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DRUM BRAKES WILL IVE

2nd September 1955
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Page 56, 2nd September 1955 — DRUM BRAKES WILL IVE
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Which of the following most accurately describes the problem?

DESPITE prophecies to the contrary, drum brakes on Commercial vehicles are unlikely to be displaced by disc brakes within the predictable future. That is the conclusion reached after a prolonged survey which has embraced operators and drivers of varied types of vehicle and the manufacturers of Girling, Lockheed, Clayton Dewandre, Bendix-Westinghouse and Ferodo equipment.

The object of my inquiries was to find out from operators their criticisms of current braking systems and what improvements they would like to see. The manufacturers were then invited to comment and give pointers to future developments. From operators 1 was able to gather that, exceptional conditions apart, most transport managers were generally content with the existing brakes, but would prefer a greater degree of standardization of units, improved trailer and semitrailer brakes and freedom from fade.

Fade has always been held against drum bt.akes when comparing them with disc brakes, but future developments are likely to rule out this objection and at the same time provide more simple systems. The ideal brakes for a heavy vehicle should, in the opinion of several manufacturers, consist of 15/-in.-diameter drum brakes with two-leading-shoe actuation at all wheels, preferably in both directions; exhaust brakes with hand operation; and transmission hand brakes.

By using a 15i-in. drum a satisfactory cooling air flow can be maintained, even within 20-in.-diameter twin rear wheels, and the lower braking temperatures produced will not only reduce fading tendencies, but also cut down wear. These agreeable conditions can be further enhanced by the use of an exhaust brake which, when intelligently applied, can reduce the work of the friction brakes to such an extent as to increase facing life by many thousands of miles and leave the brakes cool enough on long . down gradients to cope with emergency conditions.

The use of a transmission hand brake, for long practised overseas, is not viewed with favour by many operators in this country, but its advantages are threefold. It gives a powerful brake capable of holding any heavy vehicle on extremely steep gradients; it provides a reliable emergency brake which, as tests have shown, can produce repeated retardation rates of up to 0.5 g without damaging the transmission; and, by removing the need for mechanical linkages to the rear brakes, it can simplify the design of these units, thereby helping standardization and reducing maintenance problems.

Other developments likely to be seen within the next few years include the greater use of air pressure systems on medium-capacity vehicles,

which should please many of the operators to whom spoke. These systems will not necessarily be full air pressure, but are more likely to be air-pressurehydraulic, the air working either directly on to the hydraulic master cylinder or, on lighter vehicles, through a small servo motor. Air-pressure-hydraulic systems have already beet proved to be powerful, responsive, light and reliable, and are simple enough to find favour with fleet operators who may not consider vacuum servos to be sufficiently powerful.

The future objects of brake manufacturers are to increase the reliability of brakes generally, thereby promoting safety; to reduce the maintenance necessary: to combat fade; and to diminish physical effort without necessarily increasing deceleration rates to the danger of goods or passengers. These four characteristics are likely to be achieved without recourse to completely-new systems, although work on new units, such as disc brakes, will continue to be done so long as there are possible advantages to be gained.

For simplicity's sake I have taken the different sections of braking separately, starting with the three main systems in use in this country. The second part (to be published later), will contain comments on trailer brakes, auxiliary brakes and facing materials.

UNASSISTED FOR light commercial vehicles up HYDRAULIC to 5-1, tons gross weight, the

unassisted hydraulic system has become universally adopted in this country since its introduction 27 years ago. With increasing standards of retardation and the growing demand for light pedal pressures, its use without power assistance should be limited to 3-ton-payload vehicles, however, and there are few vehicles above this capacity which now have an unboosted hydraulic system.

Even on 3-tanners, a servo has become advisable, and it is unfortunate that there are one or two small oil engines used in this class of vehicle which cannot be adapted to take an exhauster or a compressor. Lack of space adjacent to the engine ancillary drives often prohibits this, and the two obvious solutions are either to drive the exhauster from the transmission or to use a vacuum generation by-pass valve (as practised by Albion Motors, Ltd.) in the induction manifold, The latter method cannot be used if the fuel-injection pump has a vacuum governor.

