NSK goes to considerable lengths to adapt a couple of its standard
spindle bearings for critical duties in a most unusual application
The efficiency of the drive system was a vital consideration in the
building of this car, says Colin Fallows, describing the design of his
and partner, Mark Newby's impressive UK entrant for a world electric land
speed record attempt in South Africa next year. A great deal of effort
has gone into maximising the electrical, mechanical and aerodynamic
efficiencies of the 'e=motion' car, and the bearing solution is
particularly illustrative of those efforts in the development of the
drive train. e=motion co-sponsor, NSK came up with the solution, based on
two of its standard 'Super Precision' spindle ball bearings. The smaller
contact area of the balls, compared with rollers, result in lower
friction and provide space within the cage for a non-pressurised oil
lubrication system.
Ensuring a consistent delivery of lubricant to these otherwise completely
dry bearings is absolutely critical to their success in this application.
Sealed grease-lubricated bearings would have added stiffness, affecting
the vehicle's acceleration from rest. The car accelerates relatively
slowly up to 100mph as it overcomes the initial inertia of a 2,400lb
rolling mass. Beyond this point, e=motion accelerates better than a high
performance road car , according to Mr Fallows.
Lubricant is gravity fed from a reservoir, so no pump is required, which
would have added weight. Indeed, the lubrication system is just
sufficient to dissipate heat and maintain a reasonable margin of bearing
life in this highly unusual application. In machine tool spindle
applications, these NSK bearings would normally have a life in excess of
10,000 hours. However, the belt drive (see photograph) imposes a severe
radial loading, which considerably reduces their life expectancy. NSK
engineers had to reach a compromise by balancing the weight, load,
frictional losses and physical size of the space available to achieve an
optimum solution. Mark Newby pays tribute to their efforts:
NSK's support for this project has been fantastic. The engineers
remained flexible and inventive enough to bend the normal rules of
bearing specification to achieve an ideal solution for our unusual
requirements.