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The Value of Accelerated Vehicle Testing
Page 139
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APPROVAL for the system of taxing commercial vehicles on. unladen weight was expressed by Mr. John Alden, A.M.I.Mech.E., commercial vehicle experimental engineer of Vauxhall Motors, Ltd., in a paper on vehicle test procedure which he read before members of the Automobile Division of the Institution of Mechanical Engineers_in London on Tuesday.
In 1949, he said, over 80 per cent, of the goods vehicles being operated came into the 3-ton-unladen class. Taxation on unladen weight caused great emphasis to be placed on the weight of components, and it was largely true that British designers of 30 m.p.h. vehicles had produced types that weighed much less, for a given payload, than contemporary foreign machines.
This opening led Mr. Alden to the main theme of his paper, which was the accelerated testing of chassis. Whilst it was not yet possible greatly to accelerate the durability test on the road of the moving parts of such components as engines, gearboxes and rear axles, it was possible to obtain valuable experience quickly by bench and rig testing. The time taken to test the large number of parts Subject to fatigue through road shocks could be greatly reduced by running vehicles on a rougb track.
Road Work Essential
Mr. Alden emphasized, however, that lest vehicles would still have to run on the road, as this was the only known method of ass-ssing, under working conditions, engine life, brake-facing wear and tyre life. Life testing was therefore effected in three ways—rough track, duration on the road and bench and rig tests.
It was easy, he said, to wreck components if one set out with that idea. Care had to be taken to ensure that accelerated tests were not too severe and that the amplitude and frequency of the deflections nroduced did not give misleading results.
Rough-track running was ideal for testing parts subiect to repeatd. stressing caused by road shocks. These included cab and body and sheet-metal parts, steering, susnension and axle units, engine mountings, frame assemblies, fixines of lamps and batteries, gearbox mountings and propeller-shaft sunnorts.
During the 1Q19.45 war, vehicles operating in the Libyan Desert suffered failmes mostly of road snrings and it was decided that tests should be carried out under conditions as similar as nossible to those under which failure
' occurred. 8vqntually a test course at Hungry Hill, near Farnham, was located. This had been built originally for testing tanks and the surface of the test track consisted of a series of granite blocks 9 ins, high, set in a concrete base.
Some vehicles were driven over this course, but the net effect was extreme, as it was only just possible for the driver to maintain control. This test track was eventually filled in with gravel and sand until the blocks protruded only 2-3 ins.
Under these conditions front springs lasted for about 250 miles, the type and location of the fracture being identical with that occurring in Army service. An intensive development programme was instituted and as it was possible to obtain up to 100 miles' running per day, ihe comparative lives of many different designs of road spring were known in a few weeks.
Practical Results
The final design gave over 2,000 miles' trouble-free running on the' test track, and the new springs were in production within five months of the original complaint being received.
Road springs were taken as an example of rough-track development to show the initiation of this particular type of testing. Experience gained on the vehicle as a whole was found to be invaluable, and tests at Farnham were continued for many years as part of the general development of the many different types of military wheeled vehicle produced by Vauxhall Motors, Ltd.
A temporary rough track built by the company was located in a disused gravel pit. (It was described and illustrated in "The Commercial Motor" on April 20, 1951.] The blocks used varied in height and none exceeded 2 ins. They were cast in pairs and were made of a special concrete mix reinforced with steel mesh. There were wide, flat verges, so that should control over the vehicle be lost, plenty of manceuvring space would be available.
With a block spacing of just over 2 ft. with the special block arrangement chosen, the road wheels received between 500 and 600 impacts per minute. All vehicles operating on the rough track were run at the maximum gross vehicle weight for which they were designed.
The target aimed at was 1.000 roughtrack miles for the vehicle as a whole, but all axle and steering parts should survive at least 2.000 rough miles. It was believed, said Mr. Alden. that for many components the 1.000 rough
track-target mileage was equal to 100.000 miles' running over hardtopped roads in this country.
One of the worst surfaces a wheeled vehicle could encounter was the washboard dirt or gravel road found in West Africa. Under suchconditions the ratio between road mileage and roughtrack mileage could fall as low as 10 to I.
On duration test, 25 per cent, of the total mileage was run with the vehicle unladen This, said Mr. Alden, was a comparatively recent innovation, all duration test running having formerly be_ I carried out with fully laden vehicles.
The procedure was changed because some failures which occurred in service were with unladen vehicles running over " rippled " roads. A rippled road, he remarked, had ridges of much finer pitch than a washboard surface, and the ripples were much shallower. Such a road could give rise to serious trouble.
Type of Load
A body suffered most when carrying a loose Or flexible load: with solid loads the conditions to produce high stresses in the frame did not occur unless the vehicle was travelling over very uneven surfaces.
• It was believed that the average brake-facing life of 16,000 miles under particular test cbriclitions, and taking one particular vehicle as an example was between 30 and 50 per cent, of the -average life of brake facings on a similar vehicle used in normal service.
It had been possible to compare the life of other components in the field with that obtained on the 95-mile test route through the eastern Chilterns, and it was considered that the severity factor of the road test was approximately two, or that 50,000 Miles on the test route. running 75 per cent, of the time loaded to the maximum gross vehicle weight. was equal to 100,000 miles in normal service.
The average speed for a day's running varied between 22 m.p.h. and 28 m.p.h. and the mileage completed in any shift was between 200 and. 250. Given reasonable weather, and the absence of major breakdowns, the total weekly mileage would be about 2,000 per vehicle, so that it was possible to obtain 100,000 miles running in a year. For various reasons, however, the practical annual mileage was something between 30.000 and 70,000.
Tipping vehicles were operated in collaboration with a number of local contractors on the kind of Work for which they were designed. Tippers did this continuously during the day, operating empty for over 60 per cent. of the time. At night the same vehicles ran fully laden over the test route. Under these conditions the tipping gear was operated on the average 12 times a day. .
Mr. Alden declared that if failures identical to those which were liable to occur in service could be produced, then the test rig provided the best test of all, because the prevailing conditions could be controlled to absolute uniformity.