A fastener is a part that mechanically joins two or more components. The types of fasteners fall into two main groups: threaded fasteners (bolts, screws, studs, nuts and threaded inserts) and non-threaded fasteners (washers, rivets, pins, retaining rings, keys and anchors). Fasteners are also classed by whether the joint can be taken apart: removable, semi-permanent or permanent. This page walks through each fastener type, the screw head and drive types, how a bolt differs from a screw and a stud, how to choose, and a worked shear example for a pin or rivet.
Types of fasteners: how they are classified
| Class | What it means | Examples |
|---|---|---|
| Removable (temporary) | Can be undone and refitted many times without damage | Bolts and nuts, machine screws, studs, clevis pins, circlips |
| Semi-permanent | Can be removed, but with effort or some damage, or only a few times | Self-tapping screws in plastic, split pins, roll pins, taper pins, nyloc nuts |
| Permanent | Can only be removed by destroying the fastener | Solid and blind rivets (welding and adhesives also make permanent joints, but are not fasteners) |
The second split is by thread. Threaded fasteners clamp parts together by the wedge action of a helical thread. Non-threaded fasteners hold by deformation (rivets), by fitting into holes or grooves (pins, keys, rings) or by spreading load (washers).
Threaded fasteners
Bolts
A bolt passes through clearance holes in the parts and is tightened with a nut. Hex, carriage, flange, U, J, eye and T-head bolts, bolt parts and grades such as 8.8 and 10.9 are covered in detail on the types of bolts page.
Types of screw fasteners
A screw goes into a tapped hole, or cuts or forms its own thread, and is tightened by turning its head. The common fastener screw types are:
| Screw type | How it holds | Typical use |
|---|---|---|
| Machine screw | Uniform thread into a tapped hole or a nut; usually small, M2 to M10 | Electrical panels, appliances, instruments |
| Cap screw (hex or socket head) | Precision thread into a tapped hole; socket head cap screws are often class 12.9 | Machine parts, jigs, fixtures, engine covers |
| Set screw (grub screw) | Threaded full length, often headless; its point presses on a shaft | Locking pulleys, collars and gears to shafts |
| Self-tapping screw | Hardened thread that forms or cuts its own thread in a pilot hole | Sheet metal and plastic housings |
| Self-drilling screw | Drill-bit point drills the hole, then the thread forms in one step | Roofing sheets and purlins, steel framing |
| Wood screw | Tapered or coarse thread that grips wood fibre | Furniture, joinery, doors |
| Sheet-metal screw | Sharp, fully threaded, coarse pitch | Ducting, thin panels |
| Lag screw (lag bolt) | Large hex-head wood screw, driven with a spanner | Heavy timber, fixing brackets into wood |
Screw head types and drive types
The head decides how the screw sits: pan and button heads sit proud, cheese heads are tall and cylindrical, countersunk (flat) heads sit flush in a coned hole, and raised countersunk heads leave a small dome. The drive is the recess or shape the tool engages:
| Drive | Shape | Plus points | Weak points |
|---|---|---|---|
| Slotted | Single straight slot | Simplest, any flat blade fits | Driver slips out sideways; low torque |
| Phillips (PH) | Cross with tapered recess | Self-centres, fast with power tools | Driver “cams out” under high torque |
| Pozidriv (PZ) | Cross plus four small ribs between the arms | Less cam-out than Phillips, more torque | A Phillips driver fits badly and damages it |
| Torx (hexalobular) | Six-lobed star | Very little cam-out, high torque, long bit life | Needs the exact size bit (T10, T20, T30 and so on) |
| Hex socket (Allen) | Hexagonal hole | High torque in a small head, reachable in deep bores | Small sizes round out if the key is worn |
| External hex | Hex head for a spanner or socket | Highest torque | Needs space around the head |
A student tip: PZ screws have four fine lines stamped between the arms of the cross, which is how you tell them apart from Phillips.
