Is your hydraulic system overdesigned?

I once configured a hydraulic power unit for an automotive customer that had so many components attached and installed that it looked like the engine bay on a new BMW M5 with the plastic cover removed. I used the word “configured” rather than “designed” because the end customer had a Functional Requirements Specification document that was over 30 pages long — it didn’t leave me much guesswork other than how big everything was going to be.

I mean, this thing had an oversized, elevated reservoir with an electrically monitored, locking ball valve at every fluid port, and pressure, return, and kidney loop filtration with a desiccant breather. Of course, the pressure filter was a duplex design that could be switched during operation, and every filter had electrically monitored bypass indicators.

The BMW M5
Image courtesy of AdobeStock

When you run an elevated reservoir, a locking ball valve is just plain smart — with a two-inch suction line, that eye-level tank can drain faster than a nepo-baby’s trust fund. But when your locking, monitored ball valve costs more than the kidney loop pump you’re protecting from cavitation, maybe you’ve overdesigned your hydraulic system.

Don’t get me wrong — the customer paid good money for their excess, and as a hydraulic designer myself, it’s fun building a cost-no-object power unit. And if you’re one of my own customers reading this, look away now, because a power unit made for a third of the price would have worked just as well. Instead, excessive capital investment displaces net profit, while providing only marginal benefit to the production line.

And while we’re at it, can we stop with all the just-in-casing? I’ve been guilty of recommending this in my missives, so that just shows you how widespread our thinking is. Say we build a circuit with a required 25 gpm, but because we couldn’t get the precise displacement for that volume, we go one size up in the manufacturer’s catalog. Great, we now have 30 gpm capacity, which, for a fixed pump running 3,000 psi, is wasting nearly nine horsepower if we try to force this through a priority flow control (which none of you are dumb enough to do, right?).

Okay, okay, nobody’s going to do what I mentioned, I know. Because gear pumps offer finer displacement increments, such excess is unlikely unless you move up to a piston pump with coarser steps to choose from. Regardless, most piston pumps have max volume adjustment, so you don’t have to settle for the extra displacement.

My point is, have you ever tried to under-design something? Seriously, what would happen if you had just barely enough for everything? Try selecting an efficient pump design that’s barely enough flow rather than a hog that wastes 20% of your energy. Instead of multiple filter locations, what about a single, low-micron, high beta ratio return filter? Rather than oversizing the plumbing, make it “big enough” and realize the machine only runs at 80% of its max capacity, anyway. How about, instead of buying enough food for an Italian wedding, we get just what we need for the party and accept that we might run out? Sorry, that last one was for my wife.

Overdesigning has effects that compound in the form of higher initial investment, extensive maintenance, or irresponsible use of fuel or electricity. So when you’re designing your next system, stop and think about whether you’re buffing it up for the sake of it, and that every decision matters to cost and performance. Otherwise, you’ll end up like the new BMW M5, which is 1,000 lb heavier than the old model and slower to 60 mph despite having three digits more horsepower.

Filed Under: Components, Engineering Basics