Reduction of Noise in Motor Omnibuses.
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By Douglas Mackenzie.
The great advances made in the construction of silent motor omnibus have been noticed by everyone. The reduction in noise dates from the spring of Iwo, and was the subject or remark as early as May or June, prior to the sudden police activity, and the wholesale refusal to issue licenses unless the vehicles complied with a new and arbitrary stendard of permissible sound. The view: that is responsible for the police action is that of the frontagers and householders on the motor-omnibus routes, who have made such serious complaints about the noise and vibration resulting therefrom. There was, undoubtedly, goo:I cause for complaint, in the early days, when motor onmibuses were run in a very bad state ot repair, with badly-worn racks and driving pinions, slack chains, gears damaged by the learners' efforts to change gear, knocking big ends, worn valve stems, and loose, rattling frame bolts and brake fittings. This state of affairs has, however, long since disappeared, in most cases, not through the efforts of the police (for indeed it had ceased before the recent police activity), but because the operating engineers had organised the routine of the maintenance staff, secured good, reliable charge hands from amongst the many fitters who had presented themselves, obtained a large stock of spare parts for easy replacements, and had learnt, from bitter experience, exactly how to adjust certain mechanical details. In the early days, owners tried to get as big a mileage as possible out of each vehicle, and endeavoured to keep it on the road, regardless of condition, provided it would work at all. It took time for them to realise that noise was inseparable from wear, and that, to allow a motor to run in bad condition, was to cause the maintenance costs to rise until the working costs exceeded the takings. It has been one of the most curious features of the motor-omnibus boom that practical men should have forgotten every lesson that other vehicles had taught. Firms which had worked horsed omnibuses for years, and were accustomed to have these examined periodically, and kept them up with such care and attention, that they lasted for zo or 30 years, appeared to consider that motor omnibuses would require only an annual overhaul. Similarly, engineers who were familiar with railway practice, and who knew the attention that is bestowed on a locomotive after every run, seemed to expect motor omnibuses to require no such attention, provided they were given plenty of oil. A few months' running, however, proved the fallacy of such a system, and good organisation had to be developed to cope with the proper maintenance of the vehicles. Such organisation could not be created instantaneously, and it was, naturally, little by little that the management of motorbuses reached its present efficiency. The noise of the engines and gearing decreased, as each detail received proper attention, and, gradually, attained that " low hum of machinery e' which is the present state of affairs.
There-are, still, some people who complain that motor omnibuses are noisy, but they are, evidently, born grumblers, and, • if angels, visits become frequent, they would doubtless complain that the swish of their wings was an intolerable nuisance. The improvement, nevertheless, is real and undoubted, and it will be interesting to readers of "
COMMERCIAL Mom OR "to know the alterations that have been made which result in the much more quiet running that is now the rule. A knowledge of the exact amount of adjustment required in various parts of the mechanism has had a great deal to do with this. An example, that may lie. cited in this connection, is the adjustment of white-metal big ends. The early practice was to fit these to the crank pin so that the connecting rod could be swung freely, but, after two or three • days, they developed a knock. The present practice is to make them so tight that considerable effort is necessary to swing the connecting rod. The engine is then boxed up, and run for half an hour, in the garage, with plenty of oil. However accurate the fittings may be, the white metal can only be touching the crank pin at a number of points instead of the entire surface. These high places are hammered out flat during the half-hour's run in the shop, and, consequently, the omnibus goes out with a thor oughly-bedded big end, which will run from three to four weeks without knocking. Another source of noise, in the engine, is end play on the big-end and crankshaft-bearing bushes. There is always a risk of seizing, if there is no clearance, but a sixty-fourth of an inch is ample for this purpose, whereas, some makers and engineers have been allowing a sixteenth of an inch on each side. This gives the bush an eighth of an inch play between the crank webs, and there is, then, a great tendency for the bush to slam across from one side to the other, when the exhaust changes to the suction stroke, or the suction to the compression stroke. These are minor details, but they all have their influence on the reduction of undue noise. The timing wheels, by which the crankshaft drives the camshafts, are apt to be noisy, even when helical teeth are employed, because the camshaft is doing work whilst it is lifting the valve, and the valve spring is actually driving the camshaft while the valve is closing. The consequence is a transference of the loads from one side of the teeth to the other side eight times in a revolution. Fibre wheels have been introduced to reduce this source of noise, and they appear to run silently and well, whether in oil or not, although they require to be removed more frequently than gun-metal or phosphor-bronze wheels. Some clearance is necessary between the tappets and the valve stems, to ensure that the valve always closes, and, therefore, there is a slight blow when the tappet rises against the valve stern. This is overcome, in some of the latest designs, by holding the push rod against the valve stem by means of a spring, keeping them always in contact, and transferring the clearance to the cam itself, so that the minute blow takes place inside the camshaft casing, and is also cushioned by the oil that the cam picks up. Those engines in which the valves are operated by tappets from an overhead camshaft are the most difficult to silence, effectively, in this respect : the introduction of a spring, between the tappet and the valve stem, might have the desired effect. The next source of noise is the gear wheels in the gear box. The most silent forms are, undoubtedly, those in which the gears are always in mesh, and, in such cases, helical gear wheels can be employed with advantage. Excellent tools are now made for cutting the teeth of helical wheels with extreme accuracy. The police have suggested to several makers that they should adopt gun-metal or phosphor-bronze for the driven gear wheels, retaining steel for the driving gears, arguing that they would be much more silent than steel upon steel. Their reason for the suggestion is that one maker has consistently used gun-metal on steel. The gears, however, must be designed for this, with broader and stronger teeth, and it is impracticable to make the alteration in existing gear boxes.
