§1Two opposite jobs
Cap screws and set screws are grouped together because both are driven by a hex key, but they do opposite things — one pulls two parts together, the other pushes one part against another.
A cap screw is a bolt in the fullest sense: it has a head, it passes through one part into a thread in another, and its purpose is clamping — stretching itself to preload the joint exactly as the torque-and-tension page describes. A set screw (grub screw) has no head at all, is threaded along its whole length, and sits entirely inside a tapped hole; it clamps nothing, and its purpose is locating — pressing its point against another part, most often to stop a hub, pulley or collar rotating or sliding on a shaft. The distinction is worth holding firmly, because it explains why one is the strongest fastener in the toolbox (§3) and the other one of the weakest holds in engineering (§6). They share a socket drive and a hex key; that is all.
Contents§2The socket head cap screw
The socket head cap screw is the machine designer’s standard fastener: a cylindrical head with an internal hex socket, made to a high strength class, and able to sit in a counterbore flush with the surface.
Its advantages come as a set. The hex socket takes far more torque than any screwdriver drive because it grips positively and cannot cam out (the machine screws page), so the screw’s full strength is actually usable — a hex key or bit reaches it in confined spaces where no spanner would fit, and needs no swing room around the head. The cylindrical head drops neatly into a counterbored hole, so it can finish flush or below the surface, giving a clean machine with nothing to catch — and the counterbore’s wall also supports the head. And the high strength class (§3) means a smaller screw does the work of a larger bolt, which matters when a machine is dense with fasteners. Variants cover the rest: button head for a lower profile with less torque capacity, countersunk socket for a truly flush finish, and shoulder screws with a precision unthreaded shank that serves as a pivot or a location dowel while the thread merely retains it.
Contents§3Why 12.9
Socket head cap screws are normally made to property class 12.9 — the top of the ordinary range — and it is worth computing what that buys and what it costs.
Decoding the class as the metric fasteners page does: 12.9 means 12 × 100 = 1200 N/mm² tensile, yielding at 0.9 of that, 1080 N/mm². For an M10, with its 58.0 mm² stress area, the yield load is 1080 × 58.0 = 62.6 kN, against a class 8.8’s 640 × 58.0 = 37.1 kN — 1.69 times as much load from an identically-sized screw. That is the whole case for the class: the same hole, the same spanner, two-thirds more clamping capacity, which lets a designer use fewer or smaller fasteners: the same hole and the same hex key deliver two-thirds more clamping capacity.
§4Set screws
A set screw is a headless, fully threaded screw that runs down inside a tapped hole and presses its point against a mating part — usually to lock a hub to a shaft.
The classic arrangement is a pulley, gear, collar or coupling with a tapped radial hole through its boss: the set screw is run in until its point bears on the shaft, and the reaction pushes the shaft against the opposite side of the bore. Nothing is clamped in the bolt sense — the screw is not stretched into a spring, and there is no preloaded joint — so the hold comes entirely from the friction generated by that radial force, plus whatever indentation the point makes in the shaft. Being headless, the screw sits wholly inside the boss with nothing projecting, which is why it suits rotating parts where a protruding head would be a hazard and an imbalance. The point shape decides how it grips (§5), and the arithmetic of how much it actually holds (§6) is the part most often skipped — and the reason set screws come loose so famously often.
Contents§5Set screw points
The end of the set screw is its working face, and the shape it is ground to determines how it grips, how much it damages the shaft, and whether it can be undone again.
| Point | Grips by | Suits |
|---|---|---|
| Cup | a sharp annular rim that bites in | the general default — good hold, marks the shaft |
| Cone | digging deep into a matching drilled spot | permanent location, highest hold, damages the shaft |
| Flat | friction over a flat face only | a shaft that must not be marked, or frequent adjustment |
| Dog | a projecting plain nose entering a hole or flat | positive location — the nose does the work, not friction |
| Oval | a rounded nose on a shaft flat | parts adjusted often; light marking |
| The pattern is a trade of hold against damage. A cup point bites and holds well but raises a burr that can make the hub hard to remove; a flat point holds only by friction but leaves the shaft clean. The two that escape the trade are the cone and dog points, because both engage a feature — a drilled spot or a milled flat — rather than relying on friction, and so are the only ones that give positive, non-slipping location. That is the practical answer to §6’s problem: if a set screw must be trusted, give it something to key into. | ||
§6What a set screw can hold
Work out the torque a set screw actually transmits and the answer is sobering — it is a small fraction of what the shaft itself can carry, which is why a key does the real work.
Take an M6 set screw seated at a reasonable 4 N·m. Its axial force follows the same torque relation as any fastener, F = T/(K·d) = 4 ÷ (0.20 × 0.006) = 3333 N pressing on the shaft. With a friction coefficient of 0.15, the tangential force it can resist before the hub slips is 0.15 × 3333 = 500 N, and on a 20 mm shaft — a radius of 10 mm — that is a holding torque of 500 × 0.010 = 5.0 N·m. Now the shaft itself: from the shafts page, T = τπd³/16, so at a modest 60 N/mm² allowable shear a Ø20 shaft carries 94.2 N·m. The set screw holds 5.3% of that — the shaft is 19 times stronger than the thing supposedly locking a hub to it. The conclusion is not that set screws are useless but that they are light-duty locating devices, not torque-transmitting elements: fine for a collar, an instrument knob, a light pulley or stopping axial drift, and wholly inadequate for driving torque, which is what the keys and keyseats of the machine elements section exist for. Where a set screw must hold more, give it a flat to bear on, use two screws at 90°, or — properly — fit a key and let the set screw merely retain it.
§7Quick reference
The working core of the page on one card rack.
Two jobs
cap screw pulls (clamps)
set screw pushes (locates)
Cap screw
hex socket · no cam-out
counterbores flush
Class 12.9
1200 tensile / 1080 yield
M10 → 62.6 kN (1.69 × 8.8)
Points
cup default · flat spares shaft
cone/dog = positive location
Reality
M6 on Ø20: holds ~5 N·m
shaft carries 94 — use a key