Removing vibrations from a bench grinder

[BazyleParticipant @bazyle]

With regard the transducer and oscilloscope method can you use an external slow speed motor and bits of blue tack as test weights on the rotor?

[Bazyle]

On Bazyle Said:

With regard the transducer and oscilloscope method can you use an external slow speed motor and bits of blue tack as test weights on the rotor?

You mean to spin the rotor outside with another motor?

[Dave HalfordParticipant @davehalford22513]

Sounds like poor quality new bearings to me and a surface that drums, even standing it on cardboard would help.

[John PParticipant @johnp77052]

Hi,

This seems to be a familiar story with grinders and other items using
small induction motors .
The problems that you have go back the the very first post in which you
state

” Since the grinder vibrates even without wheels ”

Once you go past this step without correcting this any thing else you do
will only involve luck to provide a solution.

The photo here shows the rotor assembly from a coolant pump,as you
can see this is is on a balancing machine,the two plasticene weights are
nearly 180 deg apart but of different sizes.
If you are unlucky enough to have a rotor where the unbalance masses
are 180 deg apart and of similar weight testing on knife edges will show
as in balance but running at speed will vibrate .

The problem in how to correct this without grinding away at the rotor
drum is seen here in the next photo,two split collars are clamped
around the shaft to allow material to be removed to balance the
rotor.As you can see they are as far away from the rotor as possible.
In this example when balanced the speed of the motor increased
by nearly 100 rpm.

I have had a bench grinder that i bought 40+ years ago that also
vibrated badly until i was able to balance it properly using the same
method as above.

The downside of this you need a dynamic balancer or as in my case
make your own.

 

 

 

 

 

[John PParticipant @johnp77052]

Hi

Had some problems with previous post photo’s
just some examples of the indication of balance
on scope screen .

As before this rotor from a centrifuge before and
after balancing.

The first photo
there is a clear imbalance showing and the reference
pulse at the bottom of the screen.

The second photo

The trace is not so clear although you can still see a rotational
cycle between the the reference pulse at the
bottom.

When the signal is so small the trace is corrupted by small
mechanical vibrations within the balancer.
At this point the imbalance is just a few millgrams.

As a matter of interest a ball of plasticene about 1/8 inch
diameter is about 100 mg by cutting it in half and
half again your down to 25 mg if you cut it again
it is nearly too small to see .

Refering back to my previous post the example
shown in the first photo shows how much imbalance
there was on that rotor.

John

 

 

 

[John P]

On John P Said:

If you are unlucky enough to have a rotor where the unbalance masses
are 180 deg apart and of similar weight testing on knife edges will show
as in balance but running at speed will vibrate .

I’m not sure I understand this. If the unbalance mases are 180 degree apart and of similar weight then why there are going to be vibrations? The rotor doesn’t have to be homogenous.

What sensor you used? It looks like a pickup coil. Is it sensitive enough or you had to amplify it?

 

[John PParticipant @johnp77052]

 

 

 

This video explains the types of unbalance and the causes it is worth
watching all through at around 5 min 30 sec describes the situation
you may have.

 

If you have just a disc to balance the knife edge method works well,
when the object in question has a length that is equal to the diameter
it is then really considered to be a drum and can have this dynamic
imbalance.

Somehow when i made the fist post some of the text vanished relating
to the vibro scope method by Jorg Hugel ,the sensors used are
relatively cheap from Ailiexpress and could be used on the balancer
that i have, at the moment the sensing coils that i use come from
the solinoid coils from an old washing machine valves and
round magnets the signal is very small and an amplifier is used
TL 74 i think .

The downside of having a balancer like this is that to make
the balancing spindles for some of the items such as the rotor drum
seen earlier , you need to have a cylindrical grinder
as they are hardened and cannot be turned as seen here the
balancing spindle for the centrifuge rotor the one with o rings on.
However the the rotor seen in the first post was balanced in its
own bearings which works well enough but produces a more
noisy signal.

John

[Sonic EscapeParticipant @sonicescape38234]

Now I understood what you mean, thanks!

[Author]

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