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I would advise against him stopping the fan with his hand for a few reasons, some of which have nothing to do with the fan motor. He is correct in that stopping the fan with his hand will probably not damage the motor directly. However, there are two other factors to consider... the type of ceiling mount and the fan blades.
If the fan is mounted strictly in accordance with the current NEC (national electrical code) which requires using a specially installed ceiling box bridging across two ceiling joists, he probably won't damage the mount. However, if the fan is located on an ordinary ceiling box, there is the risk that his maneuver will eventually loosen the box or even cause the fan to abruptly drop from the ceiling. Even with the more solid mount, there may be eventual loosening of the bracket which might not cause the fan to fall but may cause some wobbling. The bracket is not designed to take that sort of stress.
Even if the fan is securely mounted, there is the issue of the fan blades... specifically the balancing of the blades. Constant sudden force on the blades can cause them to become misaligned and out of balance. This can be caused through slight bending or movement in the blade mounting brackets themselves or even in slight bending in the blades. Though slight imbalance is not uncommon and hardly noticeable in most ceiling fans, increased imbalance may cause the fan to wobble or shake, especially at higher speeds. This misalignment will become more and more pronounced making the fan become noisier and putting unnecessary stress on the motor due to the uneven force… in the worst case possibly leading to motor damage. Though less likely, there is also the chance that one of the blades may crack, requiring replacement, as they are not designed to take this type of abuse.
To close out, what your friend is doing may not have any effect at all on your fan or it may have a disastrous effect. Considering these two possibilities and the fact that stopping the fan abruptly is a choice and not a necessity, I think that a better choice can be made... don't touch!
Ceiling fans are both energy saving and aesthetically a hoot! When I walk into a room with a slowly revolving ceiling fan, I just want to throw off my shoes and collapse on the lounge chair… awaiting the next Pina Colada. Of course, if the lounge chair in under the fan you want to be sure that the fan is going to stay where it belongs… on the ceiling.
A wobbly fan is usually (but not always) caused by imbalance in the blades and/or the blade brackets. Though it would be easy to blame poor manufacturing and quality control, the fact is that perfect balance is almost impossible, especially since the unit is assembled onsite under "human" conditions. Just a slight variation in a blade caused by "play" in the mounting holes can cause a wobble. Fortunately, unless the imbalance is substantial, the problem only appears when the fan runs at higher speeds.
The first thing to determine is if the fan itself has loosened from its ceiling mount. Try to move the body of the fan… does it appear to solidly attached? Be careful… if the mount is very loose it could fall into your hands! The fan will have a cover or plate that will allow you access to the mounting screws should you need to tighten them.
If the fan seems firmly attached but the mounting box seems to be moving or sagging… RED FLAG! You fan may have been mounted onto a non-code electrical box. Nowadays, all ceiling fans must be mounted onto a special electrical box that is firmly attached across two ceiling joists with special mounting brackets. In contrast, typical electrical boxes are only attached to the side of one joist… great for low weight chandeliers or normal lighting fixtures but not so great for heavy moving objects! If this appears to be the case, you should get an electrician to examine the box and if necessary install the proper supports… or replace the box entirely.
If you are a lucky pumpkin and the fan mounting is secure, examine the blades. Are all screws tight? Tightening a few loose screws can be all that is needed to end the wobbling. If all the fasteners are firmly attached, then your mission, should you decide to accept it, is to balance the blades.
Blade balancing is similar to automobile tire balancing… except you don't have fancy computerized equipment to help you! It is a matter of trial and error… attaching small weights to the blades one at a time to determine which blade is heavier or lighter. There is an excellent FAQ on ceiling fans at Hansen Wholesale at http://ceilingfans.hansenwholesale.com/. They explain the entire procedure used to balance a ceiling fan, though they recommend purchasing a balancing kit. Another website, All About Fans, has a really gritty and down-to-earth method of balancing that does not require a kit. Their address is http://www.faninfo.com/.
If I had all day I'd list my pet peeves about the lack of instructions and common sense in product design today. But I don't, so here's a simple answer...
Ceiling fan blades have an obvious tilt. To push the air downwards, the top of the blade should be leading when it turns. To pull the air upwards, the bottom of the blade should be leading the way!
Generally, most people like to have the air pushing downwards in the winter to bring the hotter ceiling air to the floor. In the summer, there are differing opinions. Some folks like to feel the cool breeze, so set the fan to blow downwards. Other people set the fan to blow upwards, equalizing the room temperature without as much of a "breeze".
As always, personal preference rules. I'll take mine strong with milk and sugar, please!