Floating bearings 1

[pontiacjack]

What was (were) the perceived advantage(s) of full-floating insert-type plain bearings in connecting rods of high performance engines? I ask this past-tense because the last time I saw them advertised (by Potvin, Moon, Fobert, and maybe others) was about 1957. They were available for then-popularly-raced OHV V8 engines (Olds, Cad, and Chrysler hemi as a minimum). They were similar in design to the stock '32-48 Ford bearings- each insert pair was as wide as the crank journal and served a pair of rods.

I have no experience with them, and want to understand any possible inherent advantage. All input is welcome.

 

[patprimmer]

I can't see a single advantage other than maybe other than 1/2 the number of bearing shells to be used.

Regards
Pat
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[BrianGar]

So how did the retaining/anti spin tangs work? I presume they had none?
Tangless bearing shells make me nervous.

Brian,

 

[PJGD]

By floating bearings, are you referring to the type of bush that is/was sometimes used on built-up crankshafts where the typically bronze bush was not pegged to the connecting rod but was free to spin between the rod eye and the crank pin? Early Bristol radial engines used this arrangement, encouraged by Ricardo thinking at the time; the idea being that the bush would precess at roughly half the speed of the relative motion between the journals and also there would be two oil films over which the load would be spread, which in those days allowed a higher load factor before failure.

The heavily loaded roller tappets in our in-line injection pumps had a similar construction at one time, but the pin, intermediate bush, and the roller were all hardened and ground steel.

PJGD

 

[pontiacjack]

PJGD- I've never seen the aftermarket inserts for OHV engines, but the '32-'48 Ford pieces were similar to 'modern' tang-located inserts- a soft babbitt-type overlay on a steel backing (on both sides, of course). The backing was thicker than is now typical, for the extra strength required.
The reduced surface speed may have been the key attribute; just thought I might be overlooking some other advantage.

 

[BigClive]

I may be a little dim - but just what are these plain bearings you are writing about?
Do you have a picture or drawing etc.?

 

[Tmoose]

http://books.google.com/books?id=pa...age&q=ford full floating rod bearings&f=false

 

[MikeHalloran]

Thanks for that, Tmoose.

I thought flatheads were weird, even before I learned about full floating bearings.

Okay, not weird, but different.




Mike Halloran
Pembroke Pines, FL, USA

 

[Pancholin]

Some aircraft engines, Allison and Merlin come to mind and used floating bearings with a bifucated rod and a narrow one in the middle and each rod used the full width of the crankpin. I think Harley V twins also use this arrangement, and the inserts are quite thick, not the thin inserts.

 

[dicer]

I see them as twice the leakage rate. The need for a very nicely finished big end, and the rod and cap would need to be accuratly located, kinda like a 2 stroke with needles, some are split or cracked at the mating surfaces. Inother words more unnecessary costs involved in manufacture. Must be why the system that has been used for years is still used.

 

[Tmoose]

All the Indian and Harley V-twin lower ends I've seen used rollers for the connecting rods. Some back well into the 1930s.

A long time ago I assembled a few JAP V-Twins as used in Morgan trike "cars' and as best as I recall they had curious arrangements of shared/inner/outer inserts among the fork/blade rods. I think the insert for the blade rod was captive in the forked rod ???

 

[pontiacjack]

Yes, Allison and Merlin V12 engines both used fork-and-blade style connecting rod pairs. The Merlin did use floating (bronze) bearings, but the Allison didn't- it employed steel-backed bearing inserts (lead/tin/silver overlay) that were long enough to be clamped into the forked rod, with the center area having overlay for the knife rod to run on.

Dicer- Oil leakage from rod journal/bearing is determined solely by the end clearance of the rod(s) to the journal's cheeks.
Rod big-end bore "finish" for a floating bearing is only slightly more critical than for a "normal" rod.
Locating-precision requirement of the rod caps is identical.
I'd say that the 1932-1948 period constituted "used for years" (Ford V8 engines with floating bearings).

 

[patprimmer]

Then why does increasing clearance between the journal and the shell while maintaining the same end clearance cause oil pressure to drop.

Regards
Pat
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[pontiacjack]

Pat- You're right, my statement was incorrect. The rod end clearances are typically less restrictive to oil flow than the nominal bearing clearance.

 

[Tmoose]

I think some factory hi performance American factory literature from the 60s and 70s warned that excessive rod side clearance would increase volume of oil flung onto the cylinder walls and make oil control more difficult for the piston rings.

I have never done any tests, but at the time, after thinking about the bearings' oil clearance (~0.003" diametral ) vs the normal side clearance ( ~0.007" each/ 0.014" per pair) the logic/cause-and-effect seemed "wrong" to me.

 

[tbuelna]

pontiacjack,

The perceived advantage of floating rod bearings was probably equalized fatigue/wear at the bearing surface. But that was only an advantage when the bearing ID was cylindrical as manufactured, and the shell had a constant wall thickness. This is no longer true with modern engine journal bearing shells. They now have a variable wall thickness. The idea is to give the optimum bearing bore shape under operating loads and temperatures.