Shifting Gears: A NXT Vehicle with a Transmission

By Sivan Toledo
November 2006

A NXT car with a transmissionThis is really just a fun project, and it is more about modelling cars than about robotics (I'll explain why later). One NXT motor drives the car, a second NXT motor steers it, and a third controls the transmission. Apart from the NXT brick, from the three motors, the touch sensor, and the motor/sensor cables, the parts are not from the NXT kit. Most of the parts are from Lego set 8421, a large Technic crane.

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This car uses somewhat exotic Technic transmission parts. Technica is a good place to learn about these and other Technic parts. You can build transmissions without these parts, but I had them (from 8421) and wanted to try to use them in a powered transmission (I found that they are not perfect for powered transmissions; this is explained below).

The next image (on the left) shows the underside of the car, and the one on the right shows the car with the NXT brick removed to reveal the transmission. The yellow beams above the transmisison, which go from one side of the vehicle to the other, both provide structural stiffening and they support the NXT brick.

A NXT car with a transmission (NXT removed)A NXT car with a transmission (bottom)

I think that this car is pretty cool, but it is not very sophisticated relative to some Lego vehicles and machines that people have built. Take a look at the web sites of Jennifer Clark, Beat Felber, and Thomas J. Avery, for example, to see how sophisticated homebrewed Lego models can get. You'll be amazed.

The Transmission

Closeup of the transmission (top)

The first picture shows the tranmission from the top. The motor that powers the tranmissoin (near the top of the image turns an 8t gear, which rotates the 24t gear you see at the top. This gear is connected to the 4k yellow knob gear, which causes another gear of the same type mounted at 90 degrees to turn. The reason I used these 4k knob gears is that they can transfer an incredible amount of torque. I first tried to use various bevel and crown gears, but they kept slipping. After some experimentation I discovered that the 4k knob gears are ideal for such situations: because of the large knobs, it's almost impossible for them to slip. The 4k knob gear is mounted on the same axle as a 20t double bevel gear (hidden below the yellow beam on the left side of the image), which turns another 20 double bebel, which turns the red changeover catch. This catch moves the red driving ring sideways, and this is what operates the transmission.

The axle that rotates the driving ring should be located on a half-stud position relative to the frame of the car. I mounted the axle in this position using a 2x2 split double cross block connected to 1x2 cross block.

Closeup of the transmission (bottom)The second picture shows the bottom side of the tranmission. The motor that powers the transmission is now on the top of the figure. The two 20t double-bevel gears that operate the changeover catch are visible on the right. The driving ring engages either of the dark gray 16t gears with the axle that is connected directly to the driving motor (the bottom one). These gears have a round hole for the axle, not a cross-shaped hole, so they normally rotate freely on the axle, but a driving ring can engage them. Therefore, either the gearing on the bottom is powered, or the one on the top. The one on the top consists only of freely-rotating 20t gears. If this chain is engaged, the differential turns once for every rotation of the motor. If the gear chain on the top is engaged, the axle rotates once for every 3 rotations of the motor.

The transmission is fairly reliable, but it requires careful tuning. It is pretty easy for the catch to turn too much and escape the driving ring. This causes the transmission to fail. To avoid this, you have to use an appropriate amount of rotation (that is, to tell the motor to rotate the "correct" number of degrees), and you have to carefully set the starting position of the transmission. This problem led me to conclude that the Technic transmission parts are not ideal for transmission that are powered by NXT motors. 

Shifting gears also requires a lot of torque, so the shifting mechanism must be robust. This is why I used the 4k knob gears. Perhaps using a lever to move the changeover catch would have eliminated this problem, but I did not have any idea how to construct an appropriate mechanism.


The front of the car, showing the steering mechanism

The steering motor is connected to a pair of 1x3 liftarms (the end of the lifarms is the gray part showing between the ends of the two yellow parts). The rotation of the motor and of the liftarms causes the long gray beams toward the back of the vehicle to move left or right, which causes the wheels to turn.

The touch sensor allows the NXT to return the streering to the straight position. The beam activates the touch sensor when the wheels are still slightly turned. To correct for this, the program stops the motor when the touch sensor closes, but then turns it again 7 degrees, which returns the streeting exactly to the straight position.

The touch sensor is mounted on the streeting motor, not on the frame of the car.

The large pulley is used to position the yellow beam that activates the touch sensor at a half-stud position relative to the touch sensor. This allows the beam to press the sensor without getting stuck.

The next images show the underside of the steering mechanism, and the way the steering engine is connected to the chasis.

The underside of the steering mechanism The connection of the steering motor to the chassis

Cars Versus Robots

Are these car-like mechanisms good for moving a robot around? Probably not. A differential drive (as in Tribot) provides more power to the wheels (two driving motors instead of one), allows for more accurate turning, and it allows for turning in place or almost in place. The transmission is not that useful either in most cases. It can allow the vehicle to go fast, but in most situations, going fast makes it more difficult to achieve precise movements.

© 2006, Sivan Toledo