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BRITISH TRANSMISSIONS Offer Maximum Efficiency
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Says P. A. C. Brockington, A.M.1.Mech.E. All Systems Tend to Simplify Driuing and Improve Efficiency. Some Makers of .Medium-steight Vehicles Favour the Auxiliary Gearbox, VC/ hil3t Others Prefer . the Two-speed Axle IF the policy of feather-bedding the driver were to be approved in the commercial-vehicle held in the same way as it has been accepted in the design offices of the private-car manufacturers, any review of transmission principles would be concentrated on the methods proposed to beat the Americans at their own game in the production of fool-proof systems. Fortunately, the commercial-vehicle driver is a highly skilled operator, and it should be unnecessary in the foreseeable future to complicate the transmission unless it is done for improved efficiency or to overcome " reasonable" driving difficulties associated with urban bus operation.
Variations in the routes over which a vehicle may be driven are virtually infinite, and for this reason operator opinion on the advantages of any transmission type is often contradictory. Expert technical opinion is equally diverse.
There have been no torque-convertor transmissions exhibited post-war in Britain, and it would seem improbable that such a system could become generally acceptable pending a large increase in power-to-weight ratios. .
The Hobbs epicyclic gearbox featuring hydraulically operatedclutches and reaction brakes, as fitted to the Dennis Dominant chassis, has already proved its efficiency and operator experience will be watched with
E4 interest. Fully automatic control with an over-riding
device giving manual gear changing when required would appear possible. The air-operated box, with governor control, developed by the Self Changing Gear Company for use with a fluid coupling has been operating successfully in Sweden for over six months. By changing a set of cam plates, the response of the control to torque-speed requirements can be varied to provide any desired characteristic, and manual operation is available at the will of the driver.
The performance of a heavy load-carrying vehicle is more nearly an exact function of power output and transmission efficiency than the performance of a vehicle in any other class. The power-to-weight ratio is low, and because speed is comparatively unaffected by traffic conditions, road bends and surface irregularities, the best use of engine power is almost continuously dependent upon the availability of a suitable ratio.
The Foden eight-wheeled overseas chassis, powered by a Gardner 8LW engine developing 150 b.h.p., has a constant-mesh four-speed gearbox with a two-speed cpicyclic unit attached. The important feature of the auxiliary box is the use of a change lever operated by a throw-over toggle spring which is in effect a preselector mechanism. The lever actuates a sleeve, splined to the output shaft, with external clutch teeth cut on the forward end which mesh with the internal teeth of the sun-wheel extension to give direct drive and with the rear end of the planet cage to provide a reduction of 3.29 to I. When the driver throws over the lever the spring tension is insufficient to move the sleeve until the load is released by synchronization.
The overseas chassis also features double-reduction driving hubs which may be adjusted to give direct drive. The cpicyclic gear incorporates a sliding member, splined to the driving shaft, which meshes internally with the input bevel pinion to provide a reduction or with the hub casing for straight drive. The change-over is by means of a trunnion projecting from the end cap, which is turned through 180 degrees with a tommy bar. A trend on the spacing of ratios and the optimum number of gears required is shown in the David Brown design. The concern's live-speed box, developed for vehicles of 15 tons capacity, provides geometrically progressive ratios with similar steps
between each With this arrangement, the speed range of the engine when accelerating through the gears varies between the maximum-torque speed and the maximum speed on the engine characteristic curve. According to the designers, closer ratios between the higher gears are not advantageous for heavy vehicles because of the deterioration of performance in the lower gears.
The gearbox is of the constant
mesh type with helical gears. The aluminium casing has. a horizontal joint through the shaft centres, and an intermediate wall supports a centre bearing. This increases rigidity, reduces resonance and obviates the use of a spigot bearing It also eliminates the need to undercut the splines to prevent slipping out of mesh.
Simplicity and robust construction are outstanding characteristics of the Albion gearboxes. The absence of helical gears obviates end thrust, and grinding the straight-spur teeth to a high standard of accuracy ensures quiet operation The dog-clutch engagement of the constant-mesh gears gives light gear changing without introducing complications and is considered preferable to synchromesh.
Because the provision of an overdrive ratio encourages its use for normal running in preference to direct drive and reduces efficiency, a direct top with a high final-drive ratio is employed. Comparatively heavy transmission shafting is required to withstand the increased torque, but this is not considered a serious disadvantage.
The Crossley designers maintain that a gearbox for universal service, capable of withstanding a great deal of abuse, must be based on orthodox practice. The Crossley -five-speed box with overdrive top, which is particularly suitable for high-speed coaches, has constant-mesh gears mounted on double rows of needle roller bearings, and both the main and layshafts are supported on intermediate bearings. The primary and fifth gear trains are of helical form, and the remaining gears-are straight spur with a 20-degree pressure angle. The aluminium casing has a wall thickness of 1 in. and is amply ribbed. Sound radiation has been reduced to a minimum by the elimination of large, flat surfaces The avoidance of appreciable torque interruption is of increased importance when a vehicle is continually operating off the road on rough or muddy ground The transmission of the Aveling-Barford 9-cubic-yd. shuttle dumper features cone engagement of the constant-mesh trains, controlled hydraulically and interlocked hydraulically with the main clutch to prevent overloading with engine torque. A similar quick change is obtained in the self-changing gear air-operated box by overlapping the brake-band operations, and this has been found invaluable when applied to Canadian logging trucks with power units of 200 to 300 b.h.p The main clutch is of the friction type.
