The sight glass was milky looking and at first I thought it might have water in it which is possible but I also noticed what looked like foam so I figured it might be sucking air or maybe some kind of cavitation happening. The pumps did seem a little noisy sounding a little like marbles rolling around inside. Just to be sure, I drained the hydraulic oil and cycled the system to help clear any contamination in the cylinders and hoses. After running for a few minutes I drained the tank the second time but this time the oil was real foamy especially after letting it sit in the pail a few minutes. One of the operators told me the sight glass has been foamy like that for a long time. The ST1520 calls for 30W which is what I replaced the oil with and filled it to the proper level. The machine has two large tandem gear pumps that sit a little higher than the hydraulic tank. One is for brake/steer and one for dump hoist/brake cooling. The #20 suction hoses that supply the pumps look good and dry. Even though they look dry I will check for tightness of flange connections at both ends, as well if the suction hoses collapse and check the strainer for restrictions in the tank too but that’s a pain because I’d have to drain the tank a third time and pull a pile of return lines off the inspection port. The hydraulics seem strong so I don’t think there’s a pump issue but who knows. I don't have access to a vacuum pump. Any tips or clever ideas to help isolate a hard to find suction leak or cavitation?
Not likely caused by cavitation as the hydraulic systems on pretty much any machine is engineered and designed to not suffer from cavitation, unless one changes the original design or adds components that don't insure cavitation prevention. In most cases, in my experience, ingress of air into the system is most commonly on the suction side of the pumps. Fittings, hoses, clamps, and yes, the pumps. Once the oil leaves the pumps under pressure there's no way air can ingress the system, instead, you'd have an oil leak. Gear pumps are very common culprits of this aeriation. Over time/years, the pressure being produced by the gear pumps shoves the shafts to one side and eventually wears a groove into the shaft bushings, this opens up a gap on the aged input shaft seal, which sucks air into the pump on the suction side, that air will and does find its way to the pressure/output side of the pump. Result, air bubbles in the reservoir. I've seen this more than once.
Your schematic is helpful except that it doesn't really explain "tank" too much. It just shows pumps drawing from tank. It doesn't say if the tank has a pressure cap or a compressor and regulator to maintain positive pressure in the tank. Particularly since you stated that the pumps sit higher than the oil level. This would require the pumps to draw oil up to feed themselves vs having positive tank pressure push the oil to the pump. That compounded with what Willie59 has explained could really aerate a system quickly. Good luck
Have no idea whatsoever about this machine, but in addition to what the other guys have said, I think you need to evaluate the strainer going to the pumps, as well as the plumbing/connections.
Could the unit have an oil cooler leaking water inot the hydraulic oil inside the cooler. It would depend on the design. Simon C
That's what I did on a Komatsu D31 having this same problem. Before I tore into the pumps I opened the tank, made sure the strainer was clear, replace rubber suction hoses/clamps, replace manifold o-rings, anything that would allow air into the suction lines. After doing that and still aerating pumps in short order after starting I ripped into the pump, that's when I found the wear. New pump, problem fixed.
Completley drained the tank and removed the inspection cover. There are two open suction ports without strainers. Both #32 elbows from tank to hoses were pressure tested ok. Both #32 rubber suction hoses, all o-rings were replaced. After filling hydraulic to proper level and test ran a few minutes all seemed ok. Moved unit out of the shop and noticed tank full of foam again. If i get back on this job I'll check the pumps for wear.
I didn’t get the honor of finishing this job or checking the pumps myself but both tandem pumps ended up getting changed and the hydraulic oil cleared up. Learned a few things here… thanks!
Thanks for sharing. Too often guys leave stuff open-ended. Glad to see a resolution.