Safety and Quality of Shackles

Introduction

Shackles have established themselves as the major connection elements in slackline setups. In contrast to carabiners they have the significant safety advantage that they are less susceptible to multi-axial loading. Regarding shape and closure - shackles have also some advantages compared to quicklinks. Their symmetrical and large, round shape is ideal for connecting textile slings, webbings and ropes. Especially for use in the main load path of long and highlines, shackles are regarded as irreplaceable. They often serve as key elements of the whole slackline setup.

Therefore shackles are very safety-relevant components. Their function must be guaranteed to 100% under all conceivable load scenarios and throughout their period of use. The used material, the manufacturing process and the performance must meet these requirements (the breaking load) and should be carefully defined and controlled continuously during the production.

Materials for shackles

Steel is the common material for shackles. A distinction is made between corrosion-resistant stainless steel and non-corrosion-resistant steel. Both stainless steel and non-corrosion-resistant steel are available in a range of variants (so called alloys). Especially for non-corrosion-resistant steels very high-strength variants are used. The high-strength properties of the Van Beest Green Pin shackles are achieved by a special hardening process (the so-called tempering). The Van Beest Green Pin shackles are also protected with a hot dip galvanizing surface layer against corrosion.

Since stainless steel is generally more expensive than a non-corrosion-resistant steel,  the focus for the most stainless steels is on untempered variants. These offer a good compromise between cost and breaking load / weight. Shackles made of high-strength and tempered stainless steels (for example 17-4 PH), such as the Petersen Hi-Mod are available at a much higher price. These products have the advantage that a significantly higher breaking load at the same weight can be achieved.

A new development, especially for slackline applications are shackles made of high-strength aluminium alloy, such as the Landcruising Airbow which is primarily characterized by a very low weight, slackline specific design and simplified handling.

Manufacturing processes

For the production of shackles two manufacturing processes are essential. First, the hot-forging and secondly the investment casting process. Both methods differ primarily with respect to manufacturing costs and the resulting material properties. Investment casting is used for inexpensive shackles because it is a more cost effective process compared to forging. However, investment casting has several safety-related disadvantages. The following table shows the advantages and disadvantages of both manufacturing processes comparatively:

The much better material properties in forging base upon the molding under high pressure and high temperature. In forging the metal structure aligns along the geometry and matches the load requirements. In contrast, at investment casting a liquid metal cures in its form, the resulting metal structure is more crystalline and not aligned to match the load requirements. This difference has a big impact on the deformation, fracture, and fatigue strength. Forged shackles have clearly better values, which in turn is very important for the performance of a shackle. Forged steel shackles are durable, robust, efficient and safer. Especially for forged shackles the following applies: they do not break suddenly, but start to deform when exceeding the maximum workload. A significant safety aspect, because overloading is visually recognizable.

In addition, the scatter of the strength properties for forged shackles is substantially lower than for investment casted shackles. Casting products can have a varying microporosity and even larger internal flaws (called blow holes). These variations are not recognized in most inexpensive shackles from the Far East production. So there must be assumed a much bigger variance of the strength values, even if the shackles look nearly identical from the outside.

Quality assurance

In addition to the choice of material and the appropriate manufacturing process a thorough quality assurance has a prominent role in the production of safety components. The basis is a material certificate and a certificate of origin for the raw material used. In the case of forged stainless steel shackles, the starting material is usually a long, round steel bar. Furthermore the manufacturing process together with its parameters (e.g. heating temperature, press pressure, dwell times, number of forging steps etc.) must  be controlled and documented too. This is the only way to guarantee a continuous product quality. Finally the strength of the shackle is checked at regular intervals, either batch-related (eg, 3 pieces of 1000), or as for example in Petersen products every shackle is individually  "Proof Load" tested.

For the Van Beest Green Pin shackles, made in the Netherlands, the production is very closely defined by the standard EN 13889. In this standard the requirements and steps of quality assurance are precisely defined. Therefore Van Beest Green Pin shackles have a very high product safety.

