SIX APPR( MAINT LCHES TO TANCE
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A Careful Investigati Procedure Follow& Fleet Owners Shows Commendable Ito the Maintenance. a Number of Big erence to a Pattern ; Thoroughness
DURING the past few months I have been investigating closely the Maintenance methods of various public service vehicle operators. In particular, I have studied the systems employed by four municipal undertakings and two companies represented
on the P.S.V. Maintenance Committee, West Midland Area, which meets regularly to discuss servicing, over haul problems and operational matters. The operators compare . notes on the , best measures to promote economy, and talks are given by established authorities on technical subjects. The differences in fleet size, route conditions and services provided add to the usefulness of the discussions.
Of all engine-maintenance jobs, those relating to the cylinders and pistons are perhaps of the most interest. The different methods which may be used to prolong the life of the parts until complete reconditioning becomes essential are varied, and the subject of controversy. Docking periods are often determined by pistonor block-overhaul requirements, and the servic ing of other components is organized accordingly.
A Safety-first Policy
When, for other reasons, a. vehicle must be docked at a certain mileage or date, the block may be stripped for attention to obviate trouble before the next dock. A particular type of overhaul may, therefore, be evaluated in terms of the extra mileage the engine is enabled to run between docks, or it may be judged economic if the cost be low and the additional useful life of the parts be sufficient for a predetermined but limited mileage. .
Route conditions, average speed, stopping frequency, traffic congestion, gradients and so on are variables which seriously affect engine wear, and comparisons between the maintenance methods of different operators should not, therefore, be based on major-overhaul mileages. The mileages between intermediate overhauls as percentages of the total distance covered before dismantling, and the factors which decide the type of overhaul, may be of far greater significance.
When the pistons of the Coventry Corporation bus BS engines arc removed for examination at the " heavydock " stage of 25,000 miles, the rings are renewed as necessary, the determining factors being oil consumption and ring wear. If the rings are replaced, the wear ridges in the bores are smoothed-out with a hand hone. At the 50,000-mile dock, the cylinder block is exchanged for one which has been resleeved to the original bore size and the same pistons are refitted after the grooves have been enlarged to take oversize rings. Chromium-plated top rings are always used.
The engines of the 253 Daimlers in the fleet of 291 double-deckers and 16 single-deckers comprise Gardner 51,W, Daimler 8.6-litre and A.E.C. 7.7-litre units. Other vehicle makes include 39 Guys, 9 Maudslays, 4 A.E.C., a Crossley and a Bristol. Recent additions are 20 Daimler CVD6 double-deckers, the first of 40 to be delivered.
Turning Cylinder Liners All engines are overhauled according to the same " schedule. An interesting practice relating to the Daimler and A.E.C. engines having pre-finished, slip-fit liners, is the removal of the sleeves at 25,000 miles and turning them through a 90-degree angle before replacement. In this way a more even wear pattern is obtained, blow-by is reduced and oil consumption improved. The maximum sleeve wear allowed h 0.015 in.
Replacement pistons with standard rings are fitted at 75,000 miles after the resleeved block has been in service for 25,000 miles. The sumps of the Daimler CVD6 engines are fitted with inspection plates of sufficient size to allow the connecting rods to be dismantled and the main bearings to be examined without sump removal. This feature was originally stipulated by the corporation and was later standardized by the Daimler concern. All engines are stripped for a major overhaul after 125,000 miles.
Interesting piston experiments include the use, in a service engine, of Wellworthy Alfin pistons, which have bonded cast-iron ring belts. Results, so far, indicate that groove wear is lower than normal and several units are to be similarly fitted. In another experiment,
the top ring grooves were machined out and fitted with Roebuck Armouring rings of thestandard size in combination with side packing rings. The engine has operated satisfactorily, but has not yet been stripped for wear measurements.
The Gardner 5LW and 6LW engines of the Wolverhampton Corporation bus fleet, comprising 116 double-deckers and 10 single-deckers, normally operate without reboring or resleeving up to the major overhaul mileage of 120,000 miles. New rings are fitted at 40,000-mile intervals, chromium-plated top rings being stipulated as original equipment and for replacements. Bore wear is about 0.0035 in. at 40,000 miles and 0.017 in. at 120,000 miles. The bores are resleeved with Hepworth and Grandage H and G22 liners, and Hepolite pistons are the standard type for replacements; other makes of piston are, however, periodically tried.
Of the 74 Guy Arab and 52 Daimler chassis, all except two Daimlers are fitted with Gardner units, of which the majority is 6LWs. The only pre-war vehicles are three Daimlers.
