Ai

Rich, I believe mean piston speed is:

Sp (in ft/min) = stroke x RPM x .166

I think max acceleration is:

PAmax = ((stroke x RPM^2)/2189) x ((1/2(rod/stroke))

EDIT: Should mention that the units for the max accel is ft/sec^2

As far as your rods being fine.. I wont say that, but I will say that everything fatigues... and if I were building a 1000hp engine, I'd be running carillo's probably... then again im paranoid sometimes . Dunno.. low rpm.. forced induction so high avg cyl pressure as opposed to crazy peak....

I'm far too indecisive to help =P

rskrause

Phil: thanks. I also found http://www.slowgt.com/Calc2.htm#PistSpeed

I am no engineer, but if F = MA I can work this out. The calculator indicates a maximum negative piston acceleration of 2,550g for 6,000rpm with my 5.7" rode and 3.75" stroke. Since 1g = 9.8m/square second:

1. for a 500gm piston+ring package the peak is 12,495 kg/m/sec2
2. 400gm = 9,996

For 7000rpm the peak is acceleration is 3,471g!.
3. 400gm = 13,606 kg/m/sec2
4. 500gm = 17,008

So as was stated, the weight of the piston is a major determinate of rod stress but that in the ranges we are dealing with, rpm seems "more" important.

BTW: isn't kg/m/sec2 a "newton"? It's been >30y since I took physics and that was kinda before metric/SI units.

Rich Krause

More: 1 newton = 0.225lbs. So consider my 500g/6,000rpm scenario. The stress on the rod bolts would also be subject to the weight of most of the rod, so lets double the force figuring ~500gm for the rod. This means that the force is ~25,000n which = 5,600lb. If the rod bolts were 7/16" they would have a combined cross sectional area of 1.2sqin. So why do rod bolts need to be so strong?

Will an engineer help me with this? I must be off by a factor of 10, at least.

Rich

Mindgame

tensile loads

tensile loads


tensile loads

Just thought I'd point out a trend in case it wasn't too obvious.

Well slap my hand and send me to bed without dinner..... why not?
In the context of this discussion, the types of stresses that cause connecting rod failure Mr. Bell's opinion as a mechanical engineer are of some value are they not? hmm...

As for the 'equation'........ The book I have, Automotive Math Handbook by Forbes Aird, shows a 'slightly approximate but widely accepted formula' for piston acceleration as follows:

PaMax = rpm^2 x stroke/2189 (cosa + r/l cosb)

where:
r = crank throw radius (1/2 the stroke)
l = rod center to center distance
cosa = cosine of the crank angle
cosb = cosine of twice the crank angle

Mr Aird goes on to explain, much the same as Mr Bell did, that the greatest inertial loads are seen at TDC where the crank angle is zero. Which simplifies the equation (since the cos of 0 = 1) to:

PaMax = rpm^2 x stroke/2189 (1 + r/l)

He also mentions a theoretical limit of between 100 and 150,000 feet/second^2, for what that's worth.

Academically intriguing.

-Mindgame

My94RedZ28A4

I see two ways to calculate whether or not you can safely run the rods.

1. Talk to your Rod manufacturer, find out what is the max acceleration the rods can handle. If they cannot say, ask them what weight (piston + wristpin) and what RPM they calculate their hp ratings for. Then knowing their max accel (or calculating it, using their weights and rpm's) recalculate yours, knowing the weight of your piston/wristpin combo, and max acceleration, and solve for maximum RPM. That way, you know how fast you can spin what you got...maybe you'll be good to 5700 rpm, and if you can live with that... Or, maybe you'll be good to 6200...

2. Other way (more of a PITA) would be to determine the minimum cross sectional area of the rod (the thinnest part). Then, knowing that, calculate the max strain given the tensile strength of the material the rod is made from. It's a dtretch, but I think you want to pick the strain right before the "deformation point" If I can remember correctly. Anyways, once you know the max strain, then you can solve for either RPM or piston weight based on the formula given for max piston acceleration.

I don't have my materials science books anymore, but that should give you the equations for calculating strain given a certain force, and since f=ma, you already can calculate A, and then use mass given for the combo. The book should also give the constants you need for the particular material used for the rod.

Of course, this will not take into effect any "processes" done to the rod, such as shot peening, anealing, hardening, etc... (making the number smaller than "in real life") however it also will reflect the (ideal case) maximums, whereas I am confident that the rod companies calculate a maximum hp, then apply a safety factor, effectivly lowering the hp rating.

