Buttress Threads

§1The one-way bargain

Every symmetric thread pays for its two-way ability on both flanks at once. The buttress form asks a sharper question — which way does this joint actually load? — and spends everything on the answer.

A vice closes; a press presses; a jack lifts; a breech holds pressure from one side; a lid stays down. In each, the thread’s working flank sees load in one direction for the whole service life, and the opposite flank exists only to be dragged past during motion. The Acme page’s trapezoid treats both flanks equally anyway, because a lathe leadscrew genuinely drives both ways. The buttress refuses the symmetry: it gives the pressure flank the geometry a power thread dreams of — nearly perpendicular to the axis, so the load passes almost straight along the screw with hardly any wedge — and gives the trailing flank a steep 45° slope whose only jobs are to make the tooth a stubby, shear-proof cantilever and to let a cutting tool in and out of the form. It is the section’s recurring dial (the flank angle) turned asymmetrically for the first time: one flank set near the square thread’s zero, the other set steep for strength — and §3 prices exactly what that buys.

§2The form

The classic American buttress leans its pressure flank 7° and its trailing flank 45°; the metric saw-tooth thread runs 3° and 30° — same idea, different accents.

The 7° on the pressure flank is not quite the square thread’s 0°, and deliberately so: a few degrees of lean lets the flank be cut and ground with clearance, gives the mating surfaces a whisper of centring, and — as on every form in this section — makes wear adjustable rather than terminal. The 45° behind it transforms the tooth’s structure: where a square thread’s rib is a slender rectangular cantilever loaded at its tip, the buttress tooth is a broad-based wedge, thickest exactly where the bending moment is highest, so the form carries brutal loads on coarse pitches without shearing teeth — the reason the largest screws in engineering, on presses and jacks measured in hundreds of tonnes, are buttress cut. The metric saw-tooth S form takes the same philosophy further, standing its pressure flank at just 3° with a 30° back; the proportions differ but every conclusion on this page transfers. Designations follow the local convention with the form named — and, as with Acme against Tr, the American and metric forms are cousins that never intermarry.

§3The flank-angle arithmetic

The pressure-flank angle sets how hard the thread pushes its nut radially outward — and lining the families up makes the buttress’s case in one column of tangents.

§4Square efficiency, kept

Friction on a leaning flank is friction magnified by the lean — and at 7° the magnification all but vanishes, which is the quiet half of the buttress’s case.

The mechanism is the one the systems and Acme pages used: the load presses the nut onto flanks that lean φ from square, so the normal force — and with it the friction — is inflated by 1/cos φ. At the vee’s 30° the surcharge is 15.5%; at the trapezoid’s 15°, 3.5%; at the buttress’s 7°, 0.75% — friction effectively at its bare sliding value, which is why the table’s η column shows the buttress within a fifth of a point of the square ideal. Everything the Acme page said about living with a power screw therefore transfers intact, with the percentages nudged in the buttress’s favour: the same T = F(d₂/2) tan(λ + ρ′) sizing, the same self-locking test, the same 1.7-odd units of heat per unit of lift at small lead angles, the same bronze-nut-and-oil economy. What does not transfer is the trapezoid’s indifference to direction — the buttress’s numbers hold only while the load stays on the 7° flank, and §6 is about the day it does not. Within its direction, though, the conclusion of the table stands plainly: the buttress is the square thread, made makeable and made strong.

§5Where buttress serves

Look for the buttress wherever the load is heavy, the direction is fixed, and the nut’s hoop strength is precious.

The classic homes are the big one-way machines: screw presses and jacks, where §3’s radial relief lets the nut survive; vice and clamp spindles in serious sizes, closing hard and opening light; artillery breech blocks, the form’s famous ancestor, holding chamber pressure through interrupted buttress segments that lock in a part-turn; and hydraulic and aircraft actuator screws whose service load is overwhelmingly one-sided. Two humbler relatives complete the picture. Thin-walled tube and casing connections use buttress-style forms because a hoop-stressed pipe wall is precisely a nut that cannot afford 0.577 F of burst. And the moulded closure — the plastic bottle cap — carries a rounded buttress-style thread for the same reasons scaled down: the load (cap pulled down onto its seal, pressure pushing up from inside) is one-way, the plastic “nut” is thin-walled, and the asymmetric form moulds and strips from its tool cleanly. It is a satisfying pair of bookends: the same geometry holding a field gun’s breech and a lemonade bottle’s fizz, for identical reasons.

§6The reverse-load caveat

Everything this page has praised applies to one flank. Load the other, and the buttress becomes the worst thread in the section.

Reverse the load and contact jumps across the backlash onto the 45° trailing flank — a flank steeper than anything a symmetric thread would tolerate. The §3 arithmetic runs immediately and unkindly: the radial push becomes tan 45° = 1.000 F — the nut is burst outward with a force equal to the axial load, nearly double the 60° vee’s wedging and eight times the form’s own working figure. Friction inflates by 1/cos 45° = 1.41, efficiency collapses, and the load crossing the backlash arrives on that flank as an impact, on a surface finished as a clearance face, not a bearing. The rule is therefore absolute and defines the form: a buttress thread must never carry sustained or repeated reverse load. Occasional light reversal — backing a press off, unscrewing the cap — is what the 45° flank’s strength is for; a duty cycle that genuinely loads both ways belongs to the Acme page, full stop. And the same asymmetry writes the assembly rule: a buttress screw or segment has a handed orientation, and fitting one backwards installs the failure mode. The buttress is the section’s purest specialist — supreme in its direction, and only in it.

§7Quick reference

The working core of the page on one card rack.