POWER
Page 38
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Behind the
BRAKES
By S. H. Edge M.I.Mech.E.
Chief Engineer, Clayton Dewandre Co., Ltd.
TO operate the brakes on a heavy vehicle, it is desirable to provide power in excess of that which the driver can supply. This short article is an elementary study of the employment of such power on commercial vehicles, and an attempt to explain why it is often used.
Let it be said right away that it is quite possible to have braking systems on heavy vehicles which will retard them satisfactorily without power assistance, provided that no expense be spared in manufacture and maintenance. The question of power for braking works. out in much the same way as the question of engine power.
Engine Analogy
The engines which drive our heavier vehicles may approach 10 litres in capacity, yet the horse-power is no more than has been obtained from car engines of about a tenth of the capacity. We know very well, however, that the small engine would not function satisfactorily if continuously propelling the heavy vehicle.
Similarly, with brake systems, power is required not only to supplement or to save the driver's efforts, but also to make the braking more reliable and reduce maintenance costs. This point may not be generally appreciated, but it is no more economical to endeavour to curtail power on a brake system than it is to curtail engine power. .
By long usage, drivers have become accustomed to expect that the amount they push on the pedal shall have some consistent relation to the amount of retardation obtained. We all know of those instances when we have pushed on brake pedals without effect, just as we know of those when we have pushed and "obtained a most fierce and startling retardation.
Consistency in Braking
Therefore, above all, and especially on a heavy goods or public-service vehicle, consistency is important, more so even than a high maximum retardation. So far, we have referred only to the effort needed to apply the brakes, but to bring the friction surface in contact at pressures required for retardation, it is necessary also for movement to take place.
The travel which has to be allowed between the pedal pad and the floorboard can be conveniently divided up into three portions:—(1) To bring the shoes into contact; (II) to allow for elasticity in the operating mechanism; (III) to allow for wear at the friction surfaces.
Because of the total. movement made up of these three components,
power braking becomes desirable. Let us analyse further the reasons for the necessity of these movements. First, it is not possible to obtain brake shoes and drums which run perfectly true. Even when suitably stiffened with ribs, as most modern drums are, a 17-in.-diameter brake drum is a comparatively elastic component, and, bearing in mind that it is subject to excessive heat, it is not to be wondered at that there must be a suitable clearance to make sure the shoe does not contact unless the brake is used.
Secondly, whatever may be the means for transmitting force from the power source to the place where it is applied for retardation, the transmitting system will extend in some way or another, and, as stated previously, the brake drum, or receiving end of the force, is a comparatively elastic component.
Effects of Wear
Thirdly, there is the allowance necessary for wear. Work is done at the shoes and wear takes place, thus making it necessary for the shoe to travel farther in order to contact the drum.
Every credit must be given for the various methods by which designers seek to eliminate waste movement. In the private-car field, certain cars have their brake shoes permanently in contact with the brake drum, and this is a bold attempt which should be watched by commercial-vehicle designers.
Another way of eliminating waste movement is to employ a shoe having a wrapping or servo effect, and this makes it possible to employ a lower mechanical advantage. The possibility of disc brakes has also received attention, and these eliminate elasticity.
Automatic adjustment of brakes to compensate for wear is a feature
A Single Explanation of the Need on Some Vehicles for Supplementing the Driver's Efforts
of many passenger vehicles. It is not possible, however, to embody all these methods of eliminating waste power in one system, such as, for example,.. a self-energizing shoe in constant contact with the drum, gr disc braloa, without some other attendant complications, and it has been found that automatic adjustment needs somewhat wide shoe clearance.
Power systems are, therefore, introduced. So far, we have spoken rather glibly of power and compared it with engine power. It is now desirable to examine the nature of our brake-power requirements a little more closely. Engine power is required to overcome rolling friction, air resistance and gravitational resistance due to climbing a gradient. Brake power is required, on the other hand, only to reduce the speed of the vehicle.
"On the Level"
In passing, it should be noted that no power is required to hold it on a gradient once it has stopped, although, naturally, more power is required to retard or stop it on a gradient than on the level. The rolling resistance is beneficial to the vehicle retardation, as also is the wind resistance, but the latter plays a very minor part at commercial vehicle speeds.
Engine power is required at all times except when descending hills where the gradient is sufficient to overcome the other resistances. Even when the gradient is sufficient to overcome all other resistances with ease, it is still not always necessary to apply the brakes, or, at least, not continuously, as the vehicle descends a hill.
Therefore, it is obvious that the total work to be done by the brake power will never be as high as the engine power, but it is most interesting to consider that often when this brake power is needed, it must be expended at a far higher rate than an engine could possibly deliver. Because the power is needed at intervals in the manner described, it is possible to store brake power, in the same way that electrical energy may be stored in a battery for use when required.
Eliminating Fatigue
This statement is, of course, equally true even if the only power for braking is that supplied by the driver. In such a case, the driver is the accumulator, but with the demand for labour saving and elimination of driver fatigue, power systems are becoming desirable even on vehicles of much less weight than those that at present are equipped with them.