ARRESTING NO ITS SOURCES.
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A further Article Concerning a Suhp Some of the Means and Methodt 1 Importance, in which we Deal with Various Sounds can be Reduced.
NEARLY every part of the chassis of a motor vehicle can at some time in its existence constitute a source of noise, and in an article entitled, "Whence Comes All This Noise?" which we published in our issue dated September 18th, we dealt fairly fully with this subject in so far as the actual causes of noise are concerned; but we did not, to any great extent, go into the problem of how the sounds could be reduced In volume or altogether silenced.
There are, of course, certain noises which cannot be reduced to any considerable extent, as, for instance, the whirr whiCh occurs between the tyres and the road, but these are, in the main, not really objectionable or great in volume.
It is, however, often minor items-which are the worst offenders, and, in many instances, it is the comparative silence of the modern power unit which has, as it were, exposed the guilt of such parts and rendered their silencing a matter of far greater importance than was formerly the case. For instance, the light rods used for the control of such parts as the carburetter, ignition, etc., are frequent offenders.
Of all the various kinds of joint for these which have come before the notice of the writer, that which has been designed by the engineers of Bentley Motors, Ltd., appears to possess So Many good points that it
is surprising that it has not been adopted on other makes, particularly as we understand that it has not been protected by the company. This joint takes the form of a pressed-steel clamp or fork, in which are spherical cavities which form a socket for the ball end of the other part of the joint. A small bolt above the arms of the fork enables the joint to be tightened when slackness occurs, although this is seldom, owing to the natural spring of the device. A particular advantage is that it provides for a ifniversal movement, so that an arm at the end of a slanting steering column can be connected direct to a lever on the carburetter.
Where ordinary fork and pin joints are employed they can be silenced fairly effectively by means of Thackray washers, one of which is compressed and mounted on each pin between one side of the fork and B311 Many engines are practically silent, but in others certain details cause annoyance. For instance, an appreciable amount of noise may arise from contact . between the tappets and the valve stems, as a certain amount Uf clearance between these parts is essential. Often the sound can be greatly 'reduced by providing cups which collect oil, the oil acting as a cushion. It is true that if the tappets be properly adjusted, allowing only the smallest permissible gap, the trouble is not great, but, even then, an accumulation of ell will he found beneficial.
No definite rule can•be laid down as to how tappet oil cups can be added to existing engines, as the types vary so widely. An essential point which must Le borne in mind is that where cups are provided as sn addition they must be removable to permit tappet adjustment, so that a feeler gauge can be introduced between each stem and its tappet.
We illustrate two ways in which such cups can be made to act fairly well. In one case a cap made of leather is fitted over the upper hexagon of the tappet . and is provkied with a hollow to catch a small amount of oil. In the other case a metal washer is inserted between the two hexagons, so that a leather ring can rest upon it, the ring being a good fit over the upper hexagon. In qther type the leather can be lifted when making adjustments, whilst friction can be relied upon to hold it in place.
a (lb cillo With sleeve-valve engines the problem is, of course, simplified as there are no tappets.
If several engines of different mate be tested by removing the valve springs So that the lower part of the stem of the valve can be gripped by hand and the valve pulled sharply down so that it strikes the seating, it will be found that the noise resulting is far greater in some engines than in others. This is
probably .due to the different acoustic ptOperties of the cylinder castings, and at this point it would be as well to mention that this is a matter which is at present very little understood. Apparently the lighter thevalve the less the noise produced ny the collision between valve and seat. A gentle slope on the falling side of the cam is said by some to lessen the sound, but cam profile cannot be influenced to any great extent for this reason, as, to obtaia power, large valves with fairly high lifts of long duration are essential.
Hum from the Timing Gears.
The noise produced by timing gears constitutes one of. the most difficult problems with which the motor engineer has to contend, whether in the construction of the engine or concerning its overhaul and repair.
Many gears start life moderately silently, but finish noisily; others, like certain members of the human race, commence by being noisy, but quieten with time.
Where a single pair of gears is employed, as in a side-valve engine, the problem is difficult enough, but In cases where the overhead camshaft has to be driven it becomes still harder. Bevel gears for this purpose have been found difficult to silence; in fact, this applies to nearly all forms of metal gearing.
Chains probably give the best all-round results, but they must be kept in correct adjustment and not run for too long a period. Heavy metal gear or chainwheels, together with large, plain bearings for the camshaft, have been found to steady revolving parts and to stop snatching.
One of the best remedies that the writer has found for noise or humming gears where a side camshaft is employed is to make the larger gearwheel of Celeron, a material made by the Diamond Fibre Co., Ltd., 75a, Queen Victoria Street, London, E.C.4. It con, gists of a largo number of layers of a woven material resembling muslin, which is held together by a special bonding agent. Layers of stabilizing material are inserted at intervals to prevent warping. Pinions made from this material run very silently and give a reasonably long life.
Pinking and Other Engine Knocks.
