Constant LS pressure at no load

[flipmoore82]

hello, i am in need of assistance. my brother purchased a komatsu pc45r-8 with blown motor, that i have fixed, but hydraulics are weak. at idle no load and cold, ls pressure is 500psi and pump pressure is 1000, when warm ls pressure 1000psi and pump pressure is 1500. when controls are operated both increase, ls pressure little faster then pump pressure, up to ls pressure of 2200psi and pump pressure of 2500. manual states with all levers at hold should have 0 ls pressure and 500-700 psi pump pressure. max pump pressure at relief is supposed to be 3500 psi. if adjust ls pressure any higher then it goes into constant loading but does have more power. when operate two funstions at once it does gain psi. when operate all but swing levers ls and pump pressure drops for a split second before rising. the two travel levers when operated have the most significant drop. when barely move either one i can get the pump and ls pressure to drop about 3-500 psi, but its a very small movement window before presure risies and tracks start moving. i have been through the pump and valve block and have not found anything obvious. though some spools and pressure reducing valves have polished sealing surfaces showing slight wear, nothing that measures out of spec from rest of the body. any help is appreciated and if i am missing needed info please let me know and i will do my best, thank you

 

[flipmoore82]

 

[FluidPowerUser]

Your standby pressure at the pump should be 34.3 BAR (497 PSI). This set by the LS compensator on the pump. The actual load sense pressure should be 0 when nothing is moving.

Your pump pressure will always be 34.3 BAR above the LS pressure when a cylinder or motor is stalled, and perhaps a little less when the system is moving, maybe 30 to 31 BAR.

The 34.3 BAR is the static margin

The 30 - 31 BAR is the dynamic margin as the pump tries to keep up with the demand of the system.

According to the data that you have shared the main relief is 245 BAR (3552 PSI).

Based on your description, the machine working as expected, apart from the max pressure, which seems to be 69 BAR (1000 PSI) low. Maybe the main relief is a little low?

 

[flipmoore82]

Sorry, forgot to mention that if I turn up the ls setting ls setting on pump I can get 3500 psi, but to do it makes the machine big the motor down and standby pressure gets up around 2000.

 

[flipmoore82]

Forgot to mention that when unhook ls line, and start machine, there is a full stream coming out of 1/4-5/16 I’d hose, and pressure drops to 400 psi

 

[flipmoore82]

Your standby pressure at the pump should be 34.3 BAR (497 PSI). This set by the LS compensator on the pump. The actual load sense pressure should be 0 when nothing is moving.

Your pump pressure will always be 34.3 BAR above the LS pressure when a cylinder or motor is stalled, and perhaps a little less when the system is moving, maybe 30 to 31 BAR.

The 34.3 BAR is the static margin

The 30 - 31 BAR is the dynamic margin as the pump tries to keep up with the demand of the system.

According to the data that you have shared the main relief is 245 BAR (3552 PSI).

Based on your description, the machine working as expected, apart from the max pressure, which seems to be 69 BAR (1000 PSI) low. Maybe the main relief is a little low?

I can turn up ls pressure and get 3500, but standby pressure is way high.

So even at unload, pump pressure will cause ls pressure it balance out and increase out of specs? Manual says all levers at hold no load, that ls differential pressure should be same as unload pressure, which I assumed would make ls pressure zero. But having set to high can cause it to increase?

 

[akkamaan]

This is just an attempt to shed some light on how an LS system works. LS pressure is supposed to be 0 (zero) if no hydraulic function is activated. The pump goes down to a standby mode at a low pressure. That pressure setting can vary depending on the type of machine. Approx 35 bar sounds reasonable. Now, when we activate one hydraulic function, the load pressure from that cylinder or motor is fed back into a complex signal grid inside the directional valve package and from the valve in a (usually) 1/4" hose connected to the pressure compensator valve pack (2 different size pressure regulator valves) on the pump. The standby pressure (35 bar) is set by a spring the smaller of the two and the larger one is a safety cut-off max pressure valve. The LS pressure that is provided by the load in the cylinder will add to the spring force in the standby spring chamber. So if the standby pressure is 35 bar and the load pressure is 65 bar the pump pressure will be 35+65=100 bar. If you activate two or more hydraulic functions, the one with the highest load pressure will go in the LS line to the pump.

So, it seems like the LS grid inside the directional valve pack is compromised. You mention that you have an LS pressure of 500 psi when oil is cold and 1000 psi when oil is warm. That LS pressure should be 0 (zero) when idling the system. So, something has possibly blocked one of the LS grid valves so it stays open and that way sending out LS pressure signal to the pump. That is also why oil is pouring out of the LS hose when disconnected. Plug that disconnected LS hose and you will probably see a normal standby pressure. IF not the pressure compensator on the pump is compromised, too. And that makes sense if the system internally has been exposed to a lot of debris from failing seals etc

 

[flipmoore82]

This thing has issues in the valve stack. I have disassembled and couldn’t find anything obvious. Before and after disassembled, the Ts (tank seal line, o ring that catches any internal looks between valve block sections) has what I think is way too much flow. 5/16 fitting and it is pouring out full stream all the time. Only supposed to be load sense bypass and self pressure reducing valve ports using that return line. Blocked off self reducing valve and did not effect it any. And just so you know, between the valve sections, there is no oring for the port that takes pump pressure from top to bottom. Only metal to metal to seal all the hydraulic pressure. Checked flatness with high spot blue and surface plate and all were touching where needed to. I am at a loss on this thing