Introduction
Riveting is often taught as the old way of joining metal the method that built the Eiffel Tower and the Titanic before welding made it obsolete.
That is only half true. Welding did replace riveting in boilers, bridges and ships. But walk around any aircraft and you will find hundreds of thousands of rivets holding the skin to the frame, because welding aluminium alloy would destroy the heat treatment that gives it strength. Look at a modern aluminium car body and you will find self-piercing rivets doing the job spot welding cannot.
Riveting did not lose. It retreated to the places where melting the metal is a bad idea and as cars and aircraft use more aluminium, it has been quietly expanding again.
This guide covers how riveting works, the types of riveted joints, how such joints fail, and how to calculate joint efficiency with a full worked example. Written in plain language for mechanical and production engineering students.
What Is the Riveting Process?
Riveting is a permanent mechanical fastening process in which a metal pin, called a rivet, is inserted through aligned holes in two or more parts and its free end is deformed plastically to form a second head, clamping the parts together.
The key word is permanent. A bolt can be undone; a rivet cannot. Removing one means drilling or chiselling it out and destroying it.
Riveting is a cold or hot working process depending on rivet size. Small rivets up to about 10 mm are driven cold. Larger structural rivets were traditionally heated to red heat, driven hot, and then contracted as they cooled which pulled the plates tightly together and produced a genuinely pre-loaded joint.
Rivet Nomenclature
| Part | Description |
|---|---|
| Head (factory head) | The pre-formed head, made at the factory |
| Shank (body) | The cylindrical shaft passing through the holes |
| Tail | The free end that gets deformed during installation |
| Shop head (bucktail, closing head) | The second head formed during installation |
| Mandrel | In blind rivets, the internal pin pulled to form the far-side head |
During upsetting, the tail expands to roughly 1.5 times the original shank diameter, which both forms the head and fills the hole.
Types of Rivets
Solid rivets — the oldest and strongest type. A plain shank deformed by hammer, press or rivet gun. Requires access to both sides. Standard in aircraft structures.
Blind (pop) rivets — installed entirely from one side. A rivet gun pulls the mandrel, which buckles the tail into a head on the far side, then snaps off. Essential where the back of the joint is inaccessible.
Semi-tubular rivets — partially hollow tail, so less force is needed to form the head. Common in brakes, ladders and general assembly.
Tubular and full tubular rivets — hollow through the length, weaker but able to pierce soft materials such as fabric, leather and plastic.
Split (bifurcated) rivets — the tail is split lengthwise so it spreads on installation. Used for soft materials and light assemblies.
Drive rivets — a short mandrel is hammered in to expand the body. Quick, low-strength, used for nameplates and trim.
Flush (countersunk) rivets — head sits level with the surface. Essential on aircraft skins where aerodynamic drag matters.
Self-piercing rivets (SPR) — no pre-drilled hole. The rivet punches through the top sheet and flares into the bottom sheet without piercing it. Now the standard method for aluminium car body assembly.
Rivet nuts (blind rivet nuts) — install like a blind rivet but leave a threaded hole, allowing bolts to be used later in thin sheet.
Rivet Head Shapes
Snap (round), pan, countersunk, flat, and mushroom heads are the standard forms. Countersunk is chosen for flush surfaces, snap heads for maximum strength in structural work.
Steps in the Riveting Process
1. Hole preparation. Holes are drilled or punched slightly larger than the rivet typically 1 to 1.5 mm oversize. Drilled holes are preferred for critical work, since punching work-hardens and micro-cracks the hole edge.
2. Deburring and cleaning. Burrs prevent the plates seating together and start fatigue cracks.
3. Alignment and clamping. The parts are held in position, often with temporary fasteners in adjacent holes.
4. Rivet insertion. The rivet is placed through the aligned holes.
5. Holding up. A dolly or bucking bar supports the factory head so force can be applied to the tail.
6. Upsetting. Force is applied to the tail, deforming it plastically to form the shop head. The shank simultaneously expands to fill the hole.
7. Inspection. The formed head is checked for diameter, height, concentricity and cracks.
Types of Riveting by Method
| Method | Description | Typical use |
|---|---|---|
| Hand riveting | Hammer and rivet set | Repair, small workshop jobs |
| Pneumatic riveting | Air-driven rivet gun with bucking bar | Aircraft assembly |
| Press riveting | Steady squeeze from a hydraulic or mechanical press | Production assembly |
| Orbital / radial riveting | Tool head rotates in a wobbling path, forming the head progressively | Low force, delicate parts, clean finish |
| Impact riveting | Single high-energy blow | Fast production of small rivets |
| Self-piercing riveting | Rivet punches its own hole and flares | Aluminium automotive bodies |
Orbital riveting is worth understanding. Because the tool forms the head gradually through a rolling action rather than a single blow, the required force drops by around 80%, and delicate assemblies such as electronics and plastic parts survive the process.
