Calculating rev limit

[evorsx]

I'm in the process of stroking my 4G63 from 2.0l into approx. 2.2l New billet steel crank, forged steel rods, uprated rod bolts, forged pistons. I'm going from a standard 88mm stroke to 94mm. I've gone to the extent recently of calculating the max force on the rods/rod bolts at TDC exhaust stroke. What I find is that the lighter pistons and rods of my new configuration make up for the increased acceleration due to the increased stroke.

My question is :- assuming a correctly preloaded rod bolt, what is the accepted max. force (% of the bolts yield force) in a high performance application? ie. what is the maximum force that I can allow the bolts to experience (considering cyclic/fatigue loading, temperature expansion, etc.) and hold up?

 

[ivymike]

if you use snug torque + angle of turn tightening (most precise), then you can get away with about 85% of the bolt proof stress. Don't forget to consider the crush force of the bearing shells in your calculation, it's important.
You'll have to look up the proof stress yourself (easy enough to find that online, given the bolt grade), and look up or calculate the load-carrying area of the bolts (inside the thread root). Multiply 85% * proof stress * tensile load area to get force per bolt. Subtract from that your bearing crush force per bolt. Compare the force you end up with to the external tensile load applied to the joint, and what you have is a big-end joint separation cover factor. 1.0 is too low. 1.5 is a bit more cozy. Use judgement.

 

[evorsx]

Thanks Inhiding, this is just what I was looking for. My initial results using about 45% of bolt yield (fatigue limit) was giving me a rev limit of 10500rpm which seemed a little high. I had not considerd the bearing crush force, so I will add that. The big-end separation factor is the real difficult one as this seems to be dependend on the stiffness of the big-end joint compared to that of the bolt. I will try 1.5 with the addition of the bearing crush to see where that leaves me.

 

[patprimmer]

Inhiding.

I have always thought that bolt stretch was the most accurate was to preload a bolt, but angle gauge is better than a toque wrench.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SMOKEY44211]

Get an ARP fastener cataloge. They have a mini course in proper method of fastener preload. Lot to be learned there. --------Phil

 

[ivymike]

pat - you're right, but who's going to drill little holes in every bolt? I meant "most precise of the garage methods." (vs torque wrench tightening with or w/o lube, etc)

 

[patprimmer]

I

The good aftermarket bolts used in many "hot rodded" engines come with the dimples in each end. Most rods have the cap bolt holes all the way through, but as you say, very few people (including myself) bother with the bolt stretch method. If I had doubtful bolt strength, or a few bolt failures it might change my habits though.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SammyDaFish]

How would one determine bearing crush force? I've never come across this before.

Thanks.

 

[SMOKEY44211]

Sammy just consider the bearing shell to be a solid cylinder. On some engines where the crankshaft is pressed together the bearings are made this way. The housing usually is sized .001-.0015" smaller in diameter then bearing shell assembled diameter thus providing an interference fit or "crush" as its commonly reffered to.-------Phil

 

[SammyDaFish]

Yeah, I understand the concept of bearing crush. What I'm referring to is InHiding's post. He mentions to take into account the bearing crush force. Since we're using the amount of torque applied to the bolt during tightening as a reference of how much force is applied to stretch the bolt, some of that force doesn't stretch the bolt, but crushes the bearing. My question is how do you determine how much force is needed to crush the bearing?

Maybe I'm thinking about this wrong. If so, please clarify.

 

[GregLocock]

Get a conrod. File off the mating surfaces. Torque up the big end bolts until your plastigauge says you have the right big end bearing clearance. The torque you use has been entirely absorbed in setting the bearing shells. Any extra will go into holding the conrod together.

Cheers

Greg Locock

 

[ejit]

10,500 is going to break, it's 32.9 m/s - way over F1 (Ilmore: 26 m/s, Honda 23.3). There are too many variables missing from your math - such as the rod bolt isn't necessarily the weakest link.
More reliable: 28 m/s (5500 f/s) max piston speed = 8935 RPM.

 

[ivymike]

re bearing crush force:
for a rough cut, which is all you'd probably use for a hand calculation of large-end joint separation, use 215 MPa * bearing shell (backing) thickness * bearing shell backing width * 2

If you want to get fancy, add 0.5 * bearing lining thickness to the backing thickness.