When it comes to arborist rigging, few topics are misunderstood as often—or as dangerously—as gear ratings.

Two numbers appear on nearly every rope, block, and carabiner: Minimum Breaking Strength (MBS) and Safe Working Load (SWL). They are related, but they are not interchangeable. Confusing them—or designing rigging systems around the wrong one—has led to damaged equipment, near misses, and serious injuries across the industry.

Let’s clear it up properly. Because arborists don’t work in labs. We work in trees.

Minimum Breaking Strength (MBS) is the lowest force at which equipment fails during controlled laboratory testing. It is not a working recommendation—it is a failure point measured under ideal conditions.

During testing, equipment is brand new and pulled slowly in a straight line without knots, bends, shock loading, wear, dirt, or UV exposure. MBS answers only one question: How much force does it take to break this item under perfect conditions?

It does not indicate how much load is safe in daily use, how the gear behaves in dynamic tree work, or whether it is appropriate for rigging or life support applications.

That’s where SWL comes in.

Safe Working Load (SWL), sometimes called Working Load Limit (WLL), is the maximum load that should ever be applied to equipment during normal real-world operations. SWL is calculated by dividing the MBS by a safety factor. In arboriculture, typical safety factors are 10:1 for life support systems and 5:1 for rigging and lowering operations.

For example, if a rope has an MBS of 10,000 pounds and a 5:1 safety factor is applied, its SWL would be 2,000 pounds.

SWL answers the question arborists actually need answered:
“How much can I load this safely, day in and day out, in real tree work?”

Tree work does not produce smooth, static loads. In real-world rigging, wood falls, swings, catches, redirects, and shock-loads systems instantly. A piece weighing 600–700 pounds on the ground can easily generate forces of 2,000 pounds or more when slack is present, when it free-falls briefly, or when it swings before being controlled.

MBS ignores these dynamic realities. SWL exists specifically to account for them.

Shock loading—often caused by slack in the system, timing errors, or abrupt catches—can multiply forces several times beyond the static weight of the load. MBS provides no margin for these unknowns. Safety factors do.

Safety factors exist because arborists are human, trees are unpredictable, and gear degrades over time. Knots can reduce rope strength by 20–40 percent. Tight bends, small-radius hardware, abrasion, glazing, UV exposure, moisture, contamination, and even unseen internal fiber damage all reduce real-world strength.

Industry guidance supports this approach. The Arborist Industry Safe Work Practices Guide emphasizes operating within rated working load limits rather than approaching breaking strength. Likewise, ANSI Z133 – Safety Requirements for Arboricultural Operations reinforces designing systems within working limits, not theoretical maximums.

Consider a rope with an MBS of 12,000 pounds. Applying a 5:1 safety factor results in an SWL of 2,400 pounds. On a job, a blocked-down piece weighing approximately 700 pounds is lowered with a small drop, redirected through a block, and allowed a slight swing as it clears.

That 700-pound piece can easily spike near or beyond 2,000 pounds of force. If the system was designed using SWL, you remain within a predictable safety margin. If it was designed assuming proximity to MBS, you are operating on assumptions rather than engineered limits.

Rigging failures rarely result from one catastrophic mistake. They result from stacked assumptions.

Manufacturers such as Samson Rope Technologies and Teufelberger publish MBS because it is a standardized, comparable laboratory value. It provides the baseline from which safety factors are calculated. However, it is not a recommended working limit.

Manufacturers expect arborists to apply appropriate safety factors, design systems around SWL or WLL, and understand that breaking strength is not a target—it is a boundary.

When you design systems around SWL, you:

• Choose the right rope diameter (not just the strongest)
• Match blocks, slings, and hardware correctly
• Reduce unnecessary wear on gear
• Build balanced, predictable systems
• Make better decisions under pressure
• Train new climbers with confidence

It shifts rigging from “strong enough” to “designed correctly.”

Minimum Breaking Strength tells you how gear fails.
Safe Working Load tells you how arborists work safely.

Tree care is not about how strong your gear could be under perfect conditions. It is about how reliably your system performs when things do not go perfectly—and they rarely do.

Design below SWL.
Inspect often.
Retire gear early.

If you ever have questions about gear ratings or building safer rigging systems, the team at The Arborist Store is always here to help.

• Samson Rope Technologies – Understanding Rope Strength & Working Loads
• Teufelberger – Technical Product Specifications & Safety Factors
• Arborist Industry Safe Work Practices Guide – Arborist Safe Work Practices
• ANSI Z133 – Safety Requirements for Arboricultural Operations