Slew motor non return valves question

[ROAMER_AUS]

Hi all,

I have this configuration of slew motor on Terex mini excavator. Upon activation of the command, my pilot pressure drops from nominal 25 bar to 5-10 bar and it is very slow and powerless in rotation. I have checked everything else, and it is good. My only suspect are these two non return valves. Connection is marked T but it is connected to pilot hydraulics. Maunal states that pressure on return line of the motor should be 30 bar, which is more than 25 and no loss of pressure there. But somehow I'm still loosing it. Does anybody know what are these two valves for?

Thanks.

 

[John C.]

They are basically anti cavitation valves. When flow is sent into the motor through V1 or V2 the pressure in the supply circuit will hold the appropriate check valve closed. When you stop flow to the motor the energy of the turning house will keep trying to turn the motor and pressure will go over the opposite side relief valve. In order to keep from pulling a vacuum on the other supply circuit which now is closed, one of those check valves will open and allow oil to be pulled from the reservoir.

Your problem could be one of those check valves or the motor itself. Have you checked the return filter and pump suction strainer in the tank for debris?

 

[ROAMER_AUS]

Thanks John.
These valves are connected to pilot hydraulics pump, so I take it that pilot pump makes up for the vacuum as you mentiond. Doesn't draw from the tank. It has to be something with return lines. My distributor has only 2 valves, slewing and blade. When I force the blade to it's upper limit and keep the blade handle pulled, rotation picks up on strength and speed. I can't figure out how is this 30 bar achieved on the return lines of the slewing motor. I think I will try another distributor valve and see how it goes. I moved all my slewing hoses to stick valve, and it was working fine. What pisses me off, is that I can't figure out how is this happening.

 

[John C.]

The drawing you posted shows the anticav check valves with a line which is marked with a T which in general means tank. The drawing represents the plumbing that I have seen on all the excavators I've worked on. The blade circuit is an add on to the swing circuit and is usually powered by a gear pump. The pilot system is usually separate from any of the working systems on the machine. I would be very surprised if you could show that the pilot system were in any way connected to the swing and blade system but I am not familiar with this particular Terex product. The 30 bar setting on the swing cross over valves can happen when you have the house turning and then let go of the control so it suddenly moves back to the neutral position. You will note that the house will try to keep turning but will actually slow down quickly. This is because the oil is going over relief in the motor. It is the inertia of the house turning and not the action of the pump that creates the high pressure. Moving the stick valve hoses to the slewing indicates that there is a problem in the swing motor.

 

[ROAMER_AUS]

Hi John,

Here is the schematic. At first I taught that it was a mistake in the schematic, because even the motor case drain is connected to the pilot line. But I checked on the excavator and it is really connected this way. Doesn't make much sense to me neither. But again, I'm not that experienced. Must be a reason behind it.

Why would it indicate that swing motor is a problem if it works fine when I move it all to the stick valve?

 

[John C.]

That is a very poor schematic and is typical of second and third tier manufacturers. If you look closely at the pressure lines coming into the swing motor you will see a port marked T. The lines are wrong in the schematic in that it looks like V1 is joined to T. In order for the system to work, they cannot be connected. . The pilot lines are the small dotted lines that are coming from controllers 27, 28, 29 and 33. The lines or connections that are labeled Y or T are return to tank lines. The schematic does not show them hooked up to anything. Another example of something wrong with the drawing is the line from the pilot pressure reducing valve directly to the swing brake. By that logic the swing brake would always released anytime the engine was running.
All motors will have some type of case drain hose and those on the swing will be relatively large coming off the top or the side of the motor. The only pressure on that line will be the pressure built up in the tank. Post some photos of the motor and hoses that connect to it. Most now days have a small line for the swing brake and another for the anticav valves.
The last pump on the left feeds the swing and blade valve. As you say the swing works better when holding the blade over relief might indicate some pump flow is not being directed to the swing when that valve is operated. Since the blade circuit is a direct stop on the flow of oil when the cylinder is at the end of its stroke, the oil bypassing in the swing motor would have nowhere to go so you have more power in the circuit.
You still need to do something to check the motor. Pull your filters and cut them open, pull the strainer in the tank and check for debris. The last issue is to remove the motor and take it apart for inspection.

 

[ROAMER_AUS]

Thanks on your taughts John. I will check the hoses again where they go, to be 100% sure.
My taughts were also that valve for swing is not opening fully and bypassing oil to the tank, and blade forcing is stopping this oil. But this doesn't explain why my pilot pressure is dropping. Then again, valve is not opening fully, maybe because of low pilot pressure, which makes sense. half open valve, does not cause pilot pressure drop! This is a close vicious circle that is making me chase my tail and driving me crazy.

 

[John C.]

I've spent so many hours chasing my tail that if hooked up to a grind stone, I could have ground up a thousand bushels of wheat. We learn by grinding through a problem like this and don't forget it.

Which valve are you talking about? The spool valve or the valves in the motor?

 

[ROAMER_AUS]

spool valve

 

[John C.]

I lost track of this thread so I apologize for getting back to you so late.

How do you know you are losing pilot pressure and are you only losing it when you are using the swing function?

 

[ROAMER_AUS]

No problem John. I didn't get to excavator in last few days anyway. too busy with other stuff. I know because I monitor it on the pressure gauge, mounted on the pilot pump. Yes, only when using swing.