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An Advanced Worm-gear Final-drive Assembly

25th December 1936
Page 26
Page 27
Page 26, 25th December 1936 — An Advanced Worm-gear Final-drive Assembly
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Which of the following most accurately describes the problem?

WHILST by no means universally employed on commer

cial chassis, worm gearing is regarded by many as the -preferable form of final drive. There are, of course, numerous considerations that have to be taken into account, and powerful arguments can be put forward in favour of bevel drive.

The fact remains, however, that an undertype worm is attractive where low floor height is important, and, in addition, the worm, wheel, differential and housing are particularly adaptable for manufacturing as a unit which can be incorporated in a wide variety of axle designs.

A manufacturing concern which has taken full advantage of this unit principle in designing its recently introduced worm-final-drive assembly, is David Brown and Sons

(iiudd.), Ltd., Huddersfield. This gear is entirely selfcontained and, when mounted in position, is isolated from stresses caused by deflection of the axle, and is thus able to operate under conditions calculated to promote maximum efficiency and durability.

The unit shown above, incorporated in an axle, is of the type built for use with a torque tube, but models for chassis with open propeller shafts are more generally used on commercial vehicles. The design is suitable for .employment as either an overtype or undertype gear, only slight modifications being necessary to adapt it for one or the other.

Notable among the features of the David Brown worm B20 gear unit is the construction of the housing, which is split and formed of an aluminium alloy. To ensure interchangeability and true registering of the top and bottom parts, elaborate jigs and fixtures are used in the machining operations. Upon this depends the location of the unit in the axle casing; the collars around the main bearing studs at the case joint also provide ample basic registers.

Among the accompanying illustrations are views showing some of the manufacturing operations. One of these is the machining of the joint face. The method of chucking the work is noteworthy, distance pieces being used to locate it axially, and a central fixing also being employed.

In another view the boring of the worm line is shown. In this case the work is mounted on an angle fixture, location being from the main spigot. Subsequently, with the top and bottom " halves " assembled, the wheel line is bored on a horizontal boring machine.

Construction of Worms and Wheels.

Centrifugally cast from Taurus bronze, the worm wheel is roughed out with a profile-ground tapered hob, and finished with a parallel serrated hob on a David Brown worm-wheel generator. The teeth are of the shape developed and patented by the company.

In the case of the worms, individual nickel-steel forgings are used, great care being taken to insure a uniform thickness of the hardened case. The multiple wheel threads are profile ground both before and after hardening, whilst the portions required to be soft are copper-plated before carburizing. By this means sudden changes of surface structure are avoided.

For the differential cages, 0.4 per cent, carbon steel is employed, whilst the differential bevels al's of nickel steel.

Although of aluminium alloy, the casing is exceedingly strong. This is due partly to the substantial ribbing and the provision of deep radiating Ens, and partly to the generous overall dimensions. The latter, incidentally, have the added advantage of affording a capacious oil reservoir.

To enable the worm wheel to be set axially, relative to the worm, to give the ideal tooth marking, adjusting plates are provided outside both bearings on the differential cages. This adjustment is made initially, and does not need to be altered if the assembly subsequently has to . be removed from the axle. The unit, complete with its bearings, is self-contained, so there is no possibility of error of alignment in assembly.

To suit various applications, certain differences of design have been adopted. One that is proving popular allows the unit to he inserted from one side of the banjo casing, the aperture on the opposite side being covered by a plate.

Noteworthy among the incidental items of the unit are the combined dip-stick and oil filler, and the oil seals used.

Turning to the actual construction of th6 unit, this merits a brief description. Four models are standardized, these having distances between centres of 3f ins.. 4 ins., 4+ ins. and 5 ins. The largest, being the most suitable for commercial chassis, may he taken as representative of the range. It weighs 55 lb. and has a worm case of 10f ins. diameter; In this part the worm tunnel is formed, the worm shaft being supported at the front and 'rear on roller bearings, which take axial loads, and at the rear on a deep-groove ball race, having a split inner member, which takes the thrust in both directions.

It is provided with a circular flange for attachment to the axle casing, and on its Upper portion incorporates the halfhousings of the worm-wheel bearings. In these ate inserted four studs, by means of which the top halves, formed in the upper part of the casing, are secured.

In the standard design a second flange is formed on this upper portion, which corresponds to that on the lower. These two flanges make the joints with the machined surfaces on the axle banjo, which is held between them, short bolts, fixed with their heads on the inside, constituting the means for attachment of the lower flange to the axle casing.

In the alternative arrangement, mentioned above, the upper flange can pass through the lower aperture, and merely registers with the upper opening, the pressed-steel plate forming the covering.


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