Original Year
# of pieces
Cars & Trucks: Big Rigs
Steering, V-6 engine, dual differentials,
pneumatic compressor, slewing cab,
luffing boom and jib, grasping claw
Other Views

8868 Air Tech Claw Rig

iso revolve
Click the image to download the LDraw file of this model.
Model by Eric Albrecht

Here it is folks, the king, the godfather, the most complex pneumatic set of all time (with the possible exception of 8455).  Released in 1992, set 8868 has the first and only motorized pneumatic compressor, four full size dual-acting unbalanced pneumatic actuators, and one of the new mini actuators.  Just in case anyone thought this was not enough, there are also dual rear differentials, a V-6 reciprocating engine, and rack and pinion steering.  If you don't have this set, you need to mortage your house, ransom your pets, or sell your liver.  Do whatever it takes to get this model.  You will not regret it.

This set is unique and special in so many ways.  It was the first big dual axle truck.  It was the first (and only) set with a compressor.  It was the first pneumatic set of any kind since 1989.  It was the first (and only) to include BOTH an electric motor and pneumatics.  And this is no simple build.  With over 900 parts, it was the biggest set to date, even bigger than the 8865 super car.  And with all that complexity, it still only has 32 steps in the instructions, which are wafer thin.  With the detailed construction and huge numbers of pneumatic hoses to route, this thing takes some time to get right.  But it is surely satisfying when you do.

The chassis of this truck is extremely rigid due to the fact that it is constructed from multiple layers of beams running both horizontally and vertically.   Because the beams are relatively far apart, the second moment of inertia of the frame is quite high.  So  even with the long wheelbase, this model does not sag at the middle.

This model also has quite a number of nice aesthetic touches which were somewhat unusual for Technic at the time.  There are nice white fuel tanks on either side, vertical exhaust stacks, compression braking vents on the sides of the hood, air horns on the roof (too bad they don't work), and a nice "slammer bar" hanging from the front bumper.  The loader even has a realistic operator's seat.

"Air Tech Claw Rig" is a bit of an odd name, but this seems to be a model of a truck used in logging.   It can go off-road deep into the woods and pick up heavy logs to load onto other special purpose trucks.


The pneumatics on this model are as complex as you will find on any LEGO® model, and in fact this is the only set with a motorized compressor.  With a compressor, 4 switch valves, 4 large actuators, one small actuator, and four rigid tubes, there are a total of 23 flexible pneumatic hoses to route through this behemoth.  As you can see in the pictures, there are two switches mounted horizontally on either side with  three hoses each passing through holes in the Technic beams.  Many of these hoses then pass aft through another beam and up through the center of the turntable for the loader.  The color coded computer images can be used to decipher the routing of the hoses in this system, which can then be used to derive the logic of this mechanical computer.

A single compressor is integrated into the body.  The pressure is split and fed to the inlets of four selector valves (switches) with two outputs each.  Selection of the switches in either direction allows pressure to flow to either chamber of the actuators.

There are five pneumatic actuators which have ports at the head and rod ends to accept input from the switches.  Head end pressure extends the piston, while rod end pressure retracts it.  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.

Because there is continuous supply of pressure, this model operates very smoothly and multiple functions can even be run concurrently.  One (or two) of the pneumatic air tanks which came out years later make great replacements for the fuel tanks and really bump the functionality up to the next level with a virtually limitless source of pneumatic power.  I'm sure no one will have any trouble finding a place to route even more pneumatic hoses!

Incidentally, if you are wondering how long it took me to model all of those pneumatic hoses on the computer, the amount of time is too long to comprehend.
tubing1    tubing2

tubing1    tubing2
Click for a high resolution rendered animation of the pneumatic connections.
(1000x1000, 9 Mb)
Motorized Compressor
Feast your eyes on the only Technic pneumatic compressor.  Why they never used this again is one of the great mysteries (and tragedies) of our time.

For the first time, a 9V battery box was actually integrated into the body of a model instead of being remote.  This serves to make this model pretty heavy.  There's a 9V motor mounted under the battery box which drives a large pulley through a belt.  On the axis of the pulley is a crankshaft part with an offset of 1/2 stud.  Reciprocation on this crank gives the compressor cylinder a stroke of 1 stud.  Due to the small gear ratio, it operates very fast.

The compressor cylinder itself looks like a small actuator but has a port only at the head end.  When driven to retract, it forces air out through the port.  When extending, it draws in air from the atmosphere.  There is an integral relief valve which prevents the pressure from building above a safe level, although the torque required to drive the compressor to relief pressure slows the motor considerably.

The computer image shows the compressor's input to the four switched  via three T-fittings.
compressor    compressor
Click for an animation of the compressor in motion.
Pneumatic Slew
By this time, there had been quite a few cranes which featured a slewing turntable, but this is the first time it was controlled pneumatically (and it only happened one more time many years later).  Even though the model had a motor, the turntable could not be driven through a gear system because LEGO® had no synchronized transmission to switch between compression and gear functions.  (Note the foreshadowing.)

