This is probably a frivolous purchase but I've wanted to try one of these mini excavators for several years now. This particular one was being sold by a church friend so all the better to help him out and also get something I've wanted. I didn't know he was selling it until I ran across it on Facebook Marketplace. The friend purchased the excavator a year or so ago and was using it at his place when all of a sudden there were no hydraulics to speak of. He said there is probably 4 hours' worth of use on the machine. He bought and installed a new pressure relief valve and hydraulic pump, but neither of those solved the problem. He decided to put it up for sale because he was done trying to fix it. Here's a photo from the ad.
I'll try to explain here how a pilot operated relief valve works but it might get muddy. The next two drawings below show a representation of such a valve, and I'll reference them.
The main important parts I'd like to point out are 1) the pressure relief valve, top right, with an adjustable spring and pilot poppet; 2) main poppet with 50 PSI spring behind the poppet; 3) balance orifice.
The red color indicates system pressure, in this case 706 PSI showing on the gauge, which is below the pressure relief valve set limit of 1000 PSI. The main poppet in the middle of the drawing blocks flow between the (red) high system pressure and the (blue) low pressure return port. As you can see, there is the same 706 PSI on both sides of the large poppet. Since system pressure is on both sides of the main poppet, a light 50 PSI spring is all that is needed to keep the poppet closed.
Now we get to the part that a lot of folks, me included until recently, don't know about these relief valve systems: the balance orifice. This is the passageway that supplies system pressure to both the top side of the poppet and to the pressure relief valve.
The main poppet will stay closed until system pressure reaches the set point of the pressure relief valve, in this case 1000 PSI. Once that 1000 PSI pressure set point is reached, the pilot poppet comes away from its seat and bleeds off the pressure behind the main poppet. How, you ask? Well, the pilot poppet orifice is larger than the balance orifice, so while system pressure is still being forced through the balance orifice, the orifice is smaller than the pilot poppet and pressure is flowing out faster through the pilot poppet. This creates a lower pressure on the top side of the main poppet (orange color) and the poppet lifts off its seat and excess hydraulic fluid is bypassed via the return line to the tank.
Now that the explanation is out of the way, let me explain what was going on with the excavator. A piece of debris plugged the balance orifice. I don't know how small the balance orifice is but it's pretty small, maybe 0.050" in diameter, so it doesn't take much to plug it up.
The clog meant that the back side of the main poppet was getting very little or no system pressure to it and so the hydraulic system's high pressure was easily overcoming the main poppet's 50 PSI spring and simply dumping all pressurized fluid back to the tank.
One thing I should point out is that the orientation of the excavator's balance orifice is vertical, with system pressure acting on the top of the balance orifice. The problem with this orientation is that any piece of debris coming from the pump could easily fall onto the top of the balance orifice and clog it. System pressure then keeps the debris forced onto the top of the balance orifice. It seems like the designers of the relief valve could have easily made it so flow through the balance orifice was going from bottom to top, and not top to bottom. That way any debris can't just fall onto the orifice and easily plug it up.
When I first took off the access plugs for the balance orifice, I shot carb cleaner through the orifice and put things back together. I started up the machine and it had good hydraulic pressure for a second but then it quickly disappeared again. I assumed the piece of debris had been dislodged from the balance orifice but was still close by, so it quickly plugged the orifice again. I again took out the plugs and shot copious amounts of cleaner through, then reassembled things.
This second cleaning must have blown the offending piece of debris out, because now the excavator works great. I've run it for 1/2 an hour and it is still doing well.
The below photo is from the net. It shows the main valve block (with an added filter on the right). The pressure relief valve is on the far left end of the valve block. The large hydraulic line going into the relief valve is system pressure from the main hydraulic pump.
The yellow line indicates where the balance orifice resides in the relief valve. The large silver fitting on top of the photo is the incoming hydraulic fluid. You can see by the orientation that any piece of debris can easily fall onto the top of the balance orifice.
This next photo shows my micro excavator on the trailer. I fixed the issue before unloading the thing.
This next photo shows the excavator all back together. A lot of panels and the top "roll cage" had been removed by the previous owner.
This excavator looks to be one of the smallest mini excavators on the market. It is labeled as an 800 kg unit, which is a bit less than one ton. Most mini ex's are 1000 kg and up. This one has a Rato 420cc gas engine rated at 13.5 hp.
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