Industrial Revolution & Mechanical Age (1760–1870)Mechanical Locks & Security Systems

Yale Pin-Tumbler Cylinder Lock & Flat Bitted Key

US 48,475

Shear-Line Pin Alignment, Flat Corrugated Keyway, Rotating Plug, and Modular Threaded Mortise Cylinder

Inventor(s)Linus Yale, Jr.

Grant Date1865-06-27

Filing DateNot recorded

LocationShelburne Falls, Massachusetts

Linus Yale Jr.'s 1865 master patent revolutionized physical security worldwide by miniaturizing the pin-tumbler cylinder lock and pairing it with a small, lightweight flat bitted key. By dividing each tumbler into an upper driver pin and lower key pin that align at a cylindrical shear line, introducing anti-pick circumferential serrations, and driving the bolt through a lost-motion lazy-arm cam, Yale rendered heavy Victorian warded keys obsolete and established the universal architecture of modern commercial and residential door locks.

USPTO PDF

Engineering Analysis & Physical Principles

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

Before Linus Yale Jr.'s 1865 invention, doors and safes were secured by heavy iron warded locks or early lever tumbler locks that required massive, cumbersome keys weighing up to several pounds. Because the key had to reach entirely through the door thickness and physically throw the heavy iron bolt, larger and more secure locks required exponentially larger keys. Linus Yale Jr. broke this paradigm by separating the keyway cylinder from the bolt mechanism. By utilizing a compact cylindrical plug with five spring-loaded split pins elevated to a microscopic shear line by a featherweight flat steel key, Yale achieved unprecedented cryptographic permutation security in a lock that could be installed in doors of any thickness.

The Core Breakthrough Mechanism

The lock operates through five distinct mechanical stages governed by precision geometric tolerances: (1) In the resting locked state, helical compression springs push five upper driver pins across the cylindrical shear boundary between the stationary outer housing and the revolving plug, mechanically pinning the plug in place. (2) When the authorized flat steel key is inserted into the narrow keyway slot, its serrated bottom bittings directly lift the five bottom key pins against spring resistance. (3) Because the key notches match the varied lengths of the lower pins, the division lines between every lower key pin and upper driver pin align exactly flush with the cylindrical plug circumference (shear error $\Delta y_i < 0.09\text{ mm}$ ). (4) With all pins clearing the shear line, turning torque applied to the key rotates the plug smoothly. (5) As the plug rotates, an axial knob drives a lost-motion lazy-arm cam through a smaller angle (approx. $90^\circ$ ), throwing the heavy sliding deadbolt and deadlocking it against external forced retraction.

Interactive Real-Time Physical Simulation

Drag to rotate · Scroll to zoom · Shared controls update the displayed model

INITIALIZING THREE.JS WEBGL SIMULATION...

Host-Model Telemetry/Computed Readout

Mechanical Shear-Line Kinematics & Pin-Tumbler Dynamics

Shear Line Alignment

Aligned (Shear Cleared)Status[1]

Max Pin Shear Error

0.000 mmΔy_max[1]

Plug Rotation Angle

2.5°θ_plug[1]

Bolt Extension / Deadlock

0.5 mm x_bolt[1]

Pin Spring Force

2.93 NF_spring[1]

Theoretical Combinations

7,776 (6⁵)perms[1]

Key Blade Insertion Depth1 fraction

Turning Torque on Plug0.15 N·m

Detailed Component Architecture

1Revolving Plug & Eccentric Cylinder Housing

The inner cylindrical core that contains the keyway slot and lower pin chambers, mounted eccentrically inside an externally threaded brass housing.

The inner plug ( $R_{\text{plug}} = 6.35\text{ mm}$ ) revolves within a precision-reamed bore in the outer cylindrical casing C. The outer casing features external machine threads that screw directly into tapped nuts in the mortise lock case, enabling continuous adjustment for doors from $1.25\text{ in}$ to over $3.0\text{ in}$ thickness while remaining flush with the door escutcheon.

