From a power transmission standpoint, Electric Adventure Vehicles (EAVs) pose an interesting design problem. On one hand, they require transmission components that are not too heavy or bulky. On the other, they need lots of torque or they won’t be much fun to drive. When Outrider USA, which manufactures a line of EAVs, looked to solve this torque density problem, it chose a Neugart planetary gearbox.
Outrider makes two basic models of EAVs: the Alpha, built for everyday use on gravel and streets, and the Horizon, for off-road cycling. Both models have four variants, each with a variety of design features for able-bodied and disabled riders alike. These configurations include: off-road handcycles, foot pedals, rising seats and adaptive tri-pin hand controls. Each model can be outfitted with 1 to 4 kWh.
For high-speed road applications, Outrider chose the single-stage version of the Neugart PLQE 60-mm gearbox, which has a 5:1 gear ratio. In applications where lower speeds are needed, the design team used a two-stage version with a 15:1 gear ratio. Both gearboxes have identical mounting configurations, the difference being that the two-stage is slightly longer.
“It’s a nice, simple system that directly couples with the planetary transmission and transmission drive,” said Outrider Co-founder, Lead Designer and CTO Tommy Ausherman.
Designing for torque density
Outrider engineers initially struggled to find transmission components that met their performance standards, packaging requirements, weight and cost targets. They considered several options, including a two-stage belt drive. However, this option didn’t provide the necessary reduction for all of Outrider’s different models. Had they gone down this route, they would have needed a three-stage belt drive that would have taken up almost three times the space of the PLQE planetary gearbox.
One of the biggest challenges Outrider engineers faced was finding a gearbox with enough torque density to handle up to 13,000 rpm. Since EAVs use a high-performance motor, the gearbox they selected had to meet both rpm requirements and desired output torque. Most smaller gearboxes can handle the rpm requirements, but offer less torque than needed in this application. With up to six planets, the PLQE 60 has a higher torque density in its given frame size than gearboxes with fewer planets.
Outrider engineers also took advantage of the ability to overload the gearbox, cranking up the speeds and torque. The gearboxes in this size have a regular torque rating of 40 Nm. Even when overloaded, the gearbox can maintain its performance.
Since the PLQE 60-mm gearbox weighs only about 2 lb, it helps keep the drive system weight low at roughly 7 lb. Outrider’s power-dense drive system puts out about 5.5 hp out of a 7 lb package—almost a pound per horsepower.
For most gearboxes, you need an adaptor for connecting the motor and gearbox. This adds extra costs, weight and dimensions to the system. However, the PLQE 60 comes with a direct motor attachment (DMA) for easy, direct mating to the EAV’s motor. This integrated, space-saving design reduces weight and costs, and cuts down on size. The PLQE 60 also comes with an attached flange for easier mounting and rigidity, creating a stiff connection for the drive system.
The PLQE 60-mm gearboxes have other features that were useful in this EAV application as well. Outrider needed a low-noise model that didn’t sacrifice other important attributes—they didn’t want a gearbox where the wind up overpowered the electric motor’s hum. Efficiency was also crucial.
In a single stage, these gearboxes have a 97% efficiency rating. On the other hand, their double-stage gearboxes are 94% efficient. Outrider knew that using an efficient gearbox would increase both the power density and lifespan of their system.