Set 8854, released in 1989, is one of the best examples of high function density: lots of functionality packed into a small package. This mid size mobile crane features rack and pinion steering, outriggers, a slewing turntable, pneumatics, a luffing and telescoping boom, and a cable hoist.
This is the first Technic model to feature the metal hook. Despite its relatively small size, it is every bit as capable as the next couple of much larger cranes which would follow, and remains one of my favorite Technic models. Besides it has some resemblance with the German Unimog brand utility vehicles like the set 8848 - Power Truck, though that set had way less technical functions.
Pneumatics
This model contains a double acting pneumatic system. The components of this system are connected with rubber tubing routed through and around the holes in the beams.
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 parallel connected via T fittings.
Features |
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Steering |
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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 through a universal joint. The second axle drives a rack via an 8 tooth pinion gear. The steering mechanism itself uses the steering arms and toothed links as shown in the computer image. |
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Luffing Boom |
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The boom can be pivoted from a position parallel to the ground up to an angle of about 45 degrees. A pair of pneumatic actuators drive this motion. They operate in parallel and extend to luff the boom. The stroke of the actuators limits the extend to which the boom can pivot. The actuators are quite powerful and have no trouble luffing the boom, even when extended. |
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Telescoping Boom |
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The boom can telescope to approximately 160% of its original length. A smaller inner boom is constructed of beams topped by the new 1x20 rack. Where set 855 - Mobile Crane had a set of 1x4 gear racks stack along each other here a special part is introduced. A crank on the right side drives a an axle parallel to the boom via a worm gear and an 8 tooth pinion. Another 8 tooth pinion then drives a long axle. This axle runs the entire length of the boom, spliced with a u-joint. At the upper end of the boom, the rotation is turned 90 degrees through a set of 14 tooth bevel gears. The cross axle contains a final 14 tooth bevel 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 carried by a force couple. The end of the inner boom is prevented from moving down by contact with the main boom, and is prevented from moving up by contact with the gear and with a vertical clamping assembly. 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. |
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Hoist |
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A hoist is available which uses LEGO® string to lift a sheave. Pulley wheels on either side are used as cranks and directly rotate an axle to which the string is affixed. This axle acts as a winding drum and raises and lowers the sheave. A 24 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. |
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Slewing |
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The rotation of the boom is accomplished via a crank on the left side. This crank passes torque through 3 sets of 8 and 24 tooth gears, resulting in a final reduction of 27:1. The final gear turns a threaded axle which passes through an old 4x4 turntable. Axle nuts on the top and bottom of the threaded axle provide clamping force which keeps the moment of the boom from tipping itself free of the turntable. Because the pneumatic hoses pass between the base and the boom, the boom can only slew about 180 degrees before the hoses become twisted. |
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Outriggers |
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There are a pair of outriggers behind the rear wheels which are constructed using the new 1x4 liftarms and standard beams. The feet are made from small pulley wheels. A crank on the right side drives a worm gear, which then drives a 24 tooth spur gear resulting in 24:1 reduction. The second axle drives a pair of 14 tooth bevel gears. One bevel gear turns an outrigger, while a parallel gear drives the other outrigger in the opposite direction at the same time. When constructing the outriggers, care must be taken to keep these two gears in phase so the outriggers will be even. 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 rear tires so that the tires are lifted off of the ground. |
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Wheels and Tires |
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This set contains 4 sets of wheels and tires of type:
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Source Technicopedia