If you have to change your hydraulic oil - then do this ...

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Bill Vogel, one of our members from California sent in this question, recently:

"Brendan, here's a suggestion for a newsletter topic: Changing a mobile hydraulic system from mineral to biodegradable oil."

"We would like to change a piece of mobile equipment from petroleum-based hydraulic oil (AW46) to a biodegradable Ester-based fluid. In general, what steps should we take and what precautions should we observe? Can you recommend an oil"

"We are leaning towards HEE (ester) fluids as recommended by the OEM, but what are the relative merits of HETG (vegetable-based) and HEPG (poly-glycols) fluids? There seem to be differences between the different manufacturer's fluids."

"Some imply no sheen on water and are not hazmat. While others mention there is a sheen and clean-up and reporting of spills the same as mineral oil. We'd prefer the most environmentally friendly fluid (that's the whole point) that performs as needed and is readily available."

Hmmm ... well firstly, with respect to Bill, this isn't an idea for a newsletter topic - it's more like a subject for a book. With that said, I'm going to start by answering a question with a question:


Why does Bill want to change from mineral based, anti-wear hydraulic oil to a biodegradable?

The most common reason, in this day and age, is because he has to. That is, some regulatory authority has mandated that if you want to use hydraulic equipment in our patch of pristine wilderness the system must be filled with biodegradable oil.

Of course the law makers are smart enough to know that most hydraulic equipment users are lousy at looking after their machines and so they leave a trail of oil everywhere they go. You reap what you sow.

When you think about it, why else would a hydraulic equipment user, most of whom are cheap to the penny, want to pay between 3 and 10 times the cost of mineral oil to replace it with a biodegradable? If the reason is not mandated by regulation, then based on my experience with hydraulics end users, it's unlikely to be founded on a highly sophisticated maintenance or reliability objective.

Anyway, my first piece of advice to Bill is: if you don't have to - then don't. And it's not only about the cost of biodegradable oil. As you'll see in a moment, Bill only needs a couple of 20 liter drums of the stuff, which ain't gonna send him broke.

No, the reason is, when I think of biodegradable hydraulic fluid, I think of my wife. Because they're both high maintenance. And most hydraulic equipment users I come into contact with are just not sophisticated enough in their maintenance practices to take care of a biodegradable oil. If this high-maintenance lube isn't looked after, the eventual result is a maintenance disaster - read: catastrophic failure.

You see, no one told these biodegradable oils they're not suppose to break down until they've managed to find their way OUT of the hydraulic system. No, given the right (wrong) conditions, they're just as happy to break down while they're still IN the hydraulic system.

So unless the hydraulics equipment user has a proper oil analysis program in place - and by this I mean he knows what to look out for and so is able to specify an appropriate test slate, then changing to biodegradable oil is a disaster waiting to happen. You can get away with a lot of things with a mineral oil that you won't with a biodegradable. They are NOT "fill and forget".

So here's my second piece of advice to Bill: If you DO have to change - get an appropriate oil analysis program in place first. And for the uninitiated, taking regular hard particle counts doesn't constitute an oil analysis program.

With that off my chest, let's look at the specifics of Bill's application:

"The machine is used in California's Sierra Nevada Mountains. The manufacturer recommends Fuchs Plantohyd S46 or Mobil DTE16. At air temperatures from -10 to +40 degC VG68 is recommended (it's required above 0 degC), and below -10 degC VG32 is recommended. The importer recommends VG46. Even though this area gets a lot of snow, typical winter temperatures are not very severe. Our normal temperature range is from -5 to +5 degC."

These vague and general viscosity recommendations are misleading at best and will be plain wrong in a significant number of applications. Following them without qualification can cause a heap of trouble. Most hydraulic equipment manufacturers are clueless when it comes to getting this issue right, but they don't let that stop them from publishing this nonsense.

The whole issue of temperature and viscosity becomes even more important when changing to an exotic oil - it's essential to do your homework. I spend nearly two hours explaining and discussing this whole issue, including a worked example, in my 'Hydraulic Breakdown Prevention Blueprint'. But I've neither the space nor time to go into it here.

So my third piece of advice for Bill is: Invest in my 'Hydraulic Breakdown Prevention Blueprint' - to avoid a costly mistake (yeah, I know, many of you will be horrified at the very idea of investing in your own technical education - but the fact is, if you don't know this stuff you're leaving a LOT of money on the table).

"The machine has two 18 liter hydraulic reservoirs. One is dedicated to the hydrostatic drive (axial-piston variable pump and variable motor operating at 300 bar nominal, 350 bar maximum). The second reservoir feeds a 14cc pump operating at 180-190 bar shared by the hydraulic steering and implement lifting & control (spool valves controlling cylinders and a small intermittent-use vane motor). Each circuit is equipped by the OEM with a 100 micron suction screen in the reservoir and a 25 micron (absolute) pressure filter."

Uh oh, the machine manufacturer needs another kick up the butt. Do YOU know why?

Read Part II here

Related articles:

Biodegradable hydraulic oil - its application and use
Hydraulic fluid - getting the viscosity right
Which hydraulic oil should I use?

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