Page:Popular Science Monthly Volume 89.djvu/173

 Trend of Motor-Truck Design Toward Worm Drive

��MORE than sixty per cent of the American motor-trucks listed on the market at the |)rescnt timeare worm-driven. Last year twenty-two per cent of the trucks Hsted were worm- driven, thus showing that the popularity of this form of drive has increased.

To understand the reason for this great increase, one must first know the cause for any form of gearing for transferring the power of the truck motor to the rear wheels in order to make the truck move. The average gasoline motor of the truck of today revolves at the rate of from I, GOO to 2,000 revolutions per minute. It is out of the question for the rear wheels to revolve at any such speed be- cause they would simply spin around and not secure enough traction between the tires and the ground to make the vehicle move. The necessary reduction between the speed of the motor and that of the wheels under varying conditions of roads is secured through some form of change- speed mechanism and the form of gearing used between the motor-shaft extended and the axle of the driving-wheels.

The latter type of gearing may be divided into four main classes as shown

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��Fig. 2. Worm drive. The bevel-pinion and wheel are substituted by a worm- gear and worm-wheel

��in the accompanying illustration, al- though there are some few other types used on special vehicles. The four most common types arc: i. Bevel drive; 2, Worm drive; 3, Double-chain drive, and 4, Internal-gear drive.

In the bevel drive the power of the motor is transmitted through the clutch

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���Fig. 1. Bevel drive. The bevel-pinion on

the end of the shaft meshes with a large

bevel-wheel in the rear

and change-gear mechanism to a longi- tudinal shaft at the rear end of which there is mounted a bevel-pinion. This meshes with a larger bevel - wheel to which are connected the ends of the rear wheel axles. When the bevel -pinion is made to revolve, the bevel -wheel re- volves, which in turn sets the wheels in motion and causes the vehicle to move. This construction is shown in Fig. i.

The method of worm drive shown in No. 2 is exactly the same except that a worm-gear and worm-wheel are used in- stead of a bevel-pinion and wheel.

The double-chain-drive method is based upon the same principle as the former methods except that instead of the extended motor-shaft reaching the rear axle of the truck, it ends at a rear- axle unit or jackshaft attached to the frame forward of a stationary axle on which the rear wheels are mounted. The jackshaft is the same as the rear-axle unit shown in No. i except that instead of having wheels mounted on the ends of its shaft, it has sprockets. Endless chains are passed around these sprockets

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