Turbo Shaft Variations in Journal Bearing Area?

[MY1PATH]

I've seen some shafts with a cutaway in the bearing area near the big oil holes. Most are subtle but some are quite noticeable. If anyone knows better please chime in.

Here's my best guess:

+more cooling from increased drainage
+less bearing seep from lower pressure
+Faster spool and top speed from decreased surface area
-increased wear from less surface area and lower oil pressure

 

[Old.Tony]

Your reasoning sounds good to me.

 

[tweak'e]

I've seen some shafts with a cutaway in the bearing area near the big oil holes. Most are subtle but some are quite noticeable. If anyone knows better please chime in.

Here's my best guess:

+more cooling from increased drainage
+less bearing seep from lower pressure
+Faster spool and top speed from decreased surface area
-increased wear from less surface area and lower oil pressure

certainly better drainage but i would not expect better cooling unless you increase the oil flow rate. might actually be less cooling due to less surface area for oil to contact. also due to less heat conduction through the shaft to the cooler side of the turbo shaft. however does that matter on a diesels whose egt's are cooler than petrols.

not sure what you mean by "bearing seep".
i doubt the surface area will make much difference to speed, maybe a tad faster due to reduced weight.
its not a wear surface so i would not expect any difference.
oil pressure won't change as thats set by input side not outlet.

downside is weaker shaft.

 

[MY1PATH]

By "Bering seep" I mean more pressure inside the turbo causes the bearings to leak oil into the exhaust and intake.

On the straight shafts, they do make contact with the bearing in that area.

As far as drainage affecting pressure, it's harder to build up pressure when it's all flowing out the other side...

Yes, weaker shaft but the compressor end is already smaller than that.(broke one off not too long ago)

 

[tweak'e]

drainage side is always big so it doesn't add extra pressure, unless you flow more than normal.
they may simply reduced shank to allow more oil flow to increase cooling, which can make sense in race style petrol motors. in diesels not so much as pistons will die from high egt's before the turbo.

 

[tweak'e]

another thought,
i wonder if they run different bearings. i'm use to dual bearings, one each end.
tho you can get dual with a spacer between them or a single long bearing (with the bearing surfaces at each end). they have a hole in the center to allow the oil out. of course the oil has to go along the shaft to get out. you can either machine the oil relief into the bearing or the shaft (or both). notice on the thick shaft the center is machined down slightly.

 

[MY1PATH]

I'm working with one piece bearings that run the full length. The thick one in the picture is stepped ever so slightly The ones I have on hand have no stepping at all. Some of the bearings have an oil groove on the inside running from one oil hole to the other oil. A step in the shaft certainly would move the oil around the shaft to the other hole faster and I'm guessing because it has a easier path less of it would be pushed outward towards the ends of the bearings and the thrust bearing plate. I imagine it's still enough, otherwise they wouldn't make them that way.

 

[MY1PATH]

I can see how working with different bearings would change how you perceive the oil flow.
I could sure should state that I'm working with Garrett t20 VNT The bearings look like this.

 

[tweak'e]

my old turbos only feeds oil to the two bearings (and thrust bearing). (my really old one even has a carbon seal)
where as that style feeds to the two halves but also put a lot of oil down the center hole. the necked down shaft would allow more oil flow, probably more cooling. maybe less drag on the oil.
for a completely straight shaft i suspect there would be an oil relief machined inside the bearing, which is not a cheap easy thing to do compared to machining a shaft down.

 

[MY1PATH]

Upon closer inspection, my single piece bearings have a relief inside the same area.
Now I'm thinking its possible the shaft with the bigger cut might be paired with a straight cut bearing to cut down on machining costs of the bearing...