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Modern Transmission Developments

17th February 1950
Page 50
Page 50, 17th February 1950 — Modern Transmission Developments
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Which of the following most accurately describes the problem?

THE chief engineer of David Brown and Sons (Huddersfield). Ltd.. Mr. F. J. Everest, M.Sc.. M.I.Mecti.E., A.M.I.E.E., gave a lecture yesterday before the London members of the Institute of Road Transport Engineers.

Introducing the subject, the author said that the transmission problem is, broadly speaking, that of connecting the engine with the road wheels so as to provide the optimum operating conditions with the maximum mechanical reliability.

With an engine designed for good topgear performance, varying-ratio transmission is essential for acceleration, hillclimbing, and to avoid engine stalling at low speeds. For urban bus work, acceleration may be most important, whilst for heavy vehicles with low power/weight ratios this is a minor factor compared with high tractive effort when hill-climbing.

Laysbaft Gearboxes (a) Sliding gear and dog-clutch engagement: Used on commercial vehicles requiring simplicity, reliability, low first cost and maintenance. Improvements are replacement of sliding gears by dog clutches and constant-mesh pinions, and increase in ratios, five speeds being common.

A questionnaire issued a few years ago to 130 users of goods and passenger vehicles showed equal division on the advantages of synchromesh and other means for easier gear changing on passenger vehicles. : For goods vehicles the majority favoured constant-mesh pinions and dog-clutch engagement.

(b) Synchromesh: This development is extremely versatile, and latest designs leave little to be desired. The system is extensively applied to coaches and buses, but seldom to big goods vehicles where extra cost, weight and complication are not considered to be justified.

(c) Synchro-self-shifting gear change: Developed by the Hydraulic Coupling and Engineering Co., Ltd., this provides semi-automatic designs giving further ease in ehanaing. There is no clutch

B 1 6 pedal; changing is effected by helically splined dog clutches controlled by the accelerator pedal and a small selector lever on the steering. It is suitable for very heavy commercial vehicles, as driving torque continues during changing.

• R M. • Mr. Everest analyzed 0 9 18 27 36 45 54 63 72 8 planetary-type gearboxes M.P.H.

as:—

• (a) Multiple-band-brake operation: The planetary gear unit gained much popularity here when adopted by Daimler in conjunction with a hydraulic coupling. The Wilson gearbox, as designed by the Self-Changing Gear Co., Ltd., is the best example, giving easy gear changing in city bus work, where frequent and rapid acceleration is required. A further suitable application is developing in connection with very heavy vehicles required for export.

(b) Multi-plate-clutch operation: A performance similar to that of a Wilson box is given by the Hobbs transmission. Doubts as to the ability of the friction clutches to stand up have been set at rest by. tests. but according to the author, the design tends to be expensive and complicated.

(c) Operation by electro-magnetic clutches: The best-known unit is the Cotal. Performance is similar to that of the Wilson and Hobbs, but electro magnets are heavy and expensive. It is popular in France.

Epicyclic units were said to compare unfavourably with layshaft types as regards size, weight, efficiency, quietness, ease of maintenance end manufacture, and first cost.

The performance of torque converters has been inferior to gearboxes with or without hydraulic couplings, as regards acceleration and efficiency. and have not found much application here owing to our low power/weight ratios.

Over-speed gears are justified only on vehicles covering long distances lightly loaded or longer distances so heavily laden that top gear cannot normally be used.

Rear Axle Gear Units Axle gears follow car practice up to about 20,000 lb. gross weight. Above this, worm drives are numerous, with occasional spiral-bevel with doublereduction giving a ratio of 7-1 or lower. Second-reduction helical gears are embodied in differential units or as planetary trains in hubs. A development is to use a two-speed axle.

In the most rigid• designs of axle, deflections of +0.03 in. often occur between gears, and much skill is needed in cutting to allow for them.

Temperature rise in heavy loading often reaches 150 degrees F., and differential housings require ample oil capacity and external cooling area, particularly in worm drives where starting is frequent, as in trolleybuses.

In many transmissions rattling occurs at low engine speeds. The problem is that of matching the torsional characteristic of the whole transmission with that of the engine. It is often necessary to increase the flywheel,

introduce a flexible clutch centre or mount a vibration damper behind the gearbox.

In the commercial field layshaft gearboxes will continue for years, but all forward gears will be constant-mesh, and on large vehicles changing may be eased by synchronous devices. There may be auxiliary gears instead of twospeed axles: The possibilities of oil engines with constant-volume superefrarge, or gas turbines, must not be overlooked.

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People: F. J. Everest
Locations: London

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