Superseding the Shoe Brake for Heavy Vehicles
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With Possible Increase in Vehicle Dimensions and Loading, There May be Need to Exploit a New Form of Brake. Our Contributor Makes Claims for the Increased Efficiency of Those of the Disc Type
By A. W. Haigh,
FOR many years past there has been considerable agitation in the commercial-vehicle world for alterations to the laws governing vehicle dimensions. It has been, and is still being, urged that overall width be increased from 7 ft. 6 ins. to 8 ft., and that the length of four-wheeled passenger vehicles be reconsidered.
If the size of our largest vehicles is to be increased, it is reasonable to assume that their weight must also rise. The adequacy of existing brake units must, therefore, be revised in the light of added duties.
It is a well-known fact that brakes for the heaviest commercial machines, including p.s.v.s, whilst doing all that is required of them, are, nevertheless. on the border line between efficiency and inefficiency. The main criticisms levelled at them, irrespective of the type of unit or the means tor operation, are that, owing to their large diameter, they almost completely fill the wheel dishes, and that shoe widths are such that it is difficult to ensure that the flanges do not distort, thus causing additional facing wear at the centre of the shoe where it is reinforced by the web.
The filling of the inside of the wheels has a detrimental effect on the rear tyres. Experiments carried out by the Institution of Automobile Engineers show that unless a rear wheel be adequately ventilated, tyres overheat and wear more rapidly. Front wheels, being steerable, are not affected by the lack of circulating air, as the frequency of corners—when the back plate is presented almost directly to the air stream—ensures that cooling is adequate.
The limitation of shoe width adds further to the temperature of operation of the brakes, for, whereas 2 h.p. energy dissipation per sq. in, of facing is an accepted figure for heavier vehicles, it is extremely difficult to work to this value on the majority of vehicles of 12 tons gross weight and upwards. If, then, the work done per unit area of facing is above that suggested by the manufacturers of the friction material, it follows that the temperature generated during deceleration must either be at the limit for efficient operation or above it, and, therefore, the wear must be more rapid than is necessary.
In addition to these disadvantages, which are applicable to all shoe brakes, there are others peculiar to individual types of unit. Of the three types, into which all shoe brakes can be classified, no matter what means be employed to expand the shoes, the fixed-anchor brake is the least efficient. The shoes are pivoted on fixed pins and can be operated by cam, toggle, wedge or hydraulically. When the shoes are expanded each contacts the revolving drum, but the leading shoe is energized additionally, due to the servo action of the drum which tends to drag the shoe into closer contact with it.
The effect on the trailing shoe, on the other hand, is to force it away from the drum. Hence the leading shoe does more work than its trailing counterpart and its facing,
HYDRAULIC CYLINDER PUSH ROD BACK PLATE
in consequence, is worn away more rapidly. Furthermore, unless the mechanical operating mechanism (not hydraulic) be allowed to float, the additional wear of the leading-shoe facing reaches such proportions that only the trailing shoe does any work until that, too, becomes worn and allows the leading shoe to contact the drum once more.
The duo-servo brake, the second of the three types, is the most powerful. In this instance, the shoe anchors are allowed to float, that of the leading or primary shoe being connected to that of the secondary shoe. When the brakes are applied the primary shoe, as before, is subject to servo action which, added to the energy from the operating mechanism, is passed on through the floating anchors, to the secondary shoe.
WHEEL
Here the entire effort is multiplied once more by servo action, due to the fact that it is applied at the leading end of sthe shoe. This double increase of the pedal effort inside the brake makes the unit extremely powerful, but it also introduces further disadvantages in addition to its failure to eradicate those associated with the fixed-anchor assembly.
Due to the high internal mechanical advantage, the brake is prone to snatch and, on long hills, where lengthy applications are necessary, it suffers from fading.
Fading is a direct result of heat generation due to high unit pressure on the facings which, in consequence, lose some of their frictional properties at high temperature. This shortcoming is not entirely confined to the duo-servo unit, but is also present, to some extent, in both the fixed-anchor brake and the two-leadingshoe unit, the third of the three types.
The difference in work done by the shoes of the two units described gives rise to a further fault, which is not immediately obvious. It will be appreciated that for two identical shoes to do the same amount of work, each must bear on the drum with the same pressure, but if one does more work than the other that one must exert more pressure on the drum. The forces acting on the drum, therefore, are unbalanced.
In the case of the fixed-anchor brake, the direction of the resultant force is roughly from rear to front of the vehicle and, in the case of the duo-servo unit, it is from front to rear. Thus, in both cases, the brake drum and the back plate are constantly being pushed off centre while the brakes are being applied. Stresses, in addition to those due to braking torque, are applied to the fixing studs of the drum and back plate if these be well fitted in their holes, or, where clearance is allowed, the two units can actualy be displaced in opposite directions, the one providing reaction to the other.
It has been found, in some instances, that this shifting of the drum and back plate was responsible for rubbing -after the brakes were released, a feature which is both detrimental to the overall efficiency of the vehicle and irritating to the driver.
