Don't Push the Limit

by Bill Sapsis

Bill Sapsis
Bill Sapsis
Our rigging Q&A column debuts, with a wellspring of info on WLL

Greetings readers. Welcome to my new adventure. I’ve always wanted to do a Q&A column and now Stage Directions magazine has given me the chance. My thanks to Jacob Coakley for pulling this together. Here’s how this is gonna work. You send me questions (This email address is being protected from spambots. You need JavaScript enabled to view it.) and I’ll answer them to the best of my ability. Fair enough?

Bruce D. asks:

My shackles are stamped with a WLL of 4.5 tons. Who decides what the WLL is and how do they determine the WLL?

Thanks, Bruce! That’s a good question to get us started.

Before I can answer your question I need to give you the definition of WLL. WLL stands for Working Load Limit. This is the currently acceptable way of defining the highest amount of weight or force that can be applied to a piece of hardware. It used to be called the Safe Working Load (SWL) till someone decided that the term was too ambiguous and gave users too much wiggle room. So now we use WLL.

OK then. Off to your question.

The company that manufactures the shackle gets to decide the WLL for that shackle and, for that matter, all of the hardware they make. They do this for a variety of reasons, but the primary reason is liability. Or, more to the point, the avoidance of liability. What they are doing is providing a margin of error over and above the Working Load Limit so that when you “accidentally” overload the shackle it won’t suffer a catastrophic failure. Catastrophic failures usually bring out the lawyers—and manufacturers desperately want to avoid lawyers.

Manufacturers use an equation to determine the WLL. That equation looks like this:

In this equation BS is the Breaking Strength, DF is the Design Factor, and WLL is the Working Load Limit.

To get a BS that’s no bull you need a testing machine and some time, because you’re going to be breaking a lot of hardware. A shackle is put on a pull test machine and is pulled until it breaks. Then a bunch more shackles are broken in the exact same way. Once you have broken enough shackles you take the average of all the tests and that gives you the Breaking Strength. It’s important to note that the pull test is a steady increase in force to avoid any chance of a shock load. Shock loads are not good as they mess up the math considerably.

After you get the Breaking Strength you then need to find yourself a Design Factor (DF). A Design Factor is a number, an arbitrary number, used to develop a margin of error, or cushion, if you will, between the Working Load Limit and the Breaking Strength. The Design Factor for hardware is chosen based on industry and ANSI standards and on the level of comfort the manufacturer wants to achieve. The Design Factor used by the three major suppliers of shackles to the entertainment industry (Chicago Hardware, Columbus McKinnon and The Crosby Group) is 6.

You now have the two components of the equation that you need and you can start plugging numbers into it. Let’s assume the Breaking Strength of the shackle in question is 50,000 pounds. Using a Design Factor of 6 you get a Working Load Limit of 8,333.33 lbs. (50,000/6 = 8,333.33)

As you can see, there’s a huge difference between the Working Load Limit and the Breaking Strength, so there might be a temptation on occasion to load up the shackle higher than the Working Load Limit. Don’t. The Design Factor, as represented by each and every one of those 41,366 pounds, is there to protect you from yourself. It’s there to compensate for all the variables that you may have overlooked.

For starters, how old is that shackle you’re using? It doesn’t have a serial number, so I’m pretty sure you have no clue unless it happens to be brand new. Normal wear and tear will take its toll. Shackles also get beat up in the way they are handled. I’ve watched technicians toss shackles across a room trying to get them into the road box. I’ve also watched as the shackles, and the steel slings they’re attached to, get kicked off of a 15-foot-high stage onto the concrete floor below. This treatment does the shackle no good.

But shock loads are the worst of it. Shock loads happen all the time. Simply pushing the Go or Stop button on a chain hoist adds a significant shock load to the hoist. We seldom carry that thought, however, over to the shackles and other hardware. Depending on the speed of the hoist you can add up to about 180% of the load when you start or stop a single speed hoist.

Add up all these variables and you can see how quickly that 41,366 lbs. can get used up. Couple that with the fact that we, as an industry, are dealing with significantly higher loads moving at faster speeds than ever before and you can see that a Working Load Limit is really important.

My advice? Don’t exceed the WLL on shackles or any of your other hardware. And don’t use hardware that doesn’t have a WLL stamped on it.

Happy rigging!

“Ask Uncle Bill” will be a quarterly feature in Stage Directions magazine, so send in your questions about rigging to This email address is being protected from spambots. You need JavaScript enabled to view it..