The importance of oil Temperature in Determining what Viscosity of oil to run.

[SubieRubyRoo]

A follow up with the info on journal bearing cavitation (for @SubieRubyRoo), and to add to my post 57. Google search for "The effect of viscosity on the cavitation characteristics of high speed sleeve bearing" to find the bearing cavitation study PDF download.

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From another source:
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Fig 1.2

"When the bearing operates in the hydrodynamic regime, a fluid pressure builds up in the bearing by a converging geometry effect in the lower portion of the bearing, shown in Fig. 1.2. Following the minimum clearance portion of the bearing, the clearance increases (the surfaces diverge). This diversion results in a pressure drop in the lubricant that leads to cavitation (this portion of the bearing is often referred to as the ’cavitation zone’)."

From King Bearing - Info on journal bearing failure modes; cavitation erosion.
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Thanks Zee! Not fully understanding how this happens in the journal area itself; it seems to imply that most of this is incoming bubbles from the pump, and then depending on the rotating speed and viscosity, this can create enough surface interaction to induce a localized pressure low enough to help the entrained bubbles pop?

If so, this sounds like it may have been part of the failure mode for the Brotella crowd in those racing engines @RDY4WAR has mentioned!

 

[ZeeOSix]

Thanks Zee! Not fully understanding how this happens in the journal area itself; it seems to imply that most of this is incoming bubbles from the pump, and then depending on the rotating speed and viscosity, this can create enough surface interaction to induce a localized pressure low enough to help the entrained bubbles pop?

If so, this sounds like it may have been part of the failure mode for the Brotella crowd in those racing engines @RDY4WAR has mentioned!

The one source description repeated below, along with a sloppy freehand annotation (lol) of the bearing figure by me shows that the red zone is where the pressure drops inside the journal bearing occurs and can become the "cavitation zone". There doesn't have to be incoming bubbles from the pump, it just takes a low enough pressure in the "cavitation zone" to make vapor bubbles form because the pressure gets below the vapor pressure of the oil. Having air bubbles in the incoming oil to the bearing certainly wouldn't help reduce cavitation and could certainly aggrivate any possibility of cavitatioin after the oil with air bubbles aleready in it gets squeezed down in the high pressure load supporting MOFT wedge. The green arrows show the oil supply feed and flow, including the side leakage. There is a pressure distribution around the whole bearing circumference.

Item 2) in the conclusions of the one study says that a higher input oil supply pressure to the bearing will help reduce cavitation. So yes, the oil feed pressure is a factor too. But if the load (meaning more pressure in the MOFT wedge), speed and viscosity are all high enough, then cavitation could occur in the red "cavitation zone" where the pressure drops after the oil is compressed into the high pressure supporting MOFT wedge.

"When the bearing operates in the hydrodynamic regime, a fluid pressure builds up in the bearing by a converging geometry effect in the lower portion of the bearing, shown in Fig. 1.2. Following the minimum clearance portion of the bearing, the clearance increases (the surfaces diverge). This diversion results in a pressure drop in the lubricant that leads to cavitation (this portion of the bearing is often referred to as the ’cavitation zone’)."

 

[Hohn]

A follow up with the info on journal bearing cavitation (for @SubieRubyRoo), and to add to my post 57. Google search for "The effect of viscosity on the cavitation characteristics of high speed sleeve bearing" to find the bearing cavitation study PDF download.

So there's only one study? "*the* bearing cavitation study? Is there a reason a link cannot be provided?

How high is "high" viscosity and how low is "low"?

 

[Hohn]

The one source description repeated below, along with a sloppy freehand annotation (lol) of the bearing figure by me shows that the red zone is where the pressure drops inside the journal bearing occurs and can become the "cavitation zone". There doesn't have to be incoming bubbles from the pump, it just takes a low enough pressure in the "cavitation zone" to make vapor bubbles form because the pressure gets below the vapor pressure of the oil. Having air bubbles in the incoming oil to the bearing certainly wouldn't help reduce cavitation and could certainly aggrivate any possibility of cavitatioin after the oil with air bubbles aleready in it gets squeezed down in the high pressure load supporting MOFT wedge. The green arrows show the oil supply feed and flow, including the side leakage. There is a pressure distribution around the whole bearing circumference.

