Two numbers describe every piece of rigging hardware, and confusing them is how gear gets overloaded "by the numbers." Breaking strength is where the metal fails; working load limit is where you're allowed to operate. The gap between them is the design factor — and it exists for reasons a spec sheet can't show.
The Definitions
Minimum breaking strength (MBS): the load at which the component is expected to fail in a straight-line pull, new, under laboratory conditions. Working load limit (WLL): the maximum load the manufacturer rates for service — the number forged on shackles, stamped on hooks, and printed on sling tags. Design factor: MBS ÷ WLL. "Safe working load" (SWL) is an older term for the same idea; WLL is the modern marking.
Standard Design Factors
| Equipment | Typical design factor |
|---|---|
| Wire rope & chain slings | 5:1 (4:1 for alloy chain per some standards) |
| Synthetic web & round slings | 5:1 |
| Shackles & rigging hardware | 5:1 (6:1 on some series) |
| Hoist rope (running) | 5:1 typical, 3.5:1 minimum per code |
| Personnel lifting | 10:1 |
Why the Factor Is That Big
The design factor absorbs everything the lab pull didn't include: dynamic loading (a load snatched off the ground can momentarily double its static weight), sling angle multiplication, wear and abrasion accumulated in service, bending losses (D/d), corrosion, temperature, manufacturing variation, and the uncertainty in your load-weight estimate. The factor is not spare capacity to spend — by the time real-world effects stack up, a rig at WLL can already be flirting with much of the margin.
Field Applications
Never rig to breaking strength — catalogs list MBS for engineering reference, not for rigging math. When a component is marked with only one number, it's the WLL. Proof test loads (typically 2× WLL on manufactured slings) verify workmanship; they're a factory event, not an invitation. And when a lift plan needs more capacity, size up — don't rationalize into the design factor.
Certs and Traceability
Rated hardware comes with the paperwork — proof test certificates, material traceability, and the manufacturer's charts that state WLL by configuration and angle. We supply certs with slings and hardware on request.
Rated Gear, Documented
Slings, shackles, chain, and hardware — all rated, marked, and certifiable. Capacity, configuration, and quantities into the quote form at the top of the page — quotes answered during business hours.