Types of Riveted Joints
There are two fundamental configurations, and everything else is a variation on them.
Lap Joints
The plates overlap each other and rivets pass through both.
- Single riveted lap joint — one row of rivets
- Double riveted lap joint — two rows
- Triple riveted lap joint — three rows
Multi-row joints are arranged in one of two patterns:
- Chain riveting — rivets in successive rows sit directly opposite one another
- Zigzag (staggered) riveting — rivets in alternate rows are offset
Zigzag is generally stronger, because the reduced cross-section is distributed rather than concentrated along one line.
The weakness of a lap joint is that the load path is offset. Because the two plates are not in line, the load tries to bend the joint straight, introducing bending stress on top of the tension.
Butt Joints
The plates are placed edge to edge and one or two cover plates (straps) bridge the joint.
- Single strap butt joint — one cover plate; rivets in single shear
- Double strap butt joint — cover plates on both sides; rivets in double shear
Double strap butt joints are the strongest configuration, for two reasons: the rivets carry load across two shear planes instead of one, and the load path is symmetric so there is no bending.
Joint Terminology
| Term | Meaning |
|---|---|
| Pitch (p) | Distance between centres of adjacent rivets in the same row |
| Back pitch (pb) | Distance between two consecutive rows |
| Diagonal pitch (pd) | Distance between rivets in adjacent rows, measured diagonally |
| Margin (m) | Distance from the rivet centre to the plate edge |
The standard design rule is m ≥ 1.5d, where d is the rivet hole diameter. Any less and the plate simply tears out at the edge.
Failure of Riveted Joints
A riveted joint can fail in four ways. Design means checking all of them and finding which one governs.
1. Tearing of the Plate Between Rivets
The plate tears across the line of rivet holes, because the holes have reduced the cross-section.
Tearing resistance: Pt = (p − d) × t × σt
where p is pitch, d is hole diameter, t is plate thickness and σt is allowable tensile stress.
2. Shearing of the Rivets
The rivets are cut across their shank by the plates sliding past one another.
Shearing resistance: Ps = n × (π/4) × d² × τ
For double shear, the rivet resists across two planes, so the resistance is doubled:
Ps = 2n × (π/4) × d² × τ
In practice the factor used for double shear is often taken as 1.875 rather than 2, to allow for uneven load sharing.
3. Crushing (Bearing) of the Plate or Rivet
The rivet or the hole wall is crushed where they press against each other, elongating the hole.
Crushing resistance: Pc = n × d × t × σc
4. Tearing of the Plate at the Edge (Margin Failure)
If the margin is too small, the plate shears out between the rivet and the edge. This is prevented by the m ≥ 1.5d rule rather than by calculation.
Riveted Joint Efficiency
Punching holes in a plate weakens it. Joint efficiency measures how much of the original plate strength survives.
Efficiency (η) = lowest of (Pt, Ps, Pc) ÷ strength of solid plate × 100%
where the solid plate strength per pitch length is p × t × σt.
Note carefully: efficiency uses the lowest of the three resistances, because a chain fails at its weakest link. Whichever mode gives the smallest value is the one that will actually occur.
Sizing the Rivet: Unwin’s Formula
For plates thicker than 8 mm, rivet diameter is estimated using Unwin’s formula:
d = 6√t (mm)
The result is rounded up to the nearest standard rivet size.
Worked Example
Given: A single riveted lap joint. Plate thickness t = 10 mm, pitch p = 60 mm. Allowable stresses: σt = 80 MPa, τ = 60 MPa, σc = 120 MPa.
Step 1 — Rivet diameter (Unwin’s formula) d = 6√10 = 6 × 3.162 = 18.97 mm → use d = 20 mm
Step 2 — Tearing resistance Pt = (60 − 20) × 10 × 80 = 32,000 N = 32 kN
Step 3 — Shearing resistance (single shear, n = 1 rivet per pitch) Ps = 1 × (π/4) × 20² × 60 = 314.16 × 60 = 18,850 N = 18.85 kN
Step 4 — Crushing resistance Pc = 1 × 20 × 10 × 120 = 24,000 N = 24 kN
Step 5 — Solid plate strength P = 60 × 10 × 80 = 48,000 N = 48 kN
Step 6 — Efficiency The lowest resistance is shearing at 18.85 kN. η = 18.85 ÷ 48 × 100 = 39.3%
What this tells you. The joint retains barely 40% of the plate’s strength, and shearing governs the rivets give way before the plate tears or the hole crushes. Increasing the pitch would make it worse, since tearing resistance rises but shear stays fixed and efficiency falls. The genuine fixes are adding more rivet rows or switching to a double strap butt joint to put the rivets in double shear.