The principle of the baulking ring, which literally makes the conventional type of gearbox , crashproof, is new to most British commercialvehicle users, and it should be equally appreciated by drivers negotiating heavy or hilly ground in tippers and so on and, by drivers of coaches who value private-car ease of control. The fourspeed baulking-ring synchromesh box, fitted to the big Bedford 7-tonner and to the 11to 5-ton models in the Bedford range, is, therefore, of widespread interest because its application is spread ovel a large variety of load-carrying and passenger vehicles.
In the top and third gears an extension of the meshed gear is coned externally and mates with an internally coned ring gear rotating with the engagement-sleeve clutch mechanism. The drive is transmitted to the ring gear by keys which are allowed a limited free movement in slots cut in the cone shoulder.
When the load is first applied to the cones with the initial movement of the gear lever, the keys are held offset in the slots by the torque transmitted to the spinning gears, and the external teeth of the cone are held in line with the internal clutch teeth with which, in this position, they cannot engage. Upon synchronization, the keys return to the centre, and the clutch is free to slide over the ring and to engage with the meshedgear extension. It is impossible to force the change.
The second-speed synchronizirig cone is housed inside the first-speed sliding gear, and a synchronizing drive and lock are provided by two long tongues projecting into shouldered apertures in the second-speed gear. This also gives full baulking-ring synchronization, and the compactness which the arrangement affords is an important factor in the robust construction and reduced E6 size of the unit. Particularly quiet operation has been achieved. .
Although the design of the Scammelf six-speed gearbox for cross-country _vehicles has riot been modified for 15 years, the method of gear compounding is still a good example of modern practice. Only eight gear wheels are employed for six indirect ratios, the four pairs being first used in sequence, and then compounded to give two extra low gears. The method of lubrication is also noteworthy, in that deep immersion and churning are avoided and thorough wetting of the rubbing surfaces is assured by a pump and sprayer system.
Comparing the advantages of the auxiliary gearbox and the two-speed axle, as applied to similar vehicles, it can be claimed that the use of the former should give a lower unsprung weight and that the latter gains by virtue of the lower torque transmitted by the propeller shafts. Both enable the gear range of a standard box to be increased without re-designing the existing unit, and whilst some operators claim that a five-speed gearbox offers better " progression" than a four-speed box with either auxiliary box or two-speed axle, the usefulness of the available gears is partly dependent upon the method of control.
The Morris-Commercial two-speed auxiliary box, of the layshaft type, is used in conjunction with a fourspeed constant-mesh main box. If an overdrive is required it is installed "back-to-front," and this may be done without complication of the drive or controls.
An all-important advantage of the Eaton two-speed axle is the simple push-button cbntrol actuating the vacuum throw-over mechanism, which affords the use of all gears in the range progressively. As an alternative, an electric control is available which, acting on a torsion spring, causes the clutches to be actuated when the load is released by the driver closing the throttle or withdrawing the engine clutch The weight of the gear-change selector gear on an underfloor-engined chassis tends to increase the manual effort required, and to offset this disadvantage Leyland Motors offers, as an optional fitting, a Clayton Dewandre
!rvo in the linkage of the standard four-speed ,nchromesh gearbox of the Royal Tiger and Olympic. is mounted on the casing, and the movement of the :ar lever opens one of two distributor valves that mtrol the passage of air from the brake reservoir to the auble-ended actuating cylinder. After engagement, the istributor valve is closed and the cylinder pressure released. Should the air-pressure supply fail, the gear tange can still be operated in the normal way.
As an alternative to air operation of the epicyclic 3x, the Daimler company offers a hydraulic servo perated by the Lockheed continuous-flow accumulator rake system. The latest Daimler unit has live speeds id the number of plates in the top gear clutch has been [creased from 10 to 12.
Whilst the benefits of the fluid coupling for certain 'pes of bus work are well established, the design of the ry-plate clutch has undoubtedly reached such a high ate of development that only a revolution in transtission systems could cause its decline.
The claims for the Fraser hydraulic dutch, featuring :ciprocating radial pistons, cannot be overlooked. A mtrollable amount of slip is provided and tests have town that when used with a conventional gearbox in
56-Seater double-decker bus the top-gear speed may be !duced to 4 m.p.h. The drive is smooth at this speed, Id two ratios are sufficient for normal service. During 4,400-mile test run the saving in fuel was 0.5 m.p.g. ver the average obtained when the vehicle was fitted ith a friction clutch.
Final-drive developments are as important as new :arbox features, and whilst the production of the irkstall Forge double-reduction back axle for the Thornycroft Mighty Antar represented a .specialized design, the principles employed could precipitate a general trend in the heavy vehicle field. The first reduction is by worm gearing and a planetary system is employed for the second reduction, which is compounded in such a way that differential action is obtained without a separate differential unit.
Four interacting members are used in place of the normal three, one member of each of the two trains being combined on a common shaft. The road-wheel drive may be taken from the member with the higher or lower torque, and it is possible so to mount these members that they are interchangeable, thus giving a choice of two ratios.
The advantages of the hypoid axle are not sometimes appreciated by those who recall its original application to private cars for the purpose of lowering the floor height. It is the practice in. Moss axles to employ a hypoid crown wheel and pinion which are developed with a spiral angle of 45 to 50 degrees as compared with the former spiral-bevel angle of 30 to 38 degrees. With an increase in the pitch-circle diameter the teeth are
much more robust, and there is also a considerable increase in the pitch-line diameter and in the diameter of the shank at the rear of the pinion head. Consequently, larger bearings may be fitted at the front Of the pinion and a spigot bearing at the rear
The advantages of worm drive are supported by the Albion engineers, who emphasize that it is not so sensitive to exact setting as the bevel and hypoid drives and that it does not require a special lubricant.