Disc Will Replace Drum Brakes
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Ban on Transmission Brakes on Buses Should be Lifted : Effect of Centre of Gravity on Braking
S0 great are the advantages of the exhaust brake and disc brake that the present type of drum brake will eventually cease to be used on public service . vehicles. This opinion was voiced. by Mr. S. C. Vince, M.I.Mech.E., A.M.Inst.T., chief engineer, Birmingham and Midland Motor Omnibus Co., Ltd., in his excellent paper, "Brakes as Applied to Public Service Vehicles," which he read at the Blackpool Conference of the Public Transport Association on Wednesday.
Moulded facings, he said, were to be preferred to the woven type and he considered that they could be bonded to the shoes with every confidence. There was, however, a need to provide simple and cheap means for bonding.
Yesterday, Mr. A. A. Jackson, M.Inst.T., F.R.S.A., general manager, Bolton Transport Department, dealt with bus garages and bus stations. He described a number of layouts planned to secure the utmost' efficiency for operators with fleets of different sizes. He was reminded, he said, that in view -of the suggested possibilities of using a bus station as a garage, the two subjects might have a closer relationship than had been considered.
Mr. E. L. Taylor, a director of 12 B.E.T. companies and chairman of the P.T.A., presided at the conference. The discussion of the papers and portraits of many of the delegates will be published next week.
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IN the opinion of Mr. Vince it would
be a big step towards greater braking efficiency of public service vehicles if the 24-year-old ban on the transmission brake were lifted. Results with the disc brake so far obtained were sufficiently promising and the advantages it possessed over the drum brake were such that he expected it to be widely adopted.
In many cases where, after an accident, doubts were cast on the brakes of a vehicle, the cause could be traced to the road surface. All roads, he said, should have a uniform non-skid surface and he could see no reason why all road surfaces should not conform closely to specifications laid down by the Ministry of Transport.
In a public service vehicle, regard had to be paid to passenger comfort, and whilst in the case of a private car a braking efficiency of 0.7g. to 0.8g. could be accepted. a more reasonable figure for a public service vehicle would be in the region of 0.4g. to 0.5g. It had been shown that a 0.25g. stop caused discomfort to standing passengers.
Vehicle stability was affected by the distribution of braking effort between the front and rear wheels. This was governed by a number of factors including the degree of deceleration, the total weight of the vehicle and the ratio of the wheelbase to the height of the centre of gravity.
It was the centre of gravity which had a marked effect on braking, and to keep the wheels just on the point of locking, which was the ideal, was almost impossible of achievement.
Bad Features
Some of the worst features of present brake design were inadequate cooling, fading, excessive wear of facing and drum, squeal and lack of protection from leaking oil seals.
Referring to the limitations of drivers' skill, Mr. Vince said that it would be of great assistance to drivers to know that by applying a similar effort on the pedal of two dissimilar vehicles, they could obtain the same braking results. He thought it would be a good thing if there were some agreed standard whereby a given pedal pressure would produce a given degree of deceleration.
The reasons which prompted the ban on transmission brakes no longer existed. The better design of axles and propeller shafts, coupled with improved standards of maintenance and the close control exercised by Ministry of Transport examiners, were put forward as reasons why the ban should be lifted.
Mr. Vince reminded his audience that it was the late J. G. Parry Thomas who was the first man to become interested
in power-assisted brakes, and that his patent was owned by Clayton Dewandre up to the date of its expiry.
In a résumé of present-day braking systems he said that the first set of continuous-flow servo-brake equipment was fitted in 1940 to a Midland "Red" bus and was now standard on over 1,000 of that operator's vehicles.
The position regarding the hand brake was unsatisfactory, because whereas the Ministry called for a brake having a high degree of efficiency--one. in fact, that could be used as an alternative to the foot brake—vehicle makers regarded it merely as a parking brake.
The transmission brake was the only answer to this problem. Were it designed in disc form and mounted en the rear-axle casing, with the disc carried behind the rear propeller-shaft coupling, excellent results would be obtained. In America and on the Continent practically every public service vehicle was fitted with a transmission brake.
Squeal Still Elusive
Brake squeal was as elusive today as it was 10 years ago. Such palliatives as there were usually had a detrimental effect and it might be that, to ensure freedom from squeal, facings having a lower coefficient of friction would have to be used.
The merits or otherwise of woven and moulded facings revealed that the moulded type was better at elevated temperatures, stood up well on heavy duty and was less prone to temperature fade. It was, however, harder on the drums.
Woven facings, being more flexible, would stand rougher treatment in fitting. One of the present difficulties with the moulded type was in providing convenient facilities for their attachment to the shoes by the bonding process.
Great heat was developed during braking, and one application to bring a bus weighing, say, 10 tons to rest from 30 m.p.h., was sufficient to raise 5-6 pints of water from 60° F. to boiling point. In existing designs, said Mr. Vince, if it had been the purpose to insulate the drum from the air, it could not have been done better.
After discussing the various methods of testing brake efficiency on the road, he said that the stopping-distance test using either a gun or a fifth wheel, and one of the available deceleration meters, would provide all the information required by a practical engineer.
Work which was already being conducted on exhaust and disc brakes would revolutionize vehicle braking. Among the advantages of the exhaust brake were increased efficiency, less wear of facing and drum, less need for frequent brake adjustments, and increased tyre life.
Each of the many beneficial features of the exhaust brake was individually desirable, but to obtain all of them by such relatively simple means reflected great credit upon those who had had the foresight to develop the scheme. The speaker mentioned Mr. Slee, chief engineer of the Rhondda Transport Co., Ltd., in this regard.
Mr. Vince was referring to the automatic system in which the full effects of this type of brake were obtained regardless of the driver's efforts. As adopted on the Continent, the brake was employed quite independently and
was an optional method of retardation.
In the exhaust brake the valve was fitted between the exhaust manifold and the silencer, and when operated caused the build-up of back pressure on the engine side of the valve.
The disc brake, ton, offered a number of material advantages, including the elimination of thermal expansion and much reduced fading. It also lent itself to automatic adjustment, did not squeal under any condition and, above all, was simple to maintain. The " feel " of the disc brake on the road had to be experienced to be believed, said Mr. Vince.
The existing type of expanding shoe and drum brake had reached the limit of its usefulness, hence his expectation of the wide adoption, in the future, of the disc brake.