Leveraging Software for Advanced Gearbox and Drivetrain Development

Due to the need for mass reduction, tight packaging, and overall car performance, it is necessary to reduce the weight and dimensions of every component of the car, including the gearbox and other drivetrain components. It is also very important to ensure that gears, bearings, and shafts can withstand the whole racing season and have the necessary safety factor. Using KISSsoft software, FSB Racing Team was able to develop a new gearbox for the AWD Formula Student car, which can easily fit inside upright and withstand 50 hours of racing.

Input Data For all calculations, the load spectrum was used. The team got the information from the Vehicle Simulations module, and then we modified it to suit the input for the KISSsoft software. Figure 3 shows how the load spectrum looks for one electric motor.

First Stage Gear Design and Calculation During gear design, it was important to contact the manufacturer to get information on what tooth cutter tools they have in stock, since new custom tools are expensive and take weeks to manufacture. Considering other manufacturing constraints, it was decided to make the sun and planets for the first stage out of nitriding steel.

Considering high loads on the car wheel and, at the time of designing, unknown wheel hub and upright stiffness, gears had to be as thin as possible. The inner diameter of the sun gear dictated a higher number of teeth, too, and therefore planets with a greater diameter. Using the data from the manufacturer's tool, the number of teeth for the sun and the planet is determined. General gear geometry data can be seen in Figure 3. After determining gear geometry, the strength calculation was carried out. Calculation was done according to the newest standard ISO 6336:2019 and considering the load spectrum prepared by the Vehicle dynamics module. To ensure that gears will withstand, safety factors for the tooth root of 1,1 and 0,65 for the tooth flank were set. The reason for such a small tooth flank safety is that pitting develops after 50x106 cycles according to the norm, and the gearbox will never achieve this number of cycles. Additionally, the gearbox on the previous car had also tooth flank safety below 1, and it has endured more than its required service life without signs of pitting. The required service life was also 50h. After calculation, it was determined that safety factors are higher than required (Figures 4 and 5), and the gearbox can withstand longer than is necessary. That is desirable considering the unpredictability of real racing conditions.

Second Stage Gear Design and Calculation Since the second stage module is going to be 1 mm, the standard gear profile 1.25 / 0.38 / 1.0 ISO 53:1998 Profile A is going to be used. The planet is going to be made of case-hardening steel, but the ring gear will be made of nitriding steel, considering manufacturing challenges. Gear geometry data can be seen in Figure 7. As for the first stage, calculation was carried out according to the newest standard ISO 6336:2019, considering the load spectrum. Required safety factors and service time are the same as for the first stage, too. In Figure 8, the achieved safety factors can be seen. Planet Bearing Calculation Choosing a planet bearing was challenging, considering various design constraints. The inner diameter of planets was limited by tooth root strength and the diameter of pin-by-pin bending. Since the pitch diameter of the planet is very small, a high bearing load was expected. It was decided to use two needle roller bearings. They are light, have small dimensions and can withstand high revolutions. The planet assembly model can be seen in the figure. KISSsoft bearing database was of great use, since there was a large number of bearings and a lot of parameters could be adjusted. Since the gearbox is to be driven at high revolutions and loads, it was necessary to take into account the influence of higher temperatures on the lubricant. As for the other parts of the gearbox, the required service time for the bearings is 50h. They are calculated according to modified life (ISO 281) to simulate real conditions as well as possible. Results can be seen in Figure 11.