Home made BH hydraulics became "soft".

[Redneck in training]

I bought homemade BH while ago and used it to dig about half mile of trench.
It worked reasonably well in the beginning but then all the cylinders became "soft? When the valve was in neutral I was able to move the cylinder to some degree and when the valve was moved there was a delay before the cylinder started to move. It all felt like there was air in the system. The BH has six Prince valves and six Prince cylinders, 10 HP engine and single stage pump. The tank was full of hydraulic fluid. My suspicion was the suction strainer being clogged and the pump sucking air in someway. Is it possible that the pump sucks in air around the shaft?
Thanks Ladia.

 

[Brad_Blazer]

Air doesn't have to get in the system for the pump to starve. A 2500psi pump has no trouble pulling a full vacuum (15psi) on the suction side resulting in cavitation. Stuff like teflon tape or degradation of the wrong type of suction hose are other things to look for.

 

[J_J]

Bleed off several of the cylinders, and see if you have any air . The air should go back to tank after several cycles. Look in the tank as you move the BH and see if any bubbles appear. Air can get in at the cylinders rod seals also. Put a pressure gage on the pump and you should be able to see your problem. If you have a filter on the suction side, check it. If you have a lot of, or continuous capitation, you will ruin the pump.

 

[Farmerford]

Ladia:

I also live in the country and prove it every day!

As you suspect, you probably have air entrained in the fluid that is entering the system somewhere in the suction circuit. That circuit begins at the tank outlet and ends just inside the pump inlet. The suction circuit is under some negative pressure (relative to atmosphere) whenever the pump is running. It may only be a fraction of a psi, but that is enough to allow some air into the circuit through a loose hose connection or pinhole in the hose. More than likely there is an obstruction in the suction line that causes the negative pressure to increase substantially. As Brad points out, the vacuum can rise to several psi.

If the hoses, fittings, etc. are tight, then the air could be entering through the pump seals.

If the problem occurred after some use I suspect (especially since it is used and homemade, like lots of my stuff) that the strainer inside the tank on the suction line is clogged. You probably stirred up sediment in the tank and system that had been resting for some time.

Some strainers have a valve that bypasses at about 3psi vacuum, but many do not. If there is no bypass and the strainer clogs the vacuum increases and air enters the system either through imperfect connections and lines or through the pump seals.

 

[Redneck in training]

Thanks for the input. The machine has filter in the return line but it has a strainer in the tank on the suction side. It was also my suspicion that it might be restricted. I am going to change the hydraulic fluid and filter. While doing it I will plumb in a pressure gauge and remove and clean the strainer. I will report if it fixed the problem and post few pics. Since I will be on a business trip one more month it might take for a while though.

 

[Brad_Blazer]

Whan you change the oil, first try draining from the pump inlet to see how well the suction plumbing flows

 

[Redneck in training]

Whan you change the oil, first try draining from the pump inlet to see how well the suction plumbing flows

Good idea. I will do as you advised me. It will take at least a month before I get to fixing it.