Corey Rasmussen, Author at Mentored Engineer

Corey describes several ways to have two or more cylinders move at the same time. This is not an easy task! https://youtu.be/lVmPPCKv-EA Transcript Welcome to the Mentored Engineer. In this video I'm going to talk about timing hydraulic cylinders. There's been several times that I've had to time hydraulic cylinders, and that means they extend and retract at the same rate with each other. There are problems with that because guess what? The world isn’t perfect. You may have two perfectly identical cylinders coming from the same manufacturer, the same batch, and they're going to perform just a little bit different each time. or you're going to have different loads on each cylinder and you're not going to be able to balance for that. So, if you have a schematic like this where you have flow coming into your directional control valve and you go ahead and extend it and this one's really loaded and this one's not loaded, the not loaded one is going to extend first, and once it gets to the end of the stroke, then this one will go, or if it builds up enough resistance, then it will go. I once worked on a project where there was two cylinders pushing up a roof section of this device, and we noticed that they were cocking, and one of them was going up faster than the other. So, if I draw this from the side, we had a section here that had a pivot and then another section where the cylinder attached and then the other cylinder was further out here. You know, one would go and then the other would go and it would kind of do like this and every once in a while, get jammed. That's a bad situation. So, when you're first starting to look at why do you have two cylinders, you want to mechanically tie those together as much as you can. All right. So, one thing is to think about first is can I stiffen up something to do this? And one of the things that we considered doing was having a giant tube here sage said that you know the tube really wouldn't let it flex one side of the other so if one cylinder was going and the other one wasn't able to push up the load this one would even out the load and they would be fighting each other a little bit but the pressure difference between one cylinder and the other would be very small because both of them were lifting the load you can't always do that though So right here, one of the ways that we can do this, probably one of the most ineffective ways is to have a motor type flow divider. So, if we go back to like a gear pump, we have two sets of meshing gears. Yeah, it's a gear. In a cavity, and oil comes in these gears go opposite directions and then we will have one gear go out and another one go out so that as they spin it takes oil pushes it around half the oil goes out that way half the oil goes up that way and as the cylinder retracts both of them bring the oil back through there you can basically expect around 10 percent efficiency with that or as far as, you know, how well they're timed. So not really on, you know, the precise end here. They do have cartridge valves where they will they split and some of these are higher percentage, you know, higher, higher accuracy or higher precision flow divider and combiner. So, you can actually get a little bit better result with that. So, you know, a cartridge valve may be, you know, 10% or lower depending on the make and manufacturer of it. So, another way you can do this is by adding needle valves to our flow here. Now you may already have a needle valve in your system and you don't even know it yet for example if when you come to the split right here all right if the hose on this side is you know four feet and the hose on this side is 20 feet um you've already got a restriction in there even if it's just the you know the force that it takes to move the oil through the hose more oil is going to go to this four foot section because it doesn't have as much pressure drop it may just be you know if it's 30 psi that could be a big enough difference between the two So one thing you can do is even out your hose lengths after the tee. Even if you just have, you know, if you had to have 20 feet of coiled hose over here on this cylinder, even though you didn't need it, it's going to make a difference. Okay, so what you want to do here is add a needle valve in on each side. And that gives you, you know, a good delta P over each one. They're going to try to be the same as much as possible. You're isolating this half of the circuit from this half of the circuit. It'll make it better. So, you're probably looking in the 5% error one to the other cylinders just because it's not a fully compensated system. one thing that's nice about this is it will allow re-timing at both ends so once you fully extend, they'll both bottom out because there won't be any pressure going here it'll just be flying over here so another way to do this and it's not very common is to have a double rotted cylinder so this is a cylinder with the rod sticking at both sides and it allows equal pressure. Now, if you're, you know, having an application where you're pushing and that's when you need your force, this is going to help you because it's going to reduce your area by the size of the rod. But what you do here in this case is instead of teeing these both together, oops, that one, that one stays, you actually feed the extend port into the retract port of the other cylinder. So, oil will come out here, go into this one, extend it. That oil will now push this one out, and then that oil will return back to tank. And it kind of works nicely. You're going to lose a lot of force because this is return oil now working against this, so you're going to have pressure intensification. So, your cylinders are going to have to be way oversized for your application to lift the loads you want because you don't get the full pressure of each one. But they will be timed. They'll probably be timed within 2%. Another way to do this, also not done very much, is to have two drive systems. uh which would include two different motors two different directional control valves uh one controlling each one and you would just basically say like extend and you're relying on your pump's flow rate which also can get out of time Very expensive. Two motors, two directional control valves, and you're not very much better than 2%. Alright, so the last one is the most accurate one and also the most expensive. And that is to put some sort of position detection in your cylinder. So, there are several kinds. Some of them you can put in the base of the cylinder and it's a sensor that measures how far the piston is away. You got LVDTs, which you can put in the cylinder. And there's a sensor here that will tell how far the piston is away or how far the piston is along that line. And it'll send back LVDTs or what those are called. And basically, you got to feed that back in the control system. You have two directional control valves, one for each cylinder, and you can extend and retract those. Very, very precise. I mean, we're talking down to less than a thousandth of an inch that you can get with that. But you got to come up with the control system to do it. And you have all that componentry. So very expensive. But if you need it, it's the way to go. I mean, you're going to you're talking about you know almost no error in those cylinders there's one thing we need to talk about and that is re-timing we've kind of talked about it a little bit as you can imagine with this system there is oil that is going to sneak past the piston over time lots of time pistons are very sealed you know they're not going to drift very often unless there's an issue inside the cylinder But eventually oil is going to go from one side to the other if it's set up under pressure for a long time. And eventually these are going to get out of sync and there's no way to re-sync them. When I say out of sync, I mean, you know, this one might be fully retracted and this one might still be extended an inch. You either have too much or too little oil in one of the cylinders. It's because of how things work, especially since the pressure in this side of the cylinder will be different than the pressure in this side of the cylinder. So, there's going to be things that they just get un-synced and we want to be able to retie them. The gear pump is not something that can be re-synced. So, what we end up doing is we'll put a small little needle valve here um so let's just say this is eight gpm this maybe it's going to be less than one or sorry that's greater than one less than one gpm all right and that's just a way to retime it where one gets to the end and then it forces the oil to go out through here to the other side But we need to be thinking about that too. How do we re-sync these? How far off will they need to be before we need to re-sync them? Well, those are the most popular ways to synchronize cylinders. Just remember two things. If you can find a mechanical solution for this first, do that. If not, make sure you have a way to re-sync these things so that if they get out of whack a little bit, they're able to get back into order. All right, thank you for watching this episode of The Mentored Engineer and please subscribe to our channel. Thank you.

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