Original Year	1978
# of pieces	512
Categories	Construction Equipment: Cranes
Dimensions	39(L)x17(W)x18(H) retracted position
Features	Luffing and telescoping boom, Hoist, Slewing, Outriggers
More Information

855/955 Mobile Crane

Click the image to download the LDraw file of this model.
Model by Rene Kok

Set 855 was released under the "Technical Set" series in 1978 in Europe. It finally made it to the US in 1979 with the number 955 as an "Expert Builder"set. This crane packs an impressive list of features including telescoping outriggers, a 360 degree slewing turntable, a luffing and telescoping boom, and a hoist. It does NOT feature steering, and is one of the only wheeled models to ever lack this feature. Traditional studded construction is used almost exclusively.

LEGO^® has built a lot of mobile cranes over the years, but this was the last one I purchased. The only thing I knew about it prior to that was what I had seen in pictures, which left me with the impression that it was rather small and boxy compared with the others. While this is true, this is still an extremely impressive model. The number of features which were able to be included while still using primarily standard bricks and plates is simply amazing. This model also marks several firsts. This was the first appearance of LEGO^® string, a sturdy braided type of cord which would show up on most of the cranes after this. This model uses the first rubber band to control a ratchet and pawl system. This model also first uses what would become one of the most standard elements, an axle pin, to support the wheels.

Features
Luffing Boom The boom can be rotated from a position parallel to the ground up to an angle of about 50°. This rotational motion is actually accomplished via a translating rack and some push rods. The boom angle is maintained by a ratchet and pawl system. The computer image at the right has been color coded to make the mechanics easier to understand. The boom itself pivots on an axle (the black horizontal axle). A crank drives an 8 tooth gear (red axle), which in turn drives a 24 tooth gear. A second parallel axle (green) crosses the mechanism and drives another pair of 8 and 24 tooth gears for a total gear reduction of 9:1. The final axle (blue) passes over a set of gear racks and drives them with a pair of 8 tooth pinions. The gear racks are part of a translating rack assembly. A pair of push rods connect the rack assembly to the boom. As the rack is translated aft, the push rods drive the boom up. At the beginning of motion, the mechanical advantage is quite poor because the push rods are almost parallel to the boom. For this reason, a lot of force is required at the crank to get things moving. Once the boom has been raised about 15 degrees, the crank turns easily. Note that a boom rest was added in later versions of this model to prevent the boom from going all the way down.¹ The 24 tooth gear directly behind the crank (on the green axle) which acts as a ratchet. A rubber band pulls a 1x4 beam used as a pawl down onto the gear. This beam effectively locks rotation in one direction, but allows it in the other direction. So as the crank is rotated, the pawl skips over the gear teeth, but when the crank is released, the ratchet cannot turn backward. The pawl must be manually lifted off of the ratchet to lower the boom.	Click for an animation of the boom luffing.
Telescoping Boom The boom can telescope to approximately 180% of its original length. A smaller inner boom, constructed of plates and covered with rack gears, is nested inside the main yellow boom, constructed of bricks layered with tiles. A crank on the left side drives a long axle parallel to the boom via a triplet of two 8 tooth pinions and a 24 tooth crown gear. This axle runs the entire length of the boom, spliced at two points with u-joints. At the upper end of the boom, the rotation is turned 90 degrees through another set of gears. The cross axle contains an 8 tooth pinion which translates the entire inner boom axially. 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 a second pinion gear supported by an axle pin. 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.	Click for an animation of the boom telescopic motion.
Hoist A hoist is available which uses the new LEGO^® string to lift a sheave. A pulley wheel is used as a crank, and directly rotates an axle to which the string is tied. This axle acts as a winding drum and raises and lowers the sheave. Because the string is simply tied to the axle, it tends to just slip when the cable is fully payed out. A certain amount of the cable needs be manually wound around the axle before it works properly.	Click for an animation of the hoist in motion.
Slewing The 360 degree rotation of the boom and cab is accomplished very simply. An old 4x4 turntable is used, surrounded by a series of tiles for extra overturning support. An axle runs up the center of the turntable and is fastened above and below by bushings to help keep the upper assembly from falling off the turntable. Actual rotation is accomplished manually; there is no drive system for slewing.	Click for an animation of the boom slewing.
Outriggers Preparing the outriggers on this model is rather labor intensive. First, 1x8 bricks covered with tiles extend from the main body. A 1x1 plate on the bottom inboard corner prevents the outrigger from extending too far and falling out. Next, a pair of 1x1 cylinders are attached to the existing cylinder which is already stored at the outboard end of the outrigger. Finally, a pad consisting of a pair of 2x2 plates is extracted from a compartment in the side of the base and attached to the cylinders. All of this must be accomplished manually for each outrigger. Despite the difficulty in deploying them, the outriggers work quite well. They provide a wide base for stability and are very slightly taller than the tires so that the tires are lifted off of the ground. Because there are four outriggers and because the feet must be parallel to the body, they only work on perfectly level ground.	Click for an animation of the outrigger in motion.
Wheels and Tires This set uses four rubber 17x43 tires and standard old gray wheels. The wheels are supported by the new axle pins.