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Why are these holes made in crankshafts?

General engine tech -- Drag Racing to Circle Track

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Re: Why are these holes made in crankshafts?

Post by Tuner »

Kevin Johnson wrote: Wed Jun 09, 2021 9:15 pm
j-c-c wrote: Wed Jun 09, 2021 6:58 pm ...

Whatever that is, I can't read/view it in that format.

Can you summarize if possible how it addresses how any solid cylinder has significantly greater torsional strength when its drilled, and effectively becomes a tube?
I just double checked by downloading an image. They are of sufficiently high resolution to read if you download them and magnify them on a PC screen. If you are trying to use a smart phone -- I can see how that would be difficult.

These are pages from C.F. Taylor. The text is available new. If you have a university near you with an engineering program they might have it in the stacks. Mine had it but removed it. Check the library catalog before wasting a trip.

https://mitpress.mit.edu/books/internal ... d-volume-2
If you right-click on those images and choose "Open Link In New Tab (or New Page)" when they are on the new tab or page you can zoom in and read them.
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Re: Why are these holes made in crankshafts?

Post by BILL-C »

PackardV8 wrote: Mon Jun 07, 2021 10:28 am IIRC, the early 427" Ford race engines had forged steel crankshafts with those lightening holes drilled and then plugged. One theory was to keep oil out of there; reduce windage. Since Ford was doing extensive dyno testing in those days, may have been a thing.
Actually the plugs are to keep the oil "in". The round cavity intersects the oil feed holes. The cavity sees oil pressure. The Boss and Tunnel port 302's are the same way. Some of the cranks use threaded plugs and others use cup plugs with retaining clips.
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Re: Why are these holes made in crankshafts?

Post by CamKing »

Kevin Johnson wrote: Mon Jun 07, 2021 9:28 am There have been some epic discussions about this. I purchased a surplus text from the Library of Congress that contained the original published research from 1930s Germany wherein various crankshaft designs and materials were tested. It is the original work that was translated by the US Government and provided to their researchers (and was cited by C.F. Taylor). That design and others were part of the experiment.

The translated work (in Taylor) and numerous other engineering references were pitched by my university library a number of years ago.

So, yes, some companies use(d) it as lightening/balancing strategy which of course it plainly is. Other companies with well educated employees and retained institutional knowledge will know about the 1930s research. Now this type of analysis is done by computer simulation.

This is the German researcher:

https://de.wikipedia.org/wiki/Karl_L%C3%BCrenbaum

Cited in:

https://mitpress.mit.edu/books/internal ... d-volume-2

Here is the original work (in German):

Luerenbaum pp 128-129 (1).JPG

Luerenbaum pp 130-131.JPG
I'm well acquainted with Taylor(required reading in the Jones family), but this always bothers my screwed up mind.
It reminds me of what Michelangelo said, when asked how he sculpted his works of art out of simple pieces of stone. He said that finished piece was always inside it, I just removed what didn't belong.
When they say that boring a hole, increases the strength of a part by adding the surface of the bore, my first thought is, isn't that bored surface already there, just filled with metal ?
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Re: Why are these holes made in crankshafts?

Post by j-c-c »

I have yet to closely review the linked supporting argument for the holes. I suspect the final outcome will be, the lightening holes reduce the loading on the crank in many forms, and making for a better end solution. My original contention was primarily with the statement "that drilling pins and mains increased torsional strength of the crank". Pin drilling as method to increase torsional strength is IMO still questionable at this moment for me. Main drilling of the crank to gain torsional strength is still a bridge too far for me. Now I suspect we might be arguing semantics here, in that the renowned research paper might have discovered/reported an improvement in the "operational" (my word) strength of the crank, but I can't yet accept that is due to the claimed torsional strength increase with a drilled center hole. The paper is still physically hard to read for me, but I will plow thru it, as it seems to be rather credible if it has stood the test of time so far.
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Re: Why are these holes made in crankshafts?

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Re: Why are these holes made in crankshafts?