Studs
A stud is a headless rod threaded at both ends, or along its full length. One end stays screwed into a tapped hole, and a nut on the other end clamps the part. Engine cylinder heads and pipe flanges use studs because repeated removal then wears the steel nut, not the tapped thread in the casting.
Nuts
A nut is the internally threaded partner of a bolt or stud. The main kinds:
- Hex nut: the general-purpose nut; IS 1364 covers the common hex bolts, screws and nuts.
- Nyloc (nylon insert lock) nut: a nylon ring grips the thread and resists loosening under vibration.
- Castle (castellated) nut: slots on top take a split pin through a hole in the bolt, used on wheel hubs and steering joints.
- Wing nut: two wings for hand tightening, for covers and clamps that are opened often.
- Flange nut: built-in washer face, often serrated, spreads load and resists loosening.
- T-nut: slides into a machine table T-slot, or is pressed into wood, to give a threaded anchor point.
The nut vs bolt page covers nut grades and how a nut and bolt share load.
Threaded inserts
An insert gives a strong steel thread in a soft or thin material. Helical coil (wire thread) inserts repair stripped threads in aluminium castings, heat-set or moulded-in brass inserts go into plastics, and rivet nuts are fitted blind into sheet metal to give a thread where a nut cannot be reached.
Non-threaded fasteners
Washers
- Plain (flat) washer: spreads the clamping load and protects the surface under the nut or head.
- Spring washer: a split, twisted ring. It is widely used, but transverse vibration (Junker) tests show it does little to stop vibration loosening once the joint is fully tightened.
- Tooth (star) washer: internal or external teeth bite into the nut and part; also used to make electrical earth contact through paint.
- Belleville (disc spring) washer: a shallow cone that acts as a stiff spring, keeping clamp load when parts settle, expand with heat or creep.
Rivets
A rivet is a permanent fastener: a plain pin with a head, placed through the holes and then deformed to form a second head. Main types:
- Solid rivet: hammered or squeezed, needs access to both sides. Used on aircraft skins, older bridges and boilers, and shipbuilding.
- Blind (pop) rivet: fitted from one side. Pulling the mandrel expands the far end, then the mandrel snaps off. Used on sheet metal, ducting, bus bodies and signboards.
- Semi-tubular rivet: a hollow tip that curls over with less force. Used in brake linings, hinges and leather goods.
Welded and high-strength bolted joints have replaced hot riveting in most Indian structural steelwork, but rivets remain common in aircraft and in sheet metal fabrication.
Pins
- Dowel pin: hardened, precise pin that locates two parts in exact alignment; the bolts do the clamping.
- Taper pin: tapered 1 in 50, driven in for a tight, self-holding fit; used to fix hubs and levers to shafts.
- Split (cotter) pin: a doubled wire pin whose legs are bent apart after fitting; locks castle nuts and clevis pins.
- Clevis pin: a headed pin with a cross hole for a split pin, used as a hinge in a clevis joint such as a brake or clutch linkage.
- Roll (spring) pin: a slotted steel tube that compresses as it is driven in and holds by spring pressure.
In machine design, “cotter” also means a flat tapered bar that joins two rods in a cotter joint, which is a different part from the split pin.
Retaining rings
Retaining rings (circlips) snap into a groove to stop a bearing, gear or pin sliding along its shaft or bore. External rings fit on shafts and internal rings fit in housings; both are fitted with circlip pliers. E-clips push on sideways and suit small shafts.
Keys
A key sits half in a keyway in the shaft and half in the hub, so a pulley, gear or coupling turns with the shaft. The common types are the sunk rectangular (parallel) key, the gib-head taper key, the Woodruff key (a half-disc, common in automotive shafts) and splines, which act as many keys cut into the shaft.
