Why Does an Aerial Silk with 1.5 Tons of Strength Have a Working Load Limit of Only 215 kg? The Engineering of Safe Flying on Aerial Silks

When you purchase a professional aerial silk and look at its technical documentation or label, you might experience a bit of cognitive dissonance.

In the section presenting laboratory test results, you read: Breaking Load Limit (MBS/BLL) = 15.1 kN (approx. 1540 kg). On the tensile testing machine, the fabric withstood the weight of a large passenger car! Yet, just one line below, the manufacturer and the safety engineer specify: Working Load Limit (WLL) = 2.15 kN (approx. 215 kg).

Where does such a huge difference come from? Does the fabric really require a 7-fold "power reserve"? How does a knot on a figure-8 component alter the fabric's strength, and why can the dynamics of a drop turn a 60-kilogram aerialist into a half-ton load?

As an aerial equipment engineer, I clear up doubts and explain what actually happens within the silk’s fibers during training.

1. Slang vs. Engineering: MBS and WLL in the World of Textiles

To understand aerial silk safety, we must distinguish between two key concepts used in certification procedures (e.g., based on the PN-EN ISO 13934-1 standard for tensile properties of fabrics):

• MBS / BLL (Minimum Breaking Strength / Breaking Load Limit): This is the absolute ultimate breaking force. In static tests on a tensile testing machine, the puller stretches a doubled silk suspended on rigging hardware until the fibers physically rupture. For our popular JUST fabric, rupture occurred at a force of 15.1 kN (1540 kg).

• WLL (Working Load Limit): The maximum static mass you can safely suspend from the silk during daily use to ensure the material retains its structural integrity and suffers no damage.

For textile products in the aerial industry, a Safety Factor / Design Factor c = 7 (and in certain industry standards even $10:1$) is applied.

Simple engineering mathematics calculates the WLL parameter as follows:

 

Hammock Rigging (U-Shape): Why Does the Same Fabric Hold More as a Hammock Than as a Silk?

It is worth noting an important structural detail that distinguishes an Aerial Hammock from an Aerial Silk, even when they are made from the exact same fabric (such as our JUST or AIR material).

When the same fabric is suspended in a U-shaped loop configuration (hammock) on two carabiners or a single anchor point, its real Breaking Strength (MBS) in laboratory tests increases nearly twofold!

Let us look at the test results from the tensile testing rig:

• JUST fabric as a silk (tied on a figure-8 descender with two carabiners): ruptures at a force ofF max = 8.507 kN (approx. 850 kg).

• The same JUST fabric suspended as a hammock (loop suspended on carabiners): withstands up to F max = 15.1 kN (approx. 1540kg)!

• The stronger AIR model as a hammock: achieves a result of F max = 18.1 kN (over 1840 kg) on the testing rig!

Where does this difference come from? Engineering Explanation:

1. Force Distribution Across Loop Legs (V / U-Shape): In an aerial silk, the entire mass of the performer hangs on two free ends originating from a single knot. In a hammock, the fabric forms a continuous loop supporting the body, distributing the load equally across two load-bearing tails of the fabric. According to vector mechanics, each leg carries roughly half of the total weight in a vertical setup.

2. Gentler Bending Radius (No Knot Strain Concentration): A classic knot tied on a figure-8 descender creates a stress concentration point (notch effect) – the fabric bends at a sharp angle over the metal element, causing localized stress concentration and reducing yarn strength by 30% to 50%. In a hammock, the loop rests flat across carabiners or webbing slings, resulting in a much gentler stress distribution across synthetic fibers.

2. The Devastating Force of Knots and Figure-8 Hardware (Loss of Strength)

Unlike steel wire ropes or hoops, an aerial silk does not operate in isolation. To suspend it, you must tie it around an aluminum figure-8 descender or a dedicated bell hook (Silks Hook).

Every bend, knot, or hitch in the fabric creates a stress concentration point. At the bend point, outer polyester fibers are stretched extremely hard, while inner fibers bear almost no load.

• Tight cinch knots (e.g., on a figure-8): Can reduce the original breaking strength of the fabric by 30% to 50%!

• While the strength of the "raw" fabric strip test is high, laboratory test reports measure the entire assembly connection (tested on figure-8 hardware with carabiners). The material failure almost always occurs directly at the knot next to the figure-8 descender.

A Design Factor of c = 7 acts as your engineering shield, ensuring that loss of strength caused by knots does not compromise user safety.

3. The Physics of a Drop: When 60 kg Becomes 350 kg

The single greatest "consumer" of silk strength is movement dynamics. An aerial silk is designed for dynamic drops, slack releases, and high-energy catches.

Let us analyze the physics based on official biomechanical research published by Dr. Marion Cossin (National Circus School in Montreal):

• Dynamic Load Factor (d): During a free fall and rapid deceleration on the acrobat's body, dynamic overload is generated. Laboratory measurements using load cells at the rigging point showed that the dynamic factor for Aerial Silks averages $d = 5.6$ (meaning gravity force is multiplied nearly 6-fold!).

• Impact Force Calculation:
  If an aerialist weighing 60 kg executes a dynamic drop from height, the instantaneous peak impact force acting on the rigging point and fabric fibers is:
  F max= 60 kg times 5.6 = 336 kg (approx 3.36 kN).
  
  

Consider that number: 336 kg of dynamic peak force generated by a 60-kilogram person!

If the WLL of an aerial silk were established without accounting for the dynamic safety factor c = 7 (e.g., set at 500 kg), a single deep drop could cause micro-tears in the synthetic yarn structure and rapid material fatigue.

4. Fiber Aging, Sweat, and Friction (The Invisible Enemies)

A third reason why aerial silks require a rigorous engineering approach is the chemical and physical nature of synthetic polymers (polyester/polyamide):

• Friction and Micro-Deformation: Fast slides along the silk generate high thermal energy. Both polyester and polyamide can melt and lose structural integrity at temperatures exceeding 80*C.

• Material Fatigue: Knit fabrics subjected to repeated stretching and releasing lose elasticity over time (fiber creep).

• Environmental Degradation: Moisture, sweat, chalk residue (magnesium carbonate), and UV exposure gradually degrade yarn strength over time.

For these reasons, user manuals for aerial equipment explicitly define a textile service life limit (e.g., maximum 3 years or 300 hours of intensive use) and mandate monthly tactile and visual inspections.

Summary of Aerial Fabrics at a Glance

The table below shows how static tensile test results translate into safe operational limits in training studios:

Golden Rule for Safe Aerial Props:

As an instructor, performer, or studio owner, never base your safety decisions solely on the laboratory Breaking Load Limit (MBS). Your safety and that of your students is guaranteed by the Working Load Limit (WLL) – because it accounts for knot reduction, material aging, and the relentless physics of dynamic drops!