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'[EE]: Centrifugal forces on a motor'
2001\02\09@050309 by Jinx

face picon face
Hi all, I have a machine white-boarded and I'm unsure
about how a motor is going to react when the machine
is running.

It's basically a centrifuge arm 1m long (negotiable to a
degree) turning at 7 revs/sec so that the end of the arm
is travelling at 100mph/160kph. This is not negotiable.
Attached to the end of the arm, also travelling at 100mph
and subject to the centrifugal forces, is a smallish motor
(at present of undetermined orientation and probably a
DC or stepper about fist-sized) spinning a mechanism
next to it at 600rpm. The spinning in this mechanism must
be parallel to the axis of rotation of the main arm, IOW
imagine a helicoptor rotor blade with another little rotor
blade at the end of it (although the second mechanism
isn't a blade)

Can anyone tell me what effect the centrifugal forces will
have on the small motor if it were to be mounted in various
orientations ? I'm guessing probably not a lot but I'd like
some opinions. One thing that does occur to me is that
perhaps the lubricant will fly out. How sealed are sealed
bearings ?

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2001\02\09@053218 by Alan B. Pearce

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If I have got your description right, the small rotor (I will use the
description "disc") rotates such that as the large arm rotates a point on the
circumference of the disc is going to describe a spiral path on the same plane
as the large arm rotates. Yes?

Whatever axis this rotates on, I think I would look at mounting the motor so all
centrifugal forces from the rotation of the large arm would act on a thrust
bearing in the motor, and then arrange some form of gearing from the motor to
the disc. I have not worked out the G forces involved, but suspect they will be
quite significant, to a point where your thinking seems to be you will need
sealed ball bearings. I think you are correct here.

But this does beg the question, a 1 metre radius rotating at an outer edge
linear speed of 100mph?

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2001\02\09@055749 by Jinx

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> If I have got your description right, the small rotor (I will use the
> description "disc") rotates such that as the large arm rotates a
> point on the circumference of the disc is going to describe a
> spiral path on the same plane as the large arm rotates. Yes?

Correct

Only reference I have to hand for bearings is the RS catalogue.
I'll have a browse through that to get an idea of the various types,
but that doesn't necessarily mean that a motor would have them
would it ? I see a few motors that have self-lubricating sintered
bronze bearings, which will help if I can find the right size. One
option that's a distant possiblity if a fair portion is redesigned
(more or less starting from scratch really) is to have the minor
motor at the main axle and send up a drive rod or flexible shaft,
but there are complications with that, perhaps more so than
putting a motor where it's needed

> But this does beg the question, a 1 metre radius rotating at an
> outer edge linear speed of 100mph?

Oooer, is this not Euro-compliant ? 160kph = (160000/3600) =
44.44 metres/s = 44.44/(pi * 2R) = 7.07 revs/s for 1m R. If that was
your concern hopefully I've got GCHQ off the blower to Brussels

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2001\02\09@061630 by Alan B. Pearce

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>Oooer, is this not Euro-compliant ? 160kph = (160000/3600) =
>44.44 metres/s = 44.44/(pi * 2R) = 7.07 revs/s for 1m R. If that was
>your concern hopefully I've got GCHQ off the blower to Brussels

Well I was not worried about the mixed units (some of us kiwi's are versatile in
this respect - you would not want to see the uproar in the UK at the moment
about having to sell meat and produce in kilograms instead of pounds weight). I
used to tell people the property I used to own in Auckland was an acre plus 10
sq m. if they did not mind mixed units. Most did not know what I meant.

My major concern with using sintered bronze bearings would be they are not
sealed, and the centrifugal force would force the lubricant out, which is why I
figured you would need sealed ball bearing units. However if using a standard
motor near the centre of rotation, with an extension shaft where you can specify
the bearing type may get you out of this problem.

Remember that sintered bronze bearings are only self lubricating because they
are porous, and so hold a quantity of oil, they still need to be soaked for a
while periodically to replenish them. The standard recommendation I used to see
was to soak them in a bath of oil for 24 hours.

The other problem I foresee is lateral wear on the shaft of the disc due to the
centrifugal force. This is why I suggested mounting the motor so the length of
the shaft is along the line of centrifugal force, and this force is absorbed by
some form of thrust bearing, to stop lateral wear of the motor bearings. Again
using suitable ball races for the disc shaft with suitable preloading possibly,
should help here. It may not be a problem without knowing the expected life of
the project, but even for a short lived project I suspect the motor lubrication
will be a problem

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2001\02\09@061642 by Graeme Zimmer

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> I have not worked out the G forces involved, but suspect they will be
> quite significant, to a point where your thinking seems to be you will
need
> sealed ball bearings

Surely the Precession forces are going to be more significant than the G
forces.

If the little rotor is turning rapidly, the precession will rip it apart
unless it and its shaft are very strong.

Also the little motor will need to have sufficient torque to overcome the
precession forces.
It will act as if it was locked up. Most likely it will just expire in a
cloud of smoke...

How do I know this? About a hundred years ago I built this antigravity
machine.
I was so sure it was going to work......


..................... Zim

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2001\02\09@073022 by Jinx

face picon face
> Well I was not worried about the mixed units (some of us kiwi's are
> versatile in this respect - you would not want to see the uproar in
> the UK at the moment about having to sell meat and produce in
> kilograms instead of pounds weight).

I saw that greengrocer hauled off before the beak last week. I believe
his defence was that it still isn't illegal to sell by the pound

> My major concern with using sintered bronze bearings would be
> they are not sealed

OK, forget the SBB for now then. If lubrication really is going to
be a problem then it may be better to go to Plan B, putting the
minor motor at the center and use a shaft drive

> The other problem I foresee is lateral wear on the shaft of the
> disc due to the centrifugal force. This is why I suggested
> mounting the motor so the length of the shaft is along the line of
> centrifugal force, and this force is absorbed by some form of thrust
> bearing, to stop lateral wear of the motor bearings.

