A new reduced-weight spur differential could dramatically increase the fuel economy of passenger cars.
In the 1950s, a mid-sized passenger car weighed around 900kg. Fifty years later, this figure had markedly increased to around 1,600kg. This steep rise was a result of a host of new safety and comfort features that were partly driven by consumer demand, yet ran counter to both the manufacturer’s and the consumer’s demand for lower fuel consumption and emissions.
But in today’s energyand emissions-conscious world, every kilogramme that can be saved from the overall weight of a vehicle is critical, putting increasing pressure on both vehicle manufacturers and their OEM suppliers to minimise the weight of all the components in a car not in the least those in the vehicle drivetrain.
Recognising the importance of that fact, engineers at Schaeffler, a manufacturer of precision bearings and automotive engine parts based in Herzogenaurach, Germany, recently set about designing a differential for automotive applications that would address these concerns.
According to Thorsten Biermann, the engineer responsible for the development of differential systems at Schaeffler, the standard bevel gear differentials commonly found in vehicles have changed little over the years. Such differentials comprise a mechanical assembly of planetary and sun gears that enable each of the driven wheels of a vehicle to rotate at different speeds to prevent one or both of them from sliding when a car is making a turn.
Nevertheless, Bierman’s design team saw an opportunity not only to reduce the weight of such differentials, but to create a design that would also sport a smaller, slimmer mechanical envelope. Better yet, they were determined to ensure that their new design would be less expensive to manufacture, have a higher reliability and be less noisy than existing products.
“The differential can be produced cost effectively in high volumes using pressing and cold forming”
THORSTEN BIERMANN, SCHAEFFLER
From a purely functional perspective, however, there is no difference between a traditional differential and the new Schaeffler design; they both perform exactly the same role, splitting the torque from an engine in two ways. The major dissimilarity is that, while traditional differentials use bevelled gears, the new Schaeffler differential makes use of spur gears for the inner gearing of its planetary gear set.
As far as weight is concerned, however, there is a big difference between earlier incarnations of such differentials and the Schaeffler design. While the new differential is claimed to be 30 per cent lighter than standard bevel gear differentials, it is still able to produce the same torque.
Indeed, if the Schaeffler differential were to be incorporated in a car with a manual transmission with an input torque of approximately 350Nm, it would come in at a mere 6kg, compared with earlier bevelled gear differential designs that would weigh nearly 9kg. According to Biermann, this weight reduction would not only have a dramatic effect on increasing fuel economy and reducing CO2 emissions in such a vehicle but, as an added benefit, reduce emissions during the manufacturing processes used to make the differential too.
He added that this is because the decision to base the differential on spur, rather than bevel, gears had some significant implications on the choice of processes used to manufacture it. It was possible, for example, for the planetary gears to be pressed, rather than forged, while the housing of the differential could be manufactured from deep-drawn sheet metal parts.
Presently, only the sun gears in the differential are still forged parts, but Biermann said the company is working on developing technology so that, in the future, those could be cold formed too.
’Unlike a traditional bevel gear differential where the housing is normally cast and the inner gearings are forged, the new differential can be produced cost effectively in high volumes, for the most part using pressing and cold-forming manufacturing techniques, which are more energy-efficient processes,’ he said.
The company has designed two types of the new differential a symmetrical and an asymmetrical version after the designers recognised that the asymmetric design would be particularly useful in high-volume applications where car manufacturers need to save as much space in the vehicle drivetrain as possible.
Although Biermann admitted that the symmetrical version of the spur gear differential is not an entirely new idea a similar design was proposed by US-based Audel and Company as early as 1911 the Schaeffler differential with asymmetrically arranged planets and straight-cut gear teeth is most certainly a novel development.
However, as the asymmetric design demands different-sized sun and planetary gears and even different-sized planetary carriers, while the symmetric design does not, Biermann added that the cost of manufacturing it is likely to be higher than its symmetric brother.
On the spur gear design, since both bearings are at the same distance from the ring gear, they will wear equally
Despite the cost implications, the design envelope of both new differentials is considerably smaller than earlier designs, in part because the support bearings of the differential are on a plane with the main bearing of the output shafts.
’In the case of the asymmetrical design, the differential gearing even remains completely within the design space width of the final drive gear, meaning we can free up 70 per cent axial space in the gearbox,’ he said.
The short bearing support found on the new differential design also brings its own advantages. Biermann said that, because of where the bearings are located on the new differential, the radial forces at the bearings that support the differential are nearly equal.
On a traditional bevel gear differential, however, it is a different story. There, the bearings support the differential at different distances from the ring gear that drives them, with the result that one will experience higher stresses than the other. On the new spur gear design, since both bearings are at the same distance from the ring gear, they will wear equally.
However, as a standalone entity, the new differential has a lower stiffness in comparison with a bevel gear differential, because it enabled the Schaeffler engineers to redesign the complete transmission system including the bearings and the differential housing. The overall stiffness of the complete system is actually greater, with the result that the complete assembly produces less noise. Biermann said that, at the present time, Schaeffler is building prototypes of both symmetric and asymmetric differentials for several car manufacturers. This will be followed by an intensive design optimisation stage, after which Schaeffler will decide whether to move into series production of such units.
Presently, it is too early in the development cycle for the company to comment on what new model of car the new differential might be found on first. Nevertheless, having already received several orders for prototypes, Biermann is hopeful that the differential will make its way into production vehicles by 2016.
’Ultimately, the lightweight differential could save valuable space that might be used for either larger dual clutches or transfer gearboxes, as well as making room for electrical components on hybrid electric vehicles,’ he said.