Anchors
Anchors fix things to concrete, brick or plasterboard. Plastic wall plugs expand as a screw goes in, sleeve and wedge anchors expand steel into concrete as the nut is tightened, chemical anchors bond a stud into the hole with resin, and cast-in foundation bolts are set into wet concrete.
Bolt vs screw vs stud
| Feature | Bolt | Screw | Stud |
|---|---|---|---|
| Head | Yes | Yes (except set screws) | No |
| Needs a nut | Yes | No | Yes, on the free end |
| Goes into | Clearance holes | Tapped hole or forms its own thread | One end in a tapped hole |
| Tightened by | Turning the nut | Turning the head | Turning the nut |
| Best for | Through joints with access to both sides | Blind holes, one-sided access | Parts removed often from soft castings |
How to choose a fastener
- Will the joint be taken apart? Often: bolts, screws or studs. Never: rivets.
- Access to both sides? If not, use a screw into a tapped hole, a blind rivet, a rivet nut or an anchor.
- Load type. Tension and clamping suit bolts and cap screws. Pure shear across a pivot suits pins. Torque from a shaft to a hub suits keys or splines.
- Vibration. Add proper preload first, then nyloc or flange nuts, split pins with castle nuts, or thread-locking adhesive.
- Base material. Soft metal or plastic needs a longer thread engagement or an insert; wood needs wood or lag screws; masonry needs anchors.
- Corrosion. Zinc-plated steel indoors, hot-dip galvanised or stainless (A2, A4) outdoors and near the coast. Avoid pairing stainless or copper with aluminium in wet conditions without insulation.
Worked example: shear capacity of a pin or rivet
A pin or rivet loaded across its axis fails in shear. Its capacity is:
F = τ × n × πd2/4
where τ is the allowable shear stress, d the pin diameter and n the number of shear planes: n = 1 for a lap joint (single shear) and n = 2 for a clevis or double-strap butt joint (double shear).
Take a 12 mm steel pin with an allowable shear stress τ = 80 MPa (80 N/mm2).
- Area of one shear plane: A = π × 122 ÷ 4 = π × 144 ÷ 4 = 113.1 mm2
- Single shear: F = 80 × 1 × 113.1 = 9,048 N = 9.05 kN
- Double shear: F = 80 × 2 × 113.1 = 18,096 N = 18.1 kN
So the same pin carries twice the load in double shear, because the load is shared across two cross-sections. To carry 40 kN in a lap joint, you would need 40 ÷ 9.05 = 4.42, so 5 rivets in single shear, but only 40 ÷ 18.1 = 2.21, so 3 rivets in double shear. A full joint design also checks bearing (crushing) of the plates and tearing of the plate between holes, and a structural design follows the rules and stresses in IS 800 rather than a single assumed τ. Fastened joints are part of the design of machine elements course in B.Tech mechanical engineering, and NPTEL has free lectures on riveted and bolted joints.
FAQs
What are the main types of fasteners?
Threaded fasteners (bolts, screws, studs, nuts and threaded inserts) and non-threaded fasteners (washers, rivets, pins, retaining rings, keys and anchors). They are also grouped as removable, semi-permanent or permanent.
What are the types of screw fasteners?
Machine screws, cap screws, set (grub) screws, self-tapping screws, self-drilling screws, wood screws, sheet-metal screws and lag screws. Each comes in several head shapes and drive types such as slotted, Phillips, Pozidriv, Torx and hex socket.
Which fasteners are permanent?
Rivets, both solid and blind, are the main permanent fasteners, because they must be drilled or cut out to remove them. Welding and adhesive bonding also make permanent joints, but they are joining methods rather than fasteners.
What is the difference between single shear and double shear?
In single shear the pin is cut across one plane, as in a lap joint. In double shear it is cut across two planes, as in a clevis, so the same pin carries twice the load.
What is the difference between a bolt and a screw?
A bolt passes through clearance holes and is tightened by turning a nut. A screw goes into a tapped hole or forms its own thread and is tightened by turning its head.