Hypoid Gear and Two-speed Axles
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r-IN March 4, at 6 Mr. E. B.
4../Wilson will read a paper, "Review of Developments in Driving Axles," before _members of the Institution of Automobile Engineers, at the Institution of Mechanical Engineers, Storey's Gate, St. James's Park, London, S.W.1.
Mr. Wilson, who devotes most of his paper to private-car design, has, nevertheless, much of interest to say concerning commercial-vehicle axles.. He classifies these into two main groupsspiral-bevel and worm-driven units. It is interesting, he says, to compare the position in this country with that in the U.S.A.
American Practice
There, he points out, light vans, in the majority of cases, use hypoid gears. In the 14-ton and 2-ton classes, spiralbevel-driven axles are employed, of which 50 per cent, in the 2-ton class are of the two-speed type. In the larger categories, the majority, uses two-speed spiral-bevel units, those of the wormdriven type being confined, more or less, to six-wheelers.
The hypoid gear, the author says, is rapidly gaining in popularity in America. Although there are no British commercial vehicles at present fitted with the hypoid gear, this system, Mr. Wilson declares, forms a probable line of development. The excellent results obtained with spiral-bevel gears are, however, likely to cause some reluctance on the part of the manufacturers ' to abandon this form of drive.
In considering the possible development in lorry axles in this country, one cannot ignore the trend in America, and, in particular, the rise in popularity and success of the two-speed axle. This has come about in spite of some initial reluctance on the part of vehicle makers to fit it.
The author makes reference to a popular type of two-speed unit in which the spiral-bevel primary reduction is followed by an epicyclic secondary reduction, which can be engaged as required. The choice of ratios, says Mr. Wilson, is of the utmost importance. In the first place, the difference between high range and low range in the axle should give approximately half the drop in speed of one change in the gearbox, thus providing twice the number of ratios fairly evenly divided. It is preferable that the spiral-bevel assembly should have a higher gear ratio than is usually employed, so as to give a' lower engine speed with greater economy when running light. The low range will then provide more power and tractive effort, and when used in conjunction with the low gear in the gearbox, a slightly greater overall reduction than normal will be obtained.
In the U.S.A. the two-speed axle has shown that marked economies and im
provements in performance over normal transmission systems arc possible, provided that the use of the additional ratios can be assured by an easy change device.
Although the two-speed doublereduction axle has not appeared in this country, certain makers employ singlespeed double-reduction drives that usually take the form of a primary reduction by spiral bevel, followed by a secondary reduction utilizing some form of helical or spur gearing.
Epicyclic Secondary Reduction
In other instances, an epicyclk secondary reduction is used. This form of drive is more widely employed in the U.S.A., and, except for certain specialized requirements, such as on battery-electric vehicles, machines of the mechanical-horse type and certain classes of 'heavy," it is more usual for British makers to employ a worm-drive when the reduction is 7:1 or lower.
The author concludes his paper by saying that the hypoid type of gear will probably come in for consideration when commercial vehicle makers arc reviewing new designs; and that the hypoid will gain ground at the expense of both spiral-bevel drives and, in some instances, worm drives. He is also of the opinion that two-speed axles will be introduced, and will find a substantial market in this country.