M.P.T.A. Recommendations for Improved Bus Design
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T HE views and experience of six maintenance engineers of. large • municipal transport undertakings are summarized in a .memorandum entitled "Motorbus Design and Construction.which is available at 4s. from the Municipal Passenger Trans-. port Association, Aldwych House, Aldwych, London, W.G2. The memorandum, issued this week, outlines recommendations for improvements in oil-engined buses, ,which have been made from experience gained in the operation of the fleets Of 95 under
takings. • •
Cooling Systems' Poor It is suggested that insufficient attention has been given to engine cooling as •a whole. All cooling systems can give trouble . because of insufficient capacity for summer conditions, unreliable thermostats and unsatisfactory blanking arrangements in cold weather. Recommendations for improvements include, larger-capacity radiators and automatic shutter controls for winter running. As an interim. move, some approved method of cOvering the radiator is held to be necessary to maintain. engine temperature and to prevent freezing-at the base
of the tube stack. : • A 10-year life is said to be desirable in a radiator, and the life of present radiators is too short, as the materials used corrode nnder. the varietiei of water encountered throughput the country. Experience has shown that aluminium alloys are not entirely satisfactory. Non-corrodible materials are more desirable, and construction should be sufficiently rigid to obviate leakage from the tubes, without being dependent upon the tube stack for that rigidity. • It is considered that provision should be made initially for the attachment of electric imnierson heaters to the bottom tanks of radiators. Drain cocks should be of larger bore for speedier emptying, and hose diameters should be standardized. Radiator-support brackets should be rigid and, where rubber packing is used, it should be in straight compression, and should have ample bearing area.
Liner Wear Excessive
M on obloc construction of the cylinder block is 'favoured, and the water inspection doors should be of corrosion-resistant material. Cylinderliner wear is alleged to be excessive, and it is necessary that liners should have higher wear-resisting properties, although no recommendation can yet be made as to whether ordinary or hardened liners should be used. No preference can be expressed for dry or wet liners, but it is suggested that in the absence of a satisfactory seal for the wet variety,' easily removable dry liners should be fitted initially, with means to secure them against movement.
Criticism is made that cylinder-head holding-down nuts or setscrews are not always accessible. Setscrews are held to be preferable, but more care is required in their positioning. Cylinder-head fracture is being experienced, and it is suggested that steps should be taken in design to prevent this fault. Material used for cylinder heads should be suchas will assist in repair by welding. Core plugs, which in their present form become loose in service, should be screwed in position. Renewable inlet and exhaust-valve inserts are desirable, but no recommendation can be made as to their composition.
' It is considered necessary that the crankcase should be Of cast iron, where the engine is not of monobloc construction. Should the sump be made from an aluminium alloy, a brass insert should be provided to carry, the drain plug, which should be of a large diameter.
Engine timing chains have insufficient life and require an undue amount of adjustment. Gear . drive, although theoretically better and preferable to maintain, is noisy by comparison with chains. The question whether manual or automatic chain adjustment should be provided is left undecided.
Good Breathing Needed
Emphasis is placed on the need for adequate provision for breathing of the engine, and fumes from the crankcase and timing case should be directed to the inlet manifold. Rigid mounting is required for fuel-injection pumps and exhauster motors, and they should be fitted independently, to dispense with the tandem system which is frequently 'employed. Fuel-pump couplings are usually a weak link in the drive and should be designed in a more robust fashion.
Few suggestions are made for improvements in engine components, except that water pumps should be made from corrosion-resistant materials, and seals should be of carbon composition in preference to soft packings. The oil dipstick should be more accessible and its guide should be bell-mouthed. To assist servicing staffs, the dipstick should be calibrated to show the amount of oil to be added, instead of indicating the quantity remaining in the sump.
Many difficulties of clutch maintenance arise from heat; and in this connection clutch cooling calls for improvement. If. a plate clutch be fitted, it should be of single-plate pattern, with a flexible centre. Frictionmaterial should be on the spinning plate, and a renewable driving plate should be secured to the flywheel.
With the present type of unit gearbox mounting, the design and lubrication of the clutch spigot bearing are considered-unsatisfactory, and improvement is needed to give a longer life to this component. Improvement is desired in the toggle mechanism, which in the present form gives inadequate life and calls for too frequent adjustment. It is observed that drivers tend to become careless in their use of a Fluid Flywheel and gear change in such a manner as to increase transmission shock, thus causing structural damage to the vehicle body. In some quarters it is considered thatwith a Fluid Flywheel, only a three-speed gearbox is required. The life of the epicyclic box is thought to be greater than that of a crash gearbox, but the overall economy is less than with the orthodox transmission. Although generally satisfactory, it is considered that there is still room for improvement in design to reduce breakages of gear bands and toObtain longer life from the bushes.'
Transmission Criticisms
No preference is expressed for any pattern of universal joint on propeller shafts, but with all-Metal joints the methods of lubrication are said to be unsatisfactory and require improvement. It is noted that with the increased loads and powers to be dealt with, the worm-pattern final-drive unit gives insufficient life, and could be enhanced by increasing the worm-wheel centre distance to not less than 8 ins., by improving lubrication and providing more adequate cooling.
The use of light alloys for wormwheel casings is regarded as unsatisfactory. No opinion is passed on the type of oil seal that should be fitted to wormshafts, although the bellows pattern is generally regarded as good. Of the two methods of adjusting races in the final-drive unit, . screwed-collar adjustment is preferred to shimming.
It is generally agreed that the pot type of rear-axle casing is satisfactory, because of its rigidity. Criticism is made of the inaccessibility of the securing nuts on the differential pot and of the poor breathing arrangements of the axle. Spring carrier brackets and brake anchor brackets would be preferred in one piece, and the use of ferrous metals in place of light alloy_for their construction is considered desirable.
Oil Seals
The use of spring-loaded oil seals in hubs is regarded as essential, although provision should be made so that oil passing the seals should be prevented from reaching the brake drums. External lubrication of wheel hubs by grease is not thought to be desirable.
Reference is made to the three types Of steering gear—screw-and-nut, wormand-sector and cam-and-roller. Wormand-sector steering has proved satisfactory, against the general experience of stiff steering with the wormand-nut pattern and over sensitivity of the cam-and-roller type. Attachment of the steering box to the top flange of the chassis frame is regarded as bad practice, because it gives insufficient rigidity, and attachment to the dumb iron or cross-member is preferred. Arrangements should be made for easy dismantling of the steering-box unit from its mounting.
All mechanical and electrical units of the chassis frame are discussed in the memorandum, which concludes with observations on conditions in the driver's cab as regards control.