So in the next few lectures, we'll be discussing hydraulic fluids. Now the most common hydraulic fluid is a petroleum-based mineral oil, and we're going to find that there's actually a lot of different additives that can be added here that will change the properties fairly significantly, so let's dive in and start talking about, first of all, what are the roles of hydraulic fluid. Well, the primary role simply of fluid power is to transmit power, so, pressure times flow rate, transmitting power so, that's the role of the hydraulic fluid. Now, that said, there's a, a large number of secondary roles., first, is that this fluid is acting as a lubricant, so we have hydraulic pumps, and cylinders, and other components where we have metal to metal coming very close to each other, and this is acting as a lubricant between those components. Next, is that it's also sealing so again, a piston to a cylinder or other, other locations where we have small asmall clearances. Well the fluid provides enough viscosity, that we are sealing those small gaps in our system, and then also provides heat transfer because as we know we have inefficiencies in hydraulic systems, and we need to transmit that heat out of our circuit some way, well, the fluid is what carries the heat out of our system. So those are our primary and secondary roles of the hydraulic fluid, but what about the properties? What should we pay attention to as we're picking a hydraulic fluid? Well, the first thing you look at is what is the Viscosity of the hydraulic fluid? So, normally this is rated at 40 degrees C, what's the Viscosity? So this one happens to be 60 centistokes at 40 degrees C, and we can pick a large different varieties of viscosities at that temperature. Now, now we have the viscosity at that temperature, but viscosity is a function of temperature, so, as the temperature increases, our viscosity drops as the pressure increases, the viscosity goes up. So, we have to pay attention to what range of viscosities are acceptable, for the components that are in our hydraulic circuit. Next, we're going to be looking at how compressible is the hydraulic fluid. So, any fluid, any, whether or not it's a gas or a liquid, is going to have some compressibility, and as we add air to our system, and that would be some entrained air. So I shake this up and you see small little air bubbles, this fluid becomes even more compressible. So we need to pay attention to how compressible is it and we'll talk more about that in a future lecture. And then also the density of the hydraulic fluid, because this is the mass or the inertia of the fluid moving through our lines. Some secondary fluid properties that we need to pay attention to, heat capacity, how much heat can we, can we carry with the fluid what's the vapor pressure of the fluid? The reason that this is important is because, normally, we have an atmospheric pressure in our reservoir and then, as we're intaking fluid into a pump, often that pressure drops. And if the pressure drops below the vapor pressure, then we're going to get the oil turning into a vapor, and then vapor cavitation, where we are compressing that, that vapor in our hydraulic pump, and then those vapor bubbles collapse and cause microjets, which cause major major component wear. So, we have to pay attention where the vapor pressure is, the lubricity, again, we have tight clearances where we often have components nearly touching and sometimes a boundary lubrication regime in the,and the fluid needs to provide lubricity there. We need to pay attention to fire potential, so flash point is one measure of that, and this is the temperature that the the fluid can combust with an ignition source. Things like oxidation and foaming, and again foaming is related to how much air does this hold and how easily does it release that air, for a large number of properties we need to pay attention to and it could be a bit mind boggling sometimes to, you know, say what oil should I pick with what additives and what not, so again, these properties can be modified in a fluid. So I can take a base stock mineral oil, and I can put additives in it that will change its properties. Sometimes either up to about 5% by mass additions to the fluid, and these are things that will change the viscosity index, meaning how much the, does the viscosity change with temperature change the, the foaming behavior of the fluid, might change the lubricity of the fluid. You know, so a lot of different things that we can do to a hydraulic fluid, just by putting additives in it, so a lot of chemistry involved here that we won't really get into in this class. Now if a mineral based oil is not the, the ideal choice, well other options are to go to some fire resistant fluids, where we start to add water to the, the fluids. And so we might have some emulsion of water and oil, or oil and water, and those mixtures reduce the, the flammability hazard. Another option for reducing the flammability hazard is looking at some synthetic fluids like, such as a phosphate ester. There's also a large push towards biodegradable fluids, and this becomes very important in applications where, you know a spiller or leak would cause damage to the environment. So, think about applications like agricultural systems or perhaps the the turf applications where you have a very precision lawn, at a golf course that you don't want to be spilling any oil on. Well, in those cases, a biodegradable fluid might be, a good choice and they're quite a few different, different options there. So, with that said, you might start thinking, why not use this? Why not use water as the hydraulic fluid? It has a lot of great properties, first of all, it has a very low viscosity, which means low pressure drops through our components. One problem with a low viscosity fluid is it's difficult to seal, and so we have to tighten up the clearances there that we're sealing on, It does have a higher bulk modulus, which means that it's less compressible. So that increases our resonance frequencies and increases the, the precision that we can control position. So it's, it's great from that perspective, It also has great thermal properties that are not touched by mineral oil. That said, there has to be a but, and the but is first, it has 100 times higher vapor pressure than a mineral based oil. So, this means that water based hydraulic systems are very prone to cavitation at the inlet to a pump or other locations where we're seeing a drop in pressure, and this is especially true as you start to raise the temperature of the water. Have poor lubricity, and again if we get into a boundary, lubrication type of regime, that will become a problem, and as I mentioned, we'd have to decrease the clearances in our seals, to seal this, this, low viscosity fluid, so we might get into a boundary lubrication regime, so we have to pay attention to that. And corrosion of our parts, so while it sounds like a great thing, it can be very challenging to use water hydraulics, and there's not a whole lot of components that are commercially available that do use water hydraulics today, but it would be be a great option. So, hopefully this quick scratching the surface introduction to hydraulic fluids, gives you an appreciation that there are drastically different properties that hydraulic fluids can have, and we can modify those properties by adding a variety of additives to the fluid, and not only can we have mineral based oils, we can have synthetic based oils, or oils that have water added to them for fire protection.
