Original Year	1988
# of pieces	332
Categories	Construction Equipment: Loaders
Dimensions	35(L)x14(W)x20(H)
Features	Steering, Lifting and dumping scoop
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8853 Excavator

Click the image to download the LDraw file of this model.
Model by Eric Albrecht

Set 8853, released in 1988, is for some reason called an "Excavator" in the USA even though it is clearly a loader. It is an example of an increasingly advanced construction vehicle which features rear rack and pinion steering, and a raising and dumping bucket using clever mechanics.

This is the first true Technic loader, but many more would follow with very similar features. The bucket used on this model appears only in one other set, the 8862 Backhoe Loader. The rear of the model is rather densely packed with gear systems. The compactness of the worm gear makes this possible.

Features
Steering The rear 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. The second axle drives a rack via a pair of 8 tooth pinion gears. The steering mechanism itself uses steering arms and toothed links as shown in the computer image.	Ldraw file courtesy of Benjamin Wendl. Click for an animation of the steering in motion
Bucket Lift The front bucket can be lifted via a crank in the rear of the vehicle. The crank drives an 8 tooth pinion which passes through a 24 tooth crown gear and then another 8 tooth pinion, cancelling out the gear ratio change. The torque the passes through a U-joint and turns a worm gear. The worm gear mates with a 24 tooth crown gear, resulting in 24:1 gear reduction. The crown gear axle attaches to toothed connectors which rotate a pair of crank arms. The crank arms push and pull 6L beams used as links which drive the bucket boom up and down. 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 bucket to be supported so that it does not fall under its own weight. Because the boom pivot and the dumping mechanism share an axle, their movement is dependent. As you can see in the animation, even though the dumping mechanism link remains fixed in space, it rotates with respect to the boom as the boom lifts resulting in the bucket dumping.	Ldraw file courtesy of Benjamin Wendl. Click for an animation of the bucket lifting.
Dump The front bucket can be dumped via a crank on the top rear of the vehicle. The crank drives a pair of 14 tooth bevel gears and then a pair of 8 tooth spur gears, one of which is connected to a worm gear. The worm gear mates with a 24 tooth crown gear, resulting in 24:1 gear reduction. The crown gear axle attaches to toothed bushings which rotate a pair toothed links used as crank arms. The crank arms drive a set of push/pull rods which control rotation of the bucket. 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 bucket to be supported so that it does not fall under its own weight. The orientation of the toothed bushings and connectors in this system is very important. If the bushing faces a link, they lock together and move with the axle. If the bushing faces away from the link, the link can rotate freely on the axle. The top view shows this clearly.	Ldraw file courtesy of Benjamin Wendl. Click for an animation of the bucket dumping.
Wheels and Tires This set uses four of the smaller size foam 20x30 tires and wheels.

Other Views
Side View	Top View
Back View	Front View
Computer Render

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8853 Excavator

Features

Other Views