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Drilling a really long hole

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  • Rolland-Christians
    replied
    I want to thank all of you for your input-Gives me a lot more information when dealing with the engineers on this job. My primary hesitation on bidding this part of the job (fabricating and boring a shaft) is of course liability, I am too small a company to be able to afford mistakes. The best I can do is to point these people in the proper direction, my job really starts on assembly, I am responsible for all the alignment, placement of the machinery, and proper base materials, etc. This is also a government owned facility, and there is a lot of red tape and many people to go through for each change. I will advise what the final solution is when it is made. (The other thing about mechanical tubing is the keyways that have to be cut on the outside, so am still not sure what the final material will be.) Again, I thank all of you for your input, it has been very helpful. Rolland

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  • J Tiers
    replied
    Originally posted by Evan
    You already know the answers to those questions Jerry. You also know that driveshafts are tubular, not bar stock. For a simple quick calculation of the required torque transmission capability calculate the area of a 4.5 inch shaft with a 3.5 inch ID. Then take the shear strength of that amount of material and compare to the probable torque rating of a 1200 hp diesel at 2000 rpm.

    By a basic rule of thumb you can count on ductile steels to have a shear strength at least half the ultimate tensile strength. For mild steel that is around 30ksi. You can do the rest of the math. A half inch wall is grossly overrated even allowing for a ten times safety margin.
    Very silly....... that straw man was not the point.........

    The point was that:

    1) the equipment mentioned was obviously not suitable for a proper job of boring the hole.

    2) make-shift boring means might likely produce stress-risers in the form of gross irregularities in the bore, which is not a good thing when dealing with a shaft at this power level, especially one with the very good possibility of reversing stresses. AND pressure in the bore besides.

    3) Not all jobs are worth taking on...... there are specialists for a reason.

    The exact likelihood of a problem is less important than the obvious fact that it's a PITA to do, might have a bad result, and can be easily and suitably done by a company with the proper tools.

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  • Evan
    replied
    Hi Evan-That was my original suggestion too, but the engineers are afraid it will take too long for the air to reach full pressure on the clutch with the extra volume
    That is why you stick a length of pipe down the center with occasional disks welded to it to prevent whipping. Caps or plugs at each end with the pipe fitted through the caps and the volume problem is solved.

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  • Rolland-Christians
    replied
    Originally posted by lwalker
    I'm confused, are you saying Ford made their own steel up there?
    Ford owns taconite mines and processing facilities on the Iron Range-They make taconite pellets and sell them to the highest bidder, not sure how much of their own steel they get back-Rolland

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  • Rolland-Christians
    replied
    Originally posted by Evan
    Seems to me that thick wall pipe would serve just as well.
    Hi Evan-That was my original suggestion too, but the engineers are afraid it will take too long for the air to reach full pressure on the clutch with the extra volume-Now they are looking at moving the air clutch to the fan shaft, and then I won't have to bore the driving shaft, it can be solid or mechanical tubing-Each solution brings up another problem-but if it were easy, anyone could do it! Thanks, Rolland

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  • Evan
    replied
    You already know the answers to those questions Jerry. You also know that driveshafts are tubular, not bar stock. For a simple quick calculation of the required torque transmission capability calculate the area of a 4.5 inch shaft with a 3.5 inch ID. Then take the shear strength of that amount of material and compare to the probable torque rating of a 1200 hp diesel at 2000 rpm.

    By a basic rule of thumb you can count on ductile steels to have a shear strength at least half the ultimate tensile strength. For mild steel that is around 30ksi. You can do the rest of the math. A half inch wall is grossly overrated even allowing for a ten times safety margin.

    Leave a comment:


  • J Tiers
    replied
    Originally posted by Evan
    Perhaps, perhaps not. Not many driveshafts in that sort of application are made from bar stock. Have a look on a large hiway tractor. Same application but much more demanding. The drive shafts are tubes.
    Ho-hum..... Also on lots of trucks, cars, etc. Old, old old OLD news.

    Demanding? In what way? At what power level? What if it breaks?

    How is the application LIKE a truck driveshaft? How is it UNLIKE a truck driveshaft?

    What is the peak torque? What is the torque "ripple"?

    Answer those questions, and the problem begins to shape itself.