Gardner Engine Performance
The Gardner engines of the 151 Guy Arab doubledecker chassis in the Walsall Corporation bus fleet of 205 vehicles, are not dismantled for decarbonizing or attention to the pistons until a mileage of 60,000 has been covered. The cylinders are then resleeved with Wellworthy liners and reconditioned Wellworthy pistons are fitted. The vehicles normally cover the remaining 90,000 mites of the major-overhaul mileage of 150,000 without further engine stripping. The Gardner 5LW and 6LW engines have to their credit a total of over 4m. miles, and individual engines have operated up to 350,000 miles.
None of the 21 Leyland PD2 chassis in the doubledecker fleet has yet covered more than 50,000 .miles, and most of the 32 single-deckers are due for replacement in the near future. The Leyland enginei are decarbonized and the valves reground after 30,000 miles running.
Engines of the Burton-on-Trent Corporation bus fleet are rebored to 0.030 in. oversize at the 50,000-mile overhaul and are resleeved to the original bore size after 100,000 miles. The cycle is repeated throughout the life of the unit, thenormal vehicle mileage before the unit is changed being 200,000. As an experiment, a small number of engines has been kept in service for much larger mileages and some remarkable results have been achieved. The pistons are returned to a specialist concern for reconditioning whenever possible. Cami faces and valve seats are Stellited.
Reliable Service Comes First The maintenance of the engines of luxury coaches engaged on tours and excursions is partly a function of seasonal opportunity, and when vehicles are operating a long way from their base, maintenance costs come secondary to a high standard of reliability. Of the 70 oil-engined vehicles in the fleet operated by Don Everall, Ltd., of Wolverhampton, 50 are coaches and 17 are double-deckers which are operated on contract-hire work, Makes include Daimler, Guy, A.E.C., Maudslay, Leyland and Tilling-Stevens, the Guys and some of the Daimlers being fitted with Gardner engines and the Tilling-Stevens with Meadows units. Many of the vehicles form recent additions to the fleet.
The pistons of the coach engines are removed at 40,000-50,000 miles and are fitted with new rings or replaced by reconditioned units according to the degree of groove wear. If the engine is using an excessive amount of oil prematurely, the bottom rings are
leplaced by Wellworthy Duallex scraper rings. The period between major overhauls varies between 75,000 and 140,000 miles according to the type of unit. The engines are bored and honed in the workshops at the first major overhaul with a borer made by Whatton and Sons Ltd., and a Buma honing machine; resleeving is performed at the second major overhaul.
Cylinder heads are checked for distortion on a flat plate, and are machined when necessary at intervals of 15,000 to 20,000 miles.. The causes of a number of "strange" faults have been revealed by this means, The engines are decarbonized and the valves ground-in at the same time, whilst connecting-rod alignment is also checked.
An average distance cf over 100,000 miles before decarbonizing or piston overhaul, is the record of the 7.58-litre and 8.6-litre engines of the 14 Crossley singledeckers in the fleet of 17 buses operated by G. Cooper and Son, Oakengates, Shropshire. The vehicles are engaged on mines-contract services, stage-carriage work and coach tours. A mileage in excess of 220,000 is often completed before an engine is removed for a major overhaul.
Until recently, new pistons were invariably fitted when an engine was decarbonized. The old pistons with new rings are now re-fitted, and results so far give promise
that the full period to the major overhaul will be covered without any operational disadvantage.
No trouble has been experienced from scoffing, ring sticking or groove wear. The average cylinder wear after 100,000 miles running is about 0.008 in.
Non-detergent or semi-detergent oil is used by the four Corporations mentioned; an anti-oxidation lubri cant is used by Coventry and Burton-on-Trent undertakings. Don Everall uses Vigzol detergent oil
S.A.E. 30 in summer and S.A.E. 20 in winter, and G. Cooper and Son uses Snowcern S.A.E. 30 detergent oil throughout the year; the oil is filtered for topping-up
purposes and a substantial saving is achieved.
All the Corporations filter the non-detergent oil in a Stream-Line plant, Wolverhampton and Coventry
adding the purified oil to the stock supply. Wolverhampton employs S.A.E. 25 in summer and S.A.E. 20 in winter, and is experimenting with S.A.E. 5 oil. The Burton-on-Trent summer lubricant is of S.A.E. 30 viscosity and the winter oil is S.A.E. 20. Experiments are being conducted with S.A.E. 15. Walsall, Coventry and. Don Everall Ltd., change oil at 5,000-mile intervals, Wolverhampton and G. Cooper and Son at 4,000 miles, and Burton-on-Trent at 10,000 miles.
The mileage run between major overhauls by the Coventry buses has been increased to 125,000 from 100,000 miles since the war, and this improvement is partly attributed to the type of oil used. The majoroverhaul mileage of 150,000 operated by the Walsall buses shows an increase of 20,000 miles compared with three years ago.
