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Discussion in 'Heavy Equipment & Pavement' started by AWJ Services, Oct 13, 2013.
This is very true.
Pressure goes up= flow goes down. Simple physics. The specs on brochures are a bench specs meaning that the pump displacement is designed for a certain amount of flow and the system will operate up to a certain amount of pressure.
I have yet to put a flow meter on a machine- CAT, Bobcat or ASV that will put out a high flow that equates to what the brochure states at the pressures on the brochure- gear pump, load sensing piston pump, direct drive or belt drive.
I understand that restriction can limit flow and I have seen as pressure spikes when mowing where I lose flow and stall head but I thought that was the relief kicking in not a complete reduction of flow. If I just had flow and not enough pressure behind the flow, I'd not have any torque in the mulching head, right?
It gets confusing. When I was a firefighter many moons ago, we turned the rpms up on the gear driven pumps and increased gpm. I don't remember what pressure we were at (probably 300 to 500psi???) but restrictions in our nozzles created extra pressure too. Take the nozzle off, and it was just high flow out of a limp noodle which didn't do any good.
There is no need for the machines to run at constant high pressure if there is no load on the system (just creates heat), correct? But they have to be able to compensate for resistance without dropping too much flow as I stated above or I think I'd stall the head's rpm. I don't know how they accomplish that exactly.
You guys are cracking me up. The two most common types of pumps are: 1) positive displacement and 2) velocity.
The type used in virtually all mobile hydraulics is positive displacement. The two most common types of these are piston, or gear pumps. The pump "curve" for these is fixed as the swept volume per revolution is constant and therefore is a straight line (ignoring internal leakage).
Gear pumps cannot adjust flow per revolution and if there were no relief pressure would either stall the motor, crack the case, sheer the shaft, whatever,...
A piston pump can have a fixed or variable swash plate. The variable swash plate version can adjust swept volume per revolution by adjusting the swash plate angle. There is a piston used internally to vary the swash plate angle and this can be adjusted with springs and other feedback mechanisms (i.e. pressure).
The pressure at which the swash plate starts to destroke can be adjusted so that a curve may be produced, but is more likely that this curve will be linear in this range. For newer machines with computer controlled proportional valves you could theoretically map any curve you want.
Yes the pump flow is a straight line in regards to flow vs rpm however it is not in regards to pressure. Take the pump at 2000 rpms and it will out put x amount of flow. Raise the pressure from 0 to 3000 psi and map it and the flow will drop exponentialy in regards to pressure rise. Glad we can make you laugh.
Yeah yeah yeah....you're not telling me anything I don't know...the internal piston that moves the swash plate is called the servo piston and it is usually shifted via charge pressure shot through galley ports on each side of the piston housing blah blah blah. And typically they don't call it internal leakage, they call it inefficiency, piston pumps being more efficient.
The simple fact that we were discussing before you came on here in an attept to ridicule us by demonstrating your immense hydraulic knowledge was that flow is impacted by increasing pressure. It can only be as linear as the increasing pressure and the ability to drive the pump under a load allow it to be.
That was my point, I think you are confusing a direct displacement pump with a dynamic pump (like one with an impeller). Dynamic pumps make maximum pressure when they are dead-headed. The only way you can vary flow rate with a positive displacement pump is to change the rpm of the pump. The output curve is virtually a vertical straight line in a pressure vs. discharge curve.
There is more than one type of efficiency for a pump. There is hydraulic, mechanical, and volumetric. The overall efficiency is the product of all three. I was referring to volumetric but I didn't want to over complicate things. And for a high efficiency positive displacement pump, flow is not impacted by rising pressure, only rpm.
But flow decreases as rpm decreases which typically drops due to resistance the system feels????? If not, please enlighten us on why the rpm of a pump would decrease during the course of normal operations with an attachment such as a mulcher.
What you are observing is flow at the motor (not the pump) decreasing as resistance increases. This is due to either insufficient input power (bogging the engine) or the limitation of system pressure (destroking the swash plate, relief valve, or other. Things are more complicate than this as there is usually more than one pump and the flow can be combined in different ways provide more or less flow when desired.
A skidsteer generally can run larger implements simply because it has more power available. This isn't a rule as the pumps may not be configured to utilize all the horsepower available by converting it to flow (i.e. high-flow). Generally, mini excavators are more limited by their lower horsepower engines. Excavators also generally favor articulation hydraulics over others as what good is an excavator if you can't move the boom or arm?