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Automotive Can two motors rotate about the same axis?

  1. Aug 11, 2018 #1
    can we add two motors to rotate same object on same axis or is it impossible?
    for a stronger rotation and torque two motors rotating object in same direction
     
  2. jcsd
  3. Aug 11, 2018 #2

    Baluncore

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    You can use another identical motor to make a pair, but it is probably easier to use only one motor with twice the capacity.

    The output shaft of a motor is usually only sufficient to handle the power of one motor. That means you cannot expect to mount the motors in line with the load at one end.

    Since motor shafts are designed to provide power output from one end, you would need to run one of the two motors in reverse, with the driven load between the motors. Any slight misalignment or vibration could damage the motor bearings so you would need to use a flexible coupling on at least one of the motors.
     
  4. Aug 11, 2018 #3
    The custom-built tractors used in tractor-pulling competition sometimes use multiple engines, with pairs of engines sharing a common shaft axis. The rear crankshaft flange of one engine is bolted directly to the front flange of the other.

    Also, Cat Cracker units in oil refineries do something along those lines. Typically a flue gas expander/turbine is used to recover energy to drive the main air blower, and an electric motor is mounted with its shaft on the same axis to drive the blower when the expander power is not sufficient (typically on unit startup). When the expander has excess power, the motor can be used as a generator feeding into the refinery electrical grid. In addition a steam turbine may be mounted, again on the same axis, to assist in starting up the blower if the refinery electrical system cannot support the motor's initial power draw.
     
  5. Aug 11, 2018 #4

    CWatters

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    Two identical electric motors geared to a common shaft have been used to power RC model planes. However it's generally more efficient just to use one big one. I don't recommend this if the motors aren't identical.

    It's easiest if they are brushed motors. If they are brushless you need two well matched speed controllers or they won't share the power equally...or worse.
     
  6. Aug 11, 2018 #5
    Um, didn't some experimental piston-engined aircraft have dual engines set 'tandem' with concentric prop shafts and counter-rotating propellers ??
     
  7. Aug 12, 2018 at 12:20 AM #6

    CWatters

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    Some production aircraft had large radial engines with two banks of cylinders one behind the other. I believe there were essentially separate engines.
     
  8. Aug 12, 2018 at 1:32 AM #7

    Baluncore

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    It would really help if we knew what type and size of motor the OP was considering.

    Can you find an example?

    Counter-rotating propellers are driven by a single gearbox bolted to one end of the motor. The torque on the counter-rotating propellers cancel in the gearbox which removes the torque from the engine mounts on the airframe. Counter-rotating propellers are noisy and to avoid destructive oscillation, must limit the RPM. The shaft torque on big slow, many bladed propellers can be so great that hydraulic torque converters are needed to handle the power.

    There are also systems that have two motors in the same nacelle, but with independent shafts. One drives a tractor propeller at the front, while the other drives a pusher prop behind. One advantage of independent counter-rotating engines and propellers is that some gyroscopic dynamics are reduced. Examples include the 12 engined Dornier Do X; https://en.wikipedia.org/wiki/Dornier_Do_X and the Dornier Do 335 twin-engine push-pull fighter. https://en.wikipedia.org/wiki/Dornier_Do_335

    A V12 engine can be seen as two straight 6 engines or two V6 in-line engines sharing one shaft. A straight 6 can be seen as a pair of straight three cylinder motors in tandem.

    Multiple parallel crankshafts get interesting. The Junkers Jumo 223 engine was a four crankshaft, 24-cylinder, opposed-piston, two-stroke diesel aircraft engine. https://en.wikipedia.org/wiki/Junkers_Jumo_223
    Collaboration between Junkers and Napier ended with WW2. Napier went on to developed the successful 18 cylinder Deltic, with three crankshafts,, an opposed piston, two-stroke diesel used for railway locomotive and marine propulsion. https://en.wikipedia.org/wiki/Napier_Deltic
     
  9. Aug 12, 2018 at 6:16 AM #8
    thank you all for the replies, i was just considering the idea of adding 2 of any type of motor for a design as to give more torque but as you all said its more efficient to put one big motor.
     
  10. Aug 12, 2018 at 10:24 AM #9

    Baluncore

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    With radial engines, 9 cylinders was most common, followed by 7, then less often 11. That gave a two row, (twin bank, or duplex) arrangement of 14, 18 or 22 cylinders. Have you ever wondered why those radial engines always had an odd number of cylinders in a bank? Could it be that they were all four-stroke engines?

    The aircraft piston engine built in the largest numbers (173,618) was the P&W Twin Wasp radial engine. It had 14 cylinders, making a capacity of only 30 litre.
    https://en.wikipedia.org/wiki/Pratt_&_Whitney_R-1830_Twin_Wasp
     
  11. Aug 18, 2018 at 3:29 PM #10

    Klystron

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    Since the OP expressed satisfaction with the answers -- a single large engine may be more efficient than twin engines mounted on the same rotor -- I'd like to expand on the beauty of the Pratt & Whitney engines.

    Waxing nostalgic, my first airplane flight was as a passenger in a DC-3 "Gooney Bird" powered by twin R-2000's turning enormous wooden propellers. After pre-flight engine checks, the first officer completely spun down the port engine to allow passengers to board the aircraft. At cruising altitude the sound and synchronized vibrations of the engines dominated the interior of the airframe; an effect felt as much in the chest cavity as heard by the ears.
     
    Last edited: Aug 18, 2018 at 4:03 PM
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