SILENCING THE PASSENGER VEHICLE CHASSIS.
Page 37
Page 38
If you've noticed an error in this article please click here to report it so we can fix it.
The Importance of Reducing Mechanical Noise and How This Can Be Done.
THERE can be no question that one of the prin
cipal factors which have assisted the development of motor coaching, and the use made .by the public of motor transport vehicles in general, is the silence with which the chassis carries on ifs work. In the towns, noisy transport vehicles are a source of annoyance, not only to those who ride in them but also to people living near the streets in which they are used,. and the present great popularity of the motorbus, as compared with the tram; is undoubtedly due, in part, to the silent running of the former as compared with the terrific clatter which is usually set up by the latter.
For country bus services one might at first be inclined to think that the question of noise was not so important, but, on the other hand, the passenger in the country is in the bus for,a longer period—perhaps as much as an hour—so that noise is apt to be very fatiguing. Lastly, there is the motor coach which is used for holiday tours, and it is common experience that other things, such as suspension, being equal, the public will tend to use the type known to be the most silent.
In addition to these important considerations, it is as well to remember that,in most mechanisms, the • emission of noise is a definite indication that undue wear is taking place. Noisy gears, for example, wear more rapidly than those which run silently. Cense.quentlya in endeavouring to secure silence; the designer is also working towards durability and reduced running costs.
When one comes to consider it, the silence of the modern petrol chassis is an extraordinary achieve meat in a vehicle which is actually propelled by a series of explosions. This• degree of quiet running has, of course, only been attained after many years of development. In the old days the engine made suffi
• cient noise to drown all other components ; then, as the power unit became quieter, noises not previously beard were clearly audible, such as die screaming of gears, the rattle of chains, and squeaks from various minor parts. So the work went on, every time one part was silenced other sources of noise were revealed, until the present high degree of silence was attained. Nevertheless, there is still room for iin--provement, particularly in the directions of silencing . the axle and the gearbox: . Taking the components one by one, the engine explosions have now been efficiently muffled by suitable silencers without loss of power, and not much development can be looked for here. It does, however, devolve upon the owner to examine the exhausting system from time to time, paying particular regard to the joints, which, if allowed to become loose and blow out, will net only give rise to noise, but Will .alsci permit the escape of poisonous fumes. An other
point is that the exhaust piping is apt to rattle when the vehicle is traversing rough roads. . As regards mechanical noises from the engine, the valve-operating mechanism is now the chief offender, providing, of course, that the principal bearings are in good condition and are not giving rise to knocking. It is essential that there should be a small clearance between the tappet and the valve stem, but if this be allowed to increase unduly, the tappet will accelerate to a considerable speed before meeting the stem, so that impact will occur and an irritating series of clicking sounds result when the engine is running. The remedy, of course, lies in adjusting the tappets from. time to time.
Another point is that valve . springs are apt to become weakened in use, so that, when the engine is running at full speed, the springs may not close the valves properly and bouncing will occur, this giving rise to a characteristic clattering sound.
Then there is a noise sometimes produced by the
gearing which drives the camshaft. On many engines this has been reduced b,yfitting an idler wheel made of some fibre compound in place of cast-iron or steel, and such wheels are found to stand up very well in practice. One of the reasons why timing gears are apt to wear somewhat rapidly and b.ecome noisy is that the effort required to turn the camshaft is continually fluctuating, according to the position of the
cams, so that the engine designer might well consider • the fitting of a torque balancing cam working against spring plungers such as is used on many pleasure-car . engines. • •
• The next point to consider is the gearbox, and many of the comments to be made on the gears in this : component are equally applicable to the back axle. Smooth and continuous transmission of l'eawee is essential.if -noise is to be avoided and although the use of an involute form of tooth Pro-file should theoretically result in this smooth drive, there are many factors in -practice which tend to produce noisy running. Aa regards the gears themselves, these are almost invariably heat-treated after being cut, and, in the course of the heat-treatment become slightly distorted. Consequently, the practice of grinding the gears subsequent to hardening is increasing in popularity and is now used by several prominent commercial vehicle manufacturers.
