Hydraulic question.

Quote:

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Originally Posted by Gerokent

"MOST pumps require a pressure head to work effectively without cavitation."

Just being pedantic, but this is aeration. Cavitation is usally caused by restriction in the inlet, or to thick or cold oil. Both cause damage. Aeration will cause faults further into the system (jerky movement, overheating the oil, ect), whereas cavitation usally only affects the pump.;)

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When I did fluid mechanics, I was taught Cavitation is a collapsing pressure wave generated within the fluid caused by a sudden pressure drop (after the impeller blades) due to; low inlet flow, a restriction in the inlet, too high a pump speed, or too high a viscosity fluid. This will erode and even destroy surfaces within the pump as each collapsing pressure wave is equivalent to a miniature explosive force.

AFAIR this LEADS to aeration, another serious pump efficiency scenario.

JC

and that would be the near textbook explanation of cavitation.

Cavitation doesnt cause areation of the oil but when the 2 combine.... the areation tends to cancel out the cavitation as the oil becomes compressible..

heres a simple demo to try...

cup a strong steel rod in your hand have someone smack the other end with a large hammer... hurts yeah? well done, thats the end result of cavitation.

now do the same thing but first cup the rod in a large carwash sponge, the impact is the same but its softened.

Now try to manouver the rod, you can do so far more precisely without the sponge ( assuming the rod is long enough and heavy enough) and that follows the effect on your hydraulic system.

with the pump cavitating the oil coming out of it is still continuous oil (no bubbles) so the system will still work, with the oil getting aerated you loose accuracy in the system as the "oil" is now compressible due to the air in it, as the system pressure changes the oil bubbles shrink and grow

For my money...

Cavitation is worse than aeration because you cant see the pressure waves coming out of the pump at better than ludicrous speed that shortens the service life of joints, hoses, pipes, seals, valve spools pretty much everything thats not a nice fat chunk of unforgiving cast metal.

I stand corrected. :bangin:

Another question for all the gurus, does it/will it harm the pump if I run the motor with out any oil flow for more than a few minutes? Just wondering for while I'm in the process of setting the hydraulic connections and motor up.

Cheers, JP

if your operating without bypass oil relief, yes.

if you have bypass oil relief no

If you have a 4 port 3 position DCV no because that will usually be set up for normal flow, reverse flow and return to tank

If your pump has in internal pressure relief return to in put no.

[ame=http://www.youtube.com/watch?v=77gWkl0ZUC8]Shatter beer bottles: Bare-handed bottle smash - YouTube[/ame]

special thanks to carjunkieanon in the interesting pics thread for the link to the machine gun rocket pack on the XKCD website and my natural curiosity to read the whole what if section and blindly follow relevant links.

start watching at about 44 seconds.

Thanks for all the help Dave, you're invaluable as always.

I've pretty well finished assembly and encountered another problem tonight, as soon as the engine goes under load it starts surging, once I engage the hydraulic drive I get about two revolutions out of the auger and the engine stalls. I tried adjusting the governor but that only made the problem more obvious. If I hold the governor open the problem pretty well disappears, fuel supply isn't an issue so I think I might have to tackle the carby tomorrow. Any other ideas?

I know the rules around here so I thought I had better add some photo's, you'll all have to excuse the mess under the auger, it's where I'm storing my tools in the neighbour's shed while I'm doing the work.

governor tension not set correctly for the application.

Choke spring

air filter

blocked emulsion tube in the carby.

If the engine checks out ok.........

Is it enough HP for the application? Was the engine sized to closely to the pump input HP requirements? From experiance if the pump needs 8HP @3600 rpm to give the required pressure, this would meen at least a 10-11HP petrol engine. Adjusting the goverened speed up will increase the the required input HP the pump needs to deliver the required pressure.

There can and will be a sudden line pressure spike when you engage the auger that will load up the engine and show up if the engine has a problem or is too small for the job. Not exact, but as a very basic rule of thumb, every 1(us) GPM of oil flow @ 1500PSI needs 1HP to drive it. So 5gpm @ 1500psi needs 5HP, 10gpm @ 3000psi needs 20HP.

I have some charts that give grain screw auger HP requirements and delivery rates at differant speeds/angles at work if you need to varify anything.



On a side note, bit hard to see in the pic but that return line into the top of the tank. The oil returns below oil level yes? If not, fit a drop tube to the fitting into the tank so it does. Filter ????

Hi Rick, The V-twin is a 16HP Briggs, when I made some initial inquiry with the hydraulic supply company they believed that it was big enough for the job. That was over 18 months ago now (this has been a long, slow project). Regarding the return line, no it doesn't return below fluid level, is this quite important? The filter is on its way, I was going to place it inbetween the high pressure line and the spool.

Cheers JP