DECKMUKR 15, 191E 



1037 



for biU'kinjj tlio t motor, a keyway must be 

 cut in tlie couiUor.sliaL't a little more than 

 twice as long- as llie thickness of the pin- 

 ion, and a smooth straight key set in se- 

 curely, so there will be no jilay. The cutting 

 may be done at home by means of a quar- 

 ter-inch drill, the series of holes being care- 

 fully sunk in exact line for the required 

 depth, and then dug and dressed out by 

 means of a cold-chisel and small tiles. Drive 

 the key in snuylv its full length and then 



c=l 



Fia. 2. 



slip the jiinion uj^on the sliaft with the key- 

 way dressed out to slide easily upon the 

 kej^ but without wabble. To the pinion 

 must be attached a shift-gear collar such 

 as can easily be secured from a mowing- 

 machine or almost anything else with pro- 

 vision for shifting a gear-wheel. Frequent- 

 ly a pinion and collar cast together can be 

 secured. 



At each end of the differential shaft we 

 now fit a sprocket-wheel that will work on 

 a chain of the same number required for 

 the bull-sprocket on the main drive-wheel 

 of the tractor described in the last number 

 of this series. Both of these small sprockets 

 nuist be alike in size, and preferably both 

 should be mates of the large sjirockets in 

 the same binders; then there will be no 

 question about everything working proper- 

 ly and in absolute step. For instance, if 

 we get our two drive-wheels out of two 

 old Champion binders, and at the same time 

 secure with each the small sprocket over 

 which the drive-chains worked, Ave are sure 

 of avoiding possible trouble through some 

 little ditterence in the intch of the two 

 wheels. If these sjirockets are one-thii'd the 

 size of the bull-sprockets we have our speed 

 divided by tlnce again, and so get the drive- 

 wheel rate of revolution down to 22 per 

 minute, which will be found fast enough. 

 If the difTerence between this last pair of 

 wheels is greater or less than three to one. 

 we must vary one of the other pairs of 

 pears a little to suit the occasion, unless we 

 are able to change the speed of the engine 

 or remodel our own ideals as to speed. In 

 making any variations it should always be 

 remembered that it is bettor to increase the 

 size of the driven wheel rather than to 

 decrease that of the driving pinion, because 

 there is more tendency to cramp and bind 

 in the smaller diameters of wheels. This 

 rule holds good alike with gear and sprock- 



et wheels, and, for heavy duty, the wheel 

 that gets below twelve to fifteen teeth must 

 be considered in the light of a necessary 

 evil which should be avoided as much as 

 can be without too greatly increasing the 

 size of the larger wheels. 



By connecting the sprocket-wheel on the 

 engine with the countershaft by a chain, 

 and attaching the two sprocket-wheels on 

 the differential shaft to the two drive- 

 wheels of the tractor in the same manner, 



Fig. 3. 



wo now have in Fig. 1 a tractor that would 

 move forAvard in a straight line whenever 

 the engine is running. To crank the en- 

 gine, however, we would have to move the 

 entire tractor, while there could be no grad- 

 ual picking up by the engine of the load 

 and no variation of speed. It would be 

 almost impossible, too, to change direction, 

 because both of the tractor drive-wheels 

 would be driven forwaid directly from the 

 engine at equal speed. We could not turn 

 a corner without sliding the Avheels, and 

 would find the tractor \ery hard to handle 

 on rough ground. 



The differential permits one of the drive- 

 wheels to turn faster than the other, and 

 one of some sort should by all means be 

 used. One from a Avorn-out steam-tractor 

 is occasionally available at junk prices, 

 though generally the differential is pretty 

 Avell Avorn out by the time the rest of the 

 lig is useless; also it is pretty lieaA-y for a 

 light tractor. At a very small cost an auto- 

 mobile differential of some sort can gener- 

 ally be obtained of any 

 garage or second-hand 

 automobile shop, 

 though often these are 

 pretty Avell worn. They 

 answer quite Avell, 

 when in fair repair, 

 for a tractor up to 

 about ten-horse poAver. 

 The entire shaft 

 should be purchased 

 together, and mounted 

 in the place shoAvn for 

 the differential shaft, 

 and then cut to length. 

 Probably a sprocket - 

 Avheel Avill have to be 



substituted foi' the gear-pinion on the coun- 

 ter-shaft, and a chain-driA-e be used betAveen 

 the latter and the differential. 



Fig. 4. — This dou- 

 ble-Rear wheel turns 

 upon its shaft, which 

 need not turn. The 

 relative speed of the 

 forward and reverse 

 motion of the tractor 

 depends upon tlie rel- 

 ative size of the two 

 niemliers. 



