After reading on this site about the effects of oil shear I am a little confused on something or maybe I am not reading it right? When motor oil shears down does it thicken and form sludge?
Oil shear ?
- Thread starter Camprunner
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No, not sludge/thickening. Thinning.
https://www.oronite.com/paratone/shearloss.aspx has a whole essay on this topic. Cool subject.
An excerpt: "During routine engine operation and through continued use, engine oils are exposed to shearing mechanisms that break down the polymer molecules, reducing the oils molecular weight. This can lead to viscosity loss and a subsequent decrease in oil film thickness. If too severe, this can cause undesired friction and engine wear with oils that are not formulated with the proper viscosity characteristics.
Molecular shearing of lubricating oils typically occurs in very tight spaces between moving parts that are in close proximity to one another. Bearings and ring faces are classic examples of where engine oil shearing can occur. Polymer coils also react to changes in shear or flow-rate in the oil."
https://www.oronite.com/paratone/shearloss.aspx has a whole essay on this topic. Cool subject.
An excerpt: "During routine engine operation and through continued use, engine oils are exposed to shearing mechanisms that break down the polymer molecules, reducing the oils molecular weight. This can lead to viscosity loss and a subsequent decrease in oil film thickness. If too severe, this can cause undesired friction and engine wear with oils that are not formulated with the proper viscosity characteristics.
Molecular shearing of lubricating oils typically occurs in very tight spaces between moving parts that are in close proximity to one another. Bearings and ring faces are classic examples of where engine oil shearing can occur. Polymer coils also react to changes in shear or flow-rate in the oil."
MolaKule
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The major component in engine oils that shear are the polymer molecules of Viscosity Index Modifiers, or as we generally call them, Viscosity Index Improvers (VII's).
The majority of the newer VII polymer molecules resist shear and a minority of newer sheared VII molecules reform after being sheared.
When the oxidation inhibitors within the oil-additive mix start to wane and the lighter fractions of base oils evaporate, the oil thickens.
The majority of the newer VII polymer molecules resist shear and a minority of newer sheared VII molecules reform after being sheared.
When the oxidation inhibitors within the oil-additive mix start to wane and the lighter fractions of base oils evaporate, the oil thickens.
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In a nutshell - oil will thin out after oil change for some time (and usually during OCI cycle) and then will start thickening (usually outside OCI - after say 6-10K miles).
Different oils will have different 'life' before they start thickening and sludge-ing up.
Different oils will have different 'life' before they start thickening and sludge-ing up.
First it thins as the viscosity index improvers are sheared, if the oil isn't changed for long enough the oil starts oxidating and "soot" from the combustion process builds up in the oil, then the soot starts building up and settles in the cooler parts of the crankcase as sludge.
Originally Posted By: oil_film_movies
No, not sludge/thickening. Thinning.
https://www.oronite.com/paratone/shearloss.aspx has a whole essay on this topic. Cool subject.
Thanks for posting, that was a very good read. I found this very interesting "Larger coils are therefore more easily pulled apart and broken than smaller coils, so therefore, high molecular weight molecules are more vulnerable to permanent viscosity loss"
So higher viscosity oils are more likely to experience shearing. Facinating...but make sense. Ironic that one would go with a higher viscosity oil to counteract shearing.
I'll ask a follow-on question. In theory, if shearing dropped the viscosity down and sludge started to bring it back up, you could run your used oil analysis when the sludge itself puts the oil back into the normal viscosity range. What component of a used oil analysis would you look at to see the sludge?
No, not sludge/thickening. Thinning.
https://www.oronite.com/paratone/shearloss.aspx has a whole essay on this topic. Cool subject.
Thanks for posting, that was a very good read. I found this very interesting "Larger coils are therefore more easily pulled apart and broken than smaller coils, so therefore, high molecular weight molecules are more vulnerable to permanent viscosity loss"
So higher viscosity oils are more likely to experience shearing. Facinating...but make sense. Ironic that one would go with a higher viscosity oil to counteract shearing.
I'll ask a follow-on question. In theory, if shearing dropped the viscosity down and sludge started to bring it back up, you could run your used oil analysis when the sludge itself puts the oil back into the normal viscosity range. What component of a used oil analysis would you look at to see the sludge?
Originally Posted By: Onug
I'll ask a follow-on question. In theory, if shearing dropped the viscosity down and sludge started to bring it back up, you could run your used oil analysis when the sludge itself puts the oil back into the normal viscosity range. What component of a used oil analysis would you look at to see the sludge?
Can't really see that in a used oil analysis since only certain elements are shown, and oxidized oil (thicker components) are just oxygen on the usual hydrogen and carbon molecules. Indirectly you can tell how much acids have been eating up the buffers by looking to see if the TBN has fallen to less than 2. If its that low, acids have been attacking, and the viscosity may have pushed upward from too much oxidation as additives get destroyed.
I'll ask a follow-on question. In theory, if shearing dropped the viscosity down and sludge started to bring it back up, you could run your used oil analysis when the sludge itself puts the oil back into the normal viscosity range. What component of a used oil analysis would you look at to see the sludge?
Can't really see that in a used oil analysis since only certain elements are shown, and oxidized oil (thicker components) are just oxygen on the usual hydrogen and carbon molecules. Indirectly you can tell how much acids have been eating up the buffers by looking to see if the TBN has fallen to less than 2. If its that low, acids have been attacking, and the viscosity may have pushed upward from too much oxidation as additives get destroyed.
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High viscosity can shear down if it depends on VII - Mercury likes the 25w40/25w50 semi synthetic without VII ...
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