The increasing speed of CD-ROM drives requires a good balance to have smooth operation. The forces applied on the disc clamper, caused by a disc with an unbalance, increases fast when the reading speed goes up. A disc with a significant unbalance will cause the player to make noise and vibrations can be noticed. A reduction of the lifetime of the CD drive is to be expected.

The specification (sept 1998)
CD-DA (Red Book): Disc unbalance <10 g.mm is recommended
CD-ROM (Yellow Book):
- Disc unbalance =<7g.mm mandatory
- Disc unbalance of < 5 g.mm is recommended.

CD-R (Orange book): Disc unbalance <2.5 g.mm

- Disc unbalance <10 g.mm is rmandatory.

Every rotating object has in practice some degree of unbalance. Methods of measurements for determining the unbalance of rigid rotors can be found in the “International Standard ISO 1940/1”. Although a disc in a CD drive is part of a rotating body composed of a motor rotor in its bearings, the disc itself and the disc clamping mechanism, only the unbalance of the disc will be determined here. The main cause of unbalance present in discs is thickness variation due to moulding.

Substrate Thickness Variations
The differences of the Polycarbonate substrate thickness over the surface is an important cause of unbalance.

Use of labels
The application of a printed disc label can have its effect on the unbalance. When the amount of printing is asymetric and rather heavy the influence can not be neglected. You will get similar effects when adding a self-adhesive sticker to the disc. A sticker with a dimension of 50mm x 31mm and a weight of 0.157gr will give an increase of the unba-lance with 5.6 g.mm.

Measurement of the disc unbalance can be performed e.g. by measuring the vibration of the measuring set-up with an accelerometer in co-operation with a Fast Fourrier Transform (FFT)-analyser or another special analyser. In most cases the measuring results will be represented by vector quan-tities. A check of the calibration of a system can be made by measuring a disc, add a known weight at a certain diameter, measure again and calculate in a vector way both results. Modern systems use this simple procedure during calibration.

Disc clamping
Special attention should be paid to the clamping of the disc during the measurement. The disc should be centred using its centre-hole. A cone will deal with the tolerances of the centre-hole. This also happens in a normal disc drive, but putting it on a disc-table with in the centre a cone as part of the table will centre the disc. The cone is on the same side of the disc as the read-out laser. Especially for double-sided discs (DVD), two discs of 0.6 mm glued together to a 1.2 mm disc. The disc side that is clam-ped determines the unbalance of the combination. Considering the tolerances of the two centre-holes and the gluing of the two disc-halves, it is advisable to use a cone with an inclination e.g. 20°.

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