Crankshafts of the Burton-on-Trent bus engines are normally reground after 200,000 miles running. Walsall engineers report that crankshafts of Gardner engines are seldom worn more than 0.002 in. after 150,000 miles and often complete a further term of service before machining is required. Line-boring by a skilled fitter, using a Newton hand-cutting bar, is considered preferable to power-operated boring.
Mileages up to 360,000 are also obtained by Wolverhampton Corporation before crankshaft machining. In the Coventry workshops the crankshafts are ground to a standard undersize after 125,000 miles. The engines of the Don Everall fleet generally operate for 240,000 miles before crankshaft reconditioning, but the bearings are sometimes replaced at an earlier stage.
The injectors of the Coventry bus engines are serviced every 5,000 miles, and injector pumps are calibrated at 25,000-mile intervals, Of particular interest is a modification to the pump which is made when it is reconditioned at the major overhaul. The standard ball bearings are replaced by the Timken taper-roller type, and the results to-date are promising.
Injectors of the Wolverhampton engines are tested on a C.A.V. nozzle tester after 3,500 miles, and when necessary reground on a Merlin reconditioning machine. The pumps are not recalibrated until the 40,000-mile dock unless sub-normal engine performance can be traced to a pump fault. The pumps are overhauled and recalibrated in the workshops.
Walsall Corporation returns injectors to the makers for reconditioning after 15,000 miles, and injector pumps are replaced after 30,000 miles. The injectors of the -Burton-on-Trent buses are reconditioned at 12,500-mile intervals. When injectors have been reconditioned on a Merlin machine in the Don Everall workshops, they are tested on Armstrong equipment fitted inside an open-ended horizontal drum. There is a fan at the far end to extract the fumes, and a light is so placed that the sprays are clearly shown against
a black background. The injectors are serviced every 15,000-20,000 miles, and the pumps are dispatched for overhaul at stages of 50,000 to 60,000 miles.
The injector nozzles of the Crossley engines of G. Cooper and Son are serviced according to a strict routine every 10,000 miles, and the pumps are returned to the maker for reconditioning at 50,000-mile intervals.
Lack of garage space for the Coventry buses necessitates outside parking of the majority, and to overcome • starting difficulties all radiators are fitted with immersion heaters. Overnight heating in cold weather reduces fuel consumption and bore wear.
Immersion heaters are also fitted to the engines of vehicles operated • by Wolverhampton Corporation. Cock-controlled by-passes are fitted to the radiator thermostats in winter, and these continually pass a minimum amount of water through the core to prevent freezing.
Mobile Heater for Easy Starting
Cold-weather starting is facilitated in the Walsall garages by the use of a mobile electric heater, from which heated air, passing over an electric clement, is blown into the engine intake through a flexible pipe. The heater was designed and constructed in the operators' workshops. Trolley booms collect current from overhead wires carrying d.c.. and the equipment can be taken to any part of the garage. During the winter months the fan belts are removed, and this is con-' sidered to be beneficial with, regard to warming-up, fuel economy and performance. This measure was originally adopted to prevent freezing in the lower radiator tank when the vehicle was on the road.
Before .being fitted in the. chassis, all the Wolverhampton engines are motored for 8 hours and run under their own power for a further 8 hours on a test bed developed and built by engineers of the Corporation from old trolleybus components. A
202B5 compound-wound generator, rated at 63 kW, is wired to a circuit which includes resistances for heat dissipation. This generator is employed as a motor for turning the engine at 350 to 400 r.p.m., and as a generator it is capable of absorbing the maximum engine power at the rated speed. Whilst calculations of Output are not exact, the readings are accurate on a comparative basis, and the machine is a valuable piece of .test apparatus. It is not used to feed back current into the mains because of the electrical complications this would involve.
Ensuring Fuel Cleanliness At the completion of a major overhaul, engines in the Don -Everall workshops are run on a test bed for 12 hours, divided into 4-hour intervals, up to speeds of 1,200 r.p.m.
What is virtually a settling tank is employed in the fuel-storage system on the premises of G. Cooper and Son. The transfer pipe from the overhead storage tank to the service tanks is at some distarrce above the base, and' this reduces the amount of'sediment reaching the Zuichy filter in the delivery line. Freedom from injector-nozzle -and pump troubles, is partly attributed to the resultant improvement in fuel purity.
Part two of this article, which will be published next week, will deal with a number of interesting modifications which have been carried out on various makes of chassis. Such modifications have not only made for improved performance, but have materially lengthened the periods between servicing. Brake squeal can be an elusive trouble: an ingenious method of curing it will be described.