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111 in the form that it is usually employed on gas tractors.

In this application the differential gear is mounted on a cross shaft, which is the usual method of installing it on both gas and steam-propelled traction engines. The bull pinion A is attached to a continuous shaft which passes through the center of the differential gearing and which is driven by the bevel gear A. The bull pinion B is attached to a sleeve forming part of bevel gear B and can revolve independently of the continuous through shaft when desired. The differential driving gear, which is turned by the source of power, is mounted independent of the continuous shaft and is coupled to bevel gears A and B only through the medium of bevel pinions C and D, which are attached to and revolve upon studs carried by the differential spider.

Assuming that all the gears are in mesh, as outlined, if the resistance to traction is the same at both rear wheels the power applied at the differential drive gear will be directed to both bull pinions A and B, and the entire assembly, which is comprised of the differential spider and pinions attached to it and the bevel gears A and B which drive the bull pinions, will revolve as a unit.

Should the traction resistance against the driving wheels vary, as is the case when the tractor turns a corner, or deviates otherwise from a straight course, so that one wheel tends to revolve faster than the other, the small bevel pinions C and D will not only turn around on the studs on which they are mounted but at the same time will run around the gears A and B because the differential spider is being rotated by the engine. When turning a corner the other wheel must revolve so much faster than the inner member that it is just as

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Fig. 112.—The Differential Spider With Bevel Pinions and

Spur Drive Gear Attached. though one of the wheels was held almost stationary and the other turned.

The action of the differential pinions may be clearly understood by giving due consideration to the following principles: The same resistance at the point of contact between the driving wheels and the ground prevents the pinions from revolving on their own studs, and in this case they act simply as keying members between bevel gears A and B and are carried around by the differential spider. If the resistance upon bull pinion A is

greater than that on bull pinion B the differential spider will rotate forward with the wheel offering the least resistance and the differential pinions will turn on their studs and run over the surface of the gear which tends to remain stationary, this obviously being the one against which there is the greatest resistance. The differential pinions can thus turn independently of one bevel gear wheel and run over its surface without turning it and at the same time act as clutching members of sufficient capacity to carry the other bevel gear and the bull pinion

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Fig. 113.—Countershaft With Differential Spider Outlined at

Fig. 112 Removed.

attached to it in the same direction as the differential spider and at a ratio of speed which will depend upon the difference in resistance between the friction members and the ground.

While the differential described is of the bevel-pinion type, and is the form generally used on gas tractors, the differential effect can be obtained by a combination of spur pinions and spur gears just as well. The appearance of the differential spider and the manner in which the bevel pinions are carried around with it is clearly outlined at Fig. 112. This also shows the method of

bolting the drive gear to the differential spider. At Fig. 113 the other parts comprising the differential gear assembly, when the differential spider shown at Fig. 112 is removed, are clearly shown.

It is sometimes desirable to lock the differential gear so that it will be temporarily out of commission. This condition would be desirable if one of the wheels was in a mud hole where the resistance to its turning was slight, while the other was on hard ground which offered considerable resistance to its moving. When the power was applied the wheel that was in the mud hole would revolve without driving the tractor, as the difference in traction resistance between the ground and the two wheels would be too great. If the two bevel-gear members A and B and the differential spider were locked together by a bolt passing through the three of them, or its mechanical equivalent, then the power would be applied to both wheels and the tractor would pull itself out onto the firm road surface again. Obviously, before attempting to turn any corners or deviate from a straight path it would be necessary to remove the locking bolt so that the differential could function properly again.

CHAPTER VIII.

THE TRACTOR FRAME, WHEELS AND AXLES.

Construction of Tractor Frames—Typical Frames Described

Why Three Point Support is Needed—Facts Concerning
Tractor Wheels—Methods of Construction-Action and
Advantages of the Caterpillar Tread-Tractor Front
Axles—How Tractors are Steered-Automatic Steering
Arrangements—Methods of Final Drive-The Conven-
tional Method-Use of Chains and Sprockets-Live Axle
Forms.

Construction of Tractor Frames.—One of the important components of the modern gas tractor and that which determines whether the machine will be enduring and efficient, is the frame. This serves to tie and support all other parts of the machinery and is depended on to maintain proper alignment between the various components of the power generating and transmission groups. If the design of this foundation is neglected or slighted the efficiency of the tractor will be materially reduced. A light, poorly braced frame will permit the various parts to get out of line if the tractor is operated over irregular or rough surfaces, such as obtain in most fields.

The frame must be heavy enough to furnish a substantial foundation for the power plant as a frame that was inadequate in strength would produce rapid deterioration of the mechanism because it would subject it to strains that it was not designed to withstand. When bearings cramp out of line much more power is con

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