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RECENT CHASSIS REFINEMENTS.

1st December 1925
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Page 25, 1st December 1925 — RECENT CHASSIS REFINEMENTS.
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A Study of •'some of the Detail Improvements to be Observed in Present-day Chassis, Making for Longer Life and Economy.

IN the article which • was published in the issue of The Commercial Motor for November 17th last a careful survey was made of those changes In design In chassis, components and fittings which gave useful clues to the progress which is being made in every direction. The purpose of the present article, from another pen, is materially different, as it is desired in the notes that follow to consider and to analyse such improvements or alterations in certain mechanical details as will tend to a lengthening of the life of a chassis,. to render easier the work of the maintenance staff attached to a fleet, thereby assisting in keeping

the vehicles on the road and in obtaining the maximum of running in the course of a year, resulting, of course, in a reduction in operating costs.

The " Servo " and the Self-assisting Brakes.

Considering the number of patents which have recently been granted for improvements in brakes, the writer is somewhat surprised to find that what changes have recently been introduced are mostly in the nature of details rather than constituting any radical alteration. Servo, or self-assisting, brakes are to be found on several of the 1926 models, and before considering these it might be mentioned that some distinction could with advantage be made between the two classes of brake by giving them different names. The word " servo " implies a slave, or someone who does the work. The appellation " servo" is more fitting to brakes where the power for application is derived from some source other than the brake itself, as in the Westinghouse, Dewandre, Peters, etc. For those brakes where a wind-on action, Similar to that of a capstan or warping drum, is employed to augment the power of the driver, the term " self-assisting " would be more descriptive. Of the former type several examples occur in 1926

productions. The luxury coach of the Guy Co., which is said to be capable of 65 miles an hour, is fitted with a Dewandre vacuum brake, and rightly so, for if ever this speed is attained with a vehicle of this size and weight, I should imagine that something more than the driver's muscular effort will be necessary in a tight place. The Peters brake was recently shown at Olympia, but not fitted to a vehicle. In this arrangement compressed air is employed, the Compression being obtained from a small pump which is mechanically driven, and the air is stored in a chamber or reservoir. Diaphragms are employed for the air to act upon, and the brake is applied by means of a valve operated by the driver.

The Maudslay Self-assisting Brake.

Of the self-assisting type there are again several examples among the new chassis, the Maudslay and the S.P.A. being specially worthy of notice. Both of these examples are really self-assisting, as the supplementary power is obtained from the application of a brake which is, in itself, exerting a retarding effect. The illustration of the Maudslay is, for the purpose of clearness, shown without any other parts than those actually appertaining to the brake. The view is taken looking at the rear end of the gearbox, the casting shown being mounted on a nose formed on the gearbox. This casting carries the fulcrum pins on which the usual form of brake shoes swing ; it also carries the expander cam and lever for operating same. A pair of ordinary shoes are mounted on this member, and an ordinary brake drum on the shaft from the gearbox completes the brake so far.

The casting, with all its appurtenances, is free to swing on the nose of the gearbox, so it will be seen

that, when the brake shoes are expanded inside the drum, they will be drawn with the drum, no matter In which direction it may be revolving.

Two connecting rods will be seen extending downwards from the ball Joints on the casting to levers 'mounted on sleeves on the cross-shaft which operates the front brakes. It must. be borne in mind that these levers are connected to sleeves only, and not directly to the shaft itself. In the centre of the shaft will be seen a muff, which is provided with a set of dogs at each end. This muff is keyed to the shaft, whilst the sleeves are free. The dogs are so arranged that there is considerable play between the teeth.

The working of this arrangement is as follows:— When the brake is applied a certain amount of retarding effect is produced on the rear wheels through the propeller shaft ; the reaction of this causes the casting

to try to revolve with its brake drum. This tendency is transmitted to the levers, according to the direction of revolution, and causes one of them to carry with it the muff and the shaft on which the latter is keyed, and thereby to operate the front brakes. It will be seen that, as one of the connecting rods is pressed d o w nwards, the other is raised, so, no matter in which direction the casting swings, it sets up a rotary motion in the muff and its shaft in one direction only. This effect is obtained by means of the play allowed in the ' Fig. 4.—The conveniently placed adjusting hand-wheels on the brake-operating gear on Sauter lorries.

dogs. A compensating device is employed on the cross-shaft, but to make matters clear it is not shown.

It will thus be seen that in the case of the Maudslay system the driver's foot pressure applies a useful brake on the transmission system of the vehicle, and this, in turn (but after the brake has begun to operate on the rear wheels), applies a powerful braking effort on the drums on the front wheels—a system which appears to be quite sound.

A Self-assisting Floating . Brake.

The self-assisting brake on the S.P.A. chassis is a very interesting example of that class in which the fulcrum is of the floating type ; that is to say, the fulcrum pin on which the brake shoes swing is not fixed, but can be diverted from a central position, by the inclination of the brake shoes, to follow the revolving drum when expanded.

An abutment is provided against which the ends of the brake shoes are drawn by means of springs. Hinged to the shoes are two levers, which have a projection in their centres against which the expander cam bears when operating. The effect of the rotation of the cam is to force apart the two levers, by so doing separating the ends of the brake shoes and• causing them to bear on the drum, which sets up a self-assisting action, no matter in which direction the drum may be revolving. The fact that the fulcrum is allowed to move sideways enables the shoes to follow the direction of rotation, and consequently the only thing that can arrest its progress Is the abutment shown, the levers being only bound together by means of the link. This appears to be a very simple and well-thought-out example of this class of brake.

