The differential mechanism allows a vehicles’ driving wheel to revolve at different speed when going around a corner.
The following diagram represents a bevel wheel differential:
A and B are the two sun gears or equal size which are keyed to the two halves of the rear axle. The planetary gears revolve on pins carried by the casing or the planetary-gear carrier, shown by the letter C. The drive is transmitted from the driving shaft to the casing through the pinion and crown wheel. The planetary gears mesh with the sun gears which enables the sun gears to revolve in opposite directions if the planetary-gear carrier where to be stationary.
When the vehicle is moving in a straight path, the sun gears and planetary gear carrier revolve at the same speed, whereas the planetary gear remains stationary relative to the planetary gear carrier.
If the vehicle follows a curved path, the sun gears revolve in different speeds and the planetary gear carrier will revolve at a speed that is the arithmetic mean of the speeds of the sun gears. The planetary gears will revolve on their pin.
A conventional differential has the disadvantage in that the power from the engine is directed to the wheel with the least resistance. Furthermore, fitting differing size tyre to the driving wheels would result in continuous operation of the differential mechanism.
A detailed analysis will now be looked at of an unusual form of final drive and rear axle: the rigid axle found in front engine cars with rear wheel drive. Different arrangements exist in independent rear wheel suspension or where the engine is located at the back driving the rear wheels, or where the engine is found at the front driving the front wheels.
There are two functions which the final drive must perform.
1. Provide a right-angled drive from the propeller shaft to the road wheels and,
2. Provide constant speed reduction regardless of the gear engaged in the gearbox.
Fulfilment of the functions is achieved with the use of one set of gears known as the crown wheel and pinion.
The pinion wheel is mounted to the end the propeller shaft and meshes with the crown wheel which is a larger gear. As these are bevel gears, they execute the first function of the final drive in providing a right angled drive. Speed reduction that may be attained is dependent on the ratio of the number of teeth in each wheel. For instance, a ratio of 4:1 means that in top gear, the engine turns 4 times faster than the road wheels. These days, in order to reduce the height of the propeller shaft hump in the floor of the car’s rear compartment, hypoid gears are mainly used.
The drive to the road wheels is not taken directly from the crown wheel, but from a cluster of gears known as the differential. If 2 wheels were driven by one shaft connected directly to the crown wheel, the outcome would be both wheels revolving at the same speed. The implications of this are slipping and sliding, rapid wear of the tyres, and difficulty in steering. The solution to counteract this scenario is having the wheels driven through the differential and joined to it by shafts known as half shafts.
The crown wheel is attached to a frame carrying a pair of bevel pinions. These bevel pinions are free to rotate on their bearings. Two bevel gears, which are mounted on the half shafts, mesh with the pinions. The crown wheel is therefore not connected directly to the half shafts. When the car is travelling in a straight line, the turning crown wheel carries round the frame and bevel pinions with it, and in turn rotates the bevel gears, and therefore the wheels are driven at the same speed.
A cornering car needs the outside wheel to speed up and the inside wheel to slow down. Since the crown wheel and frame are still being driven at constant speed, the bevel pinions are forced to roll round the slowed up bevel gear. As these pinions are rolled, they turn on their bearings and so turn the bevel gear on the outside wheels half shaft, ultimately causing this wheel to turn faster. The amount of speed increased in one wheel is always equal to the amount of speed reduced in the other wheel. The average of the two wheel speeds is equal to the speed at which the crown wheel and frame are rotating.
In the case that one wheel does not have grip, the other will remains stationary and causes the slipping wheel to rotate twice as fast as the crown wheel. Slight operation of the differential can occur with differing tyre pressures or uneven loading.
The final drive, differential, and the half shafts are enclosed in a casing called the axle housing, with the exception of using independent rear suspension.
Other types of final drive
When the engine is mounted at the rear of the car, there will be no propeller shaft as such, and the gearbox is also located at the rear. The differential is either attached to or included in the gearbox, therefore, there is no space for a rigid axle and it cannot be allowed to move up and down with the road wheels. In this case, the half shafts are fitted with universal joints similar to those used on the propeller shaft. This arrangement also applies to cars with independent rear wheel suspension.
Engines mounted at the front and driving the front wheels need a transmission system capable of adapting to the independent suspension as well as the wheels to be steered.
To determine the layout of the front wheel system, the way in which the engine is mounted must be known. The engine may either be mounted in line with the chassis or across it.
For an engine mounted in line with the chassis, the gearbox will also be in line and the conventional type of final drive is required. Both the final drive and gearbox may be in the same casing.
A transversely mounted engine may have the gearbox bolted to it in the conventional way, or the gearbox may be found in the engines sump. If a gearbox is in the engines sump, there is no need for a right angled drive and the crown wheel and pinion are replaced by, for example, a pair of helical gears, one of which carries the differential.
Given that the drive to the wheels must allow for vertical movement of the front wheels on their springs, and, lateral movements of steering, the half shafts are connected to the differential and the wheels using special universal joints known as constant velocity universal joints.
The planet pinions also mesh with the reverse sun gear. For all the forward gears, the turbine of the torque converter drives the forward sun gear, and in reverse, the reverse sun gear.
Power is always transmitted via the ring gear connected to the output shaft, to the final drive. The selection of the various gear ratios required is performed by engagement of hydraulically-operated clutches and brake bands.
The front clutch connects the turbine to the forward sun gear.
The rear clutch connects the turbine to the reverse sun gear.
The freewheel clutch prevents over-run engine braking.
The front brake band holds the reverse sun gear stationary.
The rear brake band holds the planet carrier stationary when engaged.
In a brief and simple summary, the following text will describe how the gearbox works with reference to 5 manually selected conditions of operation.
1. N- Neutral
All the clutches and brake bands are released; therefore, the gear set is disconnected from the torque converter. There is no transmission of power from the engine to the final drive.
2. D- Drive
Moving the selector lever to D engages the forward clutch and the turbine drives the forward sun gear. The freewheel clutch prevents the carrier from rotating backwards. The ring gear and output shaft are driven through both sets of planet pinions in the same direction as the sun gear. The reverse sun gear freely spins in the opposite direction while the freewheel clutch prevents over-run engine braking. This occurs in first or bottom gear.
In second gear, the drive is through the forward clutch and forward sun gear, however, the reverse sun gear is held stationary by the front brake band. Consequently, the planet carrier with the pinions is compelled to revolve in the same direction as the forward sun gear. The ring gear is now being driven faster. Over-run braking is provided by the freewheel clutch.
In third gear, the drive remains as previously, but now the rear clutch has operated to connect the drive also to the reverse sun gear. Virtually, both sun gears are locked together and there can be no movement of the gears in the train. The whole set revolves as a solid unit providing direct drive from the turbine to the output shaft.
3. R- Reverse
With the selector lever at R, the rear clutch is engaged and the turbine drives the reverse gear. The planet carrier is held stationary by the rear brake band so that the ring gear and output shaft are driven through the long planet pinions only. This results in rotation in the opposite direction at lower speed.
4. L- Lock-up
In L, the drive is via the forward clutch and forward sun gear but the rear brake band holds the planet carrier stationary. The ring gear is driven through the planet pinions as in all forward gears except that now there is no freewheel. This allows for maximum engine braking when descending steep hills.
5. P- Park
In addition to releasing all clutches and brake bands, a pawl is engaged with the outer teeth of the ring gear, therefore, locking the output shaft and immobilising the vehicle.
