What I have learned about our hydraulic system is that I do not know a heck of a lot about them. I usually take a thorough look when I work on something for the first time and then sort it out and be done with it. So here is what I learned...
The original filter head was made by LHA company that was bought out by Donaldson. The original filter was an LHA 15-10 as is listed in my Beaver manual. Because the filter has no by pass valve, it is expected that the filter head has either a 15psi or 25psi by pass built in. I noticed on Ebay that these filters and heads (old stock) were being sold with original 15lb by pass valves, but that does not mean we have that configuration.
I decided to follow Gerald's lead and use Transynd. It took a little over 4 and a half gallons. There was no drain plug so I drilled a 5/16 hole in the bottom front edge of the filter and held a funnel arrangement in place with a piece of wire. It took about a half hour to drain, worked well, did not need a pump and there was no mess. Photo attached.
Filter options range across many brands. There are filter versions using synthetic filter media that will filter down to two microns but those are more likely to plug over time and I was looking for a long service life. I also do not think that level of filtration is necessary here. There are downsides to using too fine a filter in some systems.
In the cellulose versions I chose a Donaldson 551551 as the best of the bunch, Beta 2 of 7 microns (50% chance of removing a 7micron particle in one pass) and an absolute Beta 1000 (99.9% chance of removing a 23 micron particle in one pass). Fleetguard HF6510 version had a Beta 2 of 14 microns, Baldwin BT839-10 had a Beta 2 of 10 microns, the Wix51551 had a Beta 2 of 20 micron as does the Napa Gold 1551. There is also a Cat 077-3492 with the exact same Beta specs as the Donaldson (possibly made by Donaldson) but it has a 25psi bypass valve built in, so ruled that out given the original had no bypass valve, also I could get the same filtration level out of the Donaldson.
In our filter head there was an Allen plug port for a gauge. I anticipated a 15psi bypass valve but it could have been 25psi. I bought a suitable gauge and mounted it anticipating 5 to maybe 15 psi normal head pressure. There is a return line that comes down the chassis rail around the back of the engine to the inlet side of the filter head where the gauge was. The other side of the filter head has a pipe that exits to the bottom of the tank. So I filled with Transynd and started up. At idle I had 16lbs of pressure, at 1000rpms 24 lbs, and at 1500rpm 36lbs. The Donaldson filter is rated at 30 gallons a minute and up to 150psi working pressure so no issue with that. Usually you have a suitable range pressure gauge and when you get near the bypass valve pressure rating of 15 or 25lbs (25 is more common) you would change the filter. I removed the gauge given it did not seem to provide useful information regarding filter changes and with such high and varying pressures I could not interpret anything about the state of the filter.
So the question I have in the end, does the head have no bypass, a 15psi, or a 25lb by pass valve and are we going into bypass all the time and just a portion of the fluid is passed through the filter here? It is typical in these arrangements to use either a bypass in the filter head or one in the filter and our original filter, the LHA SPE 1510 had no bypass. My first guess is that the head has a 15 or 25lb by pass valve. If there was no bypass valve, as the filter plugged with age, the pressure would rise and could then destroy the filter media contaminating the system. It makes sense that the higher the RPM, the more fluid pressure we would have, but I am puzzled as to how this system is designed and works. Any ideas, Lee, Gerald, and the rest....
Gerald, you mentioned that one of the Beaver designs had ruptured filter issues and a bypass in the filter might be advised. Could it be that we have no bypass in the head or the filter and the pressures is whatever PSI the pump is returning to the tank. I am thinking the 36lbs at 1500rpm might indicate that, given if it was a 25lb bypass valve, it would seem the pressure should have stabilized closer to 25lbs ....I have no hydraulic experience at all so this is a bit of a mystery to me. These systems have been reliable for the most part, and the fluid pressures were for fluid at 70 degrees so as it warms it will come down some. It would be good to know if there is a bypass built in to this system head or should we consider the Cat filter with the 25lb bypass and let it bypass as needed.
Later Ed
