If workers don’t understand WLL, breaking strength, design factors, or sling angles, it becomes easy to overload equipment, and that’s how gear fails, loads drop, and workers get hurt.
This guide explains WLL in simple terms so your crew can make safe decisions on the job.
What Is Working Load Limit (WLL)?
Working Load Limit (WLL) is the maximum weight a piece of rigging equipment can safely lift under normal conditions.
You’ll also hear the terms:
-
Safe Working Load (SWL)
-
Rated Capacity
-
Allowable Load
All of these refer to the same thing: the safe limit, not the point where equipment breaks.
The manufacturer assigns the WLL based on testing and industry standards.
What Is Breaking Strength?
Breaking Strength (also called ultimate strength or tensile strength) is the point at which the gear actually fails during destructive testing.
Breaking strength is NOT the number you use in the field.
Why?
Because breaking strength has zero safety margin built in. If you rig to breaking strength, the gear will fail.
WLL vs Breaking Strength
| Term |
What It Means |
Safe to Use in the Field? |
| Breaking Strength |
The point where gear physically breaks |
❌ No |
| Working Load Limit (WLL) |
The maximum safe load under normal use |
✅ Yes |
| Design Factor |
The built-in margin between WLL and breaking strength |
Informational |
The Formula: How WLL Is Calculated
Manufacturers determine WLL using this simple formula:
WLL = Breaking Strength ÷ Design Factor
Common design factors:
| Equipment Type |
Typical Design Factor |
| Chain Slings (Grade 80/100) |
4:1 |
| Wire Rope Slings |
5:1 |
| Synthetic Web Slings |
5:1 |
| Round Slings |
4:1 or 5:1 |
A higher design factor = more built-in safety.
Examples: How to Calculate WLL
Example 1: Wire Rope
Breaking Strength: 20,000 lbs
Design Factor: 5
WLL = 20,000 ÷ 5 = 4,000 lbs
Example 2: Synthetic Sling
Breaking Strength: 10,000 lbs
Design Factor: 5
WLL = 2,000 lbs
Example 3: Chain Sling
Breaking Strength: 40,000 lbs
Design Factor: 4
WLL = 10,000 lbs
These simple calculations help workers understand how gear capacity is determined.
Sling Angles: The Most Overlooked Safety Problem
Even when the sling is rated correctly, angles can instantly reduce its capacity.
The wider the angle, the more tension is placed on each sling leg.
Tension increases as the sling angle decreases.
Example:
| Sling Angle |
Load on Each Leg |
| 60° |
115% of load |
| 45° |
141% of load |
| 30° |
200% of load |
So a 1,000 lb load at 30° can put 2,000 lbs of force on each sling leg.
This is why workers need to understand angles; it’s one of the leading causes of overloaded rigging gear.
What Reduces WLL in the Field?
Even if equipment is tagged properly, the actual WLL can be reduced by:
-
cuts, tears, abrasion
-
knots in slings
-
chemical exposure
-
extreme temperatures
-
UV breakdown on synthetic slings
-
side loading shackles
-
wrong hitch type (choker, basket, vertical)
-
damaged or missing tags
If the tag is missing, frayed, unreadable, or questionable → remove it from service.
OSHA and ASME Requirements for WLL
These standards define how WLL is determined and used:
Key rules:
-
Employers must ensure rigging is used within WLL
-
Tags must be readable
-
Workers must be trained to calculate loads
-
Damaged gear must be removed from service
-
Inspections must be performed regularly
Training isn’t optional, it’s part of compliance.
Does Your Crew Understand WLL, Breaking Strength, and Sling Angles?
Most rigging incidents come down to lack of training, not bad equipment.
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FAQ: Working Load Limit (WLL)
Can you exceed the WLL?
No. Exceeding WLL can cause immediate equipment failure.
Is breaking strength the same as WLL?
No. Breaking strength is the maximum force before failure.
WLL is the safe working limit.
Is WLL the same as Safe Working Load (SWL)?
Yes, these terms are often used interchangeably.
How often should rigging be inspected?
What happens if a sling tag is missing?
Remove it from service immediately.
Do workers need formal rigging training?
Yes, OSHA requires workers to be competent and trained.
Conclusion
Understanding Working Load Limit (WLL), breaking strength, design factors, sling angles, and inspection requirements is essential for safe lifting.
If your team rigs loads, operates cranes, or works around lifting equipment, training isn’t optional — it’s what keeps people safe and keeps jobs moving.
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