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  1. #21
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    Hi Terry,

    My dyno used 4 inch Dia 4140 flywheels. They slid onto the shaft and were driven by a 3mm pin via a slot on one end of the flywheel. Did this as a safety weak link, in case of engine failure, sudden stop of engine would shear the pin and flywheel would spin on shaft and not tear anything else apart.

    I started with 1 inch long 1/4 keysteel as a link between the engine and shaft, they twisted into some fancy looking metalwork in one run. Replaced it with our usual 1/4 flex shafts.

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    Quote Originally Posted by Terry Keeley View Post

    Thanks for checking. The 0.35 is just the example given, their wheel is 350 mm in diameter and is turning at 0.3 rad/sec. You got the same O/D velocity I did tho at 239.4 m/s. Do you end up with 184.4 N/mm2 or 26,700 psi?

    Again, it seems way too low considering Machinery's Handbook said I was way over-revving it with 60 ksi material.
    Thanks for making me go to the source page to double check your other stuff, too. I got the same as you for radial stress in the flywheel.

    Then the best thing I can tell you after looking over the engineer's edge page is that the formula we just went over only accounts for radial stress in the flywheel due to the speed of spinning. One would assume that some kind of torque will need to be applied to the flywheel to achieve this, and that will make your problem a combined load type problem, and your 184 MPa is not the only load on the disk.

    The other likely possibility is that Machinery's Handbook is applying a large safety factor due to the dynamic nature of the problem. Again, our magic number 184 MPa or 27 ksi only applies to a perfectly balanced wheel. Safety factors of up to 8 are not unheard of in dynamic loading situations.

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    Quote Originally Posted by Kenre View Post
    Hi Terry,

    My dyno used 4 inch Dia 4140 flywheels. They slid onto the shaft and were driven by a 3mm pin via a slot on one end of the flywheel. Did this as a safety weak link, in case of engine failure, sudden stop of engine would shear the pin and flywheel would spin on shaft and not tear anything else apart.

    I started with 1 inch long 1/4 keysteel as a link between the engine and shaft, they twisted into some fancy looking metalwork in one run. Replaced it with our usual 1/4 flex shafts.
    Thanks, I'm quite sure now I'll be OK. Found some more info in the Machinery's Handbook:

    "Centrifugal Stresses in Flywheel Rims.

    In general, high speed is desirable for flywheels
    in order to avoid using wheels that are unnecessarily large and heavy. The centrifugal
    tension or hoop tension stress, that tends to rupture a flywheel rim of given area,
    depends solely upon the rim velocity and is independent of the rim radius. The bursting
    velocity of a flywheel, based on hoop stress alone (not considering bending stresses), is
    related to the tensile stress in the flywheel rim by the following formula which is based on
    the centrifugal force formula from mechanics.

    V = square root (10 × s) or s = V squared ÷ 10


    where V = velocity of outside circumference of rim in feet per second, and s is the tensile
    strength of the rim material in pounds per square inch."

    This for a one piece wheel which is what I'm using.

    My surface speed at 30K will be 785.4 ft/sec so the minimum tensile strength with a factor of 10 works out to 61.7 ksi, 4140 in it's various conditions comes in well above that.

    Going to couple the motor to the main shaft with a short length of 1/4" flex cable, that plus a one way bearing and the clutch should give enough of a fuse if the motor locks up at 30K.

    What's the max rpm you turned? How wide was your biggest 4" wheel?

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    Quote Originally Posted by BoxcarPete View Post
    Thanks for making me go to the source page to double check your other stuff, too. I got the same as you for radial stress in the flywheel.

    Then the best thing I can tell you after looking over the engineer's edge page is that the formula we just went over only accounts for radial stress in the flywheel due to the speed of spinning. One would assume that some kind of torque will need to be applied to the flywheel to achieve this, and that will make your problem a combined load type problem, and your 184 MPa is not the only load on the disk.