That orientation was my preference too. I'm thinking now that it
would be pushing it to get 10rps from a stepper. Think I'd be
happier with a DC motor, about 10:1 reduction

> It may not be a problem without knowing the expected life of
> the project, but even for a short lived project I suspect the motor
> lubrication will be a problem

If it works the machine would get regular, but not continuous, use for
many many years.

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2001\02\09@073645 by Jinx

face picon face
> I have not worked out the G forces involved, but suspect they will
> be quite significant, to a point where your thinking seems to be
> you will need sealed ball bearings

The one book I could have looked in to work out G has disappeared
for some reason, and I got a whole pile of 404 errors searching the
web. A lot of the non-calc ones I did find said centrifugal forces
don't exist

> Surely the Precession forces are going to be more significant than
> the G forces

Can you explain precession please ? Is this some gyroscopic effect ?

> It will act as if it was locked up. Most likely it will just expire in a
> cloud of smoke...
>
> How do I know this? About a hundred years ago I built this
> antigravity machine. I was so sure it was going to work......

I was assured a few years ago by a friend that his friend was on
the run from some US military agency because he'd wandered
off with details of an anti-g machine. He'd holed up in Titirangi
W. Auckland and had made a small electrostatic generator that
proved the principle. So he said

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2001\02\09@080012 by Bob Ammerman

picon face
If the angular velocity of the minor axis is constant, you could eliminate
the second motor altogether.

Simply use a non-rotating bevel-gear pinion, coaxial with the main drive, to
drive a shaft running along the arm of  centrifuge. A second set of
bevel-gears at the outside end of the centrifuge arm drives your 600 RPM
load.

btw: What is this thing? Some kind of killer amusement park ride for
hamsters? :-)

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)



{Original Message removed}

2001\02\09@080217 by Alan B. Pearce

face picon face
>If it works the machine would get regular, but not continuous, use for
>many many years.

Ah so you will be able to have it on maintenance contract for many years
(guaranteeing income etc) oh it's in-house, never mind someone else can do the
maintenance...

> Surely the Precession forces are going to be more significant than
> the G forces

>Can you explain precession please ? Is this some gyroscopic effect ?

It is a gyroscopic effect. In effect it is what causes a gyro to keep pointing
the same way along it spinning axis when you try and rotate it. This can result
in very strong forces on bearings. It is not an aspect I had considered in my
earlier responses to your post.

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2001\02\09@081053 by Roman Black

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face
Alan B. Pearce wrote:
>
> >If it works the machine would get regular, but not continuous, use for
> >many many years.
>
> Ah so you will be able to have it on maintenance contract for many years
> (guaranteeing income etc) oh it's in-house, never mind someone else can do the
> maintenance...
>
> > Surely the Precession forces are going to be more significant than
> > the G forces
>
> >Can you explain precession please ? Is this some gyroscopic effect ?
>
> It is a gyroscopic effect. In effect it is what causes a gyro to keep pointing
> the same way along it spinning axis when you try and rotate it. This can result
> in very strong forces on bearings. It is not an aspect I had considered in my
> earlier responses to your post.


If I read Jinx' post right the two shafts are parallel,
(two discs on same plane) so there is no precession.
-Roman

We all want to knwo what it's for!!! 'Fess up! :o)

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2001\02\09@081459 by Jinx

face picon face
> Simply use a non-rotating bevel-gear pinion, coaxial with
> the main drive, to drive a shaft running along the arm of
> centrifuge. A second set of bevel-gears at the outside end of
> the centrifuge arm drives your 600 RPM load.

If only it were that simple. The 600rpm is the max speed. It
may be anything from 600rpm CCW to 600rpm CW. The
main drive can be from 50 to 400 rpm

> btw: What is this thing? Some kind of killer amusement park
> ride for hamsters? :-)
>
> Bob Ammerman

Not exactly. Ever heard the phrase "not enough room to swing a
cat in here" ? Well, here's a machine that makes room (batteries
and stunt cat not included)

Make a great product for an infomercial. Cue lady with glum face
looking depressed at sad old juice machine with no oomph.
Voice-over - "Does your old juicer take more than 0.2 seconds to
juice 100 pounds of oranges ?"...........

Seriously, if I told you, you'd all be making them ;-)  This is some-
thing that *should* work if the practicalities can be overcome

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2001\02\09@082456 by Jinx

face picon face
> We all want to knwo what it's for!!! 'Fess up! :o)

Aw, gimme a head start !!! And then you shall knwo

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2001\02\09@084131 by Roman Black

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face
Alan B. Pearce wrote:
>
> If I have got your description right, the small rotor (I will use the
> description "disc") rotates such that as the large arm rotates a point on the
> circumference of the disc is going to describe a spiral path on the same plane
> as the large arm rotates. Yes?
>
> Whatever axis this rotates on, I think I would look at mounting the motor so all
> centrifugal forces from the rotation of the large arm would act on a thrust
> bearing in the motor, and then arrange some form of gearing from the motor to
> the disc. I have not worked out the G forces involved, but suspect they will be
> quite significant, to a point where your thinking seems to be you will need
> sealed ball bearings. I think you are correct here.
>
> But this does beg the question, a 1 metre radius rotating at an outer edge
> linear speed of 100mph?


