Specific HTHS for specific bearing clearences?

Messages
497
Location
VA
Is there a specific viscosity for certian main and or rod clearences? Would HTHS be more important here? And would it be different for different bearings throughout?
 
Messages
3,756
Location
CA
I'm not an engineer but I would guess jorunal diameter and width along with the powerband and torque rating would play a larger part on HTHS reqirements than clearances would. I would think higher pressure or thicker oil would be needed for looser clearances. I could be wrong.
 
Messages
691
Location
Aridzona
I don't think raw journal clearance is all that important. Bearing surface area and load (PSI), journal surface speed, and viscosity are the big factors. Oil pressure and flow rate are important in that oil flow removes heat and replenish lost oil, but they too have little instantaneous bearing (groan!) on the oil film thickness.
 
Messages
3,756
Location
CA
 Originally Posted By: Geonerd
I don't think raw journal clearance is all that important. Bearing surface area and load (PSI), journal surface speed, and viscosity are the big factors. Oil pressure and flow rate are important in that oil flow removes heat and replenish lost oil, but they too have little instantaneous bearing (groan!) on the oil film thickness.
Speaking on journal speed, I've always wondered how some of the Porsche engines I've seen got away with such large diameter journals coupled with the high rpms. Wouldn't this send the journal speed way up?
 
Messages
79
Location
Canada
Higher journal speed is a good thing, it creates a stronger hydrodynamic wedge of oil in the bearing. The higher the journal speed the higher the load that can be supported without journal to bearing contact for a given oil viscosity. It's the high load + low rpm (ie low journal speed) + low viscosity oil combination that creates a challenge to maintaining an hydrodynamic oil film in the bearings.
 
Messages
43,650
Location
'Stralia
Taylor, journal diameter, width, clearance, load and speed all relate to actual viscosity of the oil, not greatly to the HTHS. I've seen some 21" diameter, 19" wide 3,000RPM bearings that will have a 500% increase in vibration with a 2C change in oil temp (viscosity change). ArcticCat, speed is not always your friend, and it can reduce bearing stability criteria.
 
Messages
35,000
Location
NY
Now I'm not sure how this plays in, but I was corrected a while back on this board that oil does compress somewhat because of the additives in it. What effect would that have here? Or is that calculated into the design and specs?
 
Messages
43,650
Location
'Stralia
"incompressible fluids" are used when carrying out theoretical design. But when you get to actual testing, there's no such thing.
 
Messages
35,000
Location
NY
Thanks, so then oil does compress, and would it be safe to "assume" that it is calculated into bearing design?
 
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Messages
43,650
Location
'Stralia
When theoretical people talk about "ideal" fluids, gas equations etc., they are non real world things that bring the equations down to a manageable level. Not much is "ideal" So things are "tuned" in the real world to non ideal conditions. Every bearing in you engine has a different diameter, clearance, oil pressure, temperature and flow rate. Nobody is going to design each and every one of the, but aim for the worst, and assume everything on the "better" side is better
 
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