98 POWER AND THE PLOW 



cut teeth. The shafting must be made of the highest quality 

 of steel and carefully machined to size. If a sliding gear 

 transmission is used it is the practice of the leading de- 

 signers to cut from two to four keys out of the solid steel shaft 

 for the gears to slide upon. This is an extremely expensive 

 construction, but, since the keys are a part of the shaft itself, 

 it overcomes stripping or damage to gears from loose keys. 

 The sliding gear and those which it engages, usually have 

 the teeth beveled on the side so as to slide into mesh more easily. 



CHAIN DRIVE 



The chain drive has the advantage of flexibility and may 

 be combined to good advantage with the spring-mounted frame. 

 A well-designed chain and sprocket form an efficient trans- 

 mission, but the number of joints renders it subject to greater 

 wear than gears, and only a few tractors employ this method. 

 A pinion and master gear are sometimes used in place of the 

 final chain for driving the wheel or axle. In one three-cylinder 

 tractor both bevel and spur gears are used in connection with 

 a chain drive from countershaft to master sprocket. 



FRICTION DRIVE 



Few friction-drive constructions have been attempted, and 

 few of these survive, except on tractors designed for lighter 

 work than plowing. In one of the most successful, fibre-covered 

 disks take the power from the inside rims of both flywheels 

 for the forward speed and from the hub for the reverse. This 

 construction is simple and easily managed, and it eliminates 

 the use of a clutch, but it is doubtful if the disks can be made 

 durable enough for heavy plowing service. The experience 

 of automobile manufacturers has been that the best friction 

 material is too short-lived, even for such light work. 



REVERSE 



Easy manoeuvring requires some provision for reversing the 

 direction of movement. In the steam engine this is accomplished 



