precision shaft, final challange - balancing.

[colin]

Have you considered a strobe light for finding the angle that produces
the most deflection?
Richard

ok so ive got some vibration sensors now, and a strobe led wich fires at the
peak output from the sensor,
the signal is processed with a dsp to generate the strobe.

the shaft is 1.2M long with 3 shaft bearing blocks mounted on 2"x4"
stainless steel box section, wich is rubber mounted by 4 points to a 2"x4"
alu section and I have a place to put sensors at either end and the middle,
I can measure any 2 at once.

the vibration sensors are peizo sounders, atatched to the isolated beam, and
a bolt screwed up from the lower non isolated beam to make contact with the
inside of the piezo disc, so it gets a very strong signal of the movement of
one beam relative to the other.

it resonates at 5400rpm, I find that the peak position moves about 180' from
one side of resonance to the other, and the difference between forward and
reverse is upto 180' depending on speed, however at some speed just over
resonance I think the spot will be in the same place in either direction,
also it does wander quite a bit as resonance builds up over a few seconds.

So how do I work out where to put the weight to balance the shaft ?
im just playing about at the moment, and adding some weights notcibly
reduces the vibration, but the position wanders about too much to make a
connection between the spot and where to put the weights.

ofc I dont know what polarity of the piezo transducers are either so this
may also be 180 out of phase.

however it does run quite sweet with just a couple of 3mm nuts taped onto
the shaft with pvc tape although thats not very safely secured.

do I have to vary the speed from one side of resonance to the other in
forward and reverse and process the vibration signal to find the mid spot ?

are there any pitfalls in trying to balance for all speeds in either
direction ?

thanks
Colin =^.^=

[Mike H]

On May 22, 10:30 am, "colin" <***@ntworld.NOSPAM.com> wrote:
...

Post by colin
So how do I work out where to put the weight to balance the shaft ?
im just playing about at the moment, and adding some weights notcibly
reduces the vibration, but the position wanders about too much to make a
connection between the spot and where to put the weights.

Have you considered removing material, rather than adding material to
balance it? when balancing a automotive crankshaft, the material
added is actually mallory metal which is heavier than the original
metal it replaces.

Post by colin
are there any pitfalls in trying to balance for all speeds in either
direction ?

Interesting website I found:
http://www.brad.ac.uk/research/mbdmst/rotordynamic/unbalance/unbalance.html

[colin]

Post by Mike H
...

Post by colin
So how do I work out where to put the weight to balance the shaft ?
im just playing about at the moment, and adding some weights notcibly
reduces the vibration, but the position wanders about too much to make a
connection between the spot and where to put the weights.

Have you considered removing material, rather than adding material to
balance it? when balancing a automotive crankshaft, the material
added is actually mallory metal which is heavier than the original
metal it replaces.

I dont want to remove metal as it would weaken the discs, and i might get it
wrong , then i would end up with several holes, plus Im still needing to
take it apart to modify it every now and then.

Post by Mike H

Post by colin
are there any pitfalls in trying to balance for all speeds in either
direction ?

http://www.brad.ac.uk/research/mbdmst/rotordynamic/unbalance/unbalance.html

thats realy cool thanks, altough it covers the fundamental physical
principals such as centripetal force etc wich im fairly familiar with, the
links on that page dont seem to be working atm.

I think the case of the vibration signals is far more complex however as
there is resonace etc. im only just begining to understand this, I initialy
thought it was the shaft acting like a violin string but now realise its
dependand on the mass and stiffness of the all the components involved.

they mention bearings mounted on springs, however ive got all 3 bearings
mounted on a 2x4 steel beam, wich is itself rubber mounted, with a sensor on
the beam under each bearing. I can feel the vibration but this probably
makes it more dificult to analyse, im not sure if I can use springs as I
have to dismantle the shaft to remove the springs and re align the bearings
without the springs. however the beam its mounted to is also vibrating wich
probably is rather unhelpfull, I gues il have to clamp it to something more
solid.

thanks
Colin =^.^=

[Brian Whatcott]

On Tue, 22 May 2007 15:30:33 GMT, "colin"

