How efficient are the brakes in your fleet?
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By R. D. CATER A M Inst B E
recently had the opportunity to drive a couple of 24-ton-gross I vehicles belonging to a large oil company and discovered that neither of them could be stopped safely in an emergency. Both of these machines were in daily service and one had in fact been recently overhauled.
I considered it significant that on neither vehicle had there been a report from the driver of the day that the brakes were bad. Whilst the brakes on the machine that had been overhauled were in reasonable adjustment, it was obvious that there had been little care taken when setting up the linkages, which were stiff and badly set, a good deal of the effort applied to them being lost through friction and insufficient travel. The other machine—which, I might add, was handled by what seemed to be an extremely competent driver—was in such bad adjustment that when I tested the tractive unit handbrake it ran off the end of the ratchet and promptly fell to pieces. In both cases Tapley-meter readings well below those required for safety were all that could be obtained and, had I been a Ministry Examiner, these would in themselves have constituted grounds for immediate GV9s.
Checking efficiency Officials of the company concerned informed me that they did not possess a Tapley meter and, whilst I do not want to give the impression that the Tapley is the only method of checking brakes unless some such instrument is used there cannot be much uni:
formity of braking efficiency in a mixed fleet.
A few months ago I wrote an article entitled "In Search of m.p.g.". Whilst this portion of the engineer's duties has a direct bearing on the economics of a fleet, the efficiency of a braking system seldom does—until, that is, one of the flock finds that he cannot stop in an emergency. When this happens there is often a spate of bad feeling between the driver concerned and ,those responsible for looking after the vehicle, with each accusing the other of being incompetent. Rightly, of course, the driver should be able to stop a vehicle in an emergency. But—and this point is most important—the engineer cannot be expected to test every vehicle daily and he must of necessity be informed by the driver that a vehicle has brake trouble, before the necessary action can be taken.
Should the unfortunate "I could not top" accident occur, it is highly likely that for a period afterwards there will be intense activity going on around the braking systems of vehicles brought into the shop. Much of this activity will be wasted effort if a few simple but important factors are not taken care of.
Clean it first
One often sees vehicles going in for service smothered with road dirt and, during the winter months, caked with salt-filled snow and ice. Nobody can imagine that a fitter will give of his best when lying underneath a vehicle that is in this condition. Still less likely is it that the job in hand will be well done, because unsealed moving pans cannot be lubricated properly and be seen to be working correctly.
It is. however, equally unsatisfactory to spend such long periods cleaning vehicles that insufficient time is left for servicing to be carried out properly. Whilst talking of service time, it should always be reckoned that at least a full day is required to get through all the work that may be needed. If it is possible to set aside a full 24 hours, so much the better. Should this period be allowed and everything goes like clockwork, the operating side of the outfit will stand to gain anything up to 20 additional hours working time. Leave only the minimum four hours for a service, however, and it is more than likely that operating arrangements will be completely disrupted.
The latter method will result in a slipshod rushed job, which will in turn result in most cases in an unscheduled stoppage before very many days have elapsed. Jobs that invariably will be left undone at times like these are the partially seized brake linkage or adjusters. and before the next service becomes due there will be trouble in store, either with brake units not working, brakes binding or pulling up unevenly, and the side effects of all three defects are multiple. By far the most important of these is the loss of efficiency in, stopping. Other troubles that will arise directly from binding are oil-seal failures caused through overheated drums, which in turn will lead to hub or even differential failures, to say nothing of being the cause of premature renewal of linings. Fuel consumption will be adversely affected, the driver will become discontented and, in the event of a roadside check, the inevitable GV9 will appear on the company records.
Trouble deferred Should it turn out that the gods smile and no trouble is experienced during the period leading up to the next service, it is a fairly safe bet that when this is undertaken the laxity will catch up with you. If the driver has carried on driving the vehicle without noticing anything wrong, then invariably the linkage seizure is complete and, if one is very unlucky the adjusters will be solid and—in the case of Girling cross-pull units—the bisector will have run right through the pegs, leaving no alternative but a complete strip-out. Luck, of course, does not really play a part in these problems. If servicing is carried out regularly and correctly, little or no trouble will be experienced with the mechanical side of braking equipment. All that needs to be done is to see that every component is set right back to its zero position before starting to carry out adjustment. The only reason for this not being possible is that something, somewhere in the system, is either seized up or broken. Should it prove impossible to move everything back to zero it is obvious that it will not be possible to set the linings correctly and centrally to the brake drums. This will cause a considerably shortened "efficient period" before the brakes go out of adjustment again.
