ISSUE 012 Winter 2021 Sigma Powertrain EMAX transmission dossier l In conversation: David Hudson l 48 V systems focus l 2021 Battery Show North America and Cenex-LCV reports l Everrati Porsche 911 digest l Switching insight l Motor laminations focus

power-on in traction and regeneration, and also that it can rotate freely in both directions without generating power; ‘00’ signifies ‘zero-zero’. Kimes comments, “That is the same functionality as a dog clutch, but our clutches require much less force to change state. We have another variant that is 11/10/01/00, a four-state clutch that can be locked in both directions, or as a one-way clutch functioning in either direction, or be completely off. “That allows heavy EVs to have functions such as hill-holding, where half the clutch is ‘on’ and the other half is ‘off’, or parking where both sides of the clutch are ‘on’.” Engagement of the bidirectional rocker clutches is typically achieved using a pair of electrically actuated solenoids installed inside the electronics box next to the ECU. Each solenoid acts on one rocker, which locks the controlled element in one direction of rotation. “If the torque capacity needs to be higher than a single strut and rocker can supply, we’ll install multiple rockers,” Kimes says. “This is known as dual or triple engaging, where two EMAX anatomy Assuming a two-motor solution is requested, the customer’s choice of two electric motors (and inverters, if these are packaged with the motors) directly sit on the input side of the EMAX powertrain. Various components are used to transmit and multiply the input torque from both motors at the back of the EMAX to a single output shaft at the front. These include numerous fasteners and ittings, a total of 12 bearings, a dynamic gear assembly and a static gear assembly. While many of them are installed in the powertrain by hand, the static gear assembly weighs 68 kg and is therefore lifted and lowered into each system’s enclosure by crane (after being built separately and held together by snap rings). The enclosure is roughly 0.25 in thick, and is typically cast from A390 aluminium alloy for strength and machineability. On the side of it is an electronics box containing two electromechanical solenoids and two microcontrollers. Integrating the components into the housing starts at the motor input side. First is the assembly responsible for third gear, which consists of a stator assembly, then a dynamic clutch assembly and a notch plate. This plate is made from a toothed ring, sintered to shape using powdered metals, and a hub, which is forged before being sinter-brazed to the ring in a furnace (ensuring a joint stronger than its parent materials). The stator is fastened to the housing, while the dynamic clutch assembly largely sits freely until engaged with the notch plate, which sits on a bearing that is integral with the dynamic clutch assembly. Installed after these is an input shaft made from carburised 8620 steel, which is driven by motor A, with the clutches acting separately on this shaft to put motor A into irst, second and third gears. Motor B drives a secondary ‘input shaft’, essentially a pocket plate installed coaxially around the input shaft and inside the notch plate. To mate the motors with their respective input shafts, the motors are con igured with output shafts that extend into the EMAX housing via radial bearings, secured in place by an input shaft seal on the outer side and a scarf cut seal on the inner side. Installed around the notch plate is a larger, dual-clutch assembly, which also mounts a carrier (made from iron in two parts, also sinter-brazed together) holding four planet gears. These run inside a ring gear and around a sun gear, with a roller bearing installed inside the sun gear for mounting on the input shaft. Also, two more clutches are installed here. One grounds the sun gear (and by extension, its planet gears and ring gears) to the outer housing, while the other grounds the sinter-brazed notch plate to the irst planetary carrier. The clutch assemblies are made largely from powdered metal, formed and hardened through sintering. SPT notes that sintering powdered metals is key to its complex part geometries, which so far have not been achievable with casting or CNC machining. A second planetary gearset, largely identical to the irst, sits beyond this. Lastly, an output shaft sits at the end; it is mounted into a cast aluminium housing cover with an O-ring, two tapered roller bearings and a seal ring. A di erential end-yoke is installed on the outermost end of the output shaft. All the gears in the powertrain – sun and ring gears, and the eight planet gears – have rounded teeth. These are ground after being heat treated, to prevent heat treatment-related problems such as warping from a ecting the tooth geometries or causing noise during operation. This latter point is critical to suiting the gearbox to EV powertrains, which run far quieter than IC powertrains. Some surfaces of the input shafts are also ground after heat treatment in order to mount bearings more securely. Exact materials and treatments for some parts (particularly the gears) are not yet fully decided, although the company does say it is considering nitriding heat treatment on the ring gears for optimal hardness. 5140 alloy steel could be used for the input shaft, as it might be more cost-e ective than 8620. Metals and treatments used in the EMAX’s parts may change in the future, although SPT is confident that its supply chains are fully decided Winter 2021 | E-Mobility Engineering 25 Dossier | Sigma Powertrain EMAX transmission