road and
Page 50
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workshop by Handyman
Noises off and what they mean
THE DAYS are gone when a transport driver who heard an unfamiliar sound from his vehicle could coast for a few yards in order to make up his mind whether it was safe or serious. Now whether he is inside his cab or if he halts and gets out he is bombarded by noise that gives him little chance even to hear a new sound, and when he does, the roar of the passing traffic may defeat his attempts to arrive at a reliable diagnosis. All he can do is to pull off the road straightaway and seek assistance; certainly he should avoid the temptation to run on to some official pull-off.
Even a mechanic finds it difficult to determine the point of origin of many an unusual sound, yet it is most important for him to pinpoint its general area — and if possible the cause of the noise — before the vehicle is driven on, or towed.
It is so easy for the driver to be misled by road and tyre noise; new sections of road surfaCe have often led a man into falsely believing that he had serious transmission trouble. Rough concrete surfaces can give the impression of failing wheel bearings, only for this to disappear on running over smooth tarmac. Where there is clear evidence of some part of the vehicle becoming unduly noisy, it is better to be safe than sorry, and stop.
A driver may feel that his differential is failing, although the source of the noise may be elsewhere; where the inspecting mechanic has the room to manoeuvre in safety, noise in the rear end can be detected and located by running the vehicle slowly, first in a straight line, and then making a slow turn. in each direction. Obviously, the mechanic will need to be near each hub in turn as it revolves but if there is trouble in a hub bearing, the sharp turn left or right will emphasize the noise on the side in question, and this is a guide as to whether the noise is from cliff or hubs. If there is any doubt at all, each wheel should be jacked up in turn and individually checked.
Noise from the differential crown wheel and pinion, due to excessive wear, bad meshing, or lack of lubricant will be more pronounced on light drive or over-run. Drive noise itself is heavier during acceleration, although through a particular speed range the noise can come in and out as the speed changes. The early stages of over-run noise can again be detected at certain periods or speeds, and the noise while travelling through this period can . become so pronounced as to amount to a distinct vibration at the worst point.
Certain makes of rear axle using crown wheel and pinion when running can give off a light noise at a constant speed which will disappear the moment engine speed is changed, and often no appreciable amount of wear can be found to account for this. However, a pinion bearing on the way out makes a harsh and constant noise, soon changing to a pronounced vibration felt throughout the chassis. Wear in front wheel bearings in usually noted before any appreciable level of noise develops and as a rule the driver is soon conscious of roughness from the front end — this can often be felt up into the steering.
If there is any appreciable degree of slackness at a bearing, there will be a tendency for some brake snatch as the vehicle is about to stop; this is usually worse under light braking. Worn propeller shaft joints produce their own particular vibration which if ignored soon develops into a rattle like a loose motorcycle exhaust by which time the end caps will have probably fallen out and the needle bearings disappeared.
Obviously, the wrongly aligned prop shaft coupling will be felt by the driver in a very few yards of running, as the flopping of the coupling and shaft at slow speed is unmistakable, so only on rare occasions does a vehicle with wrongly aligned couplings proceed very far unnoticed.
Common failing
A common failing with the universal joint is that the grease nipple leaks so that the grease is flung out as soon as put in, resulting in early wear of the needle bearings. Therefore the grease nipple, and, if fitted, the relief valve need regular inspection to avoid this loss of lubricant.
A fairly easy fault to recognize is a worn clutch withdrawal bearing, as this will give Out noise and roughness on the pedal where mechanically operated through linkage. However, if the clutch is air and or hydraulically operated one has to rely upon noise for diagnosis. Normally, a withdrawal race should last the life of the clutch plate without any trouble at all, especially if automatically lubricated. Bearing in mind that the life of any such race can be seriously shortened by the driver riding the clutch pedal, so causing the bearing to run continuously, instead of only when the clutch is needed. Preserving the withdrawal bearing also saves wear to the clutch stop. Instead of a driver staying in gear at traffic lights with the gear in and clutch held out, he should slip into neutral and back into gear again. Bad clutch clearance from neutral can indicate some abnormal use of the clutch mechanism, such as holding the vehicle in gear on a slope without brakes, making the withdrawal race and the clutch stop suffer in the process.