Apart from the great mechanical advantage to be derived from hydraulics. which enables small cylinders to be used, one of the properties of a hydraulic system which is particularly appreciated is its self-compensation effect. Self-compensation is often apparent, however, only when all the wheels have their brakes equally adjusted, and uneven skidding and tyre wear can still occur if this is not the case. I heard some unfavourable comments about the different methods of brake adjustment, and whilst it was acknowledged that an enclosed type of adjustment scored on the grounds of not being affected by dirt, it was thought that such units were not always easily accessible, especially on large brakes.

Doubts on Automatic Adjusters Many operators were in favour of automatic adjusters, but were not satisfied with their practical application. They believed that as much time was required to check the adjusters as was normally needed to adjust the brakes. Automatic adjusters in use in large fleets have been shown to function perfectly well, provided that the correct initial clearance between the shoes of the drums has been observed and only colloidal graphite lubricant is used.

A few operators have been troubled by leaking cups, which made the facings useless before they were worn out, but it was generally agreed that cup life had improved greatly during the past few years. Certain cups are now being reinforced against premature failure by the inclusion of fabric inserts into the heels.

It has not been unknown for the expander units of rear brake assemblies to become rusty or even seize up because water gets into the actuating units along the hand-brake rods. The cure here seems to be in ensuring that the protecting rubber boots are in good condition and securely clipped at each end, and that the expanders are periodically greased.

The question of standardization raised much comment, and understandably so. Operators of mixed fleets of similar-capacity vehicles begrudged allocating a great deal of space to innumerable types and sizes of brake spare which all had the same effect but were not interchangeable. The ideal would be for brake manufacturers each to make a range of brakes and for each size of brake to be recommended for a certain class of vehicle with no pandering to the designer who wants his own different size.

The new Standards department . of the Society of Motor Manufacturers and Traders might assist in this respect, certainly in relation to standardization of backplate drillings, brake shoes, operating cylinders and cups.

Apart from any move in the direction of standardization, it is unlikely that any revolutionary change will be made to straight hydraulic braking systems. Steady improvements will continue to ensure greater reliability, and with the advent of transmission hand brakes; it is not unlikely that identical units on front and rear wheels will be used, certainly on light vehicles.

0F the three types of servo

system applicable to hydraulic braking—vacuum, air pressure and power hydraulic—the vacuum booster has so far. been the most popular. Not so Tong ago, when all heavy n18

ASSISTED HYDRAULIC

vehicles had mechanical brakes, vacuum servos were invariably incorporated to provide satisfactory retardation without excessive effort, but nowadays the unofficially recognized weight limit for a vacuumhydraulic system is 11-121tons, depending upon the efficiency of the brake units themselves.

The principal manufacturers of vacuum servos, Clayton Dewandre Co., Ltd., are shortly to produce a new servo for 14-ton-gross vehicles, which, by careful port design, will have a reduced time lag, this being the limiting factor in the design of vacuum servos. The larger the power cylinder, the longer it takes to exhaust it, hence a greater delay.

There were no serial's criticisms of vacuum systems during my survey, it being sensibly appreciated that if a vehicle is used outside the lirnits for which it was desiened, the brakes Will naturally appear underpowered. A fewoperators would like to see a greater reserve of power When crawling through heavy traffic without using the engine much.

Large reservoirswere suggested, but these would not effect a cure and the only satisfactory solution would be to fit larger exhausters. This could be done only to oil engines and there is no answer to the problem when it occurs on petrol-engined vehicles. Incorrectly adjusted brakes are a frequent cause of rapid loss of power, the excessive movement of

the power cylinder rigidly exhausting the reservoir.

The . Bendix Hydrovac servo, which offers several advantages, is rapidly gaining popularity. Being selfcontained, it does not have to be lined up with the brake pedal and' can be mounted anywhere on the chassis. Time lag is reduced because the servo works on the suspended-vacuum principle. In certain cases, it can be used without a reservoir.

