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Original Year
 1986
# of pieces
 349
Categories
 Farm Equipment: Tractors
Dimensions
 38(L)x38(W)x22(H)
(with sprayer)
Features
 Steering, Lifting sprayer,
PTO, pendular front axle
Other Views

8849 Tractor
iso revolve
Click the image to download the LDraw file of this model.
Model by Jerome Boulanger
Set 8849 was released in 1986 as the third large tractor in only 10 years.  It features rack and pinion steering on a pendular axle, a PTO, and lifting rear implements including a sprayer.  Both studded building techniques and pinned construction are used throughout.

This tractor is slightly smaller than the last and no longer has a reciprocating engine or a differential.  However, it adds a very nice worm driven implement lift.  This model packs a lot of functionality into the rear of the chassis with several gear systems crossing and one rotating axle used as a support for the steering.  This is one of the first sets to use the worm gear, and the first Technic model with a pendular axle.

Features
Steering
The front wheels can be steered using a wheel in the cabin or a "hand of god" control disguised as a vertical exhaust stack.

The wheel drives a pair of 8 tooth pinion gears through a universal joint.  The pinions drive the steering rack.  The vertical control connects to the same main steering axle via a set of 14 tooth bevel gears.  The steering mechanism itself uses control arms and toothed links as shown in the computer image.

In this case, the entire front steering assembly can pivot on the longitudinal steering axle, allowing this vehicle to traverse terrain in which the front wheels are in a considerably different plane than the rear (see Front View).

 steering    steering 
Click for an animation of the steering in motion.
Implement Lift
The implements can be lifted with a 4 bar linkage.  Input is a crank on the right side of the vehicle (red axle in computer image) which drives a set of 14 tooth bevel gears (purple axle).  Next is an 8 tooth spur gear mating with a 24 tooth crown gear (yellow axle).  This axle uses a worm gear to drive a 24 tooth spur gear (blue axle).  These second two gear stages result in a gear reduction of 72:1!  Behold the power of the worm.  Finally, a set of 14 tooth bevel gears drive the implement lift (green axle).  This final axle uses connectors with toothed links to drive the 4 bar linkage which keeps the implements parallel to the ground.

The use of a worm gear results in a system which cannot be backdriven (because the axial friction is higher than the backdriving torque due to the screw pitch angle) which allows the sprayer to be supported so that it does not fall under its own weight. 
lift    lift
Click for an animation of the lift in motion.
Sprayer
The rear of the tractor supports a foldable sprayer.  The tanks are made from 4 20x30 Technic wheels.  The nozzles are 1x1 cylindrical bricks.  The arms can be folded on simple finger hinges.
 plow
Click for an animation of the sprayer folding.
PTO
The rear wheels are on two separate axles.  The right hand wheel (red axle in the computer image) drives another axle (blue) through a set of 24 and 8 tooth spur gears.  A pair of 24 tooth gears drive a final PTO axle (green).

This PTO does not drive anything in the basic model, but various attachments could be added.
 diff   pto
Wheels and Tires
This set uses two foam 24x43 tires and wheels, the biggest foam tire that LEGO® made, on the rear.  The front are 20x30 foam tires and wheels.
 Wheels
 

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

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