Typical Joint Efficiencies
| Joint type | Approximate efficiency |
|---|---|
| Single riveted lap | 45–60% |
| Double riveted lap | 60–72% |
| Triple riveted lap | 72–80% |
| Single riveted double strap butt | 55–60% |
| Double riveted double strap butt | 76–84% |
| Triple riveted double strap butt | 85–94% |
The pattern is clear: more rows and double shear raise efficiency. Even the best riveted joint never reaches 100%, because the holes are always there.
Caulking and Fullering
A riveted boiler or tank has to hold pressure, and rivets alone do not produce a leak-tight seam. Two hammering operations were developed to close the gap.
Caulking uses a blunt-edged tool, roughly 5 mm wide, hammered along the edge of the plate and around the rivet heads. It burrs the plate edge over and presses it hard against the plate beneath, closing the leak path.
Fullering uses a wider tool shaped to match the full plate thickness, so the whole edge is driven down uniformly in one operation rather than only the corner.
Both are cold working operations that seal by mechanical deformation, not by adding any filler. Plates thinner than about 8 mm cannot be caulked, since the hammering would damage them.
These processes are largely historical now welded pressure vessels made them unnecessary but they remain standard exam material and explain how the great riveted boilers of the industrial era held steam at all.
Riveting vs Welding
| Factor | Riveting | Welding |
|---|---|---|
| Joint type | Permanent mechanical | Permanent metallurgical |
| Heat input | None (cold) or localised (hot) | High |
| Material properties | Unchanged | Altered in the heat affected zone |
| Distortion | Minimal | Significant |
| Joint strength | 40–90% of plate strength | Can equal or exceed plate strength |
| Weight | Higher rivets and straps add mass | Lower |
| Dissimilar metals | Easy | Difficult |
| Heat-sensitive alloys | No problem | Can destroy heat treatment |
| Leak tightness | Needs caulking | Inherently sealed |
| Inspection | Visual, straightforward | Requires NDT |
| Fatigue performance | Holes are stress raisers | Weld toes are stress raisers |
| Skill required | Moderate | High, often certified |
| Noise | Very high | Moderate |
| Disassembly | Possible by drilling out | Very difficult |
| Cost | Higher labour, holes must be made | Generally cheaper |
Why Riveting Survives
Welding beat riveting on cost, weight and strength which is why boilers, bridges and ships switched. But riveting persists wherever those advantages do not apply:
Heat-treated aluminium alloys. Welding destroys the temper of the 2000 and 7000 series alloys used in aircraft. A rivet applies no heat at all.
Dissimilar materials. Aluminium to steel, metal to composite, metal to plastic combinations that fusion welding handles badly or not at all.
Inspectability. A rivet’s condition can be judged by eye. A weld needs ultrasonic or radiographic inspection to prove it internally.
Modern aluminium car bodies. Aluminium resists resistance spot welding because of its conductivity and oxide layer, so self-piercing riveting took over. A modern aluminium-intensive car body may contain several thousand SPRs, often combined with structural adhesive in the same joint.
That last point is the interesting one. Riveting is not a survivor from the past in automotive it came back, because the shift to lightweight aluminium made welding harder rather than easier.
Common Riveting Defects
| Defect | What it looks like | Main cause |
|---|---|---|
| Loose rivet | Rivet moves or rattles in the hole | Insufficient upsetting force, hole too large |
| Cracked head | Cracks radiating in the formed head | Rivet material too hard, over-driving, cold rivet too large |
| Eccentric head | Formed head off-centre to the shank | Tool misaligned, rivet not held square |
| Undersized head | Shop head too small | Rivet tail too short, insufficient force |
| Oversized head | Head too flat and wide | Tail too long, excessive force |
| Plate distortion | Sheets bulge or lift between rivets | Over-driving, poor clamping before riveting |
| Elongated hole | Hole deformed into an oval | Bearing overload in service, hole crushed |
| Edge tearing | Plate splits toward the edge | Margin less than 1.5d |
| Gap between plates | Sheets not pulled tight | Burrs left in the hole, poor clamping |
The rule most often broken in practice is margin. Rivets placed too close to an edge look fine on assembly and then tear out under load, because the plate simply does not have enough material to shear through.
