Electrification & Early Modern (1870–1920)Mechanical Fasteners & Precision Manufacturing

Sundback Interlocking Scoop Zipper

US 1,219,881

Nested Scoop Teeth, Staggered Cords, and Converging Y-Slider Cam Fastening

Inventor(s)Gideon Sundback

Grant DateMarch 20, 1917

Filing DateAugust 27, 1914

LocationMeadville, Pennsylvania

Gideon Sundback's 1917 Separable Fastener patent established the universal modern zipper: identical interchangeable cup-shaped metal scoops clamped along reinforced fabric tape cords that smoothly interlock and disengage through a Y-shaped sliding cam.

USPTO PDF

Engineering Analysis & Physical Principles

How It Works: Step-by-Step Mechanical & Physical Breakdown

Before Sundback's 1914 breakthrough, continuous garment fasteners like Whitcomb Judson's 1893 'Clasp Locker' and early hook-and-eye chains were unreliable, bulky, and prone to popping open under minimal transverse bending. Sundback revolutionized fastener engineering by replacing complex hooks with identical, precision-stamped metal scoops. Each scoop features a rounded convex projection on one face and a complementary concave hollow recess on the reverse. Crimped at a high linear density (10–11 teeth per inch) along corded textile tapes in a staggered pattern, the teeth are forced into alternating nested engagement by a Y-shaped sliding cam, creating an airtight, flexible closure that stays locked even when folded completely double.

The Core Breakthrough Mechanism

When the Y-shaped slider advances in the closing direction, its converging interior side walls press opposing staggered teeth together at a precise engagement angle (θ ≈ 15°–20°). As each tooth enters the slider's narrow neck, its rounded upper projection nests into the hollow underside recess of the preceding tooth on the opposite stringer. The crimped corded edges of the fabric tape absorb lateral tensile stress (F_pull), while the nested pocket geometry converts transverse peel forces into compressive shear across the metal scoops. Sliding in the opposite direction drives the internal diamond wedge between the tooth rows, levering each nested projection out of its pocket sequentially.

Interactive Real-Time Physical Simulation

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INITIALIZING THREE.JS WEBGL SIMULATION...

Host-Model Telemetry/Computed Readout

Interlocking Scoop Cam Kinematics, Bending Flexibility & Burst Resistance

Engaged Scoops

Source

42 / 65teeth[1]

Cam Wedge Force

Modern Model

18.8N[ML/T²]

Burst Resistance

Modern Model

1046.4N[ML/T²]

Tape Core Strain

Modern Model

2.4%[1]

Lock Status

Source

SECURE LOCKstate[1]

Slider Position65 %

Pull Tab Force15 N

Transverse Tension40 N

Bending / Folding Angle25 deg

Tooth Density11 TPI

Claim 1 Stagger Alignment1 state

Detailed Component Architecture

1Interchangeable Cup-Shaped Scoops

Precision-stamped metal teeth with top convex projections and bottom concave nesting pockets.

Each scoop is stamped from sheet metal with a truncated conical projection (10) on its upper surface and a matching internal socket (11) on its underside. The teeth are identical on both stringers, eliminating asymmetrical left/right manufacturing dies and ensuring uniform load distribution.

19th-C. Term: locking members of elongated cup shapeModern: zipper teeth / interlocking scoops

2Corded Fabric Stringer Tape

Textile tape reinforced with cylindrical stitched cords providing a compressive anchor for metal jaws.

Woven fabric stringers (1) have longitudinal cords (2) stitched along both faces. The spread metal clamping jaws (17) are crimped around the corded bead in a die press, distorting the textile fiber to lock the tooth permanently in place without slipping under longitudinal tension ( $F_{\text{shear}} > 300\text{ N}$ ).

19th-C. Term: beaded or corded edgeModern: beaded zipper tape cord

3Y-Channel Cam Slider

Dual-plate sliding cam with converging guide channels and central separating diamond wedge.

The slider (6) consists of upper and lower stamped flanges (7) spaced apart to allow tape passage. A central separating diamond wedge (8) splits the closed chain when pulled backward, while the outer converging side walls force opposing scoops to rotate and seat into full engagement when pulled forward.

19th-C. Term: sliding cam operating deviceModern: slider body and pull tab

4Wedge Top and Bottom Stops

Mechanical limit stops preventing slider derailment and unintended chain splitting.

Bottom stop links (4) permanently bridge the two stringers at the base, while top stop members (5) abut against each other inside the slider throat. Because the combined height of the abutting top stops exceeds the exit channel width, the slider is physically blocked from running off the tape.

