v twin crank pin damage

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Dan Timberlake
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Re: v twin crank pin damage

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motionper wrote:have tried s&s crank with push in pin,now running a thundercrank with hexed shaft the rockwell idea sounds like a good idea. tried carillo top fuel steel rods and mclure steel rods, also thinking about a 4 inch stroke to take some of the rotating pressure off the pin.it turns the needle bearings into dust i have thought about contacting a bearing company and having a caged bearing made with more rollers never thought about zero clerance
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http://www.schaeffler.com/remotemedien/ ... _de_en.pdf
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Re: v twin crank pin damage

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Baprace wrote:Is it possible to put a full floater bearing insert in the application? It seems you are clearly past the limits of a roller type bearing.
Sounds to me like the rollers are skidding , from too much load. I think an insert type bearing would distribute the load over a bigger area. When I see crank pin wear on most race engines it is always the bottom side of the pin that is wearing first, I attributed the wear to the most load/power on the pin was on the down stroke not on the top of the pin. JMO
In the 70s and 80s saw many Badly maintained high mileage Passenger car engines wear their crankpins on the underside. My theory was the hydrodynamic wedge (oil film) was nice and thick near TDC shortly after oil was introduced by the oil hole, but as rotation progressed on the power stroke oil leaked out and the wedge got thinner. If it got thinner than the particles of crap in the oil then they'd make contact with the crank and cause some wear.
Most bearing calculations ( journal and rolling element) shoot for dynamic oil films ( hydrodynamic for journal bearings, and elastohydrodynamic for rolling element) thicker than surface asperities or junk in the oil. Once the journal surface was compromised with scratches or wear then the hydrodynamic wedge was cut into narrow strips (similar to the lessened capacity of a grooved main bearing) and even the bearing and crank could start to make metal to metal contact late in the rotation, on the underside of the journal.
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Re: v twin crank pin damage

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motionper wrote:help any ideas, turbo 45 degree v twin 4.375 bore-5 inch stroke 20 lbs boost methanol fuel 11.5 static compression. wipes crank pin out on the BOTTOM of the pin 300-400 feet out-have tried less boost goes a little farther out before seizing. why is the problem on the bottom of the pin????? all i can think of is the setup is beyond the mechanical limit of the roller lower end??
Here's my starting theory.
When roller bearings skid rather than roll they cause surface damage and wear. Once the roller or race surfaces are damaged then it is harder to establish elastohydrodynamic lubrication (that is the goal when dealing with rolling contact) and high contact stress can easily start spalling.
In another post to this thread I provided a link to a page at SKF that explained skidding is a problem when the load on a roller bearing is too low ( about 1/10 the basic load rating of the bearing). That is for bearings that just spin at a constant speed in an electric motor, fan or pump. Rod bearings don't spin at a constant speed even when engine rpm is constant. They speed up and slow down as the rod rocks back and forth. The higher the rpm the more violent the speed oscillation. At some point the inertia of speeding and slowing the bearing rollers is more than the friction between rollers and race can handle. A heavy steel retainer or large diameter rollers are harder to get going or stop. Indians and Harley 45s used giant 1/4 inch diameter rollers in their rod bearings.
Harley big twins changed from 18 longer square ended rollers and steel cages to 17 shorter slightly crowned rollers and aluminum cages around 1973. Despite the shorter rollers and reduced roller count big end reliability improved. The crowed rollers also helped a lot by reducing the possibility of edge loading.

Sometimes High speed 2 stroke engines were forced to use rod bearing or pin bearing cages with silver and other exotic coatings to buy survival. It was not a bearing capacity issue, it was needle to cage friction causing localized overheating above some critical rpm. Ball and roller bearings in some high speed rotating (not oscillating) military applications also use silver plated cages.

It would be good to see what the crankpin and rollers look like about halfway to failure. Clues like crankpin spalling at edges of the roller paths, or location of signs of skidding could make it a lot clearer what is going wrong.
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Re: v twin crank pin damage

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Have you dealt with spacers between the rollers? They can be hollow. They reduce the effective volume between the rollers and would allow an annular central oil feed to pressurize from the midline out and this would help ensure continuous oiling. If the spacers were large enough you would also have the opportunity to develop an wedge of oil (floating bearing). It seems fairly certain this would have been tried.

Why exactly are standard shell bearings not used?
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Re: v twin crank pin damage

Post by motionper »

it's a knife and fork rod set up,no place to put bearing shells, have been thinking about trying ceramic rollers has anyone done this before???
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Re: v twin crank pin damage

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motionper wrote:it's a knife and fork rod set up,no place to put bearing shells,

http://www.enginehistory.org/phpbb/view ... 249e6113d7 for ideas on knife and fork rods with shells.
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Re: v twin crank pin damage

Post by motionper »

great pictures on the aircraft history, my connecting rods are on piece they do not have removable ends-also maybe i could use a one piece floating bushing but who would build it?????
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Re: v twin crank pin damage

Post by OILEAKY »

Motionper,
Way back when, the Harley KR motors had a similar problem, and it was determined that they had TOO MUCH oil, and the rollers would slide rather than roll. The fix was a 1/2 speed oil pump.
Don't worry about a tight a fit, when the rod is warmed up, the fit is MUCH looser. Try measuring the rod bore at room temp, and then at 200 deg. or so.
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Re: v twin crank pin damage

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motionper wrote:it's a knife and fork rod set up,no place to put bearing shells, have been thinking about trying ceramic rollers has anyone done this before???

Similar metals are rarely run together as hard working long life bearing surfaces as there are likely to be compatibility problems if they ever come in contact.
Tricks like carburizing, nitriding or plating, magnificent geometry and surface finishes and bountiful lubrication with EP or anti-war additives are needed to make steel on steel last.

Steel on steel in roller bearings (rollers, crankpin, races) works reasonably well because the parts' design and manufacturing is so highly developed.

Hybrid ceramic ball and roller bearings have vastly improved tribological performance in part simply because the ceramic rollers are not steel.
If your crankpin and roller problems start with rollers skidding causing surface damage then I'd expect ceramic rollers to work 10X better.
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The modulus of elasticity (stiffness) of silicon nitride commonly used in ceramic balls and rollers is about 50% higher than steel.
The density is much lower than steel so the balls/rollers are about 50% lighter.
See Fig 9 here -
http://www.schaeffler.com/remotemedien/ ... _us_en.pdf

High stiffness means the balls do not conform to the steel race as well, and gently as steel balls. They sink in Kind of like riding a 10 speed bike with skinny tires on the beach. Thus with equal load and geometry the contact and Hertzian stresses are higher using HC balls or rollers. Hitting or pressing an HC bearing will dent the races before the balls are dented. Despite the higher stress Bearing manufacturers can quote greatly improved life with HC bearings operating within their rated capacity because the surface tribology is SO much better.
If your crankpin/roller problems are strictly load related then I think the stiffer HC rollers will make it worse.

The light weight can really help if oscillating speed variation, so if roller inertia is causing roller skidding and eventual surface damage ceramic rollers can help a lot there too.
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Some are concerned that being a ceramic the balls/rollers are brittle and will fracture or crush under impact.
I worked at a precision machine tool spindle company for several years. Lots of the spindles used HC bearings, and some would fail in testing, but invariably the steel races would be burnt and smeared junk but the balls would be perfect, so excellent used HC balls were available. The guys in the shop gave me some, boasting all their experiments whacking a loose ball with a big steel hammer resulted in a dented hammer, but never a damaged ball. I have a dented hammer and deeply dented steel plate having tried that experiment myself. So I'm not too worried about name brand ceramic balls and rollers being brittle.
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