The following Technic sets

  • 8043 (Motorized excavator)
  • 42009 (Mobile crane)
  • 42082 (Rough terrain crane)
  • 42110 (Liebherr)

and likely a couple of more have a double set of linear actuators to move the boom.

boom la's

For all these models, rotation is administered simultaneously on the gears of the linear actuators through bevel gears that are symmetrically placed.

bevel gear

I've been told that the symmetrical placement of the bevel gears is problematic. The reasoning being that as the boom moves up or down, additional rotation is placed on the linear actuator gears, but in opposite direction, causing one LA to slightly retract while the other to slightly extend, causing torsion on the boom. Hence it would be better to place the bevel gears asymmetrical.

My questions:

  • Is the reasoning wrong? (I think not)
  • If the reasoning is correct, why are the Lego designers seemingly not aware of this and haven't they addressed this in later Technic sets ?

1 Answer 1


Is the reasoning wrong?

Most probably it is not wrong, but its effect is small enough to be negligible (at least compared to other factors, like ensuring an error-free building experience). In serious engineering applications this effect would certainly have to be taken into account.

why are the Lego designers seemingly not aware of this and haven't they addressed this in later technic sets?

I took a look at my copy of the Bucket Wheel Excavator, and found that there is a minuscule difference in the angle of the double linear actuators between the highest and lowest boom elevation configurations:

highest boom configuration

lowest boom configuration

I don't have the relevant values right now, but I'd guess this angle of rotation does not result in a measurable difference in the lenghts of the two actuators. Or if it does, it is certainly absorbed by the flexibility of the structure without causing any harmful amounts of stress.

I think the same applies for the other such sets as well, but I'd welcome anyone providing data to either confirm or disprove this theory.

  • 1
    mkay, looking at the black gear in the back, I estimate the angle difference about 1,5 tooth on 20 tooth bevel gear, so 27 degrees. This gets geared up with 22/12 so 49,5 degrees. The LA gears go in different direction so a total difference of 99 degrees Asuming Sariels measurements of 26,5 rotations for an extension of 5 studs also apply to the new LA I get 5*99/360/26,5 = 0,052 studs or 0,41 mm difference. So yes, negligible. However, why can't Lego get it 'right'? I don't think ease of building is an argument here.I think Lego has the educational obligation to teach this stuff correct. Dec 5, 2019 at 15:54
  • Thanks for doing the research and math part :) The question "However, why can't Lego get it 'right'?" is, however, definitely out of scope for Bricks.SE, but one could always ask TLG directly.
    – zovits
    Dec 5, 2019 at 16:03

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