Some chassis are sufficiently quiet in the fast gears, but noisy in the lower gears. It is, too often, the practice to use the same width and pitch in all the gears, but the Maudslay omnibus has gear wheels 'in. on the low gear, 'Lin. on the second, and 'gin. on the third and fourth. It is obvious that the primary shaft has to transmit tne same turning moment (or angular force) through each of the drivers., which is magnified by the mechanical advantage of the gears to produce a much greater propelling force at the road wheels on the lower ones than on the higher The turning moment, being the same in each of the drivers, the force at the pitch circle will be in inverse ratio to the diameter, or in other words, if the first-speed gear wheel is only half the diameter of the fast-speed gear wheel, it will have to transmit twice the force through each tooth, and should, therefore, have twice the breadth of face. Some makers have tried to strengthen the teeth of the first-speed wheels by increasing the pitch, or decreasing the number ot teeth, and, thus, making each set of teeth larger ; but a coarsepitched wheel is always noisy, and a wheer of small diameter should have a finer pitch, making up the strength by a further increase in breadth of face. Gears can be designed to be equally silent in all speeds, but wider gears are then essential. Mr. Ravenshaw drew attention, at the last meeting of the Society of Motor Omnibus Engineers, to the advantages of gear wheels of larger diameter. The objection to this is on the score of weight, but there is no doubt that a little, extra weight in the gear box would be more than compensated by the increased durability. Great improvements towards the reduction of noise have been effected in the engine and gear box, but the greatest improvement has been in the final drive. The quietest of all, in this respect, are the live-axle omnibuses, such as the Leyland, Stirling, and Dennis. Next to these, comes the De Dion-Bouton, the axle of which was illustrated in a recent number of " THE COMMERCIAL MOTOR." The final drive, in this case, is by a pinion and an internally-toothed gear ring; these are of fine pitch, and encased from dust, and run, on the whole, very silently. The Milnes-Daimler omnibus is driven by a pinion, gearing with an internally-toothed gear ring, or rack, of large diameter, so large indeed that it catches much of the dust and mud from the road. Some engineers tried the expedient of greasing these gears, but found that grease held the dirt, and formed such an efficient grinding mixture that it was more economical to run them dry. These gears are quite silent when new, but, as they wear, they become very noisy, and as new pinions are required more often than new gear rings, the original state of quietness is difficult to regain. Most engineers have, now, replaced the original steel pinions by phosphor-bronze ones, which run much more quietly. in the 1907 type Milnes-Daimler, this driving gear is protected by a dust shield, which is, undoubtedly, a move in the right direction. Thomas Tilling, Limited, has had one Milnes-Daimler chassis altered by the substitution of a Dennis worm drive, and this car is running very well and quietly. It is rumoured that several other chassis are being similarly altered. The chain-driven cars are fairly silent, while the chains are new, but there is always a silky crackling that cannot be prevented. The proper treatment for chains is to take them off, clean them in paraffin, and then dip them in melted Dixon's graphite, so as to ensure that each roller is brushed with graphite, and that every wearing surface is thus lubricated. Proper garage organisation should provide for the removal of each chain, and its treatment in this manner, at least once a week. This means that a good stock of spare chains must be held, as the treatment requires 12 hours, and must be undertaken by the day staff. Silent chains of the Renold, or Morse, type have also been tried, and, undoubtedly, contribute materially to the quiet running of the omnibuses. Lastly, a considerable improvement is due to the attention given to frame bolts and rivets, spring shackles, steering-g-,ear joints, and such details. Proper daily inspection has been organised, and each defective part is promptly replaced. The noticeable improvement, the subject of universal comment, is, therefore, due to the natural progress of the industry, as makers and users, alike, have had to learn many lessons from hitter experience. Being engineers, they look on every difficulty as something to be overcome, and improvement has taken place all along the line, that is to say, in design, in workmanship, in material, in the application of lubrication, in systematic inspection, in garage organisation, in more skilful handling, and in public appreciation, so that any just complaint can no longer be made as to the noise of motor omnibuses.