19th-C. Term: stop members and fastening linksModern: top stops and bottom box / pin

Engineering Principles & Equations

Governing Equations & Engineering Principles

Authored explanation paired with its stated mathematical relation

Cam Wedge Normal Force Resolution

Wedge Cam KinematicsClaim 5

Mathematical Governing Law

\htmlClass{eq-term eq-term-engage_force eq-term-emerald}{\htmlData{var=engage_force}{\textcolor{#059669}{F_{\text{engage}}}}} = \frac{\htmlClass{eq-term eq-term-pull_force eq-term-sapphire}{\htmlData{var=pull_force}{\textcolor{#2563eb}{F_{\text{pull}}}}}}{2\sin\htmlClass{eq-term eq-term-wedge_angle eq-term-amber}{\htmlData{var=wedge_angle}{\textcolor{#d97706}{\theta}}} + \htmlClass{eq-term eq-term-friction_coeff eq-term-teal}{\htmlData{var=friction_coeff}{\textcolor{#0d9488}{\mu}}}}

Terms:

Plain English DecoderHover or tap any highlighted phrase

The lateral exerted on the scoops scales with the divided by twice the sine of the plus the .

F_{\text{engage}}

Lateral Engagement Normal Force

Compressive force squeezing opposing scoops into nested alignment

Newtons (N)[M L T^-2]

The converging wedge side walls of the Y-slider act as inclined planes, multiplying pull tab force into high lateral compressive force that drives the rounded scoops together.

Live Physical Value:

18.8 N

Physical Principle & Engineering Insight

Sundback's Y-channel slider converts axial pull into transverse squeeze, seating each projection into its mating hollow socket without requiring manual alignment.

Historical Context: Claim 5 protects the dual-plate slider with diverging guide channels and central diamond wedge.

Interlocking Scoop Burst Resistance

Mechanical Fastener StrengthClaim 2

Mathematical Governing Law

\htmlClass{eq-term eq-term-burst_force eq-term-emerald}{\htmlData{var=burst_force}{\textcolor{#059669}{F_{\text{burst}}}}} = 2\,\htmlClass{eq-term eq-term-num_teeth eq-term-sapphire}{\htmlData{var=num_teeth}{\textcolor{#2563eb}{N_{\text{teeth}}}}}\,\htmlClass{eq-term eq-term-shear_area eq-term-amber}{\htmlData{var=shear_area}{\textcolor{#d97706}{A_{\text{shear}}}}}\,\htmlClass{eq-term eq-term-shear_strength eq-term-amethyst}{\htmlData{var=shear_strength}{\textcolor{#9333ea}{\tau_{\text{max}}}}}\,\cos\left(\frac{\htmlClass{eq-term eq-term-flex_factor eq-term-teal}{\htmlData{var=flex_factor}{\textcolor{#0d9488}{\theta_{\text{flex}}}}}}{2}\right)

Terms:

Plain English DecoderHover or tap any highlighted phrase

Total transverse equals twice the number of multiplied by the single tooth , the , and the .

F_{\text{burst}}

Total Chain Burst Resistance

Maximum transverse tensile pull load across the closed zipper before tooth pop-out or cord tear

Newtons (N)[M L T^-2]

Because each nested cup projection overlaps behind the shoulder of the adjacent scoop, transverse loads put the metal scoops into pure mechanical shear.

Live Physical Value:

1046.4 N

Physical Principle & Engineering Insight

Unlike Judson's rigid hooks that popped open when folded, Sundback's cup sockets let adjacent teeth pivot like ball-and-socket joints while maintaining full shear engagement.

Historical Context: Claim 2 and the 1932 disclaimer protect the thin, transversely elongated scoops that enable sharp bending without automatic opening.

Cam Wedge Mechanical Advantage & Normal Force ResolutionAuthored Principle 1

Stated relation

F_{\text{engage}} = 2 F_{\text{pull}} \cot\theta

The converging flanges of the slider act as inclined wedge planes. Pulling the slider with force

F_{\text{pull}}

generates lateral compressive normal forces

F_n = \frac{F_{\text{pull}}}{2 \sin\theta}

that squeeze the staggered scoops into positive nested alignment.

Interlocking Scoop Peel & Burst ResistanceAuthored Principle 2

Stated relation

F_{\text{burst}} = 2 N \mu \sigma_y A_{\text{shear}}

Under transverse tensile load across the closed zipper, the nested projections cannot slip out because the overlapping cup lips create an interference fit. Disengagement requires yielding the metal shear area

A_{\text{shear}}

or tearing the corded tape core.

Kinematic Pitch Staggering & Stagger Phase ShiftAuthored Principle 3

Stated relation

p_{\text{stagger}} = \frac{1}{2} p_{\text{tooth}} = \frac{1}{2 f_{\text{linear}}}

To allow single-form scoops to mesh continuously, teeth on the right tape are offset longitudinally by exactly half a pitch (

p/2

) relative to the left tape, ensuring each tooth nests symmetrically between two opposing elements.

Interactive Schematic Sheet (1)

Overall perspective view of the Hookless No. 2 fastener showing fabric stringers (1), corded edges (2), interlocking teeth (17), slider cam (6), pull tab (16), top stops (5), and bottom link (4).

