TRANSPORTATION ON LAND 



297 



of in the operation of the clutch on an automobile? 

 Briefly explain the use of the clutch in the power-trans- 

 mission system of an automobile. 



Many of the modern cars are equipped with an au- 

 tomatic clutch or "free wheeling" mechanism, as it is 

 commonly called. This device disengages the engine 



Courtesy Standard Oil Co. of New York 



FIG. 493. CROSS SECTION OF AN AUTOMOBILE 

 CLUTCH AND HOW IT WORKS 



and the drive shaft automatically whenever the en- 

 gine is turning at a lower speed than the drive shaft 

 as when the car is coasting down hill. Cars that are 

 equipped with the free wheeling mechanism can be 

 changed to the ordinary transmission by the pull of a 

 handle usually placed conveniently on the dash board. 

 Figure 494 explains the operation of the free wheeling 

 device. The outside collar is coupled to the engine 



while the inside portion is connected to the drive 

 shaft. 



Operating in tapered slots between these two sec- 

 tions are three sets of four rollers. When the engine 

 is turning at the same speed or more rapidly than the 



DICTION or 

 "ROTATION 



Connected to 

 the 



Connected to the 

 drive shaft 



Courtesy the Studebaker Sales Corporation 



FIG. 494. FREE WHEELING 



drive shaft, these rollers are forced toward the narrow 

 end of the slots, thus wedging tightly and causing the 

 crank shaft of the engine and the drive shaft to turn 

 as a single unit. If the engine is idling, that is, turning 

 more slowly than the drive shaft, the rollers are forced 

 to the wider ends of the slots, thus automatically dis- 

 engaging the engine from the drive shaft. During this 

 process the clutch and transmission gears remain en- 

 gaged. When the free wheeling mechanism is in use 

 gears may be shifted at will without the use of the 

 clutch. 



Sometimes it is necessary for the automobile to 

 develop great power while the car is running slowly. 

 This is the case on steep hills and in mud or sand. 

 The sliding gears of the transmisison make this pos- 

 sible. By shifting the proper gears into the trans- 

 mission system, the car may be driven at a slow, an 

 intermediate, or a fast speed, or the motion of the 

 car may be reversed. This is made possible by engag- 

 ing gear wheels of various sizes. If two gear wheels 

 of the same size are enmeshed, each turns at the same 

 rate, while if a large gear is enmeshed with a small 

 one, the larger one turns much more slowly than the 

 smaller gear. A car running in high has two gear 

 wheels of the same size meshed ; in second gear a large 

 gear wheel is meshed with a small one ; in low gear 

 a still larger gear is meshed with a small one. Figure 

 495 is a series of pictures of a transmission gear sys- 

 tem showing the various gears and their positions. 



Exercise. Carefully study Figure 495 and show ln>u 

 the encircled insets of the man using the hoist explain 

 what results when one shifts the gears of a car from 

 "low" to "high." 



In the rear axle of the automobile is located one 



