ceiling and table fan

The direction of some fans can be reversed (summer - winter actions)

The Coriolis effect could be a reason but in SA we also use imported fans (from the northern hemisphere) and they do work.

A CCW rotation viewed from the bottom is a CW rotation viewed from the top.

I think the real reason would be the mounting flange available on a standard motor.

Believe me, there are exceptions to your statement. There are also logical reasons. A ceiling fan can be reversed - summer: fan blades push air down - winter: fan blades push air up and down walls. In general, there are two types of fan blades: pushers and pullers. As imagined, pullers create a low pressure condition (pulling in air) on the intake as they accelerate the air out the front over the blades. Ceiling fans are low evergy, low rpm pushers. Take a look at the design of the fan blades. Now to the more logical question. Motors windings are standardized. I'm sure you can figure out in which direction. Its easier to change the tilt on the blades than the windings on a standardized motor.

Air (as in Airhead) gets dizzy quickly, so it those houses where both types of fan are in use, it is customary to "unwind" the air that was passed through one fan, by reversing the direction through the next.

We want to stay on good terms with our Air, so please remember this little action and you will remain a friendly Air(head) for all time!!

Excellent answer!

I recently designed an atmospheric devortexer for that very reason. It detects the Coriolis effect within an air space and automatically emits vortices of counter-rotating eddies. These little anti-twisters negate the vertiginous effect of the air in question, rendering it stagnatable and stable.

...and if air misbehaves, there is always the dread squirrel cage.

...of course!! Well said!

I love you guys.

-A-

Wow. Very good answer JJB! (or is it JB JB? ... not sure if the "double" applies to just the J, or the JB combo)

Although all the answers I see in all these threads are usually useful, seldom are they as definitive as that one was. Doesn't seem to be any more that can be said on that subject. Mystery solved!

First guess was right it's JJB

Of course I don't know how definitive my answer was because STL fely the need to expAND on it!

I have replyed in my somewhat low key way though.

"The reason for the rotation directions being different is on your normal desk fan the blades are mounter to the armature shaft.

In ceiling fans the blades are mounted to the housing. and the armature is solid."

Double J B,

Do you know this for a fact, as a fan designer yourself, or what is your reference source for this information? Or is this just an educated guess?

I am not a fan designer, but as mechanical engineer who has worked with a lot of different motors, your answer does not entirely ring true. First of all, if you check the Wikipedia article on "Ceiling Fans", there is no mention of rotation, but there is a wide description of various type of motors and mountings used of the history of electric ceiling fans, including some, but not all, as you described.

I think a more telling reason is the axial load on the fan and the types of bearings that might be used. In many motor applications there is minimal axial load, the rotor drives and output shaft which is coupled to its application by a pulley and belt, or by gears, or even in a direct drive application, where only torque is involved. In this case only a very simple thrust bearing, such as a bronze washer, may be all that is needed, and only to keep the motor from moving axially due to vibration or other minor forces.

However a fan usually has blades attached or coupled directly to the output shaft, which may be part of the rotor itself. In a table (or floor) fan, or even the ubiquitous ceiling or wall-mounted oscillating fan, the blowing force of the blades produces an equal and opposite axial load and so the thrust bearing must be designed to resist this force with minimal rotational friction. Sometimes the radial bearings will have enough thrust capability, but often special thrust bearings or combination bearings, like tapered roller bearings, must be used.

The ceiling fan usually operates at a lower speed than the table fan and often generates much less axial force, however the main difference is that gravity is now working on the rotor in the opposite direction to the downward blowing force (which pushes the rotor upward). Due to the size and weight of the blades (and flywheels if so designed), the bearing that works against gravity must be substantial.

Of course, motors may be designed to resist axial loads in both directions, but it is certainly cheaper to use only one thrust bearing instead of two. If motor design is more or less standardized, uni-directional thrust capable motors should normally have this thrust capability to resist the output shaft being push into the motor, as in a table fans, drills, screwdrivers, etc. Therefore, the table fan will be designed so that it rotates in the common clockwise motor direction.

To design a ceiling fan in the same way, so that the larger axial force (from gravity) is countered by the same thrust bearing, the motor must be turned around and the shaft extended in the opposite direction to attached the blades (or else, as suggested, the shaft becomes fixed to the ceiling and the whole motor body turns inside a protective housing, becoming a large flywheel. In this case, without reversing the motor, the output shaft which was turning clockwise (CW) coming out the other end of the motor, now turns CCW. The motor itself did not change direction, only your perspective.

If this is not clear, picture a bicycle wheel or motorcycle wheel, up on a stand, jack, or hoist for repair. On one side (the right side) of the vehicle the forward turning wheel appears to be turning CW, while moving to the other side the wheel appears to be turning CCW. Did the wheel change its spin? Of course not, but your perspective changed!

My thinking is that as electric ceiling fans were originally developed, they borrowed from existing motor design, possibly even from table fans already in use which turned CW. Motors often have their shafts extending out in both directions. Indeed, if you check an oscillating fan you might see that the oscillating mechanism runs off the back shaft of the motor while the fan blades are mounted on the front shaft. Oscillating motion is stopped merely by disengaging the mechanism from the drive. It would be a relative simple task then to take a standard fan motor and mount it backwards so that the greater gravitational force (which would counter any upward thrust from the fan anyway) is resisted, requiring the blades to be mounted oppositely from a table fan, since the rear shaft is now used and turns CCW.

Of course, new designs of fans and motors may not have such a restriction. In fact, reversible box fans must have thrust capability in both directions (very common when box fans were placed in windows), whereas even a reversible ceiling fan will always have its major axial load due to gravity and in one direction only. However, once a standard is set, it is hard to change, especially if there is no apparent advantage in doing so.

Then of course, I could be wrong!

Ok STL now that was long winded.

First no I am not a fan designer, however I have sold them and met with a few factory reps on this and I have been told that the purpose for them being built the way they are, is as I stated in my original rendition.

"First of all, if you check the Wikipedia article on "Ceiling Fans", there is no mention of rotation"

As far as the rotation is concerned, they are reversible so its a moot point as to whether or not the rotation of the armature is the same or different than the desk fan. Typically however the housing or "field" is rotating in a counterclockwise direction and the armature is stationary. Which would translate into a clockwise rotation of the armature if the housing was mounted stationary.

"To design a ceiling fan in the same way, so that the larger axial force (from gravity) is countered by the same thrust bearing, the motor must be turned around and the shaft extended in the opposite direction to attached the blades (or else, as suggested, the shaft becomes fixed to the ceiling and the whole motor body turns inside a protective housing, becoming a large flywheel. In this case, without reversing the motor, the output shaft which was turning clockwise (CW) coming out the other end of the motor, now turns CCW. The motor itself did not change direction, only your perspective"

I believe I said this but is quite a bit more simplified language. I believe in the K.I.S.S. methodology when it will suffice.

I could have misstated the concept of the housing acting as a flywheel for the purpose of stability but I don't think so. However it could be described as more of a harmonic balancer type operation as well.

At any rate when the blades are attached and not properly balanced the blasted thing will wobble anyway. Probably not as much as if they were mounted to the armature then suspended from a single mounted strut as ceiling fans are.

Course the more I think about it, there is the possibility the the axial load on 52" celing fan blades could be quite stressful at the connection point wher the blades meet the armature shaft.

Then again there is the magnetic field that has to be considered. Which I am sure would be better explained by a physics major than myself.

You just can't stand it when I'm right can you.

The reason for it is determined by which way the motor starts.