[TowerTalk] Silly Question

[Mark, N1LO]

Mike Nash wrote
<<snip...3/16" EHS Cable 3990 lb. breaking strength. We all knew 
that...RIGHT???  3/8" x 6" drop forged turnbuckles- 1200 lb. Breaking 
strength!!! Am I missing something here? snip...>>

Hi Mike
Not a silly question at all. I think I see the source of confusion here: the 
difference between the working load of the item and its ultimate strength.

In the case of a 3/8" forged, eye-to-eye turnbuckle (IMO the best kind): 
Re-read the description and see if it says 1200 lb "safe working load" or 
the abbreviation SWL.

The Mcmaster-Carr online catalog has a nice listing of turnbuckles, and this 
size lists a "working load" of 1200 lb. The same turnbuckle is listed on 
Rohn's website with a 6000 lb "ultimate strength".

So, 3/16" EHS (4000 lb breaking) and this 3/8 turnbuckle would be a pretty 
good match. The turnbuckle is not the weakest link in a failure situation. 
The guy preload tension would be around 400 lb, well below the *safe working 
load* of the turnbuckle (1200 lb).

Safe working load, or SWL, is often 1/5 or a similar fraction of the 
ultimate, breaking strength of an item. Don't plan to use an item near its 
breaking point!
Ropes are rated this way too. You have to allow a safety factor in loading 
to account for shock loads and accidental overloads.

If in doubt, use a bigger size, and don't be tempted to use the regular 
hardware store type turnbuckles with a slotted eye or hook! Get forged 
hardware with a known rating from the vendor. If the SWL is not listed, use 
a value that is 1/5 (20%) of the breaking strength.

Hope this helped you, Mike

--...MARK_N1LO...--


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