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FROM SPOON BRAKES T 0 TWO LEADING SHOES

29th January 1943
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Page 26, 29th January 1943 — FROM SPOON BRAKES T 0 TWO LEADING SHOES
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Which of the following most accurately describes the problem?

How the Brakes Have Been Developed from the Early Days When Deceleration Depended on a Wooden Block Pressed Against the Wheel Rim to the Present Time with its Braking Equal to the Force of Gravity ,

By A. W. Haigh,

A.M.I.Mech.E., A.M.I.A.E. THE importance of the braking system on any mechanically propelled vehicle cannot be too greatly emphasized. If the engine fails it is merely inconvenient, but whenever a vehicle is in motion it is a

potential menace to lifeif its brakes be inefficient. .

In the very early days of motoring, when automobiles looked like horse-drawn carriages with the shafts renioved. a block of wood, acting on the outer rim of the metal-shod wheels, was a sufficient brake for all occasions. This type of brake held its own for a number of years, for even after cars were equipped with pneumatic tyres a spoon-type brake was used which acted directly on the tyre tread.. In 190S interest in four-wheel brakes was aroused, and in 1909 Henri Perrot marketed what was, perhaps, the first workable system and thereby raised a storm of criticism which was active as late as 1924. • The main fault found was the loss of steering control when the front wheels were braked and locked. The fault was nut merely theory, for in those pioneer days a number of car manufacturers took up the brakes with enthusiasm only to discard them after a trial.

Heavy vehicles were in no way behind private cars in the enterprise, fire-engines, among other types, being provided with four-wheel brakes in an effort to combine speed with safety where speed was so vital. ' Here again, however, the front brakes were soon discarded in support of the adverse criticism. Owing to what in these days would be put down to bad design, the front wheels did lock when the brakes were applied hard and were prone, in other circumstances, to take the steering control out of the driver's hands.

Brakes in those days were so bad that on steep hills they could not be relied upon even to hold the vehicle stationary. Evidence ot this is afforded by the fact that a aping was almost a standard fitting. It consisted of A pointed lever, hinged to the chassis, which could be lowered at will and was designed to stick into the ground when the vehicle began to run backwards.

Attractions of Transmission Brakes Outweighed

Considerable popularity, even up to recent times, has been enjoyed by transmission brakes. Engineers argued that they were far more effective than brakes on the hubs., because the shaft revolved at greater speed than the road wheels and, therefore, could slow down the vehicle more effectively. They ignored, or appeared to ignore, the fact that the whole of the transmissiOn and rear axle was subject to the braking force, and, if these units did not actually break under this force, they were considerably stressed, whilst wear of the universal joints was undoubtedly accelerated.

A further claim was that a transmission brake did not require so great a pedal pressure as a wheel brake, but, despite this acknowledged fact, the foot, which is capable of a far greater effort than the hand, in certain well-known cases was employed to actuate it. , However, the danger of a transmission brake was .eventually realized and it was recently made illegal in this country, although it is still popular in America, at least as a parking brake, for which purpose it is assuredly well suited.

The relative merits of external-contracting band brakes and internal-expanding shoe brakes provided a subject for many arguments, but it was finally proved that the contracting type, whilst being suitable for transmission work, to which it was more readily applicable than the expanding brake, was not satisfactory when used on the wheels, for a number of reasons. Shoe facings and band linings were either of metal or fabric, but metal was favoured, at least for expanding brakes, because fabric, was at the stage . when it wore rapidly and was, as it still is to some extent, adversely affected by heat, water and oil.

Whilst continuing to contest the advisability of frontwheel brakes, ,engineers used the' ref wheels for both hand and foot systems. In some cases two sets of internal shoes were used, and in others internal-expanding and external contracting brakes on the same drum. , As road speeds became gradually higher and traffic denser, it was eventually conceded that rear-wheel brakes alone were inadequate. These were either cam or -toggle operated and leverages between the pedal and the shot; were of such proportions that pedal travel on the heavier commercial vehicles was so great as to be almost impracticable. Mechanical boosters were designed, that is, mechanisms whit, took athantage of the vehicle's momentum to stop it by actuating the brakes. Vacuum servos, compressedair units and hydraulic cylinders were incorporated in the chassis, but still rear brakes alone were insufficient: Four-wheel brakes had become necessary and Perrot was justified. Many ingenious mechanisms were evolved to operate front-wheel brakes some of which are still in use. Push rods through the king pin, which automatically took care of the lock angles, were used, but the most popular type was the ball-jointed camshaft, and later the encased and .lubricated cable.

