Gearbox Genetic Optimization Tool

A flow diagram for the design tool is given in Figure 1. The following sections discuss each of the boxes in the flow diagram in order. The tool is presented in the context of a single or two-stage planetary gearbox.

Input, Constants, and Assumptions Table 1 provides the inputs and assumptions used in this paper. Along with basic output speed, power, and gear ratio, the tool requires a target life and reliability to size the gears and bearings. Gear, oil and other material information must be provided to estimate the life and power loss of the gearbox. Propeller loading information can optionally be provided for a given application. In this paper, the bulk of the propeller loading properties are based on the RVLT quad rotor concept vehicle. Hub moments in cruise are estimated based on the rotor disk loading, rotational speed, radius, and cruise velocity. These values are used as an example case. Ideally, actual values would be applied for a given target propeller design specification. Target life 5000 h Propellor thrust 0.006*Power Target reliability 99.9 percent Prop drag coefficient 0.015 Gear Material Steel Prop tip speed 167 m/s Gear steel modulus 200 GPa Prop hub moment Estimated Gear steel Poisson’s ratio 0.3 Vehicle cruise velocity 31.5 m/s Gear steel hardness 627 HB Propellor material Carbon fiber Gear steel fatigue bending strength 517 MPa Oil viscosity 100 cSt Gear steel fatigue contact strength 1.90 GPa Air viscosity 0.02 Housing material Aluminum Shaft material Steel Gear pressure angle 20 degrees Gear type Spur gears Table 1—Assumptions and inputs used in this paper. The code uses 3 genetic optimization variables for single-stage planetaries and 7 variables for two-stage planetaries. For each stage, sun gear tooth count, ring gear diameter, and number of planets are used as optimization variables. The seventh variable for the two-stage planetary is the gear ratio of the first stage. Other parameters like gear tooth pressure angle, listed as an assumption in Table 1, can also be used as genetic optimization variables if desired.

Gear Geometry Calculations The 2D geometry of each gear set is generated based on the inputs provided to the fitness function by the genetic optimizer. For a single-stage planetary, first the ring and planet gear tooth counts are calculated based on the target gear ratio and the input sun gear tooth count.