A ferrule insert is a solid machined body with an internal NC (Unified National Coarse) thread and a captive back end. A coil insert has no machined thread at all — it is a helix of wire, and the coil itself is the thread — welded to two, four or six struts. The surprise: for the same nominal diameter, the two carry essentially the same load, because failure is governed by the concrete cone, not the thread. So you don't choose on strength. You choose on bolt type (off-the-shelf NC bolt vs special coil bolt), slab thickness, and unit cost at volume — then honour the two rules that actually cause site failures: correct setback and full bolt penetration.
Ferrule Insert
Coil Insert
1. The head-to-head verdict, up front
Most articles on this topic bury the answer. Here it is:
The verdict
At the same nominal diameter, strength is a tie — the concrete decides, not the thread. Pick the ferrule when you want a permanent connection made with ordinary NC bolts your site already stocks, or when the element is thin. Pick the coil when you want the lower-cost insert at volume and you are happy to stock matching coil bolts or coil rod. Everything else in this article is the detail behind those two sentences.
| Criterion | Ferrule insert | Coil insert |
|---|---|---|
| The thread | Machined internal NC thread in a solid body, captive back end | No machined thread — a wire helix; the coil is the thread |
| Matching bolt | ✓ Standard NC bolt — off the shelf, anywhere | Needs a coil bolt / coil rod — a specialist item you must stock |
| Load at same diameter | Effectively identical — concrete cone governs, not the thread | |
| Governing failure | Concrete cone pull-out (or bolt strip if under-engaged) | Concrete cone pull-out — classic cone with base ≈ 2 × cone height |
| Unit cost at volume | Higher (machining) | ✓ Lower (wire forming + welding) |
| Thin elements | ✓ Short-body thin-slab versions available | Needs flattened / parallel-strut thin-slab geometry |
| Tolerance to fouling | Fine thread can be spoiled by grout ingress — cap it | ✓ Coarse open helix is more forgiving of debris |
| Best for | Permanent connections with standard bolts; thin panels; mixed-trade sites | High-volume lifting & handling inserts where you control the bolt supply |
2. The thread is the whole difference
Strip away the marketing and there is exactly one meaningful difference between these two products: how the hole is threaded.
Everything else follows from that:
- The ferrule needs machining, typically from free-cutting cold-drawn bar. That costs money, but you get a thread that any NC bolt on the planet mates with.
- The coil needs no machining — it is drawn wire, wound and welded to struts. That is cheap at volume, but the mating bolt is a specialist coil bolt or coil rod that your site must stock and your erector must not lose.
3. Why strength is the wrong tie-breaker
Engineers reach for the load table first, expecting one product to win. It doesn't. For the same nominal diameter, ferrule and coil inserts carry essentially the same load. The reason is structural, not commercial: the connection almost always fails in the concrete, long before the thread is in trouble.
Two consequences follow, and both are practical:
- Load tables are only meaningful with their concrete strength. Published capacities are derived at a stated concrete strength (US tables commonly at 3,000 psi ≈ 20 MPa, with a stated density). A rating quoted without its concrete grade is a number with no units — ask for the grade.
- If the insert is under-strength, don't upsize the thread — fix the concrete side. More embedment, more edge distance, or supplementary reinforcement through the cone. A bigger thread in the same shallow cone buys you nothing.
4. Struts: two, four or six
Both families are held in position — and get their cone spread — by struts: the shaped legs welded to the body or coil. This is where you actually tune capacity.
| Configuration | What it does | Typical use |
|---|---|---|
| 2-strut | Minimum spread; cheapest; easiest to place between mesh | Light, routine fixings; thin elements where a wide strut can't fit |
| 4-strut | The workhorse. Wider cone, much stiffer against being knocked out of position during the pour | Lifting and handling inserts; most structural connections |
| 6-strut | Largest shear-cone surface area, hence the highest capacity for the same diameter | High loads, or where geometry truncates the cone and you need to buy the area back |
Strut count is the reason two inserts with the same thread can be rated differently: a wider strut spread grows the shear cone, and a bigger cone is a bigger capacity. If you are load-limited but depth-limited too, going from four struts to six is usually a better move than going up a thread size.
5. Thin slabs change the answer
In a thin element the debate resolves itself: a standard-length insert simply will not fit. Forcing a long insert into a shallow slab leaves too little cover and a truncated cone, and the catalogue rating no longer applies.
- Thin-slab ferrule inserts use a shortened body designed for thin-wall elements where a standard-length insert won't fit.
- Thin-slab coil variants flatten the geometry — for example a 2-strut parallel arrangement — so the insert develops its cone in reduced depth.
6. The two rules that cause most site failures
In our experience of failure reports, the insert itself is rarely at fault. Two installation rules account for most of the "insert failed below its rating" calls:
Setback from the concrete face
The insert must sit the specified distance back from the surface — commonly around 12 mm (half an inch). Cast flush or proud and two things go wrong: the concrete cone starts from the wrong plane, and the thread gets fouled by laitance. Use the correct plug or setback former, and check it before the pour, not after.
