Corey Rasmussen, Author at Mentored Engineer

In part 16 of this series, Corey explains how a two stage gear pump works and what the benefits are. https://youtu.be/Z7z4qVQgvzY Video Transcript Hi, welcome to the Mentored Engineer. In this video, we are going to look at our drive train on the log splitter. Alright, and that consists of basically three parts. We have our gas engine here, our motor. We have our pump, and we have a shaft coupler that couples the two together.And we're going to be talking about the pump in this episode. And our pump is a special pump. It's a two-stage gear pump. Now let's focus on the gear pump first, and then we'll work in the two-stage. So, a gear pump is probably the simplest to make, manufacture, and sell.They're cheap and readily available in multiples of sizes with multiples of configurations. And let's see how it works. So, if I look in the casing, if I take a cross-section of the pump, I see a casing here with two gears. One of those gears is connected to the input shaft and the other one is just an idler. So, what happens is we have hydraulic oil come in through the inlet and go around each set of gears and goes out.Now it can't come back here because the two gears are meshed creating a barrier that it cannot get across. Alright, so this allows it to move fluid, but also allows it to build pressure. Alright, so that's pretty simple how it works. One of the key things is it's also known as a positive displacement pump. And what happens is every time I turn this gear, one revolution, I get a certain amount of fluid out.Okay, so these pumps are classified in their displacement. Alright, and that's usually categorized in cubic inches or cubic centimeters per revolution. Alright, and it's usually represented with the symbol delta. Alright, if I want to find out what the flow is, I'm going to multiply my displacement times my speed in RPM and divide that by 231 inches cubed in a gallon. Alright, and that will give me gallons per minute.Furthermore, if I want to figure out how much power I'm going to need to power this pump, I'll multiply my flow times my maximum pressure and divide that by 1714. That'll give me horsepower. For our first stage here, it's a low displacement and that's 0.194. cubic inches per revolution all right so pretty small and what happens is that will give us putting into this equation at 3000 rpm two and a half gallons per minute that's not very much all right but the problem is that takes up 4.4 horsepower when I plug it in here at 3000 psiall right so I can plot that here in my pressure versus flow and That's where I get my 4.4 horsepower there. Now what I want to do is make that cylinder move pretty fast. And it just can't.You know, not with 2.5 gallons per minute, because I do need 3,000 PSI, but I want more flow. And there's, with the gear pump, there's no other way to get there, except we use a two-stage pump. So, we kind of cheat the system. Alright, and we do this by having a second stage, or a second pump, in our primary pump, and we tell it when to turn on and off based on pressure. So, this displacement pump has much bigger, it's more than three times as big, it's .647 cubic inches per revolution.When combined with the other pump, it gives us 10.9 gallons per minute. And how we do that is we have a bigger displacement here. When we go up and calculate our horsepower, what we're actually going to do is limit our pressure. So, we're limiting our pressure to about 450 psi. And that will reduce our required power to 2.9 horsepower.So, what we've done is we've added a second curve here. Where our horsepower is 2.9 and here it's 4.4. So, we used a lot less horsepower and moved four times as much fluid. So how does this actually happen inside the pump? Let's go here through a hydraulic schematic.You'll see here we have a motor with a shaft that's turning in a single direction. We'll start down here, start at your tank, find your reservoir. That's what this U-shaped thing is. It's just to represent a device holding water, or not water, but hydraulic oil. The oil will come up into the suction lines of both pumps.That would be the inlet here. We have a triangle on the pump pointing outward to know that it's a pump and not a hydraulic motor and that the flow is coming out of it. What happens is our flow comes out of the high displacement pump and gets blocked by this unloader valve here. There's no path for it to go around. It goes through this check valve where it meets with the flow from the low displacement pump and goes out to our system.That's how we get all this flow here, 10.9 GPM. When the pressure increases to our cutoff limit, which in this case is 450 psi, we are always sending this pilot signal back up here. What happens is when our signal gets to be the same force as the spring, this block moves down and offers a path for the high pressure to go out. Okay, so we're able to keep that pressure differential between the high and the low by this check valve. Alright, so as the pressure from here gets higher than this one, this check valve will not unseat, and flow can't go that way.Alright, so what happens is this oil actually gets recirculated back into the suction line. Alright, and just keeps going and keeps going and keeps going anytime this thing is over 450 psi. All right, you'll notice here that I have a separate tank line because it just comes right back. It's the same as going to the reservoir, except it's just done internally. I don't have to run another hose.So, what this allows us to do is, as we're splitting, we're really going fast up into the workpiece. And then when we need that pressure, it's going to slow down to... Basically a fourth of the speed, break that sucker at high pressure and then as soon as it's broken it will go fast again. So that's what we're trying to accomplish here, and we do it by adding a second section into our pressure and flow chart. Thank you for joining us today.We hope that it was informative and that you are now able to feel confident just talking about pumps and knowing the displacement is important and how it affects how much horsepower is required for your application. Thank you for watching and have a wonderful day.

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