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stress involved in stopping a spinning disk.

stress involved in stopping a spinning disk. - Physics Forum

stress involved in stopping a spinning disk. - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 11-16-2004, 12:45 PM
girigiribaka
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Default stress involved in stopping a spinning disk.



I was playing a game at a friends house and when we were done I turned the
power off and opened the tray. The CD was still spinning and I grabbed it
and put it in the case. He cringed and says that that puts a lot of stress
on the spindle. Argument ensues.

If I have a bicycle wheel with pedals attached to the center and I set this
in motion, I can stop it by applying force to the pedals, or I can stop
this by applying force to the rim. It is easy to see which case I'm more
likely to break the pedals.

the freely spinning CD is the same thing. the force is applied to the edge
and the majority of it goes to stop the cd's rotation. What is left over
for the spindle is only the enough force to stop the spindle. The spindle
with a low mass and short radius has very little in the way of rotational
inertia so the forces on the spindle are slight.
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  #2  
Old 11-16-2004, 01:51 PM
N:dlzc D:aol T:com \(dlzc\)
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Default stress involved in stopping a spinning disk.

Dear girigiribaka:

"girigiribaka" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]. ..

1) Who's spindle are we discussing? Who's money bought it?
2) I would not worry about the spindle, but the "paint" on top of the CD
that protects the data. The bottom can be scratched, and be resurfaced (a
few times). The top, once scratched, is permanent data loss. "Stuff" on
your hands can do this.
3) I haven't seen anyone yet who manages to contact two diametrically
opposed points on the rim, with identical contact pressures, such that the
spindle does not see some sideways pressure... something it is not designed
to handle too much of.

SO:
1) respect the owner's wishes (player, & CD),
2) take your time,
3) have another beer.
4) Whine at your friend to get a player that has brakes... ;>)

David A. Smith


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  #3  
Old 11-16-2004, 03:34 PM
Gordon
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Default stress involved in stopping a spinning disk.

On Tue, 16 Nov 2004 06:51:41 -0700, "N:dlzc D:aol T:com \(dlzc\)"
<N: dlzc1 D:cox T:[Only registered users see links. ]> wrote:

This spindle question intrigues me. On all the CD drives I've
seen, the disk tray slides out, removing the CD from the spindle,
before it is where you could get your hands on it. There is no
spindle in contact with the CD, once the disk tray is in the open
position. Are you talking about a different CD device than the
standard configuration that is used on most computers?

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  #4  
Old 11-16-2004, 08:19 PM
girigiribaka
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Default stress involved in stopping a spinning disk.

Gordon <[Only registered users see links. ]> wrote in
news:[Only registered users see links. ]:



you find top load design in things like a gaming console or a portable CD
player.
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  #5  
Old 11-16-2004, 11:18 PM
girigiribaka
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Default stress involved in stopping a spinning disk.

"N:dlzc D:aol T:com \(dlzc\)" <N: dlzc1 D:cox T:[Only registered users see links. ]> wrote in
news:LBnmd.61113$SW3.28161@fed1read01:


but who wants to win an argument with "it's mine and I say so."

I'm ignoring the side forces, these motors are used in applications where
they take more frequent and worse side forces. Even in this application the
motor is intended to take the vibrational energy caused by an imbalanced
disk.

Applying the breaking force at the edge of the disk should put less stress
on the spindle than what happens when the console jams the motor into
reverse and the spindle has to stop the higher mass, higher radius disk.
Another alternative is letting the bearings dissapate the rotational energy
through friction. I don't see either of these as being less stressful on
the motor, and both of these are considered normal operation.
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  #6  
Old 11-17-2004, 01:06 AM
Steven Gray
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Default stress involved in stopping a spinning disk.

girigiribaka <[Only registered users see links. ]> wrote in
news:[Only registered users see links. ]:


The real issue here is the magnitude of the forces involved. You're
causing the spindle-and incidentally, the CD- to stop much sooner than it
ordinarily would. Therefore the torque applied to the spindle must be
higher than normal. Whether the stress is worth worrying about is
another question.

For what it's worth, your bicycle analogy is flawed. In a bicycle with
coaster brakes, force applied to the rim of the wheel to stop it is not
transferred to the pedals. If the connection were direct, then you'd be
right that stopping the wheel by applying force to the rim results in
less force on the pedals. However, the argument is still invalid. The
pedals are designed to take the increased stress of forcing a rapid stop.
The CD spindle is designed to coast slowly to a stop.

--
Steve Gray
[Only registered users see links. ]
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  #7  
Old 11-18-2004, 09:59 AM
operator jay
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Default stress involved in stopping a spinning disk.


"girigiribaka" <[Only registered users see links. ]> wrote in message
news:[Only registered users see links. ]. ..
this

Is that a question?

Maybe the designers foresaw this foreseeable possibility and were able to
build accordingly.


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