surface finish on bearing inserts

[JayMaechtlen]

On another forum, an interesting photo is posted of a bearing insert.
A modern vehicle with lots of miles, and the insert does not appear to ever have touched the journal.
But, what's interesting is the surface finish of the insert - comments?

http://corvaircenter.com/phorum/read.php?1,809154

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[dicer]

Cutter marks, it was likely line bored, and helped hold the oil.

 

[Tmoose]

i wonder how deep those are. Some bearing overlays are just ~ 0.0005" thick
Grooves to "hold the oil" are kind of contrary to the surface finish called for on the steel journal to estabkish hydrodynamic lubrication.
10-15 micron inch Ra

http://www.wilmink.nl/Clevite/Clevite_lagerschade_tech_info.pdf

 

[tbuelna]

Although it's a bit hard to tell for sure from the photo, it looks like there are some indications of contact on the bearing surface. Below is Clevite data for the Toyota engine in question. The AL-3 code indicates the bearings have steel backing with solid aluminum lining. There appears to be a couple undersizes available for this bearing set, and since the lining is solid aluminum it would be possible to re-size the bearings in place using line boring, as dicer noted.

The "grooves" on the bearing surface in the photo probably look much deeper than they actually are. It's likely just an optical effect from contrasting bright & dark surface appearance. The bright areas are high points where the aluminum surface gets lightly polished from briefly operating in boundary conditions, and the dark areas are low points where oxidation of the aluminum surface is not wiped away. A properly prepared journal bearing surface should have a max roughness of around 25 microinch when new. If the "grooves" were deeper than that, it would have a detrimental impact on the oil film lambda ratio.

 

[Tmoose]

I think Toyota would not elect to line bore semi-finished bearings on the production line, but finish the block accurately and install precision finished inserts, perhaps with an assortment of slightly varying thicknesses at the ready to allow selective fitting to fine adjust the clearances when desireable.

 

[dicer]

Surface finishes have always been a fad, i.e. its the thing we do. Just look at inlet ports, inlet Manifold's etc. I remember the days of wanting them highly polished.

 

[MikeHalloran]

Yeah, but I've NEVER seen a rotating bearing intentionally grooved like that.

I did do something similar once, within a hydraulic cylinder that had to take short strokes on Skydrol, which is not a good lubricant. My intention was that all of the sliding would occur over a surface that was recently wet.

... but that was for a short stroke reciprocating motion. I have no idea what would be the intent of providing quasi-circumferential grooves in a rotating bearing that's normally hydrodynamically pressurized.




Mike Halloran
Pembroke Pines, FL, USA

 

[dicer]

I have actually seen it a lot many years ago.

 

[tbuelna]

Surface finishes have always been a fad, i.e. its the thing we do.

Nothing could further from the truth with regards to hydrodynamic journal bearings. Increased surface roughness, taper/barrel/runout of the journal and bearing surfaces, and misalignment under running conditions all have an adverse effect on the operating fluid film characteristics. The thickness of the oil film that supports the journal radial load in a typical engine bearing is less than 100 microinches. Engine journal bearings are typically designed for a lambda ratio (film thickness/Ra surface roughness) between 1 and 3, which gives a good balance of efficiency and life. If the lambda ratio drops below 1.0, this indicates onset of boundary (metal-to-metal) contact conditions, with a rapid increase in wear and friction.

Let's consider the effect the depth of those machining grooves shown in the photo could have on the bearing's performance. We don't know the actual operating parameters of the bearing, but for the first case let's assume the minimum operating film thickness is 60 microinch (based on a safe POFP fatigue limit for an aluminum liner), the journal surface roughness is 16 microinch Ra (recommendation from Clevite), and the ungrooved bearing surface roughness is 32 microinch Ra (estimate). This works out to a lambda ratio of 2.5 which is safely within hydrodynamic conditions.

For the second case let's add a circumferential pattern of machined grooves covering 50% of the bearing surface area (similar to what is shown in the photo) and having a depth of just .000224". These machined grooves would be hard to see with the naked eye without any contrast, or even detect by touch with your fingertip. The grooved surface as described would have a roughness of 224 microinch Ra. Assuming everything else remains the same, this would reduce the oil film contact lambda ratio to 0.5 which is well within boundary contact conditions. A journal bearing operating in this condition would not last long.

This example is a bit simplistic, and the actual operation of a journal bearing is more complex, but it does provide a good illustration of the importance of surface roughness.

 

[dicer]

Some things are missing. Like what direction the finish or ribbing is in relation to the movement of the bearing or load on the bearing. And the viscosity of the lubricant. The spiral ribbing is helping to hold the lubricant in the clearance space, and likely increasing the hydrodynamic pressure in the clearance space. Some day I will test this.