Original Year
# of pieces
Construction Equipment: Excavators
retracted position
Pneumatically luffing boom and dipper,
Pneumatic Bucket, slewing cab,
Tracked base
Other Views

8851 Excavator

iso revolve
Click the image to download the LDraw file of this model.
Model by Jerome Boulanger

Set 8851 was released in 1984 as the flagship example of the new pneumatic system.  It does not feature any traditional Technic geared systems.  Instead, all technical functions are carried out via pneumatic actuators which makes this model less mechanically complex than many others of this size.  Despite this fact, the model is quite difficult to build.  There are many tubes in the chassis which have to be routed just right in order for the whole thing to fit together.  In appearance, the pneumatic actuators are clearly intended to look like hydraulic actuators though their diameter is vastly larger than a hydraulic counterpart due to the much lower pressure of pneumatics.

An excavator would have been a difficult model subject without the introduction of pneumatics.  This was the first attempt from LEGO.  Several other excavators would follow, some of them pneumatic, some strictly mechanical versions, and one using the new screw driven actuators.

This was the first model to use multiple pneumatic actuators.  It is one of only two sets to use the long stroke pneumatic pump.  This set also features a large number of track elements, and was only the second set to include them after 856.


This model contains a single acting pneumatic system. The components of this system are connected with rubber tubing routed through the holes in the beams.

A single pump provides air pressure.  Depression of the piston produces positive pressure, and raising the piston produces negative pressure (suction), limited by atmospheric pressure.  This particular pump uses the longer piston from the 64mm actuator which gives it a longer stroke.

The pressure is fed to a distribution block with an input (pump) and two outputs (switches).  One output has a check valve which flows out and the other a check valve which flows in.  Each of these is routed to a different inlet of a selector valve (switch) with one output.  Selection of the switch in one direction therefore allows pressure output, while selection in the other direction allows suction.  This model has three switches so the tubes from the distribution block are split with T fittings.

Finally, there are three pneumatic actuators which have a port at the head end to accept input from the switch.  Positive pressure extends the piston, while suction retracts it.  Like a real excavator, the actuators are designed to work in compression during their power stroke.  A real hydraulic actuator has significantly less output force in tension than in compression due to the fact that the annular area of the rod end of the cylinder is less than the full bore area of the head end.

The picture shows the three switches and the pump.  The distribution block is under the pump.  If you watch the animation closely, you can see the position of the switches.
pneumatics    pneu
Click for an animation of the pneumatic features in motion.
Luffing Boom
The main boom can be luffed pneumatically.  It has a fixed angle of about 135 degress in the middle, but rotates about the base.  The pneumatic actuator pushes to raise the boom, assisted by a set of rubber bands on the back of the boom (see image).

luff    luff2
Click for an animation of the boom luffing.
The dipper can be moved pneumatically.  An actuator pushes to pivot the dipper arm down.  This power stroke would be used to push the bucket into the ground.  The actuator only has a moment arm of 3 studs, so it has less mechanical advantage to produce motion than the boom actuator.
Click for an animation of the jib luffing.
This is the first model to include a specially molded bucket part.  A pneumatic actuator pushes to pivot the bucket and scoop earth (or LEGO parts).
Click for an animation of the bucket in motion.
The 360 degree rotation of the boom and cab is accomplished very simply.  An old 4x4 turntable is used.   To help keep the upper assembly from falling off the turntable, a new threaded axle is used with a pair of jam nuts at either end.  Actual rotation is accomplished manually; there is no drive system for slewing.
Click for an animation of the boom slewing.
This set uses 110 track elements for locomotion.  40 tooth spur gears are used to represent drive cogs, with 16 tooth spurs representing idlers.

Other Views

Side View
side view
Top View
top view
Front View
Back View
back view
Computer Render

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