An interesting observation with my ceiling fan

[Inkaddict]

Hello,

With the cooler temperatures I noticed an interesting thing happening with my ceiling fan when I lowered the speed to the lowest setting while still pushing the air downward.  During the summer this fan ran continuously on the highest setting with a little bit of wobble in it.  With the the fan at the lower setting the wobble has increased so much that the pull chain now swings back and forth with enough momentum to connect with hanging lights. I suspect the cause of the wobble is due to one of the arms that attach the blades to the motor is slight bent, maybe causing a different pitch to one of the fan blades or there is a weight imbalance between the blades themselves. My question is what would cause the wobble to be more pronounced at the lower speed than at the higher speed scientifically that is. I'm sure that there are centrifugal forces in play here but could the directional forces created by the fan blades circulating the air be a stabilizing factor?

[swansont]

One possibility is that there is a resonant frequency for the chassis to wobble, and at low speed you are near that resonance, so you more efficiently transfer energy into that mode.

[MigL]

At resonant frequencies vibrations can add up, and not interfere.
Think of running a wet finger around the rim of a partial wine glass; at a certain angular frequency the resonant vibrations with the glass crystal re-enforce each other and cause an audible whistle/whine. Enough re-enforcement can even shatter the glass.

Yet a little faster, or a little slower, and you hear nothing.

Edited November 12, 2020 by MigL

[J.C.MacSwell]

One other possibility, or possibly contributing factor, is that the increased straining forces on the fan blades affect the blades unequally, leading to them being better balanced.

[LaurieAG]

If your summer was hot and humid you may have an uneven buildup of dust and moisture on your blades.

[michel123456]

10 hours ago, Inkaddict said:

Hello,

With the cooler temperatures I noticed an interesting thing happening with my ceiling fan when I lowered the speed to the lowest setting while still pushing the air downward.  During the summer this fan ran continuously on the highest setting with a little bit of wobble in it.  With the the fan at the lower setting the wobble has increased so much that the pull chain now swings back and forth with enough momentum to connect with hanging lights. I suspect the cause of the wobble is due to one of the arms that attach the blades to the motor is slight bent, maybe causing a different pitch to one of the fan blades or there is a weight imbalance between the blades themselves. My question is what would cause the wobble to be more pronounced at the lower speed than at the higher speed scientifically that is. I'm sure that there are centrifugal forces in play here but could the directional forces created by the fan blades circulating the air be a stabilizing factor?

My guess is that the wobble appears the same way it appears when a spinning top slows down. https://en.wikipedia.org/wiki/Top#Physics

[swansont]

7 hours ago, michel123456 said:

My guess is that the wobble appears the same way it appears when a spinning top slows down. https://en.wikipedia.org/wiki/Top#Physics

Why wouldn’t gravity tend to straighten the fan?

[michel123456]

3 hours ago, swansont said:

Why wouldn’t gravity tend to straighten the fan?

One thing is sure: the fan has a flaw, a regular fan does not wobble. I suppose that gravity makes the flaw (some misalignement?) worse. The fan is not straighten along the axis of rotation, for whatever reason.

[swansont]

36 minutes ago, michel123456 said:

One thing is sure: the fan has a flaw, a regular fan does not wobble. I suppose that gravity makes the flaw (some misalignement?) worse. The fan is not straighten along the axis of rotation, for whatever reason.

A top’s behavior is from precession. Gravity exerts a torque trying to pull the top over. But in a ceiling fan, the torque gravity exerts would tend to straighten it.

 

[studiot]

23 hours ago, Inkaddict said:

Hello,

With the cooler temperatures I noticed an interesting thing happening with my ceiling fan when I lowered the speed to the lowest setting while still pushing the air downward.  During the summer this fan ran continuously on the highest setting with a little bit of wobble in it.  With the the fan at the lower setting the wobble has increased so much that the pull chain now swings back and forth with enough momentum to connect with hanging lights. I suspect the cause of the wobble is due to one of the arms that attach the blades to the motor is slight bent, maybe causing a different pitch to one of the fan blades or there is a weight imbalance between the blades themselves. My question is what would cause the wobble to be more pronounced at the lower speed than at the higher speed scientifically that is. I'm sure that there are centrifugal forces in play here but could the directional forces created by the fan blades circulating the air be a stabilizing factor?

Firstly further observation would be useful.

A fuller description of the wobble for instance.

Does the fan still go round smoothly, even if it wobbles ?
Or does it clunk and bump and hesitate at some point in its travel?

If the blades hesitate or bump against something , perhaps a dead fly or grit has got into the mechanism and is causing this.
Alternatively if there are too few poles on the motor the drive may actually be a series of pulses driving it round and if there is too long a time gap between pulses the rotational inertia will not be enough to keep the blade speed even at low speed.

