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8th December 1925
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The Variety of Operating Mechanism in Vogue at the Present Time. Servo Methods of Brake Application.

ALTHOUGH diverse opinions are held by designers and users as to the desirability of . a universal adoption of front-wheel braking, there can now be no question that, for-passenger-conveying vehicles (the speeds of which are often quoted as 40 m.p.h.), frontwheel brakes can be regained almost as essential. In many such vehicles, partiafarly when of the low-loadline, forward-dash type, So much as 40 per cent. of the total weight rests on the front axle when the conveyance is stationary, while a downward gradient or the forces set up by retardation will throw still more weight forward ; the weight distribution may then change to 50 per cent, or more on the front axle. It is therefore obvious that the adoption of four-wheel braking will, in such cases, result in a corresponding reduction of stopping distance amounting to nearly 50 per cent.

Even more important, perhaps; is the influence of four-wheel brakes upon the tendency to skid on greasy roads, the distribution of the retarding forces over four wheels in place of two being found almost entirely to eradicate the tendency of the vehicle to slide sideways when the brakes are applied. Then, again, although at one time it was urged by those objecting to front-wheel braking that this principle might not prove safe with solid tyres, this objection is now set aside by the great increase in the popularity of the pneumatic tyre.

In view of these incontrovertible facts it is not surprising to notice that quite a large percentage of the passenger-carrying vehicles exhibited at the recent Olympia Show were provided with four-wheel braking systems, particularly in the case of the new "safety coaches" shown by many prominent makers. At the moment there is a great diversity of operating mechanisms in vogue, and it will probably be some time before any degree of standardization takes place; it is the purpose of this article to describe the essential features of most of these mechanisms.

There are three front-brake operating mechanisms which have attained a considerable degree of popu

larity for commercial vehicles, these being the Perrot, the Kirkstall and the 'Clayton, the last named being a brake previously known as the Alford and Alder. (Rubury patents). All employ internal-expanding shoes. The principles of the Perrot mechanism are fairly widely known, the operating spindle in this case being universally jointed to the cam spindle above the steering pivot, and extending inwards to a ball joint on the side member of the frame. This spindle carries a lever which is jointed to an operating rod running to the pedal gear. By setting the operating spindle at an angle to the axis or the steering pivot an automatic differential action is obtained which relieves the braking force on the outer wheel when corners are

negotiated. Opinions differ as to whether this differential action is really necessary, but perhaps it is conducive to slightly additional safety on greasy roads. The Perrot system, with or without minor modifications, is fitted to certain passenger-carrying models of the following makes :—Maudslay, Crossley, Bean, Renault, De Dion-Bouton, Delahaye, McCurd, and Rgo. It is interesting to notice that in the case of the Renault front-wheel brakes are fitted to all models.

The Kirkstall brake is somewhat similar to the 'Perrot in design; the operating spindle being carried above the front axle and universally jointed to the cam spindle in line with the steering pivot. Instead of being carried on the frame, however, the inner end ad the spindle is supported by a bracket bolted to the "11111)nt axle. This has the advantage of eliminating one spherical joint ; but, on the other hand, the movement of the lever on the operating spindle when the front

springs deflect is greater than is the case with the Perrot system. The whole axle is very sturdily designed and is employed im certain models of the following makes :—Dennis, Guy, Karrier, W. and G., Garner, Burford, Bean and Rhode.

In the Clayton system the operating spindle is also carried on the axle, but is mounted below instead of above, the universal joint by which it is connected to the cam spindle being underneath the steering pivot. The inner end of the spindle is supported by a bearing formed in one piece with the front axle.

By an ingenious method, which consists of inclining the operating spindle and offsetting the universal joint with regard to the steering pivot, a differential action is obtained which relieves the outer brake when cornering. The levers on the operating spindles are coupled by short links to a long rod carried in bearings on the front axle, and a single lever on this rod is in turn coupled to the pedal gear. Amongst other makers using this brake may be mentioned the Vulcan and the A.E.C. concerns.

Turning to the many individual designs used by various makers, the most simple conception is, perhaps, the cable-operated type, of which a notable example is found on the Lancia Pentaiota chassis. In this case a cable is taken round a pulley mounted on the frame below the radiator and thence passes round a second pulley mounted above the steering pivot ; this cable terminates at the end of a lever, secured to the cam spindle. When the front wheel is locked over for steering purposes the cable simply' wraps round the pulley mounted above the steering pivot, so that the brakes are unaffected.

Some -Unusual Systems.

Of a somewhat different conception is the cableoperated brake used on S.P.A. chassis. The cable is taken to the top of the steering head through a flexible coiled casing, similar in principle to a Bowden control, and then passes down through the hollow steering pivot to the end of the cam, spindle lever.

A very simple cable-operated system is also used on the little 7 h.p. Austin van chassis, there being in this case a lever on the cam spindle the free end of which comes below the steering head and is ball-jointed direct to a cable running rearwards.

In the case of the 12 h.p. and 20 h.p. Austin chassis a different arrangement is adopted, notable for the fact that wedges are used to expand the shoes. A lever carried on a spindle fitted to the back plate of the brake is used to operate a rod which runs through the hollow steering pivot and operates the wedge expander placed at the bottom.

Another interesting design is represented by the Star commercial chassis, in which a special type of camcarrying trunnion blocks is employed, so that, when steering movements of the front wheels occur, the cam blocks turn on their pivots. This cam is fitted to an operating spindle carried beneath the back axle and supported by bearings secured thereto. A somewhat similar plan is adopted by Latil and Berliet, but instead of using a cam with pivoted faces the operating spindle is prOvided with a universal joint placed in line with and below the steering pivot.

In the case of the Auto-traction lorry a very unusual mechanism is employed, which operates as follows:— Mounted above and parallel to each end of the front axle is a horizontal lever, the pivot of Which is placed above the steering pivot; the inner end of this lever is jointed to an operating rod running rearwards, while the outer end passes through a slot in the back plate

and terminates in a ball which operates the front brake shoe. This front shoe is hinged to the rear brake shoe, the latter being applied by the former.

Another unusual design is that employed on the Saurer chassis, in which there is an operating spindle mounted just behind each end of the front axle. A lever on the spindle is connected by a short ball-ended inclined rod to a lever on the cam spindle, fitted just above the steering pivot, and the location of the parts is such that, when steering movements occur, the inclined shaft describes a cone-shaped path and leaves the brakes unaffected.

On the Railless trolley-bus chassis there are two bell cranks fitted near to the spring pads of the front axle, and from each a pull-rod extends upwards to another bell crank beneath the steering head. From the latter a rod is taken through the hollow steering pivot to a ball joint at the end of an arm mounted on the cam spindle.

In all the brakes so far described internal-expanding shoes of orthodox type are employed, but on two wellknown chassis contracting bands are used. One is the G.M.C. 30-cwt. model, in which the bands are operated by a lever and link connected to a spindle which is jointed to the back _plate at one end and to the frame at the other end. The other chassis is the Chrysler, on which the Lockheed hydraulic system is used to operate the bands.

It should be remembered that the adoption of frontwheel brakes increases the stresses on the front axle, there being a torque due to braking action which tends to twist the axle ends. The axle ends o have to carry a bending load, due to the weight of the vehicle, and opinion appears to be divided at the moment as to the best section to employ.

The 1-sect-ion is admirably adapted to resist bending, but must be made somewhat thick and heavy if it is to withstand torque successfully. On the other hand, a solid oval section has great strength against twisting, but is not so strong in resistance to bending. To quote examples, in the Clayton axle the I-section merges into a Solid oval section near to the steering heads, whereas in the Kirkstall axle provision for twisting is made by employing a particularly thick' web, the I-section extending to the head. In the Lancia axle an I-section is also used, but is stiffened by ribs extending from flange to flange and placed at close intervals.•As regards the layout of the operating gear, the

rods coupled to the front axle mnst, of course, allow for the movements of this part which occur when the springs are deflected. It would seem that satisfactory results can be obtained by carrying the operating rods rearwards and jointing them to levers on a cross-shaft suitably positioned with regard to the front springs.

When front-wheel brakes are employed there is an unavoidable increase in the friction of the operating gear which tends to increase the egort required at the pedal; consequently, it is not surprising to notice that servo systems of brake operation are gaining in favour. One of the first makers to use a frictional servo motor for commercial vehicles was Renault, and this system 'has been described on several occasions in The Commercial Motor.

Briefly, the servo motor resembles a clutch, several plates of which are continually driven off the main shaft of the gearbox. Depressing the pedal brings stationary plates into contact with those which are rotating, and there is a tendency to drag the former round in the direction of rotation; this tendency is utilized to apply. the four-wheel brakes. The servo motor is coupled to the operating gear by a chain ingeniously arranged in such a way that an equally effective action is obtained when the vehicle runs backwards.

The latest Dennis low-load-line chassis also employs a frictional servo motor, the design in this case being somewhat similar to that used by Rolls-Royce, Ltd., for private cars. The novelty of this system lies in the fact that the first depression of the pedal brings the rear brake shoes into contact with the drums, and until this occurs the servo motor is ineffective. Further movement of the pedal operates cams which bring the servo motor into action, with the result that the pressure on the rear brakes is enhanced and, simultaneously, the front brakes are applied.

The Maudslay low-load-line chassis embodies an ingenious method of servo application, the pedal in this case being coupled to a transmission brake; this brake is permitted to rock or oscillate between limits and, when it is applied, it tends to drag round in the direction of rotation of the propeller shaft, this tendency being utilized through suitable rods and levers to operate the Perrot-type front-wheel brakes. An ingenious servo system which has recently made considerable headway on. private cars and which is fitted to the Vulcan " safety " coach and to the sixcylinder Guy passenger chassis is known as the Dewandre and is handled by Clayton Wagons, Ltd. The principle employed is to utilize engine suction, and, as every driver removes his foot from the accelerator pedal in order to apply the brake pedal, the maximum engine suction is always available for braking.

The sectional drawing of the mechanism which we reproduce was made from the model exhibited at Olympia and is almost self-explanatory ; it will be seen that there is a large suction cylinder which can be put into communication with the inlet pipe through the medium of a valve. This valve is controlled by the pedal, and a very ingenious loose joint is employed, so that the piston in the suction cylinder follows faithfully the movement of the pedal. Furthermore, the driver can always feel the extent to which he is applying the brakes. Should the vacuum arrangement fail for any reason, depressing the pedal applies the brakes direct, although in this case, of course, additional pressure is required.

Under normal braking conditions the Dewandre vacuum system gives four times more pull on the brakes for a given pedal pressure than could be obtained mechanically in other words, for the same braking effect the pedal pressure necessary is reduced to onequarter of its normal value.


Locations: Austin

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