The problem of loss of steering through locked front wheels was still receiving much attention. Various devices for cutting out the brake on the outer wheel when cornering were introduced and claims were made that at last front-wheel braking was safe. The Hotchkiss company, however, contested the idea that outer-wheel brake relief was essential and conducted tests, whereby, first, equal braking was applied to each front wheel and, secondly, more pressure applied to the outer wheel. In neither case was it possible to detect any detrimental effect, and it was suggested that as the loss of control was experienced only when the front wheels were made to skid, they should not be allowed to du so. This, as will be shown later, can, with certain reservations, be effected.

:Careful research into the action of internal-expanding shoe brakes showed that their efficiency could be considerably increased by modification of the design. All old-type brakes, no meter what their actuation, worked on the principle illustrated in Fig. 1, which shows, diagramatically, a fixed-anchor, cam-operated unit.

When the cam is turned, pressure is applied to the shoes, which are forced against the drum At first sight the braking effort of both shoes appears to be equal, but, on taking into account the rotation of the drum, it will be appreciated that the leading shoe does far more work than the remaining trailing shoe.

When the braking effort is applied to the leading shoe, the action of the rotating drum Is to pick up the friction surface at the cam end and wedge it firmly against its

fixed anchor through the medium of the rest of the shoe. The action of the drum on the trailing shoe, on the other band, is to force the shoe away from it,

Thus the only pressure keeping the trailing shoe in contact with the drum is that applied at the cam, but that keeping the leading shoe in place is the pressure applied by-the cam plus the wedging action of the rotating drum. In other words, the manual effort on both shoes is the same ; but the leading shoe is subject to extra pressure through the wedging action of the brake drum, thus causing, besides uneven braking, extra wear on the leading shoe. It has been proved practically that the work done by a leading shoe is approximately three times that done by a trailing shoe.

No matter uhat may be the operating means for fixedanchor brakes, cam, toggle, etc., the uneven work and wear characteristics are always displayed. Steps were taken by brake manufacturers to remedy the faults and still keep fixed anchors. The facing on the trailing shoe was -decreased in arca and a different material used' to ensure the same unit pressure, or load per sq. in., on the whole of the braking surface. This, however, did not cure uneven braking. An hydraulic brake was designed with an operating cylinder embodying two different-size bores. This put more load on the trailing shoe and evened out the work but not the wear.

The idea of floating anchors was eventually hit upon and the most powerful brake ever invented was produced. Fig. 2 illustrates the action of the brake.

The effort is applied to the leading shoe, is magnified. by the self-energizing action and reacts on the floating anchor, which passes it to the leading end of the trailing shoe. As the effort is applied to the trailing shoe at its leading end, the servo action again magnifies it. Thus the manual effort is stepped up twice.

Powerful Braking Afforded by Double Servo This double-servo actIon ensured great braking power, but did not solve the problem of uneven work and wear, merely reversing the effect by making the trailing shoe do more than the leading shoe.

The most up-to-date design of internal-expanding brake, the two-leading-shoe-type is a logical development of the double-servo type. In the best examples of the latter the leading shoe works ideally, that is, all its facing area contacts the drum at the same time, whilst pressure is applied as equally as possible over the facing surface and wear is even. In the ideal two-leading-shoe brake both shoes are subject to the same manual effort and the same servo action. In consequence, they do exactly the same t mount of work and are worn at the same rate.

Figs. 3 and 4 show, respectively, Bendix and Girling two-leading-shoe units. Each brake can be designed to work as efficiently in reverse as in forward motion, but in practice. it has been found that two leading shoes in reverse are unnecessary owing to low road speeds. As fewer, parts are required for a brake giving the two-leadingshoe action in only one direction, cheaper. units are produced.

Bendix, Ltd., obtains its two-leading-shoe action as illustrated in Fig. 3, the brake shown being operated by Bendix expander. The plunger A is forced outwards by the expander and pushes on the bell-crank lever B, which is pivoted to the shoe web. The lever, in turn, passes the effort through the compression member C to a second bellcrank lever D, which is in contacf with the floating adjuster E. Plunger A1 acts in the same way on the secondary shoe, thus providing an immediate reaction to the effort from

plunger A at the floating adjuster. The direction of the resultant force of the effort and the reaction is outwards and it pushes the primary shoe bodify into contact with the drum which carries it around until the end of the shoe rr,ects the stop on the adjuster housing.