For the well- priced stainless steel shackles used in marine applications and very often in the slackline sport, there are virtually no standards. In addition, the respective manufacturers differ significantly concerning their quality assurance policy. Externally traceable documentation does not exist for the majority of cheaper stainless steel shackles. Mostly even the manufacturer is not known, because European importers do not communicate their suppliers and potentially change them frequently. Exceptions are only the high-priced shackles of renowned manufacturers like Petersen.

For shackles used in slackline setups with high security requirements (tricklines and longlines with high tension, highlines in general), a universally comprehensible documentation of quality should be an absolute requirement. No climber would use unaudited and non-standard carabiners from the home depot (to safe some bucks) for securing in heights. That would be absurd and dangerous. Currently for highlining the use of untested and non-standard conforming stainless steel shackles is a widespread practice. What security risks are implied?

Risks of non-certified products

In non-certified products, there is no proof that the shackle has the strength with which it is sold or marketed. In extreme cases this means that a low-cost stainless steel shackle, which is advertised with 60kN breaking load, can already break with a load of 20 kN (because it is made for example of a low-grade material, and has a large, internal flaw). If this shackle is heavily loaded in practice, e.g. because of an unfavorable triangular load or sustained fatigue load, their use is like a roulette game. In the worst case this shackle breaks without warning signs, gets a dangerous projectile and can harm the slackliner or uninvolved third parties. In case of a usage in highlines they can initialize the often quoted: "first term in the sequence of a tragic chain of events" (mainline fails because of a broken shackle, backup also fails because abrasion protection was not sufficient, etc.).

What consequences are to conclude from this? 

Shackles with inadequate quality assurance and documentation should only be used in slacklines with low tensions and height to be able to jump off safely. In addition, the WLL should be defined with a higher safety factor in order to obtain an additional risk buffer.

Minimum Breaking Strength (MBS) and Working Load Limit (WLL)

The two parameters minimum breaking strength (MBS) and working load limit (WLL) are firmly established terms within slacklining . Especially with the statement of a WLL a safe usage of the shackle is provided. The understanding of how a WLL should be calculated still varies greatly. How big should be the ratio: safety factor to the minimum breaking load (MBS)?

A good benchmark is the EN 13889, which applies to the Van Beest Green Pin shackles. In this standard, a minimum safety factor of 5 is given. Van Beest himself even uses a safety factor of 6. In general, a higher safety factor is preferable in slacklining, since the loads are not objects as in most cases in the lifting industry, but people balancing and falling on the tensioned setups. For this reason and in our point of view the safety factor for tested steel shackles should be at least 6. This security factor is the basis for our WLL information, stated on the website. For not certified shackles we propose a safety factor of 8, to increase the safety buffer and in combination with the limitation to low height (jump of safely) and tension. For highlines usage these shackles are no choice.

We will write more about MBS and WLL in a future blog post.

Which shackle is suitable for me?

To select the appropriate shackle you can use the following criterias in prioritized order:

1. Safety Requirements (approved vs. unaudited shackle)
2. Functional Requirements (geometry, interface, compatibility, mouth width, etc.)
3. Budget
4. Your Preferences

If the security requirements are high, we strongly recommend shackles with tested and traceable quality, which have been definitely produced by a forging process. Currently you can choose only between the low cost Van Beest Green Pin shackles and expensive, forged steel shackles by Wichard or Petersen.

We now close the existing gap in our portfolio and offer an affordable, forged stainless steel shackle with a traceable quality control with some new shackle models from Western European production. For these new EU stainless steel shackles we have certifcates for material and the forging process and a breaking load testimony from the manufacturer. In addition the EU stainless steel shackles offer a very useful feature: the so-called captive pin. The bolt remains fixed when the shackle is fully opened, thus eases the handling and prevents accidental loss.

Author info: Stefan owns a master degree (Dipl.-Ing.) in Mechanical Engineering with a focus on lightweight design and composites. Within his studies and practical work activities for the aviation industry, he has dealt in particular with the strength design and failure behavior of mechanical components and the quality of manufacturing processes.