forged eye on one end and a forged clevis on the other. It bridges the buried portion of the anchor and the guy-strand hardware above grade. The rod itself is straightforward Carbon Steel, hot-dip galvanized to ASTM A123. The interesting engineering — the part that determines whether the assembly passes inspection after a decade of wind and ice loading — sits in the two forged ends and the pin that holds them together.
What "Clevis-Eye" Means on an Anchor Rod Extension
In a guyed-tower anchor stack, the load path runs from the soil through the buried anchor, the anchor rod, and the anchor rod extension with forged eye and clevis, to the guy strand above grade. The extension is a short length of round bar, threaded at the buried end and fitted with a forged loop on one end and a forked yoke (the clevis) on the other. The guy strand hardware — a thimble, a guy grip, or a turnbuckle body — hangs from the eye end. The clevis-yoke end receives a pin that joins it to the anchor rod coming out of the ground.
Two interfaces carry the entire guy tension: the pin inside the clevis-yoke, and the bearing surface between the eye and whatever hardware is hanging from it. Every other part of the assembly exists to deliver tension to those two points without distortion, bending, or corrosion loss. The procurement spec for an anchor rod extension is not really about the rod — it is about those two forged ends.
The terminology can confuse new buyers. "Eye" is a closed loop through which a pin or shackle can pass. "Clevis" is a forked yoke designed to receive a mating tongue and to be locked by a pin through both prongs. On an anchor rod extension with forged eye and clevis, both ends are forged from the same hot billet or welded to the bar after forging. The end geometry is what determines whether the hardware above it (a guy grip, a turnbuckle eye, or a thimble clevis with steel clevis pin and cotter pin) can actually seat properly.
Forged Eye Geometry: Why Drop Forging Beats Casting for Guy Service
The eye at the end of an anchor rod extension is almost always a drop forging. There is a reason for that. Drop forging forces the steel to flow around a die cavity under high compressive load, which aligns the grain structure with the contour of the finished eye. The eye load path is continuous grain — not a weld line, not a cast interface. Under cyclic guy tension, a forged eye has measurably higher fatigue endurance than either a sand-cast eye or an eye formed by welding flat stock into a loop.
Cast eyes look identical to a forged eye on a print, and on a one-time static pull test they reach the same ultimate Tensile Strength. They diverge in service. Sand-cast eyes contain subsurface porosity, gas pockets, and slag inclusions that act as fatigue crack initiators. Over thousands of wind cycles, those defects propagate, and the eye fails at well below its rated ultimate load. The same applies to weldments: a welded eye loop has a heat-affected zone at the root where fatigue cracks initiate under cyclic load.
This is why the hot forged utility pole hardware range from established manufacturers specifies drop forging for any component that carries cyclic or dynamic load — eyes, clevis-yokes, pole top caps, ball clevises. For static-only hardware (decorative castings, some grounding lugs), cast iron or ductile iron remains acceptable because of cost and ease of machining complex shapes.
Forging quality matters. A drop forging done on worn dies produces undersized eyes, sharp internal radii, and surface laps. Buyers should require a forging drawing that specifies the eye inner diameter (not just the bar diameter), and a sample inspection that checks both the inner radius of the eye and the eye-to-bar transition — both are stress-concentration points.
Clevis-Yoke and Clevis Pin: Tolerances, Fit, and Cotter Pin Detail
The clevis end is the receiving fork. Two prongs, each with a hole, take the clevis pin that joins the assembly. The bearing surface between the pin and the inside of the clevis-yoke prongs carries the second half of the load path that takes the full guy tension.
Two tolerances matter most. The first is the inside-to-inside gap between the clevis-yoke prongs — wide enough to slip over the mating tongue without binding, but not so loose that the assembly rocks on the pin. The second is the hole diameter relative to the clevis pin diameter. A sloppy fit lets the pin rock under cyclic load, and that rocking is the dominant fatigue cycle on the inside of the hole. Both prongs and pin must resist brinelling and wear at the bearing surface — this is where the mechanical property classes in ISO 898-1 for carbon and alloy steel fasteners become the procurement spec, not just a footnote.
