How to balance an RSG rotor (long post)

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx>

Rob -

The ultimate solution is to balance all the rotating components as a set,
with the angular position ("indexing") of the RSG disk locked to the motor
armature (or rotor) via a hub with a keyway, etc. Almost all motor armatures
are balanced at the factory to a certain tolerance; i.e.there is always some
remaining uncorrected imbalance. Likewise, an RSG rotor assembly that has
been "balanced" by itself will have some residual uncorrected imbalance.
When you assemble the RSG rotor to the motor armature, the remaining
imbalance values can either be additive, resulting in greater vibration, or
subtractive, with less vibration.

A very good static balancing support can be assembled in an hour or so using
a set of four small instrument ball bearings. For best results, the bearings
should be of the flanged type (flanged outer races), as this will minimize
the area in contact between the motor shaft and the bearings. Build a simple
structure to hold the bearings in pairs to create a "rolling vee-block", and
far enough apart to ride on the armature shaft just outboard of the ball
bearings (or the portion of the shaft where the bearings seat). Make sure to
meticulously wash the bearings in solvent to remove any grease or oil that
would damp free rotation.

Next, you will need to decide if you want to correct imbalance by ADDING
mass, or by SUBTRACTING mass. Both methods are used extensively in
commercial balancing. Removing mass can be done by drilling, filing,
grinding, etc. However, it is permanent, and you better be sure you know
exactly where you need to remove mass before you attack your RSG rotor with
a cutting tool. Adding mass is a bit more forgiving, especially if you have
designed-in a series of small radial tapped holes all around the OD of the
rotor to allow the addition of correction mass by using small set screws.

Assuming that you have done all of the above, remove the motor armature from
the motor housing, set the motor armature (with RSG rotor attached) in the
rolling vee blocks, give it a slight nudge, and allow the assembly to coast
to a stop. If you are going to correct imbalance by adding mass, mark the
exact top dead center of the rotor ("light spot") with a pencil when it
comes to a rest. Repeat this several times to ensure you have accurately
located the light spot of the assembly. If you are going to remove mass,
mark the "heavy spot" at the bottom of the rotor.

Now that you have found the light (or heavy) spot, you will need to correct
the imbalance. If you are adding mass, install a set screw in the hole
nearest your "light" mark. Repeat the rotate & coast-to-a-stop process. If
the light spot still ends up on top, use a longer (e.g. heavier) set screw.
If you have installed the heaviest set screw you have and its still too
light, add another set screw in the nearest adjacent hole, or better add two
more set screws, one at each of the holes on either side of your first
correction mass. To provide some range of coarse adjustment you will need to
have a selection of set screws of masses (different lengths). Also,
adjusting the depth that the set screw is installed into the rotor gives you
the ability to "fine-tune" the balance. The closer the mass is to the OD of
the rotor, the more effect it will have.

Repeat this process until the rotor stops at completely random positions
each time it comes to a stop. Permanently mark the relationship of the RSG
rotor to the hub to ensure that it can be reassembled correctly if it is
ever disassembled.

Without an electronic balancing system, this is a pure trial-and-error
process.  As you approach (near) perfect balance, you may also have to make
subtle adjustments to the angular position of the correction masses to
achieve the best possible balance.
Obviously, if you are correcting imbalance by subtracting mass, you will
perform this exact same process, but remove material from the heavy spot.

For very small (lightweight) rotor assemblies, you can also throw together a
"knife-edge" support system using a pair of brand new box-knife blades
hot-melt or cyanoacrylate-bonded to a piece of plate glass or other
dead-flat, hard smooth surface. Some additional support on one side of the
blades is necessary so they don't just fall over under the weight of the
rotor. You can use hot-melt glue or cyanoacrylate instant glue to kludge
together small pieces of 1/2" aluminum square stock, etc to keep the blades
absolutely vertical and the edges parallel to the glass and so they can
support the weight of the rotor. Use a level and paper shims to get the
plate glass completely level on the work surface so the rotor won't roll
off. Use extreme care to VERY GENTLY set the rotor shaft onto the knife
edges. The slightest impact will create a dent in the sharp edge of the
blade and the shaft will no longer roll freely.

Now, this entire process yields a rotor assembly that is STATICALLY
balanced. This assumes that nothing deforms significantly or changes
location at the actual RSG operating speed. This assumption is probably
valid for rotors made from 1/2" thick epoxy-glass laminate ("G-10", etc) or
1/2" thick linen-reinforced phenolic. Any rotor made from 1/4" or 3/8" thick
polycarbonate or similar unreinforced polymer material WILL distort and
change its balance characteristics (and is not a sound design from an
engineering standpoint anyway).

Also, remember that as the electrodes erode, or are reground or replaced,
the balance will change. This is best controlled by very carefully matching
the weight of the electrodes initially, and keeping the set matched in
weight as they are reground or replaced. What you really want to do is
perfectly balance the rotor, and then keep the electrodes matched in weight,
even if the absolute weight of the electrodes changes over time.

Commercial balancing systems, for car tires, peanut-sized 100,000 RPM dental
turbines, or 100,000 lb powerplant generator rotors all work on the same
principal. Basically, the rotor is supported on a "table" that is suspended
so it can move very slightly in response to the imbalance of the rotating
mass. The table is equipped with two very sensitive displacement transducers
that convert the table motion into an electrical signal. Finally, there is
some sort of angular position transducer that provides an electrical signal
for rotor angular position.

The item to be balanced is rotated (it can be at very low speed, just enough
so the imbalance causes some table motion). The output from the three
transducers is processed by a computer to yield an imbalance force
magnitude, plus an imbalance force angular position. The balancing system
will tell you the angular position at which the balance correction(s) must
be made, and the magnitude of the correction mass to be added or subtracted.

Regards,
Scott Hanson



 ----- Original Message -----
From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Thursday, December 16, 2004 4:05 PM
Subject: RSG unbalance


> Original poster: Rob Maas <robm@xxxxxxxxx>
>
> Hi All,
>
> I am in the process of building an RSG. The disk spins at
> 2950 rpm (the motor is asynchronous, mains frequency is 50 Hz).
> The first trial run indicates a slight unbalance. It is
> probably OK, but if possible I like to improve the balance of
> the disk. Is there a systematic method to do this 'on line',
> i.e. with the disk mounted on the motor shaft?
>
> In a garage they use equipment to balance wheels: after a short
> spin the apparatus spits out what balance weight should be
> placed where on the rim to balance the wheel: anybody knows what
> the principle of operation is here?
>
> thanks in advance,   Rob
>
>
>
>
>
> ---
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>
>

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