Securing Rigidity in Brake Drums
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Interesting Manufacturing Methods Ensure Compactness, Silence and Strength in a New Design of Drum
ALTHOUGH the use of a servo mechanism solves some of the difficulties of the brake designer, it magnifies others. High shoe pressures, for instance, emphasize whatever weaknesses may exist in the drums. Common faults in this connection are temporary or permanent distortion, rapid wear of the tracking surface, and brake squeal. Even in brakes that are not servo-operated the last-named defect is sometimes all too audible, whilst the others, although perhaps less obvious, cause 'even greater loss of stopping power. Brake-drum design is, in short, a matter of importance.
Two midland firms, in collaboration, have evolved a new type of drum of distinct merit. Messrs. Pratt and Manley, consulting engineers, of Guildhall Buildings, Navigation Street, Birmingham, have made a lengthy study of brake design. Messrs. Holden and Hunt, of Old Hill, Staffs are, on the other ,hand, experts in the practice of welding and allied processes.
For the new brake drum a length of a suitable rolled-steel section is bent into a circle, and the two ends are.electrically welded so that a continuous hoop is formed. By a proper choice of section this hoop can be ribbed exter nally to provide rigidity and cooling surface. Similarly, by a proper choice of material, good 'wearing qualities can be ensured. Any weldable material can, of course, be used.
Into one side of this hoop a steel disc is spigoted, the two parts being then pegged and spot-welded together. The centre of the disc is shaped to mate with the wheel hub, and the tracking surface of the complete drum is ground true and concentric.
Several, results are obtained by this method of construction, which is provisionally protected. Compared with cast iron, the welded drum hasmerit of greater strength and freeom from cracking. It is also lighter. Contrasted with a pressed-steel drum, it is much more rigid, and can be made of better material than is convenient for pressing.
Normally, a 0.45 carbon steel is em
ployed for the brake track. This has excellent wearing properties and does not tear or drag. The rigidity is due not only to the ribbing, not obtainable on a pressed drum, but also to the compactness of the welded drum, which needs no radius or fillet on each side of the brake track. This point is clearly illustrated in the left-hand sketch, where the pressed-steel-type of drum is indicated by the dotted lines.
The welded drum is practically free from any tendency to squeal. When struck with a hallint" er one of these drums emits a dead sound, scarcely more than a thud. This is attributed to the fact that the hoop and disc are not continuously joined, being spot-welded. Consequently, each part damps vibrations in the other. A similar result is found, in a haphazard fashion, in a cracked bell.
Finally, it is understood that the price of welded drums is reasonable. In view of the production processes there is no obvious reason why this should not be so.