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and bevel gears. The wheels which revolve on the fixed axle are driven from the live portion by means of spur gears engaging corresponding members attached to the wheel hubs. Such a construction is not generally used because most engineers contend that this places too much unsprung weight on the rear axle.
Q. What is a double reduction live axle ?
A. A double reduction axle is one where the engine power is taken by 'compound gearing instead of the simple bevel or worm gearing ordinarily used. For example, the power from the gear set may be first delivered to a pair of bevel gears, the driven member of which is carried on a countershaft which also carries a spur pinion designed to mesh with a large spur gear on the differential
Fig. 188. McCue Rear Axle of “Full-Floating" Type Showing Use of
Single Row Ball Bearings at All Points. This provides two speed reductions, one between the driving gear on the transmission shaft and the member with which it engages on the counter and a further reduction in speed between the other two driving gears.
Q. What is a two speed direct drive axle and what are its advantages?
A. This is an axle construction of recent development and is called the two speed direct drive axle because in place of a single
Fig. 189.-Typical Simple Live Axle Construction Taken Apart to Show Components and Method of
Disassembling Axles of This Form.
bevel pinion and ring gear used in the ordinary construction, there are two bevel pinions and two ring gears attached to the differential casing. A typical assembly of this nature is shown at Fig. 189-a. The drive pinion B is integral with the drive shaft and always revolves with it while the larger drive pinion A only revolves with the drive shaft when the sliding clutch A is engaged with the corresponding clutch member attached to the pinion. When the sliding clutch A is shifted into position, the actuating bell crank shifts another sliding clutch attached to the differential gear case which releases ring gear B and allows the engine to drive the differential casing and the gearing it contains through the medium of drive pinion A and ring gear A.
When the other ratio is desired the positive clutch shifting lever is moved so that it brings sliding clutch A out of engagement which causes a simultaneous movement of sliding clutch B so that ring gear B is clutched to the differential casing. It will be apparent that the drive is through only one set of gears at a time, the set that is not in use revolving idly on suitable bearings. This construction affords two gear ratios, each driving direct from the engine to the wheels without any intermediate gearing other than the regular driving gears ordinarily employed in a one speed axle.
It is contended that no one single gear ratio can be just right for all vehicle speeds and all road conditions. The usual gear ratio of a single direct drive axle ranges from 3.5 to 1 to 4 to 1 according to the weight and power of the car. If a car is geared low, high speeds can only be obtained by racing the engine but its hill climbing abilities on the high are excellent. At the other hand, if the gear ratio is such that the car can be operated at high speed without racing the engine it will be a poor hill climber and the low gears will have to be resorted to on any gradient of magnitude. In the axle shown there are two gear ratios, one of which is low, and which is especially adapted for city driving where starting, stopping and slowing down are frequent and where cautious operation is necessary. When conditions are more favorable, the higher direct drive gearing may be brought in action and higher vehicle speeds are possible without a corresponding increase of engine revolutions.
For example, with the lower direct driving gears in operation,
Fig. 189a.- View of Austin Two-speed Rear Axle, Showing Two Pair of Bevel Drive Gears and Positive
Clutches by Which Either Gear Ratio May be Brought In Action
an engine speed of 700 revolutions per minute would mean a car speed of approximately 20 miles per hour, whereas if the high direct drive gears are used the car will travel approximately 30 miles per hour with no increase in engine speed. This increase in car speed without a corresponding increase in crank shaft revolutions means that there is a decrease in fuel consumption for a given mileage. A given quantity of gas is utilized to greater advantage and produces more actual power at the rear wheels than with the engine turning over more rapidly on the low gear ratio. Friction and wear of power plant and transmission gearing is also materially reduced because the parts are operated more slowly. When a low gear ratio is employed a car will vibrate very much at high vehicle speeds on account of the great speed at which the motor is operating, while with the higher ratios there is comparative freedom from vibration because the power plant is not operated at excessive speed.