More About Hydraulic Transmission.
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Some Queries Answered. Efficiency Figures from Tests.
By Henry Sturmey.
I am pleased to learn that what I have had to say about hydraulic transmission in a recent issue has created some interest, as the system undoubtedly presents many advantages, and I think it can now be safely said that in the Manly system, at any rate, the stage of practical commercial utility has at last been reached. I am sorry if in my article on the " Gear Range of Hydraulic Transmission" I "left so much unexplained," as "A Motor User" puts it. The article in question was not intended to be a complete disquisition on the subject, but only a few notes upon the particular phase of the matter indicated by the title, and I have now much pleasure in taking up the points raised by your correspondent.
" A Motor User" is right in understanding that, in a system like the Manly, employing adjustable throw pumps, "it can be arranged to avoid the possibility of shock." That is one of the strongest features of the transmission, as both starting and accelerating effort are applied progressively, commencing at zero, and the effect may be compared to the difference between a screw-operated wedge and blows with a sledge or battering ram. Both will shift the object to be moved, but with a very different. effect. upon the object.
My correspondent enquires, next, what efficiency can be obtained from such a system, and whether the efficiency varies with the gear ratio, and I have much pleasure in giving the results of a few tests which were made to secure data on this particular subject. They are set out in a paper read about a year ago before the American Society of Mechanical Engineers, by Mr. George Barrus, a member of that society. The power was obtained for the tests from an electric motor ; the power delivered to the transmission could be accurately ascertained, and the output at the other end of the transmission was taken by prony brake. Four tests were made, and without going into all the details of each, the net results obtained were as follow ;— Test A was made with constant power and constant speed of the motor shaft, with varying speed on the driven shaft, according as the load on the prony brake was gradually increased. With the speed of the driven shaft showing 363.0, 299.00, 240.7, 200.3 and 139.7 r.p.m., and with 24.0, 57.0, 68.7,85.7 and 116.3 lb. on the brake arm, the efficiencies registered were 85.3, 87.2, 89.4, 89.2 and 83.7 per cent. respectively.
Test B was made with power reducing from 14.3 to 4.72 h.p. delivered to the transmission from the elec
tric motor, constant speed of driving shaft and weight on brake arm, and with the speed of the driven shaft showing 352.0, 310.0, 246.0, 215.0, 160.0 and 112 r.p.m., the efficiencies came out 86.2, 87.6, 88.4, 88.2, 86.1 and 83.1 per cent., respectively.
Test C., with constant power and speed on driving shaft, and weights on the brake arm of 47.0, 56.5, 66.5, 86.0 and 103.5 lb., pulling the speed of the driving shaft down from 350.0 to 310.0, 270.0, 202.0 and 163.0 r.p.m., the efficiencies shown were 87.4, 90.8, 91.9, 89.3 and 67.0 per cent. respectively. Test D. was made with varying motor power, but practically constant speed on driving shaft, and with constant weight on brake arm, giving revolutions on the driven shaft of 350.0, 315.0, 243.0, 200.0, 145.0 and 105.0 r.p.m., and the efficiencies shown were 87.4, 90.2, 90.9, 88.6, 85.9 and 81.6 per cent., respectively. It will be noted that the average efficiency over the whole of these tests was 87.7 per cent. The first two tests were made with the oil in the transmission at a medium temperature, and tests C. and D. when it had been warmed up to its maximum by working, and it will be noted that the efficiencies are higher with the increase in temperature, which is, I believe, the opposite of what has hitherto been found to be the case with other systems, a fact which, to a very large extent, accounts for their failure, or but partial success. I question very much whether these efficiencies could be equalled under similar conditions with the conventional systems of stepped, toothwheel gearing, and I may add that the tests referred to were made four years ago, with the first five-tonwagon outfit, and, as considerable detail constructional improvements have been made since then, it is reasonable to suppose that the efficiencies will be even higher to-day. As to the cost of operation, I am sorry I am unable to give "A Motor User" any definite figures. I7sers don't give away their figures very readily. The Truck Sales Co., which has had the handling of the La France Co.'s output, says the chief economy is in the smallness of the repair bill. But, although I am unable to give figures of working, I am able to conclude this article with what I believe is the most remarkable performance ever made by a motor wagon; as, by a recent mail I received a copy of a letter from the Howes Transportation and Contracting Co., of New York, who, writing to the Truck Sales Co. on 7th January, say :— " On 4th Jan., we-used your La France hydraulic
truck in hauling a boiler from 19.91 Street and Second Avenue to 68th Street and East River, going up Broadway to 59th Street. The actual weight of the boiler was 23 tons : the weight of the truck it was on, 12i tons. We attached your automobile to the front end of the truck pole, using same as a trailer, and were surprised to find that your machine had no difficulty in handling this load, and proceeded at a rate of about four miles per hour through New-York traffic under good control." From my recollection of New York, I am able to say that the road surface over this route is none of the best, being stone setts mainly, with a mighty badness for the most part. The Truck Sales Co. amplifies this information, by saying that in addition tc. towing the boiler the truck carried its full load of five tons, and that the gross load in all shifted by the truck totalled 46 tons, which is, I believe, a totallyunprecedented performance, and very greatly extends. the "gear range of transmission" beyond the figures given in my article, and I think it will be admitted that for a wagon to do this there cannot be very much to complain about in regard to efficiency of the transmission.