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Orthoganol balancing - twinwheel grinders

A bit of extra info on orthogonal balancing.

With the balance weights mounted close to one side of each wheel the system balances out the radial unbalance component of each wheel. However it takes no account of unbalance in the axial direction i.e. the wobble if the wheel is not properly mounted square to the shaft.

This is not normally a problem but, to avoid possible trouble, be sure that the wheel mounting washers are accurately square to the shaft and also dress true both the wheel periphery AND the wheel sides before balancing.

In my initial experiments I loaded the voice coil with a

5/8" dia x 5/16" slug. This gave me a lot of trouble with the 100Hz torque variation signal from the single phase motor. This is because I had accidentally made the "accelerometer" resonant at 100Hz! I doubled the slug thickness, this dropped the resonant frequency to about 75Hz and I had no further trouble.

Your speaker may have a different suspension stiffness so it's worth checking the loaded resonant frequency. The easiest way is to lightly tap the slug and to use the 'scope to monitor the damped sinusoid that results.

Jim

Reply to
pentagrid
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We have three articles on balancing on the website

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One of the "No Phase" articles was written by Larry Meidel who is a well known expert on helicopter vibrations. He showed how a balancing was done with 4 readings on a three bladed helicopter. I modified his work for a two bladed helicopter and one of the subscribers to the Experimental Helo magazine, after reading the articles, balanced his tail rotor using just a dial indicater. All of these articles are available on our website. They are found on the left of the page down a little under special articles.

Stu

Reply to
Stu Fields

Thanks for the reference - these are useful articles. The helicopter group three axis "amplitude only" method uses the same basic approach as the orthogonal system. Pursued to a conclusion both methods should yield exactly the same results.

The helicopter group method has the advantage that the result can be a single balance weight or two weights at freely chosen spacing - this gives the least total added weight. The orthogonal system aims to finish up with 2 weights at 90 deg which may be, up to root 2, heavier than the equivalent single weight.

The 3 axis system achieves this by a careful series of amplitude measurements followed by vector calculation of the results. The penalty is that it loses the quadrature independence of the orthogonal 90 deg balancing axes - fine tuning by cheerfully adding a bit of weight to see if the balance gets better or worse is no longer an option.

The 2" speaker "accelerometer" I used is not suitable at the lower RPM of helicopter blades - more mass and longer travel is needed. I did some initial work modifying one of the self powered flashlights which use a large rare earth magnet shaken within a coaxial coil to charge up a couple of lithium cells. This has along available travel and lots of output. However I finished up with the speaker because it's a simpler modification and all that is needed at grinder RPM.

Jim

Reply to
pentagrid

I have not tried either method yet. My instinct is that the 3 axis system could do fine tuning by first doing a balance using a fairly large test weight. And after completing the balancing, do it all over again with a test weight about 1/10 of the original test weight.

Dan

Reply to
dcaster

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