Among recent detail improvements which appeal to the practical man who has to keep vehicles on the road are the following, which, although they may seem small matters, all tend to general efficiency :—

The adjustment of the brakes of the Saurer, as shown in the illustration, can both be got at without the driver leaving the cab, as by merely lifting a flap in the floorboards he can obtain any degree of tension by turning the hand wheel. The makers of the Saurer chassis have introduced an excellent brake gear for operating the brakes on trailers. It is well known that there is considerable difficulty in preventing the cable used for operating the trailer brake from snapping in certain circumstances, such as when turning shav corners or when the pull-bar suddenly comes into operation whilst the brake is on. In the arrangement illustrated a drum is operated by means of a hand wheel, and winds in a cable from the trailer. The special feature of the device is the provision of a friction disc under the hand wheel which will allow the cable to unwind should the pull become more than 1,000 lb. A knob on the handle of the hand wheel, if depressed, will release the friction disc and allow the wheel to be revolved freely.

Some of the details of the Dennis brake mechanism show an attention to small points which is worthy of notice. The cross-shafts working the various parts of the brakes are all fitted with spherical bearings, so as to prevent any stiffness in the bearings due to misalignment or distortion of the frame and consequent binding of the bearings. It is well known now that the muscular effort of a driver is none too great when applied to four-wheel brakes; consequently, any Wort to reduce friction in the working parts will be welcomed by drivers. The prevention of rattle in brake joints is a point that has not always received the attention it merits. The arrangement of the Dennis joint is a departure from ordinary practice, as the Thackray washer surrounds the bush with which all joints are fitted. The securing of fabric linings to brake shoes is a

matter which does not seem to have received Much attention; consequently, the riveting process is almost universal. That some more scientific method is wanted few will deny. The T-theaded bolt and recess, as used on the Karrier model, has proved itself to be a very satisfactory method.

Generally speaking, there seems to be a tendency towards the use of " rag " or flexible disc joints, even on some of the heavier models. In cases where the gymbal or other types of mechanical joints are used, there is a tendency towards more •

efficient means of preventing the escape of oil and the ingress of grit. The arrangement on the Sourer, in which a complete shield is formed to protect this part, seemed to be one of the best means shown for the protection of universal joints. In those universal joints in which 3 — The only means whereby sliding motion can take place is that provided by the splines of the shaft sliding within one of the members of the coupling, it is often found that much damage is done through lack of lubrication ; however, some means of introducing lubricant has been arranged in several of the new models, and, in addition, the lubricant is usually retained by a neat leather gaiter which often carries, the greaser.

Fig. 10.—The li-section rolled steel channel used for Maudslay frame members.

Flat Springs Not Popular.

Continental makers adopt the usual wide and flat springs with but few leaves, whilst British makers pin their faith to more leaves but thinner ones. Flat springs have been tried and found wanting for vehicles that have to carry heavy loads, as, if nearly flat when unladen, they will assume a camber in the wrong direction when a heavy load is on them.

This may not be a real disadvantage, but it gives the Impression that they are not strong enough for their work. Many firms have recently adopted the new method of holding the springs down by bolts or clips at an angle as shown in one of the accompanying sketches. In some instances an effort is being made to provide some means of mounting the spring ends other than by the usual shackle. An instance of this is to be seen-on the S.P.A. chassis illustrated in Fig. 9. It

is, however, doubtful whether the shackle will not hold its own, provided that more convenient and efficient means of lubrication are provided.

The Use of H-section Steel for Frame Sides.

The ordinary pressed-steel U or channel frame is employed on the majority of models, but one notable departure occurs in the case of the Maudslay coach and bus chassis, in which an H-section, 6 ins, high by 3 ins, wide, of rolled steel is employed. The writer has found that rolled sections with slightly stouter flanges at the top and bottom, and a slightly thinner vertical web with radiused corners on the inside and square on the outside, are less likely to fracture than those sections which are formed of pressed plate of equal thickness all over. The Maudslay frame member at the forward end has a V-piece cut out of it, and is then closed together and welded, forming a tapered dumbiron and greatly adding to appearances.

There is still a belief that brakes should be compensated so that, when power is applied, it is evenly divided

between the two wheels forming a pair. It is true that a scale-beam arrangement will do this, but it is no guarantee that the retarding effect will be even, as one drum may be oily whilst the other may be rusty. A. compensating arrangement is useful when setting brakes, as then it balances the pressure; but when once that balance has been obtained, it is better that , the balancing beam, or its equivalent, should be locked, or, at least, have some strong friction applied to it so as to ensure both brakes releasing together. This device has been applied to the Bentley private cars with marked success. Is there any reason why it should not become general on commercial motors? The need for something of the kind when only two brake shoes are employed is obvious, but becomes even more needed when four brakes are applied by one lever.

In spite of the objections that were raised in the past to allowing the rear springs to resist the torque of the drive and the reaction of the brakes, and at the same time act as push rods to drive the vehicle forward and as pull rods when backing, this arrangement is to be seen on so many present-day vehicles that it is now evident that such objections were without much in the way of real foundation. The writer has always favoured this construction from its introduction. It is, however, evident that the belief in the good points of this construction is not universal, as in the McCord chassis we see an arrangement as illustrated in Fig. 11. where radius rods are employed to take the push and pull, and where a link is provided to relieve the springs from the strains of the torque and brake reaction.

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