Lubricating Oil Problem

[Jnr16]

I am looking for someone to give me some info on what the problems are with heavy lubricating oil in medium speed engines, with respect to combustion, and emmissions. And also info on how to go about lunricating a crankshaft bearing.

Thanks

 

[Cockroach]

Crankshaft bearings are typically lubricated through high velocity agitation fluid in the oil pan. The splashing provides an intermitent lubrication, steady over time.

I would go to the web and search under this application. Alternately, SAE sites would probably ferrit some ideas.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[tbuelna]

Jnr16,

You need to be more specific. When you say "medium speed engine", do you mean a 1000 kW diesel?

As for the lubricating oil problems, what is the oil grade?

Most crankshaft main/rod bearings are fluid film journal bearings. They are plain bearings, where the load is carried by a hydrodynamically generated fluid film. The HD fluid film is generated by the relative motion between the bearing shell and its journal, it is very thin, and thus requires only a very small quantity of oil. However, this type of bearing arrangement is not 100% efficient, so there is some heat generated at the bearing interface. Removing this heat requires a flow of oil through the bearing, and this cooling flow rate is usually the main concern when designing a journal bearing lube circuit.

There are many factors that determine oil flow through a journal bearing: oil gallery geometry, crank rotational speed, bearing L/D dimensions, bearing radial and axial clearances, where the oil feed hole is located relative to load, etc. You must take all these factors into account and design for an oil flow rate that will maintain your bearing temperature within acceptable limits for you particular bearing materials and lubricants. As a rule of thumb, most commercial journal bearings are designed to run at temperatures of less than 350degF at the back side of the bearing shell.

Good Luck.
Terry

 

[tygerdawg]

Jnr16
Ditto what tbuelna says. Journal bearings operate on the principle of having a thin layer of lubricant in the clearance between the journal and the rod. Upon rotation, pressure of the lubricant layer builds in a non-linear manner, higher as you plot the pressures radially along the circumference of contact area. Pressure, layer thickness, all of that, depends on contact area, radial load, viscosity, relative speed, diametral clearance, other factors. From what I remember in the machine design textbooks, it is a fairly well understood phenomena.

TygerDawg

 

[rorschach]

to add to tbuelna's post. there is a happy medium for oil viscosity that is driven by bearing geometry and loads as well as temperature and shaft speed (which is why multi-viscosity oils were invented). as the oil film is developed, shearing action within the oil will heat it and generate fluid friction on the shaft. when oil heats up it's viscosity falls generally. too thick an oil and the self-heating will be excessive and horsepower will be lost overcoming the fluid friction and the oil will break down prematurely due to heat. (to give you an extreme mental example, imagine two shafts turning in two bearings. one lubricated with thick goopy grease, the other lubricated with SAE 5W oil. the shaft lubricated with oil turns easier than the one with grease, but the grease stays put in the bearing whereas the oil wants to run out and has to be replaced. that friction can result in pretty significant fuel economy losses.) too thin an oil and the oil film will not build thickly enough to prevent shaft contact with the journal bearings. at start-up, a thick oil is best because it tends to cling to surfaces and supply some lubrication until oil pressure is sufficient to maintain an oil film. but once the engine is running and has reached temerature, a thinner oil is more desireable. close bearing running clearances also mitigate towards thinner oils whereas looser (or worn) ones do the opposite.

 

[icrman]

<<some info on what the problems are with heavy lubricating oil in medium speed engines>>

By heavy is that meaning "viscosity" ?

So for a medium speed engine are you talking something like a D399 Cat, or something like a PC4.2 Pielstick ?

The D399 works just fine on SAE 30, or 40.
Most all medium speed engines and high speed as well live just fine on those oils. If the oil temps run real high then of course you can go heavier. If you have excessive wear, and low pressure you may need to go higher.
Like I think others have mentioned it all depends on temperature, clearance, pressurization.
Problems? Too thick - oil starvation-pumping losses-pump drive stresses. Like someone said, need the lube to flow to carry away heat. If you don't know what viscosity oil to use, contact the manufacture of the engine, they pay people lots of money to figure that out for you.