By Josh Cosford
We get regular reader letters, sometimes giving feedback or opinions on articles, but most often asking questions about a topic. One such reader is Ed B., who had a detailed question about filters on the hydraulic system of a motorhome:
“[What we are] interested in is preserving the components and systems that we have, given they are not making these motorhomes anymore. The hydraulic systems are one of the areas that we do not have a lot of information on. The basic components are a Cat C12 driven hydraulic pump, which drives a large side radiator hydraulic fan and a power steering unit. The hydraulic flow (to the fan) is driven with a wax valve that cuts in at about 190° and then typically stays on unless the temperature drops.
There has been a move among our group toward changing the Dexron 3 fluid to a Castrol-based Allison TES295 synthetic fluid that should have a very long life in this application. The filter is mounted below the reservoir tank, so it is difficult to change without draining the tank, so a quality filter with a long life was being sought. I did the research to look across brands and a Donaldson 551550 seemed like the best of the bunch with a Beta2 of 7 microns and a Beta1000 of 23 microns. I did not want to go to a synthetic media that filters down to Beta2 of 2 microns given we are concerned about filter plugging. The original was an LHA SPE15-10 along with an LHA filter head.
My intent was to add a gauge on the return side of the head and monitor when we reached bypass pressure, so I added one. The filter has no built-in bypass and I got the following readings at 70° with TES295 Allison synthetic fluid with the filter head that was installed by the builder:
-At idle, I had 16 psi of pressure
-At 1000 rpm, I had 24 psi
-At 1500 rpm, I had 36 psi
That implied to me that the head has no bypass valve—but I am not sure. You may have better insight. The fluids were not at temp, so the radiator fan was not running at the time.
So the question is: If you were designing this system would you have a by pass in the head or filter, are these pressures rational, anything you might advise regarding what we are migrating to for fluids, filters, heads etc.? We do not have a reliability issue per se that we are chasing, but some folks have had wax valve fails. It is just a matter of—what are the best practices and are we doing what we can to preserve what we have? Is there a concern if we do not have a bypass in the head, should we aim to have one built-in the filter then?”
Well Ed, I see one glaring issue with your experiment, and the results that came of it. By installing a stand-alone pressure gauge in the return line of your motorhome’s hydraulic system, all you’ve really measured is backpressure. In an indirect way, backpressure before the filter can give you a rough idea of how clogged your filter is, as long as you have baseline data regarding system pressure, flow, temperature, viscosity, and the amount of dirt being retained. Then, as pressure rises above the baseline, you might be able to assume backpressure is increasing. However, this method is unreliable at best.
What you really need is a differential pressure gauge. This gauge is most often installed within the filter head, because it can easily measure pressure both before and after the filter element, and the comparison pressure is what really tells you how clogged the filter is. Because it measures the pressure differential rather than the absolute pressure, it’s a better indicator of filter life.
You can get stand-alone differential pressure gauges, but to be honest, spin-on filter assemblies are so inexpensive, you’re better off buying a whole new filter assembly. In that case, most spin-on filter heads come with the bypass check valve built in. The pressure at which the bypass valve opens depends on the manufacturer, but it could be anywhere from 15-70 psi.
Less important than the bypass valve cracking pressure, is the filtration quality and beta ratio you get for your system. Your worry of using filtration too fine and clogging too quickly is unfounded. There is a slight chance you may go through the first high-efficiency filter element quickly, but that’s because it’s doing its job to remove particles.
But what you need to consider with fluid, is that dirt causes more dirty fluid. Dirty liquid travelling rapidly through a series of components and plumbing is essentially a lapping compound. The particles cause wear, and the result is even more contamination. Your best bet is to use the highest quality filtration reasonable, which will actually extend filter life by eliminating the same wear-causing particles. It’s like a reverse catch-22.
I doubt you were hoping to be told to go out to buy a whole new filter assembly, but luckily spin-on filter assemblies are inexpensive. There are no regrets when it comes to quality filtration, and if you can handle a few extra bucks for the filters with absolute ratings (Beta 75 and up), your hydraulic system will last longer (and require fewer repairs) anyway. Thanks for the letter, Ed!
Filed Under: Hydraulic equipment + components