In this lecture we'll be discussing the component that's in our course logo, which is a check valve. So, a check valve is a valve that allows flow in one direction, but prevents reverse flow. So you've likely used one of these whether it's changing a flat tire or doing some vehicle maintenance, this is a hydraulic jack. And we know from our transformer equations and from Pascal's law, that we can create force amplification. From a small diameter piston here, driving a larger diameter piston. But did you ever consider how does this create multiple pumping strokes? What's the valving that allows that? Well, as I've drawn it, if I was to just press this down it would move the larger piston up. But then as soon I move this back, it would drop the piston down. Well. We need a one way flow device to help us do this pumping. And I'm going to add these components, and these are check valves. So, as you see in this, in this circuit diagram, anytime that I am going to be moving the piston in the upwards direction, I will be having flow coming from tank through this check valve. And then, as soon as I start to move the piston downwards,. Then, the flow is going to be going through this other check valve, and into the main cylinder. So these one way valves allow this pumping action, and any time we get a pressure gradient that is against the natural direction of flow, it prevents flow through these valves. So let's talk a little bit about how a check valve actually work, and before I get there let me just mention we need one more valve, to lower this device and this is a needle valve. In this case it's a variable orifice that I just simply open this up and we can then drop this. So there are actually three valves in this hydraulic jack that we're using. So let's talk more about check valves. So there are quite a few different check valve architectures. This is the symbol for a check valve and this is the same as the ball style check valve which is really the most common and the simplest check valve. Where we have a ball that is pressed by a spring into a seat and when the pressure on the upstream side is greater than the downstream side the ball moves, in this case, to the right. And allows flow past the ball, but when the pressure on the downstream side becomes greater than the upstream side the ball re-seats and prevents the reverse flow. So, fairly simple concept, but a very useful valve here. There are also many other styles. What I have here in my hand is what's called a poppet valve and I have a diagram of it. And you might be able to see that as I push this, I have a poppet that is moving here. And so as the pressure right here becomes greater than the downstream side it will move the poppet, and when the pressure on the downstream side is greater than the upstream side, it will re-seat. Now in both of these valves we have springs, and these springs have adjustable spring rates, so we can set what's called the cracking pressure of the valve, which is the pressure differential across the valve required to cause it to open, so that's our cracking pressure and that's how we often classify one of these check valves, is what the cracking pressure is. Not only that but we can have different size valves. So I've got two different popit valves here, popit style check valves. And very often these will then go into some sort of a manifold cavity here, where this would be the upstream side, this would be the downstream side of the valve. So, we can put this in a, in a variety of different places and this is just an example of what that poppet looks like. So, this is sitting in the seat, is what provides the seal. And then as it moves away, we get flow going through these holes and out through the end of the valve. So fairly simple design. So poppet valve, and a ball style check valve, we also have disk style check valves that I have here, where it's simply a disk that's moving. And the nice about a disc is you will have a large flow area, but a fairly low mass, so you can get these to, to a respond at a very fast speeds. So this is very often used in a pump that's using a check valve. There's also reed style check valves, so there are a lot of different architectures, but they all do the same thing, allow flow in one direction. So we've talked about it in a pumping sort of a circuit. Now let's talk about it in another application where it's very commonly use which is in component bypassing. So imagine that I have this circuit right here, where I have a directional control valve controlling a hydraulic cylinder. And let's say that my goal here is that I want to have a very slow retraction of my cylinder, and so I'm metering the flow going into the cylinder with this, this adjustable, adjustable orifice or needle valve, but I want to have full speed extension of the cylinder. So I don't want to be metering the flow coming out of it when I switch the directional control valve. How can I do this? Well I can add a check valve to my circuit, so that I can bypass this needle valve when I am flowing out of the circuit. So anytime that I am flowing in this direction and extending the cylinder. My flow will instead of going through a needle valve it will be going through the check valve and will bypass this restriction. But, when I have flow going in the opposite direction, this check valve will close and I'm forcing flow then through the needle valve. So it allows me to bypass a component only under certain flow conditions. Now, another application for a check valve is in a cylinder locking application. So, imagine that I have a cylinder that's supporting a large mass like I've drawn here, and whether or not the power is shut off, or if I have a, a valve set up like this, for some reason I might want that mass to stay at its location where I left it when I shut off the power to the, the actually, or when I no longer am providing pressure or flow to the actuator. Well how can I do that? Well you've seen that I've drawn a check valve here, such that I can flow fluid through the check valve freely, and any time that I have a reversing flow, it is going to see a, a higher pressure on this side of the check valve, and therefore well prevent the, the flow through that valve. So we've locked it, essentially. But the problem is, now I can no longer actually retract my cylinder. So, we need a slightly different type of check valve in this case, and this is called a pilot operated check valve. So, what now happens here, is that when I want to retract my cylinder, and the pressure here becomes higher and the pressure on the downstream side becomes lower, so this would be P low down here. When that occurs I'm going to have this pilot pressure which is connected to my check valve. It is going to force the check valve open, and allow me to reverse flow through the valve. So, this allows me to lock this circuit, but then be able to still unlock it when I want to. So, let's talk about how this pilot operated check valve works. Here's a diagram of it, essentially this is again a poppet style valve, and the only difference now is that I have a piston that can push this check valve pop it out of the way. And so I have a third port in my valve, and whenever I send pilot pressure to this port, it will drive this cylinder, push the check valve out of the way, and allow bidirectional flow through the check valve. If I release that, that additional cylinder, that additional pilot pressure. Now this functions again as a standard check valve. So, again, it allows me some flexibility in designing this. There are also pilot to close check valves, where I can force this valve to stay closed. And, so we, we have a lot of different ways that we can create logic in our system, by modulating these pilot operated opening and closing check valves. So it, it allows us quite a bit of flexibility. But still acts as a as a passive valve in its standard operating case. So in summary, we've talked about check valves and how they allow us to do pumping applications. They allow us to do component bypassing, and allow us to do cylinder locking in a fairly simple to design valve. But again, a lot of, a lot of capabilities from this valve. [SOUND]
