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A motor success story that turns on its bearings

14 June 2011

Innovative hybrid bearing technology has enabled engineering consultancy and electric vehicle manufacturer, Delta Motorsport, to design and build from scratch an all-electric road car that is capable of 0-60mph in less than 5 seconds and a top speed in excess of 150mph

The Delta E-4 Coupe is a stylish 4-seater, 2-door electrically driven coupe that boasts a class-leading range of more than 200 miles on a single charge. The vehicle can accommodate four adults, but the acceleration is comparable to a Porsche 911 Turbo. The key to this impressive performance is the direct-drive electric motor that powers the car. Designed in partnership with Oxford University and Oxford Yasa Motors, the new axial-flux design generates well over 600Nm of torque while only weighing 23kg. Delta Motorsport technical director, Nick Carpenter takes up the story:

“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. Ultra low energy vehicles such as the E-4 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 and it opened the way to the development of a highly-efficient plug-in battery electric car that could be both stylish in its design and financially viable but also capable of overcoming the 'range anxiety' perceived by many purchasers of all-electric vehicles.

The challenge facing Schaeffler 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 assembly within a tight timescale. The first samples were delivered four months after work on the project began. The initial bearing design concept proved to be too expensive and threatened to jeopardise the project, Schaeffler’s senior applications engineer Stewart Davies recalls. “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 Mr Davies, a hybrid bearing has all the insulation qualities required for use in electric motors, including low friction, but the non-contact shield doubles the life expectancy of the grease, which further enhances the performance of the bearing.

Dr Tim Woolmer, a member of the research team that developed the YASA motor says the partnership with Delta and Schaeffler helped them to create performance prototypes of the motors very quickly. “The design has proven to be so robust at high torque and high speed conditions that Oxford YASA Motors have adopted it as part of the next generation design being finalised for higher volume production.”

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