The biggest challenge is likely going to be maintaining the axial gap (bearing system):
"The bearings were absolutely critical to enabling the modular design of the Yokeless and Segmented Armature [YASA] motor that we helped to design with Oxford," says Nick Carpenter, technical director at Delta Motorsport. "Ultra Low Energy Vehicles, such as the E-4 Coupe, rely on lightweight components to compensate for the inherent problem of the low energy density of lithium batteries when compared to gasoline.
When I first came up with the idea of the new bearing arrangement that would help to optimise the space available for the stator, while accurately controlling the air gap to the rotors, I almost dismissed it, due to the fact that standard bearings would have been too big and too heavy. However, following detailed discussions with Schaeffler UK, the company's engineers were able to offer a special design of hybrid double-row angular contact ball bearing, which has a split inner ring that was able to meet the demanding criteria of reduced weight and cross-section."
This was the breakthrough that Delta Motorsport needed, if the company was to fulfil its ambition of developing a highly efficient plug-in battery electric 'car for the people' that is both stylish in its design and financially viable, but also capable of overcoming the 'range anxiety' perceived by many purchasers of all-electric vehicles.
As Stewart Davies, senior applications engineer at Schaeffler UK, recalls: "It was a very interesting project to work on, with very demanding criteria to meet. The challenge was to keep the bearing as narrow as possible, in order to meet the dimensional constraints of the design envelope inside the YASA motor, whilst at the same time offering a commercially viable bearing solution within a very tight timescale."
Schaeffler UK was able to supply the first samples four months after it started work on the project. Davies continues: "The initial bearing design concept proved to be too expensive and threatened to jeopardise the project. Following intensive design calculations, we were able to offer a single hybrid bearing that met the key design criteria of high performance, reliability and ease of assembly, but that could also be competitively produced in volume."
One of the problems encountered by Schaeffler's design team was the high seal lip speed. With the bearing rotating at such high speeds, all calculations indicated that the seal would wear out and that the bearing would fail prematurely. By specifying a non-contact shield, this problem was solved. Another advantage of using a non-contact shield is the increased life expectancy of the grease. According to Davies, "a hybrid bearing has all the insulation qualities required for use in electric motors, including low friction, but the non-contact shield has doubled the life expectancy of the grease, which has further enhanced the performance of the bearing"
I would like to see a YASA hub motor