another or separated). This information must be very reliable, as the distance of brake disc  separation in on the order of 0.25 mm to 0.30 mm. /g21/g12/g3/g42/g72/g68/g85/g3/g69/g82/g91/g3/g68/g86/g86/g72/g80/g69/g79/g92 :   a) Gear train & bearings:   A four-stage planetary gearbox has been chosen with a gear ratio of  1: 450. The gear trains are made from SST. Nitrided steel has been used as gear material. The  thermal expansion shall be compatible with the gearwheel material, to keep the influence of  thermal deformation on the backlash at the lowest achievable level. Radial deflections at the ends should be small, they are critical for the motor assembly and gearbox. For the gear train, radial deflections affect the backlash; for the motor, they affect the torque ripple due to an adverse effect on the motor resolver. b)Number of planet wheels :  A strong preference was maintained in the design to apply three planet  wheels per stage. This is due to the fact that a three planet wheel system possesses a self-balancing capacity for the loads within the system. This acts as a load distribution system. If more than 3 planet wheels are applied in one carrier, the equal load distribution over the wheels is not ensured automatically. However, to save space, and to make sure that the stress levels do not exceed the maximum allowable Hertzian stress, it was necessary to apply 6 planet wheels in the last stage. To obtain a good load distribution over all 6 planet wheels, the planet wheels are supported by a floating system. c)Tandem configuration : Gear stages 1 and 2 share the same ring wheel, as well as stages 3 and  4. Self-adjustment of the stages in one subassembly is improved by this configuration when a  torque is applied. In a planetary gear, it is important that the load is distributed equally to the  planet wheels to take advantage of the compact arrangement. This can be done by leaving  unsupported the sun wheel, the planet wheels carrier assembly, or the ring wheel. The other parts are mutually supported by bearings. In the ERA joint, it was chosen for a floating ring wheel.It can also be considered as a floating planet wheel carrier sun wheel assembly, hence floating with respect to the ring wheel. d)The floating component has to be kept aligned with respect to the other gear wheels. In large  industrial high-performance gearboxes, this is generally accomplished by the width of the gearwheels. In this case however, the wheel widths are too short for self-aligning per stage.  Therefore, two stages are mutual connected by using a common ring wheel. The planet carriers  with sun wheels are supporting them mutually by bearings. The self-centering effect of each stage provides the alignment of the assembly, which is called a tandem. The self-aligning effect of a tandem is used to make each set of two gear stages self-supporting. These supports are a  flexible plate flexure for the planet carrier sun wheel assembly and a tooth coupling for the ring wheel. Between these interface points there is no connection between the housing and gear train.This ensures isolation from deflections of the housing. e)Bearings: The roll joint is equipped with three main bearings: for the radial and axial support one  set of two angular contact bearing is used. The single bearing is a deep groove ball bearing which gives only a radial support. The lubrication is with Bray oil. Belleville springs are implemented in the design in order to preload the bearings (to resist launch vibrations). Load on the bearing due to thermal expansion is reduced and is equally distributed on the bearing, due to the lateral elasticity of the yoke fingers that are adapted to the required minimal stiffness.   f)Flexible elements : The large number of interfacing components inside the joint requires  consideration of internal or external disturbances such as : - Thermal deformation.- Mechanical deformation during launch and during operation.- Manufacturing inaccuracies in a statically undetermined support- Radial and axial play. 327