The secondary shoe is actuated in a similar way, the effort from plunger A passing through the levers and push rod to the floating adjuster which transfers it to the levers and rod duplicated on the secondary shoe. Plunger A, provides reaction, and the shoe is forced outwards and is carried round by the drum to the stop on the .plunger housing. As the effort is applied to both shoes at their leading ends they are bath true leading shoes and both take full advantage of ,the servo action of the rotating drum to the same degree, because of the equal manual power operating them.

The methods adopted by New Hudson, Ltd., the maker of the Girling brake • (Fig. 4), whilst being entirely different from those employed by Bendix, Ltd., give the same result. Having described the Bendix model which gives two-leading-shoe action in both directions—forward and reverse—I am dealing with the model in the Girling range which operates similarly but in only the forward direction.

System Employing a FloatIng.Secondary Shoe

The leading, or primary, shoe (on the left of the illustration) is standard, but the remaining shoe is of special. construction. It consists basically of two pieces, one of which is a carrier and the other the shoe proper. The carrier is made up of two channel sections, fixed back to back, with a gap between them. 'Into this gap fits the web of the shoe, the inner faces of its flanges resting on spring-loaded' pivot levers. The two members, carrier and shoe, are held together by a bolt passing through a slotted hole at the centre of their webs which allows relative movement between them.

When the brakes are applied by the Girling wedge, at the bottom (asillustrated), the leading shoe operates in the uitial way.. The trailing shoe, .however, on coming into contact with the drum is forced around with it, sliding between the flanges of the carrier and pivoting on the levers. It moves around until its lower end meets the stop on the lirake back plate, which acts as a reaction point, and which, in effect,. transfers the operation from the trailing to the leading end of the shoe.

An examination of the four brake diagrams will show the wear which takes place on the facings of each type of brake, the black portion indicating the part worn away. With fixed-anchor brakes the wear occurs mostly at the plunger end with snore on the leading than on the trailing shoe. On the double-servo type the leading-shoe facing is evenly worn, but that on the trailing or secondary shoe is worn mostly at the end farthest from the plunger and more rapidly than the other facing, whilst wear on both facings of the two-leading-shoe brakes is evenly distributed and equally rapid. On all friction brakes, such as those described, the heat generated is. considerable. The dispersal of this. heat, at one time, received attention which modern designers have

eifher forgotten or think unnecessary. . Present-day materials for drums, friction facings and other brake items are far in advance of their predecessors and, in consequence, .

the effect of heat on them is not so marked. • Early brake fabric, for instance, was badly affected by heat, the coefficient of friction decreasing rapidly as the temperature rose. Brake drums expanded considerably so that pedal travel was increased. Often drivers adjusted their brakes while the drums were hot so that when they cooled off the brakes were permanently on, I should perhaps qualify my statement that modern designers are not worrying about cooling drums. Research work is, in actual fact, engaging considerable attention at the present time, but the object is rather to protect the tyres from heat than to avoid loss of braking efficiency.

The coefficient of friction of modern brake fabric remains practically constant throughout the range of temperatures set up under normal braking conditions, the only time when any decrease in braking efficiency can be felt being on long hills, when the brakes are in constant use and generate excessive heat causing a decrease in the coefficient.

Modern Brakes Can

Stand Up-to Much Heat

This phenomenen of "fade " is particularly loaoticeable when 'double-servo-type brakes are used, because of the

very -high pressur between shoes--and drum. Special alloys; however, have been developed which reduce drum expansion to a miniinam so that-cooling fins art no longer needed solely for their original function. They are, nevertheless, still used to avoid distortion of the drum and to prevent temperatures from being reached which might dc external damage.

Clearly, then, cooling must not be entirely neglected, but it is not now so important from the point of view of braking efficiency as it was a few years ago. Usually the brake drum occupies nearly all the available space inside the wheel so that coolibg more or less takes care of itself, but, despite this fact, modern brakes' operate satisfactorily without a great deal of time and thought being given to cooling them.

In the foregoing I have shown hta: the design of brakes has been brought to the high degree of efficiency that characterizes modern systems. There still remains to be considered the limitations' to deceleration that depend on factors lying outside the brake mechanism. The standard of retardation chOsen is the equivalent of the acceleration due to gravity, namely, 32.24t. per sec, per sec., and brakes that give this rate of deceleration are described as of 100 per cent. efficiency. But the subject is a big one, so I will leave an explanation of the matters it involves to a later instalment.

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