By Alexander Spatzig, MHI (Mitsubishi Heavy Industries)
While cost and range may be the big barriers to people buying EVs, automotive OEMs also have to be alert to the less obvious engineering challenges of EVs.
Sitting firmly in this category is the issue of Noise, Vibration and Harshness (NVH). Now NVH is nothing new to car makers, and acoustic engineers have long been employed to ensure a car sounds right and is a “smooth” ride.
However, contrary to popular opinion, EVs are not silent. Removing an Internal Combustion Engine (ICE) and replacing it with an electric drivetrain does not eliminate NVH issues. It merely reveals new challenges.
Without the sound of an ICE, the driver becomes aware of other noises, such as high frequency electric motor generator noise, power control unit high frequency switching noise, power-split system gear whine, the engine start/stop noise and vibration.
Addressing these new NVH issues requires an attention to detail that goes all the way down to assessing how individual components are made.
This is especially true when considering the transmission systems OEMs install into EVs and HEVs.
Whether they are multi-speed HEVs, one-speed EVs or those with the proposed new two-speed transmissions, it is highly likely their powertrains will rely on planetary gear systems as their key speed to torque conversion component.
These systems are also used in automatic ICEs, but the absence of engine noise in an EV means the planetary gearing’s inner workings can become a cause of NVH issues without due care and attention to the gearing components inside.
This is especially true of internal ring gears. While external gears have been optimized in production in their conventional set-up for many years, internal ring gears have been made using relatively low-cost techniques that do not include finishing processes like gear grinding, leading to a rough finish. In ICEs the NVH issues caused by the rough finish were minimal, as they were masked by the sound of the engine.
But the ascent of planetary gearing systems in EV powertrains means it is now necessary to ensure internal ring gears are of a similarly high quality as their external-gear counterparts.
Achieving this means replacing the conventional ring-gear production techniques of shaping and broaching with the alternative process of skiving.
This process, which involves the continual cutting away at the gear to create its teeth, allows for better productivity and higher flexibility. However, its industrialization on a large scale is being inhibited by a shortened tool life, due to the intensity of the process.
Super skiving and internal generating gear grinding
In response, Mitsubishi Heavy Industries Machine Tool (MAT) has developed a proprietary Super Skiving Cutting (SSC) tool with the objective of improving the cutting conditions and increasing the tool life.
The SSC is a skiving tool consisting of multiple blades. Each blade represents a pinion skiving tool with a specific amount of cutting teeth. The cutting volume can therefore be more widely distributed and the cutting load per tooth can be reduced.
Testing by the Machine Tool Laboratory at Germany’s Aachen University¹⁶ found that the SSC technique could improve the cutting of hard metals necessary for ring gears.
It could deliver a longer life for the cutting tool itself, making the skiving process more cost-effective. The process also reduces distortion in the final heat treatment that ring gears receive to give them the necessary hardness to cope with the loads in the vehicle’s gearing system.
After this heat treatment gears are finished through a grinding process that, like skiving, has until recently been difficult to scale for mass production.
The grinding of the gears refines them and eliminates any distortion that occurs in heat treatment. To make the grinding process more efficient and better suited to mass production, MAT has developed an internal generating gear grinding machine that has the world’s first continuous internal grinding process.
Automotive manufacturers and their supply chains are going through a period of unprecedented change.
While the challenge of the past 10 years has been to make vehicles more efficient, for the next decade it will be to electrify as rapidly as possible – both to meet consumer demand and to be able to withstand the banning of new petrol and diesel vehicle sales set to come into force in multiple economies from 2030.
Doing so will require car makers and their supply chains to be innovative in how they design and manufacture every single component of EVs.
It will require new levels of collaboration and a willingness to dedicate production lines to EV-specific models. Current thinking on how EV transmissions should operate will need to be challenged.
And investment in equipment that can deliver the quality and efficiencies required to take EVs mainstream will be essential.