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frictionless bearings ?

C

colin

Jan 1, 1970
0
Hi,
I gues this may be kinda off topic,
but I have an optical encoder at the end of a 5mm dia 1M long shaft,
unfortunatly the bearings in the shaft create some drag and this gives rise
to a twisting force
wich although small - gives an error of about 1 minute, im trying to get rid
of it
at the moment im using ordinary ball bearings, the shaft speed is also quite
high, ~ 10krpm

I thought of a few possibilities so far :-
1) frictionless bearings, well almost such as magnetic or air or fluid etc,
2) one shaft inside another, joined at the transmiting end, so inner shaft
shouldnt be subjected to all the drag.

a larger shaft might be stiffer but would need stiffer bearings too
probably.

plus this is getting a bit beyond the ablities of the rather limited
workshop facilities I have at hand,
ie a drill and a vice.

any other ideas ?
(obviously I cant move the encoder closer to the other end or I would have
done this already)

Colin =^.^=
 
J

John Larkin

Jan 1, 1970
0
Hi,
I gues this may be kinda off topic,
but I have an optical encoder at the end of a 5mm dia 1M long shaft,
unfortunatly the bearings in the shaft create some drag and this gives rise
to a twisting force
wich although small - gives an error of about 1 minute, im trying to get rid
of it
at the moment im using ordinary ball bearings, the shaft speed is also quite
high, ~ 10krpm

Gosh, why do you need one minute accuracy at 10,000 RPM?

Are you sure that shaft twist is the source of the error? I can think
of a couple of ways to demonstrate if it is.

10krpm is 166 Hz, 6 milliseconds per rev. So 1 minute error
corresponds to an encoder time error of 2 microseconds. Are you sure
the photodiode and signal conditioning are all that fast?

I can imagine a simple circuit that would apply a compensating time
delay (less delay at high frequencies) that would compensate out most
of the error you're seeing... IF you have a way to calibrate it.

If it is shaft twist, as it may well be, the concentric shaft thing
sounds pretty good.

Tried washing the lube out of the bearings?

What's this for?

John
 
C

colin

Jan 1, 1970
0
John Larkin said:
Gosh, why do you need one minute accuracy at 10,000 RPM?

hmm to explain would be a bit more involved than i would like right now,
and it would probably start a completly different discussion ...
Are you sure that shaft twist is the source of the error? I can think
of a couple of ways to demonstrate if it is.

10krpm is 166 Hz, 6 milliseconds per rev. So 1 minute error
corresponds to an encoder time error of 2 microseconds. Are you sure
the photodiode and signal conditioning are all that fast?

I can imagine a simple circuit that would apply a compensating time
delay (less delay at high frequencies) that would compensate out most
of the error you're seeing... IF you have a way to calibrate it.

its somewhat less than 2us I beleive, its averaged over quite some time with
a dspic,
however its too variable to be able to actually calibrate it,
the error cuased by the time delay of the circuitry can be nulled/self
cancelled.

adjusting the alignment of the bearings cuases a very wide swing of the
error.
as does lightly gripping the shaft.
its the variability wich is the problem,
particularly after reversing the rotation direction the error jumps about
wildly.
its only the change in difference of angle wich is of interest, rather than
absolute position.
If it is shaft twist, as it may well be, the concentric shaft thing
sounds pretty good.

yes but unfortunatly my usual suppliers dont have anything like the right
size tube etc that I can just use easily.
might have to find a machine shop that can do it to tight tolerances but
that sounds expensive to me.

magnetic bearings sound fun to play around with if only i had the time...
Tried washing the lube out of the bearings?

yes ... and they promptly destroyed themselves lol !
ball races also have moderate play in them unless they are preloaded, but
preloading just seems to make the problem worse.
What's this for?

il give details when ive got it working...

