HYDROSTATIC DRIVES ARE GAINING GROUND
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ALTHOUGH no direct claims were made for the advantages of hydrostatic transmission for standard types of road vehicle at last week's convention of the Institution of Mechanical Engineers, in Leamington Spa, on " The Application of Hydraulics for Mobile EqUipment for Agriculture, Construction and Industry ", a number of authors and speakers discussed recent developments that might be important in the neat future to commercial-vehicle manufacturers, particularly to the makers of heavier types that are frequently used for arduous site work.
For example, Mr. P. C. Mortenson, Director of Engineering, Vickers Incorporated Division, Detroit, referred to the problem of producing a hydrostatic pump suitable for coupling to the crankshaft of the Cummins 470-cu.-in. V-8 engine created by its high operating speed of 3,300 r.p.m. Mr. Mortenson emphasized that direct drive was advantageous because it eliminated the need for a speed-reducing gearbox, reduced space requirements and normally enabled a smaller and cheaper pump to be employed. The application was a challenge to his company, said Mr. Mortenson. and he was confident that the difficulties in producing a pump for such high-speed operation would be overcome within eight months.
Referring to a lubrication development of special importance to the hydrostatictransmission industry in America, Mr. Mortenson said that a grade of crankcase oil, known as the MS, gave better results in hydraulic units than conventional hydraulic fluids, including freedom from corrosion.
Over the past 25 years, the horsepower capacity of variable-displacement piston pumps per Cu. in. of installed space had, said Mr. Mortenson, increased nearly sevenfold, and the reduction in average price per rated horsepower was approximately in the same proportion. Operational speeds had risen by about 70 per cent.
During the dieussion, Mr. Mortenson stated that his company had run experimental units Up to pressures of 10,00015P01h. sq. in., ,about three times the normally acceptable limit of 5,000 lb. sq. in: 'Operation was satisfactory, but the units would nbt run 7,000 hours without overhaul and were not, therefore, considered reliable. Leakage represented the most difficult of all problems and it was notable that the Caterpillar Tractor company would not tolerate any leakage, which made it necessary to supply units that operated at 1,800-2,100 lb. sq. in. The lick of flexible hoses and of fittings capable of withstanding high pressures continuously was a major handicap.'
Struggle for Efficiency
Joint author of a paper on "Aspects of Hydraulic Transmission for Vehicles of a Specialized Nature ", Mr. C. K. J. Price, of Lucas Industrial Equipment Ltd., said during the discussion that it was " necessary to struggle for every ounce of efficiency" and that vehicle chassis members could with advantage be constructed to act as fluid conductors. Mr. S. A. Beasley, of the Lucas company, was co-author of the paper. .
Referring to statements made by Mr. E. H. Bowers, of Vickers Sperry Rand Ltd.. in his paper on " Hydraulic Powered Steering for Heavy Vehicles ", Mr. R. A. Jones, of Hydraulic and Pneumatics Ltd., emphasized that " feel" was allimportant to the controllability of vehicles used on the highway. At high speeds on the road servo assistance should be autoinatically counteracted to give a sense of road conditions ", otherwise the driver might corner too fast, hit the kerb or lose control when travelling over pot-holes. The job of the centring spring of a typical system was to pre-load the linkage, not to provide feel, which should be proportionate to steering load. In reply. Mr. Bowers claimed that a centring s?ring did, in practice, give some degree
of feel. P.A.C.B.