2015 Paper on Big End Bearings of WWII Engines

Status
Not open for further replies.
Thought you'd like this part on page 29.

Crankpin Bearing Lubrication
All engines described in this paper had pressure lubricated
crankpin and main bearings. Oil was supplied either directly
to the crankshaft or through an oil galley to the main
bearings and from there to the crankpin. The lube oil
serves to establish a film that prevents metal to metal contact
between the journal and bearing and to provide cooling.
The splash system that persisted in automotive practice
for a number of years could not provide an oil flow rate that
was adequate to keep the bearing temperature at an acceptable
level in the higher specific power ratings of aircraft engines.

The variables that affect the flow rate of oil are: supply
pressure, bearing clearance, and the length to diameter ratio
of the bearing. The clearance and the length to diameter
ratio are under the control of the designer and require a
very tricky balance between oil flow rate and minimum oil
film thickness.
 
But wait, you stopped half way through a paragraph....

...The flow rate increases as the cube of the clearance but the minimum film thickness decreases approximately linearly with clearance. If a bearing is limited by temperature an increase in clearance can help, but if it is limited by minimum film thickness a decrease in clearance can help.
 
edwardh1, a crankpin is a rod journal.

Originally Posted By: ZeeOSix
Thought you'd like this part on page 29.

Crankpin Bearing Lubrication
All engines described in this paper had pressure lubricated
crankpin and main bearings. Oil was supplied either directly
to the crankshaft or through an oil galley to the main
bearings and from there to the crankpin. The lube oil
serves to establish a film that prevents metal to metal contact
between the journal and bearing and to provide cooling.
The splash system that persisted in automotive practice
for a number of years could not provide an oil flow rate that
was adequate to keep the bearing temperature at an acceptable
level in the higher specific power ratings of aircraft engines.

The variables that affect the flow rate of oil are: supply
pressure, bearing clearance, and the length to diameter ratio
of the bearing. The clearance and the length to diameter
ratio are under the control of the designer and require a
very tricky balance between oil flow rate and minimum oil
film thickness.


Because you know how fervently Shannow's been arguing the case for splash lubrication
wink.gif
But seriously, the variables mentioned in the quoted text all affect side leakage. Yes pressure is mentioned. Nevertheless it remains true that only adequate make-up supply volume has 'bearing' on the lubrication integrity in the journal, and that excess flow via excess pressure serves no constructive purpose. Can you agree on that before this turns into a massive circular debate?
 
Originally Posted By: PeterPolyol
Because you know how fervently Shannow's been arguing the case for splash lubrication
wink.gif
But seriously, the variables mentioned in the quoted text all affect side leakage. Yes pressure is mentioned. Nevertheless it remains true that only adequate make-up supply volume has 'bearing' on the lubrication integrity in the journal, and that excess flow via excess pressure serves no constructive purpose. Can you agree on that before this turns into a massive circular debate?


Here's what the paper refers to on the design changes that took place...clearly to affect (increase) the side leakage...

Quote:
The trend to shorter bearings in both in-line and radial engines was due to the following:
• A desire to minimize the effects of crankshaft and crankcase deflections on bearing misalignment and conse-quent bearing wear.
• Improved oil flow through the bearing without resorting to excessive oil pressure.
 
Indeed. Therefore this passage would suggest that 'oil pressure modification' is a last resort (or non-consideration) in proper bearing design, where instead factors like clearances, viscosity, temperature rise and bearing dimensions/speed take precedence, yes?
 
A little bit unsure of the what the following implies:
Quote:
In general, any interruptions in the bearing surface
where there are high oil film pressures will cause the transition
point to move to the right in Figure 6. (higher Sommerfeld value)


In this context what does "Oil film pressure" refer to? The load applied?
 
Originally Posted By: PeterPolyol
Because you know how fervently Shannow's been arguing the case for splash lubrication
wink.gif
But seriously, the variables mentioned in the quoted text all affect side leakage. Yes pressure is mentioned. Nevertheless it remains true that only adequate make-up supply volume has 'bearing' on the lubrication integrity in the journal, and that excess flow via excess pressure serves no constructive purpose. Can you agree on that before this turns into a massive circular debate?


