Gear boxes that "hum"

Discussion in 'Diesel Engines' started by parkland, Nov 7, 2012.

  1. parkland
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    parkland Senior Member

    An old school boating guy was coaching me a bit about the boat I want to build.

    He told me to make sure I get a transmission that "hums".... ?

    I tried asking him, but he didn't really know... he just said "the ones that hum last the longest."

    What was he talking about?
     
  2. michael pierzga
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    michael pierzga Senior Member

    Whoa !...My diesel engine generates so many decibels of racket when running that I have never heard my TwinDisc transmission operate !!

    As an OLD SCHOOL guy...I recomend TwinDisc...Ive never had a problem over the several decades ive used them
     
  3. parkland
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    parkland Senior Member

    I've heard "hurth" or "ZF hurth" or something is the best, but I'd have to see how good at humming it is, lol. :)
     
  4. michael pierzga
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    michael pierzga Senior Member

    Dont know...I believe ZF owns the TwinDisc brand. My experience with twin disc is on big continuos duty powerplants. Good stuff.
     
  5. Petros
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    Petros Senior Member

    usually a hum is an indication of wear, could mean failure will soon follow. but sometimes a hum can be there for a long time.

    On older gear boxes they used straight cut gear teeth, these are stornger and still used in some industrial applications, but there is less smooth power flow and they make more gear noise. Most modern gear boxes use a spiral or helix cut gears that mesh smoother but the contact face on each gear tooth is smaller. Modern auto transmssions for example typically last a very long time, and they use the hexic cut gears (for a smooth and quite ride).

    So this "hum" might be an old timer way to identify the stronger straigh cut gears over the helix cut gears. Just a guess.
     
  6. Frosty

    Frosty Previous Member

    And ofcourse you will be able to hear that over a marine engine running.

    Thank the guy for his help and push him back in the direction of the bar he just fell out of.
     
  7. alan white
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    alan white Senior Member

    I think Petros has it. Straight cut gears hum. Their sound is like a lot of reverse gears on cars. R-R-R-R-R-R-R-R-R. Often gear numbers (of teeth) correspond in frequency with nearby ball or roller bearings. I've quieted noisy gears by replacing bearings.
    A hum is generated when the straight-cut gears mesh. It's almost impossible to make straight-cut gears that are noiseless. The culprit is a slight speeding up and slowing down of the driven gear that creates a vibration heard as a hum. The pitchline velocity of the driven gear varies, which can be seen as a mark wiggling under a strobe light driven by the driving gear.
    The good thing is that straight-cut gears can be more efficient than spiral cut gears. There's less meshing and hence less energy-robbing heat. The teeth impart motion one tooth at a time with the teeth positioned in an optimal location to efficiently "kick" the next tooth.
     
  8. parkland
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    parkland Senior Member

    I also agree. I don't know why I didn't think of that! :confused:

    I did not know stright cut gears were better at everything except noise reduction...:eek:
     
  9. Red Dwarf
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    Red Dwarf Senior Member

    In car racing you use straight cut gears for all the reasons already mentioned. At idle the gear box has so much rattle it sounds like shaking a can full of marbles.

    There is a good side to the hum you mention. We had high frequency vibration sensors and could monitor the health of each gear in real time.
     
  10. Petros
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    Petros Senior Member

    I am also familar with the straight cut gears from working for a fully sponsored race car team. and from my engineering classes from many decades ago. the hum or vibration is from the nature of the gear tooth geometry, slight speed variation as the gear teeth rotate through their travel. the helicut gears however do have more friction (more heat generated) and is therefore less efficient, and potencially could wear out faster.

    Even with those advanages, I do not think i would want a car or a boat for that matter that transmits vibration like that to car or hull. The noise and vibration would be very tiring on long hauls.
     
  11. parkland
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    parkland Senior Member

    I'm sold; can you buy transmissions with these gears, or would you have to dissect a normal transmission and source straight cut gears for it?
     
  12. Frosty

    Frosty Previous Member

    Straight cut gears have less friction, they have less teeth in contact, they are weaker.

    It will however last a race where cost does not matter, it will wear faster but cost does not matter, It is noisy but that will not matter.

    Remember when reverse gear on your car had a whine,--straight cut gear on an idler shaft for reverse.
     
  13. WestVanHan
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    WestVanHan Not a Senior Member

    And thank goodness for that, being overweight-the Panzer V's final drives were straight cut,overloaded,and weak.
    At any given time most of them were in for final drive repairs,which took a long time.
     
  14. Frosty

    Frosty Previous Member

    Straight cut gears are cheap and easy to make, but they need to be bigger.

    Bevel gears can have 3 or 4 teeth in contact at one time. Hence no definate point of contact and release which cause the whining noise.
     

  15. richard gray
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    richard gray Junior Member

    gears that hum

    i have this idea to run an auto transfer case in reverse to power a jet drive. only way to get the shafting layout i want. will the spiral(helical cut gears howl and stand up to the strain? i have a 350 chev v8 to power it. the idea is to make an amphibious vehicle,so the transfer case will drive the differential or the jet drive the case and will turn in constant reverse since it is divorced and the original input shaft is now the output. i will hot use the low gear at all just 1to 1 high.
     
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