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Technical asymmetrical (unevenly spaced) bladed engine fans

Discussion in 'The Hokey Ass Message Board' started by Cosmo49, Jul 20, 2022.

  1. Cosmo49
    Joined: Jan 15, 2007
    Posts: 1,552

    Cosmo49
    Member

    Long story short...I recently traded a 5 blade fan for a 6 blade fan that fit the arbor of my water pump. Both are asymmetrical. I was always curious as to why this was so. I finally had a "Hey, I can look that up on the internet!" I found this:

    "Unevenly spaced fan is developed to reduce the tones. The fan blades are spaced such that the center of mass is placed exactly on the fan axis to minimize fan vibration. The resulting fan noise is 11 dBA quieter in discrete noise level than the even bladed fan system."

    And:

    "The oddity is there to lessen fan noise. The uneven spaces creates air pulses that cancels out some of the noise, without lessening the ability of the fan to move air."
     
    Last edited: Jul 20, 2022
  2. Cosmo49
    Joined: Jan 15, 2007
    Posts: 1,552

    Cosmo49
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    Is it the asymmetrical type, or evenly spaced?
     
  3. 6sally6
    Joined: Feb 16, 2014
    Posts: 2,459

    6sally6
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    Sometimes...a picture is worth a 1000 words...........like Now!!!
    6sally6
     
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  4. the oil soup
    Joined: May 19, 2013
    Posts: 277

    the oil soup
    Member
    from Tucson,AZ

    The same thing is done with tire tread patterns to reduce road noise.
     

  5. Cuts the decibels by 11, heck breathing is rated at 10, that's not much when it comes to sound/noise. HRP
     
  6. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,264

    gimpyshotrods
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    Some (and not nearly enough) electronic equipment takes advantage of this technique with their cooling fans.
     
  7. greybeard360
    Joined: Feb 28, 2008
    Posts: 2,074

    greybeard360
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    The uneven spacing also reduces cavitation, a point where the fan stops moving air at certain speeds.
     
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  8. Cosmo49
    Joined: Jan 15, 2007
    Posts: 1,552

    Cosmo49
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    upload_2022-7-20_12-7-58.jpeg upload_2022-7-20_12-7-27.jpeg ]
     
    Last edited: Jul 20, 2022
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  9. Cosmo49
    Joined: Jan 15, 2007
    Posts: 1,552

    Cosmo49
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    Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid's vapour pressure, leading to the formation of small vapor-filled cavities in the liquid.
     
  10. 11 Db can be all the difference in the world when the level reaches 80+. After 80 Db prolonged exposure can cause hearing loss.

    That said the wind noise in most of what we enjoy riding and driving is 80+. LOL
     
  11. The wind, the sound of the exhaust, kind of what goes up to make the experience just a little bit better. Also, the eventual hearing loss makes it harder to hear your spouse suggest you don't need to buy another car. o_O
     
  12. Evenly spaced blades have more of a tendency to develop a harmonic or resonant vibration, or noise, when turning at a specific RPM. Multiples of a given RPM (1500, 3000, 4500 RPM, etc.) can also exhibit this behavior. Unevenly spacing the fan blades reduces the tendency of these harmonics.
     
  13. Ziggster
    Joined: Aug 27, 2018
    Posts: 1,697

    Ziggster
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    Interesting. Wondered myself. 10db reduction in noise is huge as the scale is not linear but logarithmic. In my OT project the engine fan made an incredible noise even at high idle as there was no clutch. Another reason I swapped out the motor.
     
  14. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 4,217

    ekimneirbo

    So if someone says that noise is reduced by 10 decibels .............it doesn't always mean the same amount. Lets see now, electric wire gets bigger when the numbers get smaller. When drill bits get smaller they use letters and numbers.....with an 80 being smaller than 40 rather than just saying the diameter of the drill bit. So a #80 drill is .013 and a #28 is .1405 while a 9/64 is .1406. You would think that there would be some logical spacing between these things and bigger numbers would be bigger things. I just have to wonder who comes up with all this stuff? :D
     
  15. Dave G in Gansevoort
    Joined: Mar 28, 2019
    Posts: 2,598

    Dave G in Gansevoort
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    from Upstate NY

    Scientists! That's who to blame. And don't ask me how I know...
     