19th-C. Term: cylinder D eccentric to the tumbler-caseModern: Revolving Lock Core / Plug Cylinder

2Two-Piece Split Pin Tumblers & Compression Springs

Five vertical pin chambers each containing an upper driver pin, lower key pin, and top helical compression spring.

Each chamber houses a flat-ended driver pin I ( $L_d = 5.5\text{ mm}$ ) and a variable-length key pin J ( $L_k = 2.5\text{ to }6.5\text{ mm}$ ), loaded from above by a phosphor-bronze spring ( $k_s \approx 140\text{ N/m}$ ). When the division line reaches the shear line $R_{\text{plug}}$ , the shear impedance drops from infinite normal interference to pure journal friction ( $F_{\text{shear}} \to 0$ ).

19th-C. Term: tumblers or pins made in two pieces, I and JModern: Driver Pins & Key Pins (Pin Stack)

3Flat Serrated Bitted Key Blade

A thin, lightweight blade stamped from sheet steel with serrated edge bittings that directly set pin heights.

By replacing heavy cast-iron round key shanks with a flat blade ( $1.2\text{ mm}$ thick) featuring rounded lateral corrugations and five precision-milled bitting depths, Yale reduced key weight by over $90\%$ while providing $6^5 = 7,776$ unique theoretical key differs per keyway profile.

19th-C. Term: thin slip of steel properly shaped (key K)Modern: Flat Bitted Paracentric Key

4Anti-Pick Circumferential Serrations & Racked Chambers

Notched grooves cut into the circumference of the pins and chamber walls that bind against the shear line during picking attempts.

Yale tapped micro-grooves and notches into the pin perimeters and housing bore. When a lockpicker applies rotational tension while probing pins, the serration shelves catch in the housing notch, creating a false set with high frictional lockup ( $F_{\text{bind}} = \mu_s \tau / R_{\text{plug}}$ ) that prevents feeling the true shear line.

19th-C. Term: racked pin-tumblers and notched containing-recessesModern: Serrated Security Pins / Spool Pins

5Lost-Motion Cam (Lazy-Arm) & Deadbolt Deadlock

A rotating cam ring that turns through a smaller angle than the key plug, keeping the bolt permanently deadlocked.

The stamped steel lazy-arm E rotates through approximately $90^\circ$ while the key plug rotates a full $360^\circ$ . In both the locked and unlocked terminal positions, the cam wing W rests in positive contact against the bolt talon at a $90^\circ$ mechanical angle ( $mathbf{r} \times mathbf{F} = 0$ ), preventing burglars from jimming or forcing the deadbolt backward into the case.

19th-C. Term: wing or lazy-arm EModern: Lost-Motion Drive Cam & Deadbolt Actuator

Engineering Principles & Equations

Governing Equations & Engineering Principles

Authored explanation paired with its stated mathematical relation

Pin-Tumbler Shear-Line Boundary Condition & Alignment Kinematics

Precision Mechanics & Cryptographic LocksClaim 4

Mathematical Governing Law

\htmlClass{eq-term eq-term-delta_y eq-term-crimson}{\htmlData{var=delta_y}{\textcolor{#dc2626}{\Delta y_i}}} = |\htmlClass{eq-term eq-term-y_bit eq-term-sapphire}{\htmlData{var=y_bit}{\textcolor{#2563eb}{y_{\text{bitting},i}}}} + \htmlClass{eq-term eq-term-l_pin eq-term-emerald}{\htmlData{var=l_pin}{\textcolor{#059669}{L_{\text{keypin},i}}}} - \htmlClass{eq-term eq-term-r_plug eq-term-amber}{\htmlData{var=r_plug}{\textcolor{#d97706}{R_{\text{plug}}}}}| \le \htmlClass{eq-term eq-term-delta_tol eq-term-amethyst}{\htmlData{var=delta_tol}{\textcolor{#9333ea}{\delta_{\text{shear}}}}}

Terms:

Plain English DecoderHover or tap any highlighted phrase

The equals the absolute difference between the plus and the , which must remain within the for the cylinder to rotate.

