BRAKES

Waipa Post, Volume 49, Issue 3555, 8 December 1934, Page 8

BRAKES

RETARDATION OF MOTOR , vBaiGLBs. ;

It was suggested that brake adjustment should be included in this discussion, as also trailer brakes. In these .connections, and in the first case, many years have passed since the author was engaged in passenger and goods transportation, and in the second case the extrepely rapid development during the past two years has left him rather behind in technique, so that he feels he cannot speak authoritatively on either subject. : In Point pf fact the, braking equipment of a modern mptor vehicle is not by any means as simple as it may appear to be, for apart from the brakes themselves there \is the system, of linkage With" its Compensation devices, a master. Lockheed cylinder, four auxiliary cylinders, the master cylinder again being assisted by Dewandro or ptlier vacuum cylinder so that the upkeep of the system as a whole has become a specialised jpb—but it is obvious that cleanliness, lubrication and elimination of leakage of air or fluid are the first considerations, but almost of equal importance is the adjustment of the shpes & s *^ e linings wear away, together with the balance of braking force between' pairs of brakes on each axle.

As is well known, the maximum tUrjiing movement is obtained from a lever and pull rod when the' rod is at 90 degrees to the lever; in any other position there is a loss of power, for example, if the lever moves through, an angle of 30 degrees the loss is nearly 134 per cent, and as there may be three levers in series the initial pull will be reduced by not less than 45 per cent.

This shows unmistakably how essential it is to maintain the most effective position between pull rods and levers, which can be done by moving the cam shaft lever backwards or forwards as may be required, on the spline shaft, which is arranged for that purpose, or the levers may be partially rotated by a worm and wheel, and alteration in the position of the cam shaft levers naturally, alters the position of all other levers connected to them. If a high leverage cam is fitted it may be necessary to insert packing pieces from time to time between the cams and ends of shoes, particuuarly with linings Jin thick as the effective radii of the cams may not be sufficient to follow up the movement of. the shoe for a wear of 3-16 in at the centre of each lining. Oh the other hand given a low leverage cam It is possible to take full advantage of the useful thickness of iin lining, without resorting to packing pieces, and as the labour involved in dismantling wheels and brake drums possibly three times before the lining is worn out adds to the cost of maintenance there is something to be said in favour of thinner linings and low

leverage cams. Looking at the matter from another point of view, the question arises as to the desirability of running for long periods without any attention being given to the internal parts, of brakes, if the drums are steel pressings there is a possibility . of abrasion, which if not checked in the early stages by the removal of the small particles of metal embedded in the lining may seriously affect the service of both drum and lining, hence the insertion of packing pieces leads directly to periodic inspection, against which nothing can be said. • - The "S" type of cam has the advantage of reducing friction losses to a large extent, as there is'little relative movement ( between the rubbing faces; it gives the shoes considerable radial displacement for a relatively small angular cam movement. Of the other types, one gives a constant opening and pressure; the other, the leverage increases with angular movement, but the opening v of' the shoes decreases in the same proportion, hence the brake requires adjusting more frequently when the cams reach a certain position, and as will be seen, both types slide against the end of the shoes, and if the contacting faces are not lubricated it is quite possible that the friction generated may reduce the efficiency of the brake by as much as. 10 per cent. Referring briefly to trailer brakes, the regulations in force are well known and do not call for comment, but it is not quite clear why a pressure cylinder directly connected to the drawbar is confined to trailers up to a weight of one ton. Generally the laws governing retardation apply •.equally to trailers as to self-propel ied vehicles —that is to say, the rate of retardation depends upon the power of the brakes, the weight of the wheels that are braked and the adhesion between tyres and road. In order to make steering reasonably light brakes are usually fitted to rear wheels only; they may b 6 operated by air or fluid pressure, or by cables in precisely the same way as other self-propelled Vehicles, but under no circumstances must the pipe lines or cables be subjected to any towing stresses. Where cables are used it is desirable to allow more latitude in the movement of the pperating Jever than is available with a normal lever and quadrant, the object is secured to an almost unlimited extent by -various forms of ratchet levers, of which the "Neate" is perhaps the best known; it can be locked or freed as arid .when required. \ BRAKE DRUMS. VThe modern trend is definitely in tfiy. direction of castings, either plain casKjron or cast alloy irons containing percentages of chromium, nickel fend manganese, and as, these drums possess in the highest degree