Of the two kinds of knock—the mechanical and that due to the ignition, the former is usually the result of slack big-end bearings, and there is no remedyfor this but repair. Ignition knock, however, may be caused by several disturbing elements, such as an overhigh compression for the speed of the engine permitted by the gearing of the vehicle. An engine with a high compression may work perfectly smoothly in a vehicle suitably geared for its weight, windage, etc., so allowing the engine to attain its correct speed, whilst if the same ratio of gearing between engine and axle be used with a vehicle of greater weight and larger
projected area, the power unit may not be able to get away with its load; consequently, it will always be troubled with knocking when ascending slight hills. In such engines a small reduction in the gear ratio may remedy the• trouble.
Knock, due to the deposition of carbon in the combustion chamber, is too well understood for us to dwell upon it here.
Fixed ignition is still employed on some vehicles, the drivers of which frequently complain of engine knock. It would appear that some firms do not credit, the average driver with sufficient intelligence to entrust him with an ignition device which he can control. This may be due to the fact that, in some eases, the range of control provided was too wide, and, consequently, a driver may have exceeded the proper limits, thus causing damage by over-advancing the spark. In the opinion of the writer, however, more damage is done by attempting to drive up slight inclines on. Cop gear when the ignition is fixed than would be the case if the driver could control it.
Users who are troubled with engine knock and have vehicles with fixed ignition may find relief by providing some means for controlling the ignition so that it can be retarded a little when climbing slight Inclines; Too high a compression may sometimes be remedied by fitting a gasket under the cylinders, so raising them slightly. In other cases it may be possible to bore out the valve plugs. Such expedients should, however, not be necessary with a modern and well-designed engine.
The quality of the fuel used may exercise a great effect on the pinking, and it may often be advisable, to try such mixtures as benzole and 'petrol, Ethyl petrol, etc.
Noise from Exhaust Leakage.
There are few things more annoying to passengers than the noise and the foul atmosphere due to leakage in the exhaust system, these being often due to the blowing out of an exhaust gasket.
It is not easy to see how such an occurrence should he permitted, in view of the fact that an all-metal connection has been known for over 20 years and made in its thousands. This is the spherical-face joint which was used on all the early Cornmer car models and of which we give some diagrammatic views. This joint not only prevented the possibility of a blow-out, but it kept the joint gas-tight, whilst permitting an appreciable error in the alignment of the flange faces. The making of such a flange does not entail any extra machining; in „fact, it actually reduces cost, as the faces which approach each other are not machined, that on the engine exhaust manifold only requiring the 1334
Insertion of a cutter for a depth sufficient to form a spherical sealing, whilst the flange brazed to the pipe can be touched with a gauge tool while it is being bored, so forming the spherical spigot.
An alternative method, and one which,can be applied to existing vehicles, is the fitting of a Cooper's mechanical joint, which consists of an ordinary copperaSbestos Washer with a spigot formed integral with the ring which connects • the two copper flanges. With this joint a complete blow-out becomes impossible, but leaks through misalignment may still occur.
Silencing Engine Auxiliaries..
The engine auxiliaries on certain types of power Unit, particularly those designed some time ago, occasionally cause a lot of chatter, especially when the engine is running slowly. The source of the noise usually lies in the connection from the driving mechanism, such as the ordinary form of dog coupling, but there are now better-known means for making, these connections, amongst these being the Simms vernier rubber coupling, and fabric discs such as the Hardy.
Noises Arising from Gears and Gearboxes.
The sounds which are caused by the use of the lower gears need not be expected to trouble the coaching industry to any great extent. It is probable that they Will always be with us, no matter how we try to avoid them, and passengers on these vehicles will hardly expect very steep bills to be climbed without some little extra noise. The gear that really matters is the third speed, where four are provided, as one expects a vehicle to rufrfairly silently when employing it. The remedy which is generally accepted is the grinding of the gears by one of the now well-known processes, by
means of which a perfect form as to teeth and smooth contacting surfaces are ensured. The work done by the Birmingham Gear Grinding Co., Ltd., of Birmingham, is, we believe; giving .great satisfaction in this respect, but there are other makers also giving close attention to this matter, and most modern motor vehicles have their gears properly ground.
Another factor that tends towards silence is stiffness of the shafts and the closeness of the bearings to the most important gears. In many of the newest typeS of gearbox the diameter of the shafts has been greatly increased, thus giving the required rigidity.
The provision of bearings in the middle of the shafts, although practised many years ago by some makers, is now introduced by others as a novelty.The effect produced by the extra stability thus afforded is most marked.
Attempts have been made to silence the pinions by forming internal ribs in the gearbox casting, the idea being to raise the pitch of the note produced by the vibration until it rises above the limit of human audibility. Another idea which has been tried out is the avoidance of flat walls for the gearbox.
The writer has investigated results in both these schemes, and has come to the conclusion that they, have no effect in the deadening of sounds produced by imperfect gears. Some form of third speed, in Which an internal gear is employed, such as is used in the Graham-Paige, may eventually prove to be a solution of the problem.