Open Circuit Hydrostatic Drives
Open circuit hydrostatic drives are a type of oil hydraulic motor
often used for driving flywheels in
hydraulic oil systems. We will look at them
from an operational, safety, and maintenance standpoint.
The open-circuit hydrostatic drive is an important driver in many oil
hydraulic systems. We will assume that we have a pump with a flow rate
of 20 liters per minute. This will be used later for pipe sizing.
Next, activate the pump, then deliver oil to the system via the
pressure filter, direction valve, and hydraulic motor.
Oil Hydraulic Motors
We also envision hydraulic motors driving large flywheels. We also
have the return flowing through the check valve, filter, and back to
the reservoir. To make the picture more open, let's recall that we are
showing a reservoir at each point. We could connect all these lines to
the main reservoir, but it would create clutter in the design. This is
a circuit diagram, not a pipe diagram. We know that all these lines
will return to the main reservoir at the bottom. We have one other
point. Let's just move on to the next picture.
Here are the check valves and their springs: I should remind you that
all check valves come with springs. However, a symbol with a spring on
a check valve indicates a predetermined pressure. This could be 100 or
200 kilopascals, depending on what the designer wants. The B port will
also be blocked if the directional valve is neutral. There is no way
for oil to escape. This area cannot be evacuated. This is where you
can see the pressure increasing. We would blow across the relief
The hydraulic motor also has a drain. The hydraulic motor then
receives the oil. We will review this information in a moment. It is
possible that the hydraulic engine could run dry if there was no back
pressure on the check valve that feeds oil into the hydraulic motor.
Let's look at this again. The hydraulic motor spins at a rapid speed.
The direction valve can then be set to neutral. The oil can't escape.
The back pressure is now pouring oil through the check valve, ensuring
that the oil line is full despite the low pressure. It does not affect
the hydraulic motor. It prevents the engine from running out of oil,
and it prevents cavitation damage. This is the next one.
Now we will assume that we are moving forward. To illustrate, I like
to think of a large road roller that can carry 30–40 tons or 50 tons.
This is what we are doing. The vehicle won't stop or reverse if the
driver or operator pushes the lever back. The vehicle slows down,
speeds up, and then reverses. During this time, the gas would escape
through a relief valve—the one above. We then restart the pump. We are
delivering our oil. The engine rotates clockwise. Here is where the
oil comes in. The motor now rotates counterclockwise or anticlockwise
when the oils meet and the relief valve releases air. Now imagine that
the road roller is now moving in reverse.
We have our safety valves, and the same applies during slowdowns. It
will also compensate for oil losses. This gives us an idea of the
rotational pattern of the hydraulic motor as it advances. He then
pulls the lever to turn counterclockwise and anticlockwise. He runs
for a while, then speeds up, then runs back at a moderate speed,
perhaps five to ten kilometers per hour. We will now use this digital
image to create a checklist.
It will be easy to follow. We will just breeze through it. The
pressure line filter is 1.5 times as large as the pump flow rate. The
pressure line's diameter is another factor. The pump has a flow rate
of three meters per second. Twenty-one liters per hour can be
calculated by multiplying twenty-one by two by three. Next, we get to
the directional valve. This is the pump flow rate multiplied by the
cylinder area ratio.
The cylinder is used. We now deal with the motor. This does not apply.
Next, we will determine the diameter of our return line. Naturally, we
calculate the hydraulic motor's ratio, which is one-to-one. Naturally,
the hydraulic motor does not use both cylinder lines. Let's talk about
the return-line filter. It is 1.5 times the pump flow rate multiplied
by the cylinder area ratio. The hydrostatic drive is now an open
circuit. The oil is taken from the reservoir and pumped into the
system. Finally, it is pumped back into the reservoir.
5 Step Hydrostatic Drive Checklist
We always start the
hydrostatic drive by going
through the five steps of the checklist. We now move on to the next
step. The pipe size will be estimated at 24 commas and six millimeters
long. The filter flow rate will be 1.5 times that of the pump. We also
have a 20-liter per minute relief valve, which has the same capacity
hydraulic oil piston pump.
Next, we will reach the pressure relief valve. It is 1.5 times the
price of the pump. The diameter of the pipe is then eleven and eight
millimeters. After the five steps have been completed, the suction,
pressure, and return filter sizes are 1.5 times the pump flow rate.
The suction filter, pressure filter, and return line filters are all
identical in size.
The flow rate of the pump is approximately one-half that of the
suction filter, which is 30. They are all identical. The ratio of the
hydraulic motor to oil is one-to-one. Except for some oil lost through
the case drain, the oil that enters and exits is the same. The next
step is to adjust the size of the crossover relief valves and the
crossover lines to increase the pump's flow rate by doubling the flow
The diagram shows that two oil shipments combine and then rise briefly
to the relief valve. In our case of 20 liters per minute, there will
be 20 liters of oil coming in from this side and 20 from the other
side. This gives us a total of 40 L/min passing through. We,
therefore, increased the flow rate of our relief valves from 20 liters
per minute to 40. We then calculated the pipe sizes using 40 liters.
This gives us an inner diameter of 16.8 millimeters.
Next is the pressure line. It must have the same inner diameter as
lines A, B, and the return. Remember, hydraulic motors have a
one-to-one ratio. Therefore, the A and B lines, along with the B and
return lines, will be the same size. This is what we have today. This
is the one. The ideal size for a
directional control valve
would be twice the flow rate. It should not have any unneeded
restrictions. Keep in mind that hydraulic motors have a 1:1 area
As stated many times, motors don't have the same area ratios as
cylinders. This valve now consumes 40 liters per hour. This check
valve has a one-to-one ratio, which means that it is the same size as
the pump. The
open circuit hydraulic system
circulates 20 liters of oil per minute. 20 liters are returning. We
can therefore produce at most 20 liters per minute.
Oil Hydraulics Cylinder Design & Maintenance
Oil Hydraulics & Mechanical Leverage