Fluid power has the highest power density of all conventional power-transmission technologies. Learn the benefits and limitations of fluid power, how to analyze fluid power components and circuits, and how to design and simulate fluid power circuits for applications. In this course, you will be introduced to the fundamental principles and analytical modeling of fluid power components, circuits, and systems. You will learn the benefits and limitations of fluid power compared with other power transmission technologies; the operation, use, and symbols of common hydraulic components; how to formulate and analyze models of hydraulic components and circuits; and how to design and predict the performance of fluid power circuits. This course is supported by the National Science Foundation Engineering Research Center for Compact and Efficient Fluid Power, and is endorsed by the National Fluid Power Association, the leading industry trade group in fluid power.
Pump Application: Hydrostatic transmission
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Del curso dictado por University of Minnesota
Fundamentals of Fluid Power
412 calificaciones
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De la lección
Week 3: Components and Concepts: Part 2
This will be a busy week diving into valves and pumps. We will discuss how basic valves function, how to use them in hydraulic circuits, and how to calculate pressure drop for a given flow rate, or vice versa. The videos will directly address the discussion on the forum about seeing hydraulic components working in real world circuits. In our discussion of pumps we will look at many different positive displacement pumps, exploring flow ripple and pump efficiency, look at the supporting components that form a hydraulic power supply, and see how we can make a transmission with a hydraulic pump and a motor. We are now into the heart of this course; we hope you enjoy seeing the components come together into useful circuits.
Conoce a los instructores
James D. Van De Ven, PhDAssistant Professor
Mechanical Engineering
Will Durfee, PhDMorse Alumni Distinguished Teaching Professor
Mechanical Engineering
So in this video we'll be studying a transmission formed by
a variable displacement pump, and a fixed displacement motor.
We refer to this as a hydro static transmission.
So, before we get into the nitty gritty details of how this works, let's head out
into the field and see one of these in action, and see how capable it is.
[SOUND] I'm driving a Toro Groundsmaster 4300 at the Toro test facility.
This machine uses a hydro static transmission.
[SOUND] The operator directly controls the pump displacement with a foot petal,
which directly controls the flow rate of the pump.
This infinitely variable transmission offers seamless speed transitions,
including reverse, where the pump displacement goes over center.
[NOISE] So now that we've seen our hydrostatic transmission
in the field, let's talk about how it actually works.
One component that we need before we get there is a hydraulic motor.
So a hydraulic motor,
is really just a pump where we're flipping our energy domains.
So instead of having shaft power input and hydraulic power output,
now we have hydraulic power input and shaft power output.
One thing we have to pay attention to in the design of the unit, is the valving and
making sure that we can reverse the flow, but that just comes into the details of,
of how the unit is designed.
Now, as far as the symbols go,
you'll note that in our pump, we have this triangle right here.
Filling it in means it's a hydraulic unit.
And the fact that our arrow is pointing outwards, well,
this means that it is sourcing flow, or the flow is leaving the pump.
Now, our hydraulic motor also has a triangle, but
now our triangle is pointing inwards.
So we are syncing flow in, in this hydraulic motor.
And this is the way we tell in a schematic whether or
not we're talking about a pump or a motor.
And again, we're just flipping what our two energy domains are here.
So let's now start combining these two units into a hydrostatic circuit.
So this is what we refer to as an open hydrostat.
Open meaning that, one side, or
the low-pressure side is connected to a reservoir that's vented to atmosphere.
So it's open to atmosphere.
And in this case,
we have a variable displacement pump, similar to what I have here, and
a fixed displacement motor, meaning that the, the displacement cannot be changed.
I also have in my circuit a relief valve, and
this relief valve is really just managing pressure spikes and
prevents us from creating too much pressure on the, the high pressure side.
So, let's first of all take a look at how we relate the torque of the input shaft to
the torque of the output shaft meaning the pump torque to the motor torque and the,
the speed variations as well.
So first of all, for my pump, I'm going to look at it as an ideal component,
I realize it has some many inefficiencies, but let's first of all look at it
as an ideal component and I write the relationships between flow rate and
angle of velocity and between pressure and torque.
And I rearranged them slightly from what you saw previously.
I'm going to do the same thing for
my hydraulic motor, and the one thing that you'll note that's different, besides
being slightly rearranged, is that there is this x in the, in the pump equations.
What does x is?
This is the fractional displacement.
So, when I go to a vertical condition here in my swash plate and
I have no flow of my pump, well this would be when our fractional,
fractional displacement is zero.
When I go to maximum stroke,
this would be a fractional, fractional displacement of one.
So that's just what that x is denoting.
So now I'm going to substitute the pump equations into the motor equations.
And now I can create these relationships between the angular velocity of the motor,
relative to the angular of velocity of the pump and
the torque relationships between the two as well.
What you'll notice in both of these, is that I have this relationship of
the displacement, of the pump unit times its fractional displacement, divided by
the displacement of the motor unit, or the reciprocal of that in the second equation.
And we refer to this as our gear ratio.
And this gear ratio is adjustable through our fractional displacement.
So this is what allows us to do our speed variation as we
were driving that vehicle to hydrostatic transmission.
So, this open circuit is just fine for now, but we actually need some
more components to get this to operate in the, the manner that you just saw.
So, the first question I'm going to ask is, what about going in reverse?
We realize right now I can only pressurize the high rail of, of this system, and
if I try to apply pressure to my tank, well, it's vented atmosphere.
I can't apply any pressure there.
So I can't go in reverse.
So what we need to do, is what we call closing the circuit.
The way we close the circuit is really just connect both sides of the pump and
motor to each other.