Isn't that only about 7 revs/second? I don't think
that is getting too scary mechanically. :o)
-Roman

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2001\02\09@084134 by Roman Black

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face
Jinx wrote:
{Quote hidden}

Hi Jinx, I have figures for stepper motors if that helps.
Size 23 stepper: max side load=6.8kg, max end load=11.3kg.
Size 34 stepper: 11.3kg, 22.7kg

If you have a rough idea of the rotor weight, you can
calculate Gs from the rotation and see if it exceeds
the max recommended load figures. I guess a size 23
stepper has a rotor about 200g, so load forces would
be safe to about 6800/200 = 34 Gs. Probably best to
stay well on the safe side though! :o)
-Roman

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2001\02\09@085217 by Michael Rigby-Jones

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> -----Original Message-----
> From: Jinx [SMTP:EraseMEjoecolquittspamCLEAR.NET.NZ]
> Sent: Friday, February 09, 2001 1:17 PM
> To:   RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU
> Subject:      Re: [EE]: Centrifugal forces on a motor
>
> > Simply use a non-rotating bevel-gear pinion, coaxial with
> > the main drive, to drive a shaft running along the arm of
> > centrifuge. A second set of bevel-gears at the outside end of
> > the centrifuge arm drives your 600 RPM load.
>
> If only it were that simple. The 600rpm is the max speed. It
> may be anything from 600rpm CCW to 600rpm CW. The
> main drive can be from 50 to 400 rpm
>
But using the same idea, you could have a second motor mounted on the "hub"
of the main arm, driving the outer "spinning thing" via bevel gears and a
drive shaft.  No centrifugal force problems. (or centripital reaction force
as the physicists like to say.)

{Quote hidden}

My bet is it's a high speed planetarium :o)  Or maybe a power assisted
"Spirograph" set for drawing pretty patterns?

Not sure I'd want something with a 1 meter arm whirling around at that
speed, even if it was usefull :o)  Hope it's gonna have lots of interlocks..

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2001\02\09@091035 by Alan B. Pearce

face picon face
>Isn't that only about 7 revs/second? I don't think
>that is getting too scary mechanically. :o)
>-Roman

Not mechanically, but a 2 meter diameter (1 m radius) moving at 420 rpm is not
to be messed with.

>If I read Jinx' post right the two shafts are parallel,
>(two discs on same plane) so there is no precession.
>-Roman

well yes, but I would be worried about the centrifugal side loading on the motor
bearings (unless one can buy an OEM motor with large bearings for this purpose),
which is why I suggested mounting the motor so the centrifugal force is along
the line of the motor shaft, so the centrifugal force can be taken by a thrust
washer.

However having said that I have not calculated the magnitude of the possible
forces involved, just airing some concerns that immediately come to mind.

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2001\02\09@104029 by Chris Carr

flavicon
face
Just a thought but have you investigated some of these Hi-Tech lubricants
that are a tacky material that seems to stick to any surface but are loaded
with micro-miniature teflon balls to provide the low friction between the
surfaces.

{Quote hidden}

My guess is that he is really Tim "The Toolman" Taylor
What do we need............More Power


> Seriously, if I told you, you'd all be making them ;-)  This is some-
> thing that *should* work if the practicalities can be overcome
>

Chris

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2001\02\09@131843 by Olin Lathrop

face picon face
> Oooer, is this not Euro-compliant ? 160kph = (160000/3600) =
> 44.44 metres/s = 44.44/(pi * 2R) = 7.07 revs/s for 1m R. If that was
> your concern hopefully I've got GCHQ off the blower to Brussels

That comes out to almost 200G.  The angular velocity in radians/sec = 2 * Pi
* 7 = 44.0 = W.  The force will be W**2 * R = 1934 Newtons, divided by 9.8
Newtons/G = 197 G.  Therefore a 1/2 pound object connected to the motor
shaft will pull on the motor with a force of 100 pounds, ouch!


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, RemoveMEolinKILLspamspamembedinc.com, http://www.embedinc.com

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2001\02\09@140033 by David Minkler

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face
Jinx wrote:

> Can anyone tell me what effect the centrifugal forces will
> have on the small motor if it were to be mounted in various

The required centripetal acceleration will be v*v/r or in this case
about 2000 m/(s^2) or about 200 g's.  The force will depend upon the
mass of the motor parts and will be about 200 times the force exerted by
gravity (if gravity were to have been aligned along the centrifuge
arm).  Hope this helps some.

Regards,
Dave

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2001\02\09@141933 by Bob Ammerman

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----- Original Message -----
From: Jinx <KILLspamjoecolquittspamBeGonespamCLEAR.NET.NZ>
To: <EraseMEPICLISTspamEraseMEMITVMA.MIT.EDU>
Sent: Friday, February 09, 2001 8:16 AM
Subject: Re: [EE]: Centrifugal forces on a motor


> > Simply use a non-rotating bevel-gear pinion, coaxial with
> > the main drive, to drive a shaft running along the arm of
> > centrifuge. A second set of bevel-gears at the outside end of
> > the centrifuge arm drives your 600 RPM load.
>
> If only it were that simple. The 600rpm is the max speed. It
> may be anything from 600rpm CCW to 600rpm CW. The
> main drive can be from 50 to 400 rpm

I still would be tempted to use the above mechanical arrangement, but with a
motor to drive the bevel gear. Note that the speed of the main drive would
have to be subtracted desired shaft speed of the auxiliary motor, however.

This would keep both of your motors stationary, which is good for them, as
well as making it a lot easier to bring power to them.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\09@155051 by Jinx

face picon face
> That comes out to almost 200G.  The angular velocity in radians/
> sec = 2 * Pi * 7 = 44.0 = W.  The force will be W**2 * R = 1934
> Newtons, divided by 9.8 Newtons/G = 197 G.  Therefore a 1/2
> pound object connected to the motor shaft will pull on the motor
> with a force of 100 pounds, ouch!

Ouch indeed !! And 1/2lb is on the light side. Even if I found a
motor with the bearings to take that sort of load, I don't I've
personally got the "bearings" to stand next to it. You'd be scared
to cough wouldn't you

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2001\02\09@160121 by Jinx

face picon face
> >Isn't that only about 7 revs/second? I don't think
> >that is getting too scary mechanically. :o)
> >-Roman
>
> Not mechanically, but a 2 meter diameter (1 m radius) moving
> at 420 rpm is not to be messed with.

No, I wouldn't want to cop a hit from it. I've got the main
motor drive built using a 1/2hp DC motor on a steel frame.
You definitely need it balanced and caged

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2001\02\09@182841 by Olin Lathrop

face picon face
> > Not mechanically, but a 2 meter diameter (1 m radius) moving
> > at 420 rpm is not to be messed with.

