IS BRITAIN BEHIND IN iRAKING?
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IN last week's article I gave a brief summary of the three main types of braking system in use on commercial vehicles in this country—straight hydraulic, assisted hydraulic and fully powered. It was seen that these systems were generally satisfactory, provided that correct maintenance was carried out and due regard was paid to weight restrictions. Improvements would follow the lines of increased safety, reduced maintenance, greater resistance to fade and reduced effort. It was considered that there was little chance of disc brakes being universally adopted within the near future, because improvements in drum brakes would make them unnecessary.
Mention was made of the need for improved trailer brake.s, and these will be dealt with in this section, together with considerations on exhaust and transmission brakes, disc brakes and facing materials.
TRAILER BRAKES
THE most striking feature of my
investigations was the urgency with which operators regarded the need for a greater degree of standardization of trailer brakes. Articulated units of up to 24 tons gross capacity were mainly concerned: above this weight the systems themselves appeared to be satisfactory, but the problem of drum cooling caused anxiety in some quarters.
The usual method of actuation for the brakes of a semi-trailer is a mechanical linkage on the semi-trailer, operated through the turntable by a vacuumor airpressure servo on the tractor. Operators told me that the majority of these mechanical linkages was unsatisfactory. Some of them had a geometry which quickly made the linkages loose, thereby imposing excessive stresses on certain rods and relay levers and leading to premature failure of the system.
Now that hydraulic brakes have become standard on the type of tractor which operates with 15-ton-gross units, there seems to be no reason why a separate hydraulic system should not be provided on the semitrailer itself. This could be actuated through a servo, also on the trailer, with a single pipe-line connecting the two units. This type of operation has already been used in Sweden, where one trailer manufacturer employs brake units identical to the rear brakes of Volvo tractors and actuates them through a self-contained circuit.
Greater Standardization
A greater degree of standardization of braking efficiency on different semi-trailers is also needed. The advantage of interchangeable tractors and semi-trailers is nullified to a large extent if the braking characteristics of each unit vary considerably. A lead in this direction has been given by British Road Services, who operatethree types of tractor with five types of semi-trailer.. The respective manufacturers were asked to get together and a reasonable degree of standard braking efficiency has been achieved, although certain drivers to whom Ispoke disagreed with me on this point with regard to one or two types of semi-trailer.
Larger outfits use full air pressure. This is satisfactory except sometimes in respect of time lag occasioned by the length of the pipes between the reservoir and the rear trailer brakes and the relay valve necessary in the circuit for safety reasons. A no less efficient but far quicker system would be to use air-hydraulic braking on tractor and trailer. This is a development which may appear in the future. There seemed to be a certain amount of ignorance as to the many types of trailer braking system available for heavy units and the safety features which they incorporate. I heard several requests for an inverted line system as embodied in railway rolling stock, whereby a vacuum is used to hold the brakes off and the brakes are applied when the vacuum is released. Such systems, however, have been available in this country for many years, but do not offer any advantage over two-line air-pressure braking,
Should the trailer break away from the tractor, the main consideration of •applying trailer brakes is catered for by the two-line system, and it is much lighter, more simple and cheaper than an inverted line. On the Continent, units with air-pressure brakes have to be -equipped with springs which will apply the brakes .should the air preSsure fail; but this is no more effective than the two-line system as used in this country.
The principal difficulty with heavy trailers is in cooling the brakes, and it isoften-neCess.ary to take such a unit down .a hill in easy stages,' or even to winch it down a long gradient, as the brakes become so hot as to he useless. Under Such'conditiOns, disc brakes might( provide a satisfactory answer, and even water-cooled brakes are not out of the question. Crude forms of water-cooled braking have been tried with .a measure of success in Canada, but these have mainy been adapted by operators to their own-requirements.
One operator doubted 'the wisdom of running an eight-wheeled vehicle, braked on three axles only, with a four-wheeled trailer which had brakes only on the rear wheels. Admittedly, four braking axles out of six are not sufficient, but most manufacturers can supply draw-bar trailers with all wheels braked.
IT is difficult to comprehend why .1 exhaust brakes, which have proved their worth on the Continent, have not already been adopted to any great extent in this country. Most operators realize their-advantages and appear to be on the ,point of ordering trial sets. Why they have not done so before is a mystery. Whilst an exhaust brake is primarily recommended for vehicles operating in extremely hilly country, where their use will save the wheel brakes from becoming too hot for successful application in an emergency. Ihe advantages of exhaust brakes .do not end there. On buses engaged on congested town work and on mediumand heavy-duty vehicles which maycnot travel long distances, exhaust brakes will still yield a .big
reduction in facing wear. • There are two main types of actuation for exhaust brakes: the hand control,,which manufacturers advise and drivers like, but operators regard with stispic:ont and the brake-pedal switch, which ensures that the exhaust brake is applied whenever the wheel brakes are used. The foot control, whilst it has the advantage of
EXHAUST BRAKES
being automatic, is not sufficiently sensitive to the. foot of a driver to ensure that the friction brakes are not being used when the exhaust brake is in operation.
Thus the advantage of the system is cancelled if, on a long descent, the friction brakes still become hot, and glazing or fade occurs. Foot actuation has been shown to be profitable, however, on buses, not necessarily in reducing fade, but in -saving facing wear.
