# 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.

### Check Valves

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 valve.

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.

### Direction Valve

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.

## Safety Valves

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 as our 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 rate.

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 ratio.

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.