Ideal Braking System for Heavies
Page 54
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AN ideal braking system for heavy vehicles would include an exhaust obturator, two-leading-shoe brake units at the rear and two-trailing-shoe assemblies at the front. Brake shoes would be cast and have high-coefficient moulded facings.
The units would be of small diameter and wide, with heavily ribbed drums.. Automatic adjusters for facing clearance would be favoured. For operation, the air-hydraulic • system would be used, with either a variable-ratio or multi-pull lever, with gradual release, for the hand brake.
This is the specification laid down by Mr. I. M. Simpson in a paper which he read to the Engineering Society of Leyland Motors, Ltd.
Enough Heat to Melt Iron
To show the nature of the problem that had to be solved, Mr. Simpson said that in stopping a vehicle of 12 tons gross laden weight from 30 m.p.h., sufficient heat was produced to melt 2 lb. of iron.
Weight distribution during deceleration could be anything from 45 per cent, on the front axle and 55 per cent. on the rear, to 20 per cent. front and 80 per cent. rear. If front-wheel locking was to be precluded, it was advisable to limit the potential of the front brakes to below 60 per cent. Therefore, the less the proportion of weight on the front wheels, the less the work the front brakes could be allowed to undertake. It was proposed that a foot-brake efficiency of 75 per cent. for goods vehicles and 60 per cent. for passenger vehicles be aimed for, and fade effects to be eliminated as far as was possible.
The simplest, although one of the least effective, brake was that having a fixed anchor with leading-trailing shoes and no servo mechanism. A development of this type of brake was that in which two shoes were linked together at those ends normally pivoted. This meant that any movement of the leading shoe was transmitted to the trailing one at its leading edge.
The American Duo-servo
By thus tapping the servo effect of the leading shoe the trailing one became the more powerful. It was found that even more power could be produced if the shoe expander was moved so that the shoe tips registered on a fixed abutment, whereby the nullifying effect of the trailing shoe on the expander was removed. This type of brake was widely used in the United States and was called a 'duo-servo. It was the most powerful internal-expanding brake in existence and usually had to be " de-tuned" by fitting low-friction facings.
Most of the advantages and few of the drawbacks of the duo-servo brake were found in those of the two-leadingshoe type, which was twice as powerful 1320 as those having the leading-trailing arrangement.
In passing, the speaker commended the Foden concern for being pioneers of large brakes. It was probably true to say, he said, that they were the first to produce a rigid sixor eight-wheeler, providing over 1,000 sq. in. of friction area to the brakes, Two-leading-shoe brakes were, perhaps, the best compromise of efficiency, cost and wear characteristics. The only disability was inherent proneness to the effects of fade. In the latest Girling design, whilst fade had not been reduced, its effect had. When the brake heated up, the friction coefficient was reduced, as was the force opposing the wheel cylinder. This permitted a greater proportion of thrust to be imparted to the contact pressure, thereby compensating for the loss in friction. Pedal effort was high but constant, the brake being self-compensating for fade.
Brake shoes must be rigid, especially when moulded facings were used. For this reason, it was becoming increasingly common to employ malleable cast-iron shoes for heavy-duty brakes.
Great rigidity and the ability to transfer heat readily were demanded of the brake drum. Scoring and wear resistance could be fairly easily appreciated and could be met by increasing the hardness of the drum, kit therm° checking was t more obscure. The problem was metallurgical and usually occurred at temperatures above 500' C.
High Temperature Effects
At high temperature, the drum surface oxidized and the peadite of the material took on a nodular form. At 720' C. and over, it was possible for the graphite to go into solution and should the remainder of the drum stay cool, its surface layer might become quenched, thus forming martensite. This induced heavy 'internal stresses in the drum, which would almost certainly "craze" or crack.
Now that drum diameters were being reduced, it was becoming increasingly common to use the additional clearance afforded between the drum and the wheel to cast on cooling fins, which also greatly added to the rigidity of the drum.
Disc brakes, said Mr. Simpson, had enormous advantages, not the least being their non-fade characteristics, and although they had their problems, none was unsurmountabIe.
Referring to brake operation, Mr. Simpson said that the post-war period had seen the compressed-air system increase in popularity. The air brake simplified the chassis designers' problem and tappings could be made from the brake line for tyre inflation, door operation, gear operation, powered steering and so on. As the air-brake system was roughly 2itimes as costly as vacuum equipment, it would possibly be economic only if other uses were made of the air supply than for brake actuation.
A system which would probably become increasingly popular was that in which an air-pressure boost was applied to hydraulic braking. The Lockheed continuous-flow hydraulic system was analogous to the air-pressure type. The full hydraulic system was, on many accounts, superior to the use of air pressure.
Advantages of Full Hydraulic
It did not suffer from condensation troubles, it weighed less, was more compact and less costly. Failure of the servo section did not put the brakes out of action and more scope was offered in the selection of brake units, and yet it retained all the advantages of the other systems.
Hand-brake efficiency varied from 10 per cent. to 55 per cent. The most effective basic design was one in which the slack in the linkage was taken up quickly and had some variable-ratio influence. The Foden hand brake was quoted as an example, as well as the E.R.F. and the Neate.
A multi-pull hand. brake could be avoided by using a transmission brake,. and whilst rheostatic braking was employed extensively on the Continent, in this country it had been limited to trolleybuses, where its use had extended the life of brake facings to as mud] as 250,000 miles. The use of this eddycurrent form of braking was of particular value on long, down gradients, when the retarding effect was claimed to be equivalent to two downward gear changes.
On the Turbine Principle
A form of retarder favoured in America was of the hydraulic type, the action being basically a turbine in reverse. A large header tank, however, was needed and the unit was ineffective below 1,000 engine r.p.m.
Of all the auxiliary types of brake, the most promising was the exhaust obturator which consisted of a butterfly valve located in the exhaust manifold of the engine. When the valve was closed, air was compressed in the manifold, so that the forward motion of the vehicle was arrested by the compressor action of the engine.
With such a brake in use, facings had shown less than half as much wear as was normally experienced, and this had been demonstrated by tests on an A.E.C. vehicle in this country.