Post by colin

Have you considered a strobe light for finding the angle that produces
the most deflection?
Richard

ok so ive got some vibration sensors now, and a strobe led wich fires at the
peak output from the sensor,
the signal is processed with a dsp to generate the strobe.
the shaft is 1.2M long with 3 shaft bearing blocks mounted on 2"x4"
stainless steel box section, wich is rubber mounted by 4 points to a 2"x4"
alu section and I have a place to put sensors at either end and the middle,
I can measure any 2 at once.
the vibration sensors are peizo sounders, atatched to the isolated beam, and
a bolt screwed up from the lower non isolated beam to make contact with the
inside of the piezo disc, so it gets a very strong signal of the movement of
one beam relative to the other.
it resonates at 5400rpm, I find that the peak position moves about 180' from
one side of resonance to the other, and the difference between forward and
reverse is upto 180' depending on speed, however at some speed just over
resonance I think the spot will be in the same place in either direction,
also it does wander quite a bit as resonance builds up over a few seconds.
So how do I work out where to put the weight to balance the shaft ?
im just playing about at the moment, and adding some weights notcibly
reduces the vibration, but the position wanders about too much to make a
connection between the spot and where to put the weights.
ofc I dont know what polarity of the piezo transducers are either so this
may also be 180 out of phase.
however it does run quite sweet with just a couple of 3mm nuts taped onto
the shaft with pvc tape although thats not very safely secured.
do I have to vary the speed from one side of resonance to the other in
forward and reverse and process the vibration signal to find the mid spot ?
are there any pitfalls in trying to balance for all speeds in either
direction ?
thanks
Colin =^.^=

The "homebrew delight" method is to run up the speed just past the
first resonance, then look to add weight opposite that point of max
deflection.
Varying the weight for minimal vibration comes next.
Then spin the shaft up past the second resonance, and place a small
weight at that point of max deflection on the shaft.

Repeat....

Brian Whatcott Altus OK

[colin]

Post by Brian Whatcott
On Tue, 22 May 2007 15:30:33 GMT, "colin"

Post by colin

Have you considered a strobe light for finding the angle that produces
the most deflection?
Richard

ok so ive got some vibration sensors now, and a strobe led wich fires at the
peak output from the sensor,
the signal is processed with a dsp to generate the strobe.
the shaft is 1.2M long with 3 shaft bearing blocks mounted on 2"x4"
stainless steel box section, wich is rubber mounted by 4 points to a 2"x4"
alu section and I have a place to put sensors at either end and the middle,
I can measure any 2 at once.
the vibration sensors are peizo sounders, atatched to the isolated beam, and
a bolt screwed up from the lower non isolated beam to make contact with the
inside of the piezo disc, so it gets a very strong signal of the movement of
one beam relative to the other.
it resonates at 5400rpm, I find that the peak position moves about 180' from
one side of resonance to the other, and the difference between forward and
reverse is upto 180' depending on speed, however at some speed just over
resonance I think the spot will be in the same place in either direction,
also it does wander quite a bit as resonance builds up over a few seconds.
So how do I work out where to put the weight to balance the shaft ?
im just playing about at the moment, and adding some weights notcibly
reduces the vibration, but the position wanders about too much to make a
connection between the spot and where to put the weights.
ofc I dont know what polarity of the piezo transducers are either so this
may also be 180 out of phase.
however it does run quite sweet with just a couple of 3mm nuts taped onto
the shaft with pvc tape although thats not very safely secured.
do I have to vary the speed from one side of resonance to the other in
forward and reverse and process the vibration signal to find the mid spot ?
are there any pitfalls in trying to balance for all speeds in either
direction ?
thanks
Colin =^.^=

The "homebrew delight" method is to run up the speed just past the
first resonance, then look to add weight opposite that point of max
deflection.
Varying the weight for minimal vibration comes next.
Then spin the shaft up past the second resonance, and place a small
weight at that point of max deflection on the shaft.
Repeat....
Brian Whatcott Altus OK

cool thanks for that :) ive noticed if I get the speed right the spot is the
same in both directions but its quite sensitive, its just over resonance
just as you said. seems my led flashes at the point where I need to add
weight at the top of the shaft wich is convenient.

however after ive added a few weights the spot just wanders round the shaft,
yet there is stil a fairly strong vibattion. maybe my speed isnt totaly
steady, its crystal controlled but maybe i need to tweak the control
feedback loop a bit.

looking at the vib sensor there is fundamental resonance at 5400 rpm = 90hz,
and at 1/3 or 1/5 that speed the vibration signal contains 3rd,4th harmonic
of the shaft speed, wich makes it stil the same 90hz.

when you mean 2nd resonance would that be 2 or 3 times shaft speed ? I can
manage 10krpm, 15krpm and its probably gona be singing quite a lot, although
if I could get it running realy sweet at that speed it would be cool :)

i havnt run it much past this yet although it did run away and got to 16k
rpm before I hit the off button lol ! the electroncs wil easily goto

Post by Brian Whatcott
30krpm, im not sure what speed it will fly apart. il put some sheilds

around the discs.

my motor wuch is in the middle of the shaft is from an old large disc drive,
it has 3 grub screws to centralise it on the shaft, and 3 more just to act
as weights, the discs at the end also have 3 grub screws, and il think il
try making different length screws to redistribute the weight.


Colin =^.^=