Where did the 420 RPM come from?  If 7 RPM produces 200 G at the end of the
rotor, then 420 RPM will produce (420 / 7)**2 * 200 = 720,000 G.  You'd need
a lot of power to keep it going in air at that speed, not to mention it will
be hard to find a material that can support itself, let alone a motor.  At
720,000 G a drop of water weighs about 80 pounds!


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, TakeThisOuTolin.....spamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\02\09@190834 by Steven J. Devine

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> > > Not mechanically, but a 2 meter diameter (1 m radius) moving
> > > at 420 rpm is not to be messed with.
>
> Where did the 420 RPM come from?  If 7 RPM produces 200 G at the end of the
> rotor, then 420 RPM will produce (420 / 7)**2 * 200 = 720,000 G.  You'd need
> a lot of power to keep it going in air at that speed, not to mention it will
> be hard to find a material that can support itself, let alone a motor.  At
> 720,000 G a drop of water weighs about 80 pounds!

It was 200G at 420 RPM... the seven was seven rotations PER SECOND (same as 420 RPM).

Steve

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2001\02\09@192714 by Bob Ammerman

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Olin,

It was 7 RPS == 420 RPM

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\09@194344 by Jinx

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> At 720,000 G a drop of water weighs about 80 pounds!

I think restaurants must use a machine like mine and a
calculation like yours to work out how much to charge
for a glass of water

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2001\02\10@015939 by Roman Black

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Jinx wrote:

> > The other problem I foresee is lateral wear on the shaft of the
> > disc due to the centrifugal force. This is why I suggested
> > mounting the motor so the length of the shaft is along the line of
> > centrifugal force, and this force is absorbed by some form of thrust
> > bearing, to stop lateral wear of the motor bearings.
>
> That orientation was my preference too. I'm thinking now that it
> would be pushing it to get 10rps from a stepper. Think I'd be
> happier with a DC motor, about 10:1 reduction

That orientation would cause problems with precession.
Also 10rps is fine for most steppers, even older
ones. I have a "surplus" stepper here, quite low spec,
doing 30rps no problems.

Has anyone worked out the g forces for 7rps and
a 1 metre radius yet? :o)
-Roman

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2001\02\10@034036 by Roman Black

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Jinx wrote:
>
> > >Isn't that only about 7 revs/second? I don't think
> > >that is getting too scary mechanically. :o)
> > >-Roman
> >
> > Not mechanically, but a 2 meter diameter (1 m radius) moving
> > at 420 rpm is not to be messed with.
>
> No, I wouldn't want to cop a hit from it. I've got the main
> motor drive built using a 1/2hp DC motor on a steel frame.
> You definitely need it balanced and caged

I still think this is do-able. McMaster Carr
http://www.mcmaster.com
have bearing info with side load forces listed.

Your 200g is only 6 times higher than a standard
size 23 stepper motor with standard bearings.
Use a lighter motor with higher spec bearings
and its done. Remeber that spec is for continuous
side load and is conservative, you could go many
times that load if you are prepared to reduce
bearing life. Bearings don't self destruct if you
exceed loads they just wear out quicker. Some
CNC routers replace spindle bearings every month,
they run at MANY times the specced side load.

The only critical thing is the bits at the END
of the arm, the other bits only have low forces.
How much power does the small motor need? And
what speed, and (cough...) what does it all
do again? ;o)
-Roman

PS. Don't let the 200g scare you, i've worked
around inductrial machines for years, some of
the forces would be very scary. Even my racing
motorcycle's wheels when I'm doing 260kph on the
main straight every lap.

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2001\02\11@170343 by Russell McMahon

picon face
> Well I was not worried about the mixed units (some of us kiwi's are
versatile in
> this respect - you would not want to see the uproar in the UK at the
moment
> about having to sell meat and produce in kilograms instead of pounds
weight). I
> used to tell people the property I used to own in Auckland was an acre
plus 10
> sq m. if they did not mind mixed units. Most did not know what I meant.


I know you know this, but ... :-)

Selling meat in kg is, of course, wrong.
Meat should be measured in Newtons!
kg is a measure of mass.

When meat was/is sold in pounds that was a unit of force.
As (most*) measure force and not mass, pounds and Newtons are the correct
measure.
* - I have never seen a scale which measure mass by measuring moment of
inertia or similar. The space shuttle has such a device for measuring the
mass (weight?) of its crew members).

Perhaps the British public should be informed that they SHOULD have been
selling meat measured in slugs all along (weight = g x mass. 1 slug mass
weighs 32 pounds force).

I know we (almost) all know this.
It has interested me that in the imperial system we commonly use the unit of
force for measures of both mass and force whereas in the metric system we
use the measure of mass for measures of both mass and force.



   Russell McMahon

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2001\02\12@062831 by Alan B. Pearce

face picon face
>Where did the 420 RPM come from?  If 7 RPM produces 200 G at the end of the
>rotor, then 420 RPM will produce (420 / 7)**2 * 200 = 720,000 G.  You'd need
>a lot of power to keep it going in air at that speed, not to mention it will
>be hard to find a material that can support itself, let alone a motor.  At
>720,000 G a drop of water weighs about 80 pounds!

well here is a quote from the original post.

>It's basically a centrifuge arm 1m long (negotiable to a
>degree) turning at 7 revs/sec so that the end of the arm
>is travelling at 100mph/160kph. This is not negotiable.
>Attached to the end of the arm, also travelling at 100mph

now in my book 7 rev per second = 7 * 60 = 420 rpm

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2001\02\12@063503 by Alan B. Pearce

face picon face
>Some CNC routers replace spindle bearings every month,
>they run at MANY times the specced side load

they would also be replacing them often so they do not get side play affecting
positioning accuracy.

