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IDEAL BRAKING SYSTEM?
AN interesting but necessarily incomplete paper. Some Problems in the Design of Braking Systems," was read before the Institution of Automobile Engineers last week by Mr. F. A. Stepney Acres, assistant to the chief engineer, Vauxhall Motors, Ltd. Time and space compel omission of references to servo mechanisms, air and electric brakes, automatic adjustment, brake-drum materials, heat chequering on drums, etc.
Giving a short history, the author said that about 1909 the internalexpanding brake had almost ousted the band type for wheels, both here and on the Continent, although the band brake persisted in the U.S.A. well through the nineteen twenties. It died mainly because of the impossibility of protecting it from dust, mud and water. This was regrettable, because, in many respects, the band brake was still the best form and was free from many of the troubles of the internal-expanding brake. That one objection was, however, insuperable.
Increasing Output-Input Ratio
Since the introduction of the internal type, ingenuity had been devoted mainly to attempts to increase the ratio of output to input by such devices as the Bendix duo-servo, articulated shoes, etc.. and, more recently, by two-leading shoe types. Disc brakes were under development, said Mr. Acres, but had not made much headway.
Many materials have been employed as friction facings, and about 1908 woven asbestos, incorporating copper or brass wire, began to appear, but it was much later that this type of material became really general in use.
As early as 1903. private individuals were experimenting with four-wheel brakes, and the author was in a small degree associated with such work.
Four-wheel Brakes in 1909 In 1909 Crossley and SheffieldSimplex introduced four-wheel brakes. and Argyll followed a year later. The brakes were not inter-connected as they are to-day, and Argyll strongly featured
" diagonal " braking. All these companies soon reverted to rear-wheel brakes. It was at the 1923 Motor Show that four-wheel brakes came into prominence, when about 30 per cent. of the cars exhibited were thus equipped. In 1925 the percentage had grown to 75 and a year later, to 88.
In the earliest days the brakes on the two axles were independent. one operated by hand, the other by foot. Thus, the loads were not altered. The next development was to couple them to a pedal. Then the pressure necessary was at least doubled, and this led to the introduction of various servo devices, of which the most important were the Perrot hinged shoe (front wheels), which gave a self-energizing effect, and the Dewandre vacuum servo used with 'a normal layout.
The U.S. introduced hydraulic operation, the first recorded example being the Duesenberg of 1921. The original Lockheed (then known as Loughheed) introduced was a sealed system in which the fluid was replenished under pressure. It is interesting to note that an Englishman lodged a patent application in January, 1908, covering an hydraulic system identical in principle to those in use to-day. i.e., incorporating gravity replenishment.
Few people in the private-car and goods-vehicle world now make their own brakes, but in the coach and bus group the picture is reversed, and makers do design and produce their own. It is in this sphere where the stress on the brakes is the most severe.
Curious Variations
It is notable also that when the same maker produces both goods and passenger vehicles, he nearly always uses totally different brakes and operating mechanisms on the two types. If the system of purchasing proprietary brakes were directed towards the use of a standardized product, or one of a highly specialized nature, this could be understood, but to the author it seemed that there are almost as many variations of size and type in the proprietary designs as there are varieties of vehicle.
Only in the case of the largest and heaviest vehicles are the same brakes used by more than one manufacturer. There is also no consistency in the relation between the size of brakes and that of the vehicle, as measured in square inches of facing or drum area per ton of gross laden weight. There might be swine excuse for this variation were it caused by the use of similar brakes on a range of vehicles, but this does not appear to be the case.
It has been customary to assess the capacity of a braking system in terms of pounds of gross vehicle weight per square inch of facing, or the inverse. This can be misleading. It puts a premium on long arcs of contact, whereas these are a disadvantage for several reasons.
A brake is simply a device for distributing energy by conversion into heat, and the truest method of rating is its ability to absorb the heat into it and to dissipate it at a rate adequate to conditions of operation. Some brakes, such as those on aircraft, can take their time in cooling. Conversely, it is allimportant for a colliery winding drum to dissipate heat as fast as it is generated. Some years before the war the author was experimenting and conceived the idea of making a drum full of shoes— four of about 75-80 degrees arc of contact. The purpose was to remove drum distortion and reduce loading on the facings. This brake was a failure: it overheated rapidly, with resulting early fade, and seemed as though it would never cool. The reason was that none of the really hot inner surface was exposed to any cooling influence and had to rely on the external effect.
The true yardstick by which to rate a brake facing, as distinct from the complete brake, is the amount worn off in doing a given amount of work, that amount being the area intercepted between the original and final positions of its rubbing surface multiplied by the face width. It seems that the consistent practice where the braking is heavy is to use relatively short facings.
Mechanical Strength Lacking
Generally a material with a rising co-efficient of friction is lacking in mechanical strength, sometimes associated with rapid wear.
If the surface of the normal facing be contaminated by oil or especially Lockheed fluid and subjected to high temperatures, the friction may rise alarmingly to as high as 0.8, and cause "spragging." SiAtered-metal facings experimentally used in the United States have been discontinued as being unsatisfactory. The author dislikes an ultra-light pedal, as encouraging driving on the
brakes. He is also critical of twoleading-shoe brakes, except in commercial vehicles weighing around 71 tons laden: with heavier types a servo becomes essential, when there is no reason for using the type. Mr. Acres's idea of the ideal combination is hydraulic operation at the front and mechanical operation at the rear.
Pointers from Discussion
It was claimed by Mr. W. H. Sanders that asbestos was the most suitable base for friction material and gave the best characteristics. Added cements and impregnants only coated the asbestos; they did not penetrate. The purpose was to cement the fibres, whilst the "oils" gave toughness, life and reduced wear. From certain American moulded facings he had pulled lumps of asbestos as big as peas. Synthetic materials and sintered metallic powders might come later, but the latter required very high pressures. The tyres automatically become brakes when the wheels lock. At over 400 degrees braking material breathes and deposition on the drums reduces friction. Cooling then does not raise the friction, because the products remain on the drums. Wet brakes need the water film to be broken and a hard " grindstone" material is required.
Mr. Milligan mentioned a difficulty with buses. There is little air cooling because of the low wheel arches, yet it is necessary to get rid of some 900 thermal units in four to six seconds.