three of the four essentials—tensile strength, rigidity, durability and immunity from abrasion, and are not seriously deficient in the property first named, it is safe to predict that they will supersede entirely pressed steel drums in the heavy commercial field, unless something of a revolutionary character is developed in the nature of steel sheets by which certain qualities of cast iron are imparted to a comparatively ductile metal prone to abrasion and wear. It is true that a compromise can be effected by inserting a cast iron liner into a steel shell, but as elevated temperatures are incidental to heavy and prolonged braking, composite drums as described cannot be regarded as altogether satisfactory in view of their low thermal conductivity, which is due to the resistance to the transference of heat units through the junction of liner and shell, however close the fit may be; in consequence the temperature of the lining for a given output of work is substantially higher than with a cne piece drum either of steel or cast iron, and since temperature has a very definite influence on the rate of wear and the friction property of brake lining, it follows that thermal conductivity is of prime importance in the efficiency of brakes and the service of lining. A composite drum can be made very much more efficient by coating the outside of the cast iron liner with copper, but the cost may prove to be a deterrent to its adoption. Quite the 'oest type of composite drum is known js the "Centrifuse," an American production.

It comprises a light outer steel shell, into which molten cast iron is poured and cooled and solidified in a centrifugal casting machine; as the iron fuses to the steel there is practically no interference in the transference of heat, and brake drums manufactured in this way appear to meet the requirements satisfactorily. The best known of the chrome nickel alloy •ast drums are Millenite and Chromiiium, manufactured respectively by r ,ake and Elliott, of Braintree, and The Birmingham Cylinder Castings Jo.

Another type of drum is . neither oressed nor cast, and bridges the gap between the two. It consists of a rim circled from rolled steel of any desir Ml section, with an electrically welded ioint. Into the rim is spigotted a disc }f mild steel, also electrically welded it a number of points, and as the rim ;an be made from high carbon steel — tool steel in fact, if necessary—the advantages of the method of construction are apparent, extreme hardness being combined with rigidity and light weight, in addition, as the two parts ire not entirely integral the drum is Car less resonant than those of one piece construction. The manufacturers are Messrs Holden and Hunt, of Birmingham.

One particular advantage of cast iron drums, also of those just mentioned, lies in the inability to arrange :me or more stiffening webs in the nost effective position, whereas with Irinns pressed from steel sheets, the web or flange can only be made at the ipen end, and while it does limit bell-mouthing to some extent, it canlot support the rim of the drum sufficiently to prevent distortion. BRAKE SQUEALING. Here we have a very elusive phennomenon, one that appears and disappears for no apparent reason; some cimes assuming such proportions as to oall fpr the intervention of the authorities. Squealing is the audible product of the vibration of shoes and drums; it is exaggerated by increasing brake shoe pressure, also by the resonance a chassis itself. It is obvious that a brake drum is accoustically weir designed to transmit vibration f.o the air, and this property, 3d with its own natural period and that of all surrounding mechanism, tends to increase the oscillations that are responsible for noise. It may be accepted that a certain degree of friction tension is necessary to create the initial movements, hence brake lining with a moderately high co-efficient of friction is usually associated with noise; in fact, always a brake that squeals is an effective brake, and no brake ever has squealed or could be made to do so immediately following the application of a lubricant to its rubbing surfaces. . Squealing is not the result of unequal distribution of pressure; that is high places on the lining, since the maximum noise is invariably produced when the lining has thoroughly bedded itself down to the contour of the drum, and has assumed a smooth, polished surface, for in this condition it exhibits its highest frictional efficiency. The initial vibrations in themselves are entirely insignificant, and could be eliminated by little more than the touch of a finger, but once started they increase in frequency and amplitude, at such an astonishing rate that it is practically impossible to suppress them —actually at their maximum they are responsible for a substantial increase in brake torque. On, one occasion a brake running silently, while effecting 30,000 lbs inch torque, suddenly started squealing. Immediately the torque rose to 34,500 lbs inches, an increase of 15 per cent, or the equivalent of an additional 6i h.p. to the normal rate of work. The fact that a damping band weighing a few ounces fixed round the outside of a drum weighing upwards of a hundredweight will prevent squealing indicates the feebleness of the initial vibrations. Other efficient damping devices are—to tighten shoe pivots, strengthen pull-off springs, loosely bolting pieces of lead pr iro,n to. the webs of the shoes, shoes of different weight, steel and alumin-