Full bolt penetration
The bolt must engage far enough into the insert to develop the rated load. A bolt that bites only a few turns will strip long before the concrete cone is reached — and the insert takes the blame. Specify the required penetration on the drawing, and give the crew a bolt of the right length so they physically cannot under-engage it.
And one more, on spacing
Inserts need to be far enough apart that their concrete cones do not overlap. A widely used rule of thumb is a minimum spacing of twice the corner (edge) distance. Where cones do overlap, the group capacity is less than the sum of the parts and must be reduced. Tight spacing and tight edge distance have the same cure: reinforcement through the cone.
7. The decision tree
Four questions, in this order, will settle almost every specification:
8. Frequently asked questions
What is the difference between a ferrule insert and a coil insert?
A ferrule insert is a solid machined steel body with an internal NC (Unified National Coarse) thread and a captive back end — you bolt into it with a standard off-the-shelf NC bolt. A coil insert has no machined thread at all: it is a helix of wire, and the coil itself acts as the thread, so it needs a matching coil bolt or coil rod. Ferrules are typically attached to struts or loops; coils are welded to two, four or six struts, or formed into a loop.
Which is stronger, a ferrule insert or a coil insert?
Neither, in practical terms. For the same nominal diameter, ferrule and coil inserts have essentially the same load-carrying capacity, because the governing failure is the concrete cone pulling out — not the thread. That is why strength is the wrong basis for choosing between them: choose on the bolt you want to use, the slab thickness, and unit cost at volume.
How does a coil insert fail?
Typically by concrete cone pull-out: a cone of concrete lifts out of the surface with its apex just below the coil and a base diameter roughly twice the cone height. That shape tells you the load path was through the concrete, not the steel. Solid-shaft inserts of the swift-lift type instead tend to fail by fracture of the shaft. Either way the fix is more embedment, more edge distance, or supplementary reinforcement — not a bigger thread.
What do 2-strut, 4-strut and 6-strut mean on a coil insert?
The struts are the legs that hold the coil away from the surface and spread load into the concrete. More struts spread the load over a larger cone and stiffen the insert against being knocked out of position during the pour. Two struts suit light, routine fixings; four struts are the workhorse for lifting and heavier connections; six struts are used where the load is high or the cone is constrained by geometry.
Which insert should I use in a thin slab?
Use a purpose-made thin-slab insert. In a thin element the standard-length insert does not fit, and forcing a long insert into a shallow slab leaves too little cover and a truncated cone. Thin-slab ferrule inserts have a shortened body; thin-slab coil variants use a flattened or parallel-strut geometry so they develop capacity in reduced depth. If depth is very tight, add supplementary reinforcement through the cone rather than upsizing the thread.
What are the two rules that most often cause insert failures on site?
First, setback: the insert must sit the specified distance back from the concrete face (commonly around 12 mm / half an inch). Cast flush or proud and the cone starts at the wrong place and the thread gets fouled. Second, bolt penetration: the bolt must engage far enough into the insert to develop the rated load. A bolt that only bites a few turns will strip long before the concrete cone is reached — and this is the single most common cause of an insert "failing" below its catalogue rating.
How far apart do inserts need to be?
Far enough that their concrete cones do not overlap. A widely used rule of thumb is a minimum spacing of twice the corner (edge) distance; where cones do overlap, the group capacity is less than the sum of the individual capacities and must be reduced. When either spacing or edge distance is tight, add supplementary reinforcement crossing the cone.
Can I use a ferrule insert for lifting as well as for a permanent connection?
Yes — that dual use is one of the reasons the ferrule family exists. Loop-type ferrule and coil inserts are used for lifting and handling; the same threaded body then accepts a bolt for the permanent connection after the element is set. What you must not do is assume the lifting rating and the service rating are the same number: lifting is a dynamic load with its own safety factor, so check both against the supplier's data.
What steel and what certification should I ask for?
Ferrule bodies are commonly machined from free-cutting cold-drawn bar; coils are drawn steel wire welded to struts. Ask for the mill certificate (EN 10204 3.1 or equivalent) per batch, thread gauge inspection, weld inspection on the struts, the surface finish (plain, zinc-plated, HDG or stainless for exposed use), and load tables with the concrete strength they were derived at — a rating quoted without its concrete grade is meaningless.
References
- ACI 318 — Building Code Requirements for Structural Concrete, the anchorage provisions behind concrete-cone capacity, edge distance and spacing.
- Precast handbook literature — industry load tables are published against a stated concrete strength and density; always read the header before using a number.
Need ferrule or coil inserts against your drawing?
20+ years of export experience. Ferrule inserts (standard, thin-slab, open, loop, L-leg) and coil inserts (straight, flared, 2/4/6-strut, thin-slab) in plain, zinc-plated, HDG or stainless. Mill certificates and load tables with their concrete grade. Quote in 24 hours.