    The other likely possibility is that Machinery's Handbook is applying a large safety factor due to the dynamic nature of the problem. Again, our magic number 184 MPa or 27 ksi only applies to a perfectly balanced wheel. Safety factors of up to 8 are not unheard of in dynamic loading situations.
    Thanks again for checking, found some more info in the Machinery's handbook saying a factor of 10 is common.

    I'm pretty confident if this thing is properly aligned and balanced it will stay together but I'll build a good scatter shield around it just in case!
    Last edited by Terry Keeley; 01-25-2020 at 07:36 AM.

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    Quote Originally Posted by Terry Keeley View Post
    In general, high speed is desirable for flywheels
    in order to avoid using wheels that are unnecessarily large and heavy. The centrifugal
    tension or hoop tension stress, that tends to rupture a flywheel rim of given area,
    depends solely upon the rim velocity and is independent of the rim radius.
    The bursting
    velocity of a flywheel, based on hoop stress alone (not considering bending stresses), is
    related to the tensile stress in the flywheel rim by the following formula which is based on
    the centrifugal force formula from mechanics.

    V = square root (10 × s) or s = V squared ÷ 10


    where V = velocity of outside circumference of rim in feet per second, and s is the tensile
    strength of the rim material in pounds per square inch."
    IF this were true, think about the hoop tension stress at the Equator of the Earth. The rim velocity is over 1000 mph.
    This seems to be a misleading interpretation... or something.

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    Terry
    You are good now, go for it.

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    3d3t
    The earth has gravity keeping the surface in place. Note the space shuttle launches from as near equator as possible to leverage the centrifugal forces.

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    Terry, From memory Length of the longest flywheel was about 3 inches.

    Max Rpm i seen with a .67 was about 26000. Your getting me keen on building another Dyno! Or i could send a .90 over for you to test.

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    Quote Originally Posted by Kenre View Post
    Terry, From memory Length of the longest flywheel was about 3 inches.

    Max Rpm i seen with a .67 was about 26000. Your getting me keen on building another Dyno! Or i could send a .90 over for you to test.
    Thanks, that gives me confidence mine will stay together, just havta find the "right" 4140 with certs (it's supplied in so many different conditions).

    I was wondering if you ever got to test one of your 90's, would be interesting to see how much power it made. I'm starting with 67's and will do 80's next for my straight away program. Remember off hand where the 67 made it's peak HP? CMB says 27K but I bet that's just a marketing "guess".

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    Pay attention to the speed of the "balls" and/or "needles" inside your bearings. You might have to go carbon bearings with special lubricants (maybe air under high pressure)?? The old automotive vari-speed McColloch blowers would destroy their ball bearings because of over-speeding the balls capabilities.

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    Just one note, ultimate tensile strength is NOT the only factor to consider. Exceeding the uts is in fact probably the least likely failure mode, seems more likely it would be fatigue, vibration, loosening of some attachment(s) or in the case of a catastrophic failure, caused by a flaw such as a cold lap, lamination, or other discontinuity from the preparation of the billet and or the drawing of the bar.

    A cert only pertains to the chemical composition of the metal, other factors such as grain size, extent of cold work, freedom from defects are not covered.

    It’s a super common mistake to rely on just the cert to assure quality. Most of the time there is a sufficient safety factor, and fortunate correlation between a good cert and other quality factors that such a blunder isn’t catastrophic.

    Quality is way more that a good certificate of analysis

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    Quote Originally Posted by Turnaround007 View Post
    Pay attention to the speed of the "balls" and/or "needles" inside your bearings. You might have to go carbon bearings with special lubricants (maybe air under high pressure)?? The old automotive vari-speed McColloch blowers would destroy their ball bearings because of over-speeding the balls capabilities.
    Thanks. The bearings and lubricating them will be a very important part of the project. Going with WIB "Swiss Made" 6202's for the 15 mm shaft, we use these in the motors and they hold up very well. Might even try some hybrid ceramic bearings instead. Probably use a pair of 6204's (20 mm bore) for the inertia wheel, doing some more digging it'll probably end up closer to 20 lbs. than the 8 or so I first thought.

