Driving the cylinders that lift the main boom requires taking many parameters into consideration. The bucket, arm(3.55), and boom(7.7) fully extended have a reach of 92 studs. If the longest arm were used it would be 100 studs. The pivot point used by the cylinder is 6 studs away from the base of the boom. The cylinder is clearly at a mechanical disadvantage. If the pivot were at 46 studs then the weight of the bucket/arm/boom would be the only force down on the cylinder, but it is at 6. This is a mechanical disadvantage of 7.666. Given that the bucket/arm/boom may weigh up to 1Kg fully laden, the cylinders have a lot of compression force to deal with. Any give in the design of the cylinder drive is going to show quickly.
This joint fails because the 13L dark grey liftarms that sandwich the light grey 3L cross block used for the drive shaft gears is not locked in place. As the torque goes up the beams pull apart and the whole mechanism fails.
The chassis needs to be able to handle the weight down and the gear boxing needs to arrest the dimensional forces of each gear. More on the chassis later. Two gears in the same plane acting on one another will try to push the other away in the one plane. When two gears are in perpendicular planes the forces push the gears apart in all three dimensions. The simplest way to handle perpendicular gears is to use a box on each perpendicular connection. Having more that one perpendicular connection on a single gear can be tricky as two boxes need to interact together adding to the forces that are trying to pull them apart. Ideally the two perpendicular gear connections are split into two gear sets connected by a driveshaft. This permits more room for structural components even if complexity goes up a bit.
This setup does the above.
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