Machine Screws and Nuts

§1The small end of fastening

A machine screw is a small, fully threaded fastener driven by a screwdriver or key rather than a spanner — threaded either into a tapped hole or into a nut.

The name draws a line worth keeping: a machine screw runs into a machined thread, whether tapped in the part or cut in a nut, unlike the self-threading screws of their own page, which cut their own. Machine screws are typically small — M2 to M10, or the numbered inch gauges of §4 — threaded along their whole length, and they hold together the vast bulk of manufactured equipment: enclosures, panels, instruments, appliances, electronics. Everything from the fastener pages applies to them unchanged — the thread systems, the preload, the stress area — but at this size the decisions that dominate are different: what shape of head suits the joint (§2), and what drive the head carries (§3), because at small sizes it is the driver, not the screw, that usually fails first.

§2Head styles

The head’s job is to bear on the work and spread the clamping load, and its shape is chosen for how the screw must sit and how much bearing area it needs.

§3Drive types

The recess in the head decides how much torque can be delivered before the driver slips — and at machine-screw sizes that limit, not the screw’s strength, is usually what caps the tightening.

The slotted drive is the oldest and the worst: the blade has no location, cams out of the slot readily, and cannot take much torque — but it is easily made, easily cleaned and can be driven with an improvised blade. The cross-recess family fixed the location problem: Phillips was designed to cam out deliberately at high torque, protecting assembly-line work from over-tightening in the days before torque control — a feature now widely mistaken for a defect; Pozidriv refined the geometry to resist cam-out and carry more torque, and the two look alike but should not be mixed, since a Phillips driver in a Pozidriv screw chews the recess. The hex socket takes far more torque, being a positive form grip with no cam-out, and is why socket head cap screws (their own page) can be tightened to their full class 12.9 capacity. Torx (hexalobular) goes further still: its rounded lobes spread the contact stress and eliminate cam-out almost entirely, giving the highest torque and the longest bit life — which is why it has largely taken over production assembly. As a rule: the better the drive resists cam-out, the more of the screw’s strength you can actually use.

§4Numbered sizes

Below ¼ inch, inch machine screws are sized by gauge number rather than a fraction — and the numbers are not arbitrary but follow a simple linear formula.

§5Why a nut is the height it is

A standard hex nut is about 0.8 diameters tall, and that is not a styling choice — it is the height at which the bolt will break in tension before the nut’s threads strip.

The design intent is to make the bolt the weak link, for the same reason the inserts page gives: a broken bolt is visible, predictable and replaceable, whereas a stripped thread fails unpredictably and takes the nut or the casting with it. So the nut’s height is set by the thread-stripping calculation of that page. For an M10 class 8.8 bolt with its 46.4 kN tensile capacity, the engagement page computes that roughly one diameter of steel thread is needed to just match it — and a standard M10 nut is 8 mm tall, which is 0.8D, with the extra margin coming from the nut being made of a harder grade than plain mild steel and matched to the bolt’s class. That last point matters practically: a nut carries its own strength grade, and pairing a class 8 nut with a class 12.9 bolt puts the weak link back in the nut, defeating the whole arrangement. This is also why a thin (half) nut is not a substitute for a standard one in a loaded joint — at half the height it strips at roughly half the load — and why a nut should never be filed down to fit.

§6Locking nuts

A nut with proper preload rarely loosens; where preload cannot be relied on, a locking nut adds a second mechanism to hold it.

The first defence is always adequate preload, as the torque-and-tension page argues — most loosening is an under-tightening problem. Where that is not enough, the options divide by mechanism. Prevailing-torque nuts resist turning by friction along the thread rather than by clamping: the nyloc, with a nylon collar the bolt cuts its own thread into, is the common choice but is limited by the nylon’s temperature range and should not be reused indefinitely; all-metal distorted-thread nuts do the same job with a deformed section and survive heat and reuse. Positive locking refuses to rely on friction at all: a castellated nut with a split pin through a drilled bolt is mechanically incapable of turning, which is why it holds wheel bearings and control linkages where a failure would be serious. Free-spinning devices — serrated flange nuts, spring and tab washers — bite or key against the joint face once seated. And the jam nut pair works by preloading two nuts against each other; note that the thin nut goes underneath, against the joint, with the full nut above it, since the upper nut carries the load once the pair is jammed — the reverse of how it is usually fitted.

§7Quick reference

The working core of the page on one card rack.