I think this may be the time to come out and explain something here that is being over looked in this issue and GeorgeCLS or Mola can correct me if I miss the point. So here it is,
Newtonian vs Non newtonian fluids.
In the case of a mineral oil, you start out with a base oil of 15wt(15w40 for example). You then add some VII to enhance it to the 40wt as it heats up. This is a non newtonian fluid.
In the case of a real full synth, You take a 40wt base oil synth. In most cases it has the ability to flow in lower temps to what ever it is tested to.So for example, a company like M1 can make a 40wt synth, test it to the flow propertis of a 15wt and label it, take the same oil, retest it to a 10wt property and label it,again use the same oil and test it to a 5wt and 0wt and relabel those, all being the same oil. This is not a bad thing, but in fact is a win win for everyone as it lowers the cost of production to one oil for many viscosities and also, you could have a 15w oil that actually perform down to a 5wt oil in really sub cold temps if needed. But in all, it's still a straight 40wt with no VII added, therefore it has a natural ability to resist the cold flowing properties.
This is a netownian fluid or actually a straight wt oil with the flow properties of some non netonian fluids. Now there is some so called full synth's that need some assistance to get a further spread on the numbers and there is some help added there but in the case of full synths this is the basic premise. Mobil stands out as being one of those from my understanding. Castrol on the other hand would not be.
Now given that, where these turbine bearings come into play, first lets point out that in bearings, you have a wedge effect. How that works is where you have a bearing, inside is little rollers. Oil is sitting in front of the roller and as the roller starts to move around, it pushes the oil in front of it. The faster the bearing spins, the more hydrodynamic fluid is going to be present inbetween the roller and the race as it is forcing more oil under it. The slower it moves the less hydrodynamic film in going to be present.
So, If too thick of an oil is present, this wedge will not shear down as easily, thus failing to supply enough hydrodynamic film under the roller therefore causing less film, and more wear.
This is why too thin is not good, but too thick also not good. Another point is when you get too thick, you will create a drag effect on the bearings as it will create a bigger wedge in front of the roller the faster it spins.
This priciple is the same in both mineral oil and full synths. The main reason bearings in a turbo/turbine will fail sooner is not because the synth has a better film strength over a mineral oil but because the mineral oil will start to oxidize faster due to the high levels of heat it is exposed to therefore it cannot maintain it's viscosity like a full synth and ulitmatly coking will start to appear do to all the VII's used, excessive heat and the VI curve comes into play.
So, because the full synth has a better bonding effect on the molecular level, it takes more punishment as the oxygen molecules cannot break the bond as easily unlike a mineral oils molecular chain cannot withstand these forces therefore oxygen atoms will take over the hydrogen atoms and will break down the molecular chain sooner. So, synth's molecular film strength is much better than a mineral oils, but not hydrodynamic film strength when both are new.
So, to recap, Synths' advantage in use for turbines and such is not because if produces a better film strength but maintains a more consistant file strength due to the VI index. In cases of mineral oils it heats up, oxidizes, thickens up creating more of a wedge effect, putting more drag on the bearing, less hydrodynamic film and cause more bearing failure.
Sorry George, but the idea of synth's hydrodynamic film strength being stronger than a counter part mineral is not why turbines last longer but because full synth's can supply a consistant viscosity over a mineral when an extreme amount of heat is applied.
This is One Application(jet engine turbines) that is very important to use a full synth because of the high heats vers the viscosity index. There is no way a mineral will withstand those higher temps for extended periods as it will cause coking.
As for the viscosity helping maintian the tightness of the shaft wobble is not going to fly either as the hydrodyamic film will change when the speed changes. The shaft play is a mechanical function that cannot be reliant on the film of any oil to maintain wobble free operation.
The surge load reffered to is what causes momentary shearing of the oil's film, and this is why it is necessary to have a really good barrier additive in place as shock loads do happen in every application at one time or another. As for oil whip, this is where the oil falls below the lubrication zone of a bearing and this creates an agitation or air bubbles, and as many of you know, air trapped in oil is going to generate heat and in turn more heat more stain on the oil.
