CHAPTER VTHE DIFFERENTIAL
The Meaning of Differential.—This is a term used to designate the difference in the turning movement of two wheels on opposite ends of an axle. For various reasons they do not turn at the same rate of speed, particularly in turning corners, where the outer wheel must travel a greater distance than the inner wheel.
If both wheels are fixed to the shaft the latter would be submitted to a torque, or one of the wheels would slip, and thus be destructive of tires.
On the other hand, if one wheel should be loose, then, as power is applied to the shaft, the tractive action would be on one wheel only, and this would be bad practice, and frequently cause the wheel to slip, and thus unduly increase the wear of the tire.
The differential is made up of a system of gears, which are so arranged that one wheel may turn independently of the other, and at the same time the effective driving power is utilized by each.
Various forms of this mechanism have been developed. While the differential is an exceedingly simple piece of mechanism, it is not such an easy matter to describe its operation, so that the principle will be explained by a series of illustrations.
Equalizer Bar.—Examine Fig. 28. Let A be an equalizer bar, mounted on the end of a thrust bar B, by a pivot C, so the ends will swing back and forth freely. A horizontal bar D is hinged at each end of the equalizer, which bars project forwardly parallel with each other and these are provided with right-angled bends E E, simply for convenience in describing the operation.
Fig. 28. Equalizing Mechanism.
Fig. 28. Equalizing Mechanism.
Fig. 29. Resistance in Equalization.
Fig. 29. Resistance in Equalization.
While differential gears are very simple structurally,it is not an easy matter to explain the principle on which a faster motion is transmitted to one wheel than another, and under conditions where the speed is constantly changing.
Fig. 30. Equalizer and Differential Movements.
Fig. 30. Equalizer and Differential Movements.
For instance, in Fig. 30, a cord A, over a pulley B, has weights C, D, at its ends. If the pivot or fulcrum E, of the wheel, is stationary, as in sketch 1, and the wheel is turned, say a quarter of the way around, one weight will move down below the line X the same distance that the other weight moves above it, as shown in 2.
Thus far we have an equalizer, pure and simple.But a differential requires something more. It is necessary, under certain conditions, for the weight D to move a greater distance in the same time than C, or the reverse. Or, as sometimes happens, one of the weights, as for instance, in 3, remains fixed while the other moves.
In this case, with the pivot pin E fixed, such a thing would be impossible, hence, in order to make such a relative movement between the two weights, the pin must move, and this motion is shown in 3, where it moves down from the line F. That movement, or change of position of the pivot E, is what takes place in the small intermediate gears in a train of differential gearing.
Transmission Wheel.—In Fig. 32 is shown a section of the differential housing, 1, in which, for convenience, all refinements of construction are eliminated. This shows the divided axle shafts 5, 6. In Fig. 33 is shown a side view of the same housing. This may be connected with the motor shaft by means of bevel gears, or driven by a sprocket chain. In either case the housing 1 is the substitute for the thrust bar B, in Fig. 28, and the bevel pinions 2, which are mounted within the wheel 1, represent the equalizer bar of that figure.
Fig. 31. Differential in Housing.
Fig. 31. Differential in Housing.
The gears which make up the train are usually put into a suitable casing, as illustrated in Fig. 31, which gives a good example of the construction.The housing A is fixed to the side of a large bevel gear B, this gear being designed to receive power from the motor through a bevel pinion C. One part of the axle D passes through the gear B, and is fixed to a bevel gear E within the housing, and the other part of the axle F passes through the housing and is fixed to a bevel gear G, the same size as gear E.
Intermediate the two gears is a pair of bevel pinions H, H, and these latter are mounted on pivots I, I, projecting inwardly from the housing.
The fact that the pinions are attached to housing has the effect of complicating the matter, so that it may be well to show the relative arrangement of the gears without the housing.
Fig. 32. Section of Differential.
Fig. 32. Section of Differential.
Fig. 33. Side View of Differential Wheel.
Fig. 33. Side View of Differential Wheel.
In Fig. 34 we have added to Fig. 33, two bevel gears 3, 4, which are mounted on the axles 5, 6,these representing the rear drive axles of the car.
Action of Transmission Gearing.—From the foregoing it will be seen that the axles abut each other, within the hub of the large gear 1, within which they are journaled. We might, therefore, call these pinions the counterparts of the bars E E.
Fig. 34. Top View of Differential Wheel.
Fig. 34. Top View of Differential Wheel.
As long as the resistance to the turning movements of the pinions 3, 4 is the same, the housing through pinions 2, 2, will simply carry the bevel gears 3, 4 around with it, without turning them, just the same as the equalizer bar B was moved forward without either end swinging back or forth; but the moment the wheel of the shaft 5, for instance, is compelled to travel at a higher rate of speed, or the wheel on shaft 6 meets with a greater resistance, the small equalizing gears 2 will turn, and the revoluble motion of the housing 1, while transmitting the power, and also carrying the gears, will act, in effect, the same as the push bar shown in the previous illustration.
Like the equalizing bar, the effect is to turn one wheel, say 3, with less, and the other wheel 4 with more than the normal power or speed.
Fig. 28 shows the principle on which all differential automobile gearing is based, that is, that both wheels receive half of the driving power even if one wheel should turn faster, as shown at Fig. 29, which is the case when turning a corner. This is what causes the power to drive both wheels at all times, whether going straight or on a turn.
Fig. 34a. Differential Gears.
Fig. 34a. Differential Gears.
If, however, one wheel gets on slippery ground, then A, Fig. 29, will move forward, without pulling on the lower end. As the lever A has the same action as the pinion in a differential, shown in Fig. 34a, it will be seen that if the pinion center is moved in the direction of the arrow, and if the wheel W1slips, the pinion will simply roll on the bevel gear G2without driving it on the wheel W2.
This is the disagreeablecharacteristicof a differential,that makes one wheel spin when it touches a slippery spot on the road, and stalls the car, because the other wheels cannot get any driving power.