  16. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 31,180

    Jalopy Joker
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    wonder why we get a strange sensation when things are not all even/equal - like door/fender gaps - yet, we get all excited when we see a barn find that has lots of "character" with plenty not perfect
     
  17. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 12,287

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    I always thought they spaced them like that so I could get in there to access bolts and polish stuff. :cool:
     
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  18. Mr48chev
    Joined: Dec 28, 2007
    Posts: 33,861

    Mr48chev
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    I've got one of those out hanging in the shed. It will pull a cat though the radiator but that thing makes so damned much noise that you could hear it when the truck went down the road. My buddy could tell when I pulled in the driveway while he was in his shop with the radio playing because it was so loud.
     
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  19. Cosmo49
    Joined: Jan 15, 2007
    Posts: 1,552

    Cosmo49
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    I presently have an evenly spaced blade and when travelling long distance interstate I find that I enjoy going 72 mph in a 65-70 posted speed, 1949 Chevy half ton, 235 ci, Fentons, 2x2bbl Carter-Webers on a period Edelbrock intake, 1969 Saginaw/Borg-Warner 3sp + overdrive. I've always said to myself and others that it seemed to be the harmonics. Pardon the pun, sounds like that could be true.
     
  20. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,906

    Kerrynzl
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    Decibels increase exponentially.

    For example, every increase of 10 dB on the decibel scale is equal to a 10-fold increase in sound pressure level .
    Silence is expressed as 0 dB but a sound measured at 10 dB is actually 10 times louder. If a sound is 20 dB, that's 100 times louder than silence.
     
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  21. twenty8
    Joined: Apr 8, 2021
    Posts: 2,318

    twenty8
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    :confused:
     
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  22. G-son
    Joined: Dec 19, 2012
    Posts: 1,277

    G-son
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    from Sweden

    Well, decibels (not only used for sound) is a logaritmic scale, so there's some sense in there. It makes it easy to show very weak signals together with strong signals. Say you have an electric voltage signal shown on a 0-100% analog meter or a table, it's very clear when it's at 100%, but at 1% the meter needle has barely moved from zero, and at 0.01% it's impossible to see.
    Change it to a 0-100dB scale, where we say that 100dB equals 100%. For a voltage signal, a 20dB drop moves the decimal point one step to the left, so when 100dB is 100% of the voltage, 80dB will be 10%, 60dB is 1%, 40dB is 0.1%, 20dB is 0.01% and 0dB is 0.001%. That same 0.01% we couldn't see at all on the first meter would be very obvious on the decibel meter/decibel scale table.
    If I remember it right, when decibel scale is applied to power (watts) 10dB change moves the decimal point one step. I think that's how it applies to sound pressure too.

    Apart from that somewhat confusing but sometimes very useful dB scale system you can solve ALL those problems you mention and many more simply by going metric. A 1.5mm2 electrical cable has twice the amount of copper as a 0.75mm2 one. A 10mm drill is twice the size of a 5mm one.
    A piece of 1mm sheet metal is 1mm thick, and it's 1mm thick no matter if it's aluminum, steel, stainless or something else. American sheet metal gauge on the other hand...

    I like the US, but there's some pretty good reasons it's jokingly called "the worlds most developed underdeveloped country". Sticking with obsolete, nonsensical measuring systems is one of them.
     
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  23. choptop40
    Joined: Dec 23, 2009
    Posts: 5,114

    choptop40
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    something to do with standing waves....if they interfere they reduce/cancel sound....
     
  24. Greg Rogers
    Joined: Oct 11, 2016
    Posts: 802

    Greg Rogers
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    My stock fan on my barely too new Olds is crazy loud. I only mention it cuz one of those fans maybe used on a 283 in a old Ford, or something like that. Anyway, I remember on International School busses and the like the fans were controlled and would switch on and off. When they came on, it sounded really loud.
     
  25. Ned Ludd
    Joined: May 15, 2009
    Posts: 5,026

    Ned Ludd
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    There are altogether too many small electrically-driven cooling fans in the world.
     
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  26. THIS MAN WINS THE CAKE.

    Ben
     
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  27. blowby
    Joined: Dec 27, 2012
    Posts: 8,661

    blowby
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    from Nicasio Ca

    You haven't heard me climb stairs..
     
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  28. Flathead Freddie
    Joined: May 9, 2021
    Posts: 806

    Flathead Freddie
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    Wind gusts also and acts like a relief valve for pressure . Pop the hood on a '70s Corvette you can really hear the trapped pressure in the extended shroud
     
  29. I think we may have just discovered the source of the expression "a deafening silence".
    :p
     
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