Share

Articles - Archive

05/07/2011
The transmission

From the engine to the wheels

If the wheels don’t turn, even the most powerful engine won’t be able to make the vehicle move. The conversion of engine power into movement is indispensable and is carried out by a long chain of many components: the transmission

by Nicodemo Angì

Another choice is given by the orientation of the engine and wheel axes: if the engine is positioned longitudinally, two of the gears must be conical or hypoid so that they turn the differential axis 90° with respect to the gear. If the engine is transversal, cylindrical gears can be used because the engine and differential axes will be parallel.

 

Engine power and torque were adequately covered in clutch and gears: to start to move there is just one last stage, transferring them to the wheels. It is their job to transmit to ground the torque generated by the engine in such a way that the vehicle moves forward by reaction and so a transmission chain is required to transfer movement from the gear to the wheels.
In reality, the characteristics of many electric engines are such that they do not need a clutch or gears. One of the characteristics of these engines is that they deliver maximum torque at zero revolutions and can achieve a fairly high rotational speed thanks to the absence of alternately moving parts. These features make it possible to mount them directly onto the suspension so that the wheels can be fitted directly onto them. As the space available is fairly limited, it is obvious that these engines cannot be very powerful, but this is not a very serious obstacle given that suitable torque and the possibility of mounting even up to four drives, one for each wheel, give a satisfactorily dynamic vehicle. With today’s technology, the real limitation is the excessive weight of the non-suspended components, which could have a negative effect on suspension operation.

Sending the power where it is needed
Coming back to our internal combustion engine, we can say that the transmission is absolutely indispensable: we now know that it will not work if it is connected rigidly to the wheels. In fact it needs a clutch (or devices that perform the same function) and a mechanism that will vary the ratio between the engine revolutions and the rotation of the wheels. Moreover, in practically all cases, the engine-gear unit is not very close to the wheels and it is narrower than the wheelbase, making it necessary to transfer the drive power to where it is needed – the wheel-tyre unit.
Let’s take a look at the components in the transmission, starting from the gear output. Already at this “crossroads”, four different directions can be taken depending on the position of the engine with respect to the drive wheels. If they are close together, after the gear there will be a simple pair of gears (or a “train” of 3-4 components) called final reduction gears. These are upstream of the differential and reduce by a factor of about three-four times the number of wheel rotations with respect to gear output shaft revolutions; these reduction gears give the designer another element for ensuring that engine power corresponds to the vehicle into which it will be mounted.
Another choice is given by the orientation of the engine and wheel axes: if the engine is positioned longitudinally, two of the gears must be conical or hypoid so that they turn the differential axis 90°  with respect to the gear. If the engine is transversal, cylindrical gears can be used because the engine and differential axes will be parallel.
If the drive wheels are a long way from the engine, and even if the vehicle has four-wheel drive, a driveshaft must be used to connect the gear output to the gear box. It is more or less tubular in shape and usually has at least one Cardan joint. Sometimes it has two or more sections that are connected together and an intermediate support; this is compulsory for industrial and commercial vehicles with rear wheel drive because the shaft is very long. The driveshaft has a spline joint which permits the axial movement of the two sections of the shaft: this is needed to absorb the variations in distance during the vertical movement of the suspension. This type of coupling has a sleeve with internal splines into which the end section of the shaft - splined in the same way but externally – is inserted to allow it to move longitudinally. In four-wheel drive vehicles, two drive shafts are sometimes used to connect the central distributor to the front and rear differentials. An interesting variant of these transmission layouts is the transaxle used in front-engine vehicles with rear-wheel drive. Housing the clutch, gear and differential, it is a fairly heavy component that spreads the weight better than the “all-at-the-front” configuration in which only the differential is at the rear. The transaxle sometimes has a large tube that connects the engine rigidly to the rear unit and the drive shaft rotates inside it.

Differentials & Co.
Another very important part of the transmission is the differential, the device that allows the drive wheels to turn at different speeds when turning a corner. It is essential simply because the inside wheels cover a shorter distance than the outside wheels when cornering and although this is not a problem for wheels that rotate independently, the drive wheels would spin if they were rigidly connected to a common axle. This device is housed in a cage with two axles which have two satellite gears mounted on their ends. They mesh with two planet gears rigidly connected to the semi axle shafts that make the wheels turn. The differential cage is attached to a large gear wheel called the crown, which is moved by the driveshaft. The satellite gears do not rotate when driving in a straight line, but when turning a corner they will rotate slowly around their axes thus allowing the semi axle shafts to move.
However, there is a problem with this type of differential if one of the wheels loses traction: in this case, almost zero torque will be transmitted to the wheel with more traction, but the other wheel will spin. To overcome this problem, manual or automatic locking differentials have been developed. By means of couplings, plates or clutches, they temporarily prevent the relevant two semi axles from slipping; because of the abrupt operation and sudden variations in alignment that are felt when locking, manufacturers developed limited slip differentials which do not lock completely. Various systems, including electronically-controlled systems, were developed; mechanical devices like the Torsen differential are very popular.
A particular type of differential is the torque transfer used in four-wheel drive vehicles to distribute the torque produced by the engine to the drive axles by an mount set at the design stage. In general, this is done by the same component that allows a small variation in speed between the two axles when cornering. The percentage of torque that must go to the axles can be electronically controlled or mechanically set, for example with epicycloidal gears similar to those used in automatic transmissions.
The other vital components in a transmission system are the couplings used to connect two shafts whose rotational axes are misaligned and may form an angle of varying degrees: they are used in the steering control, the drive shafts and the semi axle shafts.
The universal joint is one of the simplest (it was conceived a few centuries ago) and consists of a cross shaft with four joints and bearings. Two of these connect to a fork mounted on the driveshaft and the other two to a similar fork connected to the driven shaft. In this way, movement is transmitted even if the shafts are misaligned, but when the axes of the two shafts do not coincide movement transmission decelerates and accelerates even if the speed of the driveshaft is constant. That is why this type of joint is now used almost exclusively to control steering and the drive shafts in which the operating angle is never too high.
In front-wheel drive vehicles, constant-velocity joints are now universally used for steering systems as they allow large variable angles at constant rotational speeds. They have two elements: one is connected to the wheel and is hollow with a spherical inner. The inner face of the external element and the outer face of the inner element (excuse the tongue-twister) have grooves which guide special steel balls. In this way, movement is evenly transmitted even if the rotation axes are angled because the balls can run transversally in their tracks without any discontinuity.
There are other types of components that can be used to create a transmission chain, but to keep this short we won’t talk about them: we don’t want to fill up the entire magazine...
We’ve now come to the wheels, the kingdom of many readers of Pneurama, and the end of our mechanical journey: the car can now set off. A safe journey to you all.

back to archive