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Greg Stirling
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Platinum
An Unexpected Failure Mechanism
Greg Stirling   8/17/2011 1:06:20 PM
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Might help to keep the housing filled with water during the offseason - if that is possible.  Oil would work better - however this is drinking water.

I had a similar problem with a pool pump that only runs occasionally to power a fountian.  The iron outlet pipe will oxidize to the point that it restricts flow.

Seems like continuis running pumps suffer less corrosion due to a lack of oxygen and wet/dry cycles.

In this case it is a matter of more expensive stainless steel, zinc, or aluminum housings, or perhaps plastics such as PVC or Nylon

Or the iron housing, with a constant battle against corrosion... 

Jon Titus
User Rank
Blogger
Re: Corrosion
Jon Titus   8/17/2011 11:59:23 AM
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Hi, Dave.

I didn't install the original pump so I assume the pipe was carbon steel.  I know it wasn't stainless steel.  The pump body was cast iron and we run polyethylene pipe down to the lake.  We thoroughly drained the pipes and the pump at the end of the summer.  Perhaps the pipe-joint compound used by the installer dried up and let water into the pipe-to-pump threads, which caused the rust over the years. --Jon

Jon Titus
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Blogger
Re: Shake and Bake a best practice?
Jon Titus   8/17/2011 11:54:10 AM
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Hi, TJ:

I have seen similar failures due to conditions we can't easily test for.  Your sink drain provides a good example, although I'd think the manufacturer--knowing the use conditions--would have tested drains under different water conditions.  Maybe a cheap knock-off manufacturer went for low cost instead of quality. --Jon

Dave Palmer
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Platinum
Corrosion
Dave Palmer   8/17/2011 11:09:36 AM
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Was the pipe carbon steel or stainless steel? I wouldn't expect a galvanic couple between carbon steel and cast iron, but depending on the type of stainless, there could be a significant galvanic couple between stainless and cast iron.  A plastic or rubber isolator between the two could help prevent this. 

TJ McDermott
User Rank
Blogger
Re: Shake and Bake a best practice?
TJ McDermott   8/17/2011 11:07:34 AM
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Jon, I understand the difference between vibration, temperature, and corrosion tests, and know there are standards for each.

Applying the results of a vibration and temperature tests are MUCH easier to deciding the life of a component than it is to using the corrosion results.

The specifications for a component will say "up to 50g, 155deg F".  It may even say something about the 20% salt bath immersion for 20 days.

I saw a new sink fixture component corrode away in 5 years on city water when I was a kid.  I think it would be difficult to relate the corrosion test results to city water to come up with an expected lifetime.

Jon Titus
User Rank
Blogger
Re: Shake and Bake a best practice?
Jon Titus   8/17/2011 10:57:28 AM
Hi, TJ.

People would run special corrosion tests, which many commercial test labs know how to perform.  The "shake-and-bake" tests look for structural weaknesses at various frequencies and g levels, while the environmental temperature tests look for premature electrical and electronic failures as well as failures induced by the stresses and strains created by heating and cooling. --Jon Titus

Jon Titus
User Rank
Blogger
NIST Tests Show Carbon Nanotubes Fail Quickly
Jon Titus   8/17/2011 10:53:36 AM
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On a related subject, the US National Institute of Science and Technology (NIST) recently published information about the failures of carbon nanotubes used as electrical conductors in microcircuits.  Tests showed failures of the nanotubes after only 40 hours.  Although many researchers have touted the capabilities of these carbon structures to carry high currents, they seem to have problems that deserve more research before companies use them in integrated circuits.  You can read the complete article at: http://www.nist.gov/public_affairs/tech-beat/tb20110816.cfm#cnt. --Jon Titus

TJ McDermott
User Rank
Blogger
Re: Shake and Bake a best practice?
TJ McDermott   8/17/2011 10:50:59 AM
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Wouldn't it be difficult to determine the life of a component against corrosion in a "shake & bake" test?  How would you accelerate corrosion and still get a meaningful result for component life-time?

 

 

Jon Titus
User Rank
Blogger
Not everyone tests products
Jon Titus   8/17/2011 10:45:38 AM
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Hi, Beth.

Those types of tests should be part of a comprehensive testing program, but many small companies lack a strong background in testing or they lack the time and money to have a contract company run them.  Also, when a company custom builds mechatronic equipment, they might not think such tests deserve attention for one-off designs.  Also, I doubt many colleges or universities teach engineers about real-world product or system tests. --Jon

Beth Stackpole
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Blogger
Shake and Bake a best practice?
Beth Stackpole   8/17/2011 9:09:32 AM
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Sorry to hear about your pump, Jon. Aren't the so-called "shake and bake" tests a standard best practice for mechanical parts and assemblies or is that just another example of a no-brainer routine falling through the cracks due to engineering short cuts?

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