Balancing Leslie drum?

[David Anderson]

Has anyone come up with a good process for balancing an out-of-balance Leslie drum?

I've got one that's unusually out of balance, and, thus far, my efforts to balance it by adding or subtracting washers haven't worked as well as one would hope. It's better, but still not great. I've tried holding the axle horizontally and seeing which side ends up down. I suppose being made of natural materials that can vary in density, some are inherently better balanced than others.

[geoelectro]

Usually my replacement speaker is lighter than the original. I do horizontal for checking. I have found using 6x9's from Rodgers speaker cabinets (which I have a stock of) and adding weight to the speaker gets it close. These speakers have a magnet "sandwich" which is the ceramic magnet between two steel plates. I disassemble the bad one to get those plates. By gluing an extra plate on the replacement speaker the balance is very close.

I've tried taking weights off the other side with the lighter speaker but it's hard to get right.

Geo

[David Anderson]

I should have made it clear that this is a 122 drum, not a Rotosonic. Apologies.

[muckelroy]

David,

I've never gotten it perfect with the washers via horizontal bearing / drop test, but I've gotten it close. In those instances the improvement was massive and it was quiet on rotation.

I assume you have tried reinstalling the drum, and it is still loud or eccentric?

[David Anderson]

I assume you have tried reinstalling the drum, and it is still loud or eccentric?

If I install it and hold the upper bracket by hand, not screwed down, there's a lot of side-to-side force. I did the same thing to another one I'm working on, and it was pretty stable. The more stable one has weights in more spots. I have a feeling that the factory process on balancing was not that consistent.

[Hamman]

Remember...there is static balancing and then there is spin balancing;-)

[David Anderson]

Remember...there is static balancing and then there is spin balancing;-)

So I should take it to Discount Tire? :->

[muckelroy]

This might be sacrilege. But could putting gaffing tape in areas needing more weight help? At least, as a temporary balancing experiment, if nothing else.

(That is, in areas where no washer mounting hole is available.)

[Wes]

David, the technique I have been using on my 31H (which turned out to have a slightly tweaked axle) is based on a common technique used for balancing motorcycle tires.

1 - put the Leslie flat on its face
2 - spin the drum
3 - when the drum stops, mark the high spot with chalk
4 - repeat steps 2 and 3 a few times. If the high spot is random, you are finished
5 - add weight at the high spot (unless there is weight 180 degrees away - in which case, remove some)
6 - erase chalk, goto 2

This will be easier with the woofer out.......you could also construct a jig to hold the axle in a pair of pillow blocks outside of the Leslie, which would make it even easier. For bike tires, I put the axle in a pair of jack stands, but this won't work with a Leslie... a motorcycle axle is stationary and the bearings are in the wheel hub.

Wes

[Tim_at_Jonas]

If I install it and hold the upper bracket by hand, not screwed down, there's a lot of side-to-side force. I did the same thing to another one I'm working on, and it was pretty stable. The more stable one has weights in more spots. I have a feeling that the factory process on balancing was not that consistent.

Just hole a piece of chalk and get it gradually closer until it touches the wobbling unit. Add weight 180 degrees from the chalk mark.

[Bradley]

This is just an idea but...

The RC plane guys have used an accelerometer app running on an Android or iPhone to balance their electric ducted fan propulsion units. The fan has a different center of mass than the motor. Their effort is to rotate the fan relative to motor shaft in 45 degree increments and to find the angle that produces the minimum measured acceleration, thereby corresponding to the best angle in which the two forces partially cancel. Then they add weight at that spot until it minimizes. An excel spreadsheet does some minor juggling on the raw data dumped from the phone app. In the case of the Leslie rotor, a known weight could be added to the rotor at 45 degree angles and similar measurements taken. Then it should be possible to calculate the weight needed at that angle at a given distance from the rotor axis. Weights at two other but fixed locations should be calculable to give the same angular offset and mass as the single weight if required.

Is the accelerometer sensitive enough to achieve a reading when strapped down to the speaker baffle and at the rotor's relatively low angular velocity? (easy enough to test i suppose)

Would the motor vibration defeat the measurement?

It'd be a science project for sure. :'( How to dynamically balance EDF unit with a cell phone

[David Anderson]

I did try Wes's method and achieved some improvement.

What I would add is that not only are we dealing with balance in one horizontal plane, we're dealing with a depth dimension as well. So, after you've marked the high point with chalk, you may also have to experiment with whether to put the balancing weight in the middle of the rotor or at the top or bottom.

[toomanytoyz]

I realize that this might not help, but I use an industrial strobe light. By varying the speed of the strobe you can watch the rotor in "slow motion" to determine exactly where the heavy and light spots are. I have two rotors I need to do, (one from Leslie where a PO stuffed the rotor full of foam and removed the factory weights, one which was spliced together from from two other water damaged rotors). Maybe I should post pictures when I do it.