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2001\02\12@064947 by Alan B. Pearce

face picon face
>Selling meat in kg is, of course, wrong.
>Meat should be measured in Newtons!
>kg is a measure of mass.

Well the Brits get away with doing it in kg, as they do not serve it up on the
end of the whirling dervish that Jinx is trying to build. They stick with
standard 1G acceleration, though that does seem a bit high at times when trying
to push things through the system!!! :)

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2001\02\12@090450 by Olin Lathrop

face picon face
> Selling meat in kg is, of course, wrong.
> Meat should be measured in Newtons!
> kg is a measure of mass.

Which seems, strictly speaking, what you are buying.  Suppose there was some
spot on earth that happened to have a gravity of .8G.  The same slab of meat
would have the same mass there, but not the same weight.  It seems we have
gotten a bit sloppy about weight versus math because we are in an
environment with a constant G in human terms.

> When meat was/is sold in pounds that was a unit of force.
> As (most*) measure force and not mass, pounds and Newtons are the correct
> measure.

I disagree.  Mass is the correct measure.  I think you are arguing that the
tail should wag the dog.  If there is an error, then it is measuring with
force instead of mass.  However, measuring mass with force is valid as long
as G is known.  For the purpose of buying dinner, I think G is quite
constant enough.

> * - I have never seen a scale which measure mass by measuring moment of
> inertia or similar. The space shuttle has such a device for measuring the
> mass (weight?) of its crew members).

Balance scales measure mass, not force.  Just about all the earliest scales
worked that way, and many still do today.  Force scales are much more
recent, because most depend on the existance of reliable and predictable
springs.


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2001\02\12@091658 by Bob Ammerman

picon face
> Balance scales measure mass, not force.  Just about all the earliest
scales
> worked that way, and many still do today.  Force scales are much more
> recent, because most depend on the existance of reliable and predictable
> springs.

Balance scales measure the presumably equal _forces_ exerted by two masses
on the beam. A balance scale in a non-linear G- or apparent-G-field (like
near a black hole, or in a radial orientation in a spinning space station)
would not work very well.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\12@175919 by Russell McMahon

picon face
> > When meat was/is sold in pounds that was a unit of force.
> > As (most*) measure force and not mass, pounds and Newtons are the
correct
> > measure.
>
> I disagree.  Mass is the correct measure.  I think you are arguing that
the
> tail should wag the dog.  If there is an error, then it is measuring with
> force instead of mass.  However, measuring mass with force is valid as
long
> as G is known.  For the purpose of buying dinner, I think G is quite
> constant enough.

I'm was obviously speaking somewhat tongue-in-cheek but I disagree with you
disagreement :-).
While mass is what we WANT to measure it is NOT what we measure
ALL normal scales measure force even though the consumer would like them to
measure mass.
About the onlt time we really begin to sense mass in a significant way
independent of their "weight" is when we react with massive objects to move
them horizontally - suspended weights on cables etc, floating objects,
wheeled vehicles etc. Most time the gravitational interaction provides a
force which hides the inertial effects.

> > * - I have never seen a scale which measure mass by measuring moment of
> > inertia or similar. The space shuttle has such a device for measuring
the
> > mass (weight?) of its crew members).
>
> Balance scales measure mass, not force.  Just about all the earliest
scales
> worked that way, and many still do today.  Force scales are much more
> recent, because most depend on the existance of reliable and predictable
> springs.

I disagree. While balances do allow items of equal mass to be compared under
conditions of varying gravity this is because they are FORCE balances and
the forces still balance as long as g varies equally for both items being
compared. A balance will stop working when there is no g forces * present to
create the forces to be compared. (* - or equivalent acceleration forces). A
TRUE mass measurer will work in zero g and the space shuttle scales will do
this. None of the "weight" measuring systems that I see in common use will
work when there is no accelerating force present to create f = ma from the
product of mass and acceleration.

Mass is what we WANT to measure.
Force is what we can most EASILY measure.
Market forces (not masses :-) prevail.



Russell McMahon



cc jinx - now look what you have started :-).

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2001\02\12@225246 by Roman Black

flavicon
face
Russell McMahon wrote:
{Quote hidden}

This cool! So who's gonna be first with the PIC
controlled MASS measuring kitchen scale???
A competition? Maybe a kg (mass) of beautiful
range-fed Aussie beef as the prize??

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2001\02\12@225304 by Jinx

face picon face
We've got very humid tropical weather in NZ right now that's
blown down from the Pacific so I've dragged out all the PC
fans to keep the air moving (not for the whole country, just
my little bit of it). JFTHOI I twirled one around and found that
the pitch of the motor hardly changes whether the axis is
parallel to the axis of swinging or perpendicular to it. It does
change (decrease) though if held in one place and it's orientation
is altered by rotating your wrist, and this is done much more
slowly than the spinning. "Yawing" I guess you'd call it. You
can feel the gyroscopic effect as you do this, which I didn't feel
while spinning it as I'd want it to be in the final machine. So
possibly precession won't be a problem, just centrifugal forces

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2001\02\12@232538 by Roman Black

flavicon
face
Jinx wrote:
>
> We've got very humid tropical weather in NZ right now that's
> blown down from the Pacific so I've dragged out all the PC
> fans to keep the air moving (not for the whole country, just
> my little bit of it). JFTHOI I twirled one around and found that
> the pitch of the motor hardly changes whether the axis is
> parallel to the axis of swinging or perpendicular to it. It does
> change (decrease) though if held in one place and it's orientation
> is altered by rotating your wrist, and this is done much more
> slowly than the spinning. "Yawing" I guess you'd call it. You
> can feel the gyroscopic effect as you do this, which I didn't feel
> while spinning it as I'd want it to be in the final machine. So
> possibly precession won't be a problem, just centrifugal forces


Precession occurs on the non-rotational plane.
Didn't your machine have both rotations on the same
plane?? And what does the machine do again?? ;o)
-Roman

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2001\02\12@233942 by Sean H. Breheny

face picon face
Roman,

You mean to say that you don't consider rotating large, high-moment of
inertia objects at high speeds to be a suitable end goal of a machine? ;-)

Sean

At 03:20 PM 2/13/01 +1100, you wrote:


>Precession occurs on the non-rotational plane.
>Didn't your machine have both rotations on the same
>plane?? And what does the machine do again?? ;o)
>-Roman
>
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2001\02\12@234902 by Bob Ammerman
picon face
> Precession occurs on the non-rotational plane.
> Didn't your machine have both rotations on the same
> plane?? And what does the machine do again?? ;o)
> -Roman

Yeah, both rotations are in one plane, but he was considering mounting the
motor along a radius of the 1m circle so that the 200G's could be handled by
a thrust bearing rather than being a side load on the motor's bearings.
(obviously he'd need a couple of bevel gears to change the axis of motion).

