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Before going into the details of the different types of system it is important to realise how hydraulics transfer power.
Example 1 Generally in a hydraulic system there has to be a pump or head pressure and a restriction, cylinder or motor. The power that is transmitted is a combination of the flow of liquid and the pressure it is at, the flow is dependent on the pumps volume and the pressure is dependent on the restriction placed on it at the other end of the system. Let us say that we have a pump that pumps 20litrs a minute with an unlimited power source behind it, if there is no restriction placed on it then it will absorb no power (this is theoretical as there is always some resistance) If the pump then has a pipe put on it then the resistance of the oil passing down this will produce a small amount of pressure at the pump, the more the pressure the more power required to maintain it. The simplest way of restricting the flow is a nozzle, the smaller we make the hole in the end of it the more power that is required to pass our 20litres of oil through it The same theory can be used with a cylinder or motor. If there is no resistance put on the cylinder or motor then it will move freely and absorb no power but the more resistance we put on it the more power that is required to keep it moving. This is expressed in the formulae:
Pressure in bar X flow in litres = Kilowatts 1 bar = 1.019716 kg/cm²
So as you can see if you increase the pressure or the flow then you will increase the power required.
Example 2 Another way of viewing hydraulics is like an rubber band around two drums, as you turn one then the other will also turn, if you class the diameter of the drum as the same as the volume of the pump or motor then it is easy to see how the increase in volume of the pump will increase the speed of the motor proportionally. Two turns of A will produce one turn of B
The faster our pump is turned the more oil flows and the faster the motor goes, we measure flow in Litres per minute L/m If we apply a resistance to B then we will have to put in more effort to turn A, this we can relate to hydraulic pressure, just as the tension in the rubber belt would increase so our pressure increases, this we measure in BAR. As you can see in the example above we will need to turn the handle on A twice the speed that we wish B to rotate. The next stage in a normal hydraulic circuit is to add a control valve.
Example 3
A is the standard symbol for a fixed displacement pump and C is for a bi-directional motor. In the above example we are using a valve B to control the amount of oil that is going to the motor C, by doing this we are able to increase and decrease the speed of the motor even though the pump A is doing a set RPM. The oil that is not going to the motor C is draining from the control valve back to the tank. This is one way that a motor or ram can have its speed controlled but it is not energy efficient. The pressure that the resistance on motor C produces in the system has to be matched by a resistance in the pipe to the tank otherwise the oil will take the easiest route and head straight for the tank! If we say that the pump is producing 20L/m and the motor is placing a constant resistance regardless of speed producing 150bar of resistance then the hydraulic power required:
20 x 150 = 5kW
If we slow the motor down to half the speed it will only require half the power because the flow will have halved with the pressure remaining constant. The pump is still producing its 20 L/m so we are still using 5kW but 50% of it is producing heat by being restricted in the valve.
Example 4
Above is a representation of an open centred control valve, what this means is that the ports when the valve is not operating are connected together, so the oil when being pumped will take the course of least resistance back to the tank. The box C shows the fact that this is open centred. Box B and D show what will happen when the valve is moved in either direction so as you can see this valve would cause a motor to rotate in either direction and in its neutral position it will rotate freely.
The drawing above is of a simple circuit with a bi-directional motor, there is one fundamental component missing from this and that is a pressure relief valve, this is the hydraulic equivalent of a relief on a boiler, it will allow maximum pressure to be maintained but not exceeded and all circuits should have one. The above system is great for a bi-directional motor that has no load on it when it is not running but if we put this onto a ram or a motor with a static load then we would have no control and not be able to stop it as the oil can flow freely from one side of the motor to the other. Example 5
When we want to be able to stop the flow of oil from one side of a ram or motor to the other we may use a closed centre valve
In this example the oil from the pump would return to the tank and the ports to the motor or ram are blocked until the valve is operated. It is possible to put functions like this in series, as shown below.
There is another element that we have added to this system, filtration, dirty fluid is the most destructive thing for a hydraulic system. This diagram shows a suction strainer to stop large particles entering the pump and a return line filter for finer particles. The filter is shown with a pressure relief valve to bypass it and also a check valve, this is the ball in the V on the right hand side, these stop oil from travelling the wrong way in a pipe.
Example 6 Quite often there is another valve that is used to lock a piece of equipment hydraulically, this is a pilot operated check valve. These are designed to not allow oil to flow back to the control valve until there is a flow from the opposite port of the valve.
When there is pressure in line A the check valve B allows the pressure through to the ram and the pressure in line A opens the pilot operated valve allowing the oil to return to tank, when the pressure in A drops the pilot operated valve closes and stops the oil from returning to the tank therefore stopping the ram. There is a more refined version of this valve that allows a much smoother transition and this is called a counterbalance valve, these are used when there is a load on the ram that may cause it to want to accelerate, the valve meters the amount of return oil in relation to the supply oil so that the load can only fall at the rate required.
Variable displacement systems By now you should have a basic understanding of fixed displacement hydraulics (where the pump delivers a set amount of oil per revolution) now we will go onto variable displacement systems. There are two types of pump used in variable displacement hydraulic systems: Vane and Piston. For these examples we will say that we are using piston as these are by far the most popular in the type of systems that we build. Variable displacement piston pumps when put on a system can be set up in two basic modes, these are: fixed pressure or load sensing. There are other ways that they are used in transmission systems (read closed loop systems). For our examples we will only talk about load sensing as this is the most common way of setting them on multi function systems.
Example 7 With a load sensing system there is no or very little oil flowing when there is not a function active, a signal is sent from the control valve to the pump to tell it that oil is required, the pump then supplies only the volume of oil that is required of it. The pump has a standby pressure that it sits at when it is turning with no load, this is usually around 15 bar.
The symbol shown on the right above is an adjustable flow control, what is happening in the system above is that a small amount of oil is being bled from the function side of the valve to the pump, the non return valves are there to stop the two function pipes from being connected. The oil goes to the pump and causes an imbalance of pressure in the compensator which then increases its flow until the pressure returning from the function and the pressure that it is creating are back in balance. (for further information on how compensators work read piston pumps). With a load sensing system there is no problem with multiple functions, the pump will just keep increasing its volume up to its maximum as more and more oil is demanded.
Example 8 In the system shown below there are three independent functions shown from one pump. The valve drawn on the right hand side is a shuttle valve, this valve is basically two non return valves facing each other with a port in the middle, the supply with the higher pressure will make it through and the opposing port is blocked. In this application the function with the highest resistance sets the flow of the pump.
In a hydraulic system all the functions do not necessarily require the same flow nor are they necessarily capable of working at the same pressure. If we added extra pressure relief valves to protect the pieces of hardware that could not take the maximum system pressure, without adding any other valving, then should we try to operate two functions at once, the oil will take the course of least resistance and flow through the relief valve dropping the system pressure.
Example 9 If we take all of the control valves and modify them as the example below then we are able to control the maximum amount of oil that is allowed through to the consumer, now we can put a pressure relief valve in and it is only our consumer that is protected at this pressure. All of the examples so far have used control valves that are either open or closed, the diagram as it is drawn shows them as electrically operated (the box with the diagonal line) and the D on the end signifies that they can be opened by pushing the spool.
There are a multitude of control valves on the market all of which have different characteristics and there are thousands of ways of setting them up. We have more in depth information on a selection of hydraulic components and how they function. |
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