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Aviation History
1949
1949 - 1247.PDF
JULY JVH, 1949 Wheel Brakes ..... f LIGHT 11 \ type of brake is not more generally manufactured is the design difficulty of providing a simple adjustment to ensure that the brake will always oper- ate at maximum efficiency. In the Bendix Duo Servo Brake not only are both shoes leading shoes, but the operating force applied to the primary shoe is transferred to the secondary one, and this force is aug- mented by whatever friction effect it produces. It is an arrangement that gives great power for a small size. Apart from studying the adjustment aspect referred to above, the designer of a "brake operating on this principle has to take into special account the distortion arising from temperature changes. The ideal would be a brake drum with no "thermal expansion or distortion, and a brake lining with an absolutely stable coefficient of friction. Both factors, of course, influence all rigid-shoe brakes, but the effect is much more pronounced in a self-energizing •unit. An unusually interesting twin-shoe design is the Girling Hydraulic Self-adjusting Aircraft Brake. Here, the main departure from the orthodox is that the linings are in constant contact with the brake drum whether the brake is "on" or "off." There is neither clearance nor pull- off springs. At first this might appear to cause unneces- sary drag and wear, but the degree of contact is so fine that the drag is scarcely measurable and no heat is nor- mally detectable. In practice, this initial pressure, upon which the whole principle of the design is based, is so slight as to be harmless, while the immediate response and increased sensitivity provide a Tiigh degree of braking control. ;!r?.. . ;.^ Minimum Movement •' The brake consists of two shoes hinging on a iulcrum. Diametrically opposite is a cylinder containing a pair of short, opposed pistons. The outer end of each piston is slotted and engages with the tip of the adjacent shoe. At the back of each piston is a special cup washer and the pistons are separated by a light spring. Near the fulcrum end of the shoes is a tension spring, the main purpose of which is to hold the two shoes together suffi- ciently to prevent any tendency .of the tip of the leading shoe to '' grab'' and also to balance the outward pressure of the spring .between the pistons As the shoes .are actually touching the brake drum all the time, it will be obvious that it is only necessary for the pistons to move very slightly in order to apply full pressure to •the expanding shoes when the hydraulic pres- sure between them is brought into play by the master cylinder. Only a small operating or "master" cylinder is neces- sary to igive the desired mechanical advantage, since, due to the minute travel of the pistons, a very small fluid- displacement is required. Small displacement, a desirable feature at all times, .becomes most necessary on larger brakes if .the master cylinder or fluid reservoir is to be kept down to a reason- able size. Expanding-tube brakes differ little in principle from the Palmer Brake of 1927. Improvements have been concerned more with the development of construc- tional materials than with the basic design, which remains virtually unaltered. The changes that have been made can nearly all be traced to having been initiated to counter the effects of higher energy-dissipation requirements, resulting in higher operating temperatures to avoid too great a weight penalty. Major developments have taken (Left) Goodrich "Duplex " expanding-tube brake of the type fitted to the Boeing Stratojet : the use of twin pressings bolted together is interesting, as are the large number of blocks, which are shouldered and butt against recesses formed in the frame. (Right) Girling hydraulic self-adjusting brake, showing cylinder with opposed pistons engaging the shoe-tips. place in the brake drum to prevent the heat generated from reaching and damaging the tyre, as will be explained in detail further on. The rolled and pressed -castellated-channel brak« frame is now usually a very rigid aluminium- or magnesium- alloy casting, and the brake blocks are not attached directly to the expanding tube, but are positively re- turned and held by springs. A notable exception is the B.F. Goodrich Expanding Tube Brake, the frame of which consists of two pressings bolted together. The claim is made that this method of construction allows the brakes to be made lighter than are any other type of brake ior a given amount of kinetic energy. The penalty paid for this extreme lightness appears to be an abnormally wide brake-drum projecting beyond the wheel. Operation of tube brakes can be either pneumatic or hydraulic. If the hydraulic method is used it is advan- tageous to keep the nominal clearance between the blocks and braking service to a minimum, in order that the size of the reservoir may be kept down. The torque of the blocks can be taken either by butting directly against projections on the brake frame, or by riveting the blocks to a saddle wliich takes the load. This latter method is A = Cooling Vanes. 8 = Brake drum. C = Lining. D = Air-bag. E = Inflation valve. F = Bearing retainer. A typical Dunlop aircraft brake sectioned to show the internal construction. G — Bacfcplate. ' H = Hub. J = Detachable flange. K •= Lock-ring. L = Taper rollers. M = Staggered U-spokes.
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