Racecar Engineering – September 2019

72 http://www.racecar-engineering.com SEPTEMBER 2019

TECHNOLOGY – TRANSMISSIONS

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specifying alternate oil seals and bearings. With

the input shaft spinning at up to 30,000rpm you

need some pretty trick technologies there to

reliably survive a season of racing.

‘We’ve also seen step changes in technology

in these components over the last three to four

years,’ Blevins adds. ‘Refinements in the design

of these components have helped reduce their

losses, adding further fractions of percentage

points to your overall transmission efficiency.’

Case study

Gains can also be made in the design of the

gearbox case. Traditionally, these were cast,

but for Formula E they are now fully machined

from billet aluminium. This means that wall

thicknesses are now down to 1.5 to 2mm thick

and these are consequently heavily ribbed to

help take the gear loads. ‘Really [the casing]

wall is now just a shroud to stop the oil from

leaking out,’ Blevins tells us.

The potential gains of optimising all these

aspects of the transmission are modelled and

then combined to predict the overall efficiency

of the Formula E transmission. This is achieved

through two simulation tools specifically

developed by Ricardo. The first is called SABR,

which models the driveline to determine the

shaft deflections and bearing loads, which can

then be used to start predicting the potential

efficiency of the gearbox.

‘Shaft deflections can potentially misalign

the bearings and also affect the meshing of

the gears,’ Blevins says. ‘But there may be a

case where you will allow the shafts to deflect

as increasing their stiffness may force you to

add extra weight. Take the example of a simple

shaft with a gear in the centre and a bearing

either side. When you subject that to a torque,

the natural reaction is for the gears to start to

separate, so you effectively ‘banana’ the shaft

away from the gear mesh. This will twist the

bearings and introduce bearing drag because

the inner race and rollers are running misaligned

to the outer race. Our experience then tells

us what we can do to minimise this effect,

either through modifications to the bearing or

refinements in the case design.’

Tooth and nailed

All this is simulated by SABR, which then feeds

into another piece of software called GEAR.

This analyses everything to do with gear teeth

and tooth micro geometry. SABR and GEAR

work together in an optimisation loop, running

simulations that determine the effect of shaft

deflection on the gear meshing and then

modify the geometry to account for this. The

result is an optimised transmission efficiency

prediction which can then be simulated for a

range of motor speeds, generating a percentage

power loss matrix which is used to support and

guide further development.

‘The benefit of SABR and GEAR is that they

talk to each other,’ says Blevins. ‘For example, we

may be able to save some weight by increasing

the bore diameter of a shaft but then that might

allow the shaft to deflect too much, which could

then misalign the gears and bearings. So, we

might have saved 200g in gearbox weight, but

SABR and GEAR will show that we have lost

maybe 0.5 per cent efficiency because the gears

are now running out of position.’

However, there is no substitute for physically

making and testing parts, which is why

Ricardo has also developed a transmission rig

specifically for Formula E gearboxes. This^

allows it to run the transmission at maximum

torque, low speed and low torque, maximum

speed and everything in between to determine

at which rpm range the gearbox is most

efficient. ‘Typically, a gearbox is signed off

with a number of points within that matrix

spread throughout the full torque and speed

range of the motor,’ says Blevins.

With the season six regulations retaining

the same levels of available power from the

motors, it is likely that we will see an increase

in these impressive efficiencies as the teams

in Formula E, and their suppliers, continue to

chase the smallest of gains.

‘With the input shaft spinning at up to 30,000rpm you need some

pretty trick technologies to reliably survive a season of racing’

CNC gear-cutting equipment at Ricardo. Modern machinery allows the manufacture of extremely complex cog geometries

SABR and GEAR are simulation tools developed by Ricardo which work together to optimise gear design