In this case, the oil pressure was used to increase oil flow through the bearing certainly does server a purpose. It's clearly stated in the paper that if the bearing was adequately splash lubricated like in low power autos of the time, that these bearings would not have survive.

Did you guys miss this part?

"The splash system that persisted in automotive practice
for a number of years could not provide an oil flow rate that
was adequate to keep the bearing temperature at an acceptable
level
in the higher specific power ratings of aircraft engines."


In other words, if they didn't pressure feed these bearings they would not survive.

So the point here is that indeed oil pressure can be (and is) used to INCREASE the bearing oil flow to help keep them cooler so the MOFT remains satisfactory and the bearing doesn't fail. There are reasons why high oil pressure is used on some engines - that's the point. Not that every bearing in the world would survive without being pressure fed, as I've said many times in these discussions.
 
Originally Posted By: PeterPolyol
A little bit unsure of the what the following implies:
Quote:
In general, any interruptions in the bearing surface
where there are high oil film pressures will cause the transition
point to move to the right in Figure 6. (higher Sommerfeld value)


In this context what does "Oil film pressure" refer to? The load applied?


Yes, they are referring to the actual pressure level in the very thin oil film inside the bearing due to the load in the bearing.
 
Originally Posted By: Shannow
But wait, you stopped half way through a paragraph....

...The flow rate increases as the cube of the clearance but the minimum film thickness decreases approximately linearly with clearance. If a bearing is limited by temperature an increase in clearance can help, but if it is limited by minimum film thickness a decrease in clearance can help.


Yeah, other changes in the bearing design variables can also help ... but so can increased oil pressure to increase flow and cooling, which might be a better option then changing the physical characteristics of the bearing depending on the exact situation. This is a good example of that. Are the blinders opening up any now?
 
Originally Posted By: PeterPolyol
A little bit unsure of the what the following implies:
Quote:
In general, any interruptions in the bearing surface
where there are high oil film pressures will cause the transition
point to move to the right in Figure 6. (higher Sommerfeld value)


In this context what does "Oil film pressure" refer to? The load applied?


The oil film pressure is different to the applied pressure (Load).

Applied pressure is force over projected area.

Oil film pressure has a max pressure at (around) the centre of the oil film, that drives oil flow out the sides (and circumferentially to), the bearing sides obviously being "0psi" as that is the pressure of the ambient around them.

A geometry, speed and viscosity will have a pressure distribution across and along the bearing, thicker oils flatter and less peaky, thinner oils peakier and lower at the edges...

The "Load" has to be met by the integral sum of all the pressure/area elements that make up the film.

The "PMax/PAve" is an indicator of the above, shown here
http://www.substech.com/dokuwiki/doku.ph...engine_bearings
 
Should also add that the reference to skewing the transition point to the right is that discontinuities (like bearing part lines, circumferential grooves etc. allow an easier leakage path from the high pressure to the lower pressure, and change the shape of the pressure distribution (circ groove for example makes it "two" bearings with "four ends", and therefore two, bigger PMaxes), and so in spite of the major fundamentals being the "same", drops MOFT and pushes the transition point to the right.

Found a fitter one day "scoring" the loaded face of a 14x8" 3,000 bearing to "provide oil wells" in the surface, and hit the roof (it CAN be used to provide a stiffer oil film, but due to the negative consequences mentioned above.
 
Interestingly eccentric bit on Bristol engines, around page 28. In that special case, at least, they allegedly needed to limit oil flow through the bearing.

"Ricardo states there was “a maximum oil flow that could be tolerated” because their oil control piston rings were not able to handle the oil thrown off the master rod when that “maximum flow” was exceeded."
 
Not from this source, but related to how oil supply pressure can reduce the oil film temperature and therefore help maintain an adequate MOFT in the bearing. This is why guys who built high HP engines typically would install a higher volume oil pump with a higher relief valve setting to help save bearings. Looking at the 100 C point on the x-axis (bearing inlet oil temp), there's a delta of ~25 C between 2 bars and 9 bars of supply pressure. However, the delta of reduction in bearing exit temperature looses ground as the supply pressure goes up and the oil inlet (sump) temperature goes up.

