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Let’s say we have a 5 inch diameter wheel, we want the robot to travel at about 4 feet per second, andwe know the VEX motor spins at about 100 RPM. The first step is to calculate the diameter of the wheel,and also convert our target speed to inches per second.So now we know that we need to move 48 inches in one second, and that each revolution is15.7 inches; knowing this we can calculate how many revolutions per second the wheel needs to turnto achieve 4 feet per second.Now we know that we need the wheel to spin at 183.42 revolutions per minute, and we know the VEXmotor spins at 100 RPM, we can calculate the Gear Ratio needed to achieve our top speed.To achieve a speed of 4 ft/sec with a 5” wheel and a VEX motor; we need a gear reduction of .545:1. Wenow need to figure out what gears to use to achieve this gear ratio, or try to get as close as possible ifwe do not have the correct gears available. The VEX Robotics Design System has several GearingOptions available. The following chart shows all the gear reduction pairs possible using VEX SpurGears and VEX Chains and Sprockets.8■Unit 10: Drivetrain Design 2
Using the above table we can see the ratios closest to .545. From here it is a question of choosingwhich option best suits the design. In this case there is no combination which yields exactly the ratiowe want, but there are several which are close.You can see how one would follow this same process to calculate the gearing in other situationsas well.Motor Load GearingThe second concept to understand is calculating the maximum load applied to the motor by thedrivetrain. This occurs during the “wall push” situation; that is, when the robot is up against animmovable object and is running full throttle into it. In this situation the wheels should slip on thefloor, but the friction between the wheels and the floor will act as a brake on the motor.The first step is determining how many wheels are acting as a brake on the gearbox we are looking at.Only wheels directly linked through gearing or chain apply load to the gearbox.In this example, let’s consider the example of a 4WD robot where each side of the robot has 2 wheelslinked to the same gearbox. In this case 1/2 the robot weight will be resting on the wheels. This weightis the Normal Force which determines the Force of Friction applied by these two wheels on thegearbox.This torque is applied through the gearbox onto the motor or motors. If a gearbox has multiplemotors, then the torque will be divided evenly between them.It is important to design the gearing such that the load applied on each motor is not higher than themotor limit. VEX Motors cannot draw more than 1 Amp of current for an extend period of time. Usethe principles learned in Unit 5 and Unit 6 to ensure this limit is not exceeded.Think Phase■9
Page 1: 10:Unit10Drivetrain Design 2 UnitIn
Page 4 and 5: PhaseCreateBuildStandardScience (NS
Page 6 and 7: Think PhaseOverviewThis phase descr
Page 10 and 11: Drivetrain DesignThe two methods li
Page 12 and 13: Technical OverviewThe following Aut
Page 14 and 15: Bevel Gear OptionsThe following opt
Page 16 and 17: Create the Bevel GearsIn this secti
Page 18 and 19: 5. Press E to start the Extrude too
Page 20: Reduce the Weight of the GearIn thi
Page 23 and 24: 16. Click Circular Pattern.17. To c
Page 25 and 26: ActivityIn this activity, you desig
Page 27 and 28: Amaze PhaseOverviewIn this phase, y
Page 29 and 30: STEM ConnectionsBackgroundA pitchin

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