Colleges and universities drape the cities of our planet, and they provide a solid foundation of mathematics, communications, processes and engineering to those entering (or already in) the field of fluid power. If you have a degree in mechanical engineering, for example, I can assume your fundamental understanding of math, physics, material properties and drafting are solid. However, because few schools offer dedicated fluid power programs, little, if any, hydraulic specialization is taught during one’s impressionable years in college. Where most engineers and designers get their fluid power education is on-the-job, and although my college tenure was surely less extensive than most, I still have a list of 9 things they never taught us in college.
1. Pressure makes it go
In my very first week in class, I was fed the common catch phrase, “flow makes it go.” Not soon after, pressure was subordinated to “resistance to flow.” At the time it didn’t make sense, and I asked my instructor if “pumps only provide flow, then why do electric motors need enough torque to meet the demands of the pump?” He answered that the electric motor just needs enough torque to overcome the pressure created by the system, which kind of made sense … I guess.
But the more I thought about it, the more it didn’t make sense. And the more it came up, the more I realized it wasn’t true. The idea that pumps only provide flow and that pressure is resistance to flow is like saying the load on a hydraulic cylinder is creating its own pressure, therefore creating its own force! This is a perpetual motion idea for a sci-fi novel, not a theory for engineering.
The reality is that pressure makes it go, and that pressure starts from the pump, not the resistance to flow. The Second Law of Thermodynamics is infallible as a physical law comes, and it is for this reason energy can only move from an area of high potential to an area of lower energy. This means flow in a hydraulic system can only move from an area of higher pressure to an area of lower pressure. Flow and pressure always start at the pump; pressure makes it go. Period.
2. Cosford’s Law
I wasn’t happy with my own revelation that pressure makes it go. The “Flow Makes it Go” meme was so prevalent in this industry that I felt the need to champion truth and replace it with something more authentic. A few years ago I came up with Cosford’s Law, which states that, “Pressure makes it go. Flow is the rate in which you create pressure.”
Force is transferred from the prime mover into the hydraulic pump, which then pushes the hydraulic fluid through the circuit. You must consider that fluid is just a solid capable of taking the shape of its container, and this is regardless of its compressibility. Hydraulic fluid transfers force no differently than a pushrod or crankshaft does in a smallblock V8. Understand that any and all fluid power machines are just force transfer systems.
Sure, it was narcissistic of me to create and name a law after myself, and I used the word “law” loosely. No scientific textbook will ever mention it, but I really just wanted to create a meme to counter flow makes it go. And with my classic English surname, one might imagine a Sir Cosford of Lincolnshire worked alongside Newton and Boyle in discovering the laws of nature. I strongly suggest you envision this fantasy, as the image of my giant melon and goofy smile is not one you want recalled when you think of Cosford’s Law.
3. Patents apparently have limited reach
I’m no lawyer, so my understanding of patent law leaves much to be desired, but college definitely didn’t prepare me for the apparent free-for-all occurring in the fluid power industry. They say that imitation is the sincerest form of flattery, and in this regard, Bosch Rexroth, Vickers, Sun et al should be blushing ten shades of pink.
The Bosch Rexroth A10V series pump has long been regarded as one of the most robust and reliable piston pumps ever manufactured, so it’s understandable it should be admired and reverse-engineered by competitors. However, this pump has not only been reverse-engineered, it’s been blatantly copied part for part. You can literally rebuild an A10V pump with imitation parts available from at least five manufacturers by my last count. How Bosch Rexroth, who is displeased when their logo is reprinted without their approval, let’s this occur shows my ignorance of business law, but I know they’re aware of the issue.
The Vickers PVB series piston pump has been discontinued by Eaton, and although not a pump of the same caliber as the A10V, it is a staple of North American manufacturing, especially in the steel industry. So that just as many manufacturers copy the PVB as they do the A10V, it says a lot about the pump. However, that it’s obsolete means Eaton is losing nothing but reputation if they don’t care about the potential sales.
Sun Hydraulics is one of the largest manufacturers of cartridge valves on the planet, which is not surprising considering their massive range of high-quality cartridge valves. Other than their unmatched line of counterbalance valves, what makes them unique is that Sun uses their own cavity designs. The rest of the industry uses the “common cavity” standard, and are mostly interchangeable. However, only a cartridge made to Sun’s T-3A cavity specification will fit into a T-3A cavity.
That a hole drilled into a chunk of metal can be patented, I don’t know, but plenty of competitors make cartridge valves that fit into Sun cavities. It’s easy to make the internals of a cartridge valve differently enough to avoid patent infringement, so the valve going in the cavity isn’t exactly the same. However, this is just another example of the copycat nature of hydraulics, and if you’re an insider who can explain the apparent lack of respect for patent law in this industry, I’d love to hear from you.