In short, #1 is a lot easier, because I doubt you will be able to convince your rod comany to give you the dimjensions of the rod at it's smallest x-sectional area (and measuring your own would be too inaccurate because of production variance, IMO)

One final note, I am not an engineer (Studied it for 3 years, a few years ago) so please don't make the decision based entirely on what I've said, but it really boils down to force (Tension) of the rod vs. the maximum force the rod can handle "safely"

When I say safely, if you take a piece of metal, and pull (tension) on it, it will temporarily expand, however as long as you do not apply too much force, it will "shrink" back to it's original length. When they run tests, they do pull too hard, but doing so creates a graph that makes it very easy to determine the "point of no return" so to speak.

Now, as I sit here, I also realize that this is not the entire story either...The rod is going to experience many cycles of compression and tension, at a certain frequency, with a certain load. Now you would want to calculate your number of fatigue cycles, given the forces you expect. If I remember correctly, the part is generally considered to be "un-breakable" (fatigue-wise) if it can withstand over 1,000,000 cycles.

I think the only true way to calculate an honest "max force" method, would be to use Finite Element Analysis (CATIA, AutoCad, etc...) and do lots of modeling.

This has gotten pretty long...sorry about the length. In short, I'd look at your finances...If you got the cash to spring for a better set, I'd get them. If you don't, I'd do a basic calculation (#1) and find out how fast you can spin your motor. You might have to spin it at only 5500 RPM to be safe, but that's one way to minimize the expenses, and keep the rods - you'd just need to pick out a different cam/pulley to make your peak HP lower than you initially intended (since as you said, it's really not HP that kills rods, it's acceleration due to RPM and piston combo mass)

OldSStroker

I'm old enough that Issac, himself taught our Physics 101.

A Newton of force is a kg. m/sec^2 or [mass x acceleration].
Funny, but he taught us in British(US) units of pound(force), which we all confused with pound(mass). I still use pounds.


Area of a 7/16 dia bolt is .150 sq. in, so 2 would be .30 sq in. That's only a factor of 4 not 10, Rich. You might have forgotten the (divided by 4) part of the formula. [A= dia^2/4 x PI]

Bolts can be strong enough to take the load, but the cap may want to distort.

FWIW, yes 1 million cycles is generally regarded as the criteria for fatigue strength, I believe the Winston Cup engine of last week's Martinsville winner cycled something well over a million revs in the 3+ hours it ran from about 4700 to 9500(!) twice a lap, let alone practice, qualifying and happy hour. I believe all the rotating/reciprocating parts are junked (or sold) after a WC race just in case a million cycles isn't the magic number for fatigue life. Be careful what used parts you buy.

Also, FWIW, a 6000 hp Top Fuel engine spins about 650 total revolutions under power during a run, and then gets new pistons and rods. Big difference, huh?

rskrause

Stroker: that helps, but could you please clarify a little? What is the load on the rod bolts (keeping in mind that there are two of them) in psi with my hypothetical example of a 600gm rod and a 500gm piston+ring combo at 6,000rpm?

Thanks.

Rich Krause

81ZMouse

If anyone is interested, I have an excel spreadsheet that calculates the max piston speed and acceleration. It allows you to change rod length, stroke, even pin offset. It also outputs the load on the rod in psi, based on the inputted piston, pin, and ring weight.

The program contains a macro that steps the crank/rod/piston through one complete revolution to calculate the speed and acceleration at each degree of rotation. It also has a chart for the velocity and acceleration, along with a visual chart showing the actual rotation of the assembly during the calculation.

Again, if anyone is interested, e-mail me.

Shane

OldSStroker

Assuming your 5600 lb is accurate, and load is evenly divided, 5600/.30 = 18,667 lb/in^2 (psi) which isn't all that much. I'm having trouble calculating how much stress is in a properly torqued (or stretched) rod bolt before the engine opoeration loads it. I'm guessing it's somewhere around 10,000 - 15,000 psi.

A normal ARP 8740 rod bolt has a yield strength if 160,000 psi, so it's not being overworked here. Even 3/8 bolts would only be @ 25,000 psi or so.

I don't think ultimate load is the problem with rod bolts. It's fatigue or improper installation (preload). We had a rod bolt fail after 85.000 miles which is somewhere between 400 and 500 million cycles! It appeared to be a long-delayed fatigue failure under the bolt head.