And now we have a pump that can go over center, as this unit can,
meaning that it can produce flow on either this being the high pressure port, or
as I go over, this could be the high pressure port.
And then we can go in reverse with this unit.
And I need a bi-directional motor.
Note that my motor now has two arrows coming in,
and my pump now has two arrows going out.
So both of these can source and sink flow on either, either port.
So I can reverse the which side's the high pressure, which side's the low pressure.
So, next question is, what about overpressuring it in reverse?
Now you'll note that I still have this relief valve that takes care of
high pressure when I'm going forward, but
in reverse I could also have a pressure spike.
So what we're going to do is, we're going to make this circuit symmetric by adding
another relief valve, and this prevents high pressure on the, the lower rail.
And, let's keep asking some questions here.
What about leakage of these pumps and motors?
How do we make up for the lost flow?
Remember when we did our analysis of a, of a piston pump,
we saw that we can have some leakage between the piston and the cylinder.
And this leakage goes into the case of the pump.
And then this leakage ends up going back to tank.
Well, if we have this closed circuit,
we have nowhere to add flow back into the circuit for this fluid being lost.
So we're eventually just going to have you know, no fluid in our circuit.
We need a way to make up that flow.
And the way we do that, is by adding what we call a charging circuit.
And so, what we have here in this charging circuit, is we're adding another pump.
And this will usually be piggyback mounted to this main pump meaning it's
just a small pump mounted on the same shaft, and
the flow coming out of this pump will go to the relief valve.
This relief valve is normally set maybe at two or 300 psi,
fairly low pressure, and then the rest of the flow coming out of the pump
will go to these two check valves.
So depending on which side of our circuit is the high pressure sides, so
if we're going in forward, this would be the high pressure side and
this check valve would stay closed.
But this check valve down here would be seeing low pressure on the bottom, so
we would get the flow from our charge pump going to the low pressure side.
When we go in reverse, they would flip directions and
we would be able to provide make up flow to the opposite side of our circuit.
So this is providing the make up flow for whatever's leaked in our, in our system
and managing the, the amount of fluid that we have in our closed circuit.
Now, I've also been talking about how important filtration is and cooling is.
So anytime we make heat in a hydraulic circuit,
that heat goes into the hydraulic fluid, and we need to dissipate that in some way.
So, how do we do that?
Well, we've got a closed circuit.
We don't really have anywhere for it to go.
And so what we do is, we add another valve here.
And this valve is called a shuttle valve.
And often it is referred to as a low-side, hot oil shuttle valve.
So what happens?
Well imagining that I'm going in the forward direction, and
when I'm going forward, this high pressure rail is seeing pressure.
Or it's at high pressure, and that high pressure then will be acting on
this pilot right here, and shifting the valve to the bottom position.
As it shifts to the bottom position I then get flow from the low pressure side,
crossing over here and going to this relief valve.
So this relief valve is again setting with the low pressure.
Pressure is, and then below this relief valve,
I would add my filtration and my cooling before the fluid returns back to tank.
So this is the way that I can pull fluid out of this closed hydrostatic circuit,
and the way I would make it up is putting it back in through the charge pump.
So the charge pump will be sized a little bit larger if I had this,
this cooling circuit to make up for what I'm, what I'm losing here.
And again, we're pulling the, the hot oil off the low pressure side and
that's being done by pilot driving this shuttle valve back and forth.
So, these hydrostatic circuits are, are quite powerful and allow us to make
a very, you know, very efficient machine as far as its, its maneuverability.
Let's talk about how it compares to a mechanical transmission.
First of all, we have an infinite gear ratio in a hydrostatic transmission,
whereas in a mechanical transmission, even a continuously variable transmission,
we have certain gear ratios that we can adjust between.
But we cannot go to a true zero displacement with
a continuously variable transmission,
while we can do that with our infinitely variable hydrostatic transmission.
Second is that we have very high power density in hydraulics, low inertia,
so that we can put these hydraulic motors right into the wheels of the vehicle if we
want to and
drastically change how the vehicle is laid out as far as its power train.
So we don't have to have drive shafts going to the wheels if we put them,
if we put a hydraulic motor right there.
So it changes the plumbing of the vehicle.
We can have dynamic breaking through our relief valve.
Our engine cannot stall because of the, the relief valve in our, in our system and
as we're shifting gears, we don't have any interruption in
power as we're going from forward to reverse and changing our gear ratios.
We can, we can do that continuously and seamlessly.
Now the biggest disadvantage of a hydrostatic transmission and hydraulics in
general, is the loss of energy as we're doing all these conversions.
Our hydraulic pumps might be peaking at, you know, 91,
92% efficiency, our hydraulic motor might be at the, the same kind of efficiency.
Multiply those together.
We're doing very well if we have an 80% efficient hydrostatic transmission.
Much often, or much more often it's in the 60s and 70% efficiency.
Now a mechanical transmission could be in the, the low 90% efficiency.
So it's hard to compete as far as the, the power transmission.
And also, hydraulic systems tend to leak.
And so we just have to manage that and
make sure that we seal the system as well as we as, as we possibly can.
But again, these hydrostatic transmissions you know, give us a,
a lot of variability in, in the way that the vehicle operates, and we're doing this
by combining a variable displacement pump with a fix displacement motor.
We just as easily could have a fixed pump in a variable motor, or
a variable pump in a variable motor.
So there's lots of combinations, we've only really talked about one that we,
that we went through and analyzed here.
What we're going to be doing later in this class,
is we're going to take this same hydrostatic transmission, but
add energy storage too it in the form of a hydraulic accumulator.
And this would then allow us to do regenerative braking for
an application like a hydraulic hybrid vehicle.
So we're going to be building on what we've talked about here today in
a later lecture.
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