This is my Excavator DEC-24 (which was originally manufactured by the imaginary "Dutch Excavator Company"). It was modeled after a 24-ton typical tracked excavator hence DEC-24. Ever since I saw the Lego8851-set, it was one of my wishes to build my own tracked excavator. After aquiring 8275 and 8294 I realized that I had the parts to build a remote controlled version.
This one is greatly inspired by the JCB-220 excavator, made by Jennifer Clark. One her website she basicly explains how to make a working Lego Technic Excavator. I used many of the techniques that she used, but made my own version, using power functions.
Remote controlled driving is done using 2 PF M (!) motors, positioned in the superstructure, each motor driving a single track. I experimented with a substractor-drivetrain, but an real excavator is driven by controlling the two tracks separatly. Each of the drivetrains contains a clutch gear, is tranfered through the turntable with a driving-ring and great gear-reduction in the undercarriage. The driving ring gives the possibility to rotate the full 360 degrees. And the gear reduction in the undercarriage has a number of advantages:
1. It makes it possible to use PF-M motors instead of the room-consuming PF-XL motors.
2. Reduces the side-effect of the driving ring, which introduces a lot of backlash and causes a difference in reaction-speed of the tracks.
3. Reducing the effect of the slewing affecting the driving.
The boom was constructed with liftarms, to get the right shape and angle and covered with plates to remove the holes from sight as much as possible. I used straight liftarms onle, not the angled ones (they don't have the right angle anyway). Truss-like technique was used to make it stiff. This turned out to be a very strong and light construction. The battery box in the back of the superstructure provided more than enough counterweight.
A lot of details were added: mirrors, front and rear-lights, dummy joysticks and levers in the cockpit, even a small dashboard. The door of the cabin can be opened.
The back of the superstructure can be opened for easy access to the battery-box. The top of the superstructure can be opened as well, just like on a real one.
The remote controller contains a compressor, 2 PF-XL motors, air-pressure-regulator, battery box, and controls. The compressor uses four normal pneumatic pumps of which the springs were removed to reduce needless friction. I don't own any small pumps, so I was forced to use this method. I used 4 pumps for a nice constant air, each pump rotated 90 degrees compared to the previous pump. This needs a lot of forced to be driven, so 2 XL-motors were used. This made the remote kinda big, but that wasn't a problem for me.
The air-pressure regulated uses a pneumatic cylinder connecter to a PF-switch. The action of the cylinder is contrained by elastics. When the compressor has made enough pressure, the pump pushes the switch to off-position, when pressure drops, the cylinder is retracted by the elastics, putting the switch back to on-position.
After seeing too many all-yellow Lego-excavators on the internet, I decided to I wanted something else. I hate putting stickers on my Lego, so I used normal bricks and slope bricks to add blue and red accents to it.