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Suction Strainers, a Hungry Donkey, and the Paradox Of Choice

We’re faced with decisions every day. Decisions on minor issues are fairly easy because wrong choices carry little or no consequences. But what about technical issues, such as whether you should use a strainer on a hydraulic pump’s suction line? Opinions differ, so the choice is yours.

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A hungry donkey enters a barn in search of hay. Much to his delight he discovers two identical haystacks, each on the opposite side of the barn. The donkey stands in the middle of the barn between the two piles of hay, not knowing which one to choose. Hours go by, and then days, but he still can’t make up his mind. Unable to decide, the donkey starves to death.

This short parable comes from the French logician and philosopher Jean Buridan’s commentaries on Aristotle’s Theory of Action, and so the story is known as “Buridan’s Ass.” The moral of the story is: Not deciding has consequences. Having options can be a blessing as well a curse. Had the donkey found only one haystack inside the barn, there would have been no decision to make, and he wouldn’t have starved to death.

But life isn’t that simple. Back in 1941, the philosopher Eric Fromm wrote a book called Escape from Freedom. In it, he said that people in a modern democracy are beset not by a lack of options, but by a dizzying abundance of them. This holds even more true today. Figuratively speaking, we’re all Buridan’s ass, completely surrounded by haystacks. The options may be nice to have, but not deciding has consequences.

Decisions for Hydraulics

This concept, of course, applies to all levels of life, both personal and professional. In the hydraulics maintenance game (our main focus here), there are always decisions waiting to be made: what type of oil to use (lots of haystacks to choose from there); a planned change-out to call; which proactive maintenance tasks to do on what machines and when; a component to be declared faulty; and so on.

Often, such decisions must be made in the absence of perfect information, or worse, in the presence of conflicting information. An example that comes to mind is the installation of suction strainers on hydraulic-pump inlet lines.

As you may already be aware, I’m a loud and enthusiastic advocate of not installing suction strainers. For most applications, they should be removed and discarded when they have been installed. As a result, I’m always interested in any new information on this issue, especially if it’s anything “official” from a pump manufacturer.

On this note, the data sheet for Nachi PVS series variable volume piston pumps recently was brought to my attention. Under the heading, Management of Hydraulic Operating Fluid, on page P-2, Nachi recommends the installation of a suction strainer: “Provide a suction strainer with a filtering grade of about 100µm (150 mesh).”

Obviously, the people at Nachi haven’t read my books, Preventing Hydraulic Failures and The Hydraulic Maintenance Handbook  (although to be fair, neither has been translated into Japanese at this point in time). Seriously though, to the best of my knowledge, Nachi is in a very small minority of pump manufacturers that actually recommends the use of a suction strainer.

Contrast Nachi’s recommendation with Bosch Rexroth’s. Page 2 of its data sheet for SV-20 and SV-25 series variable vane pumps states: “Bosch Rexroth does not recommend the use of inlet suction strainers.”

That’s one pump manufacturer for and one against. Here’s another, Eaton-Vickers, sitting on the fence (bolding mine):

“[Reservoir] outlet line strainers, also called [pump] inlet filters or inlet screens are very common. This may be more traditional than functional…They are intended to keep larger solid contaminants from entering the hydraulic system. A drawback is that they are quite inaccessible for service and cleaning. If they become restricted due to excess contamination, they can cause cavitation and damage to system pumps. A more current approach is to ensure clean fluid is maintained in the reservoir, precluding the need for an outlet line [suction] strainer.

This piece of prose may well have been written by one of their in-house attorneys. It seems this pump manufacturer wants you to make up your own mind. But at least it implies that the tradition of always installing a suction strainer is outdated.

Then there’s this, from page 31 of the Contamination Control Program manual by Stauff—a company that does not manufacture pumps, but does make suction strainers (bolding theirs):

WARNING: It is advisable to check with the pump manufacturer before any type of filter is fitted to the pump inlet line…In general, suction strainers do not contribute to system cleanliness. The difficulty associated with changing strainers, and knowing if and when they are clogged [may result in pump damage].”

So if you take Stauff’s advice and check with the pump manufacturer, one says “yes,” another says “no,” and a third says “you decide” As is the case with myriad other hydraulic equipment maintenance decisions, this one’s your call. And it’s a call you have to make—one way or the other. Because as Buridan’s ass discovered, not deciding can result in the very same consequences you wished to avoid.

Brendan Casey is the founder of and the author of The Definitive Guide to Mobile Hydraulics, The Hydraulic Maintenance Handbook, Insider Secrets to Hydraulics, Preventing Hydraulic Failures, The Hydraulic Troubleshooting Handbook, Hydraulics Made Easy, Advanced Hydraulic Control and The Definitive Guide to Hydraulic Troubleshooting. A hydraulics specialist with an MBA, he has more than 25 years experience in the design, maintenance, and repair of mobile and industrial hydraulic equipment.

Download this article in .PDF format
This file type includes high resolution graphics and schematics when applicable.
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