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.