1.00x

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Why It Still Matters

Gideon Sundback's 1917 design is the exact mechanical architecture used in virtually every metal and molded plastic zipper manufactured worldwide today (over 45 billion units annually by YKK, Talon, and others). It transformed apparel, military flight gear, luggage, aerospace pressure suits, and surgical closures by providing an instantaneous, reusable, high-strength linear mechanical fastener.

Legal Claims Decoder (11 Numbered Claims)

Compare dense legalistic claims directly with decoded plain-English functional specifications.

Claim #1Independent Master Claim

1/11

Verbatim Historical Legal Text

“1. A fastener comprising a pair of flexible stringers, interlocking members secured at one end thereto in staggered relation, each member having at the free end a rounded recess on one side and a corresponding projection on the opposite side, the recessed side and the end surface of the projection of each member meeting in an edge and constituting guiding means enabling said members to ride one on the other in interlocking.”

Plain English Engineering Translation

Asserts broad patent ownership over a separable slide fastener comprising a pair of flexible stringers with staggered interlocking members, each having a rounded recess on one face and a matching projection on the reverse, with guide surfaces that allow teeth to ride smoothly over each other during progressive cam engagement.

Key Protected Innovations:

Staggered interchangeable teethConvex projection with concave nesting socketCamming guide edge for smooth interlock

Historical Legal Impact:

The foundational claim that established legal monopoly over modern interlocking scoop fasteners with nested cup geometry.

The Historical Bottleneck

In the late 19th century, clothing and shoe closures relied entirely on buttons, laces, and hooks-and-eyes, requiring tedious individual fastening. Whitcomb Judson patented the 'Clasp Locker' in 1893 (US 504,038) and the 'C-curity' fastener in 1905, but both designs used complex hook-and-socket links that jammed constantly, rusted, were difficult to manufacture, and burst open when clothing bent.

Why Prior Art Failed

•Judson's 1893 Clasp Locker: crude stamped clasp hooks that jammed in the slider and lacked transverse bending flexibility.
•Judson's 1905 C-curity: hook-and-eye chains that popped open unexpectedly when bent across the knee or torso.
•Sundback's earlier 1909 'Plako' fastener: hook-and-eye design requiring stiff fabric backing, with teeth that pulled off the tape under moderate tension.
•Asymmetrical manufacturing: prior designs required two completely different dies and machines for left and right tape teeth.

The Breakthrough Insight

“Sundback realized that reliable continuous fastening required four synchronized innovations: (1) identical, interchangeable cup-shaped scoops stamped from single dies, (2) high tooth density (10–11 teeth/inch) so individual tooth forces are minimal, (3) clamping the metal jaws directly onto thickened textile cords sewn to the tape edge, and (4) convex-to-concave rounded scoop nesting that permits full transverse flexing and folding without unmeshing.”

Patent Wars & Legal Litigations

Vs. Whitcomb Judson / Universal Fastener CompanyInfringement Challenge

Rival Claim & Defense:

Universal Fastener Co. claimed ownership over sliding shoe clasps, but their commercial products were market failures due to persistent jamming.

Litigation Conflict:

Sundback was hired as chief engineer by Universal Fastener (reorganized as Hookless Fastener Co. in Meadville, PA). After the Plako failed, Sundback spent four years developing the 'Hookless No. 2' (US 1,219,881).

Final Resolution & Judicial Outcome:

Hookless Fastener Company patented Sundback's design and built automated manufacturing machinery, establishing absolute market dominance.

Vs. B.F. Goodrich Company (Trademark Coining)Infringement Challenge

Rival Claim & Defense:

In 1923, B.F. Goodrich ordered Sundback fasteners for their new rubber galoshes and trademarked the onomatopoeic name 'Zipper' for their boots.

Litigation Conflict:

The public instantly associated 'zipper' with the slide fastener itself rather than Goodrich's rubber boots.

Final Resolution & Judicial Outcome:

Goodrich retained the trademark for boots, but 'zipper' became the universal generic name for slide fasteners worldwide.

After the Grant

Talon, Inc. produced over 500 million zippers per year by the mid-20th century. In 1932, Hookless filed a formal disclaimer narrowing Claims 1–3 to thin, flexible teeth, solidifying their patent monopoly against foreign imitators.

Civilizational Impact

Sundback's zipper became one of the most widely manufactured precision mechanical devices in human history. It fundamentally altered garment design, replacing buttons on trousers (1930s 'Battle of the Fly'), boots, jackets, and luggage. It was critical to World War II aviation flight suits and life vests, space suits (Apollo and Gemini airtight pressure zippers), and sterile medical closures.

Historical Fact

The word 'zipper' was not coined by Sundback, but by B.F. Goodrich executive Bertram Work in 1923, who loved the sound the slider made when pulling up rubber boots: 'Zip!'

Further Context

Sundback designed the automated 'S-L' (scrapless) manufacturing machine in 1914, which took a spool of Y-shaped brass wire, cut and stamped teeth, and crimped them onto cords at hundreds of teeth per minute.
The US military was the zipper's first major customer: during WWI, the US Navy purchased Sundback fasteners for canvas money belts and airtight flying suits.