Post by strokersix »

I think the missing qualifier is equal amount of material. If you just take a shaft and bore it out it won't be as strong or as stiff as it was when solid. If the shaft, however, is larger to begin with and you bore it out to match the weight of the smaller solid shaft then yes, hollow will be stronger and stiffer and more fatigue resistant that the solid shaft.

As I understand.
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Re: Why are these holes made in crankshafts?

Post by Kevin Johnson »

strokersix wrote: Fri Jun 11, 2021 3:42 pm I think the missing qualifier is equal amount of material. If you just take a shaft and bore it out it won't be as strong or as stiff as it was when solid. If the shaft, however, is larger to begin with and you bore it out to match the weight of the smaller solid shaft then yes, hollow will be stronger and stiffer and more fatigue resistant that the solid shaft.

As I understand.

See Fig. 11-36 Dimensions are given.
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Re: Why are these holes made in crankshafts?

Post by strokersix »

Yea, it's way more complicated than my simple example. Bending, torsion, harmonics, friction, thermal strains, etc, etc. all through a complex shaped object simultaneously.

I just shared that simple illustration for those who questioned why less material might make a stronger crankshaft in the simplest way I could.
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Re: Why are these holes made in crankshafts?

Post by strokersix »

Another way to look at it:

In my admittedly oversimplified example a hollow shaft makes better use of the material compared to a solid shaft. Material at the center of solid isn't doing anything except adding weight.

The solid material in the middle of a crankpin isn't doing much either except adding weight and highlighting the highest stress area in the fillet overlap. Hollow crankpins make better use of the material and distributes the strains more evenly. Result is a better crankshaft. Perhaps more costly.

I'm retired engineer from an Ag equipment OEM. This reminds me of Ag equipment frames. Analysis or test shows cracking the first response is to add material. Sometimes it's better to take a step back and remove material to better distribute the strains.
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Re: Why are these holes made in crankshafts?

Post by j-c-c »

strokersix wrote: Fri Jun 11, 2021 4:48 pm Another way to look at it:

In my admittedly oversimplified example a hollow shaft makes better use of the material compared to a solid shaft. Material at the center of solid isn't doing anything except adding weight.

The solid material in the middle of a crankpin isn't doing much either except adding weight and highlighting the highest stress area in the fillet overlap. Hollow crankpins make better use of the material and distributes the strains more evenly. Result is a better crankshaft. Perhaps more costly.

I'm retired engineer from an Ag equipment OEM. This reminds me of Ag equipment frames. Analysis or test shows cracking the first response is to add material. Sometimes it's better to take a step back and remove material to better distribute the strains.
Sorry In red above, is IMO technically a false engineering statement, unless "center" is defined as a a line between two single points .

Additionally, as I mentioned earlier, there is a difference I suspect here between drilling pins and mains, and really have no interest in exploring in depth all the stress ramifications of pin drilling. My main point, and I think easier to disprove is the statement made about drilling mains "increasing torsional strength" This has never been a discussion about sizing a hollow main with all the variables at play, regarding friction, torsional strength, stress risers, bearing area, weight reduction, grain flow, bending, ete, etc. Its been about drilling a existing mains.

The only counterpoint that might have application here is the torsional fatigue limits might be increased with all the drilling mentioned, in that the crank among other things will twist along a longer axis with all the drilling, increasing fatigue life but that does have a downside ( a flexing crank?), and not sure how its increases "torsional strength". Additionally, the drilling needs to very well thought out and determined.
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Re: Why are these holes made in crankshafts?

Post by MadBill »

j-c-c, you may have something in that last paragraph. It's possible in some cases that drilling the crankpins would result in a more uniform torsional stiffness down the length of the crank, effectively reducing "stress risers" (stiff spots). :-k
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Re: Why are these holes made in crankshafts?

Post by digger »

torsional strength appears to be in terms of number of fatigue cycles not ultimate strength.

I haven’t done my own calculations but removal of material can provide better stress flow and reduce stress concentrations (stress risers) but it can still be less stiff and weaker in a ultimate failure (I.e non cyclic)
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