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\13@022950 by Jinx

face picon face
> Precession occurs on the non-rotational plane.
> Didn't your machine have both rotations on the same
> plane??

Yes it does. Now, if the outer motor's spindle is steady at
some intermediate angle then there would be a steady
gyroscopic force that  can be compensated for by the speed
control if necessary ?

And what does the machine do again?? ;o)
> -Roman

Tut, you're like a dog with a bone aren't you ? OK Bowser,
I can reveal that one absolutely vital function is that it acts as
a support for a thin skin of green paint exactly the same size
as the machine itself

_  _
0  0
  U

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2001\02\13@050511 by Jinx

face picon face
> This cool! So who's gonna be first with the PIC
> controlled MASS measuring kitchen scale???

So how do you measure mass in space/0g ? Some sort of
accelerometer ?

> A competition? Maybe a kg (mass) of beautiful
> range-fed Aussie beef as the prize??

Is this the "prime Aussie beef" that moos or hops ?

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2001\02\13@080750 by Russell McMahon

picon face
> > This cool! So who's gonna be first with the PIC
> > controlled MASS measuring kitchen scale???
>
> So how do you measure mass in space/0g ? Some sort of
> accelerometer ?

It'll be on the web but AFAIR it uses an oscillating arm and measures the
period. This is affected by the mass being oscillated.

> > A competition? Maybe a kg (mass) of beautiful
> > range-fed Aussie beef as the prize??
>
> Is this the "prime Aussie beef" that moos or hops ?


Meows?
I believe that people who ate kanagaroo beef were generaly unaware of what
they were eating.
I suspect that I may have been fed cat at one stage (!) but didn't put 2+2
together until much later.




RM

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2001\02\13@081800 by Roman Black

flavicon
face
Bob Ammerman wrote:
>
> > Precession occurs on the non-rotational plane.
> > Didn't your machine have both rotations on the same
> > plane?? And what does the machine do again?? ;o)
> > -Roman
>
> Yeah, both rotations are in one plane, but he was considering mounting the
> motor along a radius of the 1m circle so that the 200G's could be handled by
> a thrust bearing rather than being a side load on the motor's bearings.
> (obviously he'd need a couple of bevel gears to change the axis of motion).
>
> Bob Ammerman


Hi Bob. I'm not scared of 200 Gs. :o)
My brochure here for a standard stepper
motor cites 34 Gs continuous side force
rating (by my calc), so with a lighter
rotor I would give it a go.

Does anyone here know the wear rate charts
for ball bearings? If the side force is
increased to (say) 3x continuous spec how
long will the bearings last? Normally
bearings are rated for millions of revs
within spec, and from working around
machines i've seen bearings that needed to
be replaced much quicker than that,
obviously they were run at over-spec loads.
Anyone know?
-Roman

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2001\02\13@083424 by Roman Black

flavicon
face
Jinx wrote:
>
> > This cool! So who's gonna be first with the PIC
> > controlled MASS measuring kitchen scale???
>
> So how do you measure mass in space/0g ? Some sort of
> accelerometer ?

Yep. How about a 12v hobby store solenoid,
you mount it horizontal, attach to the mass
to be measured and then power it with a
constant current. Use an opto sensor and PIC
to time the pull-in time. Have it returned
by spring and do a few pull-tests before
the mass calc. Presto, kitchen scales that
measures mass. Perfect for taking to your
next zero-G cooking class. :o)
-Roman


> > A competition? Maybe a kg (mass) of beautiful
> > range-fed Aussie beef as the prize??
>
> Is this the "prime Aussie beef" that moos or hops ?

I've eaten 'roo. Considering Aussie beef is
among the worlds best i'd go for the "moo"...
;o)
-Roman

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2001\02\13@085058 by Alan B. Pearce

face picon face
>and from working around
>machines i've seen bearings that needed to
>be replaced much quicker than that,
>obviously they were run at over-spec loads.

Or fitted/maintained to conditions other than the manufacturers recommendation?
especially for bearings that require any form of pre-loading, such as front
wheel bearings on vehicles.

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2001\02\13@085313 by Alan B. Pearce

face picon face
>I've eaten 'roo. Considering Aussie beef is
>among the worlds best i'd go for the "moo"...
>;o)
>-Roman

Well it has a decent chance of being BSE free, but then that is going [OT]:

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2001\02\13@085339 by Michael Rigby-Jones

flavicon
face
The ones that hop around down under are the roo's right?  I get a bit
confused, what with these european cows doing that....

> {Original Message removed}

2001\02\13@101030 by Olin Lathrop

face picon face
> > So how do you measure mass in space/0g ? Some sort of
> > accelerometer ?
>
> Yep. How about a 12v hobby store solenoid,
> you mount it horizontal, attach to the mass
> to be measured and then power it with a
> constant current. Use an opto sensor and PIC
> to time the pull-in time. Have it returned
> by spring and do a few pull-tests before
> the mass calc. Presto, kitchen scales that
> measures mass. Perfect for taking to your
> next zero-G cooking class. :o)

If you guys are going to be such purists about measuring mass versus weight,
shouldn't you also be worrying about gravitational mass versus intertial
mass?  The physicists seem to be saying that althought the two appear to be
the same, the laws of physics that we know don't dictate that the must be
the same.