For galvanized hardware, fit is checked after galvanizing. Hot-dip zinc adds material to all surfaces, including the inside of the clevis-yoke hole. A pin that slides freely before galvanizing may bind after; a slip fit in raw steel becomes an interference fit once zinc is added. The workaround is to specify the pin and the hole to clear each other after coating, or to oversize the pin to compensate for HDG buildup.
The cotter pin is the part most buyers underestimate. A properly sized stainless cotter pin (or spring-steel hitch pin clip) is what keeps the clevis pin from walking out under vibration. Field inspections frequently find hardware where the cotter pin is absent, bent, or the wrong diameter — the assembly looks intact from a distance but is one vibration cycle from losing the clevis pin entirely. The spec should require a cotter pin of matching material grade (stainless for corrosive environments, zinc-plated spring steel otherwise).
Raw Material Grades: ASTM A36, A572, and When to Specify Higher
The anchor rod itself is round bar to ASTM A36 for general service, or ASTM A572 grade 50 where higher yield is warranted. A36 is the workhorse — it welds cleanly, machines predictably, and is available in every drawing size a pole-line anchor needs. A572 grade 50 offers higher yield at a modest premium, with the same welding and galvanizing characteristics.
For the anchor rod extension with forged eye and clevis, the bar is ordered to a single grade (A36 or A572) and the forged ends are made from the same heat of steel. The Mill Test Report (MTR) should be traceable to the forging batch, not a generic certificate from the forge shop. That is how a buyer knows the eye and the rod came from the same chemistry lot and the galvanizer has consistent zinc-coating behavior.
Two situations justify specifying higher than A572 grade 50. The first is fatigue-critical service — guyed towers in sustained high-wind regions, or structures with large daily temperature swings translating to guy tension cycling. A quenched-and-tempered alloy bar (4140 Q&T to ASTM A829 or a proprietary grade) raises the fatigue endurance limit materially. The second is cold-region service where impact toughness at low temperature matters; the spec should add a Charpy V-notch requirement at the design minimum temperature, which neither A36 nor A572 guarantee by default.
A spec that simply says "ASTM A36" without requiring an MTR allows the forge shop to substitute any structural grade on the rack. Buyers should require an MTR plus the items below.
- Mill Test Report (MTR) traceable to heat number and bar heat.
- Forged ends: certificate from the forge shop confirming the same heat was used.
- Galvanizing: certificate confirming coating thickness (average and individual readings) measured after galvanizing.
- Dimensional inspection report for eye inner diameter, clevis-yoke prong gap, and hole diameter.
None of these documents add meaningful cost; their absence is the most common procurement gap when an extension is replaced after a service incident.
Galvanizing for Guyed Tower Service: ASTM A123 vs. A153
Galvanizing on an anchor rod extension does two things: it protects the steel from atmospheric corrosion, and it provides sacrificial anode behavior at any cut edge or scratch. The thickness of the zinc coating, the coating uniformity, and the adhesion all matter for service life — and that is why two ASTM standards apply to one assembly.
ASTM A123 / A123M covers hot-dip galvanized coatings on iron and steel products — the anchor rod, the forged eye, the forged clevis-yoke, the bar between them. Coating thickness is graded by material category and steel thickness; structural steel in the anchor rod size range lands in the middle of the table. The minimum average thickness required by utility buyers is in the moderate range, with heavier grades reserved for severe service (coastal, industrial atmosphere, sub-soil).
ASTM A153 / A153M is the companion standard for hardware — clevis pin, nuts, cotter pins, washers. A153 specifies a heavier minimum coating than A123 on equivalent mass because thinner hardware cools faster when withdrawn from the zinc bath. For matched service life, an anchor rod extension assembly should be galvanized to both: the rod and forged ends to A123, the loose hardware to A153.
