A padlock is a portable, self-contained locking mechanism with a shackle that passes through a hasp, chain, or staple to secure two objects together. It works by trapping a spring-loaded bolt or set of ball bearings against a notch in the shackle until the correct key or combination aligns the internal pins or discs at the shear line, releasing the shackle. The purpose is to provide tamper resistance without permanent installation. A boron-alloy shackle on a CEN Grade 6 padlock resists 6,000 N of bolt-cutter force.
Padlock Interactive Calculator
Vary shackle size, material strength, cutter force, and cut rate to estimate padlock resistance time.
Equation Used
The calculator converts shackle diameter into circular area, multiplies by ultimate tensile strength, then divides by tool force and the empirical cut-rate factor to estimate resistance time. In SI form, MPa is N/mm^2.
- Round shackle cross-section.
- MPa is treated as N/mm^2.
- Cut rate is empirical for the cutter and shackle material pair.
- Single-shackle cut path is modeled.
How the Padlock Works
A padlock has two jobs — hold the shackle locked under load, and release it cleanly when the right key turns. The shackle is the U-shaped bar you thread through whatever you're securing. One leg of that shackle has a notch (sometimes two, for double-locking). When the shackle is pushed down into the body, a pair of spring-loaded ball bearings or a sliding bolt drops into that notch and traps it. That's the locked state. Turn the correct key and the locking dogs retract, the spring under the shackle pops it up by 8 to 15 mm, and you can swing it free.
The key cylinder is where the security lives. In a standard pin tumbler padlock — the same architecture Linus Yale Jr. patented in 1865 — there are typically 4 to 6 spring-loaded driver pins sitting above 4 to 6 key pins of varying lengths. The wrong key leaves the pin stack crossing the shear line, blocking rotation. The correct key lifts each key pin until every pin-pair break sits exactly at the shear line, freeing the plug to rotate. Tolerances matter here — pin lengths are typically held to ±0.025 mm. Wear loosens those tolerances, which is why you start to see pickability climb sharply on a 20-year-old budget padlock.
What goes wrong? The two common failure modes are corrosion freezing the springs (the shackle stays down even when the plug rotates) and shackle attack — bolt cutters, hacksaws, or shimming. A hardened boron shackle resists cutters but doesn't help against shimming, which targets the ball bearings directly. High-security designs like Abloy disc detainers replace pins entirely with rotating discs that have no springs to bind and no pin stack to bump.
Key Components
- Shackle: The U-shaped bar that passes through the hasp or chain. Usually 6 to 12 mm hardened steel, often boron-alloyed to over 60 HRC for cut resistance. The notch on the locking leg is where the ball bearings or bolt engage — that notch geometry is held to ±0.05 mm because slop here translates directly to shimming vulnerability.
- Lock Body: Houses the cylinder, springs, ball bearings, and shackle return spring. Brass, laminated steel, or solid hardened steel depending on grade. A CEN Grade 6 padlock body resists 100 kN of pull force on the shackle without releasing it.
- Key Cylinder (Plug and Pins): The rotating plug holds the keyway. Driver pins, key pins, and springs sit in vertical chambers above the plug. Pin lengths machined to ±0.025 mm tolerance. The correct key aligns all pin breaks at the shear line, freeing rotation.
- Locking Dogs or Ball Bearings: Two hardened steel balls (typically 4 to 5 mm diameter) sit in cross-drilled holes in the body and drop into the shackle notch when the cylinder is in locked position. Plug rotation cams them outward, releasing the shackle.
- Shackle Spring: Compression spring, usually 4 to 6 N preload, that pushes the shackle upward by 8 to 15 mm once the locking dogs retract. Spring failure from corrosion is the most common reason a padlock 'feels unlocked but won't open' in marine or outdoor use.
Where the Padlock Is Used
Padlocks show up wherever you need temporary, portable security and don't want to commit to a fixed lockset. The grade you need ranges enormously — a $4 luggage padlock and a $400 ABUS Granit serve completely different threat models. Match the grade to the threat, not to the price you want to pay.
- Industrial Safety: Lockout-tagout (LOTO) on electrical panels and valves — Master Lock 410 thermoplastic padlocks are the OSHA-compliant standard, colour-coded per worker, keyed unique with no master.
- Logistics and Shipping: Container door locks on intermodal freight — high-security padlocks like the ABUS 37/55 with shrouded shackles to prevent bolt-cutter access on transit routes through high-theft regions.
- Self-Storage Facilities: Cylinder padlocks (disc-shaped, no exposed shackle) on roll-up storage doors — companies like Public Storage and CubeSmart specify these because there's nothing for cutters to grab.
