COACH ACCIDENTS AND BRAKES.
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Why Brakes Fail and Pointers on Means by Which They Can Be Made More Efficient and Reliable. by "Engineer-Designer."
METE two leading articles on the I_ above subject which appeared in The Contmercia2 Motor of August 9th should draw the attention of owners of passenger-carrying vehicles to the importance of frequently inspecting their brakes. Apart from the owners of such vehicles, however, it would he to the advantage of the general public if the makers of such vehicles were to give more attention to the details of the design of brakes and try to make some improvements which are rather badly needed. It would also be as well if coroners who have to conduct inquests on the victims of fatal accidents, not being motor experts, were made clearly to understand that they are not doing their duty to the public by accepting, without question, the hackneyed statement, "the brakes failed."
To those who understand the matter from an engineering point of view, and from an experience of maintaining vehicles, this statement is one that should never be allowed to pass unchallenged, as the probability of two brakes, which were in proper condition when the journey was started, both failing at the same time is so remote that one may safely say that such an event has seldom, if ever, happened. What really does happen is that one of the two brakes is in a weak condition, or almost useless, and that the driver relies on his one really good brake, and that when this fails the weak one proves almost useless alone,
The writer has been present at many inquests in different parts of the country, and without exception the impression made on him in each case was that only one brake was in proper condition, and that had the coroner been an expert, or had an expert to advise him, he would have attached blame to the person who had the maintenance of the vehicle entrusted to him.
The articles referred to above mention the use of cast-iron for drums or shoes. For drums with internal shoes this material would be useless except when reinforced as stated, but• for drums that are acted upon by external shoes it has certain advantages, whilst for internal shoes acting against steel drums east-iron has much to recommend it, as, unlike some friction stir -026 facings, it cannot suddenly fail providing the means by which the shoes are caused to press against the drum is such that it cannot come to a definite end, as is the case when a pedal comes right down on the floorboards, or the hand lever reaches the end of its travel in its segment.
With the present construction of brakes, when the driver has applied both brake levers until he has reached the end of their movement, and then feels that there is not sufficient retarding force to stop the vehicle, there is little more that he can do, unless it is possible to change to a lower gear, so the vehicle may take charge.
Now, this is what has happened in every case that the writer has personally investigated, excepting one, which happened in the very early days of-the industry, when a mechanical failure of one part carried away the remaining brake.
In the writer's opinion the accidents he personally investigated and in which nearly a hundred persons were either killed or maimed for life could have been entirely prevented had certain departures been made from the too generally accepted practice of brake construction, and he will endeavour to point out certain departures which, in his opinion, would make brake failures an impossibility in the future, provided that reasonable care and attention be bestowed on the vehicle.
Cast-iron is a term which means little to those who have to do with its physical properties ; some grades of cast-iron will endure long and have a high coefficient of friction when working against a steel drum, whilst other grades will commence to crumble away to dust so soon as they are put into use. Even shoes cast from the same grade of iron will often give very different results, and those that are inclined to crumble may, by careful lubrication in the first few days, be coaxed into acquiring a smooth and lasting face, but, of course, an iron
which is uncertain is not suitable for the construction of brake shoes.
Chilled cast-iron will endure long, but its coefficient of friction is too low for any practical purpose. Cast-steel shoes answer very well, but are inclined to wear away the drums unduly fast, but even this can be reduced to reasonable limits by careful lubrication in the earlier days of their use.
What appears to be needed to produce a really reliable and lasting metal shoe is more research work to ascertain the exact grade of steel suitable for the drum, and the best metal for the shoes. Even without any further research it is possible with our present knowledge to construct a brake with all metal parts, and one of the writer's suggestions is that one brake on all passenger-carrying vehicles should be of the all-metal type, whilst that for general purposes might have the usual fibrous friction material.
If we accept the idea that a metal brake cannot fail through the tearing away from rivets of a separate facing, all we have to do is to ensure that there is sufficient pressure behind the shoes to force them hard enough against their drum to produce a brake which cannot fail, in the ordinary acceptance of this hackneyed term.
The Commercial Motor has been a good prophet in many cases, and the writer could point out numerous Instances where its suggestions have at first been ignored, as most improvements usually are, and have in time become generally adopted. A brake in which the driver can take a second or third pull at his hand lever is known and has been found useful on steam wagons and is almost essential where trailers are employed. The Commercial Motor has often pointed out the advantages which such a brake would have over the usual conventional one-pulland-no-more type, when applied to vehicles generally, and the writer would not be surprised to see such a brake become general in the near future.
If brakes of this description are ever adopted the writer would suggest the following points as being essential to an absolutely never-fail brake. There should be no spring-actuated ratchet pawls to stick up and fail ; there should be no reliance on springs, except to release the brake; the expander-cam should be of such a nature that it would be operative through a longer arc than at present, so that there should be practically no limitation to the application of the brake. With all these points well and carefully carried out we should hear no more of that feeble excuse, "the brakes failed."
With regard to the employment of compressed-air, hydraulic or vacuum devices, the writer is afraid that, so far, his long experience of these agents has unfitted him for the belief that any of them is suitable for a never-fail brake, but so much experimental work is now being carried out in these directions that this opinion may be modified, and there is no doubt that the driver should be assisted.