it would not be possible to employ bevel gears carried directly in the differential housing. The use of worm gearing, however, enables the designer to employ but a single pair of reduction gears in the rear axle though a set of intermediate gearing is necessary to provide a partial reduction of speed before the power is directed to the worm gear of the axle.

The construction of a worm gear driving assembly is clearly shown at Fig. 140. The worm gear is attached

FIG. 140.-Sectional View of Worm Drive Gearing Used in Connection With Live Rear Axle.

to the differential casing and is revolved by means of the worm mounted above it and connected to the speed reducing cross shaft back of the engine. In order to reduce friction to a minimum the worm is mounted on New Departure ball bearings of the single and double row types. The end thrust of the worm is taken by a large double row bearing which is clamped to the shaft and housing in such a manner that it is restrained from lateral movement and is thus able to resist thrust in either direction and radially applied

loads as well. The single row ball bearing of the conventional annular pattern has a floating outer race, i. e., that member can move back and forth to some extent should the worm shaft expand due to heating while in operation. Gearing of this character is carried in an oil-tight container and as the differential casing may be filled with lubricant through the oil filler plug at the top of the case the gearing will run with minimum depreciation and maximum efficiency.

The view at Fig. 141 is a section through half of a worm gear driven live axle. The differential assembly is mounted on large double row bearings which locate the worm gear accurately in the center of the driving worm, a condition necessary to secure efficient driving. The rear wheels are carried by two double row ball bearings the inner members of which are clamped to and supported by the tube or housing serving to inclose the driving alxe. The drive shaft turns the wheel by means of a positive clutch attached to the end of the axle shaft and fitting into suitable depressions in the interior periphery of the wheel hub. An advantage of this method of construction is that the differential gear and driving axles may be removed if necessary without taking off the wheels. Owing to the expense of building and the fact that this method of axle construction is not adapted for use with the extremely large diameter driving members ordinarily employed on gas tractors it is obvious that the systems of drive in which the power is applied to the rim of the wheel directly instead of through the medium of the spokes and hub will continue to be most popular and generally satisfactory.

CHAPTER IX.

DRIVING AND HOUSING THE TRACTION ENGINE.

How to Start Traction Engines-How to Start Tractor-Typical Tractor Control Systems-Advice on Tractor Operation-Housing the Tractor-Combined Tractor House and Farm Workshop-Fuel Storage Methods-Tools and Equipment for Care of Tractor.

How to Start Traction Engines.-As a rule before shipment from the factory, all of the reputable makes of gas tractors are thoroughly tested with and without load to make sure that all parts are functioning properly. This includes tests of the motor on the block, running the tractor on a special testing track around the works and after the machine has been in use for some time, determining the power capacity of the motor to ascertain if it approximates the standard established by means of a brake test and determining the tractive or pulling power by some form of dynamometer test. Therefore, the tractor purchaser receives a machine that has been carefully tested and that should start without difficulty if the proper steps are taken.

The most important thing to do before endeavoring to start the engine is to see that the fuel and water containers are full and that all the electric wiring is properly connected. All mechanical oilers should be filled with the proper lubricant and if a constant level splash system is employed enough oil to insure thorough lubrication should be put into the crank case. The

open gears should be well covered with grease and all grease cups should be screwed down to make sure that all bearing surfaces fed by these members are receiving the proper supply of lubricant. Generally one or two quarts of oil will be sufficient in the crank case and nothing but the best cylinder oil should be used for this purpose. If there is too much oil in the engine, clouds of white smoke will issue from the exhaust pipe, so a certain amount can be drained from the engine case by means of petcock or drain plug provided for the purpose. In starting a new engine it would be well to remember that surplus oil will do no harm and that plenty of oil is much cheaper than repairs. When the engine is new and all bearing parts are still stiff, copious lubrication is one of the surest methods of obtaining a smooth running machine.

Before attempting to start the engine one should. become familiar with the various control levers provided both for controlling the speed of the motor and for handling the various clutches. The first point to make sure about is that the master clutch handle is set in a neutral position, i. e., with the clutch released. This will disconnect the engine from the transmission and will permit the power plant to run independently of the rear wheels. The spark advance lever is placed in the full retarded or late position, the lever that controls the throttle of the carburetor is set so this member will open the throttle slightly and the engine may be primed by pouring a little gasoline in each of the cylinders through the priming cups or petcocks, if these are provided. The switch is placed on the side of the coil marked "battery" if a double ignition system is provided. The engine is turned over, either by means of a starting handle attached to the crankshaft or by