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Original Year
 1979
# of pieces
 355
Categories
 Construction Equipment:
Bulldozers
Dimensions
 30(L)x14(W)x16(H)
lowered position
Features
 Raising and dumping bucket,
Tracks
Other Views

856/951 Bulldozer
iso revolve
Click the image to download the LDraw file of this model.
Model by Rene Kok
Set 856 was released under the "Technical Set" series in 1979 in Europe and in the USA as 951, an "Expert Builder"set.  This bulldozer features a bucket which raises and dumps via rack and pinion systems, and rides atop large tracks wound around 40 tooth gears.  The body uses mostly traditional studded construction, with the exception of the linkage which makes the bucket functional.  A large number of connectors are used to form tie rods.

This bulldozer has one of my favorite mechanisms of any Technic set.  Rather than using the expected rotational controls to move the bucket, this model uses two ingenious, interconnected rack systems which translate within the structure.  As if that wasn't enough, this model also introduces tracks, a variation on the new chain link parts included in 857.  There are 106 tracks in this set and, for a long time, this was the only set in which you could get them.  Only two other sets would ever be released which sit atop tracks, and they would be spaced out 10 years.  This model also uses a specialized piece for the side of the bucket.  This was unusual for 1979 since most parts still had many possible uses and were not highly specialized like this one.

Features
Raising Bucket
The bucket can be lifted from ground level (and slightly below) to almost straight up using a crank at the right rear of the vehicle.  This crank drives a set of 8 and 24 tooth gears, as can be seen in the computer image.  After another set of 8 and 24 tooth gears, the system drives a translating rack in the bottom of the vehicle.  When moved forward, this rack pushes a connecting rod (circled in red on the right of the picture) attached to the bucket linkage.

The bucket linkage looks like a series of two four bar linkages, but is so much more.  The system begins by pivoting around an axle (circled in red in the center of the image).  At this point, the bucket rotates and lifts at the same time, tipping upward as it raises.  Just at the point that the bucket becomes level, the linkage bottoms out on the structure, and subsequent motion raises the bucket without tipping it!  This is easy to see in the animation.  The motion is simply amazing to watch.

A pair of rubber bands right behind the bucket keep it from falling freely.
 raise    raise
Click for an animation of the bucket raising.
Dumping Bucket
When raised, the bucket can dump by rotating down about 90 degrees.  This is driven by a crank at the left rear of the vehicle. As can be seen in the computer image, torque is driven through a set of 8 and 24 tooth gears into another 24 tooth gear, then into a pair of pinions on the same axle.  These pinions drive a translating rack system in the top of the vehicle.  When moved aft, this rack pulls a connecting rod attached to the bucket linkage.

The bucket linkage, when raised, is a pair of 4 bar linkages.  The beams which make up the linkages rotate and stay parallel, tipping the bucket, and stretching the rubber  bands.
 dump    dump
Click for an animation of the bucket dumping.
Bucket
The bucket is constructed of plates and a specialized part used in only 2 Technic sets.

When the bucket is lowered, the two rack mechanisms work together in a fascinating way.  If the bucket is dumped while up, it gradually rotates itself back to level, driven by the rubber bands, as it is lowered.  This can be seen clearly in the animation.  Note that this motion is NOT reversible.  It occurs only when lowering, and cannot be duplicated when raising the bucket (unless you could get a rubber band to push instead of pull, violating the First Law of Statics [you can't push on a rope]).
bucket

Click for an animation of the bucket lowering.
Wheels and Tires
This set uses 106 new track elements.  These provide a very realistic look when "rolling".
 Wheels
 

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

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