# Back to Basics III

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#### MolaKule

Staff member
What is the name of the Graphical Representation that describes the Four (4) lubrication conditions or "Regimes?" as discussed in Back to Basics II? This question is open to all, but let's please allow non-engineers, non-Tribologists and non-chemists time to research the question and to respond before answering.

#### Danno

That would be the "stribeck curve".

#### TMoto

The Stribeck curve.

#### MolaKule

Staff member
Thread starter
Both Danno and TMoto submitted the correct answer and since we are in the Christmas spirit, we'll award both the virtual Bitog Mug with the Emblem. The Stribeck Curve is a graphical representation of the Four areas or Regimes of Lubrication. The vertical axis represents the "Coefficient of Friction" whereas the horizontal axis represent the (VelocityxViscosity)/Load equation or ratio. In the general sense, the Hydrodynamic and Elastohydrodynamic regions represent the lowest Coefficient of Friction while the Boundary and Mixed regions represent the highest Coefficient of Friction.

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#### spasm3

Thank you for the class MolaKule!

#### Shannow

Just to add... The X Axis on the stribeck curve is ViscostyXSpeed/Load, it's related to the Sommerfeld number for journal bearings In bearing design, the Sommerfeld number is the tyrpical beraing design parameter, it takes into account the bearing dimensions also, and uses a unit applied pressure as the load (so it's load/(lengthxdiameter). So for Minimum Oil film thickness, this is the typical design curve. r is the shaft radius c is the radial clearance u is viscosity N is revolutions per second (3,600RPM is 60 rps) P is the applied pressure (the load divided by the bearing projected area) L/D is the length of the bearing divided by the diameter Using the curve you can see what things move oil film thickness around, all else being equal. More viscosity - more MOFT More speed - more MOFT More load - less MOFT.

#### TMoto

Thanks to all and cheers

#### CR94

Originally Posted by MolaKule
... In the general sense, the Hydrodynamic and Elastohydrodynamic regions represent the lowest Coefficient of Friction while the Boundary and Mixed regions represent the highest Coefficient of Friction.
This definition of "elastohydrodynamic," as illustrated in Shannow's plot above, appears inconsistent with ZraHamilton's definition in your previous thread, involving rolling-element bearings.

#### MolaKule

Staff member
Thread starter
Originally Posted by CR94
Originally Posted by MolaKule
... In the general sense, the Hydrodynamic and Elastohydrodynamic regions represent the lowest Coefficient of Friction while the Boundary and Mixed regions represent the highest Coefficient of Friction.
This definition of "elastohydrodynamic," as illustrated in Shannow's plot above, appears inconsistent with ZraHamilton's definition in your previous thread, involving rolling-element bearings.
The formal definition is usually stated as:
Quote
Elastohydrodynamic lubrication is where the load is carried by a lubricant film and where the interface region between the moving parts elastically deforms under the contact pressure creating larger oil film areas and greater load carrying ability.
So maybe you can elaborate, in your view, as to what the differences might potentially be?

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#### CR94

Originally Posted by MolaKule
Originally Posted by CR94
Originally Posted by MolaKule
... In the general sense, the Hydrodynamic and Elastohydrodynamic regions represent the lowest Coefficient of Friction while the Boundary and Mixed regions represent the highest Coefficient of Friction.
This definition of "elastohydrodynamic," as illustrated in Shannow's plot above, appears inconsistent with ZraHamilton's definition in your previous thread, involving rolling-element bearings.
The formal definition is usually stated as:
Quote
Elastohydrodynamic lubrication is where the load is carried by a lubricant film and where the interface region between the moving parts elastically deforms under the contact pressure creating larger oil film areas and greater load carrying ability.
So maybe you can elaborate, in your view, as to what the differences might potentially be?
I'll try. First, I admittedly erred in attributing your Stribeck curve with the regime labels to Shannow.
Originally Posted by ZreHamilton
Elastohydrodynamic-- occurs when a rolling motion exists between two surfaces. The surfaces have a small contact area (such as a roller bearing and a race) which drastically increases the oil pressure at the point of contact, which increases the oil's viscosity. An oil film ... is still maintained. This pressure will temporarily deform the metal at the point of contact until it elastically returns to its normal shape as it rotates. ...
Upon further study, I see that prize-winning description does have some features consistent with your formal definition, but seems a very different approach. Is the "rolling motion" aspect a critical detail, as I'd assume? On another hand ZreHamilton missed your point that the elastic deformation increases surface area of contact. Meanwhile, I still don't understand how that elastic distortion of metal under concentrated high contact pressure bears much resemblance with the transition zone from boundary to full-film lubrication in a journal bearing---which is how your Stribeck zone labels appear. Surely stresses on solid parts are greater in boundary, on both micro and macro scales? I looked in old my I.C. Engines and Machine Design textbooks for a plot like that. They show small Stribeck plots ('tho without using that term) with the other three regimes duly labelled and discussed in text, but no elastohydrodynamic---at least not in chapters on journal bearings. Thanks!