Distance covered is great have often heard it said that, so long as brakes are working and eventually coming into contact with the drums in which they• are contained, they will still be efficient. To a point this is correct. But what is not generally remembered is that when travelling at 30 m.p.h. each second that passes sees 44 ft. of ground covered. When all the extra movement that must be taken up when brakes are badly adjusted is accounted for, there must inevitably be an increased delay in the braking system. As this delay—when the system is in tip-top condition—is in the region of 0.2 sec., there may be anything up to a 500 per cent increase in the time lag through the badly adjusted units before the maximum shoe-todrum pressures are obtained. Add to this driver's reaction time— the national average of which is 0.7 sec.—and, at the time when the full effect of the braking system is felt, the vehicle has travelled 83.6 ft. How often, in today's traffic, is it possible to allow that sort of margin between vehicles, even when travelling at much higher speeds than 30 m.p.h.?
It must not be forgotten that the picture does not end there, for at this point we must then add the actual distance that the vehicle will travel before it comes to rest. A quick look through COMMERCIAL MOTOR road test files shows that the mean average distance required to stop an eight-wheeled rigid or articulated vehicle is 63 ft. from 30 m.p.h.; thus the best stopping distance obtainable with an average driver under the best possible conditions is about 91 ft., and when brakes are out of adjustment this figure can rise to around 137 ft.
A second point which arises from excess travel in diaphragm chambers or wheel cylinders is the high volume of air or vacuum used each time the brake pedal is depressed. It is not unknown for the reservoir contents to be severely depleted after only a few applications of badly adjusted brakes, especially when the vehicle is moving slowly through dense traffic, or being shunted into a tight entrance. The inevitable result is one of those annoying crunches which can cost anything up to a couple of hundred pounds, even though the vehicle may have only been travelling at relatively few feet per minute.
Back to zero
So the first point to take care of when adjusting brakes is to set everything in the system back to zero. Once the linkage is attended to, the adjustment of the shoes can be tackled. The first thing that should be checked when the wheels are clear of the ground is that the hubs are in proper condition. That is to say, that there are no cracked drums and that the wheel bearings have only the specified amount of "rock". Without this being done the relative centres of the axle and the brake drum cannot be secured and correct adjustment of the brake shoes is impossible.
Taking up the full amount of slack on the brake adjuster and winding back to give the clearance required is not always sufficient to ensure correct setting of shoes. Brakes having the Girling type of square adjuster with peg-cam-operated self-centring shoes do not usually centre themselves completely, because of the lack of power that one can apply to the adjusting square. To secure the correct central position of the shoe assembly it is necessary to take the adjuster screw right hard up and give the rim of the brake drum a smart blow with a 4 lb. hammer. A couple of blows will see the drum free again and the process should be repeated until no more movement on the adjuster can be obtained. Only at this stage should the final setting be made; if this is not done the linings will wear unevenly and the whole unit will be out of adjustment in less than a couple of hundred miles. This method is the only one that I have found fully effective when dealing, with this type of unit and when the full adjustment is taken up in this way quite a useful service period can be obtained between adjustments.
Adjustment tips
Brakes having S-type cams, or others equipped with some types of slack adjusters, are extremely simple to adjust. But, like other units, they have their faults. A common one occurs when the helpful driver adjusts his own brakes without realizing that there is a locking sleeve on the adjuster head. He will arrive at the depot complaining that he is having to take his brakes up every other day, and the cause of this is that, without the built-in lock, the adjuster will keep winding off.
Each driver handling a vehicle so equipped should be shown how the lock is operated and, most important, told that the final adjustment should be made by turning the camshaft in the direction of application, not away from it. Five minutes spent in this way will save no end of trouble later. To get over the trouble quickly if it has occurred, drill a in. hole through the adjuster head, across the flats, and wire it with a fairly hard locking wire. This will keep the vehicle running until the slack adjusters can be repaired properly.