\Delta y_i

Pin Shear Line Alignment Error

Distance between the pin parting line and the outer cylinder shear boundary (

< 0.09\text{ mm}

)

Millimeters (mm)[L]

If any single pin has an error exceeding tolerance, it physically spans the shear line and mechanically blocks rotation.

Physical Principle & Engineering Insight

Linus Yale Jr.'s pin-tumbler cylinder lock separates the keyway and pin stack from the bolt-throwing mechanism. Only when all five split-pin division lines simultaneously align flush with the revolving plug circumference can the cylinder rotate.

Historical Context: US 48,475 established the modern pin-tumbler cylinder lock and small flat bitted key, making heavy, bulky Victorian bit keys obsolete and becoming the universal security standard worldwide.

Hooke's Law Spring Dynamics & Shear Binding Torque

Solid Mechanics & Friction DynamicsClaim 4

Mathematical Governing Law

\htmlClass{eq-term eq-term-f_spring eq-term-emerald}{\htmlData{var=f_spring}{\textcolor{#059669}{F_s}}} = \sum_{i=1}^5 \htmlClass{eq-term eq-term-k_spring eq-term-sapphire}{\htmlData{var=k_spring}{\textcolor{#2563eb}{k_s}}} \htmlClass{eq-term eq-term-dx_spring eq-term-coral}{\htmlData{var=dx_spring}{\textcolor{#ea580c}{\Delta x_i}}} \quad \text{and} \quad \htmlClass{eq-term eq-term-tau_net eq-term-amethyst}{\htmlData{var=tau_net}{\textcolor{#9333ea}{\tau_{\text{net}}}}} = \htmlClass{eq-term eq-term-tau_app eq-term-amber}{\htmlData{var=tau_app}{\textcolor{#d97706}{\tau_{\text{applied}}}}} - \sum_{i=1}^5 \htmlClass{eq-term eq-term-mu_fric eq-term-crimson}{\htmlData{var=mu_fric}{\textcolor{#dc2626}{\mu_s}}} \frac{\htmlClass{eq-term eq-term-tau_app eq-term-amber}{\htmlData{var=tau_app}{\textcolor{#d97706}{\tau_{\text{applied}}}}}}{\textcolor{#0891b2}{R_{\text{plug}}}} [1 - \Theta(\textcolor{#059669}{\delta_{\text{tol}}} - \textcolor{#dc2626}{\Delta y_i})]

Terms:

Plain English DecoderHover or tap any highlighted phrase

The depends on the and , while equals minus whenever pins cross the shear boundary.

F_s

Total Spring Restorative Force

Sum of downward forces exerted by all 5 pin compression springs (

2.5\text{ to }5.0\text{ N}

)

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

Continuously pushes driver pins across the shear line when the key is withdrawn.

Physical Principle & Engineering Insight

Yale's introduction of circumferential notches (spools and serrations) on the pins creates false-set gates: when a lockpicker applies turning torque, the serration catches in the housing groove, generating counter-rotation feedback and deadlocking the plug.

Historical Context: The Yale lock combined high spring reliability, anti-pick serrations, and extreme physical compactness in a single mass-manufacturable brass cylinder.

Shear-Line Boundary Kinematics & Geometric TolerancesAuthored Principle 1

Stated relation

\Delta y_i = |y_{\text{bitting},i} + L_{\text{keypin},i} - R_{\text{plug}}| \le \delta_{\text{shear}}

Rotational freedom of the inner plug requires that all five pin division lines simultaneously satisfy the shear boundary condition within machining tolerance

\delta_{\text{shear}} \le 0.09\text{ mm}

. Any single misaligned pin creates solid brass mechanical interference that withstands hundreds of newtons of rotational torque.

Hooke's Law Spring Restoration & Shear Binding TorqueAuthored Principle 2

Stated relation

F_s = \sum_{i=1}^5 k_s \Delta x_i \quad \text{and} \quad \tau_{\text{net}} = \tau_{\text{applied}} - \sum_{i=1}^5 \mu_s \frac{\tau_{\text{applied}}}{R_{\text{plug}}} [1 - \Theta(\delta_{\text{tol}} - \Delta y_i)]

Individual compression springs apply downward restoring force

F_i = k_s \Delta x_i

. Under unauthorized turning torque

\tau

, misaligned pins experience a normal clamping force against the housing wall, generating static friction that resists picking and locks the plug rigidly.