#### Shannow

Journal bearings you won't find much on EHD, nor on viscosity pressure co-efficient (although a particular BITOGer keeps trying to bring it in). The typical plain bearing simply is not strong enough to sustain very much pressure, and certainly not enough for the pressure viscosity co-efficient to be a factor (In some cased, the PVC makes the oil a near solid viscosity wise, which is only relevant in rolling elements, otherwise friction would be ridiculous) Rolling element bearings, are essentially point, or line contact depending on ball or roller/needle. The surface pressure is therefore the load, divided by the contact area, and a point or line has zero contact area, so the surface pressure is by definition infinite. That's clearly not the case, as no material has an infinite strength. So the surface (surfaces both of them) in rolling element bearings deform, such that there is now enough surface area to support the load. If the surface pressure is within the elastic range of the metals, the surfaces return to normal as the bearing rolls, and a "wave" of elastic deformation follows the element around the bearing. This reshaping of the surfaces also entrains oil, which helps distribute the pressures at the interface. (Note, this surface deformation, done cyclically and depending on loads leads to surface fatigue and spalling...load/life are negatively correlated...load it low enough, and life is long, but the components are bulky and absorb energy...load it more, more fatigue, less fatigue life) EHD in engines lives in places like cams and followers, piston rings, gears and chain/sprocket interfaces.

#### CR94

Originally Posted by Shannow
Journal bearings you won't find much on EHD,...
Shannow, thanks for verifying my objection to the strangely labelled Stribeck plot---and elaborating on the reasons! Apologies for my initial misattribution of it to you.

#### MolaKule

Staff member
Thread starter
Originally Posted by CR94
...Meanwhile, I still don't understand how that elastic distortion of metal under concentrated high contact pressure bears much resemblance with the transition zone from boundary to full-film lubrication in a journal bearing---which is how your Stribeck zone labels appear. Surely stresses on solid parts are greater in boundary, on both micro and macro scales?... Thanks!
The "Back-to-Basics" threads are just that, reviewing the basic's of Tribology, and the Stribeck Curve I posted is a very basic and "Generalized" curve. I am not sure to which Stribeck curve you are objecting. The "Generalized" curve I posted was not speaking to any specific set of circumstance(s), hence "Back-to-Basics." As far as the friction coefficient transitions on the curve you mentioned earlier for Elestohydrodynamic contact situations, a recent article in the STLE discussed this very thing and I'll attempt to summarize the main points: a. In such contacts known as Elestohydrodynamic contacts, the lubricant solidifies. b. its the high contact shear strength of the fluid as a glassy "pseudo-solid" that becomes important. c. the pressure within the oil initially captured in the contact causes the film to go from a liquid to a pseudo-solid. It then shears like a solid with relatively high traction. As the oil transits the contact under stress and shear, it heats up and goes back to a viscous low traction state. I.e., Once the film is captured the internal shear-induced thermals reduce the internal friction or traction for efficiency. So it is a very dynamic, rather than static situation in the Elestohydrodynamic region.

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#### CR94

Originally Posted by MolaKule
... I am not sure to which Stribeck curve you are objecting. The "Generalized" curve I posted was not speaking to any specific set of circumstance(s), hence "Back-to-Basics." As far as the friction coefficient transitions on the curve you mentioned earlier for Elestohydrodynamic contact situations, a recent article in the STLE discussed this very thing and I'll attempt to summarize the main points: ...
Not the curve itself, but the marking on the curve in your second post of this thread, which designates the lowest part of the curve (i.e., the transition from mixed film to full hydrodynamic) as elastohydrodynamic. I'm having difficulty understanding how contact pressures would be especially high in that circumstance, when the journal is mostly or entirely supported by fluid. Possibly related, your description of oil becoming "pseudo-solid" under high contact pressure seems a bit like descriptions of what the lubricant is supposed to do at the interface of belt on the pulleys within CVT transmissions. Is that an elastohydrodynamic situation, too? It seems a fluid that readily behaves that way would make a poor gear lubricant---in the same transmission! Thanks!

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