Physician, huh? Specialty=ER?

treyZ28

rich-
from what i understand billet stetches ESPECIALLY under heat (2 types of forced induction)
I didn't know that when I bought my rods.
I'm sure someone makes a forged rod strong enough

My94RedZ28A4

I believe that everything stretches due to heat, if I'm not mistaken, it's called the "glass transition temperature" point. I could be wrong, but I believe that the glass transition temperature is higher than the heat that rods see in an engine. I wouldn't doubt though, that the tops of the pistons and bottom of the valves do see the glass transition temperature.

There are quite a few things to consider when looking at rods:
1. Material -- Find out the exact alloy used, as this is big factor in how much it can handle.
2. "Processing" -- annealing, shot-peening, polishing, heat treating, carbonizing, anodizing, etc... -- These also can dramatically affect the stress levels of the un-processed alloy used above.
3. Design -- everybody knows that sharp corners are bad, and that the I-Beam (or H-Beam) is the best in terms of strength per weight. Also though one rod may have better webbing, larger radii, thicker walls, etc...
4. Manufacturing -- Forged is better than cast, but who's die is better, and how many tons/in^2 are they using to forge with? Do they need to work the rod much afterwords (grinding, machining, etc...) indicating a bad die, or is there minimal machining, which would stress the metal less, leading to a stronger rod.

In all honesty, for many of the things above, you'd need to compare different rods right next to each other, which is pretty much impossible to do, unless you want to buy a whole bunch of rods. I personally would check some back issues of Hot Rod, car craft, even the ford 5.0 mags, and look at the rod comparisons, and see their opinions.

rskrause

How did you guess ER (that's right)? Or did I mention it somewhere? I actually work at a Med School. My clinical practice is ER though.

As I suspected (though my math was off) it doesn't seem as though the stress on rod bolts is anywhere near their yield strength. I guess the fatigue factor plays a big role....

Rich Krause

Rich Krause

OldSStroker

Steel, or even nodular iron connecting rods change length due to heat because metal grows in all directions with any temperature change. The coefficient of thermal expansion of steel is about 6 millionths of an inch per inch of length per every deg. F. Assume a 70F room temp when the engine was built and measured.

If the rod temp gets to 300F, which is unlikely because the oil is usually 200-250F max, a 6 inch rod "grows" 6 x (300-70) x 6/1000000 or just under .010.

Now, the block is also heating up, so it too is growing at about the same rate, so the deck height is rising as fast as the rod is growing. It's about a push, and not a concern.

Heat treated steel parts like cranks, piston pins, rods, etc. are designed to operate at temperatures lower than the tempering temperature they received during the heat treating process. About 400F would be the lowest tempering temp seen by the hardest parts, like the rollers on lifters or rocker arms or maybe piston pins. If the parts operated at higher temps, they are effectively being tempered (softened), so that's not where they run.

Aluminum expands about 3 times as fast per degree as does steel, so piston to cylinder wall clearance gets tighter as the piston heats up and grows. That's one of the reason "slap" goes away when the engine get up to temp.

BTW, The bore grows also with heat. Some folks think holes get smaller with heat, but they actually get bigger.

Valve heads necessarily have a much higher minimum operating temp, especially exhaust valves, so they are made from materials which can stay strong at elevated temps.

My $.02

OldSStroker

I have lots of interests, Anaphylaxis being just one.
.
.
.
.
.
.
.
.
.
.
.
.
Not really. The internet is a wonderful tool, Doc.

http://www.emedicine.com/emerg/topic25.htm

96TAWS6

One of the properties of steel is that it has an endurance limit stress. That is, stresses below this limit will not cause fatigue failure no matter the number of cycles. Aluminum, copper, and other face-centered cubic metals do not have an endurance limit. That is to say there is no cyclic load that the material can withstand indefinitely. The best ways to combat fatigue failure are to obey the endurance limit, polish the rods, and/or shot peening. Polishing rods helps eliminate stress risers, which are the source of small cracks that grow under cyclical loading. Shot peening causes plastic (irrecoverable) deformation on the surface that results in better resistance to fatigue crack inititiation and can develop residual compressive stresses, which are always good when working against tension.

That being said, if the load is indeed roughly 20,000 psi then the rods are ok. The lower bound (conservative estimate) for the endurance limit is: .25x(ultimate tensile stress). On the other hand, if you know the grade of steel you could probably find fatigue graphs on the internet.

Harold

rskrause

Thanks for increasing my knowledge of this subject.

Rich Krause