The hydraulic oil is not your problem, it's the pump!

Dear Hydraulics Community,

Re: System efficiency

I wanted to take a moment here to share some insights that you may find useful regarding hydraulic system efficiency. We have all probably looked for ways to improve overall system efficiency by analyzing the volumetric efficiency of hydraulic pumps. This is often referred to as measuring the "internal leakage" in a pump...where there is a disparity between the theoretical amount of oil that can be pushed out vs. the actual amount of oil that leaves the pump under pressure to do useful work.

I have to assume that my experiences in trying to increase this efficiency have been similar to that of most people in the industry. This is probably a safe assumption given the focus of the hydraulic fluid manufacturers on increasing the viscosity index of a fluid (VI) as well as the shear stability of the fluid. Their catalogs and literature are full of great information on the VI of a fluid. That is...the fluid's ability to maintain a constant viscosity irrespective of its change in temperature. Similarly, shear stability is a measure of a fluid's ability to resist changes in viscosity as shear rates increase.

Many of us have gone through the gyrations of testing multiple fluids to see which ones will perform best in a given system. It feels like the oil is the only variable that can be adjusted to really affect this aspect of efficiency. And...Oh!...if only the hydraulic oil spec sheets could just be trusted on face value to answer this question! Remember, if you’re downloading a fluid spec sheet, you’re downloading something that was created by a marketing department.

What I want to share is that by testing and testing every fluid under the sun (as I'm guilty of doing), you're actually trying to solve an economics problem...and you're only looking at one variable in doing so. That's not said to be harsh. But, increasing efficiency is really about saving money for someone along the value chain. We don't chase efficiency just to gain bragging rights about how great our engineering staff is. Increased efficiency is not an end unto itself…but a means to an end that has dollars attached to it somewhere along the line.

The other (major) variable in this economic equation is the pump itself, of course. And, it's not as if short shrift is given to this component during the design process. Lots of time and effort are spent agonizing over what pump to use. But, there's probably a case to be made that each of us works in a very limited "box" when it comes to considering this critical component. We are given an industry standard toolkit and we generally accept that these are the only tools we have to work with to reach our efficiency goals.

I fell into this trap myself and spent a good seven years (off and on) testing fluids from every major supplier of every imaginable class of fluid chasing the holy grail of a fluid that had a (ridiculously) high VI and did not suffer the temporary losses in viscosity created by high shear rates found in the external gear pumps I was working with. (Look up Ted Selby's "Viscosity Index Trapezoid" paper on his company's, Savant Group, website, https://goo.gl/Dwdxr2 ) The thought was that if the fluid always stayed at a constant "ideal" viscosity that the machine would function equally well in cold and hot conditions. (Long story). I'd feel silly for trying this...except for the numerous articles published over the years sharing the stories of other people spending even more time and effort chasing the same goal of looking for a highly efficient hydraulic fluid.

But, remember, the oil is just one variable. What about the pump? The nice thing about playing with oil is that the science is quantifiable and it's relatively straightforward to test its performance in lab conditions. Pumps, on the other hand...come with a host of other challenges when it comes to figuring out what's going on inside of them. The drum to drum variations in a fluid's performance is insignificant when compared to the machining tolerance variations that come with pumps. And, measuring a pump's performance is a chore no matter what piece of machinery it's on. Lab testing a pump is expensive. Field testing of a pump is even more expensive! Measuring volumetric (and mechanical) efficiency with any sort of accuracy is also fraught with challenges and expense. And, in the end, the volumetric performance of most piston and external gear pumps have converged to a narrow band of expectations. It's not usually the deciding factor as to which brand or component series to use for your system (though there are exceptions).

You can perhaps begin to see how easily many of us have fallen into this myopic view of the options before us. Fix the oil and we've "fixed" the problem. What I'm here to tell you today is that the oil isn't the problem. The problem is actually the pump.

There aren't too many manufacturers that will be happy to read that statement. The oil manufacturers have convinced the industry that a higher VI is better. Don't get me wrong in this criticism. Hydraulic oil is my favorite component! I love it. I honestly think the technology in it is fascinating and exciting. (Yes...I'm a nerd). But, my years of testing revealed that we're putting a very expensive band-aid on a relatively low-cost pump problem.

This is best viewed in the examination of external gear pumps...but the analogy extends to the worlds of piston and vane pumps as well. As for gear pumps, the quest needs to change from looking for an oil that doesn't thin out with increasing temperature and high shear rates to looking for pumps that really are viscosity insensitive. Pumps that are volumetrically efficient across a much wider range of viscosities. As it turns out...this exists! Yeah! It's just that this other type of gear pump, an internal gear pump, is a little less common than external spur-type gear ones. The single best place to go for internal gear pumps is still the company that invented them, Eckerle. https://goo.gl/7MXdtF With more teeth in contact, no "burn-in" issues, and shaft deflection compensated for with spring loaded plates, a crescent internal gear pump is remarkably efficient across a wider viscosity range than spur-type gear pumps.

Piston pumps are a bit more challenging in every respect when they're being considered for a system. Their control strategies, pressure and speed capabilities, and other physical aspects are the driving force in their selection for a given system design. But...they're primarily made for the masses. Materials and tolerances are established to meet the widest range of needs at the lowest production cost. It's also rare that piston products are not tied to a heat exchanger in some way as well. So, temperature variations in the oil (and necessarily the associated viscosity changes) are already controlled to some degree.

The reason for my skepticism in how we as an industry hold on to our paradigms of what a piston product can do is based on an often overlooked portion of our industry where water is the fluid medium instead of an oil. Years ago I came across a pump division within Danfoss that specialized in water hydraulics. https://goo.gl/RRxJPK To me, while the idea of working with water seems like fun, I basically translate "water" into "low viscosity fluid". Water is a miracle in so many ways. And, it has a viscosity index that is literally off the charts. (Have you ever seen thick liquid water?) Any pump that works well with a non-lubris ultra-thin fluid is doing something that all of the other pump designs are not doing. So, the materials, tolerances and cooling strategy in such a device offer many things to consider in the world most of us work in...a world that uses oil for fluid power...and not water. (though fuel-draulics is another area to study)

In my (long) list of things I regret saying, I once sat down with a brand new manager of mine at a large OEM (on my first or second day at that company) and boldly stated that I wanted to develop a 2 cST high-pressure hydraulic system for a vehicle program I was on. Admittedly...my salesmanship skills could use some refining. But...I would like to throw the paradigm shift out there for each of us to consider. Why is that a radical idea? Why should we not consider that there are new ways of solving problems? How can any of us expect to create value for our customers and stakeholders if we accept the off-the-shelf design (which is basically the same as it was sixty years ago and the same thing our competitors are using) is the best solution? I understand the nature of the behemoth that is the multi-national corporation. There is only time and money allotted for a few select “science projects”. But, in an increasingly competitive world where we fight wars over energy resources and where smog and air quality is still a very real issue for billions of people, we will all be better served by challenging “the way it’s always been” not just for sake of larger profit margins, but as a way to make life better for everyone we share our planet with. (Note...this was in 2006 and ultra-thin engine oils were not being used back then...but they're all the rage now)

Here are my last parting thoughts as I wrap this up before it turns into a full chapter's worth of reading.

"The oil is not the problem; it's the pump."

"Paradigms and dogma are perpetuated by people. What paradigms are you perpetuating?"

"If you don't challenge the status quo...who will?"

I hope these thoughts are helpful.

Happy 23rd.

-Andy Hessler (23JUN2018)

(rereading this now, 29JUL2018, I see some areas that probably need more explaining to make sense. I might have to add some clarity...since this is such a high-level overview)