Lost-Motion Cam Kinematics & Mechanical DeadlockingAuthored Principle 3

Stated relation

\tau_{\text{external}} = \mathbf{r} \times \mathbf{F}_{\text{jimmy}} = r F_{\text{jimmy}} \sin(0^\circ) = 0

By decoupling the plug's full

360^\circ

key-withdrawal rotation from the cam's

90^\circ

bolt-throw arc, the lazy-arm abuts the deadbolt talon perpendicularly, ensuring external forces on the bolt head generate zero rotational torque on the cam.

Interactive Schematic Sheet (Fig. 1)

Side elevation of the mortise lock case showing the circular threaded aperture and mounted cylinder.

1.00x

Click any numbered callout pin or use arrow keys [← / →]Tap any numbered pin3 Curated Callouts

Callout Pin Inspector

Select Any Numbered Pin

Click pins on the schematic or select from the list below to inspect historical specifications.

Why It Still Matters

Linus Yale Jr.'s 1865 lock was the foundation of the modern security hardware industry. By creating a standardized, interchangeable cylinder that could be mass-produced, keyed in master-key hierarchies, and operated by small flat keys carried in pockets, Yale transformed physical security across the globe and led directly to the founding of the Yale & Towne Manufacturing Company in 1868.

Legal Claims Decoder (5 Numbered Claims)

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

Claim #1Independent Master Claim

1/5

Verbatim Historical Legal Text

“1. The contrivance, substantially as described, for holding a bolt in place.”

Plain English Engineering Translation

Claims the novel elastic spring-plate contrivance and clamping screw that holds the sliding deadbolt inside the mortise lock case, enabling the entire bolt assembly to be inserted or serviced directly through the front mortise faceplate after the lock case is mounted in the door.

Key Protected Innovations:

Front-insertable deadbolt mechanismElastic spring retention plateThrough-faceplate servicing and installation

Historical Legal Impact:

Allowed locksmiths and carpenters to install and service lock internals without disassembling the mortise door pocket.

The Historical Bottleneck

In the mid-19th century, locks were heavy iron warded mechanisms operated by giant keys that were difficult to carry, offered low key permutation security, and were readily bypassed by lockpicks.

Why Prior Art Failed

•Heavy iron keys weighing up to several pounds needed to reach through door thickness
•Warded locks were vulnerable to skeleton keys and impressioning picks
•Bramah and Chubb lever locks were expensive, large, and delicate to install

The Breakthrough Insight

“Linus Yale Jr. separated the key cylinder from the bolt throw mechanism, using five miniature spring-loaded split pins aligned to a shear line by a small, lightweight flat bitted key.”

Patent Wars & Legal Litigations

Vs. Hobbs, Hart & Co. and European Warded LockmakersInfringement Challenge

Rival Claim & Defense:

Traditional Warded and Lever Lock Patents

Litigation Conflict:

Traditional lockmakers argued that Yale's tiny flat key could never generate enough torque to throw a heavy iron deadbolt, claiming the mechanism was too delicate for robust commercial use.

Final Resolution & Judicial Outcome:

Yale proved that separating the key cylinder from the bolt throw and driving the bolt via a lost-motion lazy-arm cam allowed a tiny key to throw bolts with zero mechanical strain.

Civilizational Impact

US Patent 48,475 is the direct ancestor of over 85% of all mechanical door locks in existence today. Yale's flat serrated key, cylinder housing, split pin tumblers, and interchangeable mortise format became the universal standard for commercial buildings, residential doors, padlocks, automotive ignitions, and cabinet security across the world.

Historical Fact

Linus Yale Jr. was originally trained as a portrait painter before joining his father's lockmaking business, bringing an artist's precision to geometric mechanical lock design.