RCU Forums - How To Balance The Bell On An Outrunner

Wrote a little "How-To" message after taking a crash course in balancing today...

After recently experiencing a rough running motor on one of my planes, I figured it was time to get into prop and bell balancing. I did some research on the net and found out that I should have got into this practice a lot sooner. Not only will a unbalanced prop or bell cause the motor to run rough and at less RPMS than the motor is capable of, but the vibration is also hard on the bearings and will cause the motor to run somewhat hotter. The final straw was when I also read that a motor out of balance can suck over one amp more power, causing added battery stress and shorter flight times.

So, how to go about this? I first made myself a homemade balancer, consisting of two magnets glued between to posts on a small board. I bought a Great Planes finger prop balancer ($2 at the local hobby store) to use between the magnets. The magnets were placed so that the prop balancer would contact one magnet but have a slight gap between it's other tip and the other magnet. The magnet pull will hold the balancer and prop straight if you have the gap close enough. You must also make sure that the magnets are glued in such a way that the side facing each other will attract to the other magnet. Have one flipped the wrong way and they will repel and won't work for our project. The magnets are the silver coated type (nickle or chrome) that provides a smooth friction free surface. If using regular bare magnets you'll want to sand or polish the magnet surface to get it smooth as possible. This isn't necessary but it helps reduce the friction even more.

I balanced a few props with my homemade balancer but was worried that maybe I wasn't getting the percision I wanted. Off to the hobby store for a Top Flite $20 balancer. This unit is just, again, two magnets suspended between a plastic stand and consists of a shaft and two cones to fix the prop onto, just like the great planes finger balancer. I re-checked my props that were balanced on the homemade balancer and found no difference in them on the $20 balancer which by now I wish I wouldn't have bought. Sure, it's nice to have but a homemade unit will work just as well and takes all of about two minutes to make.

Anyway, how to balance the props? Make sure you have the prop straight and tight on the shaft. The two cones on the shaft will snug it straight. However, props with larger holes may require you to flip one of the cones so that it's fatter end is pressed against the prop rather than it's cone shaped end. Put the unit on a fairly level surface and spin the prop. Sooner or later it will come to a stop with it's heavy side on the bottom. To confirm this I like to flip the balancer shaft over so it's ends have now switched magnets. Remember that the prop balancer shaft and it's cone's themselves might be slightly heavier on one side and this could slightly effect the position of the prop.

Now you have two options...sand the backside of the heavy prop blade (careful not to get near it's edges...try in the middle mostly) with very fine sandpaper, or brush some clear nail polish or CA onto the backside of the lighter prop blade. There is some debate as to whether the front or the back of the prop should be sanded or brushed onto so make your own decision. Just stay away from the leading or trailing edge of the blade and try to keep anything brushed on somewhat smooth. I'm finding that I like to lightly sand the heavy blade for ten or twenty seconds first and then re-check the weight. If it's still heavy then I start brushing clear nail polish onto the light blade. Go a little at a time and re-check. Don't worry if you put too much on. After it has dried just sand off the excess until the prop balances. A balanced prop is supposed to stay in whatever position you place it but that's near impossible to achieve in my experience. I just balance it until the prop will come to a mostly horizontal position at rest. Again, I also flip the balancer shaft and prop completely over to switch magnet ends to re-check. What I've found is that when it's balanced about right the flipping will reveal one blade slightly heavier than the other, while the other blade will register as slightly heavier when the shaft is flipped and spun the other way. Some people go so far as to balance each side of each blade to acheive the total balance (prop will stay in any position you place it) but for me that's way too much work and kind'a overkill if you ask me. Remember that the nail polish or whatever you brush onto the blade may loose a little weight as it dries. Just put it on a little too heavy and check it after it's dried. If it's still too heavy then sand off the excess weight.

