Magnetic memory drum design

A magnetic memory drum is a convenient, relatively cheap storage device of moderate speed and of large capacity. It comprises a cylindrical rotor, whose surface is coated with a magnetic material, rotating within a shroud which supports salient heads. Proper design requires an appreciation of the effect on performance of such factors as rotor eccentricity, head-setting variations, head manufacturing tolerances, and storage surface variations. Equations are developed to describe the signal-to-noise ratio of a typical drum and the threshold (switching) voltage appropriate to the read amplifier for reliable operation. Material selection is approached on the basis of stress calculations and it is concluded that 2014-T6 aluminum is the best rotor material for general-purpose applications. It is shown that, for any rotor with a given degree of unbalance, there is a maximum stable operating speed. Problems associated with thermal mismatch of materials are discussed, and the necessity of operating large drums in a stable temperature environment is pointed out.