Vacuum Servos Popular Operators like vacuum servos because maintenance is generally small and the lower working pressures offer a safety margin. For those who require a heavier servo, air pressure provides a satisfactory force and can be used on any heavy vehicle. Air servos have the advan.tage of rapid response to pedal movement, can exert a much greater braking effort than a vacuum servo, and can be used in conjunction with any hydraulic system.

There are three types of air servo—piston, diaphragm and Bendix Air-Pac, which is similar to the Hydrovac and is widely used on the Continent and in the U.S.A. The Clayton Dewandre concentric air servo has been adopted, in conjunction with Girling hydraulic braking, on all heavy-duty E.R.F. vehicles, thereby dispensing with the need for a more expensive full air-pressure system and at the same time giving the driver some measure of control should the power fail. The air can also be used for trailer-brake operation.

• Hydraulic servos, such as the Lockheed Power Valve system and ,continuous-flow servos, have Made steady progress. A similar type of braking has been adopted extensively by the Midland " Red " and by the London Transport Executive for their new Routemaster buses. The Routemaster has a duplicate system. Hydraulic power brakes are quick, light and inexpensive, and no more complicated than other systems. The components are self-lubricated and the braking power available can be tapped for steering assistance and, on public service vehicles, door operation. Spring-loaded or spherical bladder reservoirs ensure an adequate reserve of power and the small pump required does not absorb much power from the engine.

Future developments in the field of servos are likely to lead to lighter and quicker-acting vacuum servos— possibly in larger sizes for heavy vehicles--and thz increased use of air-pressure and hydraulic servos. Already the introduction of more powerful servos has made possible the employment of high-centre-lift brake

assemblies without increasing the pedal effort. These allow a.greater shoe movement to compensate for drum expansion on severe downward gradients.

FULLY powered systems can be distinguished from power , assisted systems, in that, should the power fail, there can be no direct manual actuation of the brakes other than through the hand-brake lever. Nevertheless, air-pressure systems—which form the majority of this group—are being employed almost exclusively on current heavy vehicle's, which shows that the disadvantage is not worth considering.

Most operators of heavy vehiCles are entirely satisfied with air-braking despite its high initial cost and their sole criticisms are confinedto the weight of the gear. The system has showed itself over the years to be reliable and responsive, and to be a particular advantage overseas, where operating speeds are much higher and brakes are called upon to do much harder work than in Britain.

There are several different systems in use, hut they can be roughly classified as the wheel-cylinder type and diaphragm brake-chamber pattern. The diaphragm units have only relatively recently been adopted in this country (although they have been used on the ('ontinentand in the U.S.A. for tnatly years), and the model manufactured under licence by Clayton Dcwandre is

the Bendix-Westinghouse system, the operating units ot which are interchangeable with the American Bendix components.

The type that might oust the piston unit is the airpressure hydraulic system. This is similar to an airpressure servo hydraulic system and uses a standard hydraulic master-cylinder and wheel units. The brake pedal takes effect upon the master cylinder through the medium of an air-hydraulic actuator, which operates at a line pressure of 100 p.s.i., the power being transmitted by a flexible diaphragm. A vehicle equipped with airhydraulic brakes has all the advantages of power associated with full air braking, but the time lag is less.

The simplicity of the layout has much to commend it. Consisting basically of a brake valve and valve assembly, a small compressor, -reservoir and actuator, it can be mounted on the chassis independent of the pedal position (the nearer the actuator is to the pedal the less will be the time lag), and can, of course, be readily incorporated into an existing hydraulic system.

Full air-pressure systems operate the brake shoes

through cams instead of hydraulic cylinders, and the brake assemblies are much heavier. On large units the cams are often carried in needle-roller bearings to increase operating efficiency. The S cam, as used on Girling heavy-duty brakes, gives a shoe-centre-lift of 0.17 in., which allows ample margin for drum expansion on long downward gradients in addition to speeding up shoe reaction.—J.F.M.,


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