Book 3

I mentioned that after winding and epoxying, the poles are not
equally heavy. Static balancing, as discussed in the preceding chapter,
will correct this. What it will not correct, however, is the weight
distribution of the poles. One pole might be heavier on the com side,
another towards the opposite side. Obviously, we again have some
kind of unbalance, and this is called dynamic unbalance. Everything
is OK as long as the arm is motionless, but when it starts to turn, it
will try to whirl, as the term is. The whirling will again cause vibrations
and friction with subsequent slowing down and heating up.
NOTE: total unbalance can be seen as the sum of two unbalances,
static and dynamic. We can remove the static unbalance, but this
still leaves the dynamic kind.
As for ourselves, we cannot do anything about this with our
primitive equipment. To detect and correct dynamic unbalance, it
takes an experienced operator and a machine worth hundreds or
thousands of pounds. Luckily, there are a few balancing firms that
will do the job for us - against payment, of course.
The nearest firm for British readers would be Model Racing Car
Centre at 390 Brockley Road. London, S E 4. Their price for a balancing
job (using holes) is 10s.
Across the Atlantic, we have the famous Thorp of Pomona, who
does all the balancing for the U S. pro teams. The address is John
Thorp, 143 West Commercial, Pomona. California 91766. They charge
you $2.50 but will make an absolute perfect job and in addition true
up the com for you.
If you decide to send your armatures for dynamic balancing, don't
forget to include a cheque or money order for the correct amount plus
postage back to you, and your name and address. Make sure your
armatures are carefully packed up, so that they will survive the trip
without damage.
NOTE: dynamic balancing is, of course, superior to static, so I
strongly recommend it for your racing arms. A good knowledge of
static balancing is always an asset, though, if you are faced with
the problem of quickly fixing an armature overnight.
A few last words on balance, before we leave the subject. A
balanced arm might become unbalanced with time, because of centrifugal
forces, thermal expansion or shocks - like from hitting the floor.
Therefore, it might be necessary to rebalance the armature when it
starts losing its zing. And if your armature is dynamically balanced,
solder your pinion, don't hammer it on!
4r * ★ *
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