Harmonic balance/imbalance

[zefarelly]

Its been suggested that on high revving OHV engines you get an harmonic imbalance in the rocker shaft, apparently Cosworth used to make a 2 piece shaft to eliminate this problem.

Any one ever heard of this problem ?

I think its one that maybe the Formula Junior guys may know about ? I'll be looking to rev to around 8-9k RPM maximum

[graham bahr]

back in the early 70's a friend of mine who used to run a couple of tuning/ engine building shops and ex speedwell used to rally a 1300 X/f with nothing more elaborate tnan steel posts and spacers he never had any valve train/ rockshaft problems with an engine running A6 and A8 cams reving to a constant 10,000rpm, infact at those revs it was the rods which would occasionally let go. so i acant really see the need for special balencers.

[zefarelly]

Thanks Graham . . .thats exactly what I'm running . . . or going to . . with an A8, probably cap it at 8- 8 1/2k I have Cosworth Rods and I think some fresh ARP bolts might be a better idea than std lead soft Ford ones !

fingers crossed its all ok, the trouble is, the more I rtead and learn, the more scare stories I hear, although my current engine has behaved perfectly thus far . . .its everything round it that hasnt !!!

[graham bahr]

george used to run std rods which were polished, shot peined and tuftrided, there wasn't any such thing as ARP bolts avaiable to him back then, he used to fit twincam bigend bolts which were better than std x/f ones, your pistons should be a fair bit lighter than x/flow which will help it rev safely, but i would fit a steel flywheel and proper race clutch as you will be well into exploding clutch/flywheel revs.

[thebear]

Originally posted by graham bahr
george used to run std rods which were polished, shot peined and tuftrided, there wasn't any such thing as ARP bolts avaiable to him back then, he used to fit twincam bigend bolts which were better than std x/f ones, your pistons should be a fair bit lighter than x/flow which will help it rev safely, but i would fit a steel flywheel and proper race clutch as you will be well into exploding clutch/flywheel revs.

The shot peening process is vital as it strengthens the rod by increasing the stiffness. Many failures are due to flexing.

[zefarelly]

how does shot peening work then ? does it just de-stress the surface ? a bit like shot blasting welded fabrications ?

does that apply just to cast iron or to forged steel rods as well ?

[Revracing]

Shot peening works by introducing a "compressed" layer of material at the surface of the component, the process is used extensively in spring manufacture for increasing fatigure life.
Fatigue failures occur in instances of cyclic tensile stress, hence by introducing a compressive stress layer the net tensile stress is reduced and fatigue life increased.

I can't see how this would improve push rod stiffness but it would still prolong component life because push rods are subject to bending stress (read tensile stress at component surface) during operation due to load eccentricities..

One critical point shot peening uses spherical shot and does not introduce any sharp indentation which drastically reduce component life, shot blasting is not so controlled and can be detrimental, it is also very important to ensure complete coverage of the component as failure to do so can also lead to shortened component life.

[zefarelly]

I dont know how, but we've shot blasted simpole steel welded fabrications in the past and its helped stop fractures etc caused by vibration (this was shipping containers, not automotive in any way)

shot peening sounds similar to tuftriding, plating, polishing etc in that it just 'modifies' a very thin surface layer.

[Revracing]

it would seem that you managed to obtain a "shot peened" effect then, well done! it would have cost you a fortune if you had actually specified shot peening ..lol

[graham bahr]

when talking about shot peined rods, i ment conrods, i cant see anypoint in shot peining pushrods

[Minorist]

Must try that one

[zefarelly]

Originally posted by graham bahr
when talking about shot peined rods, i ment conrods, i cant see anypoint in shot peining pushrods

I was referring to conrods . . .If I shot peened the pushrods I'd probably bend them just bolting down the rocker gear !!!