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        riddles >> microsoft >> Ceiling Fan Balancing
        
(Message started by: James Fingas on Jul 15^th, 2003, 1:01pm)

Title: Ceiling Fan Balancing
Post by James Fingas on Jul 15^th, 2003, 1:01pm

I put this in the Microsoft section just because it's open-ended, not because it sucks like all the other questions here ;).

How would you go about balancing a ceiling fan? Suppose you have just a small selection of household tools available.

If you need to, assume that the fan has a certain number of blades (4 or 5 presumably). A fan is "balanced" when the stem hangs vertically down from the ceiling at all times.

Title: Re: Ceiling Fan Balancing
Post by Icarus on Jul 15^th, 2003, 6:50pm

Well, around my house the process has always gone something like this:

"I think that the one causing the problem. Bend it a little to the left... NO! YOUR OTHER LEFT!... Hmmm... Okay try that one... Not enough yet, bend it a little more...Bend it back a little...Well at least the blades don't hit the ceiling any more!...FORGET THIS! Leave the stupid off!"

Title: Re: Ceiling Fan Balancing
Post by towr on Jul 16^th, 2003, 12:48am

I suppose you could attach little blocks of wood with a screw sticking out to each blade, and screw the screws in or out to finely balance the blades (similar to how you get some old pendulum clocks to tick at the right frequency)

Title: Re: Ceiling Fan Balancing
Post by mike1102 on Jul 16^th, 2003, 6:42am

Balancing a spinning wheel is, at the very least, a two-step process. First: statically ballance the fan. Before mounting the fan on the ceiling, balance it from its center such that its axis of rotation is also its balance point. You can do this by resting the fan up-side-down on a screwdriver or an awl mounted in a vise. Add/subtract weight as necessary. That's the easy part. Unfortunately, the fan must be balanced dynamically too - That's the way the physics works out. There's an w x r term (where w is the fan's angular velocity) that will be zero unless the fan is rotating. Mount the now statically balanced fan from the ceiling, drop a plumb line from its center (use duct tape, a string and a small weight) and spin the fan - the faster the better - make w large. Look for periodic movement in the plumb line. As the plumb line moves in one direction, try to identify which fan blade is in the same position. Stop the fan, add small counter weights to the blades opposite, adjust the blades etc., to the fan and repeat until the motion in the plumb line is minimal. Remove the plumb line. - Good luck. I always find that a few beers, a couple band-aids and a trip or two to the local hardware store somehow manage to enter into the process somewhere along the line.

Title: Re: Ceiling Fan Balancing
Post by James Fingas on Jul 16^th, 2003, 9:26am

towr,

I suppose that would work for small imbalances, but the fan I'm thinking about is more wobbly than that. Four nickels duct taped to one fan blade did the trick, but duct tape adhesive is somewhere between a liquid and a solid ...

mike1102,

That's what I was getting at: how to determine what adjustements are needed, and how to adjust the fan.

I don't know what sort of fan you're picturing, but I don't see how I could rest mine on a screwdriver or awl. Mine looks more like this:
http://www.hansenwholesale.com/emerson/cf3900htw.jpg

You have identified an important point, though, with your static versus dynamic balancing. I would ask you though:

1) Isn't there a more scientific way to do the dynamic balancing?
2) Won't your dynamic balancing screw up the static balance?
3) How are you proposing to add counter-weights? Personally, I think Icarus' blade-bending might look nicer in the end...

Title: Re: Ceiling Fan Balancing
Post by mike1102 on Jul 16^th, 2003, 11:01am

James,
For static balancing, turn the fan up-side down, light fixture and all, and balance it on the down rod (the rod by which it hangs from the ceiling). And/or you could hang the complete fan from a rope attached to the center of the down rod such that the fan is free to hang any way it wants to, and observe the fan - it should be flat. If it isn't add/subtract weight appropriately.

For dynamic balancing (if you've ever had your tires dynamically balanced) you know the equipment is more complex. There is a mechanisim that mounts on the wheel, a strobe light with a trigger system and usually some sort of counter-weight adjustment mechanism. Very simply, the tire is spun very fast (about 80mph) the strobe triggers when the tire is at its maximum off-center point - makes it easy to identify the out-of-balance point, and counter weight is added until the off-center is reduced to acceptable limits. Often, the static balancing step is omitted completely ( I think this is a mistake). Nowadays, the whole thing is automated. The vehicle's suspension system can complicate matters a bit but that's another issue. However for a ceiling fan, angular velocity is relatively small thus, static ballance should be pretty good for all but the highest fan speed.

"If a wheel is to spin freely without exerting forces or torques on its bearings, then it must be not only statically balanced, i.e., with its center of mass on the axis of rotation, but also dynamically ballanced, i.e., the axis of rotation must be the principal axis of the inertia tensor, as any automobile mechanic knows." (from "Mechanics" by K. Symon.) This comes right out of Euler's equations of motion for a rigid body.

Unfortunately, without knowing your fan's inertia tensor, the applied torque, w and dw/dt, adding the correct counter weight is a trial-and-error sort of thing. You could probably get by without knowing the tensor and the torque but you would some how have to measure w and dw/dt. I think it's easier to go with trial and error for the weight. Finding where on the fan to put the counter weight is the hardest thing. Without the strobe, your best bet is watching the plumb line.

