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 



r:r^S 



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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