Can anyone talk something about the relationship between oil film strength and oil film thickness, or are they just the same in most case?
I am really confused with these 2 words now.
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I understand that both terms might be considered same in perfect hydrodynamic conditions but have little to do with each other in cases of boundary lubrication. In real scenarios, perfect hydrodynamic lubrication is difficult to hold. Loads, contaminants, and other work conditions make HDL almost nonexistent.
Oil Film Thickness refers to the theoretical thickness of oil film separating the surfaces
Oil Film Strength refers more to the lubricating characteristics of lubricant when the oil film breaks. It includes the ability of additives and other components, to hold lubricity even in cases of shock loads and vibration, and boundary lubrication.
Boundary Lubrication is when the oil film thickness is smaller than the average roughness of both surfaces under friction. In this case the film strength based on additives will support the lubricity and can be considered much larger than the oil film.
Oil Film Thickness refers to the theoretical thickness of oil film separating the surfaces
Oil Film Strength refers more to the lubricating characteristics of lubricant when the oil film breaks. It includes the ability of additives and other components, to hold lubricity even in cases of shock loads and vibration, and boundary lubrication.
Boundary Lubrication is when the oil film thickness is smaller than the average roughness of both surfaces under friction. In this case the film strength based on additives will support the lubricity and can be considered much larger than the oil film.
Tks for the reply. Because I am confused, so my question is also confusing. What i meant is pure oil situation, no additive concerned.
To me, film strenght should always comes from film thickness. Talking about film thickness, there should be static and dynamic situations.
Static thickness is the balanced oil film formed between oil to metal tension and oil to earth gravity.
Dynamic thickness is formed by the relative movement between solid part and oil. We may try to define a effective thickness accordingly.
While film toughness is the ability for film to resist vertical load without collapes.
Typiclly, we analyse lubrication of a shaft in a hole. In this case, it is pure hydrodynamic conditions, not much to say about the viscosity to thickness to toughness. Easy to calculate everything.
But for gear application (no worm here), if we just think about the meshing zone, typically there should be no parallel movement between gear surface. Film strength comes only from the static thickness.
My question is, for static situation (like gear, no parallel movement), how do we calculate the film strength then? It must have something to do with oil thickness.
To me, film strenght should always comes from film thickness. Talking about film thickness, there should be static and dynamic situations.
Static thickness is the balanced oil film formed between oil to metal tension and oil to earth gravity.
Dynamic thickness is formed by the relative movement between solid part and oil. We may try to define a effective thickness accordingly.
While film toughness is the ability for film to resist vertical load without collapes.
Typiclly, we analyse lubrication of a shaft in a hole. In this case, it is pure hydrodynamic conditions, not much to say about the viscosity to thickness to toughness. Easy to calculate everything.
But for gear application (no worm here), if we just think about the meshing zone, typically there should be no parallel movement between gear surface. Film strength comes only from the static thickness.
My question is, for static situation (like gear, no parallel movement), how do we calculate the film strength then? It must have something to do with oil thickness.
I understand that gears have a big component of parallel movement. The head of one tooth touches the primitive diameter of the other and slides down to the root while it starts now rolling, same movement for disengaging. A better example in trying to understand your question would be perhaps roller bearings where there should be no sliding movement, just rolling. In this case, in theory, you might be right that we speak about the same. It depends just on the viscosity.
However, in the practice, it does not exist.
Perhaps I have to get back to my physic books to review the case you question...the static thickness might be given by the oil viscosity and the thixotropy of the fluid, and the van der vaals forces in the acting materials and exoelectron effects caused by the static loads, and so many other phenomena...leave that to the PhDoctors to figure out. You just search for an oil with better additives
However, in the practice, it does not exist.
Perhaps I have to get back to my physic books to review the case you question...the static thickness might be given by the oil viscosity and the thixotropy of the fluid, and the van der vaals forces in the acting materials and exoelectron effects caused by the static loads, and so many other phenomena...leave that to the PhDoctors to figure out. You just search for an oil with better additives
Sorry for reply late, i have been leaving for a while.
Nowadays, we have saturated oil molecules and additives; besides, we also have polarized oil molecules. They are something act in-between van der vaals forces and chemical bonds to metals.
Technically, if we can build or keep several lays of polarized oil molecules on metal surface, like through electricity field, we may greatly enhance the oil film strength even. After all, 90% wear happens during startups, where no oil film exists. How do you think of this?
You mentioned "leave that to the PhDoctors to figure out". After around 10 years business life, i am little bit tired to act as "a lubricant" more. If my assumption is rational, I plan to pursue a PhD degree somewhere to see if it is practical.
Nowadays, we have saturated oil molecules and additives; besides, we also have polarized oil molecules. They are something act in-between van der vaals forces and chemical bonds to metals.
Technically, if we can build or keep several lays of polarized oil molecules on metal surface, like through electricity field, we may greatly enhance the oil film strength even. After all, 90% wear happens during startups, where no oil film exists. How do you think of this?
You mentioned "leave that to the PhDoctors to figure out". After around 10 years business life, i am little bit tired to act as "a lubricant" more. If my assumption is rational, I plan to pursue a PhD degree somewhere to see if it is practical.
quote:Originally posted by Ricky:
Loads, contaminants, and other work conditions make HDL almost nonexistent.
Where that statement comes from, and what is the basis for it? For your information, the whole hydropower industry and technology is strongly based on good old HDL. For example, typical load on thrust bearings ranges from 2-2.5 million pounds (combined load including generator, shaft, runner, and a hydrodynamic thrust). At the same time there is a load of contaminants present in oil, as the tolerable cleanliness level for lubricating oil is around ISO 17/13/10. If the HDL is non existent, as you state, how else would those thrust bearings last for 40+ years constantly supporting this huge load at speeds as low as 75-120 rpm? And by the way, this HDL is formed and sustained by every R&O oil of certain viscosity, without any AW or similar type of additives added to it. No worries folks. HDL is alive and well, and was, and will be the cornerstone of future technologies and designs.