You seem to have decided arbitrarily that intertial mass is the "true" mass.
I could just as well measure gravitational mass.  Hmm, this could be done by
fetching a large object, like a planet maybe, then measuring the
gravitational force on the mass in question.  Just a thought.


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2001\02\13@113008 by Alan B. Pearce

face picon face
>I could just as well measure gravitational mass.  Hmm, this could be done by
>fetching a large object, like a planet maybe, then measuring the
>gravitational force on the mass in question.  Just a thought.

Does this mean we have come the full circle?
Are we having fun yet? :)

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2001\02\14@064412 by Russell McMahon

picon face
> If you guys are going to be such purists about measuring mass versus
weight,
> shouldn't you also be worrying about gravitational mass versus intertial
> mass?  The physicists seem to be saying that althought the two appear to
be
> the same, the laws of physics that we know don't dictate that the must be
> the same.
>
> You seem to have decided arbitrarily that intertial mass is the "true"
mass.
> I could just as well measure gravitational mass.  Hmm, this could be done
by
> fetching a large object, like a planet maybe, then measuring the
> gravitational force on the mass in question.  Just a thought.


I think we are really all just having fun exploring the subject - defining
what something is REALLY about is a good way of finding out how much you
have unwittingly assumed and/or don't know about the subject.

However, to try to address your point. I think that a gravitational mass
measurement system would be possible but highly impractical in most cases
AND rather difficult to operate. You could do the 2 lead balls type
experiment which was very nicely used to establish a value for the mass of
the earth AFAIR - suspend two massive lead balls so that they are almost
touching and measure the attraction between them by measuring the (very
small) deflection towards each other. The problem with this - apart from the
minute deflections when you use a kg or so of sugar as your target mass, or
a dozen cream puffs (not to even think about the problem of finding the cofg
of a dozen cream puffs accurately enough for this purpose ;-)  ). A problem
is that this looks suspiciously like a force balance with an added twist.
This is now a THREE body problem - a lead ball/plate, some cream-puffs and
the earth.

A major problem with gravitational mass based systems is that any stray
gravitational "fields" (or pieces of gravitationally bent space time)(or
passing gravitons? :-) ) affect the result. By operating your apparatus
orthogonal to the vector sum of all other gravitational fields you should
get an independent result.

Presumably you are going to measure the force produced on one mass by
t'other. But, we are now back where we started, trying to measure a force to
infer the action of a mass. I can't think how one uses gravitational mass
without letting force measurement creep in (may be a way but I'm going to
bed any moment now ;-)).

We could even perhaps use the conveniently large mass we are standing on and
use it's gravitational mass effects on the target mass to determine it's
mass. Sounds suspiciously like the problem we started with.
HOW do you use gravitational mass to measure a mass's mass?
I'll go to bed now and let other inquiring minds take up the thread.

& Yes! - we are having fun yet!


Russell McMahon.

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2001\02\14@172338 by Peter L. Peres

picon face
>So how do you measure mass in space/0g ? Some sort of
>accelerometer ?

You attach the object to a spring that has another mass at the other end
and you exicte a harmonic oscillation in the system. One period is enough
to find out what you want to know.

Peter

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2001\02\14@172451 by Peter L. Peres

picon face
>Hi Bob. I'm not scared of 200 Gs. :o)
>My brochure here for a standard stepper
>motor cites 34 Gs continuous side force
>rating (by my calc), so with a lighter
>rotor I would give it a go.

I think that you should bear in mind that the lateral load spec on the
spindle is for the spindle and the bearing, not for the innards of the
motor. 200G's might shatter the permanent magnet of remove it or some
restrainging device from the spindle. It could also cause glue (which is
often used to fix the magnet on the axle) to be extruded and make the
motor permanently eccentric.

I have checked some numbers and a 200G lateral force would remove the
brushes from the commutator in a DC permanent motor for which I have
figures for the brush force and weight.

Peter

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2001\02\14@175349 by Bill Westfield

face picon face
   I think that you should bear in mind that the lateral load spec on the
   spindle is for the spindle and the bearing, not for the innards of the
   motor. 200G's might shatter the permanent magnet of remove it or some
   restrainging device from the spindle. It could also cause glue (which is
   often used to fix the magnet on the axle) to be extruded and make the
   motor permanently eccentric.

The parts of the motor do not experience 200gs.  Only things out at the rim
see that, and the motor sees corosponding FORCES based on the combination of
the Gs and the masses out at 1m radius and 7rps.  Internal issues due to
centrifugal force should all be covered by a max rpm spec, and everything
else IS up to the spindle and bearings.

BillW

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2001\02\14@183238 by Jinx

face picon face
> You attach the object to a spring that has another mass at the
> other end and you exicte a harmonic oscillation in the system.
> One period is enough to find out what you want to know.
>
> Peter

So that's similar to perturbations you'd observe between stellar
objects

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2001\02\14@214309 by Russell McMahon

picon face
This thread has split.
What say we change the title to "[EE]: Measurement of mass."

I'll post this under old and new titles so people wont lose the thread.

> > You attach the object to a spring that has another mass at the
> > other end and you excite a harmonic oscillation in the system.
> > One period is enough to find out what you want to know.
> >
> > Peter
>
> So that's similar to perturbations you'd observe between stellar
> objects


Yes. Simple harmonic motion turns up all over.
It is a function of the inverse square law - if this was out by one part in
ever so many (2.000000000000000x?)
then orbital systems would not exist. This is one of the nicer proofs that
inverse square law must be very very very ... close to 2. There is no
"reason" for gravity to work this way - it just does. Like everything in
physics (and reality) it ultimately comes down to just observing what we see
and describing it.
Rutherford (I think it was) said something like "There's Physics and the
rest. Everything else is stamp collecting." It seems to me that EVERYTHING
is stamp collecting.

