Linkbelt LX160 w D&M slashbuster hydraulic problems.

[excavator]

I recently plumbed up a Linkbelt LX160 for a D&M Slashbuster head. We used a Lynch dual pump priority flow valve going through a DO5 valve, load sense for the Lynch valve was picked up at the pump Pi2 port T'd through a shuttle valve. The 2 pumps on the machine put out a combined 72 gpm and the head should have 35 gpm to operate. Currently the head is set at 30gpm/ 4200 psi and with it running the other functions on the machine are totally dead, in order to boom up/swing/travel ect. the operator has to turn off the head. It acts like the system is not sensing pilot lever movement when the head is operating. The machine had the pumps replaced approx. 9 months ago by the local Linkbelt dealer and seems to operate fine otherwise. I admit that I'm more of a Deere/Hitachi guy so I'm not real familiar with the Linkbelt system, at this point I'm more than a bit baffled. Any help would be appreciated. Thank you.

 

[John C.]

Is that hydraulic system the same as the Quantum model machines? I seem to recall on the older machines that they were negative control for the pumps. Maybe if your return oil for the head is plumbed direct to the tank you aren't getting a NC signal back to the pumps from the main control valves.

 

[excavator]

Thanks John, I'm not sure if the Quantum is the same or not but I do know this machine uses negative flow. And to be honest, I'm not sure I really understand negative flow. How is that different than load sense? I was doing some reading on it and one page said negative flow was just a poor mans load sense.

 

[John C.]

A quick and dirty explanation is that it is an open center system that is blocked at the outlet end of the main control valve. There is a relief valve there that opens at between 300 and 600 PSI usually. There is another outlet that connects to the control valve on the pump. When all the spools on the main control valve are centered pressure builds to the relief setting on the outlet of the valve which also builds pressure over at the pump control valve and tells it to destroke the pump because nothing is happening. Let's say we want to pick up the boom so we move that spool in the main control valve which cuts off flow to the bottom of the main control valve case and pressure going to the control on the pump also drops which tells the pump to stroke up. Return oil from the boom cylinders is routed back to the tank and not to the bottom case of the main control valve. It can be to some extent a kind of load sense but it isn't very accurate. Link-Belt in fact only used it as a turn on or turn off signal. The pump turned on and went into a pressure flow compensation mode. It put out more oil than needed which made for easy, smooth and fast operation, but created heat and ate up plenty of fuel.

It kind of sounds like your load sense to your priority flow divider may be working in the opposite way that the NC control of the Link-Belt system works. Do you have any schematics on the control for the flow divider or the Link-Belt? When I was doing a bunch of that stuff, the supplier usually had some kind of instructions on how to make the component work within each manufacturer's system.

 

[excavator]

A quick and dirty explanation is that it is an open center system that is blocked at the outlet end of the main control valve. There is a relief valve there that opens at between 300 and 600 PSI usually. There is another outlet that connects to the control valve on the pump. When all the spools on the main control valve are centered pressure builds to the relief setting on the outlet of the valve which also builds pressure over at the pump control valve and tells it to destroke the pump because nothing is happening. Let's say we want to pick up the boom so we move that spool in the main control valve which cuts off flow to the bottom of the main control valve case and pressure going to the control on the pump also drops which tells the pump to stroke up. Return oil from the boom cylinders is routed back to the tank and not to the bottom case of the main control valve. It can be to some extent a kind of load sense but it isn't very accurate. Link-Belt in fact only used it as a turn on or turn off signal. The pump turned on and went into a pressure flow compensation mode. It put out more oil than needed which made for easy, smooth and fast operation, but created heat and ate up plenty of fuel.

It kind of sounds like your load sense to your priority flow divider may be working in the opposite way that the NC control of the Link-Belt system works. Do you have any schematics on the control for the flow divider or the Link-Belt? When I was doing a bunch of that stuff, the supplier usually had some kind of instructions on how to make the component work within each manufacturer's system.

Thanks again John. Unfortunately there was no info included but we'll have to see if it can be obtained.