Colin =^.^=
 
J

Jamie

Jan 1, 1970
0
colin said:
hmm to explain would be a bit more involved than i would like right now,
and it would probably start a completly different discussion ...




its somewhat less than 2us I beleive, its averaged over quite some time with
a dspic,
however its too variable to be able to actually calibrate it,
the error cuased by the time delay of the circuitry can be nulled/self
cancelled.

adjusting the alignment of the bearings cuases a very wide swing of the
error.
as does lightly gripping the shaft.
its the variability wich is the problem,
particularly after reversing the rotation direction the error jumps about
wildly.
its only the change in difference of angle wich is of interest, rather than
absolute position.




yes but unfortunatly my usual suppliers dont have anything like the right
size tube etc that I can just use easily.
might have to find a machine shop that can do it to tight tolerances but
that sounds expensive to me.

magnetic bearings sound fun to play around with if only i had the time...




yes ... and they promptly destroyed themselves lol !
ball races also have moderate play in them unless they are preloaded, but
preloading just seems to make the problem worse.




il give details when ive got it working...

Colin =^.^=
have you tried thrust bearings and a contact less tach ?
the tach is just a U sender/Reciever LED hovered around a
photo disc that is attached to the shaft.
 
C

colin

Jan 1, 1970
0
Jamie said:
...snip
have you tried thrust bearings and a contact less tach ?
the tach is just a U sender/Reciever LED hovered around a
photo disc that is attached to the shaft.

thats sounds just like an optical encoder wich is what im using.
theres little in the way of thrust on the shaft,
I dont know that these type would be any less drag ?
or if needle rollers would either.

Colin =^.^=
 
A

Ancient_Hacker

Jan 1, 1970
0
Hi,
I gues this may be kinda off topic,
but I have an optical encoder at the end of a 5mm dia 1M long shaft,
unfortunatly the bearings in the shaft create some drag and this gives rise
to a twisting force
wich although small - gives an error of about 1 minute, im trying to get rid
of it

You're partly barking up the wrong tree-- if the shaft is changing
speeds, the acceleration will result in some twisting forces, probably
larger than the bearing friction.

You could reduce the friction somewhat by using two coencentric
bearings, with the grease washed out of them and replaced by some very
light oil or light silicone fluid.

You could also reduce the twisting by using a shaft with a better
weight to stiffness ratio-- look at hollow carbon fiber arrows--
they're amazingly light and stiff at the same time.

Also note that you're asking for one part in 3600 accuracy-- even a
thousandth of an inch of radial slop in the bearings is too much if
the shaft is 1/3 inch. Most bearings have much more slop than that
unless they're carefully pre-tensioned axially. Which leads to more
friction.
 
J

Jamie

Jan 1, 1970
0
colin said:
thats sounds just like an optical encoder wich is what im using.
theres little in the way of thrust on the shaft,
I dont know that these type would be any less drag ?
or if needle rollers would either.

Colin =^.^=
the idea is to not use the shaft of the encoder but your own
to support or what ever to hold what ever it is that's spinning that fast.
i mean,i don't know what moving item you're trying to tach?
 
C

colin

Jan 1, 1970
0
Ancient_Hacker said:
You're partly barking up the wrong tree-- if the shaft is changing
speeds, the acceleration will result in some twisting forces, probably
larger than the bearing friction.

Hi, acceleration isnt an issue as the measurments are taken when it is at
constant speed.
You could reduce the friction somewhat by using two coencentric
bearings, with the grease washed out of them and replaced by some very
light oil or light silicone fluid.

