I have an axle that is currently oscillating along its axis of symmetry, and I want to now rotate it around its axis of symmetry as well using a motor. I realize this is a pretty tough problem and may involve bearings or other systems of some kind, so I thought to ask here.

I am building a rotary speaker, a very powerful but largely unexplored speaker design system.

The oscillation is from a speaker coil, which is being used to actuate small flaps (think a helicopter rotor system) at the end of the axle. The axle must also rotate in order to spin the flaps and move air across them. I would love to have a motor with an axle that simply passes right through, but I have not been able to find such a thing. See the figure below for a quick sketch of my ideas.

Fig. 1

Here is a video of one being built; there are many problems with this design that I would like to fix in my own, but the basic concept of driving an axle both longitudinally and radially remains. The OP found an induction motor with a negative shaft space for placing their own axle, but it was salvaged so no dice on finding my own.

Below is a screencap of OP's finished product. An induction motor is suspended over a speaker coil, magnet, and spider with a frame. An axle connects the speaker coil to a swash plate which moves levers that rotate 5 blades. The axle passes through an induction motor, which spins it, along with the fan blades to push air. screencapture of rotary subwoofer in attached video

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    $\begingroup$ ooh, making my own motor is an extremely cool idea; All it would have to do is drive the axle at a single, relatively constant speed. $\endgroup$ Commented Mar 20 at 2:01
  • $\begingroup$ Ah, well I am glad that it wasn't such a crazy idea after all. :-) Maybe I should post that as an answer, along with a rough diagram..? $\endgroup$ Commented Mar 20 at 4:14

1 Answer 1


Even if you found a motor that the axle does pass through, the weight of that motor would have the undesirable effect of dampening the oscillation.

That being the case, you could just as well have a "normal" motor clamped to just one end of the axis, presumably resting on the plate that is oscillating. If the weights of the motors are the same, then the damping would be the same (more or less).

A better idea, although maybe involving more work/construction, would be rigging up some coils1 in the fixed frame, around the axis, located at 360°/n from each other (where $n$ is the number of coils). Then attach some magnets to the axis - effectively creating a motor. Depending upon the size of the magnets required2, this would result in significantly less axle weight, and therefore less damping of the oscillation.

[Diagram to follow shortly]

You could pulse the coils alternatively - with the phases differing by $360°/n$ - using various output pins from an Arduino (one pin per coil). Varying the speed of the cyclic (out of phase) pulses, would vary the speed of rotation.


1 A minimum of three would be required

2 Using strong neodymium would reduce the size requirement of the magnets significantly - although some restraints upon the axis might be required to prevent the axis from being "dragged" to one side of the other.


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