Hello and welcome back to the drawing board with me, Steven Lloyd. A few months ago I talked to you about the design of hydraulic struts and I mentioned that at the end of the strut there’s always this hydraulic unit now the reason it's there is so you can pump some fluid in and adjust the length of the props so they will always meet the size requirements.
Today we are going to take a look inside the hydraulic cylinder and see how that works. I’m going to start simple talking about single acting cylinder. This basically means that you can pump it out but you can’t pump it back in, strictly speaking.
Going in to a bit more detail on that, the construction in a hollow shell made of steel usually sometimes our medium and inside that is mounted the steel piston. There are seals between the piston and the wall of the cylinder so no fluid can get past it. This area becomes like a balloon. You push fluid in and that volume expands and this is what pushes the cylinder out. So, that’s how you get your adjustment. That’ll keep moving out until you meet some resistance. And once it meets resistance you can block valve. This red here you’ve blocked it off. Then, you’ve got some force. This is pushing back, this is pushing forward and there’s some pressure inside that ram and the more you try and compress it, the more pressure builds up here and the more it will push back and it will hold everything in position. If you want to get that to draw back in, you release the valve and it’s the force pushing back on this side that actually drives everything out and pushes the fluid out of the cylinder. Now, the inherent problem with a single acting cylinder is that it is an open system. So, if you have some kind of hydraulic oil or fluid in there, you have to be careful of the environmental impact of it. If you’re just releasing the fluid into the ground it can soak away into a water course you must be very careful, or use biodegradable oils or just pure water if that’s an option for you.
Now double acting is slightly different. This is a closed system so that should make a circuit. So the fluid in this side and this side is all part of one continuous circuit. You only have a lock off valve on the full bore side, I’ll explain what that means in a second. And you don’t generally have a lock off valve on the annulus side. Now full bore and annulus over here, we’ve got a large area of the full bore and the annulus you can see there’s this not useful part in the middle. That’s where the shaft of the piston comes down there. So, you’re not actually forcing any pressure of any fluid onto that area there. Well the double actor, same as the single actor you force fluid into the full bore side and that pushes the whole piston rod out. At the same time it’s reducing this volume in here so you’re expelling fluid from there and that’s collecting in the same reservoir that this fluids coming from. So, you’re cycling fluid out of this side and into here.
Same as the single actor, once you’ve got some resistance on there, keep pumping it up, build a little bit of pressure, lock it off. You now have pressure in here that’s holding everything else so if you try and squeeze it, the pressure inside of here will hold it out. If you release the lock off valve then, you can in theory push it back by using the force you've got generated. That will draw fluid in here and push it out of there. But, the reason you use a double acting cylinder is because you want to actually drive fluid into this side to forcibly withdraw this ram and you’ll see these excavators, JCB’s and stuff, you want to lift things so you pump in that side but you may also want to drive things down so you pump it in this side so you can actually produce force in both directions. There is a little bit of tension resistance in hydraulic cylinders. But, that’s generally limited by the connection between this piston rod here and the piston head.
This bolts linking those together tend to be quite weak. They don’t usually need to generate any kind of tension and if they’re purely in compression the bolts are not really doing any work other than just physically holding it in place. So, you’ve got to be very careful if you’re withdrawing rams or if you’re putting tension on a ram not to over tension this piston rod because that can damage the ram. Have a look again at the full bore and annulus areas. I want to talk about the areas you’re dealing with. The reason the hydraulics cylinder works so well is because the pressure that can be put into the cylinder and by having a very large area of piston you can generate a large force from this pressure but because of the hoses and tubes and what you’re feeding the liquid in with is a very small area, you don’t need a lot of force to drive the fluid in to get a very large force out. So, if you’re putting a hundred kilos worth of force onto a small area, you’re getting 250 tons of force out of the full bore, or if it was the annuls side which tends to be a smaller area maybe only 125. But, like I said, be careful about tension or withdrawing with the annulus.
The strength of the cylinder is determined on this kind of trio of the variables here. The bigger pressure will give you a bigger force out. The higher the pressure, the higher the force will be, but also increasing the area on the end of that ram by increasing the diameter of the ram itself that will give you a bigger force for the same area. At the same time, increasing that diameter makes the whole shell weaker because a bigger diameter shell can take less pressure before the tension in the shell fails, so you have to make it thicker. So, ways of strengthening your ram, make it a bigger diameter and increase the thickness and that will allow either for a larger pressure or a larger area on the end and give you a bigger force.
So, you’ll notice this with like our MP250 props, very large 250mm diameter but something like a Vertishore Waler which is a single acting cylinder only a couple of inches internal bore on that. I’m not going to go into too much detail, I think that is all you pretty much need to know about the inside of a hydraulic cylinder.
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