I had thought of concentric bearings, but would this work or would only one
rotate ?
a bit like parallelling power supplies lol, or capacitors in series.
You could also reduce the twisting by using a shaft with a better
weight to stiffness ratio-- look at hollow carbon fiber arrows--
they're amazingly light and stiff at the same time.

yes a stiffer shaft would help, not sure if carbon fibre is actually stiffer
than stainless steel wich im using,
it certainly can take more load per weight, but the weight isnt an issue,
maybe extra hardened steel if I could find some, or maybe diamond :eek:
Also note that you're asking for one part in 3600 accuracy-- even a
thousandth of an inch of radial slop in the bearings is too much if
the shaft is 1/3 inch. Most bearings have much more slop than that
unless they're carefully pre-tensioned axially. Which leads to more
friction.

the end bearings are carefully shimed to remove axial play, well ok, actualy
the end blocks are just pushed in till its tight but not to tight. hovewer a
few more bearings are needed to suport the rest of the shaft or it wobbles
itself to destruction. The encoders are tiny so any play is magnified but
the new faster encoders seem to come in tiny sizes, maybe I could roll my
own fast large one, but I think this 'slop' would average out.

I seem to have made a slight error in thinking 1 minute was 1/60 of a degree
doh!
Im actually seeing an error of < 1:15,000, this is just with a home made
type prototype, im an electronics engineer not a mechanical engineer and
with limited machine tools ie a drill, and is the difference of going from
one direction to the other, and averaged over a few minutes. however its not
consistent enough to be able to calibrate out, although its much less going
from one speed to another in the same direction.

Therefore I assume I would be able to reduce this by an order of magnitude
with a better arangement and reduce the variation even more with a bit more
precision etc. so a predictablity of 1:1,000,000 would be much better.

Colin =^.^=
 
C

colin

Jan 1, 1970
0
Jamie said:
the idea is to not use the shaft of the encoder but your own
to support or what ever to hold what ever it is that's spinning that fast.
i mean,i don't know what moving item you're trying to tach?

well the encoder just fits onto the end of the shaft, its just an optical
disc, but it needs to be 1M away from the spinning object, so the shaft has
to be 1M long. as the title suggests it would be simple if I cld just use
some frictionless beatings.

Colin =^.^=
 
J

Jamie

Jan 1, 1970
0
colin said:
well the encoder just fits onto the end of the shaft, its just an optical
disc, but it needs to be 1M away from the spinning object, so the shaft has
to be 1M long. as the title suggests it would be simple if I cld just use
some frictionless beatings.

Colin =^.^=
http://en.wikipedia.org/wiki/Magnetic_bearing
SKF make's magnetic Bearings.

In fact, we are using an experimental defusing pump
at work that has some new type of magnetic bearings
in it.
it operates up to 50K RPM's, it has started bearings
that retract once the load cells on the 4 points detect
even pressure which indicates the rotor is spinning in
center with the magnetic bearings..
It actually is designed to spin up faster than that,
we were ask not to let it go any faster for now.
It has a special controller for it to run this operation.
and i can tell you that it works very nicely and has been working
nicely for some time now.
I think the company that made it either product a different
model for the original market or it may had been a closed market
that it was originally designed for. We use it to maintain a vacuum
down in the 10e-8 to 10e-9 torr.
 
C

colin

Jan 1, 1970
0
http://en.wikipedia.org/wiki/Magnetic_bearing
SKF make's magnetic Bearings.

In fact, we are using an experimental defusing pump
at work that has some new type of magnetic bearings
in it.
it operates up to 50K RPM's, it has started bearings
that retract once the load cells on the 4 points detect
even pressure which indicates the rotor is spinning in
center with the magnetic bearings..
It actually is designed to spin up faster than that,
we were ask not to let it go any faster for now.
It has a special controller for it to run this operation.
and i can tell you that it works very nicely and has been working
nicely for some time now.
I think the company that made it either product a different
model for the original market or it may had been a closed market
that it was originally designed for. We use it to maintain a vacuum
down in the 10e-8 to 10e-9 torr.

Dare I ask if they are very expensive ?

Colin =^.^=
 
well the encoder just fits onto the end of the shaft, its just an optical
disc, but it needs to be 1M away from the spinning object, so the shaft has
to be 1M long. as the title suggests it would be simple if I cld just use
some frictionless beatings.

Colin =^.^=


could you have an encoder, say, 20cm before the bearing and an encoder
after the bearing
measure angle with both and calibrate an error to subtract depending
on the difference in the two measuments?

