The Rheology of Grease

[RB Shannon]

Here's an excerpt from the book The Rheology of Grease from the ELGI.What I found interesting was this statment
" The most commonly presented idea of how lubrication occurs in a contact is that the soap structure acts like a sponge, keeping the oil loosely bonded by weak forces, and letting the oil bleed out when sheared, miraculously "sucking" back the oil again when coming out on the other side of the contact.

When using lubricating greases that are subjected to very high centrifugal acceleration forces, it becomes evident that the density of the thickener is important for the degree of separation. The smaller the difference between thickener and fluid, the less the separation will be, even at extreme conditions.

A more scientific and continuously more and more detailed investigation by several institutes, notably Lule† Technical University and Imperial College in London, have shown a much more plausible and logic process taking place. By the use of a ball and disc apparatus, where the disc can be of specially coated glass or sapphire, when investigating the lubrication contact area, the thickener is not only found to get into the contact area but also remarkably increase the film thickness. Several articles have been published on this matter"
On this note has anyone seen any papers or studies done partaining to the influence of the thickener on film thickness in rolling element or plain bearings.I know it does on reversability with aluminum complex at the top of the list,would it therefore also maintain a thicker film over lets say a polyurea which has a lower reversability which in my opinion think that is the main reason its used in electric motor bearings being a "quite"grease.

 

[PRRPILL]

RB Shannon.
Thanks, that is awesome.

Llew

 

[labman]

Just the sort of authoritative information I want to see.

I don't see centrifugal force being very important in the common steering pivot points, but a big factor in wheel bearings. More so in the sealed for a short life things on todays cars.

Can you imagine asking the salesman what type of grease the sealed wheel bearing have in them?

Electric motors are another product with their life limited by the life of their bearings. Of course, many bearings fail for lack of mantaince.

 

[satterfi]

True, and many fail because of maintenance, if you know what I mean.

quote:

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Originally posted by labman:
...
Electric motors are another product with their life limited by the life of their bearings. Of course, many bearings fail for lack of mantaince.

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[68redlines73]

quote:

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Originally posted by labman:
I don't see centrifugal force being very important in the common steering pivot points, but a big factor in wheel bearings.

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Actually, the centrifugal forces in an automotive wheel bearing are relatively insignificant. Where centrifugal forces really come into play are in industrial couplings connecting high speed electric motors with their respective applications. In that situation, you use a coupling grease in which the base oil and thickener densities have been balanced, typically with a polymer.

 

[Ken2]

I wonder what they mean by "high speed?" I've had no problem in 3600 rpm couplings with standard, good, lithium complex base EP2 grease, usually Chevron Dura-Lith, continuous running, and annual regreasing.


Ken

 

[Ken2]

quote:

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Originally posted by satterfi:
True, and many fail because of maintenance, if you know what I mean.

quote:

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Originally posted by labman:
...
Electric motors are another product with their life limited by the life of their bearings. Of course, many bearings fail for lack of mantaince.

---

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Sealed ball bearings give longer bearing life than open ball bearings unless you know the open bearings will be regreased properly. Almost any motor bigger than you can carry in one hand (and many smaller) can have the bearings renewed...why junk the motor unless the damaged bearing caused damage by the rotor contacting the stator?


Ken

 

[MolaKule]

I agree, centripital forces have little effect on automotive bears, since hub speeds and radii are small.


The article had this to say about thickeners and libricant films"


"Ideally, the thickener improves the lubricating properties, by building a
thicker film at the lubricated surfaces than can be achieved with the
included fluid alone. At the same time, it is necessary to have lubricating
grease that can replenish itself into the contact area through microscopic
flow behaviour.

The film thickness is directly dependent on the viscosity and the resistance
to flow at actual shear rates, dependant on pressure and temperature. The
film thickness in a lubricated contact is determined by the lubricants
viscosity in the inlet region of the contact at atmospheric pressure, as well
as the value of the pressure-viscosity coefficient to describe the viscosity
variation in the Hertzian pressure region.

To measure the rheological properties at very high pressures (up to 1 GPa)
and at very high shear rates (>109 s-1) are challenges that demand a lot
from the materials when constructing an appropriate rheometer. In recent
years, several attempts have been made to build rheometers for this
purpose. The technique is well under way and it is hoped to be available
soon to provide information to elucidate to the behaviour of lubricants in a
normal lubrication contact.

It is assumed that lubricants will be transformed to a "glassy" state in
highly loaded contacts, the easier they reach this glassy state the higher
the friction will be. The formation of pressure spikes, which will be
transduced into the lubricated metal surface, invoking possible minor cracks
which in turn will create wear, is dependant to a great deal on the
transformation progress of the lubricant. Simply explained, the
transformation to (and back from) the glassy state will effect the dampening
of the pressure spike, where, of course, a higher dampening effect is
favourable.

Recently, evidence has been found evidence of very peculiar behaviour for
fluids when present in layers of a few molecules in thickness. The thinner
the layer, the more "sluggish" the movement of the fluid molecules will be,
not at all represented by its bulk properties. The illustrative example with a
billiard ball separated by a drop of water from a surface shows a quick
transport of water splashing from the contact when letting the ball go. The
last two to four molecules of water layer in the contact would need
extremely high forces to be squeezed out of the contact. The same seems
to be true for all types of fluids. At the same time, the molecules of a
confined fluid can slide over each over and in-between the phase borders,
giving the fluid a lubricating effect, see for details the very recent article by
Granik [10]."

 

[RB Shannon]

"It is assumed that lubricants will be transformed to a "glassy" state in
highly loaded contacts, the easier they reach this glassy state the higher
the friction will be."

What would be the determining factor on how fast or easily the lubricant film reaches the glassy state other than speed or load.

"Simply explained, the
transformation to (and back from) the glassy state will effect the dampening
of the pressure spike, where, of course, a higher dampening effect is
favourable."

Could the thickeners reversability qualities have a direct influence on the cushioning effect.

 

[widman]

Another important factor is the Viscosity of the base oil in the grease. Whereas a "typical" grease has a base oil of 100 to 150 cSt, for low speed high pressure (rock crushers, etc.) we have greases with a base oil visc. of 320 cSt. The old addage: increased velocity requires reduced viscosity. As said, Duralith (base oil 220 cSt)works well with many electric motors with rpm around 3500 but if you want to to to 10,000 and over, Chevron's SRI grease has a base oil of 100 cSt.

 

[68redlines73]

Here a bunch of detail on SRI grease that might be of interest to our readers (you need the Adobe reader...it's a pdf file).

 

[RB Shannon]

quote:

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Duralith (base oil 220 cSt)works well with many electric motors with rpm around 3500

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That's interesting,do you have to reduce the amount used in regreasing or doesn't it seem to have any effect on excessive heat or reduced bearing life.I've thought about trying this at the plant,my theory is if you use less it would reduce drag to keep heat down but would provide a thicker film,especially in one of the hot rooms where corn syrup is stored at 120*F.