Three common mistakes show up on the galvanizing spec. The first is asking the galvanizer to coat the entire assembly in one dip — works for small parts, but for a multi-foot anchor rod extension the bath dimensions and dip time produce inconsistent coating along the length. The second is specifying A123 alone, which leaves the clevis pin and cotter pin without an explicit standard reference; the galvanizer defaults to a thinner commercial coating. The third is forgetting that hot-dip galvanizing grows the part dimensionally — the inside of the clevis-yoke hole gets smaller, the outside of the clevis pin gets larger. The hardware kit has to be re-spec'd to clear the post-galvanizing condition.
Working Load vs. Ultimate Load: Factor of Safety for Guy Hardware
Manufacturer catalogs list two numbers. The first is the ultimate tensile strength — the load at which the part physically fractures. The second is the working load limit — the load the manufacturer publishes as the maximum that should be applied in service.
The ratio between those two numbers is the published factor of safety. For guyed-tower anchor rod extensions, a working load of roughly one-quarter to one-third of the ultimate tensile strength is typical — a factor of safety between 3.0 and 4.0 for static guy service. For cyclic service, the working load is set lower because the fatigue limit is below the static ultimate. For severe environments, buyers often drop the working load below the published number to add their own reserve.
| Service condition | Typical factor of safety | Notes |
|---|---|---|
| Static guy tension, low wind region | ~3.0 to 3.5 | Manufacturer-published working load typically applies |
| Static guy tension, moderate wind, seasonal swing | ~3.5 to 4.0 | Manufacturer-published working load typically applies |
| Cyclic / fatigue service, sustained wind | ~4.0 to 5.0 | Reduce working load; verify with manufacturer's fatigue data |
| Severe environment (coastal / sub-soil / vibration) | ~5.0 or higher | Spec must call out the additional de-rating |
| Proof load test (factory or field) | ~2.0 (proof load is roughly half of ultimate) | Verifies absence of manufacturing defects; not a service load |
Three things to check on the load chart. First, whether the working load is published as a function of rod diameter, eye inner diameter, and clevis pin diameter — the honest one, because the weakest section controls. Second, whether the manufacturer publishes a fatigue rating, not just a static ultimate. Third, whether the chart is for the entire assembly (rod + forged ends + clevis pin) or only the bar. A bar-only rating is not a hardware rating.
Specifying the correct working load up front is cheaper than replacing hardware after the fact. If the design load sits in the lower-to-middle range of the chart, the part is comfortably within its working limit. If it sits in the upper third, drop to the next rod size up rather than approaching the catalog limit.
Guyed Tower Application Notes: Wind, Vibration, and Ice Loading
A guyed tower's design load is dominated by wind on the antenna and shaft, translated into guy tension through the guy strands. The design load cases are documented in the standards that govern the structure itself. ANSI / TIA-222 covers antenna supporting structures and small wind turbine support structures, including the load combinations a tower designer is expected to evaluate. ASCE / SEI 10 covers latticed steel transmission structures. IEC 60826 covers overhead transmission line design criteria used in many international markets. Each treats the anchor and its hardware as a load path that must be sized for the same load combinations as the tower above.
Hardware failure modes in service are dominated by three things: cyclic wind loading, vortex-induced vibration, and ice-load shocking. Cyclic wind is the slow fatigue cycle — thousands of small tension cycles per day. Vortex-induced vibration is the resonance the guy strand picks up from steady cross-winds. Ice-load shocking is the abrupt tension increase when accumulated ice sheds asymmetrically. Each is well below the static ultimate load of the anchor rod extension; none is below the fatigue limit of the hardware.
The forged eye and clevis-yoke are the most fatigue-sensitive parts because cyclic stress concentrates at the inside of the eye and at the root of the clevis prongs. Drop forging gives the best fatigue performance in both locations; welding gives the worst.
Vibration dampers (Stockbridge, dog-bone, or weights on the guy strand) are commonly specified upstream of the anchor rod extension. They reduce vortex-induced vibration amplitude that would otherwise reach the hardware.
Hardware replacement after a service incident is the most expensive place to discover a specification gap. Specifying forged eyes and clevis-yokes, HDG to A123/A153, MTR traceability, and a working load comfortably below the published chart maximum are the four decisions that prevent most service failures.