- Cycling: U-locks and chain padlocks for bicycle security — Kryptonite New York Fahgettaboudit pairs a 14 mm hardened shackle with a disc detainer cylinder, rated Sold Secure Diamond.
- Firearms Storage: Trigger locks and cable padlocks meeting California DOJ requirements — typically 4-pin or combination padlocks with 4 to 6 mm shackles.
- Utilities and Telecoms: Padlocks on substation gates, manhole covers, and cell tower compounds — keyed-alike systems across thousands of sites, often Medeco or Abloy with restricted key blanks.
The Formula Behind the Padlock
The most useful number to compute for a padlock isn't anything in the lock itself — it's the resistance time, the number of seconds an attacker needs to defeat it with a given tool. Insurance ratings (CEN, Sold Secure, ASTM F883) are built around this. At the low end of the typical range, a $5 brass padlock with a 4 mm shackle gives you 2 to 5 seconds against a 600 mm bolt cutter. At the high end, a CEN Grade 6 padlock with a 14 mm boron shackle holds out 5 minutes or more against the same tool. The sweet spot for most commercial use sits around CEN Grade 4 — roughly 60 to 120 seconds resistance, which is long enough that an opportunistic thief moves on.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| tresist | Estimated resistance time against the cutting tool | seconds | seconds |
| Ashackle | Cross-sectional area of the shackle | mm² | in² |
| σUTS | Ultimate tensile strength of the shackle material | MPa | ksi |
| Ftool | Cutting force the attacker can apply with the tool | N | lbf |
| Rcut | Cut rate per unit force-time (empirical, depends on tool/material pair) | mm²/(N·s) | in²/(lbf·s) |
Worked Example: Padlock in a marina gate padlock spec
A harbourmaster's office in Wellington is specifying padlocks for the security gates on a 240-berth marina. Salt-air exposure rules out plain steel. The threat model is a 750 mm bolt cutter delivering roughly 4,000 N cutting force at the jaws. They are deciding between a 6 mm stainless shackle (low end), a 10 mm hardened steel shackle (nominal), and a 14 mm boron-alloy shackle (high end). They want a target resistance of at least 60 seconds against the bolt cutter to hit the marina insurer's minimum.
Given
- Ftool = 4000 N
- σUTS (boron alloy, nominal) = 1600 MPa
- Dshackle, nominal = 10 mm
- Rcut = 0.0008 mm²/(N·s)
Solution
Step 1 — compute the cross-sectional area of the nominal 10 mm hardened steel shackle:
Step 2 — apply the resistance time formula at the nominal point. Hardened medium-carbon shackle steel runs around 1,200 MPa UTS:
That's below the 60 s target. A 10 mm shackle on hardened steel won't pass the insurer's threshold against a determined attacker with a 750 mm cutter.
Step 3 — recompute at the low end, the 6 mm stainless shackle. Stainless 316 runs around 600 MPa UTS:
5 seconds is the threat envelope of a casual thief — they cut and walk before anyone reacts. Stainless looks great on the marina but the corrosion resistance is buying you nothing if the lock falls in 5 seconds.
Step 4 — recompute at the high end, the 14 mm boron-alloy shackle at 1,600 MPa UTS:
That clears the 60 s target. In real-world terms 77 seconds is enough that the cutter needs to commit, sweat, reposition the jaws — exactly the noise and time window that gets noticed on a marina dock at 2 a.m.
Result
The 14 mm boron-alloy shackle gives a calculated resistance of 77 seconds — passing the marina insurer's 60-second minimum with margin. The 10 mm hardened steel sits at 29 seconds, fast enough that an experienced thief will commit to the cut, and the 6 mm stainless drops to 5 seconds which is essentially decorative. The sweet spot for this application is 12 to 14 mm boron with a closed-shackle or shrouded-shackle body. If a real-world test gives you under 60 seconds on the boron shackle, suspect three things first: (1) the shackle is through-hardened only on the surface and the core is soft (a cheap clone — check for a CEN Grade stamp, not just a marketing claim), (2) the shackle diameter at the cut zone has been undercut by the manufacturer to fit a cheaper body, or (3) galvanic corrosion at the shackle-body interface in salt air has thinned the shackle by 10 to 15% over a few seasons.