ium for instance; or holding the webs of the shoes between spring loaded washers so as to impose a certain amount of resistance against a very tlight lateral movement; the last resource being to fit lining with a low co-efficient of friction if all else fails.

Generally speaking, the off-side rear brake is the culprit—why this should be so no one has yet been able to explain; it is a phenomenon that has baffled all observers.

Referring to methods of testing brakes, there can be no doubt of the value of testing -equipment such as the Cowdray, by which the power of each brake can be measured simultaneously with the weight of the vehicle on the wheels. Any other method of testing, for instance, jacking up one wheel and pulling it round by nand against a light application of the brake is merely a waste of time and effort. In fact, it would be far more satisfactory to run the vehicle against the brakes and adjust the shoes according to the temperature of the drum, slacking off where the drums are hot and tightening up where they are cool. To determine the actual braking efficiency on the road, the Tapley brake testing meter is one of the most reliable instruments available. It gives the braking efficiency as a percentage of G irrespective of the speed of the vehicle, so that stopping distances from any speed can be found by a simple calculation or directly from the Ferodo chart of braking performances.

Just a few words on brake lining. Modern conditions and brake design are responsible for the creation of the many types of brake lining material. In fact, so many and varied are the requirements that we find ourselves in the not too enviable position of having to manufacture far too many varieties —not that this in itself presents any particular difficulty, but when a question of recommendation arises the choice of some twelve varieties doea lead to a certain amount of confusion, for, literally, what is "one user's food is another one's poison," and it is very difficult sometimes to reconcile the views for and against a certain type of lining—some will not have it at any price, while others will have nothing else. Motor manufacturers may standardise and recommend brake lining which they have found to be eminently satisfactory, but certain users think otherwise and go round the entire market and never get what they think they want; they are not quite certain what it is, but it appears that it must possess some charm which will overcome all the deficiencies arising from inattention, mal-adjustments and accidental lubrication and, of course, it must be cheap.

Speaking generally, the ' choice of lining lies between non-metallic wov en asbestos fabric, and woven fabrics in which brass, lead and zinc alloy wire is incorporated. Then there are the hard moulded varieties. The nonmetallic woven fabric mentioned above is very largely used as original equipment, and is generally the choice of the principal transport, companies in this country. It is suitable alike for pressed steel and cast iron drums. Returning for a moment to the divergent views of users, it is not to be wondered- at that uncertainty must arise, since brake lining mileage varies enormously, even with vehicles of the same make and type working in different parts of the country, aud between town and country working; for example, in London omnibus work 16,000 miles per set for the service brake lining is a fair average; in other towns the average may fall to as low as 3000—the same material and the same type of vehicle! Whereas in part town and part country services 35,000 miles average is not exceptional—the nature of the routes, speed of running, and above all the manner in Avhich the brakes are used being the deciding factors. It is a truism that drivers will drive according to the power of the brakes—when the brakes are really effective they will approacii stopping points at higher speeds and brake harder than they would with less stopping power. This' practice alone has led directly to the creation of a range of linings of extraordinary durability considering the duty that is imposed, and since there is a definite limit to the effective area of lining the only alternative to offset increased weight, speed and stopping power is to go on creating new and better materials.