Shifting Gears: the Transmission

Shifting gears with Lego is a complicated task, albeit with numerous effective solutions. Today I designed the transmission above, and I'm going to talk about designing your own transmission in this post.

Deciding your gear ratios is the first step. What kind of power do you want? What vehicle will be using this transmission? Is speed a concern? This transmission used two unique gear ratios:

Gear train #1:            24t:8t:40t:24t (3:1:5:3)
     - This gear train provides torque and power sacrificing speed. The 24t is part of the transmission and when you shift up, the gear train is discontinued while the 24t in the second housing shifts into place, completing...

Gear train #2:            24t:40t:8t:24t (3:5:1:3)
    - Here you increase the speed, as you utilize the mammoth 5:1 ratio you're getting in the middle. This transmission is completed when you've shifted up, and using 24t's on the same axle helps complete the train.

The basic design of all transmissions are the same. Utilize two independent gear ratios which you can switch between at the push of a handle/knob. These manual  transmissions can have an unlimited amount of speeds depending on how creative you wanna get.

Also, the two independent trains must be connected to the same axle you're attempting to drive. Failure to do this will result in no forward motion by your vehicle.

Transmissions are fun to play with, and if you can successfully build one and implement it in your Mindstorm bot, few things can match the performance. Here's a video of the transmission in action, and I look forward to using it in future bots:

Differential Drives

By far the most common drive system for Lego robots, the differential drive is probably the easiest to make. Utilizing two parallel drive wheels on either side of the robot, they can be powered separately, or use a single motor as well.

The diagram to the right shows what a basic differential drive looks like. Bevel gears placed in the central gear housing allow the axles to turn independently, enabling the robot to turn in place, and maneuver pretty easily.

The following chart is adapted from "Building Robots with Lego Mindstorms" by the Ferrari Brothers and displays the different behaviors of  a robot in accordance to its wheels (powered by the differential drive).

Using different combinations of speed and direction, the robot has the ability to cover any amount of space, and turns at any angle. All this in addition to being very easy to implement. 

I assembled an example of a standard drive, showing how you can use the bevel gears inside the housing to attach the axles, and connecting it to an RCX is as easy as sliding two beams through it. 

As always, I encourage you to send me your designs, and contact me with any questions/comments/concerns. Happy building!