-Lasse
 
N

nospam

Jan 1, 1970
0
colin said:
I had thought of concentric bearings, but would this work or would only one
rotate ?
a bit like parallelling power supplies lol, or capacitors in series.

Make concentric shafts. Outer a pipe and a rod running in bearings inside.
Drive both at one end. The encoder is on the rod at the other end.

Assuming you have the power you don't really care about friction in the
outer bearings. The inner bearings run a zero speed so you don't have to
worry much about wear and lubrication.

The mechanical equivalent of a guard ring.

You may have problems with 'siction' on the inner bearings but I imagine
very much less than you have with the current set up.
--
 
J

john jardine

Jan 1, 1970
0
colin said:
Hi, acceleration isnt an issue as the measurments are taken when it is at
constant speed.


I had thought of concentric bearings, but would this work or would only one
rotate ?
a bit like parallelling power supplies lol, or capacitors in series.


yes a stiffer shaft would help, not sure if carbon fibre is actually stiffer
than stainless steel wich im using,
it certainly can take more load per weight, but the weight isnt an issue,
maybe extra hardened steel if I could find some, or maybe diamond :eek:


the end bearings are carefully shimed to remove axial play, well ok, actualy
the end blocks are just pushed in till its tight but not to tight. hovewer a
few more bearings are needed to suport the rest of the shaft or it wobbles
itself to destruction. The encoders are tiny so any play is magnified but
the new faster encoders seem to come in tiny sizes, maybe I could roll my
own fast large one, but I think this 'slop' would average out.

I seem to have made a slight error in thinking 1 minute was 1/60 of a degree
doh!
Im actually seeing an error of < 1:15,000, this is just with a home made
type prototype, im an electronics engineer not a mechanical engineer and
with limited machine tools ie a drill, and is the difference of going from
one direction to the other, and averaged over a few minutes. however its not
consistent enough to be able to calibrate out, although its much less going
from one speed to another in the same direction.

Therefore I assume I would be able to reduce this by an order of magnitude
with a better arangement and reduce the variation even more with a bit more
precision etc. so a predictablity of 1:1,000,000 would be much better.

Colin =^.^=

(Thought a min' was 1/60th degree?.)
What about inreasing shaft diameter?. Stiffness goes up as the fourth power
of diameter, 10mm shafting would give that order of magnitude reduction in
angle of twist.
Or maybe go with 10mm but use tube. You'll lose some of that fourth power
but still result in a big gain in torsional stiffness.
 
C

colin

Jan 1, 1970
0
john jardine said:
(Thought a min' was 1/60th degree?.)

hmm so did I maybe I was right after all, someone seemed to think 1 minute
was 1/3600 of a circle.
What about inreasing shaft diameter?. Stiffness goes up as the fourth
power
of diameter, 10mm shafting would give that order of magnitude reduction in
angle of twist.

hmm 4th power thats interesting,
my hole is 16 mm wich limits the size bearing I can get in there.
the bearings with small id to od ratio tend to be a bit flimsy.

and then theres the problem of how to interface to an encoder that has a 2mm
hole.
I ground this one down to 2mm in situ. (with scrap bearings ofc)
how convenient those dremmel cuttoff discs have a 2mm hole in them.
Or maybe go with 10mm but use tube. You'll lose some of that fourth power
but still result in a big gain in torsional stiffness.

weight isnt so much of an issue, as long as it doesnt make for to much
wobbling about,
I had it described to me as "whirl mode" where it resonates at a certain
speed and vibrates like heck.
not sure i understand the exact mode of corelation between resonance and
speed.

If i could find the right place to get some tubes and long enough shafts
just the right size I would go for the concentric shafts idea.

another problem I am contending with is when the thing rotates in the other
direction the signals are on the opposite edge, thus any diference between
rise time and fall time cuases an error, although the digital signals are
~5ns the optical system may have different rise and fal times. however if it
stays consitent I can null it out.

