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Variable gear ratio [Worm gear - Worm Wheel]

  1. Jun 14, 2014 #1
    Hello fellow PF members,

    This is my first post.

    I am wondering if there is a mechanism that alters the gear ratio in worm gear-worm wheel setup.. Something like a variable pitch worm gear that can be used instantaneously to alter the gear ratios.

    If not for worm gear, is there anything in any type of mechanical gear system?

    Thanks in advance.

    Edit: BTW, when I said worm gear, I meant the worm itself & by worm gear I meant something like a spur/helical gear.
    Last edited: Jun 14, 2014
  2. jcsd
  3. Jun 14, 2014 #2


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    Welcome to PF.
    I presume by “variable” you mean “continuously variable, while transferring power”, then no, there cannot be, because the ratio is decided by the tooth count. Worm thread profile must match the wheel's tooth profile or there will be inefficiency and significant heat generation with associated wear. For mesh to be continuous while changing the gear ratio, it would require a pitch and profile change of both the worm and the wheel.

    Variable gear ratio systems invariably use friction or fluid as the coupling between the input and output shafts.

    Probably the closest to the worm and wheel would be a small variable flow hydraulic pump driving a big hydraulic gear motor. That was used in industry to vary the ratio before low cost electronic variable speed drives became available. Hydraulic components today still have a better power to weight ratio than electrical systems. Hydraulic systems are harder to control with electronic controllers than are electric motors on variable speed drives.
  4. Jun 15, 2014 #3
    Hello Baluncore,

    Thanks for your reply. Yes I meant continuously variable while transmitting power (something like a CVT but a mechanical gear system type).

    I just did some more googling and found a patent which claims that a similar mechanism is possible.
  5. Jun 15, 2014 #4


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    That patent expired a few years ago, without a product appearing in the market. Maybe that is because it cannot work. The conical worm gear is always drawn with a constant pitch.

    I believe the inventor was under the false impression that the worm diameter was important in setting the gear ratio. That is not the case, it is the pitch of the worm that sets the ratio, and that must match the driven gear wheel, so the pitch cannot be changed and the ratio is fixed.

    Did I miss something? Am I wrong?
  6. Jun 15, 2014 #5
    It is quite common in Power Transmission componentry (gearmotors, etc) to have a "mechanical variable drive" device.

    A couple of designs come to mind:
    (1) two rotating circular plates in contact, one is adjustable. The adjustment changes to point of contact radius dimension from driving-driven shafts, thus giving continuously variable ratio under load.

    (2) the other is a clutch with power transmission belt affair similar to CVT thinking.

    I have seen both manifest themselves with a device housing sporting a handcrank of some sort.

    Look into the gearmotor companies like Dodge, SEW Eurodrive, Nord, others.

    Expect low mechanical transmission efficiencies.

    The expansive use of Variable Frequency Drives has pointed this mechanical technology towards the technological graveyard.
  7. Jun 27, 2014 #6
    A common form of variable transmission is a hydraulic pump and motor combination.

    The gearing is altered by varying the pump displacement per revolution.
    Vane and axial piston pumps are typical variable displacement pumps.
  8. May 2, 2015 #7
    Thiru - interesting question. The answer is "yes" and "no". The search for an "infinitely variable gear" is a very old one - Da Vinci being one of the first to have a try (and not a very good try). It is something I have been working on for the last fifty years or so. I think you could say generally that it can't done but it depends how you define just what "infinitely variable gear" means. Brigham Young University has done a lot of work in this field (maybe the only uni to do so seriously). They have done very useful work in defining just exactly what the aims and meaning of "variable gear" are. But - notably, despite a huge amount of work, they found no solutions. If you Google "brigham young pecvt" you will find plenty of their stuff.
    I have seen it written that a continuously variable ratio gear is not possible because that would imply an "analog" device where gear or chain systems etc. are "digital" by their very nature.
    The patent USPTO 4831889 that you quote just doesn't work - the basic principal is not valid - the gear ratio would be the same anywhere along the cone.
    However just to confuse matters slightly I think the variable gear puzzle can be solved - I think I have done it in a slightly "tricky" and "unfair" (and impractical) manner but nevertheless it does satisfy the general "PECVT", all teeth and gears, rigid connection between input and output and vice versa etc. etc.
    Also of interest is the website of VMT Moongears - who claim they have a workable system - but almost certainly don't.
  9. Jan 27, 2017 #8
    If one wants a variable worm drive, you could design a micro CVT (transmission) whose output shaft is connected TO / BEFORE the input shaft of the worm drive. That way, you can vary the input revolutions on the worm drive itself, rather than having to make the worm gear somehow be 'variable', which, in this reality, it cannot be. Input power from the drive motor would also remain constant in this configuration, rather than slowing it down to achieve worm gear slowdown/speedup, which also lowers the drive motor torque input. So, the total objects involved would be:

    a) motor : (or input device of some sort) providing turning force in designated RPM range

    b) CVT transmission : (connected via software or analog input to get load sensing downstream of high pressure line)
    which would allow smooth, infinite variations based on load sensing (demand for speed/vs power)

    c) Worm Drive : (super powerful, since they're literally a 1 : 1 reduction of parent gear revs to child gear teeth)
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