Dr Haas' Motor Oil University Article

[Shannow]

Originally Posted By: turtlevette
I'm a bit overwhelmed by all the data here and am trying to understand how many of these formulas and theorems apply to a combustion engine rather than a large turbine shaft sitting stationary.

A turbine bearing sees a steady torque load from the steam jets applying steady force on the blades. An engine sees impulse loading from the explosions applying a greatly varying force to the piston.

The rod bearings "see" centrifugal force that tries to force the oil around in all different directions.

There are probably other things going on in a highly revving engine that I'm forgetting.


As an accomplished engineer, you'll understand that the theory is all ideal, and if you add things like cyclic load (as described in one of my links), things change some...and having to adapt the projected loadings to account for the inertial factors that are present.

e.g. in Electrical Engineering, Ohm's Law works for a circuit , whether the local ground is a genuine zero, or at elevated potential.

 

[Shannow]

Originally Posted By: Blue_Angel
Is there a reason why every bearing wouldn't have its supply located in this vacuum zone? Regarding your comments about excessive oil supply potentially reducing bearing efficiency, do you think car engines generally locate the oil feed to maximize efficiency? Or do you think manufacturing process/cost is more of a concern? Many crank bearings seem to feed from the top, as does the example in the article you linked to.


Bearing oil feeds are where they are to allow the engine to be built, so they are in areas that can be drilled, and straight lines....big end feed isn't all that ideal, but is works.

Not all turbine manufacturers target the vacuum point either. Some go to pains to install oil feeds at 12:00...it's not a huge issue.

Originally Posted By: Blue_Angel
A question in the back of my mind regarding pre-lube setups: With the oil feed at the top of the bearing and the shaft RPM at zero, is it possible for Hydrostatic lift to take place or does the oil feed require careful placement for a pre-lube application?


Lift pressure has to be applied directly under the direction of the load, and there has to be enough area in all directions to allow the pressure to apply over an area (F=PxA)...not practical at all in an IC engine.

Originally Posted By: Blue_Angel
Crank bearings generally have a groove in the center (for supply to the rod bearings I assume), does this groove affect the way a pre-lube setup would function? Does this groove effectively act as a radial bearing supply, allowing oil wherever the bearing requires it most (vacuum)?


The groove distributes the oil as you've described, allowing it to feed either the bearing or the rods....back in the day cranks were cross drilled.

Where the groove is, for analysis, the bearing becomes two bearings next to each other, it's load capacity is greatly diminished...but being on the top side, it's not the direction of applied load, and not an issue.

Cross drilling robs the oil film every 90 degree of crank rotation and is a worse outcome.

Originally Posted By: Blue_Angel
WRT oil viscosity, a heavier oil will have more shear drag than a lighter oil and will heat up more. Can we assume that the work done shearing the oil is in direct proportion to the oil's viscosity (i.e. a 15cst oil generates 50% more heat than a 10cst oil for a given application)? Or, does the resulting thicker fluid wedge with the thicker oil reduce the shearing somewhat, resulting in a non-linear work/viscosity relationship? Or, is it a linear relationship that varies at a different rate than the viscosity change?

On that note and using engine oils as the example, do different oil weights/blends generally have interchangeable specific heat values, or do lighter oils heat more/less for a given amount of work than heavier oils? Is this even a concern?


If considering the oil in the sump, it's not really linear, as the geometries all change, and the recirculation changes...it is pretty linear with the viscosity in the bearing, working viscosity.

e.g. a thicker oil will recirculate more, have more time to absorb the heat, and become thinner, so you can't say that it was a 15 versus 10...the 10 might be 8 in the bearing, and the 15 might be 11...

jrustles in this thread
https://bobistheoilguy.com/forums/ubbthreads.php/topics/3307626/2

gives a good account.

Oil specific heat is reasonably similar for the purposes of analysis, but changes with temperature some..., and as it's KJ/Kg, obviously with density of the oil.

Found this that you can have a a play with...it's based on the working viscosity, and you can see how different things change.

http://www.tribology-abc.com/calculators/c9_3.htm

 

[Shannow]

more stuff here
http://www.bobistheoilguy.com/forums/ubb...eor#Post3279634

 

[MolaKule]

Quote:
Cross drilling robs the oil film every 90 degree of crank rotation and is a worse outcome.


I don't think that is the case in a pressurized oil system.

 

[Shannow]

When the drilling passes through the area of min film thickness/max pressure, the oil "wedge" is diminished as pressure has to escape into the hole...

No way 60psi of oil pressure is going to prevent the localised decay of 1,000+psi of oil film.