Item 2) in the conclusions of the one study says that a higher input oil supply pressure to the bearing will help reduce cavitation. So yes, the oil feed pressure is a factor too. But if the load (meaning more pressure in the MOFT wedge), speed and viscosity are all high enough, then cavitation could occur in the red "cavitation zone" where the pressure drops after the oil is compressed into the high pressure supporting MOFT wedge.

View attachment 276008

"When the bearing operates in the hydrodynamic regime, a fluid pressure builds up in the bearing by a converging geometry effect in the lower portion of the bearing, shown in Fig. 1.2. Following the minimum clearance portion of the bearing, the clearance increases (the surfaces diverge). This diversion results in a pressure drop in the lubricant that leads to cavitation (this portion of the bearing is often referred to as the ’cavitation zone’)."

View attachment 276007

Air technically isn't cavitation-- cavitation is vapor of the liquid itself, not the air dissolved within it. This is because the cavitation phenomenon is fundamentally a phase change-- from liquid to vapor and back.

That said, you don't want emulsified oil, obviously.

Your drawing of the pressure profile seems 100% correct to me. The reason that higher pressure helps with cavitation is because the pressure gradient on the backside (your red area) is softened. You don't have nearly as rapid a pressure drop in that red area, which buys you cavitation margin.

 

[ZeeOSix]

So there's only one study? "*the* bearing cavitation study? Is there a reason a link cannot be provided?

How high is "high" viscosity and how low is "low"?

There and many journal bearing cavitation studies. Just search for the title I gave earlier, and you can do some of your own searching and research on the subject matter, and find other studies. I downloaded it a long time ago and don't have the link, only the PDF. I've found study links get broken or the PDF isn't downloadable later for some reason (access restrictions), so I just download the PDF when I find them. People can search and find stuff themselves since everyone has access to Google and the internet.

Edit: There's another study I downloaded long time ago entitled: "Fundamentals of Fluid Film Journal Bearing Operation and Modelling". Here's a snip-it below from that paper. Also this reference was shown in another study.

 

[ZeeOSix]

Air technically isn't cavitation-- cavitation is vapor of the liquid itself, not the air dissolved within it. This is because the cavitation phenomenon is fundamentally a phase change-- from liquid to vapor and back.

That said, you don't want emulsified oil, obviously.

Never said air already in the oil (aerated oil) is cavitation. Said it doesn't help, and to elaborate if there are air bubbles already in the incoming oil and it gets squeezed into the very high pressure MOFT in the journal bearing those bubbles could also collapse from the high pressure, just like cavitation oil bubbles would collapse/implode when they form and then go back though a high enough pressure region to make them implode.

From the Machinery Lubrication article that @SubieRubyRoo linked in post 31:

"Hydraulic oil contains approximately 9 percent dissolved air. When a pump does not get enough
oil, air is pulled out of the oil. These air bubbles travel into the pump and eventually collapse and
implode when they reach an area of relatively high pressure. The ensuing shockwaves produce a
steady, high-pitched whining sound and damage to the inside of the pump."

Engine oil will also contain some amount of trapped air, and it being flowed and splashed around in a running engine will aerate it to some degree. Reason that anti-foaming additives are put into motor oil.

Your drawing of the pressure profile seems 100% correct to me. The reason that higher pressure helps with cavitation is because the pressure gradient on the backside (your red area) is softened. You don't have nearly as rapid a pressure drop in that red area, which buys you cavitation margin.

Yes, as shown in post 62, the oil supply pressure to the bearing has a large effect on the cavitation factor - the higher the supply pressure, the less likely chance of cavitation inside the bearing. And in post 65 that study talks about how low oil supply (which would mean low supply pressure) can even cause cavitation before the high pressure MOFT wedge. That would be similar to the inlet of a PD pump being at low enough pressure to cause cavitation on the pump inlet. Journal bearings are like mini oil pumps with a pressure gradient across the MOFT wedge. The volume they pump is the natural side leakage flow from their rotation.

 

[ZeeOSix]

To add to post 66. From the Machinery Lubrication article below.

https://www.machinerylubrication.com/Read/690/aerated-oil

Air bubbles in the oil due to aeration can also collapse/implode in the high pressure region, but don't seem to cause surface damage like vapor bubbles created by cavitation. It can however damage and degrade the oil. But having additional air mixed in the aerated oil could add to the saturation level of air in the oil to some degree, which could cause additional cavitation as described in post 66. Same thing can happen inside a journal bearing as the pump example shown below if the oil in the engine sump is aerated and it gets squeezed in the high pressure MOFT wedge in the journal bearing.