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Aviation History
1949
1949 - 1288.PDF
FLIGHT JULY 14TH, 1949 WHEEL BRAKES Their Development, Design and Use Part II : Disc Brakes : Lining Materials : The Future—Advance of Reversible- airscrew and Other Systems By MAURICE F. ALLWARD IN the previous article the various designs of expandingor drum brakes were considered, and we now turn toa contrasting type, namely, the disc brake. Generally speaking, the brakes of this kind may be said to have three main advantages over the drum brake, as follows: — (1) A drum, when hot, may lose its cylindrical shape and expand considerably, thus adversely affecting braking performance; this expansion does not affect disc brakes to the same extent. (2) The drum dissipates heat from the wall opposite to that in which it is generated, but the disc, heated on both sides, has more even heating and cooling, thereby lessening chances of distortion. Also, the larger exposed area for cooling is in the correct attitude to the airflow for efficient cooling. (3) It is easier to isolate thermally the mass of the wheel. Heat transfer from the disc into the wheel can be kept down to a remarkably low level, as the only full metallic contact be- • tween the two is between the edges of the keys and keyways. The disc thus floats, and the expan- sion due to heat is allowed for in the design of the keyways. This, perhaps, is the most important feature of the disc brake and allows the admittance of the much-increased operating requirements of to-day, whilst even reducing the likelihood of tyre and tube failure from excessive brake-heat. Comparative weights depend essentially on the tempera- ture to which the braking surface can be taken, and the amount of cooling that can be achieved by direct dissipa- tion to the atmosphere. There is a tendency to believe that discs can more reliably be taken to higher tempera- tures than drums, and should, therefore, ultimately prove to be the lighter, but this is not entirely borne out in practice at the present time. As in most things aeronautical, a series of compromises is necessary be- tween various contradictory factors ; for example: (1) The disc will ob- viously cool more effectively when exposed to the slipstream and circu- lating air, but a disc which is not protected in some manner will be subjected to damage by dust, sand, 01 other foreign bodies, possibly re- sulting in rapid lining wear. An ex- posed disc also has the disadvantage of having projecting bulky com- ponents, sometimes necessitating longer and stiffer—and consequently heavier—axles. (2) Use of very small specific lining area under high pressure gives the maximum exposure to the disc and therefore good cooling. On the other hand, an appreciable increase in lining contact pressure may tend to make the lining performance variable, and this, in turn, may give IN Part I of this article, published last week,the author reviewed early developments and present types of expanding-diameter brakes. Here he goes on to deal with disc brakes and to discuss various general problems of brake installation and use, concluding with a consideration of other systems—e.g., reversible-pitch airscrews— designed to supplement the effect of wheel braking. rise to an undesirable increase in the rarte of wear of the brak«i (3) The greater the mass of the disc—that is, the thicker the disc—the greater is the heat it can absorb. But the thicker the disc the greater the temperature gradient, and an allowable final mean temperature of, say, 500 deg C could correspond, during braking, to an incredibly high surface- temperature. Most discs are plated with hard chroffi* to increase resistance to abrasion and oxidization and sucfi a temperature could cause the formation of chrome oxide which, having no mechanical strength, would be torn off instantly in service. The trend of present developments is to put the accent on better cooling at the risk of some damage to the disc ; the use of greater lining areas, with a consequent reduction in area exposed for cooling ; and thicker discs at the risk of very high surface- temperatures. The continually in- creasing kinetic-energy absorption requirements are likely to promote multiple discs, in order to keep the individual disc mass down to reason- It will thus be seen that, as in the case components, successful design is Duirlop disc-brake wheel pan-sectioned. The keyways for the disc are shaped in the light-alloy hub casting and flanked by steel inserts. able proportions. of many other aircraft largely a question of well-judged compromise. Basically, disc brakes consist of an operating unit which is mounted astride a disc rotating with the wheel. The operating unit is fixed to the axle or torque plate, while the disc is mounted on keys located round the rim of the wheel. Thus the disc normally runs through the operating unit, and is clamped between the linings when the brake is applied. The housing of the operating unit contains pistons, usually hydraulically operated (although some small brakes are mechanically operated), which exert pressure on the linings on one face of the rotating disc that moves axially along the keys in the wheel to contact the opposing fixed lining member on the other side. As the brake pressure is increased the rotating disc is clamped more tightly between the linings, and its rotation is arrested by such clamping action in proportion to the pressure applied. Small disc-brakes have only one lining pad, but the larger and more powerful types have three or more, arranged either closely together or spaced around the edge of the disc. The lining pads are usually circular in section, as these are cheapest to make, but sometimes they are of kidney section which, with a special form of hydraulic-motor unit, gives an increased load with the greatest economy of weight. This shape is adopted on several Palmer brakes, which use a sealed hydraulic capsule instead of a normal piston unit. On the Goodyear brakes, three pads are bunched close together; B 24
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