If however the blades just wobble then perhaps this is a similar effect to the reason you cannot stay upright on a stationary bicycle: in fact the faster you go the more stable you are.
This effect is due to the increase of rotational inertia (the flywheel effect) stabilising above a certain speed..

As to your question of air disturbance, yes this is a fair one.
I assume the fan in inside a room.
Inside the room the distance to the walls, corners, openings etc will vary as the blade goes round.
You say that air is still directed down from the centre of the fan.
For this to happen, air must be drawn in sideways (horizontally) and, as just noted, the air path for this air will be different as the blade rotates.
So the blade will encounter a varying air resistance as it goes round.
Again this may be stabilised by rotational inertia.

 

Finally, as others have said, the bearings may have worn sloppy, permitting the wobble which again will tend to centre itself at higher speed due to rotational inertia.

[Halc]

It's the same effect as my washing machine where the resonance really shows.  It is never balanced well, despite its effort to distribute the load evenly.

At very low speed, the imbalance has little effect as the forces are not large enough to move the machine.  At medium speed (when it first attempts a moderate spin) it shakes like crazy, sometimes throwing the dryer next to it off the pedestal they're on.  At high speed (1200 RPM), it's smooth as can be, but when it coasts back down to a stop, it has to pass through that moderate RPM place that makes it shake everything to death.

I had a ceiling fan that shook badly at higher speed due to imbalance (as opposed to not being aligned with its axis).  I duct-taped a thick metal washer about 2/3 of the way out from the axis and it ran flawlessly.  Took a bunch of tries to find the sweet spot.  So we're all sitting there one day and the thing lets go at high speed.  I'm 5 meters away and get this washer smacking into the furniture 20 cm from my head.  Never put it back.  We just run the thing at medium speed tops.

Edited November 13, 2020 by Halc

[MigL]

Maybe you should try that metal washer on a corner of the washing machine  .

[studiot]

 

47 minutes ago, MigL said:

Maybe you should try that metal washer  on   under ? a corner of the washing machine

[michel123456]

15 hours ago, Halc said:

It's the same effect as my washing machine where the resonance really shows.  It is never balanced well, despite its effort to distribute the load evenly.

At very low speed, the imbalance has little effect as the forces are not large enough to move the machine.  At medium speed (when it first attempts a moderate spin) it shakes like crazy, sometimes throwing the dryer next to it off the pedestal they're on.  At high speed (1200 RPM), it's smooth as can be, but when it coasts back down to a stop, it has to pass through that moderate RPM place that makes it shake everything to death.

 

Most probably the axis of your washing machine is broken, you need to call the service for repair.

[swansont]

5 minutes ago, michel123456 said:

Most probably the axis of your washing machine is broken, you need to call the service for repair.

That’s not the most probable explanation. Unbalanced load is.

The reason the wobble varies is, as I said, because of resonance. The device will rick back and forth at some natural frequency that depends on its mechanical makeup and stability. So any imbalance at or near that frequency means the imbalance is happening when it can add to the tipping - you’re tipped to one side, but momentarily at rest, ready to rock back, and the extra mass is on the other side, so you have more torque to make it tip the other way.  But now you increase the frequency. That mass is giving you less restoring torque. You get to the point where the device is tipping one way and still has a velocity, and the mass is providing a torque that makes it want to tip in the opposite direction, reducing the wobble.

(Paul traps use a similar concept to confine ions with an oscillating electric field)

[Halc]

19 minutes ago, michel123456 said:

Most probably the axis of your washing machine is broken, you need to call the service for repair.

It did this from day 1, as do all free standing front load washing machines of which I am aware.  The laundromat ones don't do it because they're bolted to the floor and to each other.

The imbalance comes from laundry not being perfectly distributed.  Nothing I can do stops that since the machine redistributes it, and tries quite hard to do it evenly.  My point is that it does it most at a moderate frequency, which is similar to what the OP described.

 

While I'm replying to your posts then:

18 hours ago, michel123456 said:

The fan is not straighten along the axis of rotation, for whatever reason.

This is a plausible problem.  If I take a disk and mount it at a slight angle relative to the axis of rotation, then it will wobble/vibrate as it spins despite not being off center.  This is why it takes at least 2 weights to balance a car tire, which in combination yield a static balance (adjust the center of gravity to be on the rotation axis) and a dynamic balance (redistribute the mass of the wheel to a plane perpendicular to the spin axis).

 

15 hours ago, MigL said:

Maybe you should try that metal washer on a corner of the washing machine  .

Or put it inside.  It is a 'washer' after all.