I am experiencing something I have not seen in my 25+ years working on equipment. the hydraulic oil level is being drawn down out of the sight glass with the engine running and machine not operating. Oil level is will return slowly to proper level when machine shut down. Machine is a 2006 Terex TXC175LC-1 which is actually a Daewoo Solar 175, now owned by Doosan. It has a Rexroth Uchida Pump with a Doosan engine and Doosan main control valve. The Hydraulic return filter has been replaced and suction screen checked and cleaned. The hydraulic oil was recently replaced due to water contamination and system filtered to remove any remaining moisture. The oil level did not exhibit this problem when system filtered. I am also experiencing a low power condition due to low main relief pressure. Other trouble shooting has led me to a main relief that will not adjust. Pressure is 200 bar and should be 310. I believe the oil intake restriction is the cause but I will be replacing the relief later today. Any thoughts or ideas are appreciated!
Did you follow the procedure for checking the oil level that is described in the owners manual ?
A main relief that will not adjust is usually a symptom of a pump that has (or is about to) **** itself. Not too often that a relief valve fails. Does the oil shows any signs of the presence of air..? Foam, lots of bubbles, etc, etc. Does the main pump sound as though it is cavitating due to lack of supply..?
weird acenes The reason I am replacing the relief is: this has happened before. low power but not the situation in the tank. The relief was screwed all the way in and talking with the terex tech support we were condeming the pump. I was going to replace the relief anyhow so I tried that first, and it restored pressures. So this time same thing. replace relief and if not that remove pump. I have no signs of air but a whine from the pump like it is starving for oil. A reply to the other suggestion. As I stated the oil level is correct .
Either the oil is being compressed under vacuum from the pump or circuits down stream are being filled that are above the oil level
Only question I can add is; Was it working correctly, despite the water contamination, before you changed the oil and filters?
Return filter, and or oil cooler higher than the tank?
And also what brand & viscosity of oil is in it right now...?
Ok update to hydraulic problems. Returned and installed new relief valve and just as before all pressures returned to normal - 310 bar- and machine working as it should. Now to determine what could be failing the reliefs! On the hyd tank issue. It seems the aftermarket filter is a lower micron rating. Running the machine without the filter and the level stays level. When dropping in the filter the oil level drops out of the sight glass and backs up in the filter side. Have a OEM filter on order. Thanks for the help. Maybe someone else can benefit from this experience.
You need to get your hands on the service instructions for setting the pumps and the main relief. If you're losing relief valves one after another it's most likely because you have oil flowing over it too often for too long. Not sure exactly what type of system that is, but typically the pumps will be limited to a max pressure and the relief will be set 20 bar give or take higher to handle pressure spikes.
I agree with lantraxco that your pump unload settings are likely too high and the pump is running over the relief every time there is a hard pull. Most all excavators built in the last 15 years have a cutout function where the pump will destroke before pressure gets to the main relief. The only time it should relieve in that case is with a sudden impact while a directional valve is open. The boom being raised and a tree falls on top might be that type of scenario. How does the engine sound when booming down, opening the bucket and running the stick out all at the same time. If you see black smoke or hear the engine lose more than a couple of hundred RPM you probably have a problem. I'm a bit puzzled by the oil level issue. The level is supposed to go up and down as you move the implements. Are you saying that just sitting still and starting the machine the level goes down? When you changed the oil did you cycle all the implements eight or ten times with the cap off the hydraulic reservoir to make sure all the air was out of the circuits? Does this machine have an air compressor the feeds into the hydraulic tank?
The whole point of a main relief valve is to relieve the pressure when normal working. A pump will not destroke to prevent the system reaching max pressure, it'll destroke only to prevent engine stall. A main relief valve is designed to reach max pressure on and off all day long. Its circuit overload valves that prevent damage by sudden impacts or spikes in pressure. The most likely cause of main relief failures in my opinion would be contamination in the oil. I'd be looking to get the oil sampled and changed asap before the pumps/motors fail. The other reason for pump pressure loss is leakage between the valve plate in the pump and the rotor or head of the pump.
You may be familiar with this type of pump and control system, I admit that I am not. It would depend on the system design, and while I am not familiar with this particular system, the pumps are apparently controlled electronically. Typically these machines with variable pumps are designed so the pumps do destroke at max pressure so as not to waste fuel and create heat flowing oil needlessly over main relief valves. It's really irrelevant as whether she is or whether she ain't, what is needed is to check and adjust pump and relief settings to factory spec. The OP stated that oil and filters were serviced, but yes, oil samples need doing to confirm things are staying squeaky clean.
Most pumps will have some sort of electronic control on them. Pump flow is regulated by pump pressure to match the machines particular PQ torque curve, this curve depends on engine power, the more power the engine has, the higher flow it'll put out at set pressure points on the curve. As the pressure reaches near its maximum, pump 1 and 2 flow will be at its minimum. On most machine normally the electronic part of the pump control, Hitachi call it a torque solenoid, cat call it a powershift solenoid, will only activate when the engine rpm decreases below a set level to maintain engine rpm. If the actual rpm decreases below the target rpm is a prime example. If i was the OP, i'd sample the hydraulic oil, change it with filters, set up ALL the relief valves, there'll be overloads for every service on the front attachment, 2 on each travel motor and 2 cross line relief valves on the swing motor, then set up the main relief valve to its spec. Then perform pump flow tests. A good test that indicates possible pump wear is get the oil up to its optimum temp, usually around 50 degs Celsius, then do stall tests on every service at low idle then again at high idle. If the pressures a good bit lower during low idle than high it usually indicates oil leakage in either pump 1 or 2. He could then check which pump it causing the leakage by using single pump services to identify where the leakage is occurring. If the sample comes back good, and the pump tests all check out i'd say the machine will be ok. If it comes back bad, it'll maybe take a couple of oil changes, many filter changes and lots of sampling to get the oil back to normal.
I've been told by people working for a Cat dealer that the newest excavators don't have a main relief, only circuit reliefs. On most of the excavators I've had my hands in the electronics play a much larger part than just high pressure cut out. Most have some connection and programing that sets flow curves based on throttle setting and actual engine speeds. Unhook those solenoids on the pumps and tell me what happens. All that I've worked on will put the pump at max flow and overload the engine. I haven't had the pleasure of putting a gauge or flow meter on a Doosan machine yet. I don't know if the systems are open or closed center but do know that they are load sensing. Since all the Korean built machines that I have had to climb on are basic copies of older Japanese built systems and all have used the older components so the information provided here was general ideas of what to check for.
Signs of a wore out pump Sorvos are un able to fully destroke pump ( due to excess case pressure from internal leakage) so it blows the relief. Ain't far from the big bang.
I appreciate all your replies to my questions. As far as relief settings go. All circuit reliefs and main are set to specs. I have operated the machine and watching the main pressure and the negatrol pressure (that is the pressure that shifts the pumps as it senses load) All is functioning proper and well below main relief setting. (320 bar) I explained in a previous post about the tank and filter issue. Thanks again.