Page:EB1911 - Volume 18.djvu/960

Rh sprockets on the ends of the differential shaft to chain rings which are bolted to the rear road wheels. Figs. 27, 28 and 29 show typical vehicles, ranging in load capacity from 30 cwt. to 6 tons, on which the side-chain method of final drive is adopted. One of the chief advantages of the side-chain drive lies in the fact that there is, with it, less weight below the springs than with any other form of final drive. The only parts below the springs are: the fixed back axle; the chain rings (bolted to the road increased efficiency, on account of the ease with which all the parts can be enclosed in an oil-tight casing. It also gives silence of running. The strongest advocate of the worm drive for heavy vehicles is the Guildford manufa.cturer, Dennis Bros., Ltd., one of which company's machines is illustrated in fig. 31. Although there are many difficulties in the matter of the manufacture of worm gearing, they are not insurmountable, and, given proper attention at the hands of the designer, followed by wheels); the road wheels themselves; the road-wheel brakes and part of the weight of the chains. The differential gear and chain sprockets are carried in a countershaft casing, which is securely bolted to the main frame.

In a number of very successful vehicles the final drive is transmitted by means of spur pinions. These are mounted on the ends of bevel-driven differential shaft, and mesh with internally toothed or externally-toothed gear rings on the road wheels. Milnes-Daimler and De Dion commercial vehicles are amongst the machines on which the internally-toothed form of gear is employed, whilst Ryknield is the most representative vehicle embodying the externally-toothed form of final drive. The direct drive, from the ends of the differential shaft, as is shown in fig. 30, is another type of final transmission that has met with a considerable amount of success, particularly on the Leyland machines of five-ton and six-ton capacity. The differential gear and the bevel-drive reducing gear are both enclosed within a casing that is bolted to a fixed back axle; the ends of the driving shaft pass through tunnels in the axle body; and claw pieces on the outer ends of the differential shaft engage with similar claws on the road-wheel hubs. The two last-named forms of gear are highly efficient, provided the pitch and shape of the teeth are carefully considered and the designs provide for the encasing of all the pinions and gear rings.

The only other type of final drive which is used to any great extent for commercial motors is that which employs a hardened and ground steel worm meshing with a machine-cut phosphor-bronze worm wheel which is bolted to the differential-gear cage of a live back axle. The employment of this type of gear for the final transmission on commercial motors generally leads to accurate workmanship, probably no other mechanical means of transmitting power can approach it for smooth and silent operation.

Both thrust bearings on the worm shaft should be on one side of the worm, to avoid lack of truth in meshing if any heating occurs between the worm and the wheel. There are many examples of the worm drive to be found in London on public-service passenger vehicles, and also on delivery vans. One of the great charms of this type of transmission is that a very large gear reduction may be obtained without making the worm wheel unduly large in diameter; this is an important factor in the design of a back axle, as every inch of road clearance is of value for operating on rough country roads. As a large gear reduction is thus rendered possible on the back axle, it will readily be understood that the change-speed gear-box may be made considerably smaller than would be necessary for a bevel-driven live axle, where a large gear reduction is not permissible, both on account of its size and because such a gear would be very noisy in its working.

Although the use of tooth wheels is still the only practical method of obtaining variable transmission for motor vehicles, the fundamental defects of transmission in this way are inherent to the system and must always be present; they are now less apparent, thanks to the remarkable improvement which has taken place in the use of suitable materials and improved design.