Colin =^.^=
 
C

colin

Jan 1, 1970
0
nospam said:
Make concentric shafts. Outer a pipe and a rod running in bearings inside.
Drive both at one end. The encoder is on the rod at the other end.

Yes thanks, this was one of the options I considerd in my first post.
just a problem finding parts to construct it out of.
Assuming you have the power you don't really care about friction in the
outer bearings. The inner bearings run a zero speed so you don't have to
worry much about wear and lubrication.

The mechanical equivalent of a guard ring.

yes guard ring thats the comparison I was looking for.
You may have problems with 'siction' on the inner bearings but I imagine
very much less than you have with the current set up.

a bit of inevitable vibration should get over that.

Colin =^.^=
 
R

Robert Baer

Jan 1, 1970
0
colin said:
hmm to explain would be a bit more involved than i would like right now,
and it would probably start a completly different discussion ...




its somewhat less than 2us I beleive, its averaged over quite some time with
a dspic,
however its too variable to be able to actually calibrate it,
the error cuased by the time delay of the circuitry can be nulled/self
cancelled.

adjusting the alignment of the bearings cuases a very wide swing of the
error.
as does lightly gripping the shaft.
its the variability wich is the problem,
particularly after reversing the rotation direction the error jumps about
wildly.
its only the change in difference of angle wich is of interest, rather than
absolute position.




yes but unfortunatly my usual suppliers dont have anything like the right
size tube etc that I can just use easily.
might have to find a machine shop that can do it to tight tolerances but
that sounds expensive to me.

magnetic bearings sound fun to play around with if only i had the time...




yes ... and they promptly destroyed themselves lol !
ball races also have moderate play in them unless they are preloaded, but
preloading just seems to make the problem worse.




il give details when ive got it working...

Colin =^.^=
Brass tubing of many sizes can be had at the hobby store; sizes allow
concentric configuration in a number of steps,depending onsize needed.
 
T

Tony Williams

Jan 1, 1970
0
Hi, I gues this may be kinda off topic, but I have an optical
encoder at the end of a 5mm dia 1M long shaft, unfortunatly the
bearings in the shaft create some drag and this gives rise to a
twisting force wich although small - gives an error of about 1
minute, im trying to get rid of it at the moment im using
ordinary ball bearings, the shaft speed is also quite high, ~
10krpm

I wonder if it is viable to set up a second (identical)
shaft, for run-time calibration. Second optical encoder
on the end, as now, driven by a dc motor at the far end,
with a third optical encoder directly on the motor.

Servo the motor so that the calibration shaft runs at
the same speed as the working shaft, as measured by the
difference between the optical encoders on their ends.

Then use the difference between the end and motor encoders
on the calibration shaft as a measure of the error on the
working shaft.

Verify the scheme by building two calibration assemblies
and compare the outputs from their motor encoders.
 
T

Tony Williams

Jan 1, 1970
0
colin said:
adjusting the alignment of the bearings cuases a very wide swing
of the error. as does lightly gripping the shaft. its the
variability wich is the problem, particularly after reversing the
rotation direction the error jumps about wildly. its only the
change in difference of angle wich is of interest, rather than
absolute position.

You might be missing a magic thing called a shaft
coupler. This will take care of forces on the
bearings caused by a) shaft misalignments, and b)
variations in shaft length (as the shaft twists).
 
C

colin

Jan 1, 1970
0
Brass tubing of many sizes can be had at the hobby store; sizes allow
concentric configuration in a number of steps,depending onsize needed.

yes, there used to be a realy good model shop nearby, with all sorts of
things like that
now the only one just sells complete RC models and stuff.

I can find 300mm lengths quite easily, in fact I have some at hand, but 1M
lengths is difficult.
although I found a place to sell 5M lengths but with a significant min order
quantity lol.

Ive been googling like mad but so many false leads or links to lists of
links wich dont realy help.
If anyone knows somewhere online in the UK that would be nice.

Colin =^.^=
 
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