During summer vacations, my wife and I stay in a cottage that draws water from a clean lake about 30 feet away. A reciprocating Southern-Deming Model S-250 pump has worked well for more than 40 summers, and we have kept the pump and motor well maintained even though we use it only for a few weeks a year. A trip to the nearest hardware store or pump-repair business takes some time, so we have spare parts on hand -- drive belt, seals, plungers, fittings, and so on. I've replaced the leather plungers several times, the drive belt once, and have had to create a new line to the pressure switch, so it made sense to prepare for a pump failure.
This year when I primed the pump, I got a nasty surprise. The connection between the steel water-inlet pipe and the cast-iron pump body had completely rusted out. I never expected that connection to rust away and fail, because it always seemed solid. Unfortunately, it would have cost more to repair the faithful pump than to replace it, so we bought a new and simpler pump, as shown in the photo.
The point here is that the most "solid" part of a device can fail even though we test it thoroughly and don't expect it to fail over the life of a product. In mechatronic systems, it can pay to subject components and complete systems to highly accelerated life test (HALT) and highly accelerate stress screen (HASS) regimens.
The electronics industry has applied these techniques for many years, and you can find a series of Web pages that describe these types of tests. Although this information and other resources on the Web apply mainly to electronic components, you can apply the underlying principles with mechatronic designs, too. Even if you contract with a testing lab to perform accelerated testing, you should know what tests you need to run, and how you want to run them.
Years ago, the US military created a handbook, "Reliability Prediction of Electronic Equipment," MIL-HDBK-217, now at revision F, and available as a pdf. This information pertains to components such as op-amps, memories, and logic devices, but it provides useful reliability-prediction guidance that can apply to other types of devices, too.
Accelerated and stress testing often involve what engineers call a "shake-and-bake" environment that literally vibrates equipment and exposes it to temperature extremes. Formally, engineers know this area as environmental stress screening (ESS). Here is a detailed "Environmental Stress Screening Tutorial." Find other ESS information here.
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?
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
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?
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
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, 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.
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.
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
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
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...
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During a recent meeting with engineering-school faculty and alumni, Contributing Technical Editor Jon Titus talked about whether colleges should educate generalists or specialists. What do you think?
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