When measuring mass by the above means, you do have to be a little careful
that you know what you are doing. If the period of the "forcing function"
(the initial perturbation) is not equal to that of the oscillatory system
(and it won't be coz if you knew what it was you wouldn't have to measure
it) then the result will be a function of the two. Very standard problem but
able to trap the unwary. Pulling the spring out, holding it until
oscillations die out then letting it go cleanly provides a pretty good
result. Note that magic is hidden in even these simple terms.  - "letting
oscillations die out" implies there is damping (which we all observe
frequently) meaning that something else is at work. if the damping function
is non-linear then life again becomes 'interesting". In practice its not too
hard to get a reasonable result simply but it's worth noting that there are
gremlins lurking in the apparently simplest system. "Letting it go cleanly"
may also pose problems in reality.



regards

           Russell McMahon

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2001\02\15@164553 by Peter L. Peres

picon face
>HOW do you use gravitational mass to measure a mass's mass?

Use the distance, Luke. The grav attraction depends on distance raised to
a power. If you can move the test body and measure force simultaneously
then you have it. You could use the Moessbauer effect (in both directions,
to cancel external field), to determine the potential energy difference
from test mass to probe mass vs. distance I think.

Peter

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2001\02\15@164612 by Peter L. Peres

picon face
>The parts of the motor do not experience 200gs.  Only things out at the
>rim

Did you not say that the smaller motor will ride at the end of the arm ?

Peter

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2001\02\16@051232 by Russell McMahon

picon face
> >HOW do you use gravitational mass to measure a mass's mass?
>
> Use the distance, Luke. The grav attraction depends on distance raised to
> a power. If you can move the test body and measure force simultaneously
> then you have it. You could use the Moessbauer effect (in both directions,
> to cancel external field), to determine the potential energy difference
> from test mass to probe mass vs. distance I think.

Except that AFAIR the proposer of using mass to measure mass was suggesting
that "gravitational mass specific" properties be used to measure mass as
opposed to using force measurement or inertial mass measurement. It was
suggested that inertial and gravitational mass MAY not be the same.

I believe that all this sprang from my suggesting that our measurement
systems all tend to measure force rather than mass and my noting that we
have moved from an imperial force/force system to a metric mass/mass system
when measuring force and mass. - Just try and buy 3 Newtons of sausage meat
or a slug bag of potatoes :-)




RM

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2001\02\16@082322 by Bob Ammerman

picon face
> Except that AFAIR the proposer of using mass to measure mass was
suggesting
> that "gravitational mass specific" properties be used to measure mass as
> opposed to using force measurement or inertial mass measurement. It was
> suggested that inertial and gravitational mass MAY not be the same.

Um, I believe that the most obvious "gravitational mass specific" property
_is_ the _force_ exerted on the object when it is in  a gravitational field.

How else would you define "gravitational mass" other than as:

"a property of an object which is proportional to the force exerted on that
object when it is placed in a uniform gravitational field"

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\02\16@083815 by Alok Dubey (OCS-BLRAKS-AVS)

flavicon
face
Actually the idea is better portrayed that mass , like charge has no meaning
if it is alone.. u can measure some quantities only by their effect on
something else.. or the interaction is necesary.. for eg.. mass is primarily
defined from the experimental results of Newton that force of attraction b/w
2 bodies is propotional to their masses and inv of square dist..

F = G m1 m2 /R^2

and its here that  m1 and m2 have a meaning.. else it doesnt make sense..

funnay actually , .. einstiens theory of relativity actually applies to all
things which have relative perception only.. although i cant say that
Minkowiski was derived coz of this( lorentz was the idea here).. i mean mass
and charge are 2 qtys which are relative and these are the 2 quantities
which are affected by relativity!!!

a nice spinoff here..

how did we 1st find the mass of the earth?.. i mean both Mass of earth and G
=gra constant were unknown!!
yeah i know the answer.. but Thanks to Cavendish.. but whew! .. weigh the
earth...

Alok





{Original Message removed}

2001\02\16@084023 by Alok Dubey (OCS-BLRAKS-AVS)

flavicon
face
-----Original Message-----
From: Alok Dubey (OCS-BLRAKS-AVS)
Sent: Friday, February 16, 2001 7:20 PM
To: Alok Dubey (OCS-BLRAKS-AVS)
Subject: RE: [EE]: Centrifugal forces on a motor


> Except that AFAIR the proposer of using mass to measure mass was
suggesting
> that "gravitational mass specific" properties be used to measure mass as
> opposed to using force measurement or inertial mass measurement. It was
> suggested that inertial and gravitational mass MAY not be the same.

Grav and inertial mass is not the same.. if ur grav is not uniform.. it goes
into General theory of relativity from there

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2001\02\18@232606 by Norman J Gillaspie

flavicon
face
There was a helicopter that had rocket engines at the end of the blades.

Norman

-----Original Message-----
From: pic microcontroller discussion list
[PICLISTEraseMEspam@spam@MITVMA.MIT.EDU]On Behalf Of Peter L. Peres
Sent: Thursday, February 15, 2001 1:10 PM
To: RemoveMEPICLISTspamspamBeGoneMITVMA.MIT.EDU
Subject: Re: [EE]: Centrifugal forces on a motor


>The parts of the motor do not experience 200gs.  Only things out at the
>rim

Did you not say that the smaller motor will ride at the end of the arm ?

Peter

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2001\02\20@221135 by Russell McMahon

picon face
Plus the Roton Single Stage to Orbit (and back) design used rotor tip
rockets to ascend and then used the rotor as a recovery device.
The company folded BUT the design is technically feasible. The rotors do not
add a vast amount of gain over a more conventional rocket system but
effectively provide themselves a 'free ride" to orbit and are then available
as a re-entry recovery device.

Ascent thrust was overwhelmingly by the effect of the rockets but the rotor
provides the pumping power.


     Russell McMahon
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