This fault is cured in the two-leading-shoe brake, as work and wear are equalized on both shoes. The principle of operation of the unit is simple, the efforts being applied to each shoe separately at its leading end, thus ensuring that both input load and servo build-up are identical. But, although the main faults of individual brake types are eradicated by the two-leading-shoe unit, the latter can be improved upon.
Before summarizing the faults of shoe brakes which should be cured by future developments, there is one further disadvantage, from which they all suffer when applied to heavy vehicles, and that is, excessive pedal travel.
Each of the three types of brake possesses an internal mechanical advantage, in that it is capable of multiplying the input effort by virtue of its design. The approximate values of these factors are 2.5 for the fixed-anchor brake, 5 for the duo-servo unit, and 4 for the two-leading-shoe assembly, excluding leverage due to the operating mechanism.
The leverage value of the external linkage, to give the same retardation for each brake, varies inversely as the internal factors, so, assuming that the leverage for the duo-servo unit to be 1 to 1, that for the fixed-anchor brake must be 2 to 1, and that for the two-leading-shoe unit 1.25 to 1.
Pedal Travel with "Unassisted" Brakes For unassisted brakes the pedal travel, employing twoleading-shoe brakes on a vehicle of 9 tons gross weight, can be as high as 8+, ins., and the pedal effort exerted by the driver 200 lb. for a retardation of 16.1 ft. per sec. per sec. For duo-servo brakes the travel, for equal pedal effort, would be 6.8 ins., but for the fixed-anchor assemblies it would reach the ridiculous figure of 13.6 ins.
External assistance would, of course, reduce these figures, but it must be borne in mind that the vehicle weighs only 9 tons, and that there are many on the road which are twice that gross weight. The fitting of a larger servo keeps the pedal travel on these vehicles down to that given above, but it is, nevertheless, on the high side.
The retardation of 16.1 ft. per sec. per sec. may seem low, when compared with the figures claimed for private cars, but heavy commercial vehicles are extremely difficult to equip with brakes. A machine of 9 tons gross weight is usually under 3 tons unladen weight. It is, therefore, necessary to provide brakes which will stop the fully laden vehicle in a reasonable distance and still not cause skidding when the vehicle is unladen.
When it is realized that the difference in weight transferred to the front axle, during braking, between a fully loaded machine of 9 tons gross and the same machine unladen, is approximately 2,000 lb., and that the permissible braking on the front wheels is directly proportional to the load on them, the task of the designer in providing brakes which will not cause front-wheel skid when the machine is running will be appreciated.
But this juggling with retardation and the size of brake to be used, is dependent on the vehicle weight and not on the type of brake unit, and can be ignored when the superseding of shoe brakes is being discussed. Let us, then, summarize the faults already outlined and attempt to find a remedy for them in the provision of a unit of entirely different conception to that of shoes rubbing on a revolving drum.
We have seen that brake units are too large to permit of adequate cooling; shoes are not wide enough to keep the unit-pressure on the friction facings down to a level which gives maximum efficiency; wear of friction facings on two of the units is very unequal and, therefore, uneconomical, whilst that of the third type could be improved; the fixedanchor and duo-servo units cause unbalanced forces to be applied to the brake drum and back plate, and pedal travel for all three units is too high even when outside assistance is provided. How can these faults be cured? The immediate answer lies in the disc brake.
Example of a Disc Unit The illustration accompanying this article shows a part section through the Girling unit which is operated hydraulically from the pedal, and by cable from the hand-brake lever, the operating mechanisms on the brake itself being so disposed that the pressure plate cannot be distorted when the effort is applied. The back plate, to which a spigoted flanged-ring is riveted, is secured to the axle in the same manner as an orthodox shoe-brake back plate. Fixed to the hub, so that it rotates with it, is a shallow drum around the periphery of which slots are cut. Located in the slots are fingers on the friction plate, which also revolves with the wheel. Pressure and reaction plates are located on opposite sides of the friction plate and ride on the flanged ring, a spring being provided to prevent rattle. On the wheel side of the reaction plate a nut is located, which provides the means for adjusting the brake. When the pedal is depressed, fluid is forced into the hydraulic cylinder and the piston is caused to push against a short rod, which passes through the back plate and moves the pressure plate so that the friction disc is trapped between it and the reaction plate, thus putting on the brake.
Servo action in the disc brake is not a fixed amount determined by the shoe anchorage, as is the case with shoe brakes. It can be varied at will as follows:—Between the pressure plate and the flange of the ring, balls are located in pits which are given a definite slope. When the brake is applied the balls are forced up the slopes and thus cause a greater pressure to be applied to the friction disc: the slope angle determining the magnitude of the additional pressure.
Now let us consider the disc brake in connection with the disadvantages of shoe brakes. It is smaller than a shoe brake and allows a freer circulation of air; the friction facings wear evenly and can be used right down to the metal; unit pressure on the facings can be regulated as there is no limit to surface area (more than one plate can be used if necessary); there are no unbalanced forces to thrOw the brake out of line; pedal travel can be regulated by adjusting the internal mechanical advantage of the brakes; no external assistance is necessary to give the brake sufficient power to stop the vehicle in the required distance.
There is one drawback, however, to the apparently perfect unit. In its present stage of development it is expensive. But even this is compensated for by the elimination of the cost of external assistance.