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Minimum Number of Gear Teeth

Discussion in 'Members Lounge' started by dknguyen, Mar 12, 2009.

  1. dknguyen

    dknguyen Well-Known Member

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    So apparently for a certain gear characteristics, there is a (or number of teeth you can have on a gear (or minimum gear size) that you can have before it no longer meshes properly becomes too much like a star or cross shape and deviates too far from that of a circle (in layman's terms).

    A bunch of the papers I've been reading say that the minimum gear teeth is 12-13 (For the gears they were looking at) and that seems to hold up for the most part at the pinion gears for RC vehicles...except you sometimes find 10T, 9T...all the way down to 6T pinion gears.

    Are these pinions sacrificing proper meshing for higher gear reductions? Because gear tooth profiles are pretty standardized and since these pinions must work with existing gears, nothing can be done to either driven or driving gear to get them to mesh better.

    Like look at this:
    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
    it barely looks like it would mesh with anything, being star-shaped and all. A whopping 60 degrees between each tooth! I might be able to barely visualize 45 degrees like on an 8-tooth pinion, but 60?!
     
    Last edited: Mar 12, 2009
  2. Mikebits

    Mikebits Well-Known Member

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    I would think pitch is the important factor. Meshing gears should have same pitch. It makes sense that as you get few teeth you pitch angle would have to increase, at some point too much pitch would result in slippage.

    Meshing gears have to have same pitch or they slip. I think pitch can be maintained within a tolerable tolerance. Fewer teeth on the pinion would imply the diameter must be smaller, at some point machining such a pitch in a small amount of metal becomes impossible.
    I am just guessing mind you, this is a Mech E problem... :(
     
  3. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Saw a documentary, showing nanomotors at the molecular level, using star gears..
    Not the sort of item for the hobbyist, but a demo on how small they can be.

    Look inside an self winding watch to see very small gearing.
     
  4. dave

    Dave New Member

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  5. Mikebits

    Mikebits Well-Known Member

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    I am still amazed when I look inside a watch. There sure are some smart peeps in the world, I am not one of em...
     
  6. duffy

    duffy New Member

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    I can tell you from experience that the 6T gears SUCK!!! We switched from a 6T to a 7T in fifty RC cars, and in spite of the reduced ratio it had more oomph and used less power (and we did very careful measurements of that power throughput).

    When I photographed the 6T gear under a microscope, it didn't look like normal gear teeth. They were sort of bulb-shaped.

    I would appreciate any references you could provide on this.
     
    Last edited: Mar 12, 2009
  7. Boncuk

    Boncuk New Member

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    Using sprockets and chains there is no problem with four teeth.

    Boncuk
     
  8. dknguyen

    dknguyen Well-Known Member

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    1. Kinematic design of machines and ... - Google Book Search

    2. https://engineering.purdue.edu/ME452/Homework/HW 8 gear geometry/hw8.sol.Spring07.doc

    3. http://www.electro-tech-online.com/custompdfs/2009/03/2_3.pdf

    4. http://forum.vtu.ac.in/~edusat/Prog6/kinematicsofmach/tvg/4-Chapter-4-Interference-Back-lash.ppt

    THe common minimum teeth is somewhere between 12-14. Obviously this depends on pressure angle, but I'm fairly sure RC gears are quite standard since people seem to be able to go to places like SDP/SI or McMaster Carr to get alternate pinions. And at these palces, the minimum number of teeth corresponds to the theretical minimum in these papers. But if you look at RC speciic manufacturesr of pinions, you can start to find pinions far far below that number (6 being the absolute lowest I've ever seen by a long shot and only from one place. 8-10 are much more common). I'm just taking a guess but it seems the 20 degree pressure angle is the most widely used.

    EDIT: Paper 1, lower down on the same page seems to say that you can undercut teeth (remove material at the root) which will weaken the gaer but remove the material which will cause gear interference if you move below the theoretical minimum...of course that doesn't help with the deviation from a circle-shape though. The paper then says that this would reduce the minimum teeth of a 20 degree pressure angle gear from 13 to 10 which seems inline most of the RC pinions that I have seen. Still doesn't explain the 6T gear though!
     
    Last edited: Mar 12, 2009
  9. dknguyen

    dknguyen Well-Known Member

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    So yeah, that undercutting might explain it resulting in your "bulb shaped teeth". If you look very closely at the root of the gear in these photos (especially the first 4 which are of the unusually low tooth count variety, the root is clearer skinner than the rest of the tooth and gets fatter as the tooth count increases. It still seems to go far below the theoretical minimum, even for undercut teeth...maybe that's why the 6T gears were so crappy.

    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
    RC4WD - The Ultimate Site for RC Off-Road Enthusiast!
     
    Last edited: Mar 12, 2009
  10. ScareCrow

    ScareCrow New Member

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    Would compressor lobe interface meet the definition of a gear even though neither lobe is actually driving the other, both being driven. It looks like the lobes of this compressor could be pressed into sevice as a gear train. Recently I took part in the rebuild of a 250 hp compressor. The driving gears on the lobes had been removed so you could turn one lobe by hand and it would drive the other lobe. Could this demonstrate a 2 tooth gear?
     

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  11. Hero999

    Hero999 Banned

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    I would assume that a pinion gear can have less teeth because it's easier to mesh with a linear toothed rack than another gear.
     
  12. Diver300

    Diver300 Well-Known Member

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    http://www.on-trak.co.uk/images/rotors.jpg

    Those are the rotors of a gear pump. One has two teeth, the other has 3. Only one is driven by the electric motor.

    The helical cut that provided the pumping action also means that some part of the gear set is always engaged at the best point.

    The ones in the picture had run for about 4000 hours and had been round somewhere around 1 billion times.

    Gerotor Pumps shows a 4 tooth gear running with a 5 tooth internal gear.
     
  13. speed804

    speed804 New Member

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    The best reference I have found for gears is the Machinery's Handbook. For gears that mesh together you need to have the same Diametrical Pitch (Imperial) or Modulus (Metric) and Pressure Angle. The diametrical pitch = number of teeth/ pitch diameter. The same diametrical pitch means the teeth are the same height on different gears. You also can't have gears with any significant amount of interference. Undercuts make up for what would otherwise be interference as stated above. So technically you can have gears as small as you want down to even probably 4 teeth or so but for most small combinations the backlash would be huge and/or the teeth would have such big undercuts they would be too weak for any practical application. It is a shame someone hasn't simply made a formula for the smallest # teeth before an undercut is required. Right now as far as I understand you just have to model it in 2d or 3d and see if there is interference when rotating or you can pick standard diametrical pitches (like 48 for many rc gears) which often have the fewest number of teeth already listed. ref http://site.petitrc.com/Tech/UnderstandGearingRatio/
     

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