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Automotive Coaxial rotors with one motor?

  1. Mar 30, 2017 #1
    hi guys, i don't really have a good background on mechanical engineering so i was hoping that someone would help me.
    i don't really get how coaxial rotors work, i saw a couple of videos and read a couple of papers on it but what i don't get is that can one motor rotate one shaft in opposite directions or is it just 2 motors installed on same shaft?
    i'm trying to make a design of a small helicopter using coaxial and i needed to know should i buy 2 motors or only one? and in general can one dc motor rotate it's shaft in opposite directions? or their has to be 2 motors or 2 shaft to rotate opposite directions
  2. jcsd
  3. Mar 30, 2017 #2
    and one other project i would like to work on is a small hand drill with the concept of coaxial rotors, drill bit would rotate in counter clockwise and drill itself would rotate clockwise with a hand at the end of the drill on bearings for grip so that the two opposite rotations cancel the torque i need to put through my hand to stabalize the hand drill and drill through wood easily without effort
    hope you get what i mean and if these projects can't be done or what i think how a coaxial works i would really appreciate the explanation.
    thank you
  4. Mar 31, 2017 #3


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    My kids have a small RC helicopter with co-axial contra rotating main lift rotors.

    The main drive is a shaft in a tube. One rotor is fixed to the shaft and the other rotor to the tube. There are two electric motors, one motor turns the shaft and it's rotor one way (say clockwise). The other motor turns the tube and it's rotor the other way (say anti clockwise).

    The vertical/height axis is controlled by speeding up/slowing down both motors together.

    Steering/yaw is controlled by speeding up one motor and slowing down the other. The torque imbalance causes the body to rotate/yaw.
  5. Mar 31, 2017 #4


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    Nice idea but unfortunately that cannot work.

    The problem is it would breech Newton's laws that state for every action there is an equal and opposite reaction. If there is a torque on the drill bit there MUST be an equal and opposite torque on the body of the drill no matter what you put in between. If you manage to eliminate the torque reaction somehow you will also eliminate the torque needed to turn the drill bit.

    The only solution is to brace the body of the drill against the wall/wood directly (eg not through you). This is frequently used when drilling large holes through concrete walls - the drill itself is fixed to the wall to stop it turning as well as supporting the weight of the drill.

  6. Apr 1, 2017 #5
    thank you for the great explanation!
    one other thing, is there a mechanism or gear to give
    as you said two electric motors with two rotors, can there be the same idea with one motor, like a figure 8 belt or some gears? i want to apply the two different rotations on same shaft and tube like you said with only one motor; does this even exist?
  7. Apr 3, 2017 #6


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    These two photos from the web show how it's usually done. There are two large gears one attached to the tube and one attached to the shaft. Each is driven by a motor and pinion gear...



    Sure, it's been done on many full size aircraft.


    One way to do it with a gearbox that uses bevel gears..

  8. Apr 4, 2017 #7
    i would like you guys to take a look at what i came up with till now, and if this will work or not.
    two motors placed opposite to each other vertically in a cylinder above a cone. two motors are on bearings inside the cylinder and each one of the motors is responsible of rotating the cone and the cylinder above the cone in opposite directions. if i used the cone to drill in huge wooden box remotely, will it rotate and drill in the wooden box with no need for a fixed frame? i'll attach a pic of my design for you guys to get a clearer picture. it's size will be no longer than elbow to finger tips and with circumference of a 90 pound man and the wooden box is like a sedan car for example, assume placing the machine inside a rat hole from the top. my question is again will it drill and go through other side on it's own without adding a fixed frame or not. would really appreciate if you would explain if you think it will work or it won't so i can get a better idea on the concept of anti torques and coaxials

    Attached Files:

  9. Apr 4, 2017 #8


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    In short.. No it won't work for the reason I mentioned in post #4.

    Your objective in post #2 was to eliminate the torque reaction that you experience when trying to drill a hole. The solution you propose in #7 amounts to you wearing a motorised "hat" that spins in the opposite direction to the drill. There is nothing to stop the "hat" turning so there is no torque reaction that would help you stop the drill turning.


    I understand you don't want to use a "frame" to fix the drill body to the work piece. There are two other solutions that would work in theory but are impractical..

    1) Put large vanes on the hat. These would have significant air resistance so there would be a torque reaction on you that would help you stop the drill turning...