One of the most serious errors in unground gears consists of variations in the pitch or spacing of the teeth ; consequently, as the gears rotate in mesh, instead of each tooth taking up the drive smoothly and progressively, the variation in the spacing will result in a series of blows being delivered. These are thought by many authorities to be the chief cause of the noise produced, and certainly must bear some relation to the noise because the pitch of the note produced by gearwheels in mesh is found to correspond to the number of teeth impacts occurring per second.
Almost equally important is the kind of distortion which results in the gear becoming eccentric with regard to the shaft upon which it is mounted. Such eccentricity results in the depth of meshing varying continually • as the •gear revolves, and the peripheral speed varies also ; consequently, smooth running is interfered with.
Quite apart from the gears, the design of the box, the mounting of the shafts, and the fitting of the box to the frame, all have their influence upon the noise produced. The gearbox casing should be as stiff.as possible, not only to resist the load, but also toprevent it from acting as a sounding board and magnifying the gear noises which may be initiated inside:
Then, again, accurate action of the gears cannot take place if the shafts are continually springing apart under the load or if the bearings tend to give. Lastly, the mounting of the box should be such that frame distortion will not affect the positions of the gear shafts. A practical point in connection with the gearbox is that the use of a really thick lubricant, while it may result in a slight loss of power, usually serves to quieten the gears considerably. At one time chains were extremely popular for the final drive, but, owing to the noise which they set up, they soon proved to be unsuitable for the passenger vehicle. Consequently,nnthe lighter chassis an open propeller shaft is corranonly used, 'While on heavier vehicles the propeller shaft is enclosed in a torque s (754
tube, and, as a rule, transmits the power to a wormdriven back Many mysterious cases of noise and vibration from the transmission are traceable to whipping of the propeller shaft at high speeds. If it be enclosed in a torque tube, the remedy consists of fitting a central steady bearing, while a troublesome open shaft will. usuallyNhave to be replaced by one of larger diameter. It may he noted that an increase of weight is not usually efficacious, rigidity with a minimum of weight being the ideal at which to aim. Consequently, the best form of shaft is a stout steel tube. So far, the use of the silent-running fabric universal joint has been practically confined to lighter types of chassis, so that universals of the metal-to-metal variety are most commonly used. It is most important to lubricate these joints periodically, because otherwise wear and noisy running are bound to occur. The Importance of Lubrication. Having dealt with the principal components, we can refer briefly to the many minor chassis parts which are apt to give rise to most annoying squeaks and rattles if lubrication be neglected. The spring shackle bolts are common offenders in this respect, as they habitually work under heavy loads and yet have not a sufficient degree of movement to draw in lubricant between the bearing surfaces. The only way to avoid trouble with these parts is to lubricate them thoroughly and often. The same remark applies to the steering gear, and, of course, periodic inspection and lubrication are more than ever necessary here to remove the possibility of failure and accident on the road. The brake. gear is often responsible for rattling sounds mined by the tendency of the long pull-rods to Vibrate.A cure for this consists of fitting a steady bearing, bushed with fibre or some similar non-metallic substanee, about half-way between the ends of the rod. In this connection we may mention what is, perhaps, one of the most annoying noises in a. vehicle, namely, the screech set up by brakes in poor condition. Very frequently this is caused by the brake ' linings having become worn and glazed, or by projesting rivets, .relining being the obvious cure. Particles of grit are also liable to become embedded in the lining, and, if of a hard nature, will produce a screeching noise when rubbed against the drum. In an old vehicle the brake drums may have laminae very thin through wear, and will then tend to amplify any noise produced ; an effective dodge for overcoming this trouble without replacing the drums is to fit tightly to the outside a strip of thick belting. This has the effect of preventing the drum from emitting a ringing sound.. This article would be incomplete without mentioning vibration as a possible source of . noise. The vibrating part may, itself, run quietly enough, but is very apt to cause parts of the bodywork, such as-doors and windows, to vibrate and rattle in sympathy ; consequently, a smooth-riinning engine and transmission are greatly to be desired. The four-cylinder engine is a tolerably well-balanced unit, but in view of the steady increase of running speeds, closer attention to the problem will undoubtedly have to be given.Tags