Onto balancing the bells on the BP21 or 12T. There are several ways to do this but they all involve first removing the bell and shaft as one unit from the motor. Pop off the c-clip on the back of the motor shaft. There may also be a small washer behind it you'll need to take off. Now pull on the bell and shaft and they should pull straight out from the motor. It will be a little hard to do at first since the magnet pull is holding the bell in place.

Once removed you've got three options to balance the bell. The easiest and probably least accurate way is to flip the bell/shaft over and insert the prop end of the shaft back into the motor. If the shaft is long enough to ride on both bearings that's good but you can probably get by with just it resting in the first one. Only stick the shaft into the motor as far as necessary to avoid any magnet attraction that may alter the bell's spin. Hold the motor horizontal and spin it. The bell should come to a rest with it's light side up. More on how to balance it in the following methods...

The next option is to place two razor blades into a board with the cutting edge sticking up. Use a level to get the blades and board perfectly level. Now place the bell/shaft onto the blades at both ends and it should roll and come to a rest with the light side up.

The final option is to use the homemade magnetic balancer you made for the props. The shaft on the motor isn't magnetic so you can't ride it between the magnets. Remove the nut on the top of the bell and pull the shaft off. Now place the bell between the finger prop balancer cones on the shaft. Again, you may have to flip one of the cones backwards to get the bell to snug up straight. Spin the balancer between the magnets and wait for it to come to a rest.

Now, this is how to balance the bell with any of the above three methods. After it's come to a rest take a felt pen and right "1" on the outside of the bell at the very top (this is the lightest spot on the bell). Now spin the bell several more times and mark any other places it comes to a rest as "2, 3, 4", etc. You shouldn't find that you have more than two or three light spots showing up at the moment. Keep spinning it remembering the numbers to find which couple numbers seem to always end up at the top.

The next move is a judgement call. If more than 2 or 3 spots seem to be popping up and the bell doesn't seem to spin slower and faster in certain places then it probably doesn't need much weight. Try putting a glob of CA or clear nail polish inside the bell lip above the magnet (towards your open end of the bell). Let it set up for a few minutes and re-spin the bell. Remember to flip the shaft from end to end on the magnets to make sure you are getting the same reading either way. If a new spot keeps popping up consistantly then you've fixed the first spot. If the old spot is still popping up most of the time then it's time to add more weight. Cut a piece of solder about an inch long and form it to the inside of the bell lip. Put a dab of CA in the middle of the solder and let it set for a few minutes. Now re-spin the bell. If the solder is now resting at the bottom of the bell then you've got too much on. Snip off some on which ever end of the solder is closest to the bottom. Spin it again. If the solder is coming up near the top but not always then just add more CA the entire length of the solder and this should give it the little extra weight. You'll need to do this anyway to secure the solder completely to the bell.

Now go ahead and re-spin the bell. You'll probably find one or two other spots that keep popping up and will need to add solder, nail polish, or CA to them as well. Once you've fixed one to three light spots on the bell you may find that the bell is now coming to a rest in more than one place now....like 4 or more spots that it may stop at after each spin. That's when I stopped. Once you've fixed one to three overly light spots on the bell and it is near balance it will have a tendency to come to a rest in various spots (Again, 4 or more). Only continue if one spot keeps popping up 2 out of 3 spins. Going any further than that you'll drive yourself crazy trying to tweak the last little bit of balance out of it.

As a sidenote to these motors, one of the weak points is the bearings can come loose in the shaft hole and will soon destroy themselves. To prevent this some people pop out the bearing and place a little CA or loctite between the bearing's outter shell and the shaft hole. Just don't get any inside the bearing. To prevent this you might want to oil the bearing inners first. Another weak point is if you over tighten the set screw that holes the motor to the mounting plate it can deform the shaft hole and blow the back bearing. Tighten it too tight and you've got a bad bearing. Tighten it too little and the motor may come free and rip it's three motor wires out.

Hope this helps people get the motor running to it's best ability. You'll be able to hear the difference in a balanced prop and motor over what it was like before.