Sourcing Checklist for Procurement Teams
Before signing a purchase order, the procurement team should answer "yes" to each item below. Where the answer is "no" or "unclear," that is where a service failure is most likely to originate.
| Checklist item | What to ask for |
|---|---|
| Eye manufacturing method | Drop forging (not casting, not weldment). |
| Clevis-yoke method | Drop forging preferred. Cast iron only for static, low-cycle service; specify as exclusion otherwise. |
| Eye ID and prong gap tolerance | Drawing callout (not just "suit the pin"). Confirm tolerance window. |
| Clevis pin fit (post-galvanizing) | Clearance class (slip, line-to-line, light interference) called out. |
| Bar material grade | ASTM A36 or A572 grade 50 with MTR. Higher grade only where fatigue or low-temp toughness is in scope. |
| Galvanizing — rod and forged ends | ASTM A123 / A123M, MTC for coating thickness. |
| Galvanizing — pin, nuts, cotter pin | ASTM A153 / A153M, MTC. |
| Working load limit | Published for the entire assembly, with stated factor of safety. |
| Fatigue rating | Published S-N curve or fatigue limit — required for cyclic service. |
| Drawing package | Fully dimensioned drawing with eye ID, prong gap, hole, pin tolerances. |
| Mill Test Report (MTR) | Traceable to heat number, supplied with each batch. |
| First-article inspection | Report required before production run is released. |
| Kitting | Clevis pin, cotter pin, nuts supplied with the rod — not as separate line items. |
| Reference installations | Manufacturer's documented supply history for utility / communications buyers. |
For buyers working from a complete pole line hardware product catalog rather than a custom print, the checklist collapses into three questions: is the eye forged, is the clevis pin galvanized to A153 with a stainless cotter pin, and is the load chart published for the full assembly? If those three are answered satisfactorily, the rest is detail work.
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Frequently Asked Questions
What is the difference between a forged eye and a cast eye on an anchor rod?
A forged eye is produced by drop forging, producing continuous grain along the load path. A cast eye is poured into a mold; the grain is irregular and the casting can contain subsurface porosity and inclusions. For guyed-tower service, a forged eye has higher fatigue endurance and is the only acceptable choice on cyclic load paths.
What clevis pin grade is typically used with an anchor rod extension?
The clevis pin is specified to a property class under ISO 898-1 (commonly class 6 or above) or to an ASTM equivalent such as A36 or A572 grade 50. The pin is hot-dip galvanized to ASTM A153/A153M and supplied with a matching cotter pin (stainless for corrosive environments, zinc-plated spring steel otherwise).
Can the eye and clevis be galvanized together with the rod, or must they be processed separately?
The rod, forged eye, and forged clevis-yoke are typically galvanized together to ASTM A123/A123M in a single dip. The clevis pin, nuts, and cotter pin are small hardware galvanized separately to ASTM A153/A153M, which specifies a heavier minimum coating. The combined assembly must cite both standards, and post-galvanizing dimensions must be re-checked for fit.
What working load should I specify for an anchor rod extension on a guyed communications tower?
The published working load on the manufacturer's chart is the starting point. For static atmospheric exposure it is acceptable (factor of safety around 3.0 to 4.0 against ultimate). For cyclic service, severe environments, or vibration-prone structures, drop the working load below the chart number, or step up to the next rod size.
How do I verify the clevis pin fits the clevis-yoke properly during inspection?
The pin should drop into the clevis-yoke holes under its own weight after galvanizing, with no rocking or binding. The cotter pin hole at the clevis pin head should be visible through the slot in the prong, and the cotter pin should pass fully through and lock. Any pin that has to be driven, or any cotter pin that does not seat fully, is a field rejection.
Is a thimble clevis the same as the clevis end of an anchor rod extension?
No. A thimble clevis is a separate pole-line fitting that sits between the guy strand and the anchor hardware above grade. The clevis end of an anchor rod extension is forged directly onto the rod that comes out of the ground anchor. They share the same interface language (clevis-yoke, clevis pin, cotter pin) but are different parts from different stages of the anchor stack.