    2) Replace the hat with a flywheel. The flywheel will only generate a torque reaction on you if it's accelerating. It won't work if you run it at a constant speed.


    Your proposal attempts to find a way around the established laws of physics. Unfortunately forum rules don't encourage us to discuss such things because it's rather a waste of time. You should try and read up on Newton's Laws and subjects such as conservation of momentum.
  10. Apr 5, 2017 #9
    isn't what i'm suggesting is just like the coaxial rotors helicopter idea.
    put in consideration that the two motors opposite to each other are attached and stuck together so if one of the motors turn the other has to turn too. won't that act like a fixed frame for the drill?
  11. Apr 5, 2017 #10
    also put in account that the cylinder above is really heavy so as it rotates it gets heavier so for the motor responsible for the rotation of the cone to not rotate the cone the motor itself has to turn and for the motor itself to turn it has to turn the other motor rotating the heavy cylinder too in the opposite direction so won't that act as a torque?
  12. Apr 5, 2017 #11
    or i just had another idea, what if there is right spiral thread on the cone and the cylinder above is the same radius as the cone not smaller with a spiral left thread on the cylinder so as motor of cylinder rotates in opposite direction the spiral of the cylinder will move in same direction of the spiral of the cone and pull to the front just like the right thread on the cone pulls to the front. will that give anti torque for the motor of the cone?
  13. Apr 6, 2017 #12


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    No because with the helicopter both rotors are loaded (air resistance). In your case one is loaded (the drill/friction) and the cap is free to rotate. There is no torque reaction from the cap.

    It takes no effort to rotate a fly wheel at constant velocity. If you want a torque reaction you have to accelerate the cylinder. That's why I mentioned an accelerating flywheel. The relevant equation is T = Iα where T is the torque, I is the moment of inertia and α is the angular acceleration.

    So the cylinder will also be a drill? Yes that works but it might be difficult to ensure the torque required to turn the cone (drill_1) is the same as the torque required to turn the cylinder (drill_2). If there is an imbalance it's likely one will stop rotating (jam) while the other continues to rotate.
  14. Apr 6, 2017 #13
    yes the cylinder will also be a like a drill but cone will encounter the wood first but let's say that the whole machine is pushed in at first and after the whole machine is inside the threads on the cylinder will grab inside the wood just like the threads on the cone. and also assume i put to motors opposite to each other vertically inside the cylinder and attach them to each other on bearings inside the cylinder. and i can manipulate each motor's power on it's own. that would do the trick right?
  15. Apr 6, 2017 #14


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    Yes that works but it's difficult to ensure the torque on the cylinder and cone are the same. It would be easier to do what a tunnel boring machine does...The cylinder just grips the inside of the hole made by the drill.
  16. Sep 5, 2017 #15
    hello again, i just wanted to ask you do you have any idea if there is a system that works like with the technique we discussed a few months ago above?
    like motors acting on each others like anti torques? i made a couple of experiments with hand drills but i would like to extend my research more.
  17. Sep 5, 2017 #16


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    The only other thing I can suggest is to look at something like a hole saw...


    Normally the whole thing rotates in the same direction. If the inner pilot bit and the outer saw part rotated in opposite directions then the torque on the user could be reduced. However to eliminate it altogether they would have to be "matched" so that the torque required to turn each part was the same. That would be virtually impossible to achieve.
  18. Sep 5, 2017 #17
    So only coaxial helicopters and the hole saw are examples of what we discussed, right?
    And as for the design of both cone and cylinder rotate in opposite directions with right and left threads to move in same path this will work but only issue is to ensure torque on both or we could use a ratio between the motors and different motors with different shaft length to ensure torque differences. Would this be possible
  19. Sep 6, 2017 #18
    The only issue here is to match the torques on both cylinder and cone or we could try to manipulate the radius of the shafts holding each and manipulate torques to "match" the Torques, right?
  20. Sep 6, 2017 #19


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    You can try and match the torques but you will find that if one starts to slip then power to the other is reduced to keep it the same. Result might be no motion at all.

    However I don't how it can be applied to something like a hand drill. Anyway the torque feed back to the operator tells him what's going on at the drill bit and helps avoid breakages.
  21. Sep 15, 2017 #20
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