Applications of Riveting
- Aerospace — aircraft skin panels, wing ribs, fuselage frames; hundreds of thousands of rivets per airframe
- Automotive — self-piercing rivets in aluminium body structures, brake linings, chassis brackets
- Construction — structural steel connections in older buildings, bridges and towers
- Shipbuilding — historically the primary joining method; now largely welded
- Boilers and pressure vessels — historical; now welded
- Consumer goods — ladders, scissors, cookware handles, luggage, furniture
- Leather and textiles — split and tubular rivets in bags, belts and footwear
- Electronics — orbital riveting for delicate assemblies and contacts
Advantages and Limitations of Riveting
Advantages
- No heat input, so material properties and heat treatment are preserved
- Joins dissimilar metals and non-metals easily
- Quality can be inspected visually without NDT
- Reliable and repeatable, easily automated
- Works on thin sheet without burn-through
- No fumes, arc radiation or fire risk
- Blind rivets allow single-sided access
- Joint behaviour under load is predictable and well documented
Limitations
- Holes reduce plate strength, so efficiency never reaches 100%
- Heavier than a welded joint because of rivets and cover plates
- Requires hole preparation, adding a process step and cost
- Not inherently leak-tight; needs caulking or sealant
- Rivet holes act as stress raisers and fatigue crack initiation sites
- Very noisy, particularly hammer and pneumatic riveting
- Higher labour cost than welding for the same joint
- Permanent; removal destroys the rivet and may damage the parts
Frequently Asked Questions (FAQ)
1. What is the riveting process in simple words?
Riveting joins parts by passing a metal pin through aligned holes and deforming its free end to form a second head. The two heads clamp the parts together permanently. No heat or filler is needed, and the rivet expands to fill the hole as it is formed.
2. What are the main types of riveted joints?
The two basic types are lap joints, where the plates overlap, and butt joints, where the plates meet edge to edge and are covered by one or two straps. Each can be single, double or triple riveted, with rivets arranged in chain or zigzag patterns.
3. How do you calculate riveted joint efficiency?
Calculate the tearing resistance Pt = (p − d)·t·σt, the shearing resistance Ps = n·(π/4)·d²·τ, and the crushing resistance Pc = n·d·t·σc. Take the lowest of the three, divide by the solid plate strength p·t·σt, and multiply by 100.
4. Why is efficiency calculated using the lowest resistance?
Because the joint fails in whichever mode is weakest, exactly like a chain breaking at its weakest link. Using any higher value would predict a strength the joint cannot actually deliver.
5. What is Unwin’s formula?
Unwin’s formula estimates rivet diameter from plate thickness as d = 6√t in millimetres, for plates over 8 mm thick. The calculated value is rounded up to the nearest standard rivet size.
6. Why are rivets in double shear stronger?
In a double strap butt joint the rivet is cut across two planes instead of one, so its shear resistance is roughly doubled. The symmetric load path also removes the bending that weakens lap joints.
7. What is caulking in riveted joints?
Caulking is hammering the plate edges and rivet head margins with a blunt tool to burr the metal over and close leak paths. It was essential for making riveted boilers and tanks pressure-tight, and it needs plates at least about 8 mm thick.
8. Why is riveting still used when welding is stronger?
Because welding applies heat, which destroys the temper of heat-treated aluminium alloys used in aircraft. Riveting also joins dissimilar materials easily and can be inspected visually. In modern aluminium car bodies, self-piercing riveting has replaced spot welding for the same reason.
Conclusion
Riveting is the joining process that hands you a clear, calculable trade-off, and that is what makes it such good teaching material.
Drill a hole and you weaken the plate. Fill it with a rivet and you get some of that strength back but never all of it. Joint efficiency puts a number on exactly how much, and the calculation forces you to check three separate failure modes and accept the worst one.
A single riveted lap joint retains around 40 to 60% of the plate strength. Add rows and put the rivets in double shear and you can push past 90%. That is the whole design problem in one sentence.
Welding won the argument in boilers, bridges and ships, where heat causes no harm and a fused joint can match the parent metal. But it lost the argument wherever heat is the enemy heat-treated aluminium in aircraft, dissimilar materials, and now the aluminium car bodies that resist spot welding.
Next time you board an aircraft, look along the wing at those neat rows of flush rivets. Each one is a joint that could have been welded, in a material that would have been ruined by it.