A NEW VACUUM BRAKE FOR ROAD VEHICLES.
Page 16
Page 17
If you've noticed an error in this article please click here to report it so we can fix it.
A Description of the Sentinel Vacuum Brake which is now Standard Equipment on the Super-Sentinel Six-wheeler.
Vi A NY CLOSE observer of the trend
of modern commercial vehicle development will already have noted the tendency for increased efficiency in the braking arrangements fitted. This movement is, of course, only the natural corollary to the great increase in the numbers of 'heavy vehicles on the road and the
greater weight now attained by individual units. With the advent of the sixwheeled vehicle, so strongly advocated by The Commercial Motor, and initially taking the form of the tractor a,na trailer, the problem of supplying adequate braking arrangements has assumed new proportions. For one thing, in order. to provide the driver of one of these multi-axled machines with proper control of his Vehicle, it is essential that the brakes be arranged to act on both the rear pairs of wheels. Unless this is done, there is great danger of skidding, even on dry roads. Since most sixwheelers are articulated between the rear pairs of wheels, this last consideration involves the use of brake-operating mechanism so designed that. it will not interfere with, or b interfered with by, the relative movement of the two rear .axles when the vehicle is turning. Over and above this point there is it limit to the amount of power that an average man can exert when applying brake effort, and this limit had already been very nearly reached before the advent, of the Aix-wheeler. Unless the application of the brakes is to be made unduly slow—a very undesirable thing, for obvious reasons—some sort of servomechanism seems imperative.
.Bearing all this in mind, the makers of the Super-Sentinel Six-wheeler decided to fit the vehicle with a pow'eroperated brake, and, in looking round for the most suitable system, selected the vacuum principle as facing the best suited 020 to their purpose. The vacuum brake is particularly well adapted for application to vehicles . driven by steam. The vacuum-producing mechanism is simple and has few moving parts. The brake is reliable and can be made as powerful in application as can be desired. The connection between the brake cylinders and' the vacuum maker and control mechanism can be made with 'flexible armoured hose. thutt fulfilling one of the requisites of its application to the articu lated six-wheeled vehicle. In designing the Super-Sentinel vacuum brake system the ideal aimed at was to evolve a vacuum brake which, whilst giving all possible progressiveness of action, would yet consist of a minimum of parts, thus ensuring the longest possible life with the smallest chance of derangement.
The brake consists essentially of two parts—a brake, cylinder and a combined pedal-operated contra] valve and steam ejector. A photograph of the brake cylinder is shown in Fig., I, and it is here seen fitted to the mar of the tractor portion of the six-vtheeler, the trailer portion having' been removed in order to obtain the photograph.
A sectional drawing . of the brake cylinder is shown in Fig. 2. It consists of two cup-shaped castingS (A), which . clamp between th.eir.',Oclges dermatimi diaphragm (B)..' Co-axial with the containing 'castings is a -piston (C), which also clamps between itstwo portions the centre part of the dermatine diaphragm. This piston is mounted on a piston rod (D), which has an extension (E) working in a guide in one of the containing castings.
The outer end of the piston rod is proaided with a fork (CO and axis pin (H) for connection to the ordinary brake mechanism. of the vehicle. The spring (J) keeps the piston (C) at one end, of the case, and thus keeps the brake "off," unless it is required to be operated. The union. (IC) screwed into a boss in the casing serves to accommodate the pipe which connects the brake cylinder to the brake-actuating mechanism. On the top of the casing is an adjusting screw (L) fitted with a locknut, and this adjusting screw controls the amount of air which can enter the brake cylinder, and thus allows the
speed of application of the brake to be adjusted. The purpose of this will be see.' later.