 

[turtlevette]

Originally Posted By: Shannow
more stuff here
http://www.bobistheoilguy.com/forums/ubb...eor#Post3279634


Have you written any papers or magazine articles? I'd like to see.

I'd show you mine but they're boring and not about oil.

 

[MolaKule]

Originally Posted By: Shannow
When the drilling passes through the area of min film thickness/max pressure, the oil "wedge" is diminished as pressure has to escape into the hole...

No way 60psi of oil pressure is going to prevent the localised decay of 1,000+psi of oil film.


What I am talking about is improved oil delivery via cross drilling:

Quote:
To have an improvement in oil delivery, a cross-drilling (Fig. 11.27B) runs straight through the big-end journal and a diagonal drilling from the main-bearing journal intersects the big-end cross-drilling. Another hole is also drilled diametrically opposite the diagonal-hole’s entry in the main journal, so that when the bearing is loaded at the top or the bottom of the stroke, the other side of the bearing permits oil to enter.


Crankshaft Lubrication

 

[Shannow]

All cool Molakule, that makes sense.

You are talking about getting better flow to the big end, I'm on stability/load capacity of the main.

As per your link
Quote:
The problem with this scheme is that the intersection of the angled hole with the rod journal produces a large elliptical interruption in the journal surface. Add the chamfering usually done around that hole, and what results is a significant interruption of the hydrodynamic surface area. Coupled with the reduced bearing widths, that divot creates a substantial leakage path for the oil to escape.


Traditional cross drilling starts with the existing oil feed holes...that described is a new adaption of the idea.

All part of the compromises that make a successful system.

 

[MolaKule]

I was not questioning your comments about pressure calculations but providing additional information visa vis your cross-drilling comment.

The oiling holes are drilled in the lower pressure areas wrt to crank angle.

The wedge is still the major lubricating process, and the relatively low oil pressure from the pressurized lubricating system is mainly there for cooling via side leakage.

Oil Wedge

As to the original post, I think BITOG needs a good Spring Cleaning to discard or move off-topic posts in various forums, especially the Science and Technology of Lubricants and Additives forum, and to delete home page topics that contain inaccurate or obsolete information.

 

[turtlevette]

This is interesting. I always thought that channel in the middle of the bearing was to distribute oil all around the bearing. So I am learning something here.

I've never seen one that was cut only partially around a bearing half. All I've seen have the channel on both halves.

Seems to me, a good compromise would be to have the top half with the channel and the bottom half without.

I wonder if you can mix and match bearing halves?

 

[MolaKule]

Originally Posted By: turtlevette
This is interesting. I always thought that channel in the middle of the bearing was to distribute oil all around the bearing. So I am learning something here.

I've never seen one that was cut only partially around a bearing half. All I've seen have the channel on both halves.

Seems to me, a good compromise would be to have the top half with the channel and the bottom half without.

I wonder if you can mix and match bearing halves?





In many engine designs that is the case.

Shannow posted an interesting article on bearings in the Interesting Articles forum but here is the link:

Bearing Technology

 

[Kiwi_ME]

Originally Posted By: Shannow
... I’ve seen a big bearing become unstable when too much flow meant too high an operational viscosity, and it fell outside stability regions (vibration could unexpectedly rise to 0.001” movement every rev on a 60 tonne load at 3,000RPM)…change oil supply by 5C, or reducing supply head made it behave.

That's interesting. It's certainly true that machines with simple bearing loading situations such as turbines are more subject to stability issues than the complex and variable loading in an IC engine. Many years ago I was asked to assess a similar instability event on a high-speed hydraulic dynamometer with 7" bearings but we got a half-speed whirl and could not make it stable over a range of oil temperatures or supply pressure, or even changing the angle of the oil entry port. We ended up substituting offset halves shells which helped a lot but some movement was still present.

 

[Shannow]

Too true Kiwi ME...the problem bearing had a 1x vibration of 6um. Could watch the spectrum, and the half speed would be 3, then 3,9,3,12,3,20...then 130um....bump up supply pressure 5C, and it would correct.

I wanted to unbalance it to about 30um, to "take it's mind" off it's half times...got over-ruled, and had to "fix it properly"...which we did by cutting supply volume.

That particular bearing on that design machine (OE and licencees) will get upset by vibration in the shaft earthing brushes exciting 60 tonnes of spinning rotor.

 

[riggaz]

In the IC engine there are a few different types of lubrication happening aren't there?

Hydrodynamic, Hydrostatic, Boundary and Extreme Pressure lubrication are all needed/happening at some point in the IC engine as far as I can work out.

Is that right?


Riggaz