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Inherant critical crankshaft harmonics

General engine tech -- Drag Racing to Circle Track

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Ed Farrell

Inherant critical crankshaft harmonics

Post by Ed Farrell » Sat May 02, 2009 11:35 pm

I read an article many years ago about crankshaft harmonics and the inherant critical RPM's where they occured. It was wrtten so a layman hotrodder could understand it. My interest then and now is an inline six cyl engine. According to the article there is a huge harmonic at 5000 RPM. Back in the '50's we couldn't turn a Jimmy much over 5300 due to valve spring availability. I had trouble keeping the flywheel tight and even had the balancer come off the engine once. Today we go right through that RPM. My question is where are the next critical rev points particularly over 6000 revs. I want to be able to drive right through them. I am sure that the points vary somewhat with piston/crank/rod weights. Some of the Hudson fellows I know build their motors to only turn 4700 to avoid this as they have cast cranks. The Jimmy has a forged steel crank. Thanks for your thoughts, Ed Farrell

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Re: Inherant critical crankshaft harmonics

Post by SchmidtMotorWorks » Sun May 03, 2009 10:00 am

Can you post a picture of the crank you are interested in?
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Re: Inherant critical crankshaft harmonics

Post by BrazilianZ28Camaro » Sun May 03, 2009 10:27 am

Here in Brazil we have plenty of Chevy Opalas that was factory equiped w/ 230ci and after 250ci six cylinder engines.

The 250ci units that are modified to drag racing spin 8000 rpms and 430 hp w/ cast crank with some compromises like durability.

They need a good harmonic damper and a lightweight flywheel. Lightwheight pistons and rods are mandatory.

The twin turbo versions push 1200hp, but they have billet cranks, main girdle...

I believe that you get a crank w/ smaller stroke and a lightweight flywheel, the harmonics can be considerably reduced.

Hope this help.
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Re: Inherant critical crankshaft harmonics

Post by D.Cecere » Mon May 04, 2009 10:28 am

I went through this problem with the Australian 265ci Chrysler Hemi in-line six cylinder. Being a relatively long stroke cast iron crankshaft (3.68") and such a long item end to end the torsional harmonics going through these engines in unbelievable.
I built a few race versions of these engines and they made close to 400hp at 6500rpm pretty easily, but keeping the flywheel on and stopping the cranks from breaking required some work. A large volcanised harmonic dampener at the front of the engine some 7" diameter but about 3/1/2" deep(made by a local specialised shop here in australia) would stop the cranks from breaking.
The only way I could keep the flywheels on these things was to drill/ream the flywheel flange and fit 6, 1/2" locating dowels which where interference fit in the flange by 0.004" and on the flywheel by about 0.0005". This would mean drawing the flywheel on with the bolts (ARP hardware) then taking the bolts out and loctite them back in. This has worked for me every time now....I tried many other ways, ie 2 dowels, 3 dowels and just broke the dowels off and rattled the bolts loose after about 4 passes down the track.
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bill@bhj

Re: Inherant critical crankshaft harmonics

Post by bill@bhj » Mon May 04, 2009 11:23 am

You are correct in that Inline 6's have a large 3rd order, 1st mode, torsion peak near 5000rpm, although this does vary a bit with engine size. Smaller engines have stiffer / lighter cranks and the peak is a little higher in the rpm range, larger engines the opposite. In the case of the 3rd order peak at 5000, the resonant frequency of the crank would be 3 x 5000 / 60 = 250 cps. (Hz). The next higher peak in the rpm range would be 2.5th order occuring at 250 cps x 60 / 2.5 = 6000 rpm. The next after that would be 2.0nd order occuring at 250 cps x 60 / 2.0 = 7500 rpm. These peaks would be of significant height, but quite a bit smaller than the 3rd order, usually 1/2 - 1/3.
I have seen the 3rd order peaks in these engine go over 2.0° twist p-p, that is 1° degree each way from neutral. Generally, the OEM's do not allow more than about 0.4° p-p for long term durability on an I6, and that is at OEM power levels.

A good damper, properly tuned, is about the only way to control these peaks and keep the torsion stress below destructive levels. The damper has a secondary effect in that it also lowers the rpm placement of the peaks by a few hundred rpm, so that the 3rd order peak with the damper may go down to 4500 rpm or so as well as being reduced in twist amplitude. All the 1st mode peaks would drop correspondingly.

Running a light flywheel will raise the frequency of the whole crank system and the placement of the peaks then would all go up a few hundred rpm. The high torsion stress point that is normally near the rear main would migrate forward a bit. That may put it forward to the 6th main or worse yet, into one of the crankpin webs that may be weaker than the mains.

Light rods and pistons are beneficial in that they reduce the recip torque loads, especially at the high rpms.

All this is mathmatically predictable, but has to start with a good math model of the crank rotating system.

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Re: Inherant critical crankshaft harmonics

Post by RW TECH » Mon May 04, 2009 9:12 pm

Bill,

Can you do modal analysis or do you know anyone who has the software that will do this?