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  • Evan
    replied
    However, I was not arguing about the loss of strength, Evan, that is irrelevant, the design is set, and the task is to MAKE the part.
    Perhaps, perhaps not. Not many driveshafts in that sort of application are made from bar stock. Have a look on a large hiway tractor. Same application but much more demanding. The drive shafts are tubes.

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  • J Tiers
    replied
    Originally posted by Evan
    No kidding? The point is to show how little the core contributes to the strength. The torque capability will be directly proportional to the bending strength, which is what I have a program to calculate.
    The core contributes NOTHING to torsional strength or stiffness at zero radius, and contribution increases as you go outwards.

    However, I was not arguing about the loss of strength, Evan, that is irrelevant, the design is set, and the task is to MAKE the part.

    I was more addressing the issue of someone's comment that the finish wasn't important. While the core contributes little strength even at that radius, it IS stressed all the way in to that bore, and a stress riser in there may have bad effects.

    Especially when a "flywheel" is coupled to a nearly megawatt piston engine. And when the "flywheel" is a device which inherently has periodic vibrations from airflow, obstructions etc.... i.e. a big fan. It is similar to a large aircraft engine application.

    So make-shift methods of boring the hole may give a worse finish than an actually suitable way of doing the job, and those finish issues might actually affect the operation negatively.

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  • Peter S
    replied
    Just for anyones interest....I have a book by Joseph Serafin (ex-Ford Motor Co.) which explains how he made accurate deep holes, I found it interesting anyway.

    See post # 13 on this thread:

    http://bbs.homeshopmachinist.net/showthread.php?t=22874
    Last edited by Peter S; 09-03-2010, 09:30 AM.

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  • becksmachine
    replied
    Yes, it is very doable.

    As for doing it in an engine lathe, it would be a long haul to go that deep with a gun drill. Other considerations aside, you really couldn't feed it manually with the tailstock, consistent and continuous chip formation is key to getting the chip to eject itself so you don't need to peck, even with the high pressure coolant.

    Oh and a coolant pump that would supply about 5-20 gpm at 500-1500 psi and the means to contain this flow when it is squirting out of the hole.

    Rich has it right, a piece of cake for the people that are set up for it, but somewhat involved for a one of.

    Dave

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  • lwalker
    replied
    I'm confused, are you saying Ford made their own steel up there?

    Originally posted by Rolland-Christians
    <snip>...The plant in Mn was at Eveleth, worked in the Fairlane and Thunderbird plants (Taconite processing plants, ...

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  • Evan
    replied
    Unless I am unable to read today, the part is a SHAFT, which probably needs little in bending, and plenty in torque capability.
    No kidding? The point is to show how little the core contributes to the strength. The torque capability will be directly proportional to the bending strength, which is what I have a program to calculate.

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  • J Tiers
    replied
    Unless I am unable to read today, the part is a SHAFT, which probably needs little in bending, and plenty in torque capability.

    However, the issue is not the design, he apparently has been told that, and much more how that hole is getting down it.

    That having been said, a 1200 HP Cat diesel, and a large fan..... I don't think that is a good place to have a rough, and particularly not a "scored" inside diameter. Torque-wise vibrations might cause those scores to become stress raisers, and lead to a fracture. With a 1200HP fan, that is probably not a good idea, it makes a spun-off chuck in the shop seem like a dropped nail paring.......

    So half-way methods are not the very best here..... Someone with a 25' bed machine could deal with it in short order, especially if they have an oil-fed drill. Not all jobs that darken the door are ones you want. or that you should do.

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  • Evan
    replied
    I did a quick calculation of the bending strength of a solid bar that size vs a pipe with 0.5 inch wall thickness.

    The solid bar with simple supports at each end and with a 2000 lb distributed load located evenly starting 20 inches from each end will produce a static deflection of 0.111" at the centre.

    The 1/2 inch wall pipe with the same loading deflects an additional 0.075".

    All that is required is a couple of end caps and a few spiders tacked to a suitable piece of air pipe that runs down the centre of the main shaft pipe. Seamless DOM should suffice for the shaft just fine.

    It will also weigh half as much which will make the stiffness to mass ratio higher.
    Last edited by Evan; 09-03-2010, 12:50 AM.

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