Set 8862 was released in 1989 as one of the most complex sets to date, and remains one of my favorite Technic sets of all time. The list of features is simply remarkable, with 4 input cranks and 3 pneumatic switches. It uses the new pneumatic system with 3 actuators to power the backhoe boom, jib, and scoop. The backhoe also slews via a gear system. The vehicle has rack and pinion steering and deployable rear outriggers. Finally, the loader can be lifted and dumped via worm driven gear systems.
This set is fairly difficult to find if you should wish to buy one, and sells for a premium. The alternate model combine is almost as good as the primary model. Depending on which year you obtain, there are two different pneumatic versions. The newer set has black caps on the pneumatics, while the older set (pictured) has yellow caps. This is the only set to have the large Technic wheels in yellow.
There has only ever been one other backhoe which is at least as complicated, if not more so.
Main model |
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Features |
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Pneumatics |
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This model contains a double acting pneumatic system. The components of this system are connected with rubber tubing routed through and around the holes in the beams. A single pump is integrated into the hook to look like an exhaust stack. Depression of the piston produces pressure. The pressure is split and fed to the inlets of three selector valves (switches) with two outputs each. Selection of the switches in either direction allows pressure to flow to either chamber of the actuators. Finally, there are three pneumatic actuators which have ports at the head and rod ends to accept input from the switches. Head end pressure extends the piston, while rod end pressure retracts it. Like a real backhoe, the actuators are designed to work in compression during their power stroke. A real hydraulic actuator has significantly less output force in tension than in compression due to the fact that the annular area of the rod end of the cylinder is less than the full bore area of the head end. |
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Steering |
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The front 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 through a universal joint. The second axle drives a rack via an 8 tooth pinion gear. The steering mechanism itself uses the steering arms and toothed links as shown in the computer image. The steering knob is designed to look like a flashing light. The photo also shows a weighted brick which is used as a counterbalance to the rear backhoe excavator. |
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Luffing Backhoe Boom |
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The boom can be pivoted from a position almost parallel to the ground up to an angle of about 60 degrees. A pneumatic actuator drives this motion. The actuator has a fairly long moment arm giving it adequate mechanical advantage. The switch controlling this motion is on the rear of the cab. |
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Luffing Backhoe Jib |
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The jib can be pivoted around an axis on the end of the boom through an angle of about 60 degrees. A pneumatic actuator drives this motion. The switch controlling this motion is on the rear of the cab. |
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Dumping Backhoe Scoop |
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The scoop can be pivoted about an axis on the end of the jib. A pneumatic actuator drives this motion through a linkage made of liftarms and rotors. The switch controlling this motion is on the rear of the cab. |
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Slewing |
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The rotation of the boom is accomplished via a crank on the left side. This crank passes torque through two sets of 14 tooth bevel gears and then into a worm gear. The worm gear drives a 24 tooth crown resulting in a reduction of 24:1. The final gear turns a threaded axle which passes through an old 4x4 turntable. Axle nuts on the top and bottom of the threaded axle provide clamping force which keeps the moment of the boom from tipping itself free of the turntable. Because the pneumatic hoses pass between the cabin and the boom, the boom can only slew about 180 degrees before the hoses become twisted. The boom can only be pivoted when the outriggers are deployed or they block its motion and lock it in the centered position. |
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Outriggers |
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There are a pair of outriggers behind the rear wheels which are constructed using the new 1x4 liftarms and standard beams. The feet are made from beams and sloped bricks. The color coded computer image shows the mechanism. A crank on the right side drives two sets of 14 tooth bevel gears (black and green axles). Next a worm gear (green axle) drives a 24 tooth crown gear (blue axle) resulting in 24:1 reduction. The crown gear on the blue axle drives a pair of 16 tooth spur gears (red axles) in opposite directions (16:24). Each red axle drives an outrigger after passing through a set of 24 and 8 tooth gears (24:8), resulting in a final gear reduction of 48:1. When constructing the outriggers, care must be taken to keep these two gears in phase so the outriggers will be even. Due to a careful selection of geometry, the outriggers are self-locking. When fully deployed, weight on the foot tends to rotate the two sets of liftarms together, locking the assembly, rather than trying to backdrive them. It is this geometry, rather than the worm gear, that prevents the outriggers from collapsing under weight. The outriggers provide a wide base for stability and are significantly taller than the rear tires so that the tires are lifted off of the ground. |
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Lifting Loader Bucket |
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The front bucket can be lifted and dumped in a manner almost identical to that used on the 8853 loader. Input is via a crank on the right of the vehicle. The crank drives a pair of 14 tooth bevel gears. The torque then 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 liftarms which act as cranks. 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 slightly. |
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Dumping Loader Bucket |
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The front bucket can be dumped via a crank on the left of the vehicle. The crank drives a pair of 14 tooth bevel gears and then 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 liftarms used as cranks. The crank arms drive a set of push/pull rods which control rotation of the bucket. The motion is transferred past an angle change in the boom using a pair of 3 blade rotors. 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. |
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Wheels and Tires |
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This set contains 4 sets of wheels and tires of type:
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