7 replies to this topic

[Alchemist]

I am only familiar with the "razor blade" method of testing an armature for "out of balance".

 

When it comes to balancing an armature after being rewound, I've always wondered and never thought to inquire until now, how do you decide where on the lamination to drill out material?

 

What is the methodology in balancing an armature please?

 

Thank you.

 

Ernie

[havlicek]

Hi Ernie,

     Static balancing ("razor blades") doesn't tell you anything about where on each pole to drill, only what pole is heavy and *about* how heavy it is (*depending on how good your jig is, and how level it is).  So with static balancing, the arm doesn't care "where" you drill on the pole.

     Dynamic balancing is a whole different thing, and I don't do it, so others will have to fill in the blanks.

[Champion 507]

Over 50 years ago I lived in Huntsville, AL. Huntsville is where the Saturn boosters were developed and built for America's moon program. My dad was one of the many engineers. He totally understood the difference between static and dynamic balancing because a number of pumps inside the Saturn booster had to spin at high rpm and smoothly.

 

He told me, take the weight out of the middle of the arm stack. With razor blades, you don't know which end is heavier. If you take the weight out of the middle, you aren't going to be too far off from it's heaviest point. If you remove weight from the comm end and the stack is heaviest on that pole at the back end, the harmonics you create will be greater than if you remove the weight from the center.

 

That's why today I still go for the middle.  

[Champion 507]

Two older Mura untagged 12 arms I did recently.

 

[Slot-Racer]

You want to get a magnetic balancer for the best accuracy. You want to balance by letting the arm spin, and the heaviest area that needs to be removed will always fall to the down position. You keep doing this until the arm starts to wobble back and forth. When this happens you are close to an ideal balance. Now you have to start removing little by little always marking with a marker noting where you are removing material. The wobbling will start to turn into complete large turns and longer and longer back and forth wobbling. This means you are on the right track. If it starts to wobble less you are off. You will get closer and closer to a point where when you spin the armature it will never fall heavy to any one spot and always stop spinning in a different position. 

 

P.S. You can look up how to balance a boat prop for RC boats as it's the same principle and concept. 

 

There is one method I used that no one has come up with which is added balancing. The principal is to help you visualize where the actual weigh it coming (to remove) from when balancing. The neat thing about this concept is that it is dynamic, meaning you can move it around to find the area needed instead of guessing. Using very small pieces of putty, I would stick small pieces to areas opposite of the area(s) that fall heavy. In doing so I can actually confirm that is the area that needs to have material removed to get a balance instead of just guessing. I did this with rigger boat props for years and always achieved a perfect balance and compared to a machine factory balanced prop that cost way more, the speed difference was insignificantly small in gain. 

 

Hope that helps. 

[Alchemist]

Thank you John, Doug, Mario for the information.

 

Doug, thank you for the photo example!

 

It's very much appreciated!

 

Ernie

[havlicek]

  Mario's explanation of static balancing is a good one.  I have a different take on a couple of points though (*but any means of static balancing should be considered as really only "OK" for very mild winds).

 

 

 

You want to get a magnetic balancer for the best accuracy.

 

I dug in to this method some years ago, and was always able to get a better finished product using razor blades.  The magnetic type balancers rely on a perfectly concentric point being ground on one end of the armature shaft, otherwise the armature is rotating on an axis that isn't the same as the armature's true center.  It "seems" like this shouldn't matter much, but it can.  On some armatures, you may wind up having to grind that point uncomfortably close to the coils or the com. 

 

Also, these balancers use a pair of very strong neo magnets acting on the ends of the armature shaft.  One end is floating, slightly away from the magnet, the pointed end is touching the opposite magnet.  In order to suspend an armature in mid air, the pointed end of the arm is being pulled pretty hard against the face of the magnet, so the armature's ability to spin easily doesn't seem any better than when rolling along razor blades.  

On the plus side, the magnetic type static balancers don't rely on having a perfectly clean and round shaft, while of course the razor blade type DOES.  With the magnetic balancer, you should be able to balance even an arm with a splined shaft.

***If I were going to use the magnetic balancer, I would have a short shaft end adapter made with perfectly centered cone ground on it's end and a snug perfectly centered 2mm bore, made out of hardened steel.  This would be no small thing, but it would at least help avoid more work and possible problems down the road.

 

 

 

There is one method I used that no one has come up with which is added balancing. The principal is to help you visualize where the actual weigh it coming (to remove) from when balancing.

I've done some of this, and documented it here somewhere back in my bazillion posts Mario.    Couple of thoughts:

1) You can definitely do balancing (*at least static balancing) by adding weight, rather than removing it by drilling or grinding.  Just on general principle though, adding more rotating mass to an arm isn't ideal.

2) The steel and copper that make up almost all of an armature's mass are both dense/heavy materials.  Whatever is being used to permanently add mass to the "light poles" of an arm (*as opposed to removing it from the heavy ones), must stay put under the great stresses exerted by a spinning arm...some sort of epoxy I suppose.  Most any commercially available epoxy, even something like JB Weld with it's powdered iron content, is far less massive than steel or copper.  That means you have to add a LOT of epoxy to make up for a little steel or copper.  Maybe there's a more dense epoxy with something like powdered lead filler, but whatever you use has to be really tough.

***I also did some (*and still do occasionally) "prebalancing" on arms when I think it *might* be a good thing, by removing a small amount off the edges of the stack.  I guess you could use additive balancing for this, and then the coils and final epoxy would help hold it in place.  It would still take a lot of material being added, versus slightly removing some, and any added material would have to no be "in the way" of where you'd want to wind.

[SlotStox#53]

Have one of your big block Padlock rewinds John that you used JB Weld as "added" weight balancing. Works great