These notes are excerpted from The 1994 6.270 LEGO Robot Design Competition Course Notes, Chapter 7, by Fred Martin and Randy Sargent
Objective: To understand Lego design principles and to build a Lego geartrain
Lego Technics are fun to play with and allow the construction of great things, but they are not always easy to use. In fact, it is often quite challenging to build a Lego device that does not fall apart at the slightest provocation.
A well-designed Lego device should be reliable, compact, and sturdy. If it makes extensive use of gears, the geartrain should be able to rotate cleanly and easily. If it is a structural element, it should hold together squarely and resist breaking apart.
The ratio between the length of a Lego brick and its height is 6 to 5. This ratio coupled with the existence of one-third height flat pieces, allows the creation of vertical spacings that perfectly match unit horizontal ones. By using these perfect Lego spacings, vertical stacks of bricks can be reinforced with cross-beams, creating sturdy structures that won't fall apart.
Making a good Lego geartrain is indeed a fine art. However, this art can be learned and having some simple information can make a big difference.
One of the first things to notice about Lego gears is their diameter, which indicates at what spacings they can be meshed together. The following table shows the radii of the various Lego gears.
Gear Teeth Gear Radius (number) (horizontal units) 8 0.5 16 1 24 1.5 40 2.5
Notice that three of the gears (namely, the 8-tooth, 24-tooth and 40-tooth) have radii that, when used together in pairs, form an integral spacing is formed.
A very high performance geartrain will be necessary in order to drive a robot; for this type of geartrain, the following rules are suggested:
The purpose of gearing, in addition to transmitting mechanical energy, is to transform it. For the purposes of a drivetrain, the gears will change the high speed and low torque of an electric motor and create the low speed and high torque that is required to move a robot. It is important to experiment with different gear ratios. The gear ratio determines the tradeoff between speed and torque.
Try creating a geartrain with 5 pairs of 8-tooth to 24 tooth gear meshings to achieve a gear ratio of 243:1 (see the figure provided at the lego building station). This is probably a bit of overkill for a robot drive, but it illustrates all the rules given above.