The beginnings of the upper structure

Starting the frame

Attaching crawler motors

Padeyes (cylinder attachment hinges)

Upper frame- this didn't work

Rebuilding upper frame

Compressor

Pressure limit switch
(uses polarity reverser)

Connecting pneumatics and motors

Mechanical Stuff

Part of the crawler gear train is located in the superstructure. Specifically, two sets of 8t and 24t gears (per crawler side) make up the 1:9 gear reduction. The crawler drive motors are mounted vertically over the gear train. This was the most direct way to attach them.

Typically, designers will place motors in the rear of machines like this to provide a "natural" counterweight, and not have to add scrap steel to make extra ballast. I estimated that my model would be heavy enough not to need the extra weight in the back.

The swing motor is located behind the crawler drives and is powered by one motor. The swing motor drives a Technic turntable at a 1:56 ratio (worm to 24t - 24t to 56t). The turntable is mounted upside down so that the driving motor can be located in the upper structure.

Pneumatics

I did a little research on Lego pneumatic compressors, and found many different designs. I decided to build Ralph Hempel's double-acting compressor, but slightly modified. It was the most compact and easiest to use. The modification is the addition of cams so that the full stroke of the cylinders is used.

C. S. Soh tested different pumps to find the best one. His pressure tests are really useful.

I decided to have a cut-off switch for the pneumatic system. This meant that the compressor could be turned on and left that way. It will operate until it reaches max pressure, and then cut off. It will turn back on when the pressure drops, and it's needed.

Again, I did some research and found much useful information. Jennifer Clark has a very nice dual pump and auto switch that doesn't use a pole reverser. See her ideas page.

Ralph Hempel also has an excellent design for a pressure switch that uses a pole reverser.

However, I ended up creating my own design for the pressure swich. It does use a pole reverser and a small pneumatic cylinder. You can see a picture of it to your left.

I've set it up so that the switch activates, breaking the circuit to the compressor motor, just as the cylinder starts its stroke. Jennifer Clark's switch used this concept, but with a small battery box instead of a pole reverser. Previous designs had the cylinder almost completing its stroke before cut-off.

Pneumatic Performance

After the model was completed, I discovered that the double acting compressor didn't produce satisfactory performance. Once the two air tanks were charged, the digging arm could be lifted and then nothing else. I'd have to wait until the system pressurized again.

Jennifer Clark and I discussed the philosophy of pneumatic systems, and she told me of her preference to omit the air tanks. She felt that with appropriate compressor output, you wouldn't need the tanks, and the machine could be operated satisfactory.

We tested this out at Brickfest by adding one of her compressors to my model. We then operated the model with 2 compressors going at once. Its performance improved quite well.

I think if I add more compressors to the model and take out the air tanks, it will make a nice improvement. Now all I have to do is save some money. Those little compressor cylinders are not cheap!

If I really wanted to beef things up, I could build something like Doug Carlson's Octopump (see his compressor page).

One thing I should note is that the long tube lengths needed for the umbilical effectively act as an air storage tank. The volume contained in the lines is significant, and even without air tanks, it takes a little while for pressure to build up.