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Maintenance Engineers
Developments in Design Have Increased Complication and Brought Greater Problems to the Engineer
WITH the title "The Problems of the Mannfacturer: and the Maintenance Engineer," an interesting paper was read at Glasgow by Mr. Sidney Wightman,' •MIA.E. (Albion Motors, Ltd.), last Friday. The occasion was the first meeting and inaugural lecture of the Institute of Road Transpoit Engineers in Scotland.
Surveying design in relation to maintenance, Mr. Wightman said that weight restriction's, increased engine outputs, improved ,transmissions "and forward-control :layouts had all been responsible for the " drowning of units in a sea of bits and pieces:" ItitpCiiiance Of 'Torque On the subject of power units, he discussed the variety of types and went on to theoretical considerations. Whilst b.h.p. and torque values were generally understood, it was not always appreciated how they could be assessed to interpret performance. They showed. why an engine having a high maximum b.h.p. was not necessarily a more powerful engine than one developing a lower b.h.p. Maximum torque was the figure required to determine the maximum gradient that could be negotiated.
Brake mean effective pressure was the mean or average pressure on the piston over the whole stroke, being stated in lb. per sq. in. This allowed comparison of the relative performances of engines with different piston sizes. Thus, if one engine gave a' higher b.m.e.p. than another, it had a better performance, although not necessarily higher horse-power.
A good modern petrol engine with a compression ratio of 7 to I developed about 120 b.m.e.p., whereas an earlier engine of 5.5 to 1 compression ratio developed about 95 b.m.e.p.
Thrust In Terms of Gradient After explaining how thrust, ox tractive effort, was calculated, Mr. Wightman showed how it could be interpreted' in terms of gradient.
For example, in bottom gear:—
Max. torque = 4,500 in./lb.
First gear = 6.5 to 1 Rear-axle ratio = 7.5 to 1 Loaded tyre radius = 18.8 ins.
Drive efficiency = 85 per cent.
Gross weight = 19 tons 4,500 x 6.5 x 7.5 x .85 Tractive effort = 18.8 x 19 = 522 lb./ton This figure could be converted to maximum gradient (after subtracting 60 lb./ton tractive resistance) by simple division, thus:— , 2,240 lb.
. – 4,85 or 1 ina4.85 462 lb. .
4,500 x 7.5 x .95 30 = 70 lb,/ton' 18.8 x 19 This was equivalent to a maximum gradient of 1 in 32. A transmission efficiency of 95 per cent. and a tractive resistance of 30 lb./ton were assumed in this case.
Quoting an instance of an engine with a power peak at 1,800 r.p.m. and a ' torque peak at 1,000 r.p.m., he assumed a difference in b.h.p. of 50 per cent. Why could the higher speed not be used to climb a hill, and what use was the high-speed power? The answer simply was that the power curve showed the rate of doing work, but when it came to sheer thrust, the rate at which the thrust was applied was of no concern, and it was only the maximum tractive effort that counted.
Clutch and Gearbox Turning to the transmission, Mr. Wightman said that the clutch should transmit torque with perfect certainty. The movement of the clutch disc between the pressure plates when under load was not always detected.
The gearbox could not be mentioned without touching on the topic of whether it should be of the crash type, synchromesh, epicyclic, etc. Aspects centred on freedom from breakdown,
ease. of servicing and repair, price and supply of replacements, and length of time between overhauls.
The low speed range of the oil engine called for an increase in the number of ratios, and auxiliary over drives or five-speed gearboxes were required. The modern method of gear lubrication by a pump-and-circulating system`Was an improifement on the old. one of hoping to force oil from the teeth towards the shaft-centre throne' holes drilled radially in the gears. In such cases there was a speed at which the centrifugal action overcame the pressure caused by the trapping of the oil in the teeth and above this speed the bearing was starved. The final drive to the road wheels necessitated a change in axis of the
transmission and was done either by worm or bevel gearing. Opportunity was taken to reduce the shaft speed at this point, and it was, perhaps, the fact that a larger reduction could be obtained by worm gear than bevel for a, given size of wheel, that had made it so popu lar in the heavy class. r
With regard to brakes, Mr. Wightman suggested that automatic adjustment was what all maintenance engineers dreamed of, but brakes refused to behave in a proper automatic sense. What was often thought More desirable was to have brakes of useful efficiency which maintained their performance over reasonable periods between adjustment and were easily and positively adjusted when required.
The testing of a vehicle's brakes was a point which gave the maintenance staff some concern. It was desirable, on test, to have the vehicle loaded to its rated capacity to ensure that not _O n 1 y did the brakes have to do the maximum work, but that the correct tyre-to-road adhesion was obtained.
Optimum Braking Undesirable
The term "brake efficiency" was commonly used, and as engineers always aimed at 100 per cent., it was sometimes thought desirable to have brakes of this efficiency. A little thought would, however, show that it was not wanted for heavy lorries or passenger vehicles. because of the effect on loads and passengers. Mr. Wightman went into details of the subject of road adhesion, mentioning that a clean tar or concrete road with a rough surface might give a figure of 85 per cent. adhesion, but a dirty road might give a figure as low as 15 per cent.
In practice, he said. 331 per cent. braking efficiency gave a good, safe brake for a fully loaded vehicle, and resulted in a stopping distance of 40 ft. from 20 m.p.h. or 90 ft. from 30 m.p.h.
Engineers Can Help Manufacturers
On miintenance in general, the speaker said that a good set of records should be kept by users to ensure effi
ciency. Where a small number of vehicles was concerned, the history of all units was easily kept, but Correct mileage figures must be obtained. Speedometers were often left unrepaired after failure.
For large concerns the task of keeping a tally of all units was a big job, and called for a good system of cross-references or card-indexing. When such was established, the compilation. became automatic.
Sometimes, however, individual parts Were changed over and their identity lost. If they were given serial numbers, their history could easily be logged. Operators having accurate and exhaustive' records could help Manufacturers by passing on valuable informa tion. . „.
In the animated discussion 'which followed, it' was suggested that maintenance .manuals should give illustrations and dimensions of any special tools required.
Answering query, the lecturer said that he considered a third differential necessary between dual-driven rear axles,436eitiie otherwise any tyre difference thieW' great stresses on the shafts.
A coloured guest referred to the danger of. tyre over-inflation in the Tropics, and a plea was made for a safety blow-off valve.