circumferentially grooved main bearing calculations

[EdOM]

I am trying to work out a design for a 360 degree circumferentially grooved main bearing. My plan was to consider it to be 2 short journal bearings, but I immediately ran into the problem of being off the chart because lambda (diameter to width ratio) is 6. I think this must be a common issue with this kind of bearing and was wondering if there was a better way to go about it? Shaft diameter is 70.4mm, bearing length is 25.93mm and I was starting with a groove width of 3mm. The reason for the groove is to provide adequate flow through the crankshaft to the rod bearing.

 

[Tmoose]

Maybe you've read all about this already, but The reduction in load capacity is pretty significant with 360 grooving.

discussed on page 28 in Mahle Clevite EB-20-11.pdf

http://www.mahle-aftermarket.com/C1...E0B180EF1BC12579AB00611F28/$FILE/EB-20-11.pdf

"better" options for improved or full time rod oiling (if you really need it) may be 270 degree grooving, cross drilling the main journals, or the heavily biased chamfer described in Chevy Power books for a while.

The "new" (1956) American Motors Rambler V8 came with full grooved mains from the factory. About 40% of the way down, here.

http://www.wps.com/AMC/Rambler-327/The New American Motors V-8 Engine (SAE Paper details).htm

 

[EdOM]

Thanks for the Mahle document, I have not seen that one before. I am aware that full grooving really kills load capacity, but the engine is a boxer and the loads on the mains are quite modest. I was hoping that it would still be adequate. Loads on the rods, however, are huge.

Because it is a boxer I cannot do the traditional grooved top shell and flat bottom shell. Perhaps I can groove the 45 degrees on the two unloaded sides. I *think* that Porsche boxer engine bearings have no grooves, yet they are still able to supply sufficient oil to their rods using what looks to be a larger drilled hole and chamfer in the crankshaft than usual. I will peruse the Mahle document to see if there is a way for me to estimate how much flow I could get to the rods using that method.

 

[tbuelna]

Tmoose- thanks for the link to that Clevite document. The graphs do show a significant drop in minimum oil film thickness (~30%) and an increase in peak oil film pressure (~80%) between 180deg and 360deg grooved main bearings. A 30% reduction in min. oil film thickness could easily cause the average auto engine main journal bearing to degrade to boundary contact for some operating conditions. And an 80% increase in peak oil film pressure would substantially reduce the surface fatigue life of the bearing shell.

 

[JayMaechtlen]

Does the oil channel have to be up the middle?
Could it be next to the edge of the bearing, with just a narrow wall to keep the oil in its channel on the way to the rods?
Obviously the original oil holes wouldn't fill it, but the the bearing capacity might be a lot better than that of two half bearings.

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[tbuelna]

Sure, in theory the oil groove in a main bearing could be offset to one side. We should also remember that a main bearing with a 360deg groove can be made wider to compensate for the loss of bearing surface area from the groove. The general rule of thumb for a non-grooved engine journal bearing L/D ratio is about 0.30 or more. Making crank journal bearings wider can create problems such as journal edge loading due to increased bending in the wider crank pins.

Sometimes it may not help to use a 360deg oil groove in the mains to increase oil flow to the rod journals. Oil will only flow from the main bearing to the rod journal when the rod discharge hole is aligned with the unloaded sector of the rod bearing.

 

[Tmoose]

"the engine is a boxer and the loads on the mains are quite modest."

The inertial loads may balance a bit, but Do opposing cylinders fire at the same time?
If not I think the closest main bearing on each side of the firing piston get an eye popping work out shortly after the plug fires. See attached rod bearing load jpg, and compare (7200 rpm) load between 10 and 40 degrees after TDCs.

============
Are you working on a Subaru engine?
Are you looking to thwart problems similar to the one shown at 0:44 herer?

http://www.youtube.com/watch?v=2gNToFY_bqQ

Looks like for a year or more King bearings ( a brand held in high regard in some Hot Rod engine circles) things some amount of extra groovery is overall good thing.

http://kingbearings.com/files/1362510928King_Introduces_New_Subaru_XPG_Bearings___News_Release_.pdf

 

[EdOM]

They do actually fire at the same time. Puts a lot of stress on the webs of the crank and the fillet where they meet the main bearing journal. But main bearing loads aren't too bad!

 

[Tmoose]

"They do actually fire at the same time...."
Which engine is that?

 

[EdOM]

A very unusual one Unfortunately it is still proprietary, otherwise I would love to describe it.

 

[tbuelna]

EdOM-

I've attached a graph from an analysis of a conrod bearing in a relatively high performance N/A 4-cycle engine. The vertical scale is oil film pressure in psi x 10^3. The reason this is important is that whenever the exit point of the oil gallery feeding the rod bearing passes within an area where the dynamic oil film pressure exceeds the gallery discharge pressure, there will be no oil flow thru the bearing. If there is no oil flow thru the gallery during these phases of the journal rotation, it is of no benefit to have an associated groove in the sectors of the main bearing to feed oil to the gallery. Thus it may be possible to reduce the circumferential length of the feed groove to less than 360deg and improve the load capacity of the main bearing.

Regards,
Terry

 

[EdOM]

Tbuelna,

Thanks. The pressure in the groove is not the same as the bearing pressure, though. It is much much lower. That being said I keep the supply hole on the less-loaded side of the rod journal.