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Discussion in 'Heavy Equipment & Pavement' started by AWJ Services, Oct 13, 2013.
I don't disagree with the results that you observed. I only wanted to correct some false assertions about pumps and the power output available. There are many configurations of pumps and the characteristics of a variable pump can configured in a variety of ways depending on what is required. If there was only one way then all the systems would behave the same and we wouldn't be having this discussion. Perhaps it's easier to think of the curve as a system curve as this more adequately describes it.
I didn't waste 4 years of my life in engineering to be wrong about the basic aspects of hydraulics. I should mention that I was also a certified fluid power specialist (though I let that slide a while ago) so I'm not talking out of my A$$. You guys can go on believing whatever works for you.
It is not about believing what ever I want it is about what the machines I am referiing too and what they actually do? Not sure why the Manufacturers do not know how to make there machines do what you say they should do?
I know we were on mini ex's but in the skid steer world, I'm finding that the machines I use are within specs on high flow attachments. I run a few low flow, high rpm attachments like mowers but on mulcher, trencher, auger, and stump grinder, the machine seems to do what it says. I hooked up my low flow mower today and at full rpm, the pressure showed 1000 psi. I did not place it under load but when i shut it off and started it up, it spiked to above 3,000 psi then leveled off.
When mowing, if the blade tip speed slows, that particular gauge maxes out but I don't know how accurate it is.
I just don't see flow decreases the way everyone is describing. In my application, if the flow would decrease, I'd stall my head even with pressure supporting it. I need the rpm and that is provided by flow, correct? I'm not afraid to learn something new.. I'm not a mechanic. I'm an operator and a damn good one but I can only report on what I experience. I know that when the pressures were lower, even with .5 gpm higher flow, my older machines did not perform as well as my newer ones. Could be the system efficiency. My specs, I believe, are taken at the auxiliary couplers and not at the pump but I could be wrong.
My motor is stock but it runs at the high end of the rpm range which in turn supports the hydraulic pump. I tried matching the torque curve listed in the book for max torque and it is not as good as full throttle in supporting my pump. What that means exactly, I'm not sure other than I always want more power..
I believe you YellowDog. Your machine being a skid, has the power to run the high flow at high pressures. You can pretty much ignore the pressure drop across the couplers because I doubt they are more than 7-10 psi each.
with this formula my T770 magicaly has 157 gpm max flow at 1000 psi
out to lunch these numbers
Flow meters/pressure gauges will only show me what the specs say in the book
max 36.6 gpm and max 3500 psi
But mind you these machines can be tweeked for better performance
That is the max theoretical flow at that pressure. Do you think I'm making this stuff up?
This theory of flow might work on a piece of paper for sure...........
But not in the real world of machines
I asked my buddy who is a service manager at a nearby Bobcat dealer to grab a machine for me and run it through its paces with a flow meter. He grabbed a T770 and here are the results he sent me. no shock that the actual flow through the meter decreases as the pressure increases. Notice that the maximum actual hydraulic hp is well below 3000psi.
40.4 GPM @ 382 PSI
38.9 @ 1000 (22.6 hp)
36.55 @ 2000 (42.6 hp)
35 @ 2500 (51 HP)
31.2 @ 2750 (50 hp)
28.6 @ 2800 (46.7 hp)
26.1 @ 2900 (44 hp)
21.1 @ 3000 (36.9 hp)
13 @ 3100 (23.5 hp)
12.5 @ 3200 (23.3 hp)
11.5 @ 3300 (22.1 hp)
11.3 @ 3400 (22.4 hp)
4.2 @ 3500 (8.6 hp)
0 @ 3550