A lot of braking power can be lost through worn camshafts or bad camshaft-bearings, and the same goes for the dry and rusted clevis pin through the cam-lever fork. These points are so often overlooked at service time, whilst a great deal of energy is spent in other directions trying to get more efficiency from the system. With all systems that have mechanical linkage the most important function that can be performed by a fitter looking for better braking on a machine is to make sure that the components that rotate, swivel or slide do just that, and are set right back to their original zero position before starting to take up on the adjusters.
Little useful can be said here about hydraulic brakes, other than when these are powered by Hydrovac units. Careful attention should be paid to the vents in the atmospheric side of the Hydrovac body. Allowing these to become blocked or partially blocked will result in a slowing down in the application of pressure on the lines to the wheel cylinders. The leather washers in the vacuum-piston assemblies should be lubricated with castor oil to ensure that the leather stays pliable and able to seal the vacuum chamber effectively as well as lubricating the movement. Again, bad adjustment will result in slow application, and, because the fluid has to be returned to the master cylinder, there will also be a slowing down in the release of the brakes.
Another cause of lazy application in hydraulic circuits is crushed pipes. Quite often one sees the main runs of the pipework flattened by accident—and sometimes by design, when a bodymaker has found it difficult to thread a U-bolt or some other fixing close by. A certain amount of this type of damage can be accepted, for instance, where the effective area of the pipe is not reduced. This is, of course, a lot more acceptable than the pipe left vibrating about, chafing or becoming brittle adjacent to the next fixing point down the line.
Pipework is vulnerable to misuse
An important point often overlooked when a vehicle having air or hydraulic brakes is in the shop, is to ensure that the flexible pipes running from the chassis to the axles are in good condition, with sufficient slack in the run to avoid tension being applied when rebound occurs in the suspension.
Here again, chafing must not be allowed and there should be no torsional stresses in the pipe when the nipples are tightened. This has been known to be the cause of nipples becoming loose, resulting in a complete brake failure.
Do not forget that most modern braking systems rely on some form of power application. Whether this be via compressed air, by vacuum or through a hydraulic pump, the unit providing the power will need servicing just as do those that it serves, even if not quite so often. Remember that compressors must get the air they need through a filter of some sort, and that they are provided with finning on the cylinders and heads to assist in keeping them cool. A quick wash down with the power hose will clean the finning; cleaning the filter, whatever type it may be, will not be a long and complicated job. Compressing air causes moisture, and if left for long periods the amounts produced in a braking system can be staggering, and harmful to that system.
If compressors are driven by a belt or belts make sure that these are doing the job that they are supposed to be doing, and not just thrashing around wearing out themselves and the pulleys upon which they run.
Many thousands of pounds are spent every month in joint efforts by the vehicle manufacturers and the brake lining makers to develop the best lining materials for every type of service. Be guided by their expert knowledge as to the type of lining that you should use. If you find that the service or efficiency which you are getting from your linings is not coming up to your expectations, do not be tempted to change from standard in a rush. If you have some ideas about what is needed, discuss it with the makers and then put out a test set of linings. Better still, get them to do it for you.
Give them a Chance
To be efficient, brakes have to fulfil three main functions: they must stop the vehicle quickly, they must not suffer from excessive fade and they must last as long as possible. Sometimes the decision to change to a different type or make of lining is made because of cost, with little regard to performance. Equally often the yardstick used is that of long life. Without the first two considerations being applied with equal influence, both of these requirements can produce disastrous results, either in ruined drums or, much more seriously, in a bad accident. Whichever way it goes, it is a safe bet that the end result will be an increase in cost of operation at the end of the financial year.
Whatever else there is on a vehicle that can be allowed to run slightly under the best obtainable efficiency, I am absolutely certain that the great majority of operators and engineers will agree that brakes must be on the top line all the time. To achieve this would be to achieve perfection, and as nobody is perfect, one can only hope to be pretty near to it. The only commodity the engineer cannot do without, when striving to be perfect, is time.