4. Cartridge valve engineers have too much time on their hands
Cartridge valve technology was covered in class, although a ton of time wasn’t spent on more than just basic pressure, flow and directional valves. College didn’t prepare me for the mind-boggling array of cartridge valve designs and the subsequent possibility of valve combinations ad infinitum.
The two largest cartridge valve manufacturers, HydraForce and Sun Hydraulics, probably have a combined range of unique valves over a thousand. HydraForce, for example, has at least seventeen sequence valves, which we were told in college would eventually be replaced by electronics. Additionally, as I sit writing this, I absolutely refuse to attempt to count how many counterbalance valves Sun manufactures … “ain’t nobody got time for that!”
5. Quality matters and doesn’t matter at all
When you learn about hydraulics, you really just learn about the generic components used in a circuit, like a gear pump or a directional valve, and less so about who makes them. Fluid power textbooks are often written by manufacturers, and Bosch Rexroth and Eaton both have excellent print material, but there’s no guarantee you’ll ever be exposed to the same components depicted in the textbooks.
Textbooks talk little of product quality, or even if it matters. Quality matters, but also it doesn’t matter. Let me explain. I learned about logsplitters in a textbook, but real world experience taught me you could purchase every component to build a logsplitter for less than $500. To say that logsplitters use poor quality pumps and valves is an understatement, but this is a case where quality doesn’t matter at all. A typical logsplitter will sit out in the elements year-round, and it will be fired up for forty hours per year every autumn.
Where quality completely matters, is with high-accuracy, high duty-cycle systems, such as on injection molding machines and forging presses. It simply makes no sense to put a $90 gear pump with expected life of 300 hours on a machine that costs $2 million and runs a 24/7 production. The range in product quality is as varied as the range in machinery, and its something college can’t teach you until you get out in the field to see these applications in person.
6. Suction filters are useless
I think the industry is finally starting to reject the suction filter concept. My old textbooks specify their use, and we were told all hydraulic systems should use them to protect the pump. But when you think about it, a suction filter can often filter no better than a 100-µm particle, which is proven to typically pass through a hydraulic system harmlessly anyway. It does nothing to prevent the wear particles most damaging to hydraulic systems, and a properly designed hydraulic system will not allow the ingression of large particles anyway.
The latest version of the Eaton Industrial Hydraulics Manual states, “It is Eaton’s contention that these devices play no part in cleaning a system…” Hopefully instructors are passing along this tidbit, because the only thing I’ve ever seen a suction filter do is cause cavitation.
7. Fluid “velocity” is meaningless
The early days of classroom learning discussed fluid velocity and we were all given the conductor sizing nomograph to help size suction, pressure and return lines, which are limited to 4, 12-25 and 15 ft/sec respectively, depending on who you ask.
But not once have I had a conversation in the design or troubleshooting of a hydraulic system where fluid velocity was part of the conversation. What really matters is pressure drop, which is how we size plumbing in the real world. We size suction, pressure and return lines within a range of acceptable pressure drop, and it’s always completely dependent upon the specifics of the application.
8. Pumps make annoying noises
I can hear a hydraulic pump from a hundred yards; their unique combination of buzz and whine is unmistakable. Other than the tame test benches in the lab, college doesn’t prepare you for the sounds you will hear coming from hydraulic power units. Chances are that if you’re in a manufacturing facility using hydraulics, the noise problems have been sorted out before the unit was installed in your plant.
However, if you are in the business of fabricating hydraulic power units, the cacophony emitted from power units upon start-up can be enough to turn you homicidal. The odd vibrations and harmonics that arise after a power unit is fired up are impossible to predict given the myriad combinations of tank design, pump design, mounting locations, plumbing materials and operating pressure possible. Sometimes as much labor can be wasted to dampen a shrill vibration as it took to build the unit itself.
9. Hydraulics and electrics are very similar
I’ve discussed the similarities between hydraulics and electrics many times in the past, and although electric and electronic control of hydraulics is covered well in most college programs, the actual similarities between the two vocations is not.
Pressure and voltage both describe energy potential, and flow and amperage both describe the rates of pressure and voltage delivery. Electric and hydraulic power calculations are similar, as is the idea of electron/fluid flow through conduits. Even the symbols used in each industry are similar. I submit that anyone who truly masters one vocation can easily learn the other, which explains why the industry leaders in one field are also heavily involved with the other. If you are an electrical engineer, for example, your mastery of hydraulics will require only the familiarization with the vernacular, symbols and applications.