# A question about STAC chain.



## cmckeeman (Jan 30, 2013)

So with all the STAC chain i have used the links are made so that two of them can't go past each other in another link, what this means is that you can get one link so it's sitting on another if you aren't careful, is there a design necessity for this, it would appear to me that the chain would be safer if links could simply pass each other without having to flip a link?


----------



## DuckJordan (Jan 30, 2013)

> Then its not a true stac chain. A true stack chain should be able to have a dead link between two live links.


Edit: This is why I shouldn't post after doing points in a 70' house. Misunderstood stac chain. Was thinking of something else.


----------



## SimplyCircus (Jan 31, 2013)

cmckeeman said:


> So with all the STAC chain i have used the links are made so that two of them can't go past each other in another link, what this means is that you can get one link so it's sitting on another if you aren't careful, is there a design necessity for this, it would appear to me that the chain would be safer if links could simply pass each other without having to flip a link?



Here are the specs for CM STAC chain. As you can see, they are designed not to pass each other, but rather to fit over the ears of a shackle. 

http://www.riggingwarehouse.com/customer/petrco/specpages/CM_STAC Chain Specs.pdf

As to why, well, its part of the standard. Yes, not passing each other is part of the specification for lifting chain, specifically the US National Association of Chain Manufacturers' specification for overhead lifting.


HTH


----------



## cmckeeman (Jan 31, 2013)

why is that a necessity for overhead lifting?


----------



## SimplyCircus (Jan 31, 2013)

The standard does not say, but section 6.2 (2003 edition), specifies the dimensions in tables iv and v. 

I didn't write the standard, I just have a copy of it.


----------



## Chris15 (Jan 31, 2013)

My guess is that if we remember that strictly speaking overhead lifting is defined as a chain that passes over the lifting wheel in a hoist, then in that instance, a possibility of a link being skipped could end up very badly...


----------



## SimplyCircus (Jan 31, 2013)

In all my years of rigging, I have never seen that definition of overhead lifting. I would love to know where that definition comes from.

Overhead lifting is typically defined as the lifting and moving of a load overhead. 

Suspension is generally defined as suspending a load overhead (notice: no movement)

Overhead lifting typically implies a QC expecting it to be the only support.

Suspension generally implies a QC that assume redundant supports.

But again, as far as I know, their are no official terms. It is not defined in any standard I have seen (I researched it quite a bit when I wrote Shackles | Simply Circus )


----------



## MNicolai (Jan 31, 2013)

SimplyCircus said:


> In all my years of rigging, I have never seen that definition of overhead lifting. I would love to know where that definition comes from.
> 
> Overhead lifting is typically defined as the lifting and moving of a load overhead.
> 
> ...



NACM defines overhead lifting for chain as:

> That process of lifting that would elevate a freely suspended load to such a position that dropping the load would present a possibility of bodily injury or property damage.



Worth noting that if something is suspended on several unique points, it's not freely suspended and therefore not overhead lifting. It may be lifting that is happening over someone's head, but by NACM's definition, "Overhead Lifting", it is not.


----------



## egilson1 (Jan 31, 2013)

Entertainment Structures Group has a good white paper about use of trim chains and talks in depth about the definition of Overhead Lifting. You can find it at http://esg.ssastructural.com/userfiles/files/fall08.pdf


Regards,
Ethan


----------



## BillConnerFASTC (Jan 31, 2013)

Interesting that when I contacted all of the members of the committee that writes the NCMA standard and showed them pictures and diagrams of a line set with multiple pick points and the trim chains, they all said it was overhead lifting and that it required alloy chain like grade 80 or Secoa's STC or Clancy's Alpha Chain.

FWIW I also disagree with some of the points in the ESG paper. You can't change the intent of a standard and disregard the intent and interpretation of the standard's writers.


----------



## SimplyCircus (Jan 31, 2013)

BillConnerASTC said:


> Interesting that when I contacted all of the members of the committee that writes the NCMA standard and showed them pictures and diagrams of a line set with multiple pick points and the trim chains, they all said it was overhead lifting and that it required alloy chain like grade 80 or Secoa's STC or Clancy's Alpha Chain.
> 
> FWIW I also disagree with some of the points in the ESG paper. You can't change the intent of a standard and disregard the intent and interpretation of the standard's writers.



Part of what makes it a difficult job is that they don't agree with each other about it. Grade 80, 100 and 120 chain can't be used to make a basket. Grade 70 is often used for that, and its overhead lifting as well. 

I am going to look into this a LOT in the next few months. Will report back my findings.


----------



## MPowers (Feb 1, 2013)

I've been following this thread and it looks like we've moved to a new/old topic, i.e., trim chains/definition of "freely suspended/definition of "overhead lifting". And lots of inconsistencies. 

The following is IMHO from 49 years and 7 months of professional rigging. I know that some agree and some disagree. FWIW, I have seen and, officially and unofficially, investigated a significant number of "incidents" and "accidents". I have never seen a trim chain fail, regardless of the chain size or grade. As part of my regular work, I do facility safety inspections. Rigging is of course the major component of those inspections. I have found chains, wire rope and other components with well over 50 years in use. Trim chains, as a group, are by far the least affected by long and hard use. I regret now that I didn't foresee this conversation and document those observations until now. 

My major observation is not unique or new. Trim chain, even the smallest size of G30 proof coil, is far stronger than the wire rope used in the system. Example: 1/4", 7x19 GAC has a breaking strength of 7,000# and a WLL of 875#. 1/4" G30 proof coil chain has a WLL of 1,300# and a breaking strength of 5,200#. As trim chain is, when properly used, rigged in a "basket" configuration, the load capabilities are doubled. What that means is that the load ratings, both breaking strength and WLL, of even the lowly 1/4" chain is over a ton greater than the aircraft cable that supports it.

My second observation is that the suspension point terminations, i.e. the wire rope thimble eye and the first link of the chain, show zero evidence of wear due to friction. This supports the definition of "freely suspended" as a single point with no restrictions to twisting or swaying. 

My major point here is I wish that, as an industry, we could make a standard that is recognized, to define, specifically, what a trim chain is and must be. To define what "overhead lifting" is. 

If we define grade 80 chain as the only acceptable chain, we will all eliminate trim chains. All riggers will go to batten clamps and turn buckles. If we create a code that specs STC or Alpha chain, fine. Over kill IMHO but we're all on the same page and playing field. If a Standard were to be drafted and approved, it would have to include a paragraph on how to deal with existing installations and what "must" be upgraded and what "shall" be upgraded.


----------



## BillConnerFASTC (Feb 1, 2013)

All good points and all the grade 30 trim chains installed on my specifications are very low on the list for causing me sleepless nights. The problem remains that the manufacturers of the chain say only grade 80 and above is suitable for overhead lifting application and they confirm that trim chains are an overhead lifting application, the total reinvention and redefinition of "freely suspended" by some theatre practioners not withstanding. Hats off to SECOA and Clancy for finding a reasonable way around this liability issue.


----------