Title: Re: Ceiling Fan Balancing
Post by James Fingas on Jul 16^th, 2003, 1:23pm

mike1102,

My kitchen fan's so unbalanced you don't need a plumb line. You just need enough courage to actually turn the thing on.

Here's a question: presuming you could actually tell which blade is the most "out of balance", is it always appropriate to put the weight on the opposite side? Do we necessarily model the fan as a body rotating around its center of rotational inertia, or could we model it as a damped oscillator leading or lagging the imbalance?

And there are definitely things you can do at home without a strobe light to determine which direction the out-of-balance point is. Though if you have a nice electronics setup at home, then you could make a strobe light ;) but not even I'm that serious about fan balancing...

It may be true that static balancing will help the problem, but I would argue that if we examine the ways in which ceiling fans probably become unbalanced, then it is very likely that naively static balancing the fan won't do a good job dynamically balancing it. Then if you naively dynamically balance it, you'll screw up your static balancing.

Title: Re: Ceiling Fan Balancing
Post by towr on Jul 16^th, 2003, 2:26pm

if the blades are easily detachable it might help to first take them off and (visually) compare them and perhaps also weighing them.
You can then try to bend and/or file them to be more similar if that happens to be the problem.

Title: Re: Ceiling Fan Balancing
Post by mike1102 on Jul 17^th, 2003, 6:25am

James,
Are you sure your fan, the down rod, the motor and/or the blades and their mounts have not been bent or damaged? Does the motor turn evenly and easily? Is it out-of-round? From what you describe, it sounds as though there is something really wrong. I've hung three or four ceiling fans and never encountered the problems you're having - never had to balance the fan.

As far as modeling goes, the wheel, fan, prop, propeller - whatever is treated as a rigid body, even if the propeller's pitch can be "adjusted" externally - as in an airplane. Its the media that can change - air pressure, water (laminar or turbulant flow and everything in between). All this will affect the motion of the turning wheel and cause it to deviate about its "ideal" axis of rotation. I suppose perturbation theory approach could be used to include these forces - never tried it or read anything by anyone who did.

As far as counter-weights go, once you know something is out of balance, you can add weight to the opposite side or reduce weight on the heavy side. Unfortunately with a 5-blade fan such as yours, there is no directly opposite side.

Another thing to consider is blade pitch - they all must be the same ( typically about 14deg) If they're not - that's your problem - one of them is moving more or less air than the others. That's something you can easily measure. My guess is that you want to hold blade pitch to less than +/- 0.25deg.

Title: Re: Ceiling Fan Balancing
Post by James Fingas on Jul 17^th, 2003, 11:08am

mike1102,

I assume that the fan has been bent in some way--it turns freely but wobbles very strongly. But just looking at it, it's not obvious what's wrong. The fan as it is is unusable. There's not necessarily anything seriously wrong: I would guess that merely unscrewing one blade from the fan and putting it back on could introduce a very large wobble.

When I said "model it as an oscillator", I am suggesting that the fan is a pendulum, with the motor/blades as the weight. The down rod is attached to the ceiling with effectively a ball and socket joint, so the pendulum is free to swing. The forcing function is simply the eccentric weight (or more importantly, eccentric moment of inertia) of the rotating bit, rotating around. All I know is that when I tried to put the weight directly opposite from what appeared the heaviest part of the fan, it didn't balance quite right, so either I was misjudging where the eccentricity was, or the wobble was leading or lagging (I guess it would be lagging) the eccentricity.

In a frictionless world, the oscillations would build up until they brought the forcing function to zero, but in the real world, there might be a significant lag.

And I never found a problem with five blades--odds are that the eccentricity doesn't line up exactly with any one blade anyways, and it's difficult to measure the location of the eccentricity to within 30 degrees.

Title: Re: Ceiling Fan Balancing
Post by grimbal on Apr 30^th, 2004, 4:51pm

There would be a device to center a fan automatically. It is a perfectly round pipe with steel balls inside. That pipe must be centered on the axis of the fan. If the center of gravity is not along the axis, the balls tend to move away from the axis, to the opposite direction in the pipe. This restores the balance.

If your fan has a single screw for each blade, and it can turn around that, just loosening the screews would restore the balance. Model helicopters are balanced like that.

First, you should make sure the arm is fixed to the ceiling somewhere in the axis of the arm. And that it can move freely. That means, the fan must really be hanging. Then, make sure the arms is vertical. If not, adjust the angle of the blade to move some mass in the direction in which the arm is off-center.

Then for fine tuning, you can fix a piece of paper to the underside, run the fan, and try to make a dot in axis of rotation. More likely you will get a circle. You want that circle properly centered. If it is not, you can move the center of mass by changing the angle of the blades towards the direction from the circle to the wanted position.

Title: Re: Ceiling Fan Balancing
Post by blue07 on Mar 6^th, 2009, 10:56pm

However, when the blades get out of balance, some of that efficiency is lost. What happens is that over time and with a lot of use, a ceiling fan can get here...

_____________________
Fanimation Fans (http://www.ceilingfantasia.com)

Title: Re: Ceiling Fan Balancing
Post by Grimbal on Mar 7^th, 2009, 9:31am

Hm... is that an advertisement I see down there?

Anyway, you seem to be a specialist. How do you guys balance ceiling fans?