In article ,
Mike Henry wrote:
My used 7x14 surface grinder appears to need a new motor, or maybe just new
bearings. The spindle is belt driven and is currently fitted with GE K163,
1HP, TEFC, 3450 rpm 3-phase motor. The motor is being driven with an
Hitachi L-100 VFD from a 220 VAC single phase line.
With the belts off the motor makes some noise on startup that sounds like it
could be bad bearings. A cup of water sitting on the spindle housing
(spindle belts still removed) has some standing waves at full speed and the
wave amplitude varies with speed. I'm assuming that the motor itself or
perhaps just the motor the bearings should be replaced.
To me, it sounds like a mixture of problems. The standing waves
are most likely from an imperfect balance of the combination of the
motor and the pulley.
The noise during startup could be bad bearings -- or it could be
the vibration from the imbalance passing through resonance with some
structural feature of the grinder.
Since you have a VFD, try starting the motor very slow, and
stepping the speed up a little at a time. If it is a problem with
balance and resonance, you will find one speed at which it is
particularly bad.
When you find this speed, start touching various parts of the
machine to see where you can damp the vibration. If you can't damp it
with pressure anywhere on the machine, then it is more likely to be the
bearings.
However, if you do find such a speed, you might try removing the
pulley (and its key, to keep that from flying away at speed). If you
have similar vibration, the motor's rotor is poorly balanced. (Though
the absence of the key will introduce a bit of imbalance.) However, if
it is much better without the pulley, then the pulley itself is the
primary candidate for balancing. Ideally, the pulley and motor rotor
should be balanced as a unit, probably after doing as good a job as
possible on the motor's rotor first. I believe that a proper job of
balancing calls for specialized equipment.
It is bad news to run a motor under load at low speeds from a
VFD for very long (without augmented cooling from an external fan), but
for the time of this test that should be within reason.
Is the GE motor a good choice for a surface grinder or should I be looking
at other brands?
I suspect that you can get many grades of motor from *any*
maker. The trick is finding someplace to get ones of the quality which
you need from whatever maker you choose. Most GE motors are sold for
appliance applications, or machine tools where the balance is less of an
important factor, so you may have to search more to get what you need.
If so, replacement bearings should be pretty cheap. If
the GE motor is wrong for this application, Baldor and Leeson motors with
the same specs above can be had for around $150 on-line.
Trace down where the vibration comes from. It may be that the
pulley was balanced for a different motor, and simply needs to be
re-balanced for this one.
Also, is it necessary to have a motor specially balanced for a surface
grinder?
Since vibration in the surface grinder can negatively impact
surface finish, I think that I would consider it to be more important
than for a big horizontal mill.
If so, is this usually specified when the motor is ordered or
should it be done after receipt with the belt pulley in place? Dreisilker
has a service center fairly close to me and would probably be my choice for
balancing should it prove advisable.
Run the tests which I suggested, and go to them if balance of
the motor/pulley assembly turns out to be the problem.
Good Luck,
DoN.
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