Redline? 2

[n85]

Other than destructive testing how would one go about determining the failure mode at extended high speed operation of an engine? (and the practical speed limit?)

The particular engine I'm thinking about (because I push it hard) is a 125cc single, 56.5x49.5mm bore and stroke, all ball/roller bearing design (implying a three-piece crankshaft). Valvetrain actuation is by gear-driven camshaft and pushrods + rockers. Two valves per cylinder with dual valve springs and old school screw adjusters. Basically a clone of the Honda CG engine.

Marked redline is 9k but the powerband extends a bit beyond this. I've hit 10.5k (indicated) with a mis-shift but I imagine the valvetrain would float significantly there.

My intuition says the valvetrain would be the limiting factor but another user reported a "snapped in three" gudgeon pin after daily extended use close to redline. Which would be more probable?

 

[dicer]

Calculations and tests. The simpler way is some of the simulation programs that exist. Redline would be much lower than the calculated over stress rpm's.

 

[patprimmer]

Piston speed is a good indicator.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[tbuelna]

N85,

Crank pin failure for sure.

The valvetrain components in that 125cc single are so small and light that it would be hard to damage them, even with an over rev to 10.5K. The roller bearing in the big end of the conrod however, would not be so happy. Roller bearing rods work OK in 2-strokes because there is no piston/conrod load reversal at TDC like there is with a 4-stroke. The running radial clearance in the conrod roller bearing will cause it to quickly beat itself and the crankpin to death in a 4-stroke that is over-revved. That's why you don't see too many 4-strokes with roller bearing rods.

The failure mode would be spalling of the crank pin case surface due to excessive contact stresses with the bearing rollers during over revving. These crank pin case surface spalls are the nucleation points for fractures to propogate outward from. The final catastrophic result being a fractured crankpin.

If you want to determine by analysis the number of load cycles at which your crank pin will fail, you will need a good understanding of rolling element bearing theory, and you will need to accurately characterize the dynamic loads, structural conditions and tribological effects your conrod bearing and crankpin are subject to.

It won't be easy, but it should present a good learning experience. Have fun!

Regards,
Terry

 

[n85]

Thanks all for the input!

Piston speed at 9K works out at just under 14.9m/s which considering that this is not a race bike seems a reasonable limit.

Regarding the valvetrain I would have thought that the mass of the pushrods and the two rockers per valve would have detrimental effects. Now I've seen from some pictures that there is a significant rocker ratio that keeps the movement of the rods quite short.

I had never thought about the crank pin bearing issue. The explanation makes sense but this is not my field of expertise. Would the oil supplied to the bearings provide any "cushioning" effect or would this be negligible since the rollers only have line contact and easily break the oil film on the crankpin/rod?

The service manual specifies the radial clearance service limit at 0.05mm and this makes sense with the problem of "hammering" mentioned.


Seeing that I have only one bike and that I use it regularly it does not make sense to dismantle the engine and measure masses, clearances etc. in order to create a model. I will restrain myself to about what the factory engineers decided and hope for no problems.

Regards,
N

 

[BrianPetersen]

I have a 125cc street bike myself, although not that particular one. They're designed to handle extended full-throttle operation near redline, and the redline is rather conservative so that the engine will last. Wide open is the only way to ride a small bike like that.

I've heard of Honda CBR125's and CBR150's going beyond 50,000 km without major problems. Mine has 22,000 km.