Hand operation has much to commend it: on a long downward gradient it can be left in position, thereby relieving`the driver of the need to keep his foot on the brake pedal, and it does ensure that the friction brakes are not being used until an emergency arises. The argument that drivers would not take _kindly to an additionat control has already been shown to be weak, asmost drivers of vehicles equipped with handcontrolled exhaust brakes are well aware of their advantages and regard the additional, iever more as a guardian angel than as" another flippirC handle."
Automatic Application The two systems already described are not the only ways of operating exhaust brakes. -They Can be 'connected to the heel of the aec.eleratdrpedal„ so-as to be automatically applied when the aceelerator' is released, or a separate foot-operated valve or 'switch can be fitted.
• Air, vacuum, hydraulic or electric operation is available, the electric system has dubious advantages, because of the presence of .a solenoid close to a .hot exhaust manifold or pipe. Electric operation, in conjunction with vacuum or .air. pressure; .is a more reasonable proposition, and allows greater 'flexibility in
placing the switch. :
lam convinced that exhaust brakes will eventually be more generally used on both. petroland oil-engined vehicles of all sizes and their adoption' Will encourage the prolonged use of drum brakes,. as Opposed to disc brakes.
TRANSMISSION 'TRANSMISSION hand BRAKES brakes are another
feature to be found in general use on the Continent but not in this country, although early British vehicles invariably had them. These early units were admittedly unreliable,which is possibly why operators are prejudiced against transmission brakes these days.
When mounted behind the gearbox, the transmission brake does throw excessive stress on the propeller shaft, but when it is located on the rear axle itself, this objection does not arise. It has already been shown that standard rear axles would withstand retardation rates in excess of 0.5g produced through a transmission brake, although 0.35g is the maximum recommended for passenger comfort.
Simplified Construction A further advantage of the transmission hand brake is that it disposes of the need for mechanical linkages to the rear-wheel brakes, thus simplifying their construedon and encouraging the. use of standard units on all wheels.
Disc or drum transmission brakes can be employed and it is possible that in normal circumstances a general preference will be shown for disc unit's. On vehicles such as buses with Underslung-worm axles, a disc transmission brake cannot be fitted, however,' because the size of the disc would not leave sufficient ground clearance, in which case a small-diameter drum brake will be essential. The mechanical advantage to be obtained by applying a brake through the reduction ratio of the rear axle should be obvious, and transmission hand brakes, therefore, provide a positive means for holding a heavy vehicle on a gradient, besides being a reliable emergency brake.
'THE objection to disc brakes so far 1 has been the rate of wear of the friction pads, and the need for overcoming this problem has been the reason why disc brakes have not yet been offered in quantity. Original units had small circular pads which did wear out quickly, but by using segmented pads, the rate of wear has been reduced to such an extent as to be less than that of some equivalent drum brakes.
As is well known, the rate of wear of friction material rises with the temperature and the good cooling available with disc brakes not only reduces the risk of fade,
DISC BRAKES
but also increases the resistance to wear of the friction material. Nevertheless, the heat generated near the actuating units is high and has caused complications, because conventional brake fluids have been found to boil under such conditions.
Two solutions are available: the first is to isolate the operating cylinder from the friction pads by the employment of linkages, and the second, which is still in the experimental stage, is to use mineral oils. This latter solution has so far met with a fair measure of success and it is not improbable that mineral oil might be employed in braking systems in the near future.
Disc brakes have proved their worth on racing cars, which travel at high speeds but do not weigh much, hut there is no reason to suppose that this success will be repeated with slow-moving heavy vehicles. Reasonably sized disc units might be feasible for coaches and lorries of up to 12 tons gross, but above that weight the size of the disc required will make the unit bulky and expensive, possibly necessitating two or even three sets of calipers per disc, or twin discs for each wheel, as used on aircraft.
TALKS with engineers at Ferodo, Ltd., have convinced me that there is scarcely any limit to the type of friction material that can be made to suit specified requirements. Progress is continually being made in respect of anti-fade and wear-reduction properties and any operator who has complaints against facings must first ensure that he is using the correct kind. There is no all-purpose quality of facing which will suit any type of vehicle.
With the advent of small drums which give improved cooling, greater pressures will be imposed upon the friction material, but tke current facings can cope with them. Similarly, the smaller drums and exhaust brakes will allow wheel brakes to run at lower temperatures, which fact in itself will reduce fade and wear.
Many people are still puzzled by the difference between woven and moulded facings and try to weigh up the merits of one as opposed to the other. This is the wrong approach, as the two are coMplementary to each other, some vehicles being more suited to woven than moulded, and vice versa. Operators are advised to stick to the type of facing already fitted and to change only on the advice of the manufacturer. Technically, -moulded facings offer more scope for development, as it is easier to alter the characteristic of a moulded facing by changing the chemical consistency.
Bonding has been receiving much attention recently and is viewed with favour by operators in this country. Whilst it is agreed that a bonded facing offers a greater frictional area to the drum and longer life because of the absence of rivets, it is not so easy to reface a bonded shoe and specialized methods of application and curing are essential.
Furthermore, to export bonded shoes is a much more bulky business than to export rivets and facings. Some operators pointed out that bonded facings reduce the risk of the drum being scored by rivets, to which I must answer that they increase the danger of the drum being
scored by the shoes. J.F. M
FRICTION FACINGS