 
What's the relevence to the article I shared, other than you chasing me from thread to thread trying to point score ?

Not much of a "Map" if it doesn't include the other operating parameters, like RPM, bearing size, eccentricity...the SBC "rule of thumb" that you like quoting as gospel would have that thing turning at 13,000RPM for 9 bar (130psi)...for reference for the casual observer, 4 bar is about 60psi.
 
You pull stuff out of your behind all day long in these discussions, but nobody else can? The relevance is to show that there can be a significant cooling effect on the bearing oil film due to the oil supply pressure, and a point that was made in the paper you linked to start with - so pretty obvious there's a relevant connection.

Obviously the temperature rise map is based on only the supply pressure and inlet oil temperature changing, and all other variables holding constant, so it doesn't really matter what the other bearing variables are in this case.

Never said the SBC rule of thumb was "gospel" ... said it was a general rule of thumb. Just so happens to be the way GM has designed the oiling systems in those engines for many decades, and still do.

You do realize right that in order to get the higher supply pressure data points in the graph they could change the pump pressure relief point and/or change the oil pump to get more volume through the bearings - just like the hot rodders would do.

Got anything more to add that's constructive before getting back on the super spinning merry-go-round?
grin.gif
10.gif

https://www.youtube.com/watch?v=Lwo9AvTA5xY
 
Given that the paper is on "BIG END" beariings, and as we've already been over, BIG END bearings don't recieve "Pressure Lubrication" for a significant part of their rotation, maybe something "BIG END" related would have been pertinent to the topic, rather than wheeling out your constant stuff about pressurised bearing and oil flow.

In regard to "BIG END", the statement in the paper that pressurised oil systems being far superior to splash, in regard to supplying adequate flow to make up for increased speeds/power density is clearly self evident.

As I've said, the role of the lubrication system is to supply adequate volumes of oil at a pressure that gets it to the remotest part of the engine.

Originally Posted By: ZeeOSix
Yeah, other changes in the bearing design variables can also help ... but so can increased oil pressure to increase flow and cooling, which might be a better option then changing the physical characteristics of the bearing depending on the exact situation. This is a good example of that. Are the blinders opening up any now?


"might" ???

Well clearly, with the war effort at their disposal, "might" didn't become "was", so they clearly found the alternative means described "to avoid the issues of excessive pressure" adequate (or even superior) to the simplistic logic of increasing pressure like hotrodders, (who aren't engine designers) do.

edit...as an example, here's a paper with calculated and observed flows to a big end bearing with a couple of different crank drillings, demonstrating that sufficient pressure is required to overcome crankshaft dynamic forces and prevent cavitation in the part of the cycle where bearing supply ISN'T pressurised...
http://www.tytlabs.com/english/review/rev383epdf/e383_044suzuki.pdf

And yes, more pressure is clearly more flow and cooling...in this paper, pertinent to what happens in BIG END bearings, the topic of the thread.
 
Last edited:
Originally Posted By: Shannow
Originally Posted By: PeterPolyol
A little bit unsure of the what the following implies:
Quote:
In general, any interruptions in the bearing surface
where there are high oil film pressures will cause the transition
point to move to the right in Figure 6. (higher Sommerfeld value)


In this context what does "Oil film pressure" refer to? The load applied?


The oil film pressure is different to the applied pressure (Load).

Applied pressure is force over projected area.

Oil film pressure has a max pressure at (around) the centre of the oil film, that drives oil flow out the sides (and circumferentially to), the bearing sides obviously being "0psi" as that is the pressure of the ambient around them.

A geometry, speed and viscosity will have a pressure distribution across and along the bearing, thicker oils flatter and less peaky, thinner oils peakier and lower at the edges...

The "Load" has to be met by the integral sum of all the pressure/area elements that make up the film.

The "PMax/PAve" is an indicator of the above, shown here
http://www.substech.com/dokuwiki/doku.ph...engine_bearings


Well-explained and makes perfect sense, thank you Shannow
 
Status
Not open for further replies.
Back
Top