journal bearings min recommeded viscosity 13 cSt

[1 FMF]

http://media.noria.com/ml/

Machinery Lubrication, July-Aug 2005

Article: Journal Bearings and their Lubrication, pg72

good article.

quote:

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A generally accepted minimum viscosity of the oil at the operating temperature for journal bearings is 13 cSt, although some designs allow for an oil as thin as 7 or 8 cSt at the operating temperature.

The optimum viscosity at operating temperature is 22 to 35 cSt, for moderate-speed bearings if no shock loading occurs. The optimum viscosity may be as high as 95 cSt for low speed, or heavily loaded or shock-loaded journal bearings.

If the oil selected is too low in viscosity, heat will generate due to insufficient film thickness and some metal-to-metal contact will occur. If the oil is too high in viscosity, heat will again be generated, but due to internal fluid friction created within the oil. Selecting an oil which is too high in viscosity can also increase the likelihood of cavitation.

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What caught my eye was *minimum* recommended is 13 cSt, with some designs allowing 7-8 cSt, at operating temperature.
Current engine oils run 200-240F normally, and a typical 5w-30 is 10-11 cSt at 212F and 5w-20 is 7-8 cSt. Does the epa and cafe have us running at the minimum to get maximum fuel economy?

What would you define as moderate speed? Article makes mention of speed defined as rpm and irrelevant to shaft surface speeds. Does a car engine which runs 1500-3000 rpm qualify as moderate speed?

 

[Mitch Alsup]

"Does the epa and cafe have us running at the minimum to get maximum fuel economy?"

Basically, yes. As the oil gets thinner the journal bearings display less friction, but the cam lobe show increasing friction. So, its a delicate tradeoff, and one that is different between an overhead cam with flat buckets and a roller cam with pushrods and rocker arms.

"What would you define as moderate speed?"

Moderate speed table (my version)
oil temp RPMs
< 140dF .. 30%
..150dF .. 50%
..160dF .. 75%
..200dF .. 75%
..210dF .. 65%
..220dF .. 55%
..230dF .. 45%
..250dF .. 35%
..260dF .. 25%
> 260dF .. does not qualify as moderate

Where RPMs are a percentage of the RedLine of that engine.

Rational: A) Until the oil gets into the 20cSt range it is simply too thick and this happens in the 150dF range. B) as RPMs rise the oil is loaded mroe and more by reciprocating and rotational loads. So, as the oil thins with temperature, you want to reduce the loads to those than the oil viscocity can manage. C) throttle position also contributes to the load but this requires a 3rd dimension to this table.

 

[mechtech]

It just can't be that we need 70W oil in our vehicles, with all due respect to the studies that show we do.
Maybe a thin 60W would suffice?

 

[XS650]

quote:

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Originally posted by mechtech:
It just can't be that we need 70W oil in our vehicles, with all due respect to the studies that show we do.
Maybe a thin 60W would suffice?

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The Noria paper was primarily refering to journal bearings in industrial applications. The industrial user wants them to last forever and aren't handicapped by needing to humor CAFE. The mere 3000 to 5000 hours automotive journal bearings last with their lubricant supply wouldn't be acceptable.

 

[Eddie]

Too many variables here to acept an absolute statement like MUST HAVE or BEST XXX viscosity. Type of bearing and shaft material, shaft surface velosity and the relationship to hydrodynamic pressure, shaft surface finish, and a host of other variables IMHO. ed

 

[AEHaas]

This article is for industrial, usually relatively low speed, high load equipment. There are however, several general statements that can be said to apply to automotive applications:

If the oil selected is too thin or too thick heat will be generated.

Selecting an oil too high in viscosity can increase the likelihood of cavitation. Cavitation pitting damages the metal.

The higher the RPM the thinner the viscosity required.

These statements made in the article do apply both to industrial and to automotive applications.

They describe bearings in the temperature range of 0 - 90 C. These are controlled to this range whereas automotive bearings must operate from much cooler to much hotter bearing temperatures, as high as 250 C or more at piston areas.

aehaas

 

[Jimbo]

According to my old textbook, Shigley and Mitchell "Mechanical Engineering Design", 4th ed, McGrawHill 1983,
there are eight variables in the design of a journal bearing.

Independent variables
viscosity
load per unit of projected bearing area
speed
bearing dimensions

Dependent variables
coefficient of friction
temperature rise
oil flow
minimum film thickness

"The fundamental problem in bearing design is to define satisfactory limits for the second group of variables and then to decide upon values for the first group such that these limitations are not exceeded."

The equations and unit conversions are too complex to reproduce here. Since computers were not commonly available to undergraduate engineering students at the time, it was not an easy task to iterate on a solution.

The bottom line is that a bearing can be designed around any reasonable oil viscosity. I do question how Ford can approve a retrofit to a different oil viscosity on older engines without changing anything else.

 

[XS650]

quote:

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Originally posted by Jimbo:
I do question how Ford can approve a retrofit to a different oil viscosity on older engines without changing anything else.

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The oil viscosity was selected with engine robustness a primary consideration. Now they are willing to trade a bit of that off to get a better CAFE.

 

[AEHaas]

quote:

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Originally posted by Jimbo:
The bottom line is that a bearing can be designed around any reasonable oil viscosity. I do question how Ford can approve a retrofit to a different oil viscosity on older engines without changing anything else.

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In the seventies I personally tested oil viscosities as I was insanely interested. What I found was that MOST oils that were labeled as 30 wt. were about a 20 wt. from the start. All oils rapidly thinned to a 10 or 20 wt. with use (if not less). Much later they thickened rapidly to 30, 40 wt. or more.

In a nutshell therefore, engines were running with 10 to 20 wt. oils in the mid seventies so a non-thinning 20 wt. should certainly be enough.

aehaas

 

[MolaKule]

See:

http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=4;t=000165

and

http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=4;t=000194