Select your language

  • Set Year: 2002
  • Set Number: 8431
  • Set Parts: 838
  • Building Instruction: No
  • Currency: na

This marvelous crane, was first released in 1995. This huge model is absolutely packed with functions. It does pretty well everything you would expect a real mobile crane to do. It has 4 wheel steering with a 6 wheel chassis. There are 4 outriggers which can actually lift the model. A pair of pneumatic actuators in series lift the boom which can then telescope with a crank. The hoist employs a ratcheting mechanism. A crank on the back slews the superstructure 360 degrees which is possible because all of the pneumatics including the pump are above the turntable. When fully deployed, the reach of this model is truly extraordinary; 69 studs!

There is a lot of traditional studded construction in this model, but also a few surprises. This was the first model to put two pneumatic actuators end to end to extend the stroke and the bottom of the telescoping boom is built upside down. All the cranks in the first model use the new handle part, only seen in the year 1995, the two consecutive ones didn't have that part anymore.

This model was released three times, which seems to indicate that this model was very popular. In 1995 it was released as 8460 - Pneumatic Crane Truck and again in 2003 as 8438 - Pneumatic Crane Truck. In both of these cases, the model was identical except that it no longer had the handle parts and had an updated box. Why they released it as two different numbers in two consecutive years is a bit of a mystery since the normal production run of a model would have spanned over this time anyway.


This model contains a double acting pneumatic system. The components of this system are connected with rubber tubing.

A single pump provides air pressure.  Depression of the piston produces pressure.  The pressure is fed to an inlet of a selector valve (switch) with two outputs.  Selection of the switch in either direction allows pressure to flow to either chamber of the actuators.

Finally, there are two pneumatic actuators which have ports at the head and rod ends to accept input from the switch.  Head end pressure extends the piston, while rod end pressure retracts it.  This model uses a pair of actuators in series connected via rotors.


The front wheels can be steered using a pair of overhead "hand of god" controls.  The steering knobs are designed to look like flashing lights.  The overhead control turns the blue axles in the computer image which then turn the green axles in parallel through a set of bevel gears.  The vertical red axle mates with both bevel gears, linking the two inputs together and transferring torque downward.  The longitudinal axle shown in tan then drives the front gear rack.   Two more pairs of gears on the brown and white axles reach the rear axle which turns in the opposite direction.  The steering mechanism itself uses the steering arms and toothed links.

The two light blue pinion gears move the front and rear axles at different rates which results in a larger steering lock in front.  This is due to the brown spur gear pair (16:8) which halves the rotation of the rear steering axle.  The center of rotation is then roughly at the central unsteered axle.

Luffing Boom

The boom can be pivoted from a position parallel to the ground up to an angle of about 60 degrees.  A pair of pneumatic actuators drive this motion.  They operate in series and extend to luff the boom.

This is the first time pneumatic actuators were used in series.  Since no special parts existed to accomplish this connection, they are attached with a pair of rotors and some axles and pins.  To keep the actuators from rolling around their long axis, another pair of black links on the side clock them to the angle of the boom.

One bit of unexpected design here is that the retract ports of the cylinders are also plumbed.  This has two effects.  First, it slows the descent of the boom since the retract ports cannot just suck freely from atmosphere.  Second, it allows the boom to actually be pressurized down to hold it down against the chassis.  This allows you to carry around the model using the boom as a convenient handle!

Telescoping Boom

The boom can telescope to approximately 180% of its original length. A smaller inner boom is constructed of beams topped by gear racks.  It rides upon a support made from another set of black beams constructed upside down.  The resulting smooth surface supports the telescoping motion.

A crank on the right side drives an axle parallel to the boom via a worm gear and an 8 tooth pinion.  This axle runs the entire length of the boom, spliced with another pair of 8 tooth spur gears.  At the upper end of the boom, the rotation is turned 90 degrees through a set of 12 tooth bevel gears.  The cross axle contains a final 16 tooth spur which translates the entire inner boom axially as it mates with the rack.

There are stops at either end of the inner boom to prevent it from extending or retracting too far.  The cantilever moment of the inner boom is significant, so a solid structure braced with vertical liftarms and rotors is required to keep the whole thing from prying apart.

As can be seen in the animation, the sheave is effectively raised by the telescoping of the boom.  Since the length of the boom has increased, less cable is available in the vertical direction.


A hoist is available which uses LEGO® string to lift a sheave.  A pulley wheel on the right side is used as a crank and directly rotates an axle to which the string is affixed.  This axle acts as a winding drum and raises and lowers the sheave.

A 16 tooth gear on the left side acts as a ratchet which mates with a pawl.  This supports the tension in the cable.

Even though the metal hook is fairly heavy, sometimes when the cable is being payed out the weight is not enough to unspool it resulting in a rat's nest of tangled cable.


The rotation of the superstructure is accomplished via a crank on the rear.  The crank turns a set of 8 and 24 tooth spur gears, then a pair of 12 tooth bevels.  A final 8 tooth pinion mates with the outer 56 tooth ring gear on the top of the turntable.  This results in a total reduction of (24:8 x 12:12 x 56:8) = 21:1.

Because no pneumatic hoses or axles pass between the base and the boom, the boom can  slew 360 degrees without becoming twisted.


There are two pairs of outriggers behind the front and rear wheels which are constructed using 1x4 liftarms and standard beams.  The pads are made from cams.

A crank on each side independently drives the front and rear outrigger on that side.  The crank drives a worm gear which then drives a 24 tooth spur gear resulting in 24:1 reduction.  An axle on the 24 tooth gear runs the entire length of the chassis and directly drives the rotation of the driving links of the outriggers.  A secondary linkage below the driving links makes this a 4-bar linkage.  Because the arms are 3 studs apart at one end and only 2 studs apart at the other, the outrigger arm rotates as it deploys.

Due to a careful selection of geometry, the outriggers are self-locking.  When fully deployed, weight on the foot tends to rotate the two sets of liftarms together, locking the assembly, rather than trying to backdrive them.  It is this geometry, rather than the worm gear, that prevents the outriggers from collapsing under weight.

The outriggers provide a wide base for stability and are slightly taller than the tires so that all 6 tires are lifted off of the ground.

Wheels and Tires

This set uses six of the smaller size 20x30 balloon tires and wheels.

By Technicopedia