The color coded computer image shows the way the dual pneumatic cylinders are cross connected.  The head end of one actuator is connected to the retract port of the other.  This way, pressure causes one to extend and one to retract simultaneously.  A pair of rack gears are connected to the actuators and are used to turn a 16 tooth spur gear.  Since the bottom half of the turntable needs to remain fixed, a unique driving method was required.  A vertical axle which connects to the 16 tooth gear drives a horizontal axle which passes through the holes in the upper turntable.  It is important to get the two actuators even and centered when rigging the system to make sure that it can rotate 90 degrees in either direction.

Because all of the other pneumatic hoses have to pass through the center of the turntable along with the driving axle, the crane can only turn so far before the hoses snag.  Extra length is required when at the centered position.

A weighted 2x6 brick is located at the back of the crane to act as a counterweight so that the turntable is reasonably balanced.
slew    slew
Click for an animation of the loader slewing.
Pneumatic Boom
The main boom can be luffed via a single pneumatic cylinder working in compression.  The actuator attaches almost half way along the length of the boom.  This gives it excellent mechanical advantage to lift the weight, but also means that the total boom rotation is small; in this case about 45 degrees.

Technic "triangles" are used as lugs to attach to the rod end of the actuator.  The compressor has plenty of pneumatic pressure to lift the boom, even at full extend.
boom    boom
Click for an animation of the boom in motion.
Pneumatic Jib
The jib has two sections, a fixed section and a movable section.  The angle of the first section with respect to the boom is about 135 degrees and is locked by triangles and lift arms.

The second section is movable and is lifted by a single pneumatic actuator driving another pair of triangles.  Compared with the boom actuator, this one has fairly poor mechanical advantage, but less is required due to the decreased moment arm of the lifted load.  The smaller arm allows a greater range of motion; in this case almost 90 degrees.
jib    jib
Click for an animation of the jib in motion.
Pneumatic Claw
A grasping claw on the end of the jib is driven by one of the new miniature pneumatic actuators.  Because of the very small area, this actuator has a small output force and the grip of the claw is not very strong, but it does the job.  Unlike the other actuators, this one has to work in both tension and compression, and in fact the lower tension force is what is used to close the claw.

The actuator attaches to the middle hinge point of the claw which allows it to drive both sides at once.  The outer angle of the claws is fixed at 90 degrees.  The claws are supported by 3 arms which are free to pivot at the ends.  This allows the claw to remain perpendicular to the ground no matter what the jib angle is.

A look at this computer image and those in the above sections will demonstrate that the hose's path to this actuator is pretty circuitous.  Flex system rigid sleeves are used as pneumatic tubes to span the sections between the pneumatic hoses.  This allows them to be clipped to hooks which makes certain that the hoses move with the boom and jib and do not become tangled.

The claw can be lowered to grasp the rear bumper to help keep the crane stable when the truck is in transit.
claw    claw
Click for an animation of the claw in motion.
The front wheels can be steered using an overhead "Hand of God" control.

The overhead control drives an axle connected to a pair of 14 tooth bevel gears.  The second axle drives a pair of 16 tooth spur gears and then the steering rack via an 8 tooth pinion gear.  The steering mechanism itself uses steering arms and toothed links as shown in the computer image.  The steering arms may seem small for a model of this size, especially since the weight of the model is cantilevered out the wheels via only a single hole as a support couple.  But it seems to work fine.

steering    steering
Click for an animation of the steering in motion.
The rear wheels are used to drive a V-6 engine.  Torque from the differentials passes through a long drive shaft to a point under the front of the engine.  A combination of an 8, 24, and another 8 tooth gear pass the torque up without changing the gear ratio or direction.  A belt then actually drives the crankshaft of the engine via a pair of pulleys which increase the rotation about 3 times.  This makes the engine turn fast.

The front of the crankshaft uses a 3 blade rotor as a fan.

The engine is made from cylindrical engine elements.  The two cylinder banks have a standard V angle of 90 degrees.  The crankshaft is offset 1/2 stud from center, giving the pistons a stroke of 1 stud.  Each pair of pistons shares a common crank pin.  Because the crank pins are each offset 180 degrees, the forward and back cylinders are synchronized.  This can be seen clearly in the animation.

A big diesel truck like this would indeed probably have a large displacement 6 cylinder engine (although probably inline), so this engine fits very well.
engine    engine
Click for an animation of the engine in motion.
The dual rear axles are both "live" and use a pair of differential gears which incorporate a built in 28 tooth ring gear which can work either as a bevel or a spur, similar to the 24 tooth crown gear.  The differential ring gears are linked via an axle with 14 tooth bevel gears at either end.  While this allows them to rotate at the same rate, it could make one differential rotate backwards.  To solve this, the ring gear of the second differential is located on the opposite side.

The differentials are made to house 3 of the 14 tooth bevel gears.  One is on each axle, and one planet gear in the middle allows the axles to turn at different rates.
differentials    differentials
Click for an animation of the differentials in motion.
Wheels and Tires
This set uses six of the smaller size foam 20x30 tires and wheels.  While tandem rear wheels would have been nice, there is no way they would have fit into the width of this model.

Other Views

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

Contact me with comments on this page.