Choosing the Padlock: Pros and Cons
A padlock isn't the only way to secure a hasp or chain — and within padlocks, the cylinder type drives most of the cost and security difference. Here's how the main options stack up on the dimensions buyers actually search on.
| Property | Pin Tumbler Padlock | Disc Detainer Padlock (Abloy-style) | Combination Padlock |
|---|---|---|---|
| Pick resistance (skilled attacker, seconds) | 30 to 300 s on standard, 600+ s on security pins | 1,800+ s, often unpicked in field | 5 to 60 s by decoding |
| Shackle cut resistance (CEN equivalent) | Grade 1 to 6 available | Grade 4 to 6 typical | Grade 1 to 3 typical |
| Cost (single unit, retail) | $5 to $80 | $80 to $400 | $8 to $40 |
| Corrosion tolerance in marine air | Springs corrode in 2 to 5 years | No springs in cylinder, 10+ years typical | Wheels seize in 1 to 3 years |
| Key control / rekeyability | Restricted blanks possible (Medeco, Mul-T-Lock) | Tightly restricted (Abloy Protec) | No keys — combination only |
| Failure mode | Pin wear, spring corrosion, shimming | Disc wear (rare), body attack only | Wheel decoding, forgotten code |
Frequently Asked Questions About Padlock
Published CEN ratings are run on the shackle in a controlled jaw alignment with a defined cutter and operator. In the field, attackers brace the lock against a hard surface and use body weight, which roughly doubles effective jaw force compared to the test rig. Cold also matters — boron alloys lose 10 to 15% notch toughness below 0°C, so a winter cut is faster than a summer cut.
If your real-world resistance is consistently half the rated value, you're probably also seeing a cutter with longer handles than the test spec (1 m vs 600 mm changes everything). Match the threat tool, not the marketing number.
Yes, and the difference is bigger than most people expect. A shrouded shackle (where the lock body wraps up around the shackle, exposing only 5 to 10 mm) reduces the cutter's mechanical advantage to near zero — the jaws can't fully close around the shackle. The same 4,000 N cutter that opens an exposed 10 mm shackle in 30 seconds may take 5 minutes or fail entirely on the shrouded version.
The trade-off is hasp compatibility. Shrouded padlocks need a hasp designed for them — a standard 6 mm hasp staple won't fit through the small exposed gap. Spec the hasp and lock together.
For a 200-cabinet rollout, disc detainer (Abloy Protec2 or similar) wins on two specific dimensions: cylinder lifespan in outdoor exposure and field pickability. No springs in the cylinder means no spring corrosion, which is the failure mode that takes pin tumbler padlocks out of service first in outdoor utility use — typically 5 to 8 years on a coastal or industrial site.
High-security pin tumbler (Medeco, Mul-T-Lock) is cheaper per cylinder and easier to source replacement keys for in North America, but you'll be replacing cylinders on a rolling basis as springs fail. For 200 sites, calculate 10-year total cost of ownership including service truck rolls — disc detainer usually comes out ahead despite the higher unit price.
This is almost always shackle spring failure or debris in the shackle bore. The cylinder is doing its job — rotating the cam, retracting the locking dogs — but the compression spring under the shackle has lost preload (corroded, broken, or fouled with dirt) and there's nothing pushing the shackle up the 8 to 15 mm it needs to clear the dogs.
Quick diagnostic: with the key turned to unlock position, lift the shackle manually with pliers. If it lifts and the lock works normally, you've confirmed spring failure. A shot of PTFE-based lock lube into the shackle bore from the top often frees a sticky spring; if that doesn't restore pop-up action within 24 hours, the spring is mechanically broken and the lock is scrap.
Two causes, both common. First, water ingress has rusted the locking dogs or ball bearings in their cross-drilled bores so they can't cam back inward when you push the shackle down. Second, the shackle notch has filled with grit or ice that physically blocks the dogs from seating.
Flush the shackle bore and dog channels with a penetrating lubricant (not WD-40 long term — it dries out and gums; use a graphite or PTFE lube). If the lock still won't relock cleanly after flushing, the dogs are likely deformed from someone forcing the shackle and the lock body needs replacement. This is why marine-grade padlocks use brass or 316 stainless dogs, not plated steel.
Mathematically, almost not at all — the keyspace of a 6-pin cylinder is around 1 in 300,000 regardless of how many locks share that one combination. The real-world weakness isn't pickability, it's key control. One lost or copied key compromises every lock in the system at once.
For fleets above about 50 locks, move to a restricted keyway (Medeco Bilevel, Abloy Protec, Mul-T-Lock Interactive) where blanks aren't available at hardware stores and cuts require factory authorisation. The cylinder cost goes up 3x to 5x but the failure mode shifts from 'lost key compromises everything' to 'lost key is cuttable but not duplicable in the wild'.
References & Further Reading
- Wikipedia contributors. Padlock. Wikipedia
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