Application of Connectors
Connectors (shackles, quicklinks and carabiners) are what hold slackline rigs together. They are an integral part of the system and it is important that every serious slackliner learn their purpose, limitations, ratings and the differences between Working Load Limit (WLL), the Minimum Breaking Strength (MBS), Safety Factor (SF) and the little known about Design Factor (DF). The following section will discuss what all these acronyms mean, why they are important to know and how they are applied to the connectors. We’ll also go over the different types of connectors and how they are used. It's worth noting before we get too deep into things that different industries use these terms in slightly different ways depending on the connector. This article is an attempt to simplify these ideas so they are more commonly known within the slackline community.
Understanding Connectors and Their Ratings
The important acronyms regarding connectors are WLL, MBS, SF and DF:
- Working Load Limit (WLL): tells you the maximum load that should apply to the connector no matter what the industry or activity during normal use.
- Minimum Breaking Strength (MBS): tells you the lowest load the connector should break at given a 3 sigma testing protocol.
- Safety Factor (SF): can be stated by the industry and is a factor derived from the MBS, for slacklining we use 5:1.
- Design Factor (DF): similar to the Safety Factor but it is a factor stated by the gear manufacturer. SlackTech uses a 5:1 DF when creating gear to align with industry standards. It can also tell you how many times higher the MBS is over the WLL.
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It is critical to know these numbers because of the phenomena of material fatigue. To illustrate this concept, imagine your ear has a Minimum Breaking Strength (MBS) of 7 pounds and a Design Factor (DF) of 7, this means your ear will have a Working Load Limit (WLL) of 1 pound as stated by the creator. If I yank outward on your ear with seven pounds or more of force, it will probably tear off rather quickly. Maybe it will take a little bit more load because the maker of your body wanted to be on the safe side, but it will probably pop off at or just above 7 pounds. However, if the load were to stay lower than one pound, no trauma happen. I can pull on it as many times as possible and nothing will happen. Your ear was designed to handle, with ease, anything 1 pound and under. But, if I were to repeatedly pull in between a 1 and 7 pound load, it will hold the first time, but eventually, after enough cycles it will tear off. The tissue would suffer from being repeatedly loaded over the WLL, become “fatigued” and fail at a lower threshold than the MBS. We can apply this ear concept to the types of connectors used in a slackline system. But really, it comes down to the same thing: If you pull on something with too high of a load enough times, it will fail.
Another reason load limits exist is because connectors are meant to be loaded in specific ways. Generally, connectors perform best when loaded on a straight axis with small metal pins. That is also how the majority of connectors are tested and how the load limits are meant to be applied. When connections are made that do not follow the intended use or loading characteristics of the connector the force can be distributed differently. Often, this can reduce the MBS of the connector if it was not intended to handle that type of load. We can apply this to the ear analogy to make it more clear: Imagine I pull on your ear with a downwards force instead of a force straight out like in the tests. It would probably tear off at a much lower force because the load is being applied in a different, unintended direction.
Material Debate: Aluminum vs Steel
There is a bit of controversy in the slackline community regarding aluminum vs. steel. There is a lot of advice floating around the slackline community concerning these two materials, mostly it’s the idea that “aluminum should not be used in any slackline system!”
Connectors are usually made from an alloy; not pure steel or pure aluminum. These alloys are typically chosen for strength and factors like corrosion resistance, brittleness, hardness and common failure modes. Every material performs differently when loaded. Some might be more brittle and show next to no plastic deformation before breaking (meaning you will not see them bend before failure). Others might bend significantly before breaking. Generally speaking, steel alloys bend before they break while aluminum will not significantly bend before breaking, although there can be exceptions. So, what it really comes down to is using the appropriate component designed for a particular application.When in doubt simply ask the manufacturer or consult the manual the piece of gear came with.