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darryl
07-24-2008, 02:40 AM
Saw some gears today made from what's called ergal. Anybody heard of this? Apparently it's 7075, maybe a special blend of ingreedients, maybe a special processing, maybe just a fancy name to raise the price of it. Is this anything special, or just another name for 7075?

Fasttrack
07-24-2008, 02:59 AM
Ergal is a term corresponding to potential energy in physics. It was used in conjunction with "virial" by Clausius in his theory of heat. Although Ergal has become a somewhat dated term, virial is still used as in the Virial Theroem.
;)


I think Ergal in machining was originally a brand name for 7075 but I could be wrong.

Evan
07-24-2008, 03:55 AM
Whatever it is it isn't a recognized name for any aluminum alloy. This is what Matweb has to say about it:



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It's the same as "Fortal". 7075 T6 aluminum has a well deserved bad reputation for stress corrosion cracking. While it is extremely strong it cannot withstand repeated stress close to the elastic limit for the alloy. If it is repeatedly stressed to near but below the elastic limit it may have a cycle life of only a few hundred such events before it either cracks or catastrophically fails. I am sure that this is behind the renaming of the alloy in order to hide and disassociate it from it's reputation for cracking. The name "Ergal" seems to have been developed particularly to hide the actual alloy name in the making of bicycle parts. As with Fortal vague and nebulous claims are made for "special processing" and "strict control" etc. At least with Fortal you can find the actual material specs which not coincidentally are precisely the same as 7075-T6 aluminumn alloy.

It's also a lot like the total BS that surrounds the use of "special" copper in audio speaker cables etc.

Here is an example of the sort of failure that occurs with 7075-T6. I was using this piece as a clamp and it had been repeatedly stressed to the point of bending slightly but without any sign of permanent deformation. One day when I was tightening it it just went BANG and popped in two pieces.

http://vts.bc.ca/pics/7075a.jpg

John Stevenson
07-24-2008, 04:38 AM
Clumsey Bastich....................:)

.

wierdscience
07-24-2008, 07:56 AM
Whatever it is it isn't a recognized name for any aluminum alloy. This is what Matweb has to say about it:



It's the same as "Fortal". 7075 T6 aluminum has a well deserved bad reputation for stress corrosion cracking. While it is extremely strong it cannot withstand repeated stress close to the elastic limit for the alloy. If it is repeatedly stressed to near but below the elastic limit it may have a cycle life of only a few hundred such events before it either cracks or catastrophically fails. I am sure that this is behind the renaming of the alloy in order to hide and disassociate it from it's reputation for cracking. The name "Ergal" seems to have been developed particularly to hide the actual alloy name in the making of bicycle parts. As with Fortal vague and nebulous claims are made for "special processing" and "strict control" etc. At least with Fortal you can find the actual material specs which not coincidentally are precisely the same as 7075-T6 aluminumn alloy.

It's also a lot like the total BS that surrounds the use of "special" copper in audio speaker cables etc.

Here is an example of the sort of failure that occurs with 7075-T6. I was using this piece as a clamp and it had been repeatedly stressed to the point of bending slightly but without any sign of permanent deformation. One day when I was tightening it it just went BANG and popped in two pieces.

http://vts.bc.ca/pics/7075a.jpg

I was told by an Alcoa rep that there is a alot of unprocessed 7075 floating around the world that carries the T6 designation.It doesn't have the same post treatment properties and artificial aging specs as 7075-T73 which was developed specifically to reduce SCC in 7075.The trade off though is it has slightly less strength than T6.See here-

http://books.google.com/books?id=iEeiQEeLOmYC&pg=PA114&lpg=PA114&dq=7075+ARTIFICIAL+AGING&source=web&ots=EHjbW0Y2Xf&sig=Vthda-wO_nRQfd4SbYdccFArzLk&hl=en&sa=X&oi=book_result&resnum=4&ct=result

I have about 25' feet of 4-1/2" OD 7075 T2 laying around with the heat treat instructions and the aging data.

The last part of the T73 process is a 300-350F soak 4hours for every inch of thickness which makes it cost intensive to process.

Evan
07-24-2008, 08:46 AM
That is called "over aging" and it does reduce strength slightly but greatly reduces the cracking problem.

airsmith282
07-24-2008, 10:34 AM
so if the stuff is so crapy then why use ,and and really what would one use it for being that it has these defects to it

NickH
07-25-2008, 11:50 AM
I suppose it would be great for washers and spacers loaded only in compression ;-)
Nick

Evan
07-25-2008, 01:02 PM
so if the stuff is so crapy then why use ,and and really what would one use it for being that it has these defects to it


There are many places where the tremendous strength to weight ratio of 7075 alloy is valuable. There are plenty of applications where the strength is potentially required someday but not called on in normal service. In a commercial jet liner for example the seat frames and seat belt brackets are 7075 T6. They have to be strong enough to withstand the weight of a 250 lb passenger when the plane pulls up to 3 gees and with a safety margin above that. That is not something the happens often and on many aircraft will never happen in the service life of the machine. But, that reserve strength must be there and nothing that is remotely cost effective and well understood beats the 7000 series alloys. That is changing though with the advent of super composite materials.

The problems that 7075 has with stress corrosion cracking are common in varying degrees to all high strength aluminum alloys. 7075-T6 is particularly bad but if allowances are made for this behavior then it doesn't present a problem. It should be used where it will not be repetitively loaded close to the structural limit of the part. It's ideal where the high strength is a requirement but not tested on a regular basis.

Aluminum alloys in general have an upper fatigue life limit, unlike steel. No matter how lightly a high strength aluminum alloy is loaded strain accumulates in the form of crystal boundary dislocations in the intergranular regions of the alloyed metals. This places an upper limit that is proportional to the percent of plastic limit stress applied times the number of repetitions and a factor for each alloy and heat treatment class. It is why aircraft have stated airframe lifetime limits.

MickeyD
07-25-2008, 02:17 PM
The British had a tough series of lessons about aluminum fatigue with the de Havilland Comet. A couple of poor design choices stressed the aircraft skin almost to the point of failure, and after repeated pressure cycles, the aircraft would come apart, generally at a high altitude over the ocean. There is a lot of information both online and in book form about these accidents and figuring out what caused them, and is well worth a read.

As a side note, the great mechanical geek British author Nevil Shute wrote a fictional book, No Highway, about a similar ocurance four years before this happened that is a very enjoyable book.