Sat 2003-06-14 06:21, Charles Angelich (1:106/2000) wrote to All:

 CA> Didn't think much of this until people wrote into Fred Langa about 
 CA> CDs exploding in their new 52x CD drives.

 CA> There are some theories as to why but no way to prevent this (yet).

On a related note, I found this:

"What is CD unbalance? Why do I keep hearing about it? 

Compact disc unbalance is similar to the unbalance of an automobile tire.
The ride can be smooth at low speeds but very rough at high speeds. CD
unbalance causes the disc and drive to vibrate. This vibration causes the
track location to shift at moderate amounts of unbalance, transferring the
rotational frequency into the radial servo channel. Severe vibration at
high speed can permanently damage the spindle motor bearings. 

Conventional CLV drives rotate at a speed that causes the pits to pass
under the pickup at constant linear velocity. This results in a 1X
rotational speed of about 580 rpm near the inner diameter at lead-in.
Rotational speed decreases as the pickup moves outward on the spiral track,
reaching 230 rpm near the outer rim. Since information always begins at
lead-in, every disc must initially rotate at the highest 1X rate of 580
rpm. This means that a 24X drive spins at 13,950 rpm when reading lead-in,
and a 48X drive screams at 27,900 rpm. Bearing life in such drives can be
very short if the disc is unbalanced. 

Unbalance is caused by an uneven distribution of mass around the disc.
Offset of the center hole can cause unbalance as can an outer rim that is
not perfectly centered or not circular in shape. Slight variations in
thickness of a disc or an asymmetric label also results in unbalance. Disc
damage that chips or deforms the rim does not affect the readout surface
but will create unbalance, while modest internal stresses can cause the
disc to shatter. 

Another potential source of unbalance is the 15.00-15.10 mm tolerance of
the center hole. The spindle motor clamping mechanism of CD drives allows
for this tolerance and attempts to center the disc. Centering is not
perfect, and resulting unbalance can have severe effects at high speeds. 

Equivalent static and dynamic methods are used to measure unbalance. Static
unbalance depends on the mass of the disc and the offset of its center of
gravity from the geometric center established by the center hole. Dynamic
unbalance is determined by the radial force of a disc spinning at constant
speed, and is proportional to the square of that speed. 

Dynamic unbalance of less than 0.04N is required by the Red Book for 120 mm
diameter audio discs rotating at 600 rpm. This speed is very close to the
actual 1X speed at lead-in. The abbreviation "N" represents
Newtons, and 0.04N is equivalent to about 0.009 pounds force. Since radial
force increases as the square of the speed, 0.04N at 600 rpm translates to
21.6N or 4.86 pounds at 24X, and to 86.4N or 19.4 pounds at 48X. 

Very high speed 40X, 48X, and faster drives can be destroyed by discs that
satisfy the Red Book unbalance specification. In response, Orange Book Part
III reduced the unbalance limit to 0.01N at 600 rpm. This still results in
very high radial forces on the bearings of the highest speed drives.
Remember that a 24X drive is spinning at 13,950 rpm when reading lead-in,
and a 48X drive is screaming at 27,900 rpm. 

Disc static unbalance is the displacement of its center of gravity from the
geometric center times the weight of the disc. Although discs are allowed
to weigh between 14 and 33 grams, most discs measure about 15 grams. The
Red Book dynamic unbalance limit of 0.04N equates to a static unbalance of
10.1 gm mm. This allows the center of gravity of a 15 gram disc to be off
center by 0,67 mm or 0.0265 inches. The Orange Book requires a maximum
static unbalance of 2.4 gm mm. This limits the center of gravity error to
only 0,16 mm or 0.0063 inches. 

The popularity of high speed drives has caused unbalance to change from an
obscure requirement to a very important quality indicator. Opportunities
for unbalance have increased as molding cycle times shorten. Overly
creative art departments can design labels with unbalanced ink
distributions. Stick-on labels and "shaped" discs obviously
create unbalance."

- http://www.mscience.com/faq17.html

 CA> The suggestion is to not have a CD drive at eye level where you 
 CA> set to use your computer.  I think this may be good advice?

I keep most of my computers in a wardrobe to keep the noise down.

-- mail{at}ozzmosis.com

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