Number of RPMs for Oil shearing

[Al]

In other words does the speed really matter? In other words my theory is that an oil will have sheared based on the total number of revolutions regardless of the speed within limits.

In other words if an engine puts out a total number of a billion revolutions oil shearing will be about the same if it is at 3000 rpm or 6000 rpm.

 

[Propflux01]

I think load bearing will have an effect in there somewhere.

 

[TurboZedd-R]

Someone with a background will have an extremely well formulated answer, but I remember seeing something on the speed of the engine creates more sheer due oil being forced through or around areas at a much higher rate. Heat would play a part too, with racing engine having a higher shear compared to something living at 2200 rpm for the same mileage.

If you race a car for 200 miles and sample the oil and then drive the same car around town for 200 miles does you think the oil will have the same viscosity?

 

[SubieRubyRoo]

Since HT/HS is supplied as a rate (centipoise per second), it certainly appears that RPM plays a factor. Also remember that as RPM increases linearly, friction and forces increase exponentially; this means at 6k RPM the shearing forces aren’t just doubled, they’re squared. At least that’s the way it appears to me?

https://360.lubrizol.com/2019/Impact-of-High-Temperature-High-Shear-HTHS-Viscosity

 

[danez_yoda]

it takes energy to shear anything. The higher the RPM the higher the energy imparted into the oil on each stroke. This will create more heat and make it break down faster.

 

[StarCaller]

In other words does the speed really matter? In other words my theory is that an oil will have sheared based on the total number of revolutions regardless of the speed within limits.

In other words if an engine puts out a total number of a billion revolutions oil shearing will be about the same if it is at 3000 rpm or 6000 rpm.

"Your theory is mostly correct within reasonable limits. The key factor in oil shearing is the total mechanical work done on the oil, which correlates with the number of revolutions and the forces applied to the oil. Here's how it breaks down:

What Causes Oil Shearing?​

• Shearing happens when oil molecules are exposed to high-pressure and high-stress conditions, such as in the engine's bearings, valve lifters, and gears.
• The shearing is cumulative and depends on the total exposure to these stresses.

Total Revolutions vs. RPM​

• At a given load and pressure, the total number of revolutions does play a major role in how much the oil shears.
• Whether the engine operates at 3000 RPM or 6000 RPM, the same number of revolutions would expose the oil to the same amount of stress over time.

Speed Matters in Specific Cases​

• Higher RPMs increase temperature due to friction and fluid dynamics, which can amplify thermal breakdown or oxidation. This could indirectly affect the oil’s viscosity, making it more susceptible to shearing.
• Oil flow dynamics at high RPMs may cause aeration, cavitation, or turbulence, which might increase shearing stress in extreme cases.

Practical Takeaway​

For normal operating conditions:

• Total revolutions matter more than instantaneous RPM within typical ranges (like 3000-6000 RPM).
• High RPMs may accelerate oil breakdown through heat or other secondary effects but don’t directly change the shearing mechanism.

Your theory is solid for engines operating within their designed range. However, extremes (like sustained operation at redline RPM) could push the oil beyond its designed limits, making RPM more of a factor."

 

[dnewton3]

In other words ....
What has the greater effect on viscosity index improvers? I say that because "shearing" really is a reference to the degradation of the lube viscosity, which is mainly controlled by the VIIs.

With sustained higher RPM, it would likely induce a greater stress on the VIs via thermal loads (higher heat from high rpm, especially if the high rpm were due to heavy loading such as WOT runs and heavy loads). With longer total run times (extended OCIs), the amount of oxidation will increase because exposure duration is increased. Obviously, the combination of the two (time and heat loading) will have some manner of increased multiplier effect not seen by only one factor alone.

As with most things in life, the quality of the VIIs plays into this. The chemicals selected (class, type, purity) for the VIs also would be a significant factor. Lower end products will not hold vis nearly as well as higher-tier products, relative to the heat and time inputs.

 

[Al]

Since HT/HS is supplied as a rate (centipoise per second), it certainly appears that RPM plays a factor. Also remember that as RPM increases linearly, friction and forces increase exponentially; this means at 6k RPM the shearing forces aren’t just doubled, they’re squared. At least that’s the way it appears to me?

https://360.lubrizol.com/2019/Impact-of-High-Temperature-High-Shear-HTHS-Viscosity

Actually High speed, low load and high viscosity lead to longer life of a bearing. So speed does not by itself contribute to bearing wear. But some good responses here. I truly do not know the answer.