In a hydraulic system, a number of components are used to convert a power source into hydraulic pressure to do a task not easily achieved with the original power source. An example; say you have only a somewhat small gasoline motor, and want to make a nice portable log splitter. From experience you know that you have to use a wedge shaped object to spread the wood from the end to cause it to split. You know that a 12# maul swung as hard as you can will not always split the wood so you want lots of force to push the wedge into the wood to effect the split. To have the small gas engine do the work you need to take the relative rotational energy at the output shaft and convert that into a linear motion to push the wedge, or some other way of getting that wedge into the wood with great force. So one way is to use a hydraulic cylinder to push the wedge. A straight linear push is what hydraulic cylinders do well. So a simple frame that holds the cylinder on one end and at the other either the wedge or an end stop. In any case the cylinder rod and the fixed end trap the wood between a flat and a wedge and the wood is split. But wait, it it is not yet split because the hydraulic cylinder needs hydraulic fluid, flowing with pressure to move. So you need a pump to make the fluid flow and to take some of the energy from the engine rotating shaft and raise the pressure of the fluid. So now on your frame you make a mount, and bolt the engine down. You then have to somehow couple the shaft from the engine to the pump, so one of several styles of coupling is used and the pump is connected to the engine. You also have to mount the pump either to the engine case or the frame to keep it in place and prevent it just spinning with the shaft. OK so now we have that engine mounted, and the pump mounted, how to get that fluid to the cylinder so it does not just spray around. Tube, pipe and hoses are used. Rather obviously they have to be able to hold the pressure. But wait, we also want to be able to split more then ONE piece of wood, so we have to be able to put oil to both ends of the cylinder as we desire to change the direction of the cylinder, so now we have to add a valve, and more of those tubes, pipes or hoses, again all able to take the full pressure we will see when the wood is strong and the pressure must be high. But what happens when the wood is TOO strong? Since most pumps move a fixed amount of oil for every revolution, if the cylinder stalls, and the fixed amount of oil is moved every revolution of the motor what happens? Since oil is almost non compressible, the pressure will rise till one of several things happens. The wood will split, the coupling between the motor and pump will fail, the frame will fail, the engine will stall or the weakest item will fail and spray oil. SOOO>>> we also need an item called a pressure relief valve that opens to let fluid flow so the pressure stays at or below the safe range. Last but not least a container of some sort is required to hold that oil. Do we need a container to hold the oil? Can't we just take the oil coming from the cylinder as it moves and feed it right back to the pump to have energy added and send it to the other side? Since we have a cylinder with only one rod coming out one end NO. The rod displaces oil so the amount of oil displaced as the cylinder moves in the direction that extends the rod is less then need to fill the side without the rod and vice versa. That tank is often the item given the least thought of the system. The lowly tank has several functions. It must hold the fluid without leaking. It gives the fluid a place to rest, to give foam a chance to float up and dissipate, for particles to settle out and also an important thing - a place to let the heat flow from the oil, through the tank walls and out into the air.
So, the need for a valve to move the cylinder both ways can be easily filled with any of a number of types. Say you don't mind moving a lever back and forth. So you use a "4-way, 2 position directional control valve" this would have 2 positions, and would either put oil to the extend side and connect the other side to tank or vice versa. This has some issues. when the cylinder is stopped at end of stroke, say retracted while you load another log the oil from the pump is going over the relief valve. Since the oil has high energy state IE high pressure, and goes to low pressure as it crosses the valve seat, that energy changes state to heat and noise. The amount of heat is nearly the same as the total energy made by the engine. Not a good choice. So you choose a "3 position, 4 way directional control vale with open center" This valve has 3 lever positions, with the middle position connecting the pressure directly to the tank, and the pump just flows oil, but can't make pressure so much less noise and heat. BUT you have to remember to put the lever in that middle position every time. And if you don't, you get to hear the relief valve scream to remind you. So a better choice is the nifty 3 position, open center, pressure detented 4 way control vale. This one has a little detent that holds the valve in the retract position until the pressure rises and it then pops over to the center position and lets the oil go to tank and all automatically. Most "Log Splitter Valves" are set up this way and even have the relief valve built in to simply the circuit and plumbing. Now that is a very simplified description. There are literally hundreds of possible valve configurations and styles.
There are many basic pump types with the more complex pumps having hundreds of control possibilities. I left out things like filters tank breathers etc. So when you decide to build a hydraulic press, a great deal of the design is done as I went through above. start with the cylinder. Then get it mounted and that leads you to the frame. Now you have to have a way to make the pressure and to control it, and that describes the circuit. Then there is the choice of prime mover. The energy source can be as diverse as a water wheel, a horse on a treadmill to a gas turbine. Not to discourage anyone from designing a press, but rather to understand that there are many many choices, and each choice effects all the other choices. Engineering is a compromise. I will write more bits on each component soon. And if this is all very confusing, you can easily buy a ready made, safe press that will work first try when delivered and you can then charge on and forge more metal.
Written by Jeff Reinhardt