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Original Year
 1977
# of pieces
 315
Categories
 Farm Equipment: Tractors
Dimensions
 24(L)x16(W)x14(H)
not including implements
Features
 Steering, Threshing,
Raising implements, PTO
More Information

851/952 Farm Tractor
iso revolve
Click the image to download the LDraw file of this model.
Model by Owen Dive
Set 851 was released under the "Technical Set" series in 1977 in Europe.  It made it to the US in 1978 with the number 952 as an "Expert Builder"set.  This tractor features rack and pinion steering, a PTO (Power Take Off) which can power various implements including the thresher of the primary model, and a linkage to raise and lower the implements.  Traditional studded construction is used almost exclusively, with the exception of the implement lift.  This was the only set released by LEGO® in the first year which used a four bar linkage.  To this day, this is one of a very small number of sets which actually use the 40 tooth spur gear.

A number of tractors have been made over the years, but this one is the most elegant in many ways.  I've seen old tractors which look very much like this one on many occasions while riding through the countryside in the Midwest of the USA (without the studs, of course).  It is a very nostalgic design.

Features
Steering
The front wheels can be steered using a  wheel made from a 24 tooth gear at the driver's position.  As can be seen in the computer generated image, the wheel drives an axle connected to a pair of 8 tooth pinion gears via a universal joint.  The pinions drive a pair of racks.  The steering mechanism itself uses 6 traditional 2x2 swivels with 2 pair allowing rotation of the wheels about the vertical axis and another pair acting as control arms attached to the rack gears.  Simple axles allow the wheels to rotate freely.

 steering    steering2
Click for an animation of the steering in motion.
Implements
The right rear wheel of the tractor drives a PTO (Power Take Off) via a bevel set using an 8 tooth pinion and a 24 tooth crown gear.  This gives the PTO a gear ratio of 3:1.  The left rear wheel is not connected to this mechanism.

The implement for the primary model is a thresher.  The PTO drives a 40 tooth spur gear on the thresher which, in turn, drives an 8 tooth spur gear.  This final set of gear reduction drives a trio of 40 tooth gears acting as cutting wheels.  Yellow bricks are inserted into some of the stud holes in the gear to act as cutters.

The final gear reduction at the cutters is 9:1 with respect to the rear wheels, so these turn very quickly when engaged (see animation).  They turn so quickly that this creates a lot of drag on the tire and tends to make it slip.
 thresher    thresher
Click for an animation of the thresher in motion.
Lift
The thresher can be raised and lowered via a lever at the driver's position on the right side.  The lever pivots about the frame and drives a push-pull link made from connectors.  This link pushes a four bar linkage which raises the implement, keeping it (almost) parallel to the ground during motion.  The reason it is not quite parallel is that the vertical distance between the upper and lower links is not equal at the front and the back, so the links are not quite parallel.

As long as the model is right side up, the push-pull linkage is always in compression so it tends to stay together.  If the model is inverted, this is in tension and can pop apart.

If the thresher is lowered too far, the 40 tooth gear will fall under the PTO and jam.  There are wheels on the thresher to make it level and prevent this from happening.

The original mechanism (pictured) had a weak spot at the aft end of the connector link.  Only the friction on the studs kept this together, so it tended to come apart often.  In 1978, LEGO® changed the design to have a vertical 1x4 beam at this location to hold things together.  This resulted in a slightly different parts list and count for this set.
 lift    lift
Click for an animation of the lift in motion.
Wheels and Tires
This set uses two rubber 17x43 tires and standard old gray wheels on the front.  The rear uses a foam 24x43 tire and wheel, the biggest foam tire that LEGO® made.
 Wheels
 

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

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