FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1951
1951 - 0265.PDF
FLIGHT, 8 February 1951 167 OPTIMUM BRAKING Westinghouse System for Wheel-skid Prevention MJHGFEABCDKL A CARDINAL requirement for good braking is highresistance to slip between tyre and surface—in otherwords, a good coefficient of friction. Thus, by im- plication, if the wheels stop rotating before the vehicle has stopped then the braking becomes less effective. A locked, skidding wheel will not stop in so short a distance as will a wheel which is braked to just short of the skidding point. Neglecting the imposition of any side-force on the tyre—such as occurs, for example, when a road vehicle is cornering— what makes a tyre skid ? It is, obviously, the application of a force greater than that which the adhesion between tyre and surface can accommodate.. It is unnecessary here'to go into the technicalities of brakingas a study in mechanics; suffice it to state that, in order to provide sufficient braking force in ideal conditions, modernaircraft-wheel brakes must be of such power that the tyre will inevitably skid on surfaces which are less than ideal—and,of course, such conditions will apply with the greater frequency. To some extent the position can be modified by skilled applica-tion of the brakes, but so many factors conspire against a pilot being able to apply the brakes with anything like the delicacyof control enjoyed by, for example, the car-driver, that there is need for some means whereby human skill may be replacedby mechanical selection. Such is the designed duty of the Decelostat which, made by the Westinghouse Brake and SignalCo., Ltd. (82, York Way, London, N.I), is the British version of an American development. Before tracing the succession of events which occur in theDecelostat it is advantageous briefly to review what the device does. This can be stated quite simply: it releases the brakesimmediately the tyre slips preparatory to skidding, and re- applies braking pressure as soon as the tyre stops skidding.This cycle is capable of rapid execution, e.g., circa six to eight reversals per second. Therefore, no matter how hard the brakesare applied, optimum retardation is ensured for the adhesion conditions obtaining, in that the tyre is maintained as long aspossible at just below the skidding point throughout the time- to-stop period. The Decelostat unit has a diameter of 6in, is 2T5sin deepand weighs 3| 1b. As will be seen in the cut-away drawing (Fig. 1) it is flange-bolted to the aircraft wheel rather in the -.;:• -51OOO A =B = C =D = E = F — pawlclutch-ring throfcc bearing clutch-spring friction annulus ring-gear DECELOSTAT G -- bearing-plate H = planet pinions J ^ ptanet carrier K = sun-gear L = flywheel M = torsion-shaft F/g. 2. Curves of wheel speed and brake pressure fluctuations, sbdwing effect of Dece/ostots fitted to rear wheels of B-26 tandem undercarriage dufng s/^f^from 79 m.p.h. on wet concrete runway. Fig. I. Sectional detail of Dicelonat ani valve. The Decelostat is bolted to the landing wheel and, via the torsion-shaft running coaxially through the stub-axle, actuates the valve. manner of a hub-cap. Diametrically opposed on the inside of the Decelostat case are a pair of pawls, which engage buttress stops on the outside diameter of a clutch-ring. On the out- board side of the clutch-ring is an eight-ball thrust-race, against which bear the radial leaves of a spring-ring reacting against the end-wall (cover-plate) of the unit. Sandwiched between the inboard face of the dutch-ring and a bearing-plate pegged to the casing is a phosphor-bronze friction-annulus, and an internally- toothed ring-gear with which mesh three planet-pinions. These in turn mesh with a central sun-gear which is, in fact, formed on the shaft of a flywheel. From the planet carrier extends a torsion-shaft to actuate the Decelostat valve. The fact that the valve is a stationary -unit anchored to the undercarriage- leg assembly means that the torsion-shaft has to pass coaxially through the hollow core of the wheel stub-axle. In that it is bolted to the wheel, the Decelostat casing can be regarded as an integral part of the wheel structure; and when, on touch-down, the wheel is spun-up, the rotation is directly applied through the pawls to the clutch-ring. At this instant, however, the flywheel is stationary, but spring pressure applied through the thrust bear- ing squeezes the ring-gear between the bearing- plate, friction annulus and dutch-ring, and tends to make them rotate together. The exertion of the spring being less than the force required to spin-up the flywheel under the high initial rate of accelera- tion, the clutch slips and rotation is therefore 'applied to the flywheel gradually. (" Gradually " is, se, a relative term; in point of fact, the time- lag is infinitesimal.) The gear ratio between ring- gear and" flywheel is 1:7 and, as a result, the latter is rotated at a speed seven times greater than that of the undercarriage wheel and in the opposite direction. Whilst the flywheel is being accelerated, there is necessarily a torque reaction imposed on the planet carrier, and it is this force which is utilized to actuate the valve by means of the torsion-shaft. .When the flywheel has attained full speed, torque reaction on the planet carrier is relieved, and a graduating spring in the Decelostat valve returns m
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