    Kinda concerned about the one way clutch bearing, the best I've been able to find is from CSK: CSK202PP(X) by Boca Bearings :: Ceramic Bearing Specialists

    I know from experience most bearings have lower rpm ratings than what works in practice but these are only rated at 8K. Might havta change it often...

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    Quote Originally Posted by Turnaround007 View Post
    Pay attention to the speed of the "balls" and/or "needles" inside your bearings. You might have to go carbon bearings with special lubricants (maybe air under high pressure)?? The old automotive vari-speed McColloch blowers would destroy their ball bearings because of over-speeding the balls capabilities.
    what are "carbon bearings"? ceramics such as silicon nitride are used in all ceramic and hybrid bearings..not sure what carbon bearings are.

    hybrids (steel race, silicon nitride rolling elements) can go up to 200k without conventional lube under certain conditions. don't think they use "air under high pressure".

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    Quote Originally Posted by Terry Keeley View Post
    Thanks, that gives me confidence mine will stay together, just havta find the "right" 4140 with certs (it's supplied in so many different conditions).

    I was wondering if you ever got to test one of your 90's, would be interesting to see how much power it made. I'm starting with 67's and will do 80's next for my straight away program. Remember off hand where the 67 made it's peak HP? CMB says 27K but I bet that's just a marketing "guess".

    Now your testing the brain memory!
    The number 22000 0r 23000 rings a bell. I know i was disappointed at first, wondering why i couldn't tune it to run like it was advertised. Experience since has taught me there just like you say, salesman talk.

    I looked at sprag (oneway) clutches but decided against it thinking the one power pulse per rev would destroy it in no time.

    I used standard 15mm ID shielded bearings, no seals to drag, very cheap to replace.
    Couldnt afford 6 of engine spec bearings at the time haha.

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    Think about the bearings used in automotive turbochargers

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    Little clip from an Internet search of Carbon bearings: Carbon Graphite Bearings & Bushings
    We custom manufacture many sizes of carbon graphite bearings and bushings used in a variety of support and motion applications. We offer many different types of bearings, including radial, axial/thrust and sleeve, especially beneficial in any type of dry-running, harsh (e.g., high temperature, chemically corrosive) and sanitary environments.

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    Do an Internet search for "air bearing" or similar words. They make closed chambers that function just like needle/roller bearing, but have high pressure air trapped inside the "bearing housing."

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    Quote Originally Posted by Turnaround007 View Post
    Do an Internet search for "air bearing" or similar words. They make closed chambers that function just like needle/roller bearing, but have high pressure air trapped inside the "bearing housing."
    well yea, I know what an air bearing is, but as a "lube" for a rolling element bearing?

    as to "carbon graphite" being a high temp bearing, carbon BURNS in air at the temperatures silicon nitride and cubic boron nitride ball bearings can operate at (red heat)

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    Digging up this old thread I know, but maybe this helps someone down the road.

    My buddy's daughter is in the final year for mechanical engineering and ran the numbers for my wheel.

    The numbers for radial and tangential (hoop) stress came out very close to that formula I used earlier.

    She also did a Solidworks mock-up that included a "von Mises" yield analysis: von Mises yield criterion - Wikipedia That showed the highest stress (and lowest FOS or "factor of safety") was right at the radius for the stub shaft. The area decreases a bunch by increasing the radius, think I'll go with the 1/4", lol.

    Looks like my wheel will hold up as long as there isn't any harmonics or balancing issues...








    1/32" radius at shaft:








    1/16" radius at shaft:








    1/4" radius at shaft:






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    Wow, I sure would build some type of scatter shield for this thing, even small components at that RPM will launch a long way.

    or leave a note..

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