Thanks.

bill@bhj

Re: Inherant critical crankshaft harmonics

Post by bill@bhj » Mon May 04, 2009 10:18 pm

RW Tech
I have software that does a full torsional analysis identifying modes, orders, and amplitudes based on a mass - elastic crank model and a cylinder pressure curve. It can include tuned or viscous dampers.
It is basically a computerized version of the classic BICERA - Ker Wilson work.
It is not FEA based and does not include bending, it assumes a rigid block.
Bill

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Re: Inherant critical crankshaft harmonics

Post by Ron Golden » Mon May 04, 2009 11:16 pm

Bill,

I'm in the same boat as Ed Farell with my 321" Jimmy. I used 308" Hudson rods, JE pistons and pins and saved 1.25 lbs/cylinder. I felt the crank counter weights needed to be cut some amount to compensate for the reduced piston/pin/rod assy but couldn't find anyone that knew how much to cut them. I've limited the rpm to 6000 but not sure that's correct.

Ed and I have discussed this several times but we're still in the dark.

Ron

rmcomprandy

Re: Inherant critical crankshaft harmonics

Post by rmcomprandy » Tue May 05, 2009 11:35 am

YEARS ago when we used to run the 292 Chevy truck six on the oval tracks, we needed to use the LARGE diameter, 8" x 1 3/4" V-8 "mall" dampner on the front to help them live.
Now I kinda know why ... lol.

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Re: Inherant critical crankshaft harmonics

Post by Dave Flanders » Tue May 05, 2009 12:06 pm

Is there any way (shadetree method) to determine just where in the rpm range bad harmonics are likely to occur? How much do the crankshaft counterweights affect it if at all?
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Re: Inherant critical crankshaft harmonics

Post by MadBill » Tue May 05, 2009 12:17 pm

I don't work with imports, but I have looked at Honda and Subaru cranks, and they have wonderfully sculpted counterweights, one on each side of each throw. If you were to cut up a scrap shaft from such a unit, you could create a single cylinder assembly, check its balance factor and use the results to guide your efforts...
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Re: Inherant critical crankshaft harmonics

Post by SchmidtMotorWorks » Tue May 05, 2009 11:12 pm

I have software that does a full torsional analysis identifying modes, orders, and amplitudes based on a mass - elastic crank model and a cylinder pressure curve. It can include tuned or viscous dampers.
It is basically a computerized version of the classic BICERA - Ker Wilson work.
Is that a commercial software or something unique? Sounds very useful.
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bill@bhj

Re: Inherant critical crankshaft harmonics

Post by bill@bhj » Wed May 06, 2009 10:30 am

They are classic works on torsion done mostly during and for WWII.
Here's a short list. Some can be hard to find now except in big libraries.

1. Practical Solution of Torsional Vibration Problems, Volumes 1,2,and 4
W. Ker Wilson
Chapman and Hall Ltd.

2. A Handbook of Torsional Vibration
E.J. Nestorides of the B.I.C.E.R.A. Research Laboratory
Cambridge Press
BICERA = British Internal Combustion Engine Research Association

3. Mechanical Vibrations
J.P. Den Hartog
McGraw Hill

BICERA was a group of people from most of the British engine mfgrs who were brought together and sponsored by the government to study and find fixes for torsion problems in the marine and aircraft areas. This was done under the gun, so to speak, as these problems were really impacting the war effort.
Ker Wilson was a major Brit contributor and wrote a series of 7 volumes
of his own. Nestorides was more the compiler of all the collected work.

The USA did the same thing for the same reasons, and the results were published by SAE.
This is still available from SAE I believe, as a photocopy for a fee.
Den Hartog was a Naval guy and was a major contributor to the USA effort, then wrote his own book. Still available, used as college textbook.

The Hartog book is fairly easy to get through with some math and is a shorter overview.
The BICERA - Wison books really get into detail with math and technique.

There are more, I have a longer list if you're interested.

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Re: Inherant critical crankshaft harmonics

Post by SchmidtMotorWorks » Wed May 06, 2009 3:21 pm

Bill thanks for the information you have posted!

what approach would you recommend for people to diagnose crank problems? Is an FEA approach advantageous?

For example recently there have been a lot of breaks in the 1st rod arm of crankshafts used in nitromethane fueled racing engines. Since the least accumulated torque is at the front of the crank and the 1st arm is the thickest vibrations seem to be a possible root of the problem. (they don't have any vibration reducing mechanisms)

Another case I'm curious about is large displacement V12 engines; 5 inch bore spacing and 4.625 stroke. Would there be an advantage or disadvantage to an even firing pulse 60 degree block or uneven pulse 90 degree block?

As a general rule, will the vibration issues be better in V8 crankshafts where there is choice between counterweight layouts that:

1. Counterweights are distributed heavily toward the ends of the crank and do not have center counterweights (resulting in a lighter crank)

or

2. Counter weights more evenly distributed to all areas of the crank, with center counterweights (resulting in a heavier crank).
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Re: Inherant critical crankshaft harmonics

Post by williamsmotowerx » Wed May 06, 2009 3:31 pm

An accelerometer with a scope is best way to determine resonance.

You can view the spikes, do the math, and determine RPM. there is an order for everything.. cam.. crank.... until accellerometer goes flaky and math won't give you the order... then you have your resonance.

Sometimes you accelerate through it, or never reach it. But when your engine spends time there... things break.

Adding weight removing weight will move the resonance around to where it won't affect problem area.

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