A photograph of the brake-operating mechanism is shown in Fig. 3, and a sectional drawing of this detail is shown in Fig, 4. The pedal (A) operates a spindle (B). The top end of this spindle is adapted to open the air valve (0) when the spindle rises, and thus allows air to enter the pipe lines through the openings (P P). On the side of the spindle (B) is cut a rim (E), and on this cam rides the stem
of a steam valve (F), which is kept steamtight by having a series of grooves turned on it, as shown in the illustra.. tion. This steam valve controls the admission of steam to the ejector (G), which is of the Holden and Brook pattern.
The action of the device is as follows : When the brake is "off," the pedal (A, Fig. 4) is up and the spindle (G) is in its highest position, The valve (C) is thus held open, and there is atmospheric pressure in the pipe lines which Connect the union (H, Fig. 4)•to the union (K, Fig. 21. The spring (J, Fig 2) is thus enabled to push the p'ston (C, Fig. 2) to the left-hand extremity of the brake cylinder, the brake thus being quite free.
On it being desired_ to apply the brake, pedal A (Fig. 4) is depressed. This causes the spindle (B) to descend, and, in consequence, the air valve (C) closes the openings (P P). Further movement of the spindle (B) now causes the steam valve (F) to open, owing to the cam (E) riding against the stem of the latter. Steam now enters the ejector (G) through pipes (J and K, Fig. 4), and the ejector then starts to remove the air from the pipe lines between the brakeactuating mechanism and the brake cylinder, and also the right-band side of the brake cylinder. As soon as this happens, atmospheric pressure acts upon the left-hand side of the brake piston (C), and forces piston (C) and piston rod (D) towards the right, compressing the spring (J, Fig. 2). 'The brake is thus applied. The actual vacuum obtainable is in the neighbourhood of 30. ins., and, since the dianieter of the brake cylinder is nearly 9 ins., this makes available a brake pull of over 800 lb., which, of course, is far greater than could ever be exerted by unaided human agency in a like short time.
In all servo-operated brake mechanisms it is essential that the application of the brake shall not be instantaneous, otherwise there is grave danger of the vehicle getting out of control, due to the locking of the wheels. In the Sentinel mechanism the system adopted of turning on steam to the ejector when the brake is required allows of a small, but quite definite, time-lag between the operation of the pedal by the driver and the application of the brake by the mechanism, and this time-lag is sufficient to enable the driver to take note of the response of his vehicle to the brake action, and to increase or diminish the amount of braking, according to what the condition of the road and circumstances may require. hi other words, the action is almost, analogous to the short time taken in operating the handworked brake mechanism and prevents any dire results front too sudden application of the brakes.
In practice the time-lag is scarcely perceptible, although it is quite definitely there. It may be Controlled in amount by the fidjusting screw (L, Fig. 21 on
the top of the brake cyjinder. This adjusting screw also has another purpose. In fitting this brake to a sixwheeler it is essential that means he
provided whereby the braking of the rearmost wheels comes into play before the middle wheels are retarded:If this were not done, the back part of the vehicle would tend to overrun the front portion, and, if the roads were greasy, or if the vehicle were not traveling in a dead-straight line, the front portion would be slowed round by the momentum of the back portion of the machine, and would become out of control.
In the -Sentinel mechanism the cylinder which operates the brakes our the rearmost pair of wheels has this adjust. ing screw (L) set so that this brake comes on ahead of that of the middle pair of wheels. 'This adjustment need only be made when the brake mechanism is first installed, after which it operates quite definitely and quite surely without further attention.
From the foregoing description and photographs it will he seen that the Sentinel •designers have evolved an extremely simple form of servo-brake; one, moreover, which comprises but two moving parts and has nothing small, fragile or delicate to get out of order. The brake is at present fitted as a standard equipment to the company's Super-Sentinel six-wheelers. but it may be bed on request, at. a small extra cost, fitted to any vehicles manufactured by the company. It is the subject of world-wide patents.
As visualized in the opening paragraphs of this article, the day is fast arriving when hani-operated brake mechanism will become inadequate, and we have no doubt that this successful solution offered in the Sentinel vacuumbrake gear will go a long way towards popularizing this most essential "Safety First" movement.