TECHNITOP1CS By P. A. C. Brockington, AMI Mech E
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Musical Trouble shooting
Midland consultant pinpoints source of gear noise by tuning in with a harmonica WHEN Dr. Henry E. Merritt was seen VI' riding in a Daimler Fleetline doubledecker playing a harmonica he was not, as supposed by some, paying his way with a tune. He was in, fact being employed as a consultant by the Daimler company on an exercise to reduce the gear noise of the company's doubledeckers in the interests of passenger comfort. In this case, a noise analysis was being made of the gear train between the engine and back axle and he was able to diagnose the sources of the noise at various frequencies.
Dr. Merritt is a consultant gear specialist of Merritt and Company (Engineering) Ltd., Claverdon, Warwicks, and emphasizes that normally gear noise is only objectionable if it offends the ear. It is, however, a relatively serious fault if produced by the transmission of a bus, coach or private car, and with the threat of regulations that will prescribe maximum noise-level limits and with the greater attention being paid to the comfort of lorry drivers, the problem is one that the makers of goods-vehicle chassis will have to probe more thoroughly in the coming years.
For some years up to 1937, Dr. Merritt was chief engineer of the David Brown company: from 1937 to 1940 he was director of tank ' design in the Ministry of Supply and later became technical director of the David Brown Tractor Company. As a consultant he is now employed by about 20 manufacturers of products which include machine tools as well as vehicles.
The harmonica currently used by Dr. Merritt is a Hohner Melodica Soprano and is particularly suitable for pinpointing the source of gear noise because of its loudness potential. Its two-octave range covers notes of 256 c.p.s. (middle C) up to 1,024 c.p.s., semi-tone keys providing variations of 6 per cent.
Dr. Merritt's technique is simple; the vehicle is driven at steady speeds on a gradient in both directions and, by trial and error, the note on the harmonica is found that corresponds in frequency to the meshing oscillations causing the offending noise—the vehicle being driven in top gear for tests of driving-axle trains. Reference to a table then reveals which pair of gears is the cause of the noise, the oscillations being proportional to speed.
Whilst the noise of a pair of gears that is located at a considerable distance from other pairs can generally be attributed to the correct source without an instrument, noise origins can be extremely elusive, and Dr. Merritt cites the example of a back-axle noise that appeared to emanate from the steering column. The harmonica technique has been applied to all types of gearbox and to driving axles in which more than one pair of gears is employed. More sophisticated means are available to determine the source of noise but the instruments are not readily portable and, of more serious importance, the complex information they provide is difficult to interpret.
Because the note produced by a noisy pair of gears is a function of road speed, the harmonica may be tuned to the note at differing speeds as a means of doublechecking the results. If, for example, the note at 20 m.p.h. is Middle C and at 30 m.p.h. it is G on the scale, it can be concluded that the noise is created by the same pair in both cases, the frequency in c.p.s. of the second note being 50 per cent greater than the c.p.s. of the first note. Normally the noise is a function of a tooth-form characteristic but may be caused by resonance of some part of the casing. Resonance can be detected with the harmonica but often presents a particularly difficult problem because of the complexity of the source. Although in a typical case the problem can be overcome by stiffening the casing or changing its physical features, this may be very costly, and the alternative but far from simple expedient of modifying the tooth form to obviate resonance excitation is generally preferable.
In Dr. Merritt's opinion, gear research seriously lags behind the research applied to power units, despite a national expenditure on gears of all types of around £100 m. a year. Although gear research is being conducted by universities and other independent bodies, their work is largely wasted because they are not sufficiently in touch with the requirements of the industry or of users. There is an urgent need, he considers, for a centralized establishment with appropriate resources that would cater for the needs of the entire industry.
Both the weight and the cost of gear units could, Dr. Merritt claims, be reduced substantially if modern research techniques were applied to evolving improved types of case-hardened steels and of gear forms that offered lower .stresses. More advanced research methods should also be applied, in Dr. Merritt's view, to providing greater accuracy in production processes. Although computers could be used more extensively for all these objectives, notably with regard to optimizing .design features and details, use of a computer would not by-pass routine testing.
Lubrication and Stressing Dealing with lubrication in relation to stressing, Dr. Merritt points out that gear teeth can withstand higher loading at higher speeds because of the enhanced wedge action of the oil, despite the fact that contact loading increases with speed. No lubrication advantage could be obtained at any speed by increasing the amount of oil fed to the gear teeth with the aid of an external supply.
As a means of reducing the cost and weight of the transmission, Dr. Merritt favours the relatively high-speed diesel engine, the weight of transmission units being proportional to input torque, and this preference gives support to the author's view that power-to-weight ratios should be related to engine